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Geology of the country around Inverurie and Alford. Memoir for 1:50 000 geological sheets 76E and 76W (Scotland)

By D Gould,  contributor: Geophysics K E Rollin

Bibliographic reference: Gould, D. 1997. Geology of the country around Inverurie and Alford. Memoir of the British Geological Survey, Sheets 76E and 76W (Scotland).

British Geological Survey

• Author
• D Gould, BSc, PhD British Geological Survey, Edinburgh
• Contributor
• K E Rollin, BSc British Geological Survey, Keyworth

London: The Stationery Office 1997. NERC copyright 1997. First published 1997. ISBN 0 11 884525 X. Printed in the UK for the SO Dd 301331 C6 1/97

(Front cover) Kenmay Quarry (Kenmay Granite) with Bennachie (Bennachie Granite) in the distance Most of the intervening ground is underlain by Dalradian metasedimentary rocks [NJ 7387 1685] (D4338).

(Rear cover)

(Frontispiece): Bennachie (Bennachie Granite) from Pitscurry Quarry, Pitcaple. Low ground in foreground is underlain by gabbroic rocks of the Insch intrusion [NJ 728 266] (D4331).

Other publications of the Survey dealing with this district and adjoining districts

Books

• British Regional Geology
• The Grampian Highlands, 3rd edition, 1966; 4th edition, 1995
• Memoirs
• The geology of central Aberdeenshire, 1890
• The geology of the country around Aberdeen, 1986
• Explanation of Sheet 75: west Aberdeenshire, Banffshire, parts of Elgin and Inverness, 1896
• The geology of the country around Banff, Huntly and Turriff, 1923
• Reports
• See Appendix 4
• Geochemical atlas
• East Grampians

Maps

• 1:625 000
• United Kingdom (North Sheet)
• Solid geology, 1979
• Quaternary geology, 1977
• Aeromagnetic anomaly, 1972
• 1:250 000
• Moray–Buchan Sheet (57°N–04°W) Solid geology, 1975
• Aeromagnetic anomaly, 1978
• Bouguer gravity anomaly, 1977
• 1:50 000
• Sheet 65E (Ballater) Solid, in press
• Sheet 66W (Aboyne) Solid, in press
• Sheet 67 (Stonehaven) Solid and Drift, 1929, reprinted 1967
• Sheet 75E (Glenbuchat) Solid, 1995
• Sheet 76E (Inverurie) Solid, 1992
• Sheet 76W (Alford) Solid, 1993
• Sheet 77 (Aberdeen) Solid, 1982; Drift, 1980
• Sheet 86 (Huntly) Solid, 1923; Drift, 1923
• Sheet 87W (Ellon) Solid, 1991

Notes

Throughout the memoir the word 'district' refers to the area covered by 1:50 000 geological sheets 76E (Inverurie) and 76W (Alford).

National Grid references are given in square brackets; those with northings beginning with the figure 9 lie in 100 km square NO and those with northings beginning with the figures 0, 1 or 2 lie in 100 km square NJ. National Grid references associated with a distance from a named topographic feature (e.g. 500 m south of Logie [NJ 505 188] ) relate to the geological locality being described and not to the named topographic feature.

Numbers preceded by the letter S refer to the Scottish sliced rock collection of the British Geological Survey (located at Murchison House, Edinburgh).

List of 1:10 000 geological maps available for 1:50 000 Sheets 76E and 76W

National Grid 1:10 000 geological maps included wholly or partly in Sheets 76E and 76W are listed below, together with the initials of the surveyors and the date of survey. These maps show the Solid geology only.

The surveyors were D J Fettes (in areas outside Sheet 76 only), D Gould and A Grout. Sheet NJ 42NW was only partially surveyed. Information for the unsurveyed part of this sheet was obtained from Busrewil et al. (1973).

The maps are not published but are available for consultation at the Library, the British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA. Dyeline copies may be purchased from the Bookshop.

Acknowledgement

For details of areas surveyed by different geologists, see the section 'History of research' in Chapter 1.

The memoir has been written by Dr D Gould. However, the 'Crustal model' section in Chapter 3 and (Figure 8) were written and modelled by K E Rollin. Chapter 11 (Quaternary) relies heavily on work by C A Auton and J W Merritt (Auton and Crofts, 1986 and Auton and et al., 1988). The advice of P J Brand in compiling Appendix 3 is acknowledged.

The assistance of Dr W A Ashcroft (Aberdeen University) in the magnetic survey and related pitting in the Pitcaple area, and in elucidating the nature of the Rhynie sedimentary basin is gratefully acknowledged. Dr Ashcroft and Dr A G Leslie (Queen's University, Belfast) are also thanked for valuable advice on the geological structure of the eastern part of the Insch Intrusion.

The co-operation of landowners and quarry owners in the area, especially Tillypronie and Dunecht estates, the Forestry Commission and Grampian Regional Council, is gratefully acknowledged.

Preface

This memoir is based on a rapid resurvey of the Solid geology of the district, together with input from resource studies for sand and gravel in the eastern parts of the district. These BGS surveys have been integrated with information from geophysical surveys conducted for metalliferous mineral exploration and university research (mostly directed to the igneous rocks of the district) to provide an updated account of the geology of a critical, though poorly exposed, part of the eastern Grampian Highlands, superseding the memoir published over a century ago.

The Dalradian rocks, derived from sandy and muddy sediments with thin calcareous layers, were deposited in late Precambrian times on the edge of the expanding and deepening Iapetus Ocean. During the Caledonian Orogeny, from late Precambrian to end-Silurian times, the Dalradian rocks suffered polyphase folding, and were subjected to a distinctive low-pressure regional metamorphism (the Buchan type). Locally, the proximity of large quantities of basic magma resulted in partial melting of the country rock to form 'xenolithic' feldspar-sillimanite-garnet-cordierite gneisses.

The late-tectonic layered ultrabasic to basic Insch and Boganclogh intrusions provide a good example of crystallisation differentiation. Lithologies range from dunite to syenite, and olivine, pyroxene and feldspar compositions change progressively with height in the layered succession.

A late-tectonic phase of granitic intrusion, c. 470 Ma, produced the Kemnay granite and the granite-granodiorite-diorite intrusions at Kennethmont and Tillyfourie, which show evidence of slight deformation during the latest phases of the Caledonian orogeny. The postorogenic granitic rocks (425–398 Ma) form part of the East Grampians Batholith, which underlies the region at depth. They comprise the earlier Crathes Suite of largely hornblende-bearing diorites to megacrystic granodiorites, and the later Cairngorm Suite of amphibole-free biotite-granites.

The Lower Devonian red beds of the Rhynie outlier, which rest unconformably on the Dalradian metasedimentary rocks and Caledonian intrusive igneous rocks, were deposited in an semiarid continental environment. The Rhynie Chert, a silicified hot-spring deposit of early Devonian age, has yielded exceptionally well-preserved remains of early terrestrial arthropods and vascular plants.

During the Quaternary, repeated glaciation affected the landscape, and several conspicuous rock-cut melt-water channels were excavated. The end of the latest, Devensian, glaciation in the area was marked by the deposition of much glaciofluvial sand and gravel in many lower-lying areas.

Traces of gold have recently been found in the Rhynie Chert, and molybdenite in veins associated with the Middleton Granite. There is a possibility of copper, nickel and platinum-group metals occurring in the basic and ultramafic intrusions of the district. Several of the granites, were quarried for building stone in the late 19th and early 20th centuries. Between 1885 and 1918, a bed of diatomite in the lacustrine deposits of the Muir of Dinnet was exploited for use by the explosives industry. At present, quarrying is restricted to the production of aggregate and reconstituted stone. Sand and gravel deposits are being exploited, largely in the eastern part of the district. However, the district is likely to retain its essentially agricultural and recreational character for the foreseeable future.

Peter J Cook, CBE, DSc, CGeol, FGS Director, British Geological Survey, Kingsley Dunham Centre, Keyworth Nottingham NG12 5GG

Geology of the country around Inverurie and Alford–summary

This memoir describes the geology of a largely rural area lying 12 to 45 km west of the city of Aberdeen, containing a mixture of arable land, pasture, heather moor and conifer plantations, and ranging in altitude from 50 m to 619 m above OD.

The district lies within the Caledonian orogenic belt and is underlain by metamorphic and igneous rocks in roughly equal proportions, with only two small outliers of later sedimentary rocks.

The Dalradian metasedimentary rocks were deposited on the margin of the spreading Iapetus Ocean in the late Precambrian (650–600 Ma). The Argyll Group rocks were deposited in fairly shallow water as sands, silts and muds, with minor calcareous sediments. The Southern Highland Group rocks were originally coarse, poorly sorted sands, grading to silts and muds, deposited by turbidity currents in deep water. Dolerite sheets were intruded into the Dalradian sediments shortly after deposition.

The Dalradian rocks suffered polyphase deformation and high-temperature metamorphism during the Caledonian Orogeny. A late Precambrian phase of deformation resulted in the formation of large-scale nappe structures, overturned to the south-east, and isoclinal minor folds, accompanied by greenschist-facies regional metamorphism.

In the early Ordovician, the second deformational episode was followed by the intrusion of large volumes of basic magma, which crystallised as a number of layered intrusions. The Insch and Boganclogh intrusions form the largest coherent mass of basic rocks in the north-east Grampian Highlands. The climax of regional metamorphism was roughly contemporaneous with these intrusions. The metamorphism was characterised by the presence of andalusite and cordierite in pelitic rocks of green-schist and higher grade, indicating a steep geothermal gradient. The metamorphic grade was highest in the south and decreased northwards. The third regional deforma tion episode occurred almost immediately after these events, while the rocks were still at high temperatures. Folding on kilometre and smaller scales was accompanied by the development of zones of ductile shear up to 500 m wide. Still in the early Ordovician, the fourth period of deformation, resulting in broad warping of north-east Scotland, was accompanied by brittle minor folds, retrogressive metamorphism in the metasedimentary rocks, and a period of regional uplift. A suite of granites to diorites, marked by extensive marginal vein complexes, was intruded, and a suite of pegmatites and aplites was intruded into the basic igneous rocks.

In the Silurian and early Devonian, (c. 425–398) Ma, many postorogenic granitic plutons, together with felsite, microdiorite, aplite, pegmatite and microgranite sheets and dykes, were emplaced. The earlier Crathes Suite ranges from diorite through tonalite to granodiorite, while the later Cairngorm Suite of biotite-granites cuts the Crathes Suite plutons.

Deposition of the largely arenaceous Lower Devonian Rhynie Group sediments in a semiarid fluviatile environment of moderately high relief was accompanied by faulting and minor andesitic volcanism, and by hot spring activity near Rhynie.

Quartz-dolerite dykes were intruded along ENE-trending fractures in late Carboniferous times (c. 295 Ma).

Extensive glaciation during the Quaternary removed most of the loose superficial material from the higher ground. Meltwater from the decay of ice-sheets cut large channels in bedrock. By the end of the Devensian glaciation, about 13 000 years ago, much of the ground was mantled in till. Large quantities of sand and gravel were deposited and then reworked by large river systems, especially in the eastern part of the district. In postglacial times, diatomite formed in shallow lakes in moundy terrain. Peat formed both in waterlogged lowland areas and as a blanket on hill tops.

Geological sequence in the Inverurie–Alford district

Chapter 1 Introduction

Location and physical features

The district described in this memoir comprises the 1:50 000 geological sheets 76E (Inverurie) and 76W (Alford) (Figure 1). It is located in the Gordon, Kincardine and Deeside, and City of Aberdeen districts of Grampian Region. The principal settlement is Inverurie (population 7649 in 1981), and the small towns or villages of Oldmeldrum, Insch, Kemnay, Kintore, Torphins, Alford, Tarland, Rhynie and Lumsden also lie within the district. The district is largely agricultural, with most of the ground below 200 m being arable, and much of the ground between 200 m and 400 m improved pasture. The higher ground is mostly heather moorland, but considerable areas above 250 m have been planted with commercial forestry since 1960. There is varied light industry in Inverurie, while quarrying is important at Kemnay, and there are whisky distilleries at Oldmeldrum and Kennethmont. Many of the people living in the eastern part of the district commute to Aberdeen to work.

The greater part of the district lies within the catchment of the River Don. However, the north-western corner is drained by the Water of Bogie, a tributary of the Deveron, and the southern third of the district drains towards the Dee, which flows a short distance to the south of the district boundary.

The altitude varies from barely 50 m to 619 m. The landscape is dominated by two main erosion surfaces: the Grampian and Buchan surfaces, typically separated from each other by relatively steep slopes. These surfaces were formed by subaerial erosion in a warmer climate than the present. They have been modified by differential uplift and by the effects of the Pleistocene glaciations. The higher of the two main surfaces is the Grampian Surface, which is represented by the roughly concordant summits of the major hills; with the exception of Pressendye (619 m), these lie in the range 450 to 530 m. The most easterly of these is the Hill of Fare (471 m), which is a relatively little-dissected plateau. Benaquhallie (494 m), together with the hills to the north-east, forms a gently rolling upland area. The Bennachie (528 m) and Cairn William (446 m) massifs together constitute a large area of high ground which has been deeply dissected by the River Don. The lower, Buchan Surface is represented by the considerable areas of rolling country where the hill summits reach 250–300 m, the height generally increasing westwards and, to a lesser extent, southwards. Such areas may be found around Muir of Fowlis, between Dunecht and Kemnay, and near Durno. However, this surface has been strongly dissected, and few plateau-like areas remain.

Between the areas of higher ground are amphitheatre-like basins, such as the Howes of Alford and Cromar, the Muirs of Dess and Dinnet, upper Strathbogie, and the area around the Loch of Skene. Their formation probably predated the glaciation of the area, but glacial erosion has greatly enlarged them. In places, they contain lochs infilling rock basins, but for the most part their floors have been the sites of deposition of glaciofluvial sand and gravel.

History of research

Sheet 76 (Scotland) was surveyed by J Horne, L W Hinxman, D R Irvine, H M Skae and J S G Wilson between 1879 and 1883; the map was published in 1886, followed by a brief memoir (Wilson and Hinxman, 1890). Due to the policy in force at the time, the area was surveyed rapidly, and Skae and Wilson mapped directly on to the one-inch (1:63 360) topographical map. Read (1927; 1928) investigated the rocks of Cromar, with particular attention being given to the 'xenolithic' gneisses of Scar Hill and the stratigraphical position of the Deeside Limestone. Apart from Read's work, the district was neglected by the Geological Survey from 1890 to 1980, and the only significant work was done by universities and mining companies.

Read's work on the part of the Insch Intrusion lying in the Huntly district (Sheet 86) to the north eventually focused attention on the remainder of the intrusion (Whittle, 1936). Later, Read and his students produced a series of papers on the Insch, Kennethmont and Boganclogh intrusions (Read, 1951; 1956; Read and Haq, 1965; Read et al., 1961; Sadashivaiah; 1954a; b). Read (1952; 1955) also published on the regional geology and structure of the north-east Grampian Highlands.

Interest in the copper and nickel potential of the basic masses prompted a detailed aeromagnetic survey of most of the district, carried out by Barringer Research for Exploration Ventures Ltd. in 1970–71. The results of these surveys, together with ground-based follow-up surveys by various mining companies, have been assessed and used in a reinterpretation of the geology of the district (Gallagher, 1983). Research on the basic masses has continued steadily (Allan, 1970; Ashcroft and Munro, 1978; Blyth, 1969; Clarke and Wadsworth, 1970; Leslie, 1984; Wadsworth, 1986; 1988; 1991). In addition to the published work, ground-based magnetic surveys of parts of the basic masses have been carried out by staff and students at Aberdeen University. Relatively little work has been done on the granites of the district, except for a survey of the Bennachie Granite (Anderson, 1971), and a major study by Walsworth-Bell (1974), who differentiated the phases of the 'Skene Complex' which had been named originally by Bisset (1934).

The Devonian (Old Red Sandstone) rocks of the Rhynie outlier were first described by Geikie (1878), whose work was consolidated during the primary survey of the area. During a reassessment of the Rhynie area (Mackie, 1914), the fossiliferous Rhynie Chert was discovered, prompting a pitting programme sponsored by the British Association for the Advancement of Science (Horne et al., 1917). Well-preserved early land plants and arthropods were described (Kidston and Lang, 1917; 1920a; 1920b; 1921a; 1921b; Scourfield, 1926; 1940). Additional material was obtained from excavations in the 1970s and 1980s. Recently, the hydrothermal system which produced the Rhynie Chert has become the focus for mineral exploration, which has prompted drilling and geophysical surveys (Rice and Trewin, 1988; Trewin and Rice, 1992).

BGS interest in the district revived shortly after 1980 with the inception of the East Grampians Project. Originally designed as a rapid geological resurvey of the country east of a line from Forres to Grantown-on-Spey and Dunkeld, targeted on specific geological problems in the region, the project has expanded and production of many updated 1:50 000 maps is planned, with memoirs to be produced for most of the districts. Sheets 76E and 76W were resurveyed at an early stage in the programme when only a rapid resurvey of the solid geology, incorporating all outside sources of information, was required. Hence almost the entire district was covered by the author between 1985 and 1987. The area of Dalradian rocks to the west of the Devonian rocks at Lumsden was surveyed by A Grout and the small portion of the Ballater Granite lying within the district was surveyed by S Robertson. (Figure 2) is a simplified and reduced version of the resulting solid geological map.

Regional geochemical mapping of the UK landmass started in the north, and stream sediment sampling of the Moray–Buchan 1:250 000 Sheet, which includes Sheet 76, was conducted between 1977 and 1980 (BGS, 1991). The data were used by the Mineral Reconnaissance Programme to locate areas of potential economic interest. The BGS Mineral Reconnaissance Programme has investigated a molybdenum anomaly centred on the Middleton granite (Colman et al., 1989; Kimbell, 1991), and has also investigated the possibility of platinum-group metal mineralisation in the ultramafic rocks at the south-east corner of the Insch mass (Gunn and Shaw, 1991).

In 1985, a commission from the Scottish Development Department for an environmental geology map for the use of local government planners was answered by a rapid BGS resurvey of both solid and drift deposits in 10 X 10 km squares NJ70 (A J Highton), NJ71 (C G Smith), NJ72 (A Grout), N069 (D Gould) and N079 (A J Highton) (Smith, 1986). The proximity of Aberdeen to the eastern part of the district has led to an interest in its sand and gravel resources. Two commissions from the Department of the Environment, via the Scottish Development Department and Grampian Regional Council, have included parts of the district. Auton and Crofts (1986) evaluated NJ71, NJ80 and NJ81, while Auton and others (1988) surveyed NJ70, NJ72, N079 and N089. These authors surveyed the sand and gravel deposits in detail, and resurveyed the remaining drift deposits on a semi-reconnaissance basis. The potential of the Old Red Sandstone of the Rhynie outlier as an aquifer has been assessed by Robins (1990). Interest in the Cairngorm Suite of granites as potential sources of geothermal power, owing to their high uranium, thorium and potassium contents, prompted geochemical and petro logical studies of the Bennachie and Ballater granites (Webb and Brown, 1984), which were followed by the drilling of boreholes for heat-flow measurements.

Summary of the geological history

The Inverurie–Alford district lies within the Grampian Highlands, part of the Caledonian Orogenic Belt. The Grampian Highlands lie within the orthotectonic Caledonides, defined as that part of the belt where pre-Caledonian, late-Precambrian sedimentary rocks have suffered deep burial, polyphase folding and thrusting, regional metamorphism, and intrusion of plutonic igneous rocks. The Grampian Highlands are bounded to the north-west by the sinistral Great Glen Fault and to the south-east by the Highland Boundary Fault, which has a large downthrow to the south-east, but probably conceals a terrane boundary.

The LISPB seismic survey (Bamford, 1979) indicates that the Grampian Highlands north of a line running approximately from Connel to Crieff are underlain by 28 to 35 km of continental crust, divisible by P-wave velocity into three layers. The uppermost layer, 12–15 km thick, is interpreted from the surface geology to consist largely of Dalradian metasedimentary rocks and granitoid plutons. The second layer, from 12–15 km to about 20 km depth, is interpreted as granulite-facies basement gneisses; the physical properties can be matched with Lewisian rocks exposed in the North-west Highlands. The third layer, from 20 km to 28–35 km depth, could be matched by basic granulites which have partly regressed to amphibolites (Hall, J, 1985).

Apart from the Central Highland Migmatite Complex, which is of uncertain status, the rocks exposed in the Grampian Highlands are metasedimentary rocks of late Proterozoic age, together with later igneous and sedimentary rocks. The late Proterozoic Dalradian Supergroup has a total thickness of 20 km, as measured from exposure, although the total thickness preserved in any one place is probably considerably less, as indicated by LISPB. The basal Grampian Group is exposed in the Monadliath Mountains and on Strathspey, but is believed to underlie the rest of the Grampian Highlands at depth.

The south-east corner of the Inverurie–Alford district lies 15 km from the Highland Boundary Fault, while the north-west corner is 70 km from the Great Glen Fault. The oldest rocks exposed in the district belong to the middle part of the Dalradian Supergroup. In Deeside, feldspathic psammites and semipelitic to pelitic gneisses of the Queen's Hill Formation can be traced into the district from the south-west, and can be correlated with reasonable confidence with the Ben Lui Schist of Perthshire, which belongs to the Crinan Subgroup of the Argyll Group. Similarly, the Deeside Limestone, which on Deeside (Sheet 66W) underlies the Queen's Hill Formation in an inverted succession, is almost certainly stratigraphically equivalent to part of the Loch Tay Limestone (Tayvallich Subgroup).

In the Alford–Tarland area, the stratigraphical correlation of the metasedimentary rocks with the Dalradian of Perthshire and the south-west Highlands is much less certain, due to the absence of a widely mappable calcareous formation at the top of the Argyll Group. The Craigievar Formation, an assemblage of psammite, semipelite and pelite, with rare calc-silicate lenses near Towie, has been defined to include all the rocks below the incoming of gritty psammites at the base of the Southern Highland Group. The southern portion of the Craigievar Formation probably youngs southwards, but the northern part definitely youngs northwards towards the Southern Highland Group rocks. In the country around Inverurie, lying to the east of the Bennachie Granite and south of the Insch basic mass, the Aberdeen Formation comprises the uppermost part of the Argyll Group. It is lithologically similar to the Craigievar Formation, though with more psammite and less pelite.

The Southern Highland Group rocks represent an overall pattern of turbiditic sedimentation, with beds of gritty psammite, best developed in the western Correen Hills, becoming important. They come to resemble the Fyvie Schists of the Ythan section (Sheets 86E and 87W), to which they are probably stratigraphically equivalent.

Evidence of four deformation episodes can be recognised in the eastern Grampian Highlands. The earliest episode, which produced large-scale recumbent SE-directed nappes, is believed to have occurred 590600 Ma ago, shortly after deposition of the Southern Highland Group rocks (Rogers et al., 1989). Within the district, this episode caused the overturning of the Queen's Hill and Deeside Limestone formations around Queen's Hill, and the major reversal of younging direction which occurs in the southern part of the Craigievar Formation. The second fold episode is now considered (Robertson, 1994; Tanner and Leslie, 1994) to be considerably later than the third fold episode, though still predating emplacement of the 'Younger Basic' masses. Its effects are difficult to separate from those of the first fold episode in the Inverurie–Alford district, due to the absence of distinctive minor folds of this age and the intense subsequent deformation.

The third fold episode produced kilometre-scale folds throughout the Grampian Highlands, and associated metre-scale minor folds with gently dipping axial planes. It occurred shortly after the peak of regional metamorphism and the approximately coeval c. 490 Ma late-tectonic 'Younger Basic' intrusions. Many parts of the basic masses are traversed by shear zones, within which amphibolite-facies metamorphism has been recorded (Kneller and Leslie, 1984). The shear zones developed immediately following D₃. The fourth fold episode produced the Turriff Syncline and the Buchan Anticline in the north-east Grampian Highlands, as well as kinks and crenulations of all earlier structures; it was probably roughly contemporaneous with the retrogressive metamorphism which affected parts of the eastern Grampian Highlands and may have been roughly concurrent with regional uplift movements.

The regional metamorphism of the Dalradian rocks of the district is of the low-pressure Buchan type, with andalusite and cordierite developed from greenschist facies upwards. The grade increases southwards, that is towards lower structural levels, with fibrolitic sillimanite replacing biotite, and muscovite and biotite replacing andalusite. Adjacent to the basic intrusions, the metamorphic grade increases still further, and garnet, cordierite, prismatic sillimanite and potash feldspar occur, while the rocks change in texture to become coarse granular gneisses with xenoliths of quartzite, amphibolite, and silica-poor hornfels indicating probable partial melting of the envelope rocks. The granitic intrusions produce only minor contact metamorphic effects, in part being masked by the previous high-grade regional and contact metamorphism. However, in a few places fresh andalusite is developed within a few hundred metres of granite contacts, as at the south-west margin of the Hill of Fare intrusion.

The basic masses of the eastern Grampian Highlands (the Younger Basics of Read) were intruded at about 490 Ma, close to the regional metamorphic peak. The magma probably crystallised in several separate magma chambers of varying size, in part controlled by tectonic features. Subsequent shearing and faulting have resulted in basic and ultrabasic rocks being separated into several discrete basic masses which now have a broadly H-shaped regional outcrop pattern. A major portion of the central Insch–Boganclogh mass lies within the district, as well as a small part of the Morven–Cabrach mass, and the smaller Tarland, Kildrummy, Lawel Hill and Lynturk bodies. Within the Insch–Boganclogh mass, most of the magma crystallised as a series of layered cumulates ranging from dunite at the base to syenite at the top, but considerable amounts of quartz-biotite-norite and fine-grained granular gabbroic rocks are developed in close association with the cumulates. Near the contacts, xenolithic cordierite-norites are interpreted as partial melts of country rock; many of the hornfelses nearest to the contact have silica-poor and other extreme chemical compositions, suggesting that they are restites. Shortly after the crystallisation of the basic rocks, an episode of locally intense shearing affected the eastern Grampian Highlands. The resultant shear zones principally trend east and NNE. Lenticular masses of serpentinised ultramafic rocks are common close to the tectonic contacts of the main masses; they are often highly magnesian, possibly reflecting a different parent magma from the main basic intrusions.

The first, late-tectonic, episode of granitic intrusion marks the first period of major uplift of the Caledonian orogenic belt in Scotland. The plutons emplaced in the district at this time were the Kemnay and Corrennie granites, the Tillyfourie Tonalite, the Syllavethy intrusion and the Kennethmont Complex. They are all, except for the Kennethmont Complex, well foliated, and, except for the Kemnay and Corrennie granites, contain a high proportion of dioritic to tonalitic material. Except where faulted, the margins are marked by extensive vein complexes. They are believed to be coeval with the Aberdeen Granite, dated at 470 ± 1 Ma (Kneller and Aftalion, 1987). Aplites and pegmatites cutting the basic rocks have been dated at 462 ± 5 Ma (van Breemen and Boyd, 1972), i.e. Middle Ordovician.

The post-tectonic granitic intrusions of the district have been divided into two suites. The earlier Crathes Suite, for which no radiometric ages are available, ranges from diorite through tonalite and granodiorite to granite, and probably predates the last major uplift of the district. The component plutons have sharp contacts, where seen, but no contact metamorphic effects have been detected. They form an almost continuous 512 km-wide east–west belt north of the River Dee (Figure 2). Some of the intrusions are multiphase, with the more acid rocks being generally the younger. They have suffered little hydrothermal alteration and cause no gravity or magnetic anomalies.

The later Cairngorm Suite, consisting entirely of biotite-granites, is the surface expression of the East Grampians Batholith (Plant et al., 1990). It was emplaced during and following the major end-Caledonian uplift. The larger members of the suite comprise several granite phases of varying grain size and porphyritic character. They show extensive hydrothermal reddening, but where fresh have a significant magnetic susceptibility. The plutons produce complex aeromagnetic anomalies, and have related negative Bouguer gravity anomalies. They are cut by veins and sheets of pegmatite and aplite, and also by north–south zones of brecciated and silicified aplite.

Post-tectonic minor intrusions, related to both suites of granitoid intrusions, are common in the district. They are typically dykes, but sills and rare plugs also occur. Felsite, quartz-porphyry and microdiorite are the commonest lithologies, but lamprophyre (spessartite and vogesite) dykes are present, and the plugs consist of appinitic diorite.

The Lower Devonian sedimentary rocks of Old Red Sandstone facies, preserved in the Rhynie and Towie outliers, are only slightly younger than the Cairngorm Suite granites. The sediments were originally deposited by rivers in depressions in a tropical, arid landscape showing moderate relief. They consist of well-bedded conglomerates, mudstones, siltstones and sandstones. A thin andesitic flow and some tuffs occur in the sequence, and the Rhynie Chert represents a silicified hot spring deposit (Trewin and Rice, 1992). Faulting was penecontemporaneous with sedimentation, and the Rhynie outlier is in part bounded by north- to NNE-trending faults, parallel to those associated with breccias cutting the Cairngorm Suite granites. No Middle or Upper Devonian rocks occur in the district, but Middle Devonian rocks occur in the Turriff outlier, extending to within 10 km of the northern boundary of the district.

A major swarm of east- to ENE-trending quartz-dolerite dykes was intruded during the late Carboniferous, notably in the Midland Valley of Scotland, though a few members of the suite occur in the eastern Grampian Highlands. Several discontinuous dykes traverse the district.

Permo-Triassic and Lower Jurassic sedimentary rocks occur along the Moray Firth coast, and Cretaceous rocks are known to crop out a short distance offshore. However, the former extent of the post-Silurian sedimentary cover of the eastern Grampian Highlands is not known. Flints occurring in the Pliocene Buchan Ridge gravels within 20 km of the north-east corner of the district, suggest that Upper Cretaceous chalk may have covered the district.

The Cairngorm (1200 m), Grampian (470–900 m) and Buchan (150–250 m) erosion surfaces are all probably of Neogene age, the Buchan surface being Pliocene, and closely related to the Buchan Ridge gravels. During the period when these surfaces were forming under warm, humid climatic conditions, patchy deep weathering, with the formation of grusses, occurred widely in the eastern Grampian Highlands. During the Pleistocene (2.4 to 0.01 Ma), the eastern Grampian Highlands were subjected to a series of glaciations separated by interglacials. The ice sheets eroded large amounts of material from the higher ground, but appear to have removed relatively little material from the lower-lying north-eastern part of the region. At the end of the last major glaciation of the district, about 13 000 years ago, large quantities of till and moraine were deposited by the decaying ice sheet, and much of this was reworked by meltwaters to produce extensive glaciofluvial deposits. Since then, hollows and river valleys have been partly filled by lacustrine deposits, alluvium and peat; some of the upland areas have also been covered by blanket peat, which has formed since a marked deterioration in the climate about 5000 years ago.

Chapter 2 Dalradian lithostratigraphy

The Dalradian Supergroup is a succession of metasedimentary rocks with minor metavolcanic units which has been recognised from Connemara to Shetland. The sedimentary and volcanic rocks from which they were derived were deposited at the eastern margin of the Laurentian continent during the late Precambrian, the youngest rocks being about 600 Ma in age (Halliday et al., 1989). The rocks of the supergroup have been subjected to at least four major tectonic episodes, some of which were accompanied by regional metamorphism, reaching amphibolite facies in places. As a result, the Dalradian rocks now range from slates and phyllites, through schists to migmatitic gneisses. Several suites of plutonic igneous rocks have been intruded into rocks of the Supergroup.

The Dalradian Supergroup is divided into four groups, the Grampian, Appin, Argyll and Southern Highland, of which only the two last are represented at outcrop in the Inverurie–Alford district. The formations cropping out within the district are listed in the geological sequence table (inside front cover). Detailed stratigraphical correlation is hindered over most of the district by poor exposure, by the presence of many large intrusions and by the absence of the distinctive marker beds which are used for correlation farther to the south-west. Consequently it has been necessary to recognise informal formations in different parts of the district (Figure 2). The basic igneous intrusions in the north of the district form an east–west gap impeding correlation with metasedimentary rocks farther north, while an almost continuous outcrop of granitic plutons separates the Dalradian rocks of the south-western margin of the district from those in the central and eastern parts.

The Dalradian rocks in the south-western part of the district, north of Aboyne and Dinnet, are continuous with those of the Aboyne and Ballater districts (sheets 66W and 65E), and the Queen's Hill (Crinan Subgroup) and Deeside Limestone (lower Tayvallich Subgroup) formations are recognised in all three districts. To the north of the belt of granitic rocks, no continuous mappable unit of calcareous rocks can be recognised, and the Craigievar Formation (to the west of the Bennachie Granite) and the Aberdeen Formation (to the east of the Bennachie Granite) are probably equivalent to parts of the Crinan and Tayvallich Subgroups.

Rocks of the Southern Highland Group are represented by the Suie Hill Formation to the south of the Insch and Boganclogh basic intrusions, and the Clashindarroch Formation to the north of the Boganclogh intrusion.

Most of the formations in the district consist of varying proportions of psammite, semipelite and pelite, the metamorphosed equivalents of sandstone, siltstone and mudstone respectively. The dividing lines between the three categories are not precisely defined because of variations in both composition and metamorphic grade. The separation between psammite and semipelite is made on the mica content of the rocks, and hence their fissility; the division is placed at about 25% total mica. The division between semipelite and pelite is placed at the point where the content of micas and other Al-rich silicates (cordierite, andalusite, garnet, etc.) exceeds that of quartz plus feldspars; it normally corresponds to about 40% total micas. Very few analyses of metasedimentary rocks from the district are available; hence a geochemically based classification of the rocks is not practical.

The BGS (1991) stream sediment survey shows significant geochemical differences between the Argyll and Southern Highland groups. The Argyll Group is relatively enriched in Mg, Fe, Co, Ni, Cr, Ca, Sr, Ti, V, Y, La, K and Rb, but relatively depleted in Be, B, Li, Zn, Ba and Ga with respect to the Southern Highland Group.

The base of the Southern Highland Group is taken as the first incoming of poorly sorted, gritty psammite. Mappable units of pelite with large andalusite and cordierite porphyroblasts are developed immediately above this level, and the sequence as a whole becomes dominantly semipelitic rather than psammitic. However, the boundary is not abrupt, and there is no evidence for an unconformity.

The depositional age of the Dalradian rocks exposed in the district is tightly constrained by radiometric work in other parts of the Grampian Highlands. Halliday et al. (1989) obtained a U-Pb zircon age of 595 ± 4 Ma from a keratophyre laccolith from the Tayvallich peninsula. The occurrence of similar keratophyres in volcanic breccias associated with the youngest member of the Argyll Group suggests that the radiometric age is close to the depositional age of the Tayvallich Subgroup. The ∈Nd values are consistent with the basalt and keratophyre being cogenetic and point to a well-fractionated magma chamber.

A high-precision U-Pb zircon age of 590 ± 2 Ma for the intrusion of the Ben Vuirich granite (Rogers et al., 1989), which was intruded after the D₁ deformational episode into metasedimentary rocks of the Argyll Group in the Pitlochry district (Sheet 55E), implies that deposition of Southern Highland Group sediments and the first deformational episode in the Pitlochry district all occurred within a period of 11 Ma or less (Tanner and Leslie, 1994). It also confirms that all of the Dalradian rocks were deposited during the Precambrian, the base of the Cambrian currently being placed at about 570 Ma (Harland et al., 1990).

Argyll Group

Queen's Hill Formation

The Queen's Hill gneisses were named by Read (1927) in the area between the River Dee and the Howe of Cromar, where they are intruded by the Tomnaverie and Cromar granites; they are bounded to the west by the Muir of Dinnet and to the east by the Muir of Dess. Within this area, a regionally inverted, ENE-striking sequence of amphibolite-facies metasedimentary rocks, intruded by a thick amphibolite sill complex on Creag Dhu, was recognised (Figure 3). The total thickness is at least 2.5 km.

Feldspar-porphyroblast gneisses of Scar Hill

This is the structurally highest, and thus, by implication, the stratigraphically lowest unit in this area. It is a unit of mixed gneissose psammite, semipelite and pelite, in which Ethological distinctions become increasingly difficult due to the high metamorphic grade. The total apparent thickness of this unit is at least 1.5 km. Around Braeroddach Loch [NJ 486 002], and extending to the south-east on Creag Ferrar [NO 493 995] (Sheet 66W), the rocks have a well-developed foliation and a degree of compositional layering, with psammitic and pelitic layers being distinguishable. On Scar Hill [NJ 482 014], the rock is a uniform, poorly layered gneiss with subhedral porphyroblasts of plagioclase up to 5 mm and garnet up to 3 mm, set in a matrix of quartz, biotite, sillimanite and cordierite (Plate 1)a. The gneiss contains rounded xenoliths of refractory lithologies, principally quartzite, calcsilicate rock, and aluminous material, with rare amphibolite. Similar rocks occur on Craigie [NJ 474 006] (Plate 2)a, while on Mulloch Hill [NJ 469 005] xenolithic gneisses have been sheared and partially retrogressed.

Feldspar-porphyroblast pelite south-east of Balnagowan Hill

Exposures of very coarse-grained, gneissic, biotite-rich pelite and semipelite with euhedral plagioclase porphyroblasts up to 10 mm occur on the south-east flank of Balnagowan Hill [NJ 505 006] and near Coull Home Farm [NJ 513 012]. This unit has an estimated thickness of 500 m.

Feldspathic psammites of Queen's Hill

Exposures of very coarse-grained gneissose feldspathic psammite with < 5% (muscovite + biotite) are widespread on Queen's Hill [NJ 530 005]. The foliation is defined by flattening and stretching of quartz crystals. In thin section, ribbon quartz is abundant, and the grain boundaries do not represent original clasts. The rocks are abundantly veined by quartz and pegmatite, both parallel and perpendicular to the foliation. Similar, though slightly finer-grained, psammites crop out on Court Hill [NJ 511 001] to [NJ 516 000]. The unit is estimated to be 500–600 m thick.

To the north-east and east of the Cromar Granite, and south of the Torphins Diorite, the rocks of the Queen's Hill Formation cannot be assigned to any of the three above units, although they lie along strike from the upper two units, and are succeeded to the south by the Deeside Limestone. They are psammitic, semipelitic and pelitic gneisses, interlayered on centimetre to metre scales.

Exposure is generally poor between Dess and Tornaveen, and the boundary between the Queen's Hill and Craigievar Formations has been taken to run from near Maryfield [NJ 602 060] to near Burnside of Ennets [NJ 616 063] through an area of no exposure. Migmatitic semipelite is exposed in a railway cutting [NJ 575 043] near the Peel of Lumphanan, and exposures also occur at Balnacraig [NJ 606 034], Tillyching [NJ 597 043], and in the Beltie Burn 1615 053]. Exposures of interbedded psammite and semipelite with minor pelite occur near Westhall [NJ 593 002] and on the Hill of Beltie [NO 635 997].

Within the Alford district, the only exposures of the formation occur in the mill lade at the Sloc of Dess [NJ 5661 0050], where a bed of pale blue-grey crystalline limestone at least 10 m thick occurs within calc-silicate rocks. The limestone is rather sugary in texture, with traces of disseminated pyrrhotite, and contains 30–40% of calcite, together with quartz, clinozoisite, and minor diopside, feldspars and sphene.

Deeside Limestone Formation

This formation crops out only in a small area within the district; it is a continuation of a larger outcrop to the south in the Aboyne district, where the formation consists dominantly of calc-silicate rock with minor psammitic layers and one or two beds of limestone. The formation has a thickness of 250 m in the Aboyne district. The talc-silicate rocks typically contain zoisite, diopside, amphibole, plagioclase and grossularite; idocrase and wollastonite are locally present.

Craigievar Formation

This formation is the stratigraphical equivalent of the Queen's Hill, Deeside Limestone and Tarfside Psammite formations of middle Deeside. The overall thickness of the Formation is may be locally as high as 5 or 6 km, but an accurate estimate is impossible because of uncertainty in the position of the major D₁ fold closure believed to occur within its outcrop. The rocks are mostly monotonous impure psammites and semipelites with some pelite, but a few distinctive rocks occur. Semipelitic and pelitic rocks are schistose in the northern part of the outcrop, but are gneissose and migmatitic farther south. Exposure in the Craigievar area is generally poor; scattered exposures occur in a few streams and in rare disused quarries. Hillside exposures are very rare, but loose blocks are widely scattered on the hill slopes. The boundary with the Suie Hill Formation to the north is marked by the incoming of beds of gritty psammite, but the semipelites in the two formations are lithologically similar, though differences in metamorphic grade make them look dissimilar.

The major part of the outcrop of the Craigievar Formation is occupied by interlayered psammite, semipelite and pelite, which vary greatly in their relative proportions between exposures, though micaceous psammite with semipelite layers is dominant. The psammite shows variations in feldspar and biotite content, which probably reflect the original bedding, and the compositional layering is mostly parallel to the dominant foliation, marked by the alignment of biotite crystals. However, in places rootless isoclinal folds are preserved within psammite layers. Where psammite and semipelite are interbedded, the psammite is bedded in units of 0.1–0.2 m, whereas the semipelite is bedded in units of 0.01–0.05 m. Pelite occurs as interbeds up to 0.05 m thick within the psammite and semipelite, and also as rare exposures consisting entirely of pelite. It is mostly very dark, biotite rich and fissile, with 1–10 mm-scale layering. The unmigmatised semipelites and pelites in the north of the outcrop contain muscovite pseudomorphs after andalusite, together with fibrolite (sillimanite), cordierite and garnet. However, in the migmatitic semipelites and pelites in the southern part of the outcrop, fibrolite occurs as wisps and streaks, andalusite is absent, and cordierite and garnet are rare, except in the large mass of pelitic rocks described below.

A large mass of pelitic rocks, possibly up to 2 km thick, containing abundant sillimanite and scattered garnet porphyroblasts, occurs on and around Pressendye [NJ 490 090]. The boundary with more normal psammitic and semipelitic rocks is sharp, and its south-eastern contact was traced over areas of newly planted forest from spoil in the ploughed furrows. The pelite contains beds of fine-grained quartzose psammite up to 2 m thick, as on the summit of Pittenderich [NJ 496 080]. On the col between Scar Hill [NJ 487 112] and Craiglea Hill [NJ 480 110], there is an occurrence of distinctive pelite containing c. 50% almandine garnet in 2–3 mm crystals. Magnetic beds within these pelites have been traced from aeromagnetic surveys by Gallagher (1983) and produce significant anomalies on a regional scale (Figure 4); they strike roughly east–west to the west of Pressendye but to the east the strike swings round to south-east and south.

Within the rest of the formation, quartzite is rare, but a 10–20 m unit crops out near Milltown of Towie [NJ 467 128]. Calc-silicate rocks are relatively common in the Towie area, and at least two beds 5 m or more thick have been traced for up to 2 km along strike. Near Mill of Culfork [NJ 4483 1057 and 4464 1098], calc-silicate rock and very impure limestone have been exposed in small pits. Beds with over 30% calcite are scarce and rarely more than 1–2 m thick (S78583), (S78585), (S78590). These calcareous beds, together with some of the surrounding psammites and semipelites, may be equivalent to the Deeside Limestone Formation. They may mark a northeastward transition from calcareous to dominantly non-calcareous sedimentation in Tayvallich Subgroup times.

Along the eastern margin of the Morven–Cabrach Intrusion and the northern margin of the Tarland Intrusion, the rocks have been subjected to extremely high temperatures with possible partial melting. Xenolithic gneisses occur, similar to those in the Queen's Hill Formation (see above).

To the west of the Rhynie Devonian rocks, the formation continues as far north as Clova Hill [NJ 439 221], where it is overlain by a magnetite-bearing pelite, taken as the local base of the Southern Highland Group.

Aberdeen Formation

This formation was defined in the Aberdeen district by Munro (1986b), where it was distinguished from the Ellon Formation by its content of clearly recognisable psammitic and semipelitic lithologies showing modified bedding, and having a well-developed foliation probably reflecting the original compositional layering. Except for a small area within 2–3 km of the eastern contact of the Bennachie Granite, the pelitic and semipelitic rocks are all migmatitic, and also some of the impure psammites are gneissose. No stratigraphical top or bottom was identified, and some of the rocks included in the formation, particularly to the south of the Dee Fault in the southern part of the Aberdeen district, are probably stratigraphically equivalent to Southern Highland Group rocks in the Alford, Banchory and Stonehaven districts. In contrast, the entire Craigievar Formation is considered, despite the absence of a good calcareous marker unit, to be the equivalent of Crinan and Tayvallich subgroup rocks farther to the south and west. The proportion of pelitic and semipelitic rocks in the Craigievar Formationis greater overall than in the Aberdeen Formation, except for the major pelitic unit extending eastward from Altons [NJ 809 204]. The total thickness of the Aberdeen Formation is uncertain. Munro (1986b) divided it into Northern and Southern units. Allowing for the probable major F₁ fold closure lying within the Northern Unit in the Aberdeen district, this unit has a probable structural thickness of about 6 km in the eastern part of the Inverurie district and the western part of the Aberdeen district. The Southern Unit, nearly all of which crops out in the Aberdeen district, is also several kilometres thick, but the lack of recognisable lithological divisions within the unit means that no reliable estimate of its thickness can be made.

The rocks of the Aberdeen Formation are generally very poorly exposed, and this prevents satisfactory mapping of divisions within the formation. However, trenches dug during gas pipeline construction in the area east of Bennachie and between Oldmeldrum and Peterculter were logged by members of Aberdeen University under the supervision of Dr M Munro, and have been of great assistance in the description of the Aberdeen Formation rocks. The Aberdeen Formation consists dominantly of interlayered psammite and semipelite, with minor pelite, the latter being less abundant than in the Craigievar Formation, and with rare, thin beds of calcsilicate rock. These appear to be considerably less abundant than in the Aberdeen district, but a 2–3 m-thick calcsilicate bed containing a 0.3 m-thick hornblendic layer occurs at Capperneuk [NJ 7071 2236]. The plagioclase is An₇₅, and epidote, sphene and diopside are abundant (S75436). The thickness of the psammite beds is generally 0.2–1 m, rarely reaching 2 m, whereas semipelite and pelite layers are generally less than 0.1 m, and mostly 0.01–0.1 m. Biotite is dominant over muscovite in nearly all the pelites and semipelites, and many of the pelite layers, which are generally coarser grained than the semipelites, contain fibrolite. Andalusite, even as pseudomorphs, is much less abundant than west of the Bennachie Granite. Cordierite and garnet are rare except in the immediate vicinity of the Insch Intrusion.

Nearly all of the Aberdeen Formation within the Inverurie district forms a western extension of Munro's Northern Unit, within which units up to 1 km thick dominated by psammitic or pelitic rocks were distinguished. Within the Inverurie district, this distinction is much less marked. However, an area of dominantly psammitic rocks has been recognised between Mains of Afforsk [NJ 692 200] and Mellanbrae [NJ 717 209], and a large exposure of psammite occurs in a quarry near East Aquhorthies Stone Circle [NJ 730 206]. Most of the other exposures to the north of the Kemnay and Tillyfourie intrusions consist of psammite and semipelite in close association. An area mapped as dominantly pelite in Sheet 77 (Munro, 1986b) extends into the Inverurie district at Isaacstown [NJ 810 210], but rapidly thins out. However, pitting conducted in the Chapel of Garioch area to aid interpretation of the Pitcaple ground magnetic survey suggests that semipelitic and pelitic rocks form a greater proportion of the Aberdeen Formation than surface exposures indicate.

Only a few isolated exposures of the Southern Unit occur, to the east of the Crathes, Balblair and Gask intrusions. They consist of interlayered psammite and migmatitic semipelite, with no individual lithology forming units more than 20 m thick (Munro 1986h). These lithologies are very similar to the rocks of the southern part of the Craigievar Formation and to the rocks of the Queen's Hill Formation on Hill of Beltie [NO 635 997].

Within 400 m of the contact of the Kemnay Granite, the gneisses become more granitoid in appearance, but with more persistent biotite-rich layers than in the Kemnay Granite proper. Compared with the typical migmatitic semipelites and feldspathic psammites of the formation, the gneisses contain more feldspar and less quartz, and K-feldspar is equally abundant with plagioclase, whereas within the regional migmatites K-feldspar is scarce, except in the granitoid veinlets.

Amphibolites, which represent pre-metamorphic intrusive sills and sheets, are relatively common in the area between Chapel of Garioch and Inveramsay. They are responsible for most of the aeromagnetic anomalies in the area of outcrop of the formation (Chapter 5).

The Aberdeen Formation rocks in the quarry on the northern outskirts of Oldmeldrum [NJ 8070 2804] lie just to the east of the fault terminating the Insch Intrusion. They are mostly migmatitic semipelites with layers of massive unmigmatised psammite. The gneissose semipelite contains pods of silica-poor hornfels with the high-temperature metamorphic assemblage K-feldspar + sillimanite + cordierite + garnet + spinel. An exposure at Balcairn [NJ 7916 2867] just to the north of the district boundary, shows silica-poor pelitic gneiss with the same assemblage in contact with amphibolitised basic rock. These rocks are continuous with rocks assigned to the Stuartfield Division in the Ellon district (Sheet 87W), and demonstrate that that unit is equivalent, at least in part, to the Aberdeen Formation.

Southern Highland Group

Suie Hill Formation

This formation lies conformably above the Craigievar Formation in the area to the west of the Bennachie Granite and to the east of the Rhynie outlier. Its base is taken to be the first occurrence of thick beds of gritty psammite in the succession. To the west of the Rhynie outlier, no gritty beds were recognised to the south of Clova Hill [NJ 439 221], about 4 km north of the north-westward continuation of the boundary with the Craigievar Formation to the east of the Rhynie outlier. Hence the base of the formation is probably diachronous. However, the magnetic pelite bed exposed on Clova Hill is correlated with that at Correen Quarry [NJ 5225 2135] , and included within the formation. The formation consists dominantly of semipelitic schist with distinct beds of slaty to schistosepelite and gritty psammite, in contrast to the ill-defined alternation of psammite, semipelite and pelite characteristic of the underlying Craigievar Formation. A more finely layered appearance becomes apparent going north, but this may be related to lower grades of metamorphism. Coarse lithological layering is confined to the Craigievar Formation. The rocks of the Suie Hill Formation show a range of metamorphic effects, and those adjacent to the Insch intrusion have suffered ductile shearing penecontemporaneous with the regional metamorphism. The rocks at the south-western and northeastern contacts of the Bennachie Granite are similar to the high-grade hornfelses of the Clashindarroch Formation around Tap o' Noth (Sheet 86W).

Main outcrop

This lies east of the Rhynie outlier west of the Bennachie Granite and south of the South Insch Shear Zone (Figure 2). Here, the sequence dips regionally north to north-west at an average of 35°. The dominant lithology is grey, muscovite-rich semipelite with diffuse cordierite porphyroblasts up to 5 mm across and scattered andalusite porphyroblasts 1–2 mm in size. A finely developed layering on 0.5–1 mm scale passes through the cordierite crystals, and the foliation is defined by the alignment of biotite flakes. A 200 m-thick unit consisting dominantly of gritty psammite extends from Ardhuncart Quarry [NJ 473 183] to Invermossat [NJ 490 186] (Plate 1)b. This unit is abruptly truncated by a fault near the Saplings of Logie [NJ 497 193]. The very poorly exposed north-western part of the Correen Hills consists dominantly of gritty psammite, cropping out on Brux Hill, Badingair Hill and Clova Hill, with thin beds of silty pelite and rare talc-silicate rock. The outcrop narrows rapidly eastwards; the grit is cut out by the South Insch Shear Zone and the main outcrop terminates at Rapplich [NJ 564 237]. Smaller pods of gritty psammite occur south of Chapelton [NJ 595 235] and south of Edingarioch [NJ 616 237]. A bed of pebble conglomerate at least 5 m thick, with flattened quartz pebbles up to 30 mm long, occurs at Gallowhill [NJ 559 158]. A rare exposure of quartzite occurs at Longbog [NJ 594 206].

Impure limestone, with up to 30% calcite in some beds, has been sporadically worked on the Limer Shank [NJ 5119 2138]. Calc-silicate rocks occur in the Carlinden Burn [NJ 4932 2305], in a small pit in Correen Wood [NJ 4916 2264], and on Clatterin' Kist [NJ 5416 2214]. These rocks are very fine grained, especially the more impure calcareous rocks, and are generally tremolite rich.

A distinctive andalusite-cordierite-magnetite pelitic schist with larger than usual cordierite porphyroblasts is exposed at Correen Quarry [NJ 5225 2135] (Plates lc, 2b), where it was quarried for flagstones. It can be traced from the Whitestone Burn at [NJ 5142 2075] to Clatterin' Kist [NJ 5416 2214]. The magnetite-rich pelite can be identified as the source of a strong aeromagnetic anomaly which extends from Mossat [NJ 472 193] to Knock Saul [NJ 581 233]. Fine-grained slaty pelite with sporadic andalusite (chiastolite) porphyroblasts up to 1.5 mm across is well exposed in the Carlinden Burn [NJ 4861 2263] to [NJ 496 229] , and similar rocks are exposed in a belt extending southwards from the Whitestone Burn at [NJ 508 210] to the Saplings of Logie [NJ 496 190].

South Insch Shear Zone

For 0.3 km to 1 km to the south of the Insch Intrusion from near Smallburn [NJ 520 240] to the contact with the Bennachie Granite near Bogside [NJ 640 242], the Dalradian rocks are fine-grained platy semipelites. They contain small (up to 1 mm) clear andalusite crystals, set in a relatively coarse-grained quartz-muscovite fabric, and minor enclaves of extremely sheared fine-grained, slightly gritty psammite. A small pod of talc-silicate rock occurs near Loanend [NJ 6044 2405], bounded to the north and south by rocks of the Insch Intrusion. These sheared and platy rocks have probably recrystallised during and after the shearing associated with the emplacement of the Insch Intrusion.

West of the Rhynie outlier

Strong aeromagnetic anomalies on Clova Hill [NJ 439 221] are attributed to a magnetic pelite similar to that of Correen Quarry. The gritty psammite exposed in the Burn of Corchinnan becomes indurated and strongly fractured near the fault bounding the Rhynie outlier [NJ 454 230]. To the north of the gritty unit, semipelite and pelite are exposed on Clayhooter Hill; these are andalusite-cordierite schists similar to those on Suie Hill. Small exposures of calc-silicate rock occur on the eastern slopes of Clay-hooter Hill [NJ 440 237] and beside the track from Tod-stone to Silverford [NJ 437 243].

NE and SW of the Bennachie Granite

A small triangle of Dalradian rocks occurs to the north of the Bennachie Granite, south of the Insch Intrusion and west of the Bennachie eastern boundary fault. Abundant large blocks and one in-situ exposure occur at Hillbrae [NJ 685 246]. These are cordierite-sillimanite-K-feldspar-garnet-gneisses with well-developed centimetre-scale compositional layering, marked by variation in sillimanite content. Similar well-layered rocks occur at Bogmore [NJ 646 155], at the southern contact of the Bennachie Granite. To the west they become tougher and more sillimanite rich, but garnet disappears. Two foliations are developed, and there is a series of tension gashes filled with muscovite-microgranite. Within these rocks on Kist Hill [NJ 638 162] occur magnetite-rich layers which produce a strong aeromagnetic anomaly. These high-grade hornfels-like rocks continue south to the southern slopes of Green Hill [NJ 640 143], where they pass southwards into rocks of the Craigievar Formation. They are juxtaposed against regionally metamorphosed rocks of considerably lower grade along a postulated fault near Dencroft [NJ 636 155]; this fault must predate the Corrennie and Bennachie granites.

Clashindarroch Formation

The only exposures of this formation in the district occur on the southern slopes of the Tap o' Noth [NJ 490 290]. They form the southernmost part of a large outcrop of dominantly semipelitic schists lying to the north of the Insch and Boganclogh intrusions. They are assigned to the Southern Highland Group, but exact correlation with the Suie Hill Formation is not possible. The rocks within the Alford district are tightly folded, finely layered semipselitic hornfelses, containing cordierite, K-feldspar and sillimanite. They lie within the northern aureole of the Boganclogh Intrusion (Plate 1)d.

Chapter 3 Dalradian structure

Regional setting

The Buchan Block, of which the Inverurie–Alford district forms part, is the north-easternmost part of the Grampian Highlands, bounded to the west by a roughly north–south line from Portsoy to Ballater and extending southwards approximately as far as the River Dee (Figure 5). The major structural features developed further south in the Grampian Highlands are not easily recognised within the block and different regional metamorphic facies are developed. Read (1955) proposed that a major tectonic junction, the Boyne Line, separated the rocks of the underlying Keith Division from those of the overlying Banff Division. At the coast, the Boyne Line was placed between the Cowhythe Gneiss (Crinan Subgroup) and the Boyne Limestone (Tayvallich Subgroup). Away from the coast, the Boyne Limestone, Whitehills Group and Cowhythe Gneiss were considered to have been progressively excised by movement along the line. Ashworth (1975), working in the Portsoy–Huntly area, postulated that the absence of these units in the inland area was due to facies variation, and that the Boyne Line did not exist. Ashcroft et al. (1984) demonstrated several major D₃ shear zones within the 'block', including one from Portsoy to the western margin of the Huntly Intrusion. No disruption was postulated along the base of the Southern Highland Group between Cabrach and Oldmeldrum. The Portsoy Shear Zone is one of several tectonic features which together comprise the Portsoy Lineament (Fettes, 1986, 1991). This lineament separates regions with different Dalradian sedimentary sequences. It has been traced south from Portsoy as far as Duchray Hill, following the western margins of the Morven–Cabrach Intrusion and the Duchray Hill Gneiss. The Boyne Line does not exist as a major structure. Of the other shear zones shown by Ashcroft et al. (1984) to cut the rocks of the Buchan Block, the most important are the Fraserburgh–Oldmeldrum–Belhelvie Shear Zone, a complex, anastomosing zone trending north–south roughly parallel to the Portsoy Lineament, and a series of roughly east–west shear zones, including those located along the northern and southern margins of the Insch and Boganclogh intrusions. (Figure 5) shows the Inverurie–Alford district in relation to the major structural elements of the Buchan region. A southern boundary to the Buchan block is difficult to locate, due to the large volumes of post-tectonic granitic material intruded to the north and south of the River Dee.

The Portsoy Lineament is a crustal-scale tectonic feature which was reactivated at several periods, controlling both the sedimentation patterns and the Caledonian orogenic development of the north–east Grampian Highlands. It separates a region of higher pressure metamorphism to the west from the region of distinctive low-pressure Buchan metamorphism (Read, 1955; Fettes et al., 1991) and marks the position of a thrust with up to 80 km lateral displacement, which has subsequently been steepened. This accounts for the greater depth of burial to the west of the lineament at the time of the post-D₂ regional metamorphism, as shown by kyanite replacement of andalusite (Beddoe-Stephens, 1990). This pressure difference ceases to be discernible to the south, possibly due in part to vertical movement along the east–west shear zones which traverse the Inverurie–Alford district.

The status of the Deeside Lineament proposed by Harte et al. (1984) is uncertain. There is no obvious shear zone or east–west fault close to the Dee valley, but the large number of plutonic intrusions and the poor exposure act to obscure any major structure. The lineament was postulated as a southern boundary to the Buchan metamorphic facies, and as the termination of the Deeside Limestone and Tarfside Psammite formations, the most north-easterly units which can be correlated with any certainty with the Loch Tay Limestone. Fettes et al. (1986) regard the Deeside and Portsoy–Duchray Hill lineaments as reflecting pre- to syndepositional boundaries which controlled the pattern of Dalradian sedimentation as well as later tectonic events.

To the south-west of the Buchan Block, the structure of the Dalradian rocks is dominated by the Tay Nappe, formed during D₁ and modified during D₂. The recumbent fold of the Tay Nappe is interpreted as causing the gentle inverted dips of the Queen's Hill and Deeside Limestone formations in the south-western part of the district. The nappe is believed to reduce progressively in amplitude to the north-east. The proposed fold hinge line in the south of the district, separating right-way-up rocks to the north from inverted rocks to the south, may continue in an ENE to NE direction through the Aberdeen district as far as Collieston, where it is much reduced in amplitude (Figure 6 ; Fettes et al., 1986).

Geophysical models

BGS has over the years conducted regional aeromagnetic and gravity surveys of the UK landmass and continental shelf. (Figure 4) is a digitised and smoothed version of the aeromagnetic map of the Inverurie–Alford district and surrounding areas, and (Figure 7) is part of the Bouguer gravity anomaly map covering the same area. Modelling of the observed Bouguer gravity and aeromagnetic anomalies along line G–G' on these figures indicates the possible disposition of the basic and granitic intrusive rocks (Figure 8). This shows part of a modelled section from near Nairn to Aberdeen across the Portsoy Shear Zone, the Boganclogh basic mass and granitic rocks of the late-tectonic and Crathes suites. The line of section crosses Sheet 76 from Whitehill [NJ 425 265] on the Boganclogh mass to [NJ 812 039] near Broadwater, just east of the Crathes Granodiorite. The properties of the polygons used to calculate the model, and the geological units represented by these polygons, are listed in (Table 1).

The deep-seated magnetic anomaly in the northwestern part of the section is attributed to magnetic (0.045 SI) basement rocks at depths of 2–5 km, which are present only to the north-west of the Portsoy Lineament (Fettes et al., 1991).

Immediately to the south-east of the Portsoy Lineament, magnetic basic intrusive rocks of the Blackwater intrusion and ultramafic volcanic rocks of the Blackwater Formation are associated with a local aeromagnetic anomaly. The observed 600 nT aeromagnetic anomaly and 40 mGal gravity anomaly over the Middle Zone quartz-biotite-norite of the Boganclogh intrusion can be explained by a body about 5 km thick with a density of 3.05 Mg m⁻³ and mean susceptibility of 0.038 SI. The olivine-ferrogabbro of the Upper Zone is modelled with a slightly reduced density (3.00 Mg m⁻³) as are the Middle Zone rocks along the south-east side of the body (2.95 Mg m⁻³), while the serpentinite of the Lower Zone adjacent to the Rhynie Devonian rocks is modelled with a reduced density (2.85 Mg m⁻³) but increased susceptibility (0.042 SI). The Boganclogh body has been modelled assuming a significant natural remanent magnetisation (NRM) with a declination of about 200° and inclination of about 50°. This model of the basic rocks is in effect a minimum volume model, since the density values used are close to the likely maximum. However, it does indicate that most of the observed anomalies can be approximately modelled assuming a basic intrusion less than 5 km thick. The layering within the Boganclogh mass is taken to be steeply dipping throughout, and the gently dipping base of the intrusion is interpreted as a down-dip continuation of the extensive shear zone which crops out along its southern margin. The model suggests that the main basic intrusion is limited to the west by the Portsoy Lineament.

Small outliers of Devonian rocks at Cabrach and Rhynie are modelled as only a few hundred metres thick. Basic or ultramafic rocks are modelled beneath both outliers of Devonian rocks. Exploration Ventures Ltd (EVL) aeromagnetic data indicate a significant anomaly associated with the ultramafic rocks adjacent to the Rhynie Devonian rocks just north-west of Lumsden, with the anomaly extending eastwards across the basin, supporting the model of ultramafic rocks at depth.

South-east of the Rhynie outlier, magnetic andalusitebearing pelitic schists within the Suie Hill Formation locally produce a clear anomaly in the low-level commercial aeromagnetic survey. This is less clear in the high-level aeromagnetic data used in the model profile, partly because of the 2 km flight line spacing, but the magnetic rocks are modelled with a generally north-westerly dip towards the Devonian sedimentary rocks.

The Bouguer anomaly map (Figure 7) suggests that the ground for several kilometres north-west of the Syllavethy intrusion and south-east as far as the Tillyfourie intrusion is underlain by granitic rocks at a shallow depth, possibly less than 1 km. Much of this ground is intruded by the Tillyfourie–Syllavethy vein complex.

A significant aeromagnetic anomaly is centred on Kist Hill (Figure 8), close to the Dencroft Fault, east of which highly magnetic semipelitic hornfels of the Suie Hill Formation crops out. This unit is modelled with a mean susceptibility of 0.04 SI. The strongly magnetic unit is of limited horizontal and vertical extent, and is believed to represent a displaced part of the metamorphic aureole of one of the basic masses. The extent of the dislocation causing the displacement is unknown. A shear zone similar to those identified by Gallagher (1983) further to the south (see below)may be responsible.

The observed gravity and magnetic anomalies over the Tillyfourie and Crathes plutons have been modelled in relation to the various phases of intrusion recognised in the field; they yield pluton depths of up to about 8 km below OD, when densities in the range 2.61–2.75 Mg m⁻³, and magnetic susceptibilities in the range 0.005–0.020 SI are used. The density of granitic rocks is directly proportional to the concentrations of a few major element oxides, especially CaO and MgO. Using mean oxide concentrations, the estimated density of the main plutons has been calculated: Crathes 2.63, Kemnay 2.61, Balblair 2.64, Tillyfourie 2.69 and Torphins (assumed to be similar in composition to Gask) 2.75 Mg⁻³.

A magnetic tonalite similar to that of Tillyfourie, with a relatively high density, has been modelled as intruding the Crathes Granodiorite at depth to account for the positive aeromagnetic anomaly over the south–eastern part of the Crathes Granodiorite. The minimum gravity anomaly lies close to the boundary of the Tillyfourie and Crathes plutons, but the contact is not associated with a significant inflection in the observed gravity profile, despite the likely density contrast between these two phases. It is likely that the Tillyfourie pluton is partly underlain by the lower-density Kemnay Granite, and that the Crathes pluton is partly undercut by the penecontemporaneous, lower-density Balblair Granodiorite. Subsequent injection of a late-phase low-density Hill of Fare magma is also a possibility.

Planar and linear fabrics

The Dalradian rocks of the Inverurie–Alford district all display at least one planar fabric. Even in the lowest-grade rocks bedding planes are always modified. The rocks show a penetrative foliation, which ranges from slaty cleavage in greenschist-facies pelitic rocks to a penetrative schistosity, marked by parallel growth of micas, at higher grades of metamorphism, and a coarse gneissose layering in the migmatitic rocks, with segregation of leucosomes and formation of biotitic selvedges. The foliation developed progressively during the early deformational episodes, but in many rocks is seen to be overgrown by later porphyroblasts, particularly muscovite and secondary biotite. The primary foliation can in places be shown to be parallel to the original bedding, but in places it cuts across mainly bedding-parallel compositional variations. Progressive growth of porphyroblasts during growth of the metamorphic fabric is demonstrated by the preservation of early fabrics in the inclusions within some porphyroblasts, especially andalusite. In the unmigmatised rocks, the dominant foliation, with good alignment of micas, is generally taken to be a composite S₀–S₁–S₂ surface, possibly incorporating S₃ in places.

Within the lowest-grade rocks, the foliation consists simply of a parallel alignment of biotite flakes within pelitic rocks. In an example from Ardhuncart Quarry [NJ 4729 1829] (S78500), compositional layering on a 1–2 mm scale, marked by variations in proportion and size of biotite, is folded into an open minor fold of 10 mm amplitude. The main foliation of the rock, marked by parallel alignment of the biotite flakes, is roughly axial planar to the ?F₃ fold. Within the low-grade gritty psammites, a foliation is typically lacking on the microscopic scale, and the degree of flattening of clasts is generally slight. On moving to higher-grade rocks, the fabric of the rock is coarser, though relics of a finer-grained, earlier fabric are preserved in andalusite and, to a lesser extent, in cordierite porphyroblasts. As the metamorphic grade increases still further with the breakdown of andalusite, muscovite and biotite have grown across the existing foliation while the original biotite has been replaced, largely by fibrolite. Simultaneously, segregation into aluminous, fibrolite-rich layers and more quartzose layers on a 2–5 mm scale is pronounced. This is the 'sillimanite overprint' of Chinner (1966) – see Chapter 4.

The earliest recognisable foliation developed in the fibrolite-cordierite-K-feldspar-gneisses of the Suie Hill Formation on Kist Hill [NJ 638 162] is marked by a segregation of fibrolite-rich and quartz-rich layers on a 5–20 mm scale, and accompanied by quartz veins running parallel to it. The early foliation is cut by a later spaced cleavage, which is associated with the realignment of quartz veins, formation of tension gashes, and intrusion of veins of muscovite-microgranite. The spaced cleavage is associated with metre-scale folds of the earlier foliation, has a spacing of 10–30 mm, and is subvertical, striking north–south. The tension gashes have a spacing of 0.05–0.3 m. The main foliation in the hornfelses of the Clashindarroch Formation is a segregation of sillimanite-rich and quartz-rich layers, 1–5 mm thick, similar to the earlier foliation at Kist Hill, but it has been folded into tight folds with amplitudes of 0.1–0.5 m and subvertical axial planes striking north-east to ENE.

In unmigmatised rocks of the Aberdeen and Craigievar formations, which are principally the psammitic layers, there is a well-marked planar foliation. This is roughly parallel to bedding features where these can be seen, but in many places tight, often 'rootless' isoclinal folds can be seen within a 'bedded' unit. An axial planar foliation runs through the fold noses in some cases, but on the limbs of the folds the bedding and axial planes are subparallel, and the status of the foliation is uncertain. These 'rootless folds' could be either slump folds or the dismembered cores of F₁ or even F₂ folds. The migmatitic rocks of these formations, and the migmatites of the Queen's Hill Formation, show a foliation marked by alignment of micas and variation in the relative proportions of micas and quartzofeldspathic material, on a 1–5 mm scale. Superimposed upon this layering, and usually roughly parallel to it, is a coarse gneissose layering, marked by the segregation of granitic veins, commonly with biotitic selvedges. However, the gneisses of the Mulloch Hill–Scar Hill area [NJ 469 005]–[NJ 482 014] in the south-west, are massive rocks with a poorly developed compositional layering, showing a rough alignment of micas and slight flattening of feldspar porphyroblasts. Segregations of more and less mafic material into layers gives a rough foliation, which passes round the plagioclase and garnet porphyroblasts. This foliation strikes north–east and dips moderately north-west. It is considerably coarser than the original bedding-parallel foliation which is preserved in the restite xenoliths.

In the area to the north of the Insch and Boganclogh intrusions, Fettes (1970) has shown that an S₁ cleavage, which in places lies oblique to the bedding, is progressively destroyed in the aureole of the Insch Intrusion, though it is preserved within cordierite porphyroblasts. The S₁ cleavage is cut by an S₃ cleavage, which disrupts and modifies the fabric of the aureole rocks in the same way as it modifies the country rocks. Evidence from inclusion trails in cordierite (Fettes, 1970) indicates that the contact metamorphism due to the basic masses was coeval with the D₃ deformation.

Folding

Four principal deformational episodes (D₁–D₄) are recognised in the eastern Grampian Highlands, although the relative importance of these episodes varies considerably in different areas. The earliest, D₁, produced the Tay Nappe, which is recognised throughout the Grampian Highlands. The nappe is a recumbent antiform/synform pair with a 20 km north–south wavelength. The D₂ deformation produced folds with amplitudes of up to several kilometres which deform the Tay Nappe. Minor folds attributable to the D₁ or D₂ episodes are scarce in the Inverurie and Alford districts, and are preserved largely as isoclines within bedding units. The outcrop pattern is believed to be strongly influenced by regional folds of the D₁ and D₄ episodes. The main metamorphism of the district postdates D₂, but there is evidence of earlier, low-grade metamorphism in adjacent parts of the eastern Grampian Highlands.

Major folds

An early major fold is inferred from the regional stratigraphy and the regional strike and dip of the main foliation to affect the Dalradian rocks of the district. Its axial plane trace is inferred to extend from near Towie [NJ 440 127] to the boundary between the Queen's Hill and Craigievar formations near Tornaveen [NJ 615 063], where it separates regionally inverted and southwardyounging rocks, to the south of the granitic intrusions and probably as far north as Craiglea Hill [NJ 476 108], from right-way-up rocks which both dip and young roughly northwards. However, from Towie to Craiglea Hill, the regional strike is NNE, suggesting that the fold is very broad, and its hinge cannot be defined readily. Structures in the area between Towie [NJ 440 127] and Kirkton of Tough [NJ 615 130] are however difficult to interpret due to paucity of exposure. Around Towie, layers of calc-silicate rock can be traced north–south along the strike, suggesting that the nose of the major D₁ fold may be preserved here. The fold is recumbent in the western part of the district where the rocks to the south are inverted and gently to moderately dipping, but it may become more upright to the east, where the rocks to the south dip more steeply, though they are still inverted. Although partly disrupted by later igneous intrusions, faults and shear zones, this major fold is possibly the north-eastern extension of the Tay Nappe into the district.

The D₃ episode occurred synchronous with and immediately following the emplacement of the late-tectonic basic to ultramafic intrusions, which have been dated at 489 ± 17 Ma (Pankhurst, 1974). The peak of regional metamorphism in most of the district was also roughly coeval with D₃. D₃ deformation produced metre- to kilometre-scale folds. Intrusion of the basic rocks was accompanied by ductile shearing, during which temperatures remained high (Kneller and Leslie, 1984).

The D₄ episode is associated with the ENE-trending Highland Border Downbend in areas to the south of the district. However, to the north of the Insch Intrusion the Turriff Syncline is an open northward-plunging D₄ syncline, which folds the metamorphic isograds. Its southward continuation into the Inverurie–Alford district is marked by a change in regional strike. The rocks to the west of a line roughly 5 km east of the Rhynie outlier mostly strike north-west to WNW, while those to the east of this line mostly strike ENE to north-east. Not only does the boundary between the Craigievar and Suie Hill formations follow this curve, but so do the limit of migmatisation and the sillimanite isograd west of the Bennachie Granite. The pattern of this fold is compatible with observed strikes and dips in the Scar Hill and Correen Hills areas, and to the east of the Bennachie Granite. The complementary Buchan Anticline occurs in the Ellon district (Sheet 87W). Its axis runs from Inzie Head to Arnage and may continue south to near the eastern margin of the lnverurie district, where it is obscured by the Clinterty and Crathes granodiorites. The brecciation of some of the granitic intrusions cutting the basic masses, mostly along the line of the post- D₃ shear zones (Read, 1951), may have occurred at this time or may be of Lower Devonian age (Chapter 10). Retrogression, with chloritisation of biotite, pinitisation of cordierite and sericitisation of andalusite and fibrolite, occurred extensively in the district during this episode, especially in the Aberdeen Formation rocks north-west of Inverurie.

Minor folds

Within the Dalradian rocks of the district, minor folding is widespread, though possibly less so than in most of the Grampian Highlands. The style of folding varies with the grade of regional metamorphism, and later shearing and contact metamorphism have notably modified the earlier generations of folds.

Within the district, it is not possible to distinguish minor D₁ and D₂ folds, due to the absence of any recognisable D₂ major fold. Folds of these two episodes are relatively scarce, and are preserved principally as tight folds and isoclines within 'bedding' units in the Aberdeen and Craigievar formations, and as centimetre-size folds with axial planes parallel to the main foliation in the main outcrop of the Suie Hill Formation. The folds in the fibrolite-gneisses of the Kist Hill [NJ 638 162] area may be of D₁– D₂ age, as they are cross-cut by a spaced cleavage which is roughly parallel to the axial plane of the D₄ Turriff Syncline.

Most of the minor folds in the district belong to the D₃ generation. The irregular, almost ptygmatic folds in the migmatitic rocks of the southern half of the district were largely developed during or after the migmatisation, which rendered the rocks plastic, and obscured folds of the earlier episodes, which in places were highly modified and incorporated into the later folds. Minor D₃ folds are well seen in the northern half of the district, although in many exposures the rocks are essentially unfolded. Also, some of the minor folds within high-grade hornfelses on Kist Hill [NJ 640 161] may belong to this episode.

A group of relatively open minor folds is recognised in the district. They can be attributed to the D₄ or later minor episodes, and are developed in the more pelitic rocks within the Suie Hill Formation, as at Correen Quarry [NJ 523 213] and in the Carlinden Burn around [NJ 490 227]. Brittle folds with rectangular hinges and well-marked crenulations occur principally in the lower-grade semipelites and pelites, which are micaceous enough to deform easily. The amplitude of the folds ranges from a few millimetres to approximately ten centimetres, and the wavelength from about a centimetre to about a metre. The folds are generally upright, with near-vertical axial planes typically striking north-east, roughly parallel to the axis of the Turriff Syncline. They are associated with chloritisation of biotite in certain parts of the district.

Shear zones

Major shear zones occur at the north and south margins of the Insch and Boganclogh intrusions, and also elsewhere in both the Dalradian rocks and the basic intrusive rocks (Figure 5). Most of them strike approximately ESE in the Alford–Inverurie district. Many of the shear zones have been identified by means of aeromagnetic surveys (Gallagher, 1983). The shearing occurred coeval with, and shortly after, the intrusion of the basic masses, while the rocks were still hot. Kneller and Leslie (1984) have documented amphibolite-facies metamorphism insheared basic rocks at the eastern end of the Insch intrusion and in the adjacent Ellon district (Sheet 87W). The fine-grained platy semipelitic schists between Smallburn [NJ 520 240] and Bogside [NJ 637 240] were recrystallised during and after the shear episodes. However, later cataclasis has affected these shear zones in addition to the granitic rocks (c. 470 Ma) intruded into the basic rocks (Read, 1951).

The northern boundary of the Insch and Boganclogh intrusions, to the north of the Inverurie–Alford district, is sheared, though the presence of a wide aureole along much of the contact implies that less displacement has occurred than along the southern contact. The northern shear zone is only about 100 m across, as evidenced by the width of the serpentinite, granite, and other bodies that occur along it. This shear zone occurs in the Alford district only between Scurdargue [NJ 475 291] and north of Windyfield [NJ 494 284], to the north of which a wide hornfels zone occurs. Sheared hornfels occurs in a borehole near Old Merdrum [NJ 4651 2950], just outside the district.

Two ductile shear zones, the Rothney and Ledikens shear belts, were identified within the Insch intrusion by Read (1956); the former trends ENE and passes just south of Insch, while the latter lies in the Turriff district (Sheet 86E). Work by Ashcroft and Munro (1978), Leslie (1984a) and Gallagher (1983) has demonstrated that shear zones within the basic rocks can be traced from aeromagnetic surveys. The sheared basic rocks have much lower magnetic susceptibilities than unsheared rocks, due to alteration of magnetite and ilmenite during the uralitisation and amphibolitisation which accompanied shearing of gabbroic rocks. There is evidence of horizontal displacement along several shear zones, but the amount of vertical movement is difficult to assess. Many of the shear zones were reactivated as faults in the Devonian.

The shearing along the southern margin of the Insch and Boganclogh intrusions affects both Dalradian meta-sedimentary rocks and basic and ultramafic rocks. Several discrete shear belts, each less than 100 m wide, occur over a north–south distance of up to 1.5 km. The shear zones have been dislocated by several later NE-trending faults. The shear zones along the south-east margin of the Insch intrusion in the Pitcaple–Oldmeldrum area, have created enclaves of high-grade hornfels of Dalradian origin within the basic rocks, as in the railway cutting to the west of Pitcaple [NJ 715 254] and at Govals Quarry [NJ 732 256] (Figure 9). The shear zones have also truncated the metamorphic aureole of the intrusion. Many of the hornfelses are highly magnetic due to development of fine-grained magnetite, and their boundaries show up well on both airborne and ground-based magnetic survey plots. Munro (1986a) described exposures in gas pipeline trenches which showed that the shear zones are relatively narrow (10–100 m); metasedimentary and igneous rocks within them have been mylonitised. Some recrystallisation, with loss of primary magnetite from gabbroic rocks, has taken place in the shear zones. The noritic rocks of the Tarland intrusion are sheared adjacent to the eastern and western contacts with Dalradian country rocks, and the shear zones also continue within the mass (Figure 15). The basic rocks of the southern part of the Tarland intrusion, from Nether Ruthven [NJ 463 020] to the boundary against Dalradian rocks west of Wester Coull [NJ 478 023], have been strongly recrystallised to coarse hornblende-gneisses, probably as a consequence of shearing across a wide belt, whose continuation is now concealed by intrusion of the later Logie Coldstone and Tomnaverie granitic plutons.

Gallagher (1983) observed that the EVL aeromagnetic survey showed a concentration of anomalies along certain ESE-striking lineaments and postulated that three shear zones traverse the Dalradian rocks of the Alford district. The displacement of metamorphic zones and the stratigraphy by the shear zones in this district is difficult to confirm, owing to the lithological uniformity of the Craigievar and Aberdeen formations, as well as the large granitic intrusions. Exposures of sheared rocks along these lineaments are lacking, but this can be attributed largely to the general poor level of exposure.

The first shear zone runs from Milltown of Kildrummy [NJ 469 165], to just south of Alford [NJ 577 160]. It forms the ESE continuation of a zone of shearing in the Morven–Cabrach intrusion in the River Buchat [NJ 368 160] in Sheet 75E, and to the east of Milltown of Kildrummy marks the southern boundary of the Suie Hill Formation. Gallagher interpreted the aeromagnetic anomaly as a shear zone although it is coincident with an exposed late-Carboniferous dyke at Milltown of Kildrummy.

The second shear zone was traced from Drybrae [NJ 508 133] through Muir of Fowlis [NJ 563 122] to near Redwell [NJ 645 106]. Exposures at Drybrae are cut by several shear planes, and the fibrolite-magnetite-cordierite-schists of Langgadlie Hill [NJ 514 137] have a blastomylonitic fabric with smeared-out cordierite and abundant fibrolite. The lineament lies a short distance to the south of the Drumallachie part of the Kildrummy intrusion and is also a short distance south of the Lynturk ultramafic rocks. Recent ground magnetic surveys (Bellman, 1988; Illingworth, 1988) do not, however, suggest that these intrusions are linked to ESE-trending structures.

The third lineament identified as a shear zone by Gallagher (1983) could be traced only for a short distance due to lack of aeromagnetic coverage. It passes through Nether Ruthven [NJ 468 022], which lies within the hornblende-gneisses at the southern extremity of the Tarland intrusion. The aeromagnetic anomaly probably reflects a broad zone of recrystallisation and deformation of the basic rocks rather than a discrete, narrow, shear zone.

Faults

There were at least two major episodes of faulting in the district; the first shortly after the major shearing episode and the second in the early Devonian (Chapter 10). The earlier faults have been obscured to a large extent by the granitic intrusions; some of them were reactivated during the early Devonian. Most of the early faults trend ENE to NNE, and are associated with the shear zones within the late-tectonic basic intrusions, or have developed along their margins. However, a few faults, trending north–south to NNE, lying wholly within Dalradian rocks are assigned to this episode, although evidence of the age of the faulting is scarce or circumstantial.

The Dencroft Fault, trending north–south, is postulated to account for the sudden change in metamorphic grade in rocks just south of the Bennachie Granite. Normal andalusite-porphyroblast schists with incipient fibrolite development to the west of the fault are set against high-grade cordierite-sillimanite-garnet-K-feldspar hornfelses to the east. The fault does not affect either the Corrennie or Bennachie granites, and therefore predates these intrusions. A similar structure, bounding the hornfelses to the east, was reactivated in Devonian times and now forms the eastern margin of the Bennachie Granite.

The northern boundary of the sillimanite-garnet schists of Craiglea Hill is displaced 1.3 km from the western slopes of Craiglea Hill [NJ 471 105] to 500 m southwest of Easterton [NJ 476 117] by a NE-trending fault.

Most of the other late-tectonic (Ordovician) faults affect the basic rocks as well as the Dalradian and are intimately involved with shearing. The Dalradian enclave at Migvie [NJ 438 069] is bounded both to the east and west by NNE-trending faults whose continuation is marked by shearing of the basic rocks. The pegmatite at Reinacharn Lodge, which is aligned along a northern continuation of the western boundary of the Migvie Dalradian block, has been brecciated. Nevertheless, there is no displacement of the northern contacts of the Morven–Cabrach and Tarland intrusions at the northern end of these lineaments.

The ENE-trending fault which extends from near Boghead [NJ 522 270] to near Glanderston [NJ 591 290] juxtaposes quartz-biotite-norite of the Insch intrusion against diorite of the Kennethmont Complex. It probably formed shortly after the emplacement of the latter body, but several sections have been exploited by the late-Carboniferous Rhynie dyke (Chapter 8). Another ENE-trending fault forms the northern boundary of the syenite outcrop between Craignook Wood [NJ 538 246] and the eastern end of Gallow Hill [NJ 606 266]. This fault runs parallel to several other faults and shear zones within the Insch Intrusion, and is displaced by the later north- to NE-trending faults which affect the ultramafic rocks along the south–west margin of the Insch intrusion.

The shear zones along the southern margins of the Insch and Boganclogh intrusions are displaced by a number of faults, mostly trending north to NNE, some of which appear to run into discrete shear zones within the basic masses. These faults are most numerous where competent rocks, such as syenite and quartz-biotite-norite, are in contact with incompetent rocks, such as serpentinite, and are presumed to postdate the serpentinisation of the ultramafic rocks. Relations are particularly complex on the Hill of Towanreef ([NJ 450 245] and vicinity) and at Knockespock ([NJ 542 243] and vicinity).

The northern and southern boundaries of the Lawel Hill Complex are thrust or shear planes which dip north at about 40°, and the complex is a thrust sheet of Lower Zone material which has been tectonically separated from the Insch intrusion. The extension of these thrusts cannot be traced in the Dalradian rocks to the east and west.

Chapter 4 Dalradian metamorphism

The Da'radian rocks, together with the pre-orogenic intrusions, have all been regionally metamorphosed during the tectonic episodes described in the previous chapter. The district forms part of the north-east metamorphic province of Fettes et al. (1986), which shows evidence of a distinctive metamorphic history. It was first described by Read (1952) as the Buchan Type of metamorphism, and distinguished by lower pressures, which resulted in different metamorphic mineral assemblages from the rest of the Grampian Highlands. The characteristic metamorphic minerals are andalusite and cordierite rather than the garnet, staurolite and kyanite which characterise the Barrovian metamorphism of the region to the south of the Mount Battock Granite (Figure 10). The Buchan metamorphism extends up to 20 km west of the Portsoy Lineament, but the southern boundary is ill defined, due to the sillimanite overprint in Deeside and neighbouring areas. The main regional metamorphism immediately preceded the intrusion of the late-tectonic basic masses (Chapter 5), and merged with the contact metamorphic effects in the contact aureoles. These regional and contact metamorphic effects were then followed with little or no interruption by the D₃ deformation episode and the formation of shear zones. Migmatisation is associated with the regional metamorphism over much of the district, being most intense in the south. As in the case of the regional metamorphism further north, the increased thermal gradient during the emplacement of the basic masses caused increased intensity of migmatisation, culminating in partial melting, particularly around the Tarland and Morven–Cabrach intrusions. No contact metamorphism can be attributed to the late-tectonic granitoid intrusions, or to the Crathes suite of post-tectonic intrusions owing to poor exposure and the high grade of the surrounding metamorphic rocks. However, the granites of the Cairngorm Suite have narrow metamorphic aureoles, with diagnostic minerals developed only in pelitic rocks. The metamorphic isograds and boundary of migmatisation within the Inverurie–Alford district are shown in (Figure 2).

Regional metamorphism

Pelitic and semipelitic rocks

The metamorphic zonation in the pelitic and semipelitic rocks of the district follows the Buchan scheme of Read (1952):

• 6 Biotite + cordierite + sillimanite (prismatic) + garnet
• 5 Biotite + cordierite + sillimanite (fibrolite)
• 4 Biotite + cordierite + andalusite
• 3 Biotite + cordierite
• 2 Biotite
• 1 Chlorite

Rocks of zones 1 to 3 occur to the north of the Insch intrusion outwith the district. The incoming of andalusite and cordierite is almost coincident, and zone 3 is very narrow in the Banff area. The local absence of the zone may be controlled by the chemical composition of the rocks.

The lowest-grade rocks in the district belong to zone 4 and occur in the Correen Hills, specifically in the Carlin-den Burn [NJ 488 227]–[NJ 495 230] and between the upper part of the Whitestone Burn [NJ 508 210] and Saplings [NJ 496 190]. Here dark grey very fine-grained pelites, with a slaty cleavage defined mainly by biotite, contain rounded cordierite spots up to 0.5 mm across and euhedral andalusite porphyroblasts which vary greatly in size (1–4 mm) and typically show chiastolite crosses (S78668). Only a short distance away, silvery muscovite-rich schistose semipelites contain anhedral sieved cordierite and andalusite porphyroblasts up to 5 mm and 3 mm across respectively, which enclose small biotite, muscovite and quartz crystals e.g. (S78661). The andalusite porphyroblasts contain inclusions which are considerably finer grained than the groundmass of the rock, and show compositional layering, with varying proportions of fine opaque inclusions. The layering in the inclusions (S_i) is parallel to or at a slight angle to the foliation in the matrix (S_e). The cordierite porphyroblasts also contain numerous inclusions; these are only slightly finer grained than the matrix of the rock, and layering is less well defined than in the andalusite porphyroblasts.

The pelite exposed in Correen Quarry [NJ 5225 2135] is muscovite rich, like the silvery semipelites, and has a similar texture, but the cordierite and andalusite porphyroblasts are larger (8 mm and 5 mm respectively) and more abundant. Euhedral magnetite crystals up to 2 mm across are common (S78663).

To the south of the Correen Hills, the semipelites show similar assemblages to those described above as far as a line from Westside [NJ 476167] to Castle Forbes [NJ 621 191], although the size of the andalusite porphyroblasts generally increases southwards. The black specks in the andalusite porphyroblasts disappear, and S_i ceases to be visible within the porphyroblasts (S78456). To the south of this line, the semipelitic and pelitic rocks develop fibrolite; relics of andalusite are present for approximately 2 km (S78669), and pseudomorphs of muscovite after andalusite are recognisable for up to 4 km south of the line. The rocks show a progressive southwards coarsening of the groundmass fabric, from 0.2 mm average grain size in the Correen Hills, to 0.5 mm where fibrolite first develops, and 1.5–2 mm where the first signs of migmatisation occur.

A complex sequence of chemical reactions accompanies the formation of fibrolite. The fibrolite grows within crystals of biotite, which is first reddened, then loses its colour. The fibrolite needles grow with their long axes within the basal cleavage plane of the original biotite. A new generation of foxy red biotite crystals grows across the foliation. At the same time, andalusite is progressively replaced by muscovite which forms randomly oriented large plates with irregular margins, and magnetite in octahedral crystals up to 1 mm is developed in the more iron-rich lithologies e.g. (S77480), (S78481), (S78482).

To the south of a line from Glaschul Hill [NJ 459 149] to Seats [NJ 621 123], the more susceptible lithologies develop migmatitic textures, with segregation of leucotonalite veins and pods, mostly parallel to the foliation. At this point, the rocks show no signs of the previous existence of andalusite or an earlier foliation.

To the west of the Rhynie Devonian outlier, the semipelites are mostly andalusite-cordierite-porphyroblast schists similar to those of the Correen Hills, but the incoming of fibrolitic sillimanite occurs farther to the north, between the Hill of Wester Clova [NJ 442 186] and Broom Hill [NJ 435 183]. Also, to the north of the Burn of Corchinnan [NJ 436 229] metamorphic grade increases, and miginatites are developed for almost 1 km south of the contact of the Boganclogh intrusion; this is largely a contact effect of the Boganclogh mass.

The assemblage in the pelites between Scar Hill [NJ 487 112] and Tarland Lodge [NJ 490 065] is quartz + plagioclase + muscovite + biotite + sillimanite + garnet ± cordierite (S78582). Both fibrolite and prismatic sillimanite occur, and garnet forms crystals up to 5 mm. Rotated inclusion trails occur in garnets on Craiglea Hill [NJ 4793 1100] (S78467), but on Pittenderich [NJ 4981 0837] inclusions up to 0.3 mm show no preferred orientation (S78514). In the extreme south of the district, the same assemblage occurs in highly aluminous pelites and in the restite xenoliths in the Scar Hill [NJ 482 013] gneisses.

Textures exhibited by the fibrolite-bearing rocks to the south of the River Don, and in several areas outwith the district, led Chinner (1966) to postulate that sillimanite growth occurred after the main regional metamorphism of the Grampian Highlands and was an overprint related to the intrusion of the basic masses. Whereas isobars are roughly parallel to isotherms in the Huntly–Portsoy area, they are widely divergent in the Alford district. Peak metamorphic temperatures appear to be partly related to proximity to the basic masses, except where disrupted by shear zones. However, pressures appear to have increased steadily southwards, as evidenced by the general presence of garnet from Craiglea Hill southwards. Garnet is absent on Donside, except for the anomalous hornfelses on Kist Hill. While it seems certain that the fibrolite in the Donside rocks is later than the andalusite, this probably reflects a prograde sequence of events occurring over a short period of time rather than two .separate and distinct metamorphic episodes.

The position of the original andalusite/kyanite boundary is difficult to trace due to the pervasive development of sillimanite in the gneisses of Deeside, but Chinner and Heseltine (1979) have placed it close to the southern boundary of the district. Relics of regional andalusite become scarcer southwards from the Don and very few occurrences occur on Deeside west of Banchory. A specimen of gneiss from near Balnacraig [NJ 6006 0402] (S77492) contains a relic crystal of andalusite, and Porteous (1973) reports an occurrence of probable regional andalusite from Mill of Cammie [NO 695 919] in the Banchory district (Sheet 66E). Rare andalusite occurs on Scar Hill [NJ 4813 0150] (S78539) and 600 m to the northwest, near the contact with the Tarland intrusion, at [NJ 4766 0134] (S71813). The andalusite forms small irregular crystals, and in a thin section described by Read (1927) it is surrounded by shimmer aggregate. No stau rolite is known from the district, but a pelitic xenolith from Craig Ferrar [NO 493 995], less than 1 km from the southern boundary of the district, shows staurolite partly replaced by prismatic sillimanite (Read, 1927, p.330). To the south of the Ballater and Mount Battock granites (Sheets 65E, 66W, 66E), the rocks show kyanite partly replaced by sillimanite in the prograde metamorphism of the area; staurolite is common and cordierite absent. Andalusite also occurs in the thermal aureoles of the Hill of Fare and Mount Battock granites on Deeside within 200 m of the granite contact.

Within 500 m of the southern contact of the Insch intrusion between Smallburn [NJ 520 240] and Bogside [NJ 637 240], the rocks are platy quartz-mica-schists with a well-developed foliation marked by the alignment of abundant coarse muscovite flakes. Andalusite and cordierite form anhedral porphyroblasts up to 1 mm across containing abundant fine inclusions; the fabric in some porphyroblasts has been rotated (S77427), (S77428), (S77429), (S77430), (S77431), (S77432). The muscovite foliation is folded in a few places by sharp kink-folds. These rocks are more coarsely crystalline than most of the semipelites in the northern part of the Correen Hills, and may have recrystallised during or after a shearing episode, possibly connected with the emplacement of the Insch intrusion.

Psammites and quartzites

Psammitic rocks, and especially quartzites, show much less change with metamorphism. Quartz and plagioclase are stable throughout the range of pressure and temperature occurring during regional metamorphism, and, although grain boundary changes have occurred widely, recrystallisation of these minerals is much less than that of micas and other aluminosilicates. Biotite and muscovite are present in psammites throughout the district, but there is generally too much potassium in the rocks for other aluminosilicates to form.

The gritty psammites of the Suie Hill Formation have a fabric which is dominated by slightly flattened detrital quartz grains up to 4 mm across, enveloped in a very fine-grained matrix of sericite, biotite and quartz; the rock also contains widely scattered, possibly detrital, magnetite. The psammites of the Aberdeen, Craigievar and Queen's Hill formations have a fabric consisting of a mosaic of quartz and plagioclase with a weak alignment of long axes, and a foliation defined by the alignment of the minor proportion of biotite and muscovite. The textural differences observed are largely due to the sorting and variable clay content of the original sediments. However, the more pervasive recrystallisation of the higher-grade rocks in the south may in places have obliterated any original gritty texture. Many psammites, especially those of Queen's Hill [NJ 528 005], have suffered extensive shearing, as evidenced by the ribbon texture of the quartz.

Calc-silicate rocks

Calcareous lithologies are poorly developed in the district, but over 30 thin sections of limestone and calc-silicate rock are now available. These rocks generally occur as thin impersistent beds, less than 10 m thick and traceable for less than 100 m along strike. Thicker and more persistent units only occur in the Deeside Limestone Formation and in the Craigievar Formation near Towie. In the Suie Hill Formation to the west of the Bennachie Granite, on both sides of the Rhynie outlier, diopside is generally absent from calc-silicate rocks and the amphibole is a pale tremolite. The most typical assemblage is quartz ± calcite + plagioclase + tremolite + clinozoisite + sphene e.g. (S76878), (S78505). The plagioclase is generally strongly sericitised and partly replaced by epidote and clinozoisite. However, diopside occurs in calc-silicate rock and impure limestone at the Limer Shank [NJ 5147 2138], along with (probably secondary) tremolitic amphibole (S78664)–(S78665). Wollastonite was recorded from a thin calc-silicate layer (S78447) at Clatterin' Kist [NJ 5416 2214].

In the rest of the district, the typical calc-silicate assemblage is quartz + calcite + plagioclase + clinozoisite + diopside + sphene ± amphibole ± K-feldspar e.g. (S78583), (S78585). Grossular garnet and idocrase are present in several specimens (S77497), but rarely form more than 5% of the rock. However, a small pod of talc-silicate rock at the southern contact of the Insch intrusion at Loanend [NJ 6044 2405] contains 20–30% of grossular and 5–10% of idocrase (S77412). Plagioclase compositions reflect the variation in bulk rock composition and normally lie in the range An_30–70, though An75 has been recorded (S75436). Where amphibole is present, it is generally a rather pale greenish blue type, but where the rock is transitional to amphibolite, it is darker green and closer to hornblende. Tremolite occurs only where retrogression has taken place.

Regional migmatites

With increasing metamorphic grade, the fabric in lithologies such as semipelite, some types of pelite and arkosic psammite, is not only coarser, but also pervasively inhomogeneous on a megascopic scale, with segregation of quartzofeldspathic and more mafic and alumina-rich material into layers. These rocks conform to Ashworth's (1985) definition of stromatic migmatites, with a prominent quartzofeldspathic component (leucosome), and a marginal mafic selvedge (melanosome). Some of the migmatites in the district are believed to have formed by diffusion of elements via a water-rich fluid phase, whereas others record partial melting of the host rock (anatectic migmatites). Only the latter type produce clots or xenoliths of more refractory material (restite).

The limit of migmatisation in the Dalradian rocks of the district (Figure 2) extends from the western side of Brown Hill [NJ 430 180], underneath the south end of the Rhynie outlier, through the Kildrummy intrusion to the end of the Corrennie Granite near Kirkton of Tough [NJ 620 125]. The limit is displaced northwards by the Den-croft Fault to run to the south of Green Hill [NJ 632 138]–[NJ 658 139]. To the east of the Bennachie Granite, all of the Dalradian rocks are generally migmatised except for an area immediately east of the granite, west of a line from Drumdurno [NJ 708 244] to Grantlodge [NJ 703 173]. The intensity of migmatisation increases southwards to the west of the Bennachie Granite and eastwards to the east of this granite.

The effects of migmatisation are first manifest in the semipelites, where quartzofeldspathic lenticles and vein-lets develop parallel to the foliation, with some concentration in fold hinges. As migmatisation increases, quartzofeldspathic segregations form an increasing proportion of the rock and cross-cutting veinlets develop. Also, biotite-rich selvedges develop marginal to the leucosomes, which become granitic in composition and coarser grained than the rest of the rock. The appearance of the most intensely migmatised rocks along the southern margin of the district, where the highest-grade regional metamorphism occurred, is exemplified by the fresh exposures in the working Craiglash Quarry [NO 622 987], just south of the district boundary. Here, granite veins form 50–65% of the rock, in a ramifying stockwork. About half of the veins are parallel to the foliation in the semipelitic gneisses. The veins are mostly 5–10 mm wide, but a few reach 50 mm. The grain size in the veins is typically 2–3 mm, but pegmatitic patches with a 10 mm grain size occur as pods and in the wider veins. The veins comprise quartz, plagioclase, K-feldspar and biotite, while muscovite occurs in the pegmatitic patches. The host semipelite/pelite has a grain size of 1–2 mm, and is well segregated into biotite-rich and quartzofeldspathic layers 2–20 mm thick. This lithology is essentially similar to the 'injection gneiss' described by Barrow (1912) from Cairnshee Quarry [NO 739 939], in the Banchory district (Sheet 66E).

To the east of the Bennachie Granite, the increase in the intensity of migmatisation culminates in the formation of gneisses of granitic composition which form an envelope ranging from 50 m to 500 m in width around the Kemnay Granite and its satellites (Crichie, Port Elphinstone, Craigforthie and Hill of Crimond). The gneisses are white to pale pink, medium- to coarse-grained rocks with a prominent foliation defined by 0.5–1 mm thick biotitic layers spaced at 1–10 mm intervals, with the intervening layers being almost devoid of mica. The biotitic layers anastomose and gentle swirling fold patterns are developed. Layers of non-migmatitic psammite occur within the gneiss. As the contacts of the Kemnay Granite and its satellites are approached, the spacing of the biotitic layers increases, but the transition to the true Kemnay Granite, with well-aligned biotite crystals scattered randomly in the rock, occurs over 20 m or less.

The garnet-sillimanite-pelites in the Craiglea Hill–Tarland Lodge area [NJ 476 109]–[NJ 490 055] are in general only slightly migmatitic, except adjacent to the northern contact of the Tarland intrusion. The degree of migmatisation of these aluminous rocks is consistently less than that of the semipelites and micaceous psammites of the rest of the Craigievar Formation. Less aluminous lithologies within the unit, however, develop narrow granitic veins parallel to the foliation, and on the Socach [NJ 483 098] patchy development of xenolithic gneisses occurs.

In the area south of the Lumphanan, Cromar and Tomnaverie intrusions, from Tornaveen [NJ 615 062] to Mulloch Hill [NJ 470 005], the migmatisation increases westwards, and evidence of probable anatexis is seen. Near Balnacraig [NJ 6006 0402], migmatitic gneiss with relatively poor compositional layering contains rounded clots of biotite and coarse sillimanite (S77492). To the west of the Cromar Granite, the pelites and semipelites of Balnagowan Hill [NJ 506 007] and Coull Home Farm [NJ 513 014] are very coarse grained (2–3 mm) with feldspar porphyroblasts up to 10 mm. They are coarsely layered on a 10–30 mm scale. The psammites on Queen's Hill [NJ 533 006] and Court Hill [NO 510 999] are the only biotite-poor psammites to be migmatised. The rocks are traversed by granite and pegmatite veins, most of which run parallel to the foliation of the host, although there are a few cross-cutting veins. The more feldspathic psammites have also recrystallised to a coarse pegmatitic texture, which postdates the ribboning of the quartz crystals in the psammite.

The amphibolites of Creag Dhu have suffered no migmatisation, but a layer of amphibolite at Balnacraig [NJ 479 005] is cut by numerous trondhjemitic veins, mostly discordant to the foliation, but ptygmatically folded in places.

The migmatites 200 m east of Braeroddach Loch [NJ 486 002] which extend on to Creag Ferrar [NO 493 995] are relatively weakly layered gneisses with a good alignment of the biotite crystals ind with porphyroblasts of plagioclase and garnet up to 5 mm and 3 mm respectively. However, clots and segregations of restite material are rare. They are transitional to the heterogeneous xenolithic gneisses of Scar Hill [NJ 482 014], Mulloch Hill and Craigie, whose regional migmatisation is difficult to separate from the partial melting due to the intrusion of the Tarland basic mass (see below).

Contact metamorphism

Metamorphism related to the late-tectonic basic intrusions

The intrusion of the basic masses produced contact metamorphic effects which varied according to the preexisting metamorphic grade of the host rocks. In the northern half of the district hornfelses were produced, whereas in the south, where the rocks were regionally migmatised, partial melting of the host rocks occurred and xenolithic migmatites were formed. These latter rocks can be explained best as the products of partial melting followed by tectonism and equilibration to amphibolite facies metamorphism during the cooling process.

The rocks on the southern slopes of Tap o' Noth [NJ 490 290] lie adjacent to the northern contact of the Boganclogh intrusion and are fine- to medium-grained semipelitic hornfelses containing the assemblage quartz + plagioclase + K-feldspar + cordierite + sillimanite ± garnet (S78702). The occurrence of relics of anda-lusite suggests that they formerly contained regional metamorphic assemblages of zone (4). Compositional layering on a 2–5 mm scale is tightly folded.

Around Clayhooter Hill [NJ 434 238] , andalusite cordierite-schists show evidence of modification of the original regional fabric and mineralogy. In thin section, clear cordierite rims with relatively coarse inclusions are seen to overgrow cloudy cordierite with numerous very small inclusions. Wrapping of zoned andalusite crystals by the clear cordierite has also been reported (Fettes, 1970; (S76875)). Fettes interpreted this texture as the modification of a regional andalusite-cordierite texture by contact metamorphism attributable to the Boganclogh intrusion. Clear evidence of a metamorphic aureole to the south of the Insch and Boganclogh intrusions can be found only to the west of the Rhynie outlier.

The high-grade rocks of Kist Hill [NJ 638 162] and Hill-brae [NJ 685 246], to the south-west and north-east of the Bennachie Granite, are similar in appearance and mineralogy to the Tap o' Noth rocks. Garnet is absent from the rocks at Kist Hill but appears farther to the east near Bogmore [NJ 646 152] (Michie, 1968). The rocks are not as tightly folded as on Tap o' Noth, and at Hillbrae and Bogmore show 10 mm-scale compositional layering expressed as variation in fibrolite content. The Dencroft Fault juxtaposes the hornfelses to the east against regionally metamorphosed semipelites with fibrolite partly replacing andalusite porphyroblasts. The transition from the high-grade hornfelses on Kist Hill to regionally migmatised rocks on the southern slopes of Green Hill [NJ 648 136] is gradational, with the loss of K-feldspar and cordierite, change in biotite colour and development of a gneissose texture. Unlike the other hornfelses in the district, there is no obviously related late-tectonic basic intrusion. Slight retrogression due to the Bennachie Granite is superimposed on the high-grade sillimanite-K-feldspar hornfelsing, but the mineralogy would have required much higher heat flows and temperatures for its formation than could be attributed to the effects of granite magma. It is believed that the hornfelses, bounded to the east and west by faults and to the north by the South Insch Shear Zone, are the displaced aureole of a basic intrusive body; this could be a buried intrusion, or part of the Insch intrusion now separated by 9 km of Bennachie Granite, or some other body now displaced by thrusting or lateral shearing.

The migmatisation of the rocks immediately adjacent to the northern contact of the Tarland intrusion from the northern slopes of Baderonoch Hill [NJ 428 092] to Broom Hill [NJ 470 093] and east of Ranna [NJ 489 070], together with those of the Mulloch Hill–Craigie–Scar Hill area [NJ 465 005]–[NJ 487 017], is attributed to partial melting of rocks already regionally metamorphosed at sillimanite grade.

The Mulloch Hill–Craigie–Scar Hill rocks consist of a relatively uniform, poorly layered gneiss with good parallel orientation of biotite, containing plagioclase as 5 mm porphyroblasts, garnet as 3 mm porphyroblasts, and rare magnetite in 1–2 mm crystals, with interstitial quartz, plagioclase, biotite, cordierite and prismatic sillimanite. The gneiss contains rounded xenoliths of restite material: quartzite, talc-silicate rock, sillimanite-biotite-garnet-rich pelitic material, and, rarely, amphibolite. These xenoliths are interpreted as the remnants of more refractory layers, while the matrix is the product of a high degree of melting of mixed semipelite and micaceous psammite, with some pelite. To produce relatively homogeneous rocks like these, the degree of partial melting would have to have been considerable, especially as the gneiss is of roughly tonalitic composition, albeit with excess Al₂O₃. This is confirmed by the notably aluminous compositions of the pelite xenoliths.

The rocks at the northern contact of the Tarland intrusion and east of Ranna are much less homogeneous, and the degree of partial melting was less. Sizeable mafic schlieren and slab-like xenoliths of semipelite and psammite float in a tonalitic to granodioritic gneiss which lacks the large plagioclase porphyroblasts of the Scar Hill gneisses. The gneiss has a foliation defined principally by the parallel alignment of the biotite with very little segregation of biotite into mafic layers Similar heterogeneous gneisses with xenoliths of silica-poor hornfels occur at the north-east corner of the Insch intrusion at Oldmeldrum [NJ 8070 2804] and Balcairn [NJ 7916 2867]. At Govals Quarry [NJ 732 256], cordierite-norites derived by partial melting of Dalradian pelitic rocks, and containing xenoliths of silica-poor and alumina-rich pelitic restite material lie along the chilled contact of the Insch intrusion.

Metamorphism related to granitic intrusions

The only granitic bodies which have produced recognisable contact metamorphic effects within the district are some members of the Cairngorm Suite of granites, and these effects are slight compared with those produced by the basic intrusions. Contact metamorphic effects are recognisable for no more than 300 m from the contacts of the intrusions. Fresh clear andalusite crystals and foxy red biotite are developed in suitable lithologies, and garnet and fibrolite are retrograded. The Cairngorm Suite granites were intruded at a high level in the crust, and intrusion was accompanied by reddening and hydrothermal alteration of the granites. Contact metamorphic effects are absent along faulted contacts, and unrecognisable against other plutonic intrusions.

To the south-west of the Bennachie Granite, high-grade hornfelses, probably the displaced aureole of a basic intrusive body (see above), have been retrogressed for up to 300 m from the contact. At Craignarb [NO 6774 9969], 200 m south–west of the margin of the Hill of Fare Granite, semipelites have been hornfelsed with reddening of the biotite and development of clear andalusite in subhedral porphyroblasts up to 1–2 mm which cross-cut the regional gneissic foliation (S74764). Due to poor exposure and the short length of the Hill of Fare Granite–Dalradian contact, this is the only exposure of aureole rocks around this intrusion. No contact metamorphism has been recorded around the Cromar Granite and contact effects related to the Ballater Granite are visible only outside the district. Effects related to the Middleton and Cushnie granites are dominated by lower temperature, hydrothermal alteration of both the granite itself and the adjacent country rocks (Chapter 7) and (Chapter 12).

Chapter 5 Pre- and late-tectonic basic magmatism

Pre-tectonic basic magmatism

A number of amphibolite sheets occur in the Dalradian rocks of the district. These have been subjected to the same tectonic and metamorphic events as the enclosing metasedimentary rocks, and are therefore of Precambrian age. They may be coeval with the Green Beds pyroelastic rocks which occur in the Southern Highland Group rocks of Glen Clova (Ballater district) and areas further west. Within the Inverurie–Alford district, amphibolite sheets occur in three principal areas: Conglass–Dilly Hill, north-west of Inverurie; Scar Hill–Craig Dhu, south of Tarland; and near Lumphanan.

North-west of Inverurie

The amphibolites of the area around Newbigging [NJ 736 216] and Dilly Hill [NJ 751 225] have been partly delineated by magnetic surveys conducted to investigate the southern contact of the Insch intrusion east of the Bennachie Granite. In addition to natural exposures and disused quarries, further records of amphibolite were obtained from pits dug while following up this magnetic survey. The sheet which crops out north of Newbigging dips gently northwards, and was considered to be the source of magnetic anomalies discovered during an investigation of the Middleton Granite. Gabbro or amphibolite may be present on the summit of Knockinglews [NJ 733 218], as basic rocks were recorded here during the primary survey of the district (Geological Survey of Scotland, 1886), but the hilltop is now unexposed and even the loose blocks are mossed over. The old quarry at Dubston [NJ 749 220] and the nearby exposures on Dilly Hill well illustrate the concordance of the foliation in the amphibolite with the principal foliation in the country rocks.

South of Tarland

Several sheets of amphibolite occur in the vicinity of Craig Dhu [NJ 490 011]. The largest, occupying most of Craig Dhu, attains a maximum thickness of 400 m. It thins rapidly WSW and appears to interdigitate with the country rock. A further three sheets, each up to 100 m in thickness, crop out to the north-west and south-east of Craig Dhu. One of the sheets continues as far south-west as Balnacraig [NJ 479 006]. The internal foliation within the sheet is generally concordant with that in the country rock. However, in places an earlier discordant foliation is locally preserved, as on the southern side of Craig Dhu [NJ 489 009] where a foliation at a high angle to the contact is overprinted within a few metres of the contact by a later foliation parallel to the contact. At Balnacraig [NJ 479 006], narrow felsic veins traversing the amphibolite pinch and swell around tight folds of the amphibolite foliation. The contacts of the bodies with the host meta-sedimentary rocks are sharp, but subsequent deformation and metamorphism have obliterated any primary intrusive relationships and fabrics.

The amphibolites are all medium to coarse grained (0.5–1 mm), with a well-developed planar fabric and a certain degree of linear alignment of the long axes of the amphibole crystals. No chemical analyses are available for amphibolites from the district, but pre-tectonic amphibolites from other parts of the Grampian Highlands have a tholeiitic composition.

Late-tectonic basic magmatism

The late-tectonic basic intrusions of the north–east Grampian Highlands constitute a major magmatic province characterised by large volumes of basaltic magma which were intruded into hot continental crust, near to the peak of metamorphism during early Ordovician times. This magma consolidated into a small number of differentiated basic intrusions, which were disrupted into large and small masses by penecontemporaneous shearing and later faulting. The regional disposition of the larger basic masses is shown in (Figure 11).

One of the most characteristic features of these basic rocks is the prevalence of layering, which reflects crystallisation from a large body of magma held in one or more magma chambers. Most of the rocks are cumulates, i.e. rocks precipitated from the magma on to the floor (rarely walls or roof) of the magma chamber (Wager et al., 1960). Several types of layering are developed:

Igneous lamination is defined by Irvine (1982) as a pervasive fabric occurring throughout the cumulate mass at a grain-size scale. It is formed during the deposition of crystals within a magma chamber, and is marked by the preferred orientation of tabular crystals or the long axes of acicular crystals in the same plane, which may or may not have been horizontal at the time of deposition. In the absence of evidence to the contrary, the plane of the igneous lamination is assumed to have been roughly horizontal at the time when the cumulus minerals were deposited.

Rhythmic layering is a variation in the relative proportions of the different cumulus phases, typically with a steady increase in the phase of lower density and decrease in the other phase (s) followed by a sudden reversion to a high proportion of the dense phase. This type of layering commonly occurs on scales of a few centimetres and several metres.

Phase layering occurs where, due to progressive differentiation of the magma in the chamber, a new phase commences to precipitate, or a phase which was precipitating ceases to do so.

Cryptic layering is the variation in composition of the precipitated phases, and typically involves the lower-temperature members of a solid solution series crystallising after higher-temperature members, i.e in the main cumulate pile the plagioclase becomes more sodic and the ferromagnesian minerals more iron-rich upwards. Reversals in cryptic layering are generally ascribed to an influx of less-differentiated magma into the magma chamber.

The phase layering was used by Clarke and Wadsworth (1970) to divide the Insch Mass into three zones, the uppermost of which was subdivided into three:

The base of MZ is defined by the disappearance of olivine as a cumulus phase, and the base of UZa by the reappearance of (much more iron-rich) cumulus olivine. The base of UZb is defined by the incoming of cumulus K-feldspar, and the base of UZc by the final disappearance of olivine and orthopyroxene. All of these zones can be recognised in the Boganclogh intrusion. The zonal scheme is slightly different in the Morven–Cabrach and Tarland intrusions.

The majority of the late-tectonic basic masses, including all of those in the district, are believed to be comagmatic. Whether they originally formed a single intrusion or several bodies which were further fragmented during the D₃ episode is uncertain. The boundaries of the basic masses are now largely tectonic following shear zones, which are also developed within the masses. There are no recognisable comagmatic minor intrusions which can be analysed to give a likely composition for the parent magma, but this can be estimated from the sequence of crystallisation of the cumulus minerals, and the composition of the cumulus minerals. By such reasoning, the parent magma is though to have been of tholeiitic basalt composition, possibly rather magnesian if the marginal ultramafic rocks of Boganclogh represent part of the same crystallisation path as the main body of the Insch and Boganclogh intrusions.

The Inverurie–Alford district includes approximately 60% of the Insch mass, 45% of the Boganclogh mass, 5% of the Morven–Cabrach mass, the whole of the Tarland mass, and several smaller basic and ultramafic bodies. This memoir includes full descriptions of the Insch, Boganclogh and Tarland masses, and brief descriptions of the smaller basic bodies lying within the district. Only a short account of the eastern part of the Morven–Cabrach mass is included; a full description of this intrusion will be found in the memoir for Sheets 75W and 75E, which is in preparation.

Insch intrusion

Of all the basic intrusions of the north-east Grampian Highlands, the Insch intrusion contains the widest range of composition, extending from dunite to syenite. It has been separated from its western continuation, the Boganclogh intrusion, by the intrusion of the later granitic Kennethmont Complex, and, in outcrop, by the superposition of the sedimentary rocks of the fault-bounded Rhynie outlier (Figure 12), (Figure 13). For the sake of convenience, the Insch and Boganclogh intrusions will be described separately. Although Read and his co-workers (see Chapter 1 for a selection of references) described many important features of the Insch intrusion, the first account of the body as a classic layered intrusion was that of Clarke and Wadsworth (1970). Further work by Ashcroft and Munro (1978), Gallagher (1983), Leslie (1984a; b) and Wadsworth (1986; 1988) has enabled the relations of the various rock units to be worked out in more detail.

Lower Zone rocks are confined to the margins of the intrusion. The largest outcrop occurs in the south-east, between Hillhead of Lethenty [NJ 762 254] and Southbog [NJ 822 264]. These rocks form a mass up to 1.2 km wide, bounded to the north and south by shear zones. Other remnants of ultramafic rock occur in the south-west and along part of the northern margin, where they are also tectonically juxtaposed against rocks much higher in the layered succession. The main body of the Insch intrusion consists of Middle and Upper Zone rocks. The Middle Zone consists of olivine-free noritic rocks of varying texture: cumulate norites and granular gabbros to the east of the Upper Zone rocks and quartz-biotite-norites to the west. Apart from a few rare granular olivine-ferrogabbros near the base of the zone, no granular rocks are seen within the Upper Zone. The Middle and Upper zones show steady variation of mineral composition (cryptic layering). Middle and Upper zone rocks have been folded into a broad northward-plunging syncline, whose axis runs approximately from near Mill of Johnston [NJ 572 245] to near Kirkton of Culsalmond [NJ 650 323]. This is the local expression of the D₄ Turriff Syncline. The olivine-monzonites and syenites of UZb and UZc form prominent low hills to the west and south-west of Insch (Plate 3). The base of UZa crops out approximately 2 km to the northwest and 2 km to the south-east of this axis, and the low hills formed from the olivine-monzonites and syenites of

UZb and UZc are aligned along this axis. This syncline has been modified by uptilting of the southern margin of the intrusion between Knockespock [NJ 545 243] and Mill of Johnston [NJ 572 245]. On the Hill of Dunnideer [NJ 612 282] and the Hill of Christ's Kirk [NJ 602 274] the dip of the layering is less than 10° to the west, while on the Hill of Newleslie [NJ 585 255], the layering within the olivine-monzonite can be seen to dip to the WNW at 50°–65°. Several ductile shear zones trending ENE–WSW to east–west affect the mass, and these are cut by north–south and NE–SW faults (Figure 13).

The Middle Zone contains extensive rafts of metasedimentary material which, in common with some of the metasedimentary rocks near the north and south contacts of the mass, have undergone partial melting to produce cordierite-norites with xenoliths of restite material (silica-poor, basic, quartzitic, and calcareous hornfelses). The adjacent norites have in many places been modified by assimilation of metasedimentary material.

Geophysical expression

The late-tectonic basic and ultramafic intrusions produce strong magnetic and gravity signatures (Figure 4) and (Figure 7). The whole of the Insch intrusion was covered by a helicopter-borne magnetic survey conducted by Exploration Ventures Ltd in 1970, whose results were made available for study by Gallagher (1983), and which are deposited with BGS, Edinburgh, where they are now on open-file access. Ground-based magnetic surveys have been conducted in the eastern part of the intrusion by Ashcroft and Munro (1978), Leslie (1984a) and Ashcroft (unpublished data held by BGS). These surveys have been followed by shallow drilling and/or pitting, and they have enabled the structure of the intrusion and its contact zones to be interpreted in considerably greater detail than would be possible from just the aeromagnetic survey and available bedrock exposures.

The most intense magnetic anomalies (Figure 4) are associated with the Lower Zone rocks, especially over Hill of Barra [NJ 802 256] and nearby. They are caused by secondary magnetite produced during serpentinisation of ultramafic rocks. A similar, though less intense, anomaly is associated with the ultramafic rocks of the Knockespock area; this can be traced, in subdued form, along the southern contact of the Kennethmont Complex and under the Devonian rocks of the Rhynie outlier, to join up with the more intense anomaly over the ultramafic rocks of the Hill of Towanreef [NJ 457 242] in the Boganclogh intrusion.

The Upper Zone rocks, particularly the olivine-monzonites and syenites, give rise to prominent magnetic anomalies located over the 'Red Rock Hills' from Hill of Auchmar [NJ 572 250] to Fallow Hill [NJ 637 316]. The magnetic intensity over the Middle Zone rocks is lower than over the Lower or Upper Zone rocks, though still considerably elevated above levels in the Dalradian country rocks.

Shear zones in the Middle and Upper Zone rocks can be traced as linear belts of low magnetic intensity within the Middle and Upper Zone outcrop. These belts of low intensity are caused by the destruction of primary magnetite which occurred during the shearing episode; this effect was restricted to rocks in the immediate vicinity of the shear zones. The shear zones shown in (Figure 13) are mostly mapped from these negative anomalies.

Bouguer gravity anomaly values over the Insch intrusion range from −20 mGal in the south-west to +18 mGal in the north-east (Figure 7). The negative values are due to the effects of the Bennachie Granite and its hidden western and eastern extensions. McGregor and Wilson (1967) attempted to interpret the three-dimensional form of the Insch intrusion by modelling the Insch and Bennachie intrusions along a north–south traverse running through Oyne (grid line 67). Using a density of 2.65 Mg m⁻³ for the Bennachie Granite, 2.74 Mg m⁻³ for the Dalradian country rocks and 2.95 Mg m⁻³ for the rocks of the Insch intrusion, they estimated that the basic and ultramafic rocks of the Insch intrusion form a sheet approximately 3 km thick, which thins northwards to 2 km and continues north-west for at least 10 km under a cover of about 1 km of metasedimentary rocks.

Character of the margins

The Insch intrusion is roughly rectangular in surface form, extending 30 km in an east–west direction and 7 km from north to south (Figure 13). To the west the intrusion is cut by the later granitic Kennethmont Complex, and in part of its southern contact it is intruded by the Bennachie Granite, but everywhere else it is in contact with Dalradian metasedimentary rocks. The nature of these contacts between intrusions is difficult to interpret, especially in view of the generally poor exposure, but there is evidence that most of the contacts are tectonic, and that the original relationships have been heavily modified.

The existence of a broad contact aureole along the northern margin of the intrusion lead Read (1923) to believe that this contact is undisturbed and that the basic rocks dips gently northwards under the hornfels. More recent evidence indicates, however, that the contact is largely tectonic. From Slack [NJ 579 311] to south of Largie [NJ 611 312], the contact is almost straight, and truncates the north–south fault which runs from Brankston [NJ 552 320] to New Leslie [NJ 589 254]. Between Brankston and south of Largie, the rocks immediately south of the contact range from syenite, through olivine-monzonite to olivine-ferrogabbro. In the vicinity of Largie, [NJ 611 312], [NJ 619 320], the contact runs approximately north-east. There is a possibility that the two kinks in the contact are related to the ESE-trending shear zones which run from there to near Old Rayne, and that the section between the two kinks may be part of the intrusive contact of the mass. The section of the northern contact of the intrusion from south of Carnieston [NJ 619 320] to near Cross of Jackston [NJ 750 328] is extremely poorly exposed. At Smiddyburn [NJ 739 330], Leslie (1984b) has shown that a thin strip of norite separates olivine-ferrogabbro from Dalradian country rock, and that the various contacts are sheared.

The north-eastern contact of the Insch intrusion has been investigated by a magnetic survey (Leslie 1984a), which showed that the contact has been affected by faulting and shearing. Most of the displacement of the contact is caused by NE-trending faults, the largest of which displaces it from near Lightnot [NJ 795 302] to south of Newton of Saphock [NJ 775 284]. In this sector, the broad hornfels zone found along the rest of the northern contact of the Insch intrusion cannot be recognised, due to the higher grade of regional metamorphism, but local developments of contaminated and xenolithic basic rock occur, e.g. near Newton of Saphock [NJ 775 284]. From near Glengarioch Distillery [NJ 806 281] to near Southbog [NJ 821 264] a fault is taken as the eastern boundary of the Insch intrusion; a displaced belt of basic rocks continues to the east of the fault from Oldmeldrum town centre [NJ 808 279] into the Ellon district (Sheet 87W).

In the south-eastern part of the intrusion, Ashcroft and Munro (1978) have demonstrated that the ultramafic rocks which constitute the Lower Zone of the intrusion occur in a narrow belt, up to 1.5 km wide, bounded to north and south by east- to ENE-trending shear zones. Within this belt, layering is subvertical and strikes roughly north–south (Sadashivaiah, 1954a). A total thickness of 1.8 km of cumulates, repeated by faulting, occurs. The bounding shear zones have been exposed in gas pipeline trenches, and Munro (1986a) has demonstrated that shearing took place under amphibolite-facies conditions. Between these Lower Zone rocks and the Middle and Upper Zone rocks which form the bulk of the intrusion, there is a belt approximately 750 m wide, consisting of sheared basic rocks with very minor sheared granite and pegmatite, which extends from Sauchenloan [NJ 770 265] to the southern part of Oldmeldrum [NJ 810 270].

The southern contact of the intrusion between Newmains [NJ 679 247] and Mains of Inveramsay [NJ 747 256] is complex, and has been affected by shearing and faulting.

An unpublished magnetic survey, linking with that of Ashcroft and Munro (1978) was carried out by Aberdeen University for BGS and followed up by pitting in 1986 (Figure 9). A bifurcating ENE-trending shear zone, roughly parallel to and 100 to 300 m to the south of the contact of the basic rocks, separates magnetic hornfelses from unhornfelsed metasedimentary rocks. Running parallel to this shear zone within the Insch intrusion are belts of hornfelsed Dalradian rocks up to 100 m wide and 1.4 km long; their southern boundaries are probably faults or shear zones. These belts of hornfels are well exposed near Knockollochie [NJ 709 254] and also near Lumphart [NJ 759 271]; in places they are associated with contaminated and xenolithic basic rocks.

From near Kirkton of Premnay [NJ 642 248] to Newmains [NJ 678 247], the Insch intrusion is intruded by the Bennachie Granite. The contact is nowhere exposed, and there are no signs of contact metamorphism of the basic rocks by the granite.

From Smallburn [NJ 520 240] to Kirkton of Premnay [NJ 642 248], the contact of the intrusion lies within a 1 km-wide zone of sheared rocks. The Dalradian country rocks are fine-grained platy schists with small (< 1 mm) andalusite porphyroblasts. There is a discontinuous belt of serpentinised ultramafic rocks, averaging 100 m in thickness but reaching 400 m in places; mostly this is in contact with Dalradian rocks but in places, e.g. near Bogs [NJ 590 245], they are separated by a thin slice of basic rocks. The zone of sheared rocks is disrupted by a number of north- to NNE-trending faults with displacements of 100–300 m, which truncate or displace belts of ultramafic rocks but cannot be traced into the main mass of basic rocks to the north. The belt of shearing cuts across structures in the body of the Insch intrusion.

To the west, the Insch intrusion is in contact with the younger Kennethmont Complex. Exposure in the area of the contact is particularly poor, and the nature of the contacts is difficult to interpret. The boundary from near Bankhead [NJ 522 270] to south of Glanderston [NJ 584 291] is a fault, which has been exploited by a late Carboniferous quartz-dolerite dyke. To the south of this fault, rocks of the Insch intrusion extend to within 200 in of Druminnor House [NJ 518 262], but there are a few isolated bodies of Kennethmont-type diorite within the Insch quartz-biotite-norite. To the north of the fault, the Insch–Kennethmont boundary runs roughly along the Shevock Burn [NJ 583 295]–[NJ 579 311]. Insch Upper Zone rocks are in contact principally with the red granite of the Kennethmont Complex, but there are small exposures of diorite and gabbro of uncertain affinity in the vicinity. The boundary between the Insch and Kennethmont intrusions is sharply truncated by the east–west shear zone described, which marks the northern boundary of the two intrusions.

Lower Zone

The main outcrop of Lower Zone rocks forms a narrow strip from Hillhead of Lethenty [NJ 762 254] to near Southbog [NJ 822 264]. Exposures are very poor, and much of the information on these rocks has come from the British Gas pipeline trenches and from shallow drilling by Aberdeen University (Ashcroft and Munro, 1978). Such natural exposures as occur were described by Sadashivaiah (1954a). Nearly all of them lie between Barra Castle [NJ 792 257] and the summit of Hill of Barra [NJ 803 256]. The upper slopes of Hill of Barra are littered with blocks of well-layered olivine cumulates (dunite, peridotite and troctolite), but in-situ material showing good rhythmic layering is uncommon. The dunite and peridotite are dark, almost black rocks with clearly discernible cumulus olivine crystals up to 4 mm long, and cumulus chromite up to 0.5 mm across. Small quantities of interstitial plagioclase and pyroxenes are present. With increase in plagioclase, at first interstitial, then with increasing abundance, becoming cumulus in certain layers, they grade into troctolite. Towards the B9170 road troctolite becomes more abundant and the proportion of feldspar increases. Cumulus pyroxene occurs in olivine-norite to the west of Barra Castle which has been proved by shallow drilling, and an exposure of olivinenorite was recorded 400 m south-east of Barra Castle [NJ 7955 2545] during the original survey (Plate 4)a. A second area of olivine cumulate occurs near Mill of Bourtie [NJ 777 258], and is succeeded to the west by olivine-plagioclase cumulate (troctolite), which is known only from drill holes.

The cryptic layering in Lower Zone rocks is defined by changes in olivine composition (Ashcroft and Munro, 1978, fig. 5). The olivine in the dunite has a composition of FO_87–83, while in the troctolite it is Fo_83–80 and in the olivine-norite it is Fo_80–78. Plagioclase crystals, as in the rest of the Insch intrusion and in the Belhelvie intrusion (Wadsworth, 1986; 1988; 1991), vary irregularly in An content, probably due to reaction between cumulus crystals and the residual liquid; the plagioclase in the troctolite is mostly An_83–85 and in the olivine-norite it is mostly An_75–81. The correlation between olivine composition and the incoming of cumulus feldspar and pyroxene is almost identical with that in the Belhelvie intrusion (Wadsworth, 1991), where olivine and pyroxene compositions are closely correlated with stratigraphical height in the layered sequence, irrespective of whether the mineral is cumulus or intercumulus.

Ductile shearing occurred along the northern and southern boundaries of the Lower Zone rocks while amphibolite facies conditions prevailed in the district (Kneller and Leslie, 1984). To the north of the Lower Zone rocks, a 1 km-wide belt of highly sheared basic rocks has been recorded from pipeline trenches and shallow drilling. These rocks have been intensely recrystallised under amphibolite facies conditions, and few relics of the original texture remain; they have also lost nearly all of their magnetite, and have much lower magnetic susceptibilities than fresh gabbros. Hence it is not known whether they originally belonged to the Lower or the Middle Zone. This belt is succeeded to the north, across a third shear zone, by the main mass of Middle Zone rocks. Similar sheared basic rocks of uncertain provenance occur to the east of a large fault at Oldmeldrum [NJ 807 279], and extend into the Ellon district (Sheet 87W).

A few tectonically bounded bodies of highly sheared ultramafic rock occur along the southern contact of the Insch intrusion between Smallburn [NJ 520 240] and Kirk-ton of Premnay [NJ 642 248]. Similar rocks also occur rather less frequently near the northern boundary of the Insch and Kennethmont intrusions, e.g. near Leith Hall Home Farm [NJ 545 303], and within the Dalradian country rocks north of Slack [NJ 575 316]. They are almost invariably completely serpentinised, and most examples show much strain, so that original textures are rarely preserved. The rocks in along the south-western margin are well described by Read (1956). Similar rocks occurring along the southern and northern margins of the Boganclogh intrusion have considerably more primitive olivine and pyroxene compositions than the Lower Zone rocks of the Barra Hill area or the Belhelvie intrusion in the Aberdeen district (Sheet 77), (Munro, 1986b). This implies either that the Barra Hill and Belhelvie rocks do not represent the earliest cumulates laid down in the Insch–Boganclogh magma chamber, or that the marginal ultramafic rocks of western Insch and Boganclogh were derived from a separate, probably ultrabasic, magma (Table 2).

Middle Zone

Middle Zone rocks occupy most of the southern part of the Insch intrusion, but are of different character on either side of the belt of Upper Zone rocks. To the east of the belt of Upper Zone rocks, the rocks are norites of three intimately associated textural types (Wadsworth, 1988): orthopyroxene-clinopyroxene-plagioclase-ilmenomagnetite cumulates; fine-grained granular gabbros; and porphyritic granular gabbros. The first two types are by far the most common. The cumulates are relatively coarse-grained (2–4 mm) adcumulates with little or no zoning of cumulus pyroxenes and irregular variation of plagioclase composition, but show little preferred orientation of cumulus grains or rhythmic variation in mineral proportions (Plate 4)b. Apatite is a cumulus phase in some norites. Alternation of light and dark layers is generally poorly developed; where seen, it dips at 10°–50° to the west, north-west or north. On Candle Hill, Oyne [NJ 662 266], well-developed layering defined by centimetre-scale variations in modal proportions of felsic and mafic constituents dips north-wards at 30°–50°. A 5–7 m-thick layer of mafic cumulates contains abundant tabular xenoliths, up to 15 cm long, of feldspathic fine-grained granular gabbro (Wadsworth, 1988).

The granular gabbros are finer grained (0.5–1 mm), with a texture varying from granoblastic to a granular mosaic of polygonal crystals (Plate 4)c. The fine-grained and porphyritic granular gabbros differ only in that the latter contain abundant plagioclase phenocrysts. The granular gabbros show a close spatial association with the cumulates, with changes from one type to the other occurring over distances of 5 to 10 m, sometimes within a single outcrop. Where contacts are seen, they are sharp, but there is no sign of either the cumulates or the granular rocks having been chilled; there are indications that the granular gabbro mostly intrudes the cumulate norite, although the exposures on Candle Hill indicate that there the cumulates postdate the granular gabbro. The origin of the granular gabbros has been discussed by Read and Haq (1963), Clarke and Wadsworth (1970) and Wadsworth (1988). Suggested mechanisms include: intrusion by a slightly different magma in bodies which were too small to differentiate; and autometamorphism of cumulate rocks under the influence of interstitial fluids.

To the west of the Upper Zone rocks, the Middle Zone is represented by quartz-biotite-norite, which is very similar to that of nearby Boganclogh as well as to that of the northern part of the Morven–Cabrach intrusion. The quartz-biotite-norite of western Insch underlies the Upper Zone rocks, and is therefore equivalent to the Middle Zone of eastern Insch. It is a plagioclase + orthopyroxene ± clinopyroxene + ilmenomagnetite orthocumulate, with minor intercumulus quartz. It is characterised by the presence of abundant poikilitic biotite crystals, reaching 8 mm in some coarse-grained examples. No layering is visible in hand specimen, and no work has been done on possible cryptic layering in this unit.

The Middle Zone cumulates do not show any clear-cut spatial pattern of variation in pyroxene composition; this is largely due to the extensive disruption of the rocks by ductile shear zones and faults. However, there is a tendency for the Mg content of the pyroxenes to decrease to the NNW over a wide part of the eastern Insch intrusion.

Upper Zone

The apparently concordant boundary between the Middle and Upper zones, which is marked by the incoming of iron-rich olivine as a cumulus phase, is the first marker horizon in the main part of the Insch intrusion. There are a few granular rocks in UZa, but they are unknown in UZb and UZc. Rapid upward iron enrichment occurs in the cumulus olivine and pyroxenes of the Upper Zone, while the plagioclase, though more sodic than in the Middle Zone, continues to show a wide range in chemical composition within individual crystals. Rhythmic layering on centimetre to metre scale can be seen in many exposures in the Leslie area, and the layering generally dips gently north-westwards, but steepens progressively towards the southern contact of the Insch intrusion.

UZa

Olivine-ferrogabbro forms the northern and most of the west-central parts of the intrusion. Where fresh it is a dark blue-black, tough rock with only poor mineral layering. In most exposures a brown weathering crust is developed, and spheroidal weathering is common. The rocks are olivine + plagioclase ± clinopyroxene ± orthopyroxene + ilmenomagnetite + apatite ortho- to mesocumulates, though pyroxenes, especially orthopyroxene, are less abundant than in the Middle Zone rocks. Reaction rims are well developed: orthopyroxene on olivine; amphibole on pyroxenes; and biotite on ilmenomagnetite.

UZb

Olivine-monzonite, with minor olivine-monzodiorite in places, overlies the olivine-ferrogabbro in the low hills between Knockespock and Leslie, as well as in the Red Rock Hills of Dunnideer [NJ 613 282] and Christ's Kirk [NJ 602 275]. It is very similar in field appearance to the olivine-ferrogabbro, except that euhedral crystals of orthoclase can be seen scattered through the rock (Plate 4)d. Zircon becomes a cumulus phase at approximately the same level as orthoclase, and almost simultaneously the An content of the cumulus plagioclase falls below 50%, so that the more plagioclase-rich cumulates of UZb are olivine-monzodiorites rather than olivine-monzogabbros. The cumulus alkali feldspar is strongly enriched in Ba in the lowest UZb cumulates, but the Ba content decreases steadily on ascending the layered succession.

UZc

Syenite crops out over an area of 3 km² to the north of Wardhouse [NJ 593 290], and forms the summits of Fallow Hill [NJ 637 316], the Hill of Dunnideer and the Hill of Christ's Kirk. Further south, syenite caps a discontinuous line of hill summits from Gallow Hill [NJ 600 262] to near the southern boundary of the intrusion at Suiefoot Quarry [NJ 552 244]. In these areas the syenite overlies olivinemonzonite of UZb. Westwards from Suiefoot, around Craignook Wood [NJ 540 244], it forms a discontinuous, tectonically emplaced strip between the marginal ultramafic rocks and the Middle Zone quartz-biotite-norite. Here the tectonic situation of the syenite is similar to that along the southern margin of the Boganclogh intrusion. It has been sheared in places, but nowhere as strongly as in Boganclogh. The syenite is considerably more leucocratic than the olivine-monzonite or olivine-monzodiorite. Orthoclase perthite is more abundant than plagioclase. Clinopyroxene is present in some specimens, but it is usually replaced and/or overgrown by amphibole, and biotite is always present. Apatite and zircon form small euhedral crystals.

Contaminated and xenolithic rocks

These rocks occur where the original intrusive contacts of the Insch intrusion are preserved, including the edges of enclaves of metasedimentary rocks within the basic rocks. In the south-eastern part of the intrusion between Harthill [NJ 684 259] and Newton of Saphock [NJ 776 288], several slices of Dalradian metasedimentary material, up to 1 km long and 50–100 m across, are contained within the Middle Zone rocks. Analogy with the southern contact of the intrusion between Firbogs [NJ 697 246] and Mains of Inveramsay [NJ 745 256] suggests that these may be bounded on one or both sides by shear zones, cutting the Middle Zone rocks. Contaminated and xenolithic rocks are well developed near the margins of these enclaves. The best developments are at Harthill [NJ 684 259], Warrenside [NJ 724 250], Govals Quarry [NJ 732 256], Cuttlecraigs [NJ 758 268], and south of Newton of Saphock [NJ 776 283]. Away from this zone, contamination phenomena are also developed at Mill of Boddam [NJ 6240 3032] (Read, 1921; 1966; Whittle, 1936; Gribble, 1970).

In the south-eastern part of the Insch intrusion, the contamination of basic magma is indicated by the occurrences of quartz-biotite-norite, which contains more biotite than normal norite, has interstitial quartz, lacks cumulus texture and in places shows fluxion structures. At Mill of Boddam, olivine-free coarse-grained norite with patches of fine-grained norite containing pelitic xenoliths occurs in an area of dominant olivine-ferrogabbro (Read, 1966). At Govals, the contaminated basic rock is quartz-monzogabbro with higher SiO₂ and K₂O than the adjacent norites. Gribble compares it with the Insch 'syenogabbro' (olivine-monzonite), but admits that it could have formed by contamination of the marginal portions of the Insch magma by a partial melt derived from Dalradian metasedimentary rocks.

The contaminated norite is associated with considerable quantities of xenolithic material. As well as quartzbiotite-norite, this xenolithic material includes cordieritenorite, which was thought by Read to represent a further stage in the contamination of basic magma by assimilation of pelitic material. However, Gribble (1968; 1970) showed that the quartz-rich and cordierite-bearing norite, which occurs as patches, and is rich in Al-rich and Si-poor xenoliths [biotite + cordierite + plagioclase + sillimanite + spinel + magnetite + K-feldspar] at Govals, and also and Arnage in the Ellon district (Sheet 87W), is likely to be derived by extraction of an Al-rich dioritic melt from pelitic Dalradian country rock. The presence of spinel and/or corundum in the pelitic xenoliths shows that they are restites (Gribble, 1970). Whittle (1936, p.89) describes an exposure at Easter Saphock [NJ 775 283] not available to Read (1921) where lit-par-lit injection of granitic material containing pelitic xenoliths into Dalradian country rock has occurred. Volatiles are believed to be very important in the hybridisation process. The geochemistry of the contaminated basic rocks and the complementary restite xenoliths is described by Gribble (1970).

Whole-rock geochemistry

Within a layered cumulate succession, bulk rock chemistry is strongly influenced by the nature of the cumulus phases and their relative proportions; hence the whole rock compositions cannot be used directly to define a liquid line of descent. Rhythmic layering often occurs over short distances in the Lower Zone and parts of the Upper Zone, and causes difficulty in estimating the relative proportions of the cumulus phases. By contrast, mineral composition varies steadily with original height in the intrusion. Hence, the order of incoming of the cumulus phases, together with the variation in composition of the cumulus minerals, can be used to characterise the differentiation trend and, by inference, the parent magma of the intrusion. The parent magma of the Insch and Boganclogh intrusions is believed to have been basaltic, with normative hypersthene, and to have followed a typical tholeiitic differentiation trend. Influx of fresh magma to the magma chamber during crystallisation is believed to have occurred at the base of the Upper Zone, as witnessed by a reversal in cryptic layering at this point (Table 2), but was otherwise insignificant.

A wide range of rocks from the Insch intrusion was analysed for major and selected trace elements by Read et al. (1961; 1965) and by Read and Haq (1963). The analyses of serpentinised ultramafic rocks are all distorted by the serpentinisation; most of the original FeO is oxidised to Fe₂O₃, and some of the Al₂O₃ in the feldspar-free rocks may have been introduced during conversion of olivine to antigorite. The Mg/ (Mg + total Fe) ratio of the rocks is consistently lower than that of minerals from comparable rocks (Ashcroft and Munro, 1978), possibly due to the presence of chromite (Cr₂O₃ was not determined). The analyses of Middle Zone rocks are all consistent with the known mineral compositions (Wadsworth, 1988). The high P₂O₅ in the Upper Zone rocks reflects the incoming of cumulus apatite, and the Ba values peak at the incoming of cumulus alkali feldspar. Zr increases from 100 ppm in UZa to over 1000 pm in UZc, marking the incoming of cumulus zircon in UZb. The higher TiO₂ values in the Middle and Upper Zones compared with the Lower Zone reflect the cumulus status of ilmenomagnetite in the Middle and Upper Zones.

Petrography and mineral chemistry

The late-tectonic basic and ultramafic rocks of the district are part of a single suite whose evolution can be best studied in the Insch intrusion. Data have been published on the petrology and mineral chemistry of the Insch Middle and Upper zone rocks by Clarke and Wadsworth (1970) and Wadsworth (1986; 1988), and on the Belhelvie intrusion by Wadsworth (1991). The variation in mineral composition in the Insch and Boganclogh intrusions is shown in (Table 2). Incomplete data from Ashcroft and Munro (1978) enable the Lower Zone cumulates from the Barra Hill area to be correlated with Belhelvie. The marginal ultramafic rocks of western Insch are believed, by analogy with those of Boganclogh, which occur in a similar tectonic setting, to be petrochemically distinct from the Lower Zone rocks of southeastern Insch and Belhelvie. The differentiation trend worked out for the Insch intrusion can be applied to Boganclogh and the smaller intrusions of the district, but there are certain differences between the Insch succession and that of Morven–Cabrach and possibly Tarland.

The mineral textures show considerable differences, not only between the cumulus norite, the granular gabbro and the quartz-biotite-norite of the Middle Zone, but also between different cumulates, depending on the number of cumulus phases, and the degree of adcumulus growth.

Lower Zone

The ultramafic rocks which are tectonically emplaced along the south-western and north-western margins of the Insch mass are olivine adcumulates, characterised by a polygonal olivine texture with olivine grains showing 1200 triple junctions in thin section. Small interstitial orthopyroxene crystals form 5–15% of the rock. Chromite forms 0.1–0.2 mm euhedra either enclosed within cumulus olivine crystals or at grain boundaries. In some specimens, the olivine has been almost completely altered to antigorite serpentine, with the expulsion of magnetite along cracks and boundaries of the original olivines, while orthopyroxene has been altered to bastite serpentine. However, where the serpentinites have been strongly sheared, the antigorite has recrystallised in parallel orientation and the secondary iron oxide has been streaked out, giving the rock a banded appearance in hand specimen. No chemical analyses of these rocks or of minerals from them are available, but they are believed to be analogues of the ultramafic rocks of the Boganclogh intrusion.

The Lower Zone rocks of south-eastern Insch are coarse grained, with olivine crystals up to 4 mm, and contain 5–20% of intercumulus material. The lower part of the Lower Zone consists of ultramafic olivine cumulates, with interstitial plagioclase, clinopyroxene and, more rarely, orthopyroxene. The pyroxcnes form large oikocrysts enclosing many rounded olivine crystals. A rough layering with larger and smaller proportions of inter-cumulus minerals is present, and there is also a planar fabric defined by the long axes of olivine crystals. Most of the olivine is serpentinised, and the intercumulus minerals, especially plagioclase, have frequently suffered partial alteration. The upper part of the Lower Zone consists of troctolites and olivine-norites, with cumulus olivine, plagioclase, and, in the highest layers, orthopyroxene (Plate 4)a. Due to the higher proportion of inter-cumulus material, the clinopyroxene oikocrysts are smaller than in the olivine cumulates.

Within these rocks a single differentiation trend, which is identical to that defined by Wadsworth (1991) at Belhelvie, can be made out. There is a hiatus in mineral compositions between the Lower Zone rocks of southeastern Insch and the Middle Zone rocks to the north of the belt of sheared rocks. However, this hiatus has been shown by Wadsworth (1988) to be smaller than postulated by Clarke and Wadsworth (1970).

Middle Zone

Wadsworth (1988) has shown that by plotting the En content of the orthopyroxene against the Mg/ (Mg + Fe) ratio (Mg#) of the coexisting clinopyroxene, separate differentiation trends can be demonstrated for the cumulate norites, the fine-grained granular gabbros, the porphyritic granular gabbros and the quartz-biotite-norites (of Boganclogh) (Figure 14). Unfortunately no mineral analyses are available for the quartz-biotite-norites of western Insch. Plagioclase compositions cannot be used to define a trend because of the patchy variation within plagioclase cores which is a characteristic of virtually all the Insch and Boganclogh rocks.

Upper Zone

The Fo content of olivine and En content and Mg# ratio of the coexisting pyroxenes show a single trend of cryptic variation (Figure 14), which is very similar to that shown by other highly differentiated tholeiitic layered intrusions, e.g. Stillwater, Skaergaard, Bushveld (Wadsworth, 1986). The most notable feature is the overlap in Mg# ratios between the most evolved Middle Zone rocks and the least evolved Upper Zone rocks. This may indicate an influx of less evolved magma into the magma chamber.

Geochronology and isotope geology

The only radioactive isotope work on the late-tectonic basic rocks of the north-east Grampian Highlands is that of Pankhurst (1969; 1970). A Rb-Sr whole-rock isochron of 492 ± 26 Ma was obtained from the Insch Upper Zone rocks (Pankhurst, 1970). All points, except one which was disregarded, lay, within error limits, on the isochron. The initial ⁸⁷Sr/⁸⁶Sr was 0.7117 ± 0.0003. An isochron of 487 ± 23 Ma, obtained from schists and gneisses from the aureole of the Haddo House intrusion, was interpreted as dating isotopic homogenisation caused by the sillimanite overprint over the regional metamorphism. The average, 489 ± 17 Ma, has been taken as the best estimate of the crystallisation age of the basic rocks of north-east Scotland. It is likely that a more accurate age could be obtained by Pb isotope work on cumulus zircons from the UZc syenites of the Insch intrusion.

Pankhurst (1969) investigated the Sr isotope systematics of the Insch intrusion. Due to the low Rb content of the basic and ultramafic rocks of the Lower and Middle zones, no geochronology was possible for these rocks, but a steady increase in initial ⁸⁷Sr/⁸⁶Sr from the Lower Zone, through the Middle Zone, to the Upper Zone was demonstrated, with initial ⁸⁷Sr/⁸⁶Sr values of 0.7045–0.7082 in Lower Zone rocks, 0.7032–0.7114 in Middle Zone rocks, and 0.7110–0.7122 in Upper Zone rocks. The Insch parent magma is assumed to have had an initial ⁸⁷Sr/⁸⁶Sr of 0.702. A process of isotopic exchange between total Sr in the country rocks and that in the crystallising magma, by a similar mechanism to that occurring in whole rocks during metamorphism, was postulated by Pankhurst (1969). Similar equilibration has been shown to occur up to 3 km outwards from the contact of the Haddo House intrusion.

Boganclogh intrusion

The Boganclogh intrusion is the now-detached western continuation of the Insch intrusion (Figure 12). All of the rock-types found in Boganclogh have analogues in Insch, but a few Insch lithologies are absent from Boganclogh. However, due to the prevalence of shearing and faulting, the field relations of the various units show significant differences from the Insch intrusion. Due to its higher altitude above sea level and thinner drift cover, exposure in the Boganclogh intrusion is considerably better than in Insch.

Geophysical expression

Both the Exploration Ventures Ltd (EVL) and BGS aeromagnetic surveys define the Boganclogh intrusion as a large area of anomalously high magnetic intensity (Figure 4), whose boundaries closely accord with the mapped contacts. Within the intrusion, the BGS survey gives little detail, but the more detailed EVL survey shows considerable detail. Gallagher (1983) has identified a sharp cut-off to the positive anomaly along the Devonian unconformity to the west of Rhynie. However, to the south of Rhynie, the anomaly due to the southern ultramafic rocks continues at a lower amplitude under the Devonian rocks and joins with that caused by the ultramafic rocks of south-western Insch. The ultramafic rocks along the southern margin of the Boganclogh intrusion give a distinct magnetic anomaly, and small anomalies are also situated over the ultramafic rocks of Hill of Creagdearg [NJ 452 260], Red Hill [NJ 457 263] and Cnoc Cailliche [NJ 472 261]. The highest magnetic intensities in the Boganclogh intrusion are located over the ultramafic rocks north of Mount of Haddoch [NJ 415 290], and the olivine-ferrogabbro between Boganclogh Lodge [NJ 436 295] and Brae of Scurdargue [NJ 478 286]. The EVL aeromagnetic survey has delineated at least three linear lows which are interpreted as shear zones within the quartzbiotite-norite and olivine-ferrogabbro. These run east–west and swing to ENE in their eastern parts to converge near Milton of Lesmoir [NJ 468 285], disrupting the boundary between the quartz-biotite-norite and the olivine-ferrogabbro. The lows along the shear zones are due to local uralitisation of the gabbroic rocks. The EVL aeromagnetic survey does not extend west of National Grid line 40, but a ground-based magnetic survey (Fettes et al., 1991) has shown that the quartz-biotite-norite extends under the Devonian rocks of the Cabrach outlier and passes into that of the Morven–Cabrach intrusion.

The highest positive Bouguer gravity anomaly in mainland Scotland is centred over the Boganclogh intrusion (Figure 7) and extends south-west to cover the quartz-biotite-norites of Morven–Cabrach. The peak value of +42 mGal occurs over the centre of the Boganclogh intrusion, and is 15 mGal higher than over Haddo House–Arnage and 23 mGal higher than the highest value over Insch. The current best estimate of the thickness of basic and ultramafic rocks underlying the Boganclogh intrusion is 5 km (Figure 8). This is comparable to the estimate of McGregor and Wilson (1967), but considerably less than that of Gallagher (1983), who postulated a thickness of 12–15 km of basic and ultramafic rocks.

Margins of the intrusion

Rocks of the Boganclogh intrusion are unconformably overlain by Devonian sedimentary rocks from Wheedlemont [NJ 475 263] to Windyfield [NJ 495 280], but to the south from Wheedlemont to the Burn of Glenny [NJ 457 232], the boundary is faulted. The northern boundary against hornfelses derived from Clashindarroch Formation rocks is tectonic, with thin pods of ultramafic rock squeezed up along it. At its western and southwestern boundaries, the Boganclogh intrusion is overlain by Devonian rocks of the Cabrach outlier. It continues under these to the south-west to join up with the Morven–Cabrach intrusion (Fettes et al., 1991), but is truncated to the west by shear zones belonging to the Portsoy Lineament. The southern boundary is a ductile shear zone, and shearing affects the thin strip of ultramafic rocks to the north of this shear zone. The boundary is disrupted by a number of NE-trending faults, and the ultramafic rocks are displaced and occasionally cut out by these faults.

Lower Zone

Almost all of the LZ rocks in the Boganclogh mass are dunites and harzburgites, analogous to those between Knockespock and Newton in the Insch intrusion, but there is one outcrop of wehrlite and two of olivinenorite, analogous to the Lower Zone rocks of the Hill of Barra area.

Southern margin

There is an almost continuous belt of ultramafic rocks from near Bodibae [NJ 393 247] to west of Contlach [NJ 470 237]; its width increases eastwards from about 200 m to 1.2 km. The rocks within this belt are very strongly serpentinised, and fresh olivine cores are rare. Shear zones are common within the ultramafic rocks; a good example occurs about 300 m north of Auchinleith [NJ 462 236], where pale yellow-green serpentinite with little secondary iron oxide exhibits a strong layering, marked by segregation of secondary magnetite. Within the southern belt of ultramafic rocks are fault-bounded blocks of quartz-biotite-norite and syenite, the latter typically partly mylonitised. An unusual exposure of relatively fresh wehrlite forms an isolated small knoll at Innesbrae [NJ 433 252]. It is apparently fault-bounded to east and west. The ultramafic rocks are in tectonic contact with syenite along their northern margin, and the rocks are sheared for a considerable distance on either side.

Northern margin

A narrow, discontinuous strip of ultramafic rocks occurs along the northern margin of the Boganclogh intrusion between Old Merdrum [NJ 458 299] and Scurdargue [NJ 488 285], while a larger body of ultramafic rocks occurs to the north of Mount of Haddoch [NJ 420 290]. Like the southern marginal rocks, they are strongly serpentinised, and in places show signs of strong shearing.

Cnoc Cailliche, Red Craig and Hill of Creagdearg

These faulted blocks of ultramafic rocks occur within the quartz-biotite-norite outcrop. They were regarded as deep-seated plug-like bodies by Blyth (1969), but the apparent gentle dips of the unfaulted margins and the small gravity and magnetic anomalies associated with them indicate that they are probably parts of a flat-lying sheet, preserved as downfaulted blocks. A roughly triangular outcrop of serpentinite forms the summit of Cnoc Cailliche [NJ 473 261]. It is fault-bounded to the east and north, but the south-western boundary is curved in plan and may be gently dipping. The rocks are all strongly sheared, probably due to their proximity to the fault bounding the Rhynie outlier. The Hill of Creagdearg [NJ 452 260] and Red Craig [NJ 458 263] ultramafic exposures are similar in geometry to Cnoc Cailliche, but the rocks are much fresher. The hills are made of dunite, which is only slightly serpentinised, with a brown weathering skin. No layering is developed. The rocks are unsheared, except for a faint gently dipping foliation of probable tectonic origin near the foot of the crags at Red Craig. Fresh olivine cores with a composition of Fo₉₂ are abundant, and the interstitial orthopyroxene has a composition of En₉₂.

Three Burnshead area

The only rocks in Boganclogh analogous to the Hill of Barra cumulates occur at Three Burnshead [NJ 410 294] and about 1.3 km further north-west [NJ 400 300]. A specimen from the former locality (S73376) is an olivinenorite with An_76–54 and Fo₇₈, similar to the material from near Barra Castle [NJ 7955 2545], and also to some rocks from the Blackwater intrusion (Fettes and Munro, 1989). These rocks lie to the north of a fault or shear zone separating them from the Mount of Haddoch ultramafic rocks, and may not be related to the rest of the Boganclogh intrusion.

Middle Zone

The Middle Zone of Boganclogh consists entirely of quartz-biotite-norite, which is well exposed in much of the central part of the intrusion: Craik [NJ 461 255], White Hill of Bogs [NJ 440 263], Blackmiddens [NJ 426 260], Nether Howbog [NJ 405 256]. Middle Zone rocks also occur as small tectonic pods and patches within the serpentinites of the southern margin. The boundary with the Upper Zone olivine-ferrogabbro dips gently NW and strikes roughly NE–SW, but is displaced by east–west faulting along shear zones identified by the EVL aeromagnetic survey. The quartz-biotite-norite is typically a massive grey rock with widely spaced joints and a pale weathering crust. The groundmass grain size varies irregularly from 1 mm to 3 mm. Poikilitic biotite crystals with random orientation are generally two to three times the groundmass grain size. In the eastern part of the intrusion, where uralitisation of the pyroxenes is more extensive, the rock tends to weather darker, and to be more closely jointed. Phase layering is absent in the quartz-biotite-norite, and the mineral composition data show no obvious pattern of cryptic layering. Wadsworth (1988) showed by plotting the En content of orthopyroxene against Mg# ratio of the coexisting clinopyroxene that the quartz-biotite-norite of Boganclogh had a slightly different trend of magmatic evolution from the Insch Middle Zone norite and granular gabbro (Figure 14).

Upper Zone

The lithologies developed in this zone are identical to those of the Insch Upper Zone. Olivine-ferrogabbro of UZa is confined to the northern part of the intrusion, where it forms a 1 to 2 km-wide strip along much of the northern contact. Where unfaulted, contacts with the underlying Middle Zone and overlying UZb rocks dip gently north or west. Olivine-monzonite and olivine-monzodiorite of UZb are confined to a few exposures in the central and northern parts of the mass, where they form flat-lying outliers on top of olivine-ferrogabbro (or rarely quartz-biotite-norite). Small areas of olivine-monzonite occur around Mains of Lesmoir [NJ 471 272] and beside the road to Boganclogh Lodge [NJ 435 287].

In the Boganclogh intrusion, syenite of UZc forms a narrow strip in the southern part of the intrusion between the ultramafic rocks and the quartz-biotite-norite. There are also a few tectonic enclaves of syenite within the main serpentinite mass of the Hill of Towanreef. Both northern and southern boundaries of the main strip are tectonic. The syenite, like the ultramafic rocks, has undergone considerable cataclasis. It was described as felsite by Blyth (1969) and was not mentioned by Busrewil et al. (1973). Much of the syenite has been strongly sheared, and it has in places been reduced to a mylonite. The shearing is most intense in narrow zones, often only 10 m wide, and generally trending east–west. All stages in this shearing can be observed between Silverford Bridge [NJ 425 250] and Contlach [NJ 470 243], and the cataclasis is well brought out in thin section.

Whole-rock geochemistry

Fifteen whole-rock analyses from Boganclogh were published by Busrewil et al. (1973). Most are of quartz-biotite-norites and olivine-ferrogabbros, but one harzburgite from the Hill of Creagdearg and one olivinemonzonite from Belhinnie [NJ 462 278] were included. The olivine-ferrogabbros from Boganclogh are very similar to corresponding rocks from Insch. High P₂O₅ and TiO₂ in the olivine-ferrogabbros reflect the cumulus status of apatite and ilmenomagnetite respectively. A wider range of trace elements was determined than in the case of Morven–Cabrach. Ni and Cr are, as expected, high in the harzburgite, but are otherwise very low, being higher in the quartz-biotite-norite than in the olivine-ferrogabbro. Rb is everywhere below 30 ppm except in the olivine-monzonite. Ba is 400–600 ppm in the quartz-biotite-norite, 750–1820 ppm in the olivine-ferrogabbro and 3420 ppm in the monzonite.

Mineral chemistry

The magmatic evolution of the Boganclogh intrusion is closely comparable to that of the Insch intrusion, particularly in the Upper Zone. The trend of cryptic variation in the two masses is shown in (Table 2). Minerals from specimens of the different lithologies within the Boganclogh intrusion have been analysed by electron microprobe. Some of these are Busrewil's (Busrewil et al., 1973) specimens analysed by Dr W J Wadsworth, University of Manchester (Wadsworth, 1988), and some are specimens collected during the present survey and analysed by the author (Table 3). The ultramafic rocks of Boganclogh are much more magnesian than the Hill of Barra or Belhelvie rocks (Ashcroft and Munro, 1978; Wadsworth, 1991), but may be related to the marginal ultramafic rocks of western Insch, for which no mineral chemistry is available. The wehrlite from Innesbrae [NJ 433 252] is the only record from in the Insch and Boganclogh intrusions, but it is comparable to the ultramafic rocks of the Craigs of Succoth (Styles, 1994). The olivinenorite from Three Burnshead, at the north-western corner of the intrusion [NJ 410 294], is similar in composition to olivine-norite from near Barra Castle [NJ 7955 2545] and also to material from the Blackwater intrusion (Fettes and Munro, 1989); however, this may not be part of the Boganclogh intrusion proper. The Middle Zone rocks of Boganclogh are petrographically very similar to the quartz-biotite-norites of western Insch, but no mineral composition data are available for the latter. On a plot of En in orthopyroxene against the Mg# ratio of the coexisting clinopyroxene (Figure 14), the trend for the Boganclogh quartz-biotite-norites differs from those derived for the Insch cumulate norites or granular gabbros (Wadsworth, 1988), but is only slightly different to that from the Morven–Cabrach quartz-biotite-norites. A certain amount of overlap in pyroxene compositions between the quartz-biotite-norites of the Middle Zone and the olivine-ferrogabbros of the lower Upper Zone occurs in both the Insch and Boganclogh intrusions.

Morven–Cabrach intrusion

Only the extreme eastern portion of this intrusion occurs within the district (Figure 15). A full description of the geology of the intrusion as a whole will be found in the memoir for the Glenlivet/Glenbuchat district (Sheets 75E and W); a brief summary will be given here, together with a description of that part of the intrusion lying within the Alford district.

The Morven–Cabrach intrusion is 25 km in north–south extent, with a maximum east–west width of 8 km. It consists of norite, hypersthene-gabbro and quartz-biotite-norite, with minor olivine-ferrogabbro, olivine-diorite, diorite and monzonite. Bodies of ultramafic rock occur at or near the margins of the intrusion. Mineral layering is well displayed in a few localities, and distinguishable in a number of others. In the south it dips moderately northwards, but the dip steepens progressively northwards and the strike changes from roughly east–west to NNW–SSW. Cryptic layering in the intrusion is only slight within the most abundant rock types, which have mineral compositions within the range of Middle Zone rocks from Insch and Boganclogh, but the olivineferrogabbros and olivine-diorites have more differentiated mineral compositions comparable to Insch UZa and UZb; these occur in relatively small pods and thin layers within rocks of more primitive mineral compositions. The eastern margin between Creag an Sgor [NJ 366 195] and Lazy Well [NJ 442 089] is marked by a zone of xenolithic partial-melt rocks which swings eastwards and occurs to the north of the Tarland intrusion.

One of the most characteristic features of the Morven–Cabrach intrusion is the intense alteration of the gabbroic rocks, especially in the southern part of the intrusion. This alteration ranges from a relatively late-stage uralitisation of the pyroxenes and saussuritisation of the plagioclase, to complete recrystallisation of the rock to a medium-grained hornblende-plagioclase rock where the texture is similar to that of an amphibolite but the grain size rather coarser.

Within the Alford district, only the southern norites and one of the olivine-ferrogabbro layers are represented. Exposures are poor, but norite can be seen at Coldhome [NJ 432 072], and blocks of olivine-ferrogabbro occur on the slopes of Baderonoch Hill at [NJ 430 086]. These rocks are petrographically identical to cumulate Middle Zone norite and Upper Zone olivine-ferrogabbros of the Insch and Boganclogh intrusions.

Petrology and mineral chemistry

Most of the rocks of the Morven–Cabrach intrusion bear chemical similarities to Insch and Boganclogh Middle Zone rocks (Allan, 1970). The southern norite is comparable to the Insch Middle Zone cumulates and the Morven–Cabrach quartz-biotite-norite is similar to that of Boganclogh, but the hypersthene-gabbro, characterised by the dominance of clinopyroxene over orthopyroxene, has no obvious parallel in Insch or Boganclogh. The Tarland norite is an eastern extension of the Morven–Cabrach norite, but the high level of uralitisation means that few mineral chemical data are available.

Tarland intrusion

This intrusion is an eastern extension of the Morven–Cabrach intrusion, to which it is joined by a narrow neck, which is faulted against the main bodies of both larger intrusions. The Tarland intrusion extends approximately 8 km from north to south and 3 km from west to east (Figure 15). The northern contact from Lazy Well [NJ 442 089] to Broom Hill [NJ 469 092] is marked by a zone of partial melting of the adjacent metasedimentary rocks, and probably represents the roof of the original intrusion. Between the Morven–Cabrach and Tarland intrusions, a zone of brecciated pegmatite indicates considerable displacement. The western boundary from Mains of Tillypronie [NJ 447 078] to near Coynach [NJ 442 060] is faulted against metasedimentary rocks, and from near Coynach to Collordon [NJ 447 053] the contact with the Logie Coldstone Tonalite is marked by a brecciated granite pegmatite. From Collordon to near Altanree

[NJ 473 013], the Tarland intrusion is intruded by the Logie Coldstone Tonalite, and from Tarland Lodge [NJ 484 055] to near Coatmore [NJ 476 025] it is intruded by the Tomnaverie Granodiorite. In the extreme south of the Tar-land intrusion, between Altanree and Coatmore, the contact of recrystallised hornblende-plagioclase-gneisses derived from rocks of the Tarland intrusion with remobilised Dalradian feldspar porphyroblast gneisses can be mapped to an accuracy of 5–10 m. It is uncertain whether this is an original intrusive contact or a fault. To the east the Tarland intrusion is bounded by zones of ductile shear from Tarland Lodge [NJ 484 055] to 1 km north-east of Ranna [NJ 489 070], and from 300 m north-east of Boig [NJ 474 080] to Broom Hill [NJ 469 092]. Adjacent to the contact north-east of Ranna, there is a development of feldspar porphyroblast gneiss.

The principal rock type in the Tarland intrusion is norite, similar to the Middle Zone cumulate norite of eastern Insch. Fine-grained granular norite occurs near the contact with the Dalradian metasediments; in this case it may represent a marginal chill. The norite, especially where cumulate, has suffered extensive alteration. In the northern two-thirds of the intrusion this consists simply of replacement of pyroxenes by acicular needles of actinolitic amphibole, the colourless centre of the original pyroxene crystal becoming blue-green at the boundary with the original plagioclase, which is saussuritised. However, near the southern extremity of the intrusion, the whole rock has recrystallised to a coarse-grained hornblende-gneiss with a poorly marked foliation.

Due to the extensive alteration, only two specimens have been subjected to microprobe analysis (Table 3), a granular norite from near the northern contact and a cumulate norite from 1 km north-east of Mains of Tillypronie [NJ 4526 0855]. The pyroxene compositions are slightly more evolved than those of analysed norites from Morven–Cabrach, but well within the compositional range of the hypersthene-gabbro and quartz-biotitenorite from Morven–Cabrach.

Smaller late-tectonic intrusions

Several smaller intrusions of basic and ultramafic rocks occur in the Inverurie–Alford district. They are bounded on most if not all sides by faults and/or ductile shear zones. The largest is the Kildrummy intrusion, with the Lawel Hill and Lynturk bodies also significant. The remaining occurrences are less than 50 m wide and less than 500 m long.

Kildrummy

This was shown on the previous edition of Sheet 76 as a single body, and due to the poor exposure, was still considered to be a single body during the present ground resurvey. However, a magnetic survey (Illingworth, 1988) has enabled the form of the body to be more closely defined. Three fault-bounded pods of gabbroic material were identified (Figure 15). The two smaller pods near Fichlie [NJ 4568 1398] consist of norite comparable with the Insch Middle Zone; electron microprobe analysis gives orthopyroxene of En_65.2, clinopyroxene Mg#_78.3 and plagioclase with a range of An_64.8–40.5 (Table 3). The larger Drumallachie body comprises material with several distinct geophysical signatures. Exposures and large blocks of troctolite and olivine-norite of Lower Zone type occur around the Clochter Stane [NJ 4734 1435]. These are olivine-plagioclase-(orthopyroxene) cumulates with clinopyroxene oikocrysts up to 30 mm in size. Electron microprobe analyses of one in-situ sample and one closely similar glacial erratic give olivine Fo_80–82, orthopyroxene En_82–84, clinopyroxene Mg#₈₄ and plagioclase An_80–88 (with rims zoned to An₅₇).

Lawel Hill

The rocks on the southern slopes of Lawel Hill were described by Whittle (1936) as the 'Lawel Hill Complex', which he considered to be pretectonic. Specimens collected during the present survey show relict olivine and pyroxenes, and strongly suggest that the intrusion is a tectonically detached part of the Insch Lower Zone.

The intrusion is situated 1.5 km south of the Barra Castle–Hill of Barra outcrops of Insch Lower Zone rocks, and it crops out as a 100–300 m-wide belt from 400 m north-east of Old Bourtie [NJ 800 238] to 300 m south-west of Pitgaveny [NJ 814 240], in low ground on the south side of Lawel Hill. Its northern boundary dips north at about 40°, and is parallel to the foliation of the overlying Dalradian metasedimentary rocks. The southern boundary runs approximately parallel to the northern boundary, but its angle of dip cannot be determined. To the west of a point 250 m north-west of Lowhillside [NJ 8052 2385], the intrusion consists principally of gabbroic rocks, with ultramafic rocks possibly forming a discontinuous belt along the northern contact; to the east of this point only ultramafic rocks occur.

The gabbroic rocks are troctolites which have suffered partial amphibolitisation. In the freshest available specimen (S75930), plagioclase An₈₀ and olivine form cumulus crystals up to 3 mm long. Clinopyroxene, forming large oikocrysts, and rare orthopyroxene are intercumulus. Brown hornblende and phlogopite are secondary. In more altered examples, e.g. (S75928), the original mafic minerals are replaced by medium-grained hornblende aggregates with minor plagioclase, and the original plagioclase is represented by a very fine-grained plagioclase mosaic with minor hornblende.

The ultramafic rocks were originally harzburgites, but the original olivine and pyroxenes are largely replaced by antigorite serpentine and tremolite. The freshest available specimen (S75931) contains rounded cumulus olivine crystals up to 4 mm in size forming 50% of the rock, intercumulus orthopyroxene in large plates, largely replaced by crystals of colourless amphibole (?cummingtonite), and minor intercumulus clinopyroxene, largely replaced by tremolite. Plagioclase originally formed approximately 5% of the rock, but is now completely altered. Dark green spinel forms 0.1–0.3 mm euhedra, but it is uncertain if this is a primary mineral.

Lynturk

Exposures of ultramafic rock occur near Lynturk [NJ 5991 1178]. As with Kildrummy, the field observations have been supplemented by a magnetic survey (Bellman, 1988). The exposed rocks have an unusual mineralogy, containing up to 40% of tremolitic amphibole and up to 5% of green spinel, as well as partially serpentinised olivine and partially chloritised pyroxenes. They have a strong magnetic signature. The magnetic survey has shown that there are at least two fault-bounded pods of ultramafic material in the vicinity.

Electron microprobe analyses were made of two specimens. The first, an olivine-chlorite-tremolite rock yielded olivine Fo72. The second, an olivine-orthopyroxene-tremolite-spinel rock, yielded olivine Fo₇₉ and orthopyroxene En₈₁ (Table 3). These compositions are more evolved than those from any other late-tectonic ultramafic rocks in the north-east Grampian Highlands (Styles, 1994).

Smaller bodies

Sheared gabbroic rock is in contact with migmatitic semipelite in a disused quarry 250 m north-west of Newseat [NJ 497 117]. There is no indication of the extent of this body. About 400 m north of Saplings [NJ 497 194], boulders and possible exposure of sheared serpentinite occur in an ENE-trending train 500 m long. This development is associated with a fault which truncates the Ardhuncart–Invermossat outcrop of gritty psammite.

Chapter 6 Late-tectonic granitic intrusions

The granitic intrusive rocks of the district have been divided into an earlier and a later group. This division has been established on the basis of radiometric ages, and relationships both with the Dalradian country-rocks and with other plutonic masses. Where such information is absent or equivocal, analogies have been drawn with dated rocks in adjacent districts to assign intrusions to a specific group. The earlier, late-tectonic, group dealt with in this chapter comprises intrusions dated between 450 and 470 Ma (Middle Ordovician) and others believed to be of similar age. These are the Kennethmont Complex, Kemnay Granite, Tillyfourie Tonalite, Syllavethy intrusion and Corrennie Granite. The allocation of the Kennethmont Complex and Kemnay Granite to this early group is based on reasonably firm evidence, but the ages of the other intrusions assigned to the group is uncertain. The Tillyfourie and Syllavethy intrusions are linked by a tonalitc-granodiorite vein complex which occupies much of the ground between them. They are closely related to each other, and the Tillyfourie Tonalite is intruded by the post-tectonic Balblair pluton at Upper Kebbaty. The relative ages of the Tillyfourie and Kemnay intrusions are unknown. However, an amphibolite which separates the two intrusions near Leschangie [NJ 743 141] appears to represent a mafic marginal phase of the Tillyfourie Tonalite, which would suggest that the Tillyfourie Tonalite postdates the Kemnay Granite. There is no evidence on the relative ages of the Tillyfourie Tonalite and Torphins Diorite, but the more intense foliation of the former suggests that it may belong to an older group than the latter. The Corrennie Granite, which intrudes the Tillyfourie Tonalite, possesses a prominent foliation, defined by streaked-out quartz aggregates. On this basis, it is also assigned to the late-tectonic group. The relationships of the two intrusions are well displayed in Corrennie Quarry [NJ 642 119].

This group of intrusions shows a greater compositional range than do the Crathes or Cairngorm suites (Chapter 7); however, the various intrusions cannot be regarded as a single suite. The diorite, granodiorite and grey granite of the Kennethmont Complex show a tholeiitic differentiation trend, and have initial ⁸⁷Sr/⁸⁶Sr ratios indicative of mantle-derived I-type magmas which have undergone progressive crustal contamination. However, the initial ⁸⁷Sr/⁸⁶Sr ratio of the pink fine-grained granite (component 3 of the complex) suggests that it is of crustal derivation. The Kemnay Granite and its satellite intrusions are biotite-granites with occasional, largely secondary, muscovites, and are the only representatives within the district of the suite which includes the Aberdeen, Strichen, Longmanhill and Aberchirder granites. It is not associated with any diorites or granodiorites, and, by analogy with the Aberdeen Granite which has an S-type isotopic character, is probably derived from a crustal source. The Tillyfourie Tonalite is geochemically more primitive than the granodiorites of the post-tectonic Crathes Suite but lies on the same calc-alkaline trend (Chapter 7).

Minor intrusions include aplites and pegmatites which cross-cut the late-tectonic basic intrusions, appinitic plugs and lamprophyre, microdiorite, felsite and porphyry dykes. These are described separately in Chapter 8.

Kennethmont Complex

This complex, which consists of rocks ranging from diorite to granite, lies to the west of the Insch intrusion and to the east of the Rhynie Devonian outlier (Figure 12). Relations with the Insch intrusion are complex, and difficult to elucidate due to poor exposure. The northern part of the Kennethmont Complex was described by Read (1923), and studies of the whole complex have been published by Sadashivaiah (1954b), Read and Haq (1965), and Busrewil et al. (1975).

Field relations

The northern and southern contacts of the complex are tectonic, being continuations of the contacts of the Insch intrusion. Both are marked by the presence of narrow, discontinuous pods of serpentinite, in places up to a few hundred metres from the actual contact of the complex, within a wide zone of shearing. The northern contact of the complex is exposed on the hill slope to the north of Leith Hall [NJ 532 303], and can be traced eastwards along the break of slope from the Home Farm [NJ 545 302] via Wardhouse [NJ 564 306] to the Shevock Burn at Slack [NJ 581 310]. The rocks to the north of the contact are sillimanite-bearing hornfelses similar to those at the northern contact of the Boganclogh intrusion on Tap o' Noth. The southern contact is less well exposed, but lies close to the northern edge of the moorland area in the Correen Hills [NJ 500 242]. The serpentinite exposed in the burn 600 m south-east of Cairnmore [NJ 507 244] lies within a zone of sheared granite and is about 100 m north of the contact with Dalradian rocks. The degree of shearing in the marginal zone is probably comparable to that seen at the margin of the Insch intrusion around Knockespock [NJ 545 241]. The western contact of the complex is believed, from magnetic surveys of the Rhynie outlier (W A Ashcroft, written communication, 1990), to he faulted between Mains of Cairndard [NJ 504 255] and near Boghead [NJ 525 274], but to the north of this point, and possibly to the south of the NW-trending fault near Mains of Cairndard, the complex is overlain unconformably by Devonian rocks.

The eastern contact of the Kennethmont Complex is poorly defined, firstly due to poor exposure, and secondly to the similarity in the field between some dioritic rocks of the Kennethmont Complex and the finer-grained quartz-biotite-norites of the Insch intrusion. The contact runs south from the Shevock Burn near Slack [NJ 580 310] to near Glanderston [NJ 582 290]. It is displaced to the WSW by faulting to a point 800 m west of Bankhead [NJ 527 271], and continues from there roughly south to the southern contact near Smallburn [NJ 524 243]. However, diorite occurs in a small area south of this fault on Hill of Glanderston [NJ 582 287], and in other isolated exposures at Kylieford [NJ 565 277] and Meikle Auchlyne [NJ 555 267].

The Kennethmont Complex can be divided into three phases: (1) a grey, fine- to medium-grained tonalite containing abundant dioritic xenoliths, which include patches of, but are also cut by, veins of coarser-grained grey granodiorite and granite; (2) a coarse-grained, pink, quartz-syenite to syenogranite; and (3) a finer-grained pink granite. The relative ages of phases (1) and (2) are unknown, but phase (3) cuts both phases (1) and (2). Phase (1) is believed to represent a diorite which has been intruded and hybridised by a possibly cogenetic granitic magma (Busrewil et al., 1975). It contains abundant typically subrounded, blocky xenoliths of diorite, which are veined by less mafic granitoid material. The density of xenolithic material is variable; in places it is such that the tonalite to granite host is present as a vein complex enclosing dioritic blocks. Phase (2) is of uncertain origin. The more syenitic parts bear a strong resemblance to the syenite of the Insch intrusion, but the more granitic parts are more acid than any proven Insch Upper Zone rocks. Phase (3) is very similar to, and may well be part of, the suite of granite and pegmatite sheets and veins which intrudes most of the late-tectonic basic intrusions of the north-east Grampian Highlands, including the Insch intrusion (Chapter 8). The three phases are included together in the Kennethmont Complex partly for historical reasons, and partly because of the difficulty of mapping separate areas of outcrop of phases (2) and (3).

To the north of the ENE-trending fault running from Boghead [NJ 522 240] to Glanderston [NJ 582 290], phase (1) occurs to the south-west of a line from near Leith Hall Home Farm to near Glanderston, and in a small patch close to the Shevock Burn [NJ 580 309]. To the south of this fault, phase (1) occurs in the south-west corner of the complex near Cairnmore [NJ 502 247], and around Mains of Cairndard and Druminnor House [NJ 515 264]. It also crops out on Hill of Glanderston [NJ 582 287], and in small patches within the Insch quartz-biotite-norite. Phase (2) occurs north of the fault and to the east of the diorite, and possibly in the poorly exposed area around Dykeneuk to the south of the fault [NJ 512 254]. Phase (3) forms both dykes and sheets which intrude components (1) and (2), and a few larger masses within the outcrop of component (2), including the area around Dykeneuk.

Petrography

(1) Grey diorite to granite

The texture of the diorite varies from fine grained and equigranular to coarse-grained with poikilitic mafic minerals. Plagioclase, zoned from An₄₆ to An₂₄, forms randomly oriented subhedral laths set in a matrix of quartz, mafic minerals and, rarely, traces of orthoclase. Biotite and hornblende typically form ragged crystals enclosing small plagioclase euhedra. Quartz, and orthoclase where present, form interstitial material which in places coalesces in pegmatitic patches up to 4 mm across. The transition from diorite to white granite is marked by increase in the proportions of quartz and orthoclase and decrease in mafic minerals. The rocks in the Dual Burn - Cairnmore area [NJ 500 250] are gabbroic in composition, and typically contain up to 10% clinopyroxene and have plagioclase cores of An_56–50. Texturally they are similar to the more basic diorites. The xenoliths are finer grained and more mafic than their hosts, but otherwise similar in texture. The more mafic dioritic xenoliths contain a pale green granular clinopyroxene.

(2) Pink coarse-grained quartz-syenite and syenogranite

Sub- to euhedral orthoclase crystals up to 5 mm form 50–60 per cent of the rock, and show well-developed Carlsbad twinning and perthite development. Plagioclase forms about 30% of the rock, in sub- to euhedral crystals, up to 4 mm, zoned from An₂₈ cores to An₁₆ at the margins. Quartz, forming 5–20% of the rock, is interstitial to the feldspars, with grains reaching 5 mm. The principal mafic mineral is a dark green-brown, probably iron-rich, amphibole; a dark brown biotite is also present. Apatite and zircon are moderately abundant accessory minerals.

(3) Pink finer-grained granite

This is a granular aggregate of quartz, plagioclase and microcline crystals in roughly equal proportions, with a grain size of 1–2 mm. Plagioclase crystals contain cores of An₃₂ zoned to rims of An₂₆. Only 1–2% of ragged, anhedral dark brown biotite is present, and muscovite, forming 2–3% of the rock, is almost all secondary. The feldspars are all strongly reddened and sericitised.

Isotopic data

Pankhurst (1974) obtained a Rb-Sr whole-rock isochron age of 453 ± 4 Ma, and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7145 ± 0.0001 for the pink granite of component (3). Busrewil et al. (1975) were unable to obtain an isochron from the grey diorites to granite of component (1). Assuming an age of 453 Ma, initial ⁸⁷Sr/⁸⁶Sr ratios ranged from 0.709 to 0.712; these ratios are similar to those of the Insch Upper Zone rocks, where the basic magma suffered progressive crustal contamination during differentiation. This raises the question as to whether the parental Kennethmont dioritic magma is related in some way to differentiates of the Insch magma.

Kemnay Granite and associated bodies

The Kemnay Granite is a foliated orthoclase-megacrystic biotite-muscovite-granite with diffuse margins. It has similar contact relations, petrography and geochemistry to those described by Munro (1986b) from the Aberdeen Granite, which itself shows close petrographical and geochemical similarities to the granitic component of the migmatitic vein complex which pervades most of the rocks of the Aberdeen Formation in the Inverurie and Aberdeen districts (Walsworth-Bell, 1974). The Kemnay and Aberdeen granites form part of a suite occurring widely in the north-east Grampian Highlands. The Aberdeen and Strichen granites have been dated by U-Pb analyses of zircon and monazite at 470 ± 1 Ma and 475 ± 5 Ma respectively.

Field relations

The Kemnay Granite crops out over an area of 36 km² around Kemnay and Kintore (Figure 16). Within a contact zone which reaches 500 m in width, the granite is heterogeneous, with a diffuse compositional layering, in places picked out by 1 mm-thick biotite-rich partings, which separate 5–10 mm layers of quartzofeldspathic material. This is attributed to partial digestion of migmatitic arkose and semipelite by the granitic magma. This zone grades outwards to migmatitic semipelite and feldspathic psammite veined by granite, and inwards to more homogeneous, unlayered and more leucocratic, but still foliated granite.

The central part of the Kemnay Granite is remarkably uniform, being a white leucocratic, biotite-muscovite-granite, with a weak foliation defined by parallel orientation of the biotite and, to a lesser extent, feldspar crystals, but no visible layering. The foliation typically strikes north-east and dips at moderate angles to the north-west, but shows broad open folds on a 1–10 m scale. However, local variations are considerable, with the strike of the foliation being locally up to 45° away from this direction. Biotite-rich schlieren, typically 510 cm wide and up to 0.1 m long, are occasionally present (Plate 5)a. White K-feldspar megacrysts are absent in some exposures and sparsely distributed in others. The muscovite forms randomly oriented conspicuous flakes up to 5 mm across. In parts of the outcrop the granite has been partly recrystallised, and the foliation has been overprinted by a later tectonic fabric, within which the secondary muscovite is concentrated and along which hydrothermal alteration of the feldspars to greenish sericitic aggregates is most pronounced. The granite in the current workings at Tom's Forest Quarry [NJ 761 169] has been hydrothermally altered, with a pale green mica partially replacing both feldspars.

At its south-western contact, from near Letter [NJ 762 115] to near Todfold [NJ 745 138], the Kemnay Granite is intruded by the Crathes Granodiorite (Chapter 7). From Todfold to south of Greatstone [NJ 717 161], the granite is in contact with the Tillyfourie Tonalite. Near Leschangie, a strip of amphibolite up to 100 m wide separates the two intrusions. It is uncertain whether this represents a raft of Dalradian amphibolite or a mafic marginal facies of the Tillyfourie Tonalite. To the north-west of this amphibolite, the contact is unexposed and the relative ages are of the two intrusions are equivocal, though it is suspected that the Tillyfourie Tonalite postdates the Kemnay Granite.

Small bodies of granite, petrographically similar to the main Kemnay Granite, occur to the north-east of the main outcrop: south of Crichie [NJ 770 193], east of Craigforthie [NJ 802 200], and in the western part of Port Elphinstone beside the B994 [NJ 775 200]. The Hill of Crimond body, although cropping out mainly within the Aberdeen district, extends into the Inverurie district [NJ 817 225]. These bodies have diffuse margins, similar to those of the main Kemnay Granite. Exposures of Kemnay Granite within the Crathes Granodiorite near Drumnaheath [NJ 751 123] are interpreted as forming part of a part of large xenolith. The only radiometric age available is a Rb-Sr whole rock–mineral isochron of 411 ± 7 Ma, with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.715 ± 0.002 (Bell, 1968), which is likely to be erroneous due to the amount of secondary mica in the rocks.

Petrography

The Kemnay Granite is a biotite-muscovite-granite, with 3–4% biotite, and up to 5% muscovite. Biotite flakes up to 3 mm across, showing good alignment, define the foliation. The biotite is dark brown where fresh, but most crystals are partly chloritised. A small proportion of the muscovite is associated with the biotite, and is interstitial to plagioclase and quartz, and is probably primary. However, most of the muscovite forms ragged secondary crystals replacing both feldspars, and is most abundant where the rocks have been recrystallised. Plagioclase forms 2–3 mm subhedral crystals with fine lamellar albite twinning. Cores are An_30–24, zoned out to rims of An_22–16. Quartz forms anhedral crystals, many of which have broad cracks running through them, interstitial to plagioclase and biotite. Some of the large quartz crystals have recrystallised to mosaics of smaller crystals. The primary potash feldspar is orthoclase, which forms crystals up to 30 mm long, enclosing small euhedra of quartz and plagioclase, with fine wisps and stringers of perthite. Myrmekite is absent. However, where the rock has recrystallised, especially where this is the result of shearing, the orthoclase has been replaced by aggregates of anhedral microcline crystals. Specimens from Tom's Forest Quarry show evidence of greisening, with biotite replaced by muscovite and feldspars replaced by fine sericitic aggregates and occasional larger muscovite crystals. Epidote is only rarely present.

Tillyfourie Tonalite

This intrusion covers about 46 km² in the area between Tillyfourie and Blairdaff (Figure 16). It consists principally of hornblende-biotite-tonalite, but ranges from quartz-diorite to granodiorite. With a colour index of 25–30, it is one of the more melanocratic granitic plutons in the district. The rock has a prominent foliation which typically dips steeply to the WNW or northwest. The foliation is parallel to that of the Dalradian country rocks close to the contact, and this is defined by the parallel orientation of biotite crystals. A linear quartz fabric is visible on weathered surfaces, and hornblende crystals are preferentially aligned with this fabric. Otherwise there is little segregation of mafic and felsic components. The foliation is parallel that of the Dalradian county rocks close to the contact, and this is taken to indicate that the Tillyfourie Tonalite underwent a deformation event after its emplacement.

Where the contact of the intrusion is seen, it is sharp, with no sign of chilling of the tonalite against country rock. Near Mossside of Coulie [NJ 700 173], a small exposure of fine-grained diorite lies adjacent to Dalradian country rock and may represent a marginal facies. The tonalite at the contact with country rock at Greatstone Quarry [NJ 717 162] is also fine-grained and rather mafic. No transition to a vein complex is observed in this part of the intrusion. At Upper Kebbaty [NJ 658 079], the Tillyfourie Tonalite is cut by fine-grained granodiorite at the margin of the Balblair pluton (Plate 5)b. The contact against the Kemnay Granite is unexposed, but the amphibolite body between the intrusions in the Leschangie area [NJ 743 141] is of uncertain origin (see above). A body of tonalite of Tillyfourie type occurs within the Crathes granodiorite 500 m south-east of Lauchintilly [NJ 748 130], and is interpreted as a xenolith.

Exposures within the main body of the Tillyfourie Tonalite are sparse. The main (No. 5) quarry at Tillyfourie [NJ 645 128] exposes uniform tonalite with a well-marked foliation. In Causeyton Quarry [NJ 656 115], a vein of graphic granite, with large crystals showing a coarse quartz–alkali feldspar intergrowth, cuts the tonalite. A few xenoliths of metasedimentary rock occur within the main part of the intrusion, as in Tillyfourie No. 1 and No. 2 quarries [NJ 647 124], on the north-east slopes of Black Hill [NJ 6'77 125] and near Muirton of Sauchen [NJ 691 109].

Tillyfourie–Syllavethy vein complex

This tonalite–granodiorite vein complex occurs over about 42 km² between the Tillyfourie and Syllavethy intrusions (Figure 2). Much of the ground, especially in the Howe of Alford, is poorly exposed, and the abundance of veins and sheets is difficult to assess. The geophysical model (Figure 8) suggests that the ground between the two intrusions is underlain by granitic rocks at a depth of less than 1 km. The veins are mostly at least several metres in width. In the railway cuttings at Tillyfourie [NJ 642 126], the tonalite and Dalradian rocks occur in roughly equal proportions, with sheets of tonalite up to 200 m wide being separated by screens of country rock 20 to 200 m wide. The contact of the Tillyfourie intrusion is drawn where the proportion of tonalite is in excess of the host rock. On Strone Hill [NJ 579 135], the veins are only 1 to 10 m thick, and contacts with country rocks can be seen. There is no sign of chilled margins to any of the veins, either at Tillyfourie or on Strone Hill. Petrologically, the rocks of the vein complex are similar to those of the plutons into which they grade. Adjacent to the Tillyfourie intrusion, they consist principally of foliated tonalite, with 10–15% hornblende and 15–20% biotite. The foliation is much coarser than that of the country rocks, and is parallel to the margins of the veins and sheets, but not always parallel to the foliation in the country rocks. K-feldspar megacrysts occur in a few exposures, but are generally absent. In the western part of the vein complex, the tonalite grades into a leucocratic granodiorite, e.g. on Strone Hill and around Cairnballoch [NJ 568 140], which is similar in appearance to the rocks of the southern part of the Syllavethy intrusion. Some of the granodiorite exposures near Kingsford [NJ 564 146], mapped as veins, may actually be parts of the Syllavethy intrusion.

A sizeable intrusion of foliated tonalite (about 1 km by 200 m) occurs to the south of Lynturk [NJ 601 118]. The fault forming the south-eastern boundary is exposed in a tributary of the Lyne Burn [NJ 603 118], where shattering and comminution are well seen. At Craich [NJ 603 134], a body of tonalite at least 100 m wide crops out on the hill.

Petrography

The typical Tillyfourie tonalite has an average grain size of 2–3 mm. This decreases to 1–2 mm in the marginal dioritic rocks but increases to 3–4 mm in the granodiorite. Biotite typically forms 15–20% of the tonalite, occurring as aligned 1–2 mm dark brown flakes. Dark green hornblende in well-aligned 2 mm crystals represents 10–15% of the rock and defines a linear fabric which in places is marked by elongate quartz aggregates. Plagioclase, An_30–26 zoned to An_24–20, forms subhedral crystals with well-developed albite and Carlsbad twinning. Quartz is interstitial to the other minerals. Traces of slightly perthitic orthoclase occur throughout the pluton, but in a few places, e.g. Tillyfourie No. 1 and No. 2 quarries, it forms crystals up to 10 mm long. These crystals look euhedral in hand specimen, but under the microscope are seen to have irregular margins, enclosing and being interstitial to quartz and plagioclase. Myrmekite is developed at plagioclase–orthoclase boundaries. The principal accessory minerals are ilmenite and sphene.

Syllavethy Intrusion

Within an area of 7 km² around Bridge of Alford [NJ 561 171], scattered exposures of granodiorite and tonalite occur, and there are no exposures of Dalradian rocks. This area is probably underlain by a pluton, here named the Syllavethy intrusion. Between this pluton and the Tillyfourie Tonalite, exposures of Dalradian metasedimentary rocks are roughly equal in abundance with exposures of granodiorite and tonalite. The latter comprise an intense complex of veins, probably genetically related to the Tillyfourie and Syllavethy intrusions.

The probable contact of the Syllavethy intrusion with the country rocks is exposed at Gallowhill [NJ 559 159]. Here, deeply weathered granodiorite has a subhorizontal cross-cutting contact with Dalradian metaconglomerate. Other exposures of granodiorite occur at Shannoch [NJ 544 160], Mains of Asloun [NJ 543 152] and near Kingsford [NJ 560 148], while exposures of quartz-diorite displaying a well-developed foliation, striking SSE and dipping at 35° to the ENE occur in the disused Syllavethy Quarry [NJ 567 176]. The foliation is defined by the parallel alignment of biotite and hornblende, but a slight layering of felsic and mafic components is also visible. Quartz-diorite crops out at Galla How [NJ 555 180].

Petrography

The foliated quartz-diorite of Syllavethy Quarry is very similar to the more basic tonalites of the Tillyfourie intrusion, except for the smaller proportion of quartz. Hornblende and biotite together form about 25% of the rock. The granodiorite exposed in the southern part of the intrusion is considerably more leucocratic than the quartz-diorite, and foliation is absent or very poorly developed. At Gallowhill [NJ 559 159] the granodiorite contains xenoliths of foliated diorite. Petrographically it resembles the post-tectonic granodiorites, but is non-porphyritic, with a grain size of 2–3 mm. However, the gradual change in the composition of the vein complex from tonalite at Tillyfourie to granodiorite near Gallowhill, makes it likely that the Tillyfourie Tonalite and the granodiorite and quartz-diorite of the Syllavethy intrusion are related to each other. Biotite forms 5–10% of the granodiorite, and hornblende forms less than 2%. Plagioclase (An_30–26, zoned to An_20–16) is dominant over alkali feldspar, and myrmekite is developed at their mutual contacts. No geochemical or isotopic data are available for the Syllavethy intrusion.

Corrennie Granite

This intrusion cuts Dalradian metasedimentary rocks and the Tillyfourie Tonalite. It is assigned to the late-tectonic group on the basis of its strong tectonic foliation. Contact relationships with the Tillyfourie Tonalite are well exposed in Corrennie Quarry [NJ 642 119], where the foliation of the Tillyfourie Tonalite has been mobilised and drawn into concordance with that of the Corrennie Granite (Plate 5)c. The latter shows a fine-grained, more mafic facies within a few centimetres of the contact (Harrison, 1987). The intrusion has the form of an exceptionally wide dyke, varying from 150 m at Corrennie to 300 m near South Tillykerrie [NJ 624 125]. At its western end, it is terminated by a zone of faulting and intense cataclasis near Kirkton of Tough [NJ 615 130].

The main body of the intrusion consists of pink, very leucocratic granite, with grain size 3–4 mm and only traces of dark minerals. The prominent linear fabric is defined by the tectonic extension of 4–5 mm quartz aggregates, which are made up of ragged, sutured 0.52 mm crystals. Potash feldspar is dominant over plagioclase, most of which is probably recrystallised exsolved perthite. The plagioclase is unzoned, with a composition of An₁₄. The rock contains only 1% of biotite, and traces of secondary muscovite occur in places. The quartz fabric implies post-crystallisation deformation. The narrow marginal zone against the Tillyfourie Tonalite has a grain size of 1–2 mm, and has 2–3% biotite whose alignment defines a foliation parallel to the contact.

Geochemistry

Kennethmont Complex

Several analyses of rocks from the complex have been published by Read and Haq (1965) and Busrewil et al. (1975). Unfortunately all the published analyses come from components (1) and (3), so the possible relation of component (2) to the Insch syenites cannot be determined. Plotted on a normative Q-Ab-Or diagram (Figure 17)a, the rocks of component (1), except for the grey granite, lie close to the Or-Ab cotectic line at P_H2O = 5 kb, but the grey granite of component (1) and the pink granite of component (3) lie close to the ternary minimum at P_H2O = 1 kb, and close to the Kemnay Granite analyses.

The homogeneous tonalite and the diorite xenoliths within the grey granodiorite of the Kennethmont Complex have a greater geochemical affinity with the quartz-biotite-norite than with any of the other components of the Insch or Boganclogh intrusions. The Kennethmont diorite and tonalite contain more modal and normative quartz, but have higher Mg# ratios than the quartz-biotite-norites.

The gradation from a relatively uniform tonalite through grey granodiorite with mafic enclaves to grey granite with large xenoliths of diorite is matched by a geochemical transition. Busrewil et al. (1975) suggested that the inhomogeneous series of rocks resulted from the hybridisation of a granite magma with dioritic to tonalitic material. The fine-grained pink granite was considered to be an unrelated intrusion. It is, however, similar to a granite vein cutting the Boganclogh intrusion in its peraluminous and moderately evolved geochemical character.

When plotted on an AFM diagram (Figure 18)a, the rocks of component (1), particularly the diorites and tonalites, show a wide scatter, suggesting that the diorites and tonalites include cumulus material. However, an overall tholeiitic trend can be made out, with the maximum Fe enrichment occurring close to the transition from tonalite to granodiorite. The pink granite of component (3) lies close to the Corrennie Granite, and probably represents the end-point of magmatic evolution for silica-oversaturated magma.

Only limited trace-element data are available for the Kennethmont Complex (Busrewil et al., 1975; (Figure 19)). The Rb/Sr plot shows a differentiation trend from homogeneous tonalite through grey granodiorite through to grey granite. However the diorite xenoliths are scattered at various points along the trend, possibly indicating partial host-xenolith equilibration of these trace elements. The pink granite of component (3) is the most evolved member of the complex, and shows a degree of depletion in Sr and enrichment in Rb indicative of a degree of volatile action (cf. the lower Rb of the Corrennie Granite). From these data, together with rare earth element abundances, Busrewil et al. (1975) concluded that the Kennethmont rocks could not have formed by addition of material similar to the Kennethmont grey granite to any Insch or Boganclogh rocks. The diorite and grey granite are richer in both LREE and HREE than the Insch rocks, while the pink granite is depleted in HREE.

Kemnay Granite

Major element analyses were carried out by Walsworth-Bell (1974), who also conducted electron microprobe analyses of biotite from a few specimens. Additional trace elements were determined on a selection of Walsworth-Bell's specimens by BGS. The major element composition is close to the minimum melt in the system KAlSi₃O₈–NaAlSi₃O₈–SiO₂ at a P_H2O of 1 kb (Figure 17)a, but contains a slight excess of Si and Al. The plot cannot be used to provide an indication of the pressure of crystallisation, because of the additional components in the rocks which are not plotted on the diagram. The high initial ⁸⁷Sr/⁸⁶Sr ratio (0.715 ± 0.002), together with the peraluminous nature of the granite, suggests that the granite has either assimilated much metasedimentary material or is an S-type, formed form a crustal, probably metasedimentary, protolith. The AFM diagram (Figure 18)a shows that the Kemnay granite lies to the alkaline side of the calc-alkaline trend defined by the Tillyfourie Tonalite and the Crathes, Balblair and Clinterty granodiorites. The trace element data (Figure 19) show no evidence of any late volatite-rich differentiates of the Kemnay magma.

Tillyfourie Tonalite

Several specimens were analysed for major elements by Walsworth-Bell (1974), and trace elements were subsequently determined on some of these specimens by BGS. No isotopic data are available for this intrusion. The proportions of the felsic minerals are intermediate between the Torphins Diorite and the Balblair Granodiorite, but are also similar to some rocks from the Kennethmont Complex (Figure 17)a, b. The compositions vary from slightly peraluminous to slightly metaluminous, depending on the relative proportion of biotite and hornblende. An AFM plot (Figure 18)a shows that the Tillyfourie analyses define a calc-alkaline differentiation trend, upon which the Crathes, Balblair and Clinterty analyses also lie (Figure 18)b.

Corrennie Granite

The single analysis (Walsworth-Bell, 1974) shows that its major element composition is very close to the ternary minimum in the system Q-Or-Ab-H₂O. The Corrennie analysis lies at the extreme felsic side of the AFM diagram (Figure 18)a. The Y, Zr, V, Nb contents are all 50–70% of those levels in the main phases of the Cairngorm Suite granites, while the relatively high Sr (134 ppm) and Ba (769 ppm) show that the Corennie Granite has not followed the extreme differentiation trend of those granites.

Chapter 7 Post-tectonic granitic intrusions

This grouping includes all of the intermediate to acid intrusions which are believed to postdate the granitic rocks described in the previous chapter. They were intruded between 425 Ma and 400 Ma (Silurian to early Devonian), compared with 470 Ma to 450 Ma for the late-tectonic intrusions (Brown, 1991, pp.251, 262). Apart from the Torphins and Gask diorites, they are unfoliated or only patchily foliated.

A group of granitic intrusions recognised in the area between Torphins and Anguston was termed the Skene Complex by Bisset (1934). This complex was studied in more detail by Walsworth-Bell (1974), who deduced that the individual components belonged to several different groups, divided by age, field relations and geochemistry. Three members of Bisset's Skene Complex, the Kemnay Granite, Tillyfourie Tonalite and Corrennie Granite, have already been described as late-tectonic intrusions (Chapter 6). Of the remainder, all, except the Hill of Fare Granite, share a certain coherence, and comprise recognisable phases within a suite whose members extend outside the area studied by Bisset and Walsworth-Bell at least as far as Logie Coldstone. This suite, here named the Crathes Suite, is virtually confined to the Alford–Inverurie district, where it comprises the Torphins and Gask diorites, the Logie Coldstone Tonalite, and the Crathes, Balblair, Clinterty, Kincardine O'Neil, Lumphanan and Tomnaverie granodiorites. The Hill of Fare Granite is a member of the Cairngorm Suite of late biotite-granites, which cut members of the Crathes Suite. The Cairngorm Suite is widely represented in the eastern Grampian Highlands, extending from the Monadhliath to the Mount Battock, and probably Peterhead, granites. They form the major components of the east–westtrending East Grampians Batholith (Plant et al., 1990). The Cairngorm Suite is represented in the district by the Bennachie, Hill of Fare, Ballater, Cromar, Cushnie, Ord Fundlie and Middleton granites.

Crathes Suite

These intrusions were emplaced after the cessation of tectonothermal activity related to the Grampian Orogeny, but predate the intrusion of the Cairngorm Suite and the onset of early Devonian faulting and volcanism. Contacts with the country rocks are sharp, though in a few cases vein complexes are developed. There is no sign of chilling at the margins of any intrusion or of contact metamorphism of country rocks. The various intrusions range in composition from quartz-diorite to granite. The granodiorites display a typical talc-alkali differentiation trend, but the diorites show geochemical indications of enrichment in cumulate phases. Individual members of the suite show only a limited range of compositional variation. The finer-grained, more basic parts of the Torphins and Gask diorites show complex relationships within slightly differing dioritic phases, while the Balblair Granodiorite is, over much of its outcrop, a vein complex which intrudes and carries xenoliths of the Crathes Granodiorite, which is itself multiphase. A limited vein complex extends for a short distance beyond the Kincardine O'Neil Granodiorite, but otherwise veining of the host rocks enclosing the plutons is rare. Chemical analyses by Walsworth-Bell are available for those members cropping out from Torphins eastwards, but no geochemical data are available for those from Kincardine O'Neil westwards.

Isotope geology

Brown (1965) obtained a K-Ar biotite age of 420 ± 2 Ma from a specimen of Crathes Granodiorite from Craigenlow Quarry [NJ 732 093]. It is likely that this represents a cooling age, but suggests that the Crathes Suite granitoids are probably older than the Cairngorm Suite, if only by a few million years.

Geophysical expression

The gravity and magnetic expression of the plutons of the Crathes Suite is relatively subdued. The gravity and magnetic model (Figure 8); (Table 1) estimates the depth to the base of the plutons at 6 to 8 km along the line of (Figure 8). The Crathes Granodiorite has been modelled with a magnetic susceptibility of 0.008–0.012 SI, slightly less than the 0.015–0.018 SI of the Tillyfourie Tonalite. The annular magnetic anomaly lying within rocks of the Crathes intrusion is attributed to a body of slightly more magnetic rock (susceptibility 0.015 SI) at a depth of 0.5–1 km.

Torphins and Gask diorites

These intrusions are very similar to each other in lithology and geochemistry, and both are intruded by the Crathes and Balblair granodiorites (Figure 16). The Torphins intrusion is the larger and better exposed. The Torphins Diorite crops out over an area of 21 km² extending from Townhead [NJ 577 009] to Wester Campfield [NJ 654 007]. The contacts are not exposed, but exposure is sufficient for the contact to be defined within 50 m in many places. The Torphins diorite is cut by the Crathes, Balblair and Kincardine O'Neil granodiorites, as well as by the Ord Fundlie Granite and a wide range of minor intrusions, including aplite, felsite, microdiorite and appinite. The larger part of the Torphins Diorite outcrop consists of coarse-grained diorite, especially around Tillyneckle [NJ 610 011] and Oldtown [NJ 578 021]. A coarse-grained (5 mm) quartz-diorite with 10–15 per cent of biotite and hornblende crops out within the northern face of Sundayswells Quarry [NJ 614 033]. A small quarry in Learney Woods [NJ 623 046] exposes a mylonitic diorite on one face, with sheared and partly mylonitised diorite about 10 m away on the opposite face of the quarry. Where the A980 crosses a disused railway line 1 km west of Milton of Campfield [NJ 637 008], a cutting, now filled in, exposes a medium- to coarse-grained (2–4 mm) diorite cut by a 25 m-wide, possibly dyke-like, body, ranging in texture from microdiorite to fine-grained (0.5 mm) diorite. It is not certain whether the latter is a microdiorite dyke or a fine-grained phase of the Torphins Diorite. Both the dyke and the country-rock diorite have up to 25% biotite and 5–10% hornblende, and the dyke rock is porphyritic with plagioclase phenocrysts up to 2 mm long.

Virtually the only exposures of the Gask diorite occur in the large, disused Gask Quarry [NJ 794 065]. The diorite in the quarry ranges in grain size from about 1 mm to 4 mm, and the finer-grained parts possess a foliation marked by parallel alignment of biotite and hornblende crystals (Harrison, 1987). Metasedimentary xenoliths are widely scattered, and biotite-rich schlieren are oriented parallel to the foliation. The diorite is cut by dykes up to 20 m wide of porphyritic microdiorite and pink micro-granite: A patch of granodiorite with a sharp contact on one side but a gradational contact on the other occurs within the diorite. The sharp contact of the diorite with the Crathes Granodiorite is exposed at the south end of the quarry. The Gask Diorite has been proved in pipeline trenches to the north of the Loch of Skene at [NJ 775 105].

Petrography

The coarser facies of the Torphins and Gask diorites consists of light grey coarse-grained (4–5 mm), unfoliated, fairly leucocratic diorite with 10–15% each of hornblende and biotite, and up to 5% quartz. The sub- to euhedral plagioclase crystals have cores of An_57–44, zoned to An_29–23. Sphene forms up to 5% of the rock, iron oxides 2–3% and apatite 1% of the rock. Quartz, and potash feldspar where present, are interstitial to the other minerals. The finer-grained parts of the Torphins and Gask diorites consist of a darker grey diorite, grain-size 1–2 mm, in which a faint foliation, marked by mafic-rich schlieren, is detectable in places. Hornblende and biotite together form 30–40% of the rock, quartz forms up to 10%, and alkali feldspar is rare. The plagioclase is zoned from An_42–38 to An_28–23.

Crathes Granodiorite

The slightly pink coarse-grained K-feldspar megacrystic Crathes Granodiorite is spatially associated with the bluish grey fine-grained non-porphyritic Balblair Granodiorite. (Figure 16) shows the principal areas occupied by the two phases, but the amount of Balblair-type granodiorite within the main area of Crathes Granodiorite is almost certainly understated due to lack of exposure. The Crathes Granodiorite covers 128 km² and is the largest pluton in the district, extending into the Banchory (Sheet 66E) and Aberdeen (Sheet 77) districts.

The Crathes Granodiorite crops out from Viewfield [NJ 666 082] and Lauchintilly [NJ 752 132] south-eastwards to Landerberry [NJ 750 042], Cairneywhin [NO 712 993] and Dalmaik [NO 804 986], and continues as far as Crathes Castle [NO 733 968]. A small area of similar granodiorite crops out to the west of the Hill of Fare Granite around Cormoir [NJ 654 022]. Exposure in most of this large area is very poor, due to the large extent of glaciofluvial deposits and peat. Most of the granitic exposures are of the megacrystic Crathes type, but in places exposures of Balblair type occur. The most instructive exposures are in Craigenlow Quarry [NJ 732 093] (Harrison, 1987). Here, the Crathes granodiorite consists of at least two phases, differing slightly in colour, grain size and proportion of megacrysts, with sharp mutual contacts, and containing subangular xenoliths of fine-grained mafic diorite similar to that at Gask. It becomes almost pegmatitic in patches near xenoliths. The Crathes Granodiorite is cut by sub-vertical sheets, up to 20 m wide, of pale bluish grey fine-grained Balblair Granodiorite, with narrow offshoots and veins, some of which cut through diorite xenoliths (Plate 5)d. This granodiorite encloses xenoliths of Crathes Granodiorite, which display all stages of assimilation. There are no signs of chilling at any contacts between the granodiorites. Microdiorite sheets and dykes cut all phases of the Crathes and Balblair granodiorites.

Petrography

The Crathes granodiorite is white to pink, coarse-grained, megacrystic, and ranges from granodiorite to granite. Potash feldspar megacrysts up to 40 mm long form 10–15% of the rock. Biotite is the main mafic mineral (5–8%), but sphene is unusually abundant (1–5%) and crystals up to 3 mm are not uncommon. Hornblende is present only sporadically and in trace amounts. Plagioclase forms sub-euhedral crystals up to 4 mm, showing well-developed normal, and sometimes oscillatory zoning from An_29–21 cores to An_19–11 rims. Myrmekite is abundantly developed at plagioclase–Kfeldspar grain boundaries.

Balblair Granodiorite

The main outcrop of the Balblair Granodiorite lies to the west and north-west of the Crathes Granodiorite (Figure 16), and covers 44 km². The contact between the two granodiorites runs from near Mill of Echt [NJ 740 050] westwards as far as Upper Kebbaty [NJ 662 080], but cannot be mapped accurately due to poor exposure. The contact of the Balblair Granodiorite with the Tillyfourie Tonalite is exposed at Upper Kebbaty. It is sharp and cuts across the foliation of the Tillyfourie Tonalite (Plate 5)b. The contact runs from Upper Kebbaty to Milton of Tolmauds [NJ 620 076], south to near Bogenchapel [NJ 618 043], and south-east to 200 m north of Meikle Maldron [NJ 650 030]. Sheets and subsidiary bodies of Balblair-type granodiorite occur at Sundayswells [NJ 610 033] and northeast of Tillyching [NJ 602 045]. There are no exposures of Crathes type within this area; however, the Balblair Granodiorite, though of roughly constant grain size, shows variations in colour from pale blue-grey to white and even pale pink, and is cut by irregular veins of white leucogranite.

Many exposures of Balblair Granodiorite occur within the main outcrop of the Crathes Granodiorite. Some of these are too small to display contacts with the Crathes Granodiorite, but a few veins and sheets from 0.1 m to 10 m wide are seen to cut the Crathes Granodiorite. However, a large area of Balblair Granodiorite occurs around Anguston [NJ 810 020], extending to Coalford [NO 817 998] in the Aberdeen district. The granodiorite in Anguston Quarry contains scattered xenocrysts of K-feldspar up to 10 mm long, probably relics of assimilated Crathes Granodiorite, as well as rounded mafic enclaves, and is cut by irregular veins of white leucogranite. In this area, only small patches of Crathes Granodiorite occur. A poorly developed flow foliation marked by trails of biotite crystals occurs in parts of the Balblair Grano-diorite, especially where it forms veins in the Crathes Granodiorite.

Petrography

The Balblair Granodiorite is a slightly more calcic and finer-grained granodiorite than the Crathes Grano-diorite. Subhedral plagioclase crystals are zoned from An_33–27 to An_21–16. Biotite forms 10–15% of the rock, and K-feldspar 5–10%. No megacrysts occur, but rare K-feldspar xenocrysts up to 10 mm long occur where assimilation of Crathes Granodiorite has taken place. Exposures of the Balblair Granodiorite in Craigenlow Quarry [NJ 732 093] contain rounded xenoliths of Crathes Granodiorite, which are surrounded by a zone up to 50 mm wide of fine-grained (0.5–1 mm) granodiorite, possibly more mafic than the typical Balblair Granodiorite, indicating that the xenoliths have suffered partial digestion.

Clinterty Granodiorite

This pluton crops out over 14 km², of which some 20% lies within the district, the rest being in the Aberdeen district (Figure 2); (Figure 16). It is roughly circular in shape. The contact can be closely constrained near South Auchronie [NJ 808 093], whence it runs NNW and then north-east to Wogle Cottage [NJ 814 116] and out of the district. It is a white to pale pink, coarse-grained (3–4 mm) granodiorite, containing rare K-feldspar macrocrysts up to 10 mm in size. It shows strong petrological and geochemical similarities to the Crathes Granodiorite, but is, if anything, slightly more evolved. A fuller description of the pluton is given by Munro (1986b).

Kincardine O'Neil Granodiorite

This largely granodioritic mass crops out in several small scattered areas, none larger than 0.5 km²: at the Sloe of Dess [NJ 566 004]; Westerton–Dess [NJ 575 005]; Craigton [NJ 595 005]; Mill of Dess [NO 570 996]; and Kincardine House [NO 603 998] (Figure 16). A granodiorite vein complex extends up to 0.5 km outwards from the Westerton–Dess and Craigton outcrops. A disused quarry in Westerton Wood [NJ 5708 0068] exposes pink coarse-grained granite to granodiorite, slightly foliated, in the north and east faces. The southern face consists of psammite and semipelite cut by a 10 m-wide body of grey finer-grained granodiorite. The granodiorite at the western margin of the Sloc of Dess exposure is shattered, and the contact against the Deeside Limestone Formation is believed to be faulted.

Petrography

The Kincardine O'Neil Granodiorite is petrographically very similar to that at Clinterty, except for the outcrop at Kincardine House, which is finer grained and more similar to that of Balblair. The coarser-grained variety (grain size 3 mm), is a white to pale pink granodiorite to granite, with rare K-feldspar macrocrysts up to 10 mm, while the finer-grained phase has a grain size of 1–2 mm. Plagioclase is zoned from An₂₈ to An₇₆; the rocks contain 5–8% biotite and 2–3% sphene, while hornblende is absent.

Lumphanan Granodiorite

An area of approximately 3 km2 to the north of Lumphanan (Figure 16) is underlain mainly by grey granodiorite. At its western margin, from Lumphanan Church [NJ 584 049] to near Mains of Glengarry [NJ 580 065], it is intruded by the Cromar Granite; it may have originally been continuous with the Tomnaverie Granodiorite. The eastern margin runs from Lumphanan churchyard northeast to Maryfield [NJ 600 060], and then probably runs north and then west to Mains of Glengarry. Exposure to the east of Maryfield is very poor, and it is possible that the intrusion joins up with the Balblair Granodiorite at Tornaveen [NJ 616 060]. The western part of the intrusion consists of medium- to coarse-grained, non-porphyritic grey granodiorite to tonalite with scattered mafic xenoliths, in which a rudimentary flow foliation is developed in places. The eastern part consists of pale grey to pale pink, finer-grained (1–2 mm) granodiorite to granite, and is similar to the Balblair Granodiorite, although slightly paler in colour. Its relationship to the western facies is not exposed.

Petrography

Quartz forms about 20% of the both coarser-and finer-grained facies. It occurs as 1 mm interstitial crystals, in places forming aggregates up to 3 mm across. The quartz is fractured and strained, but not recrystallised. The coarser-grained facies contains 8–10% of dark brown biotite and generally has only about 5% K-feldspar; hence it is in many places a tonalite. It contains up to 2% hornblende, and up to 1% sphene as 1–2 mm euhedral phenocrysts. The plagioclase is zoned from An₂₆ to to An₂₀. The finer-grained facies is a granodiorite containing 5–8% biotite and up to 20% K-feldspar, and the plagioclase is zoned from An₂₆ to An₁₆. Hornblende is absent from the finer facies and sphene is present only in trace amounts.

Tomnaverie Granodiorite

This intrusion occupies most of the low ground of the Howe of Cromar and the low ground to the north and west of Milton of Auchenhove [NJ 553 035] (Figure 2). The outcrop is split into two by the later Cromar Granite. Read (1927) recognised that the granodiorite in Tomnaverie Quarry [NJ 487 034] was different from the granite of Mortlich and Craiglich, but did not assess its extent. Exposures are small and scattered, but the boundaries can be defined with reasonable confidence to within 200 m in several places. Exposures in the Howe of Cromar occur in ditches near Woodfield [NJ 516 051], south of Easttown [NJ 496 056], around Corrachree [NJ 463 045], near Coatmore [NJ 473 025], at Tomnaverie Quarry, at Coull Castle [NJ 513 022] and in the Glen of Peat Lochies [NJ 525 030]. The slopes of Drummy Hill [NJ 47 04] are strewn with large boulders of granodiorite and there are a few possible in-situ exposures. Near Leys [NJ 465 028], large boulders show angular blocks of norite veined by grey granodiorite. Exposures to the east of the Cromar Granite are scarcer, but occur near Loanend [NJ 540 035] and on the lower slopes of Blelack Hill [NJ 555 052]. It is uncertain whether a very large block of granodiorite in a field near Milton of Auchenhove [NJ 550 035] is in situ or is erratic.

Petrography

The granodiorite in Tomnaverie quarry is white medium grained (c. 2 mm), and non-porphyritic, but reddened for 1–2 mm along joints. K-feldspar forms 10–20% of the rock, and dark brown biotite 5–10%. Plagioclase is zoned from An_28–24 to An_18–12. Quartz forms 20–30% of the rock, occurring as 1.5 mm interstitial crystals with strained extinction in places. Sphene forms up to 1% of the rock and magnetite up to 2%. Most of the central part of the western outcrop is petrographically similar, although the grain size increases to 3 mm on Drummy Hill. At Milton of Auchenhove and at Leys the granodiorite is coarser grained (4–5 mm). The Tomnaverie Granodiorite is petrographically similar to the western part of the Lumphanan Granodiorite. A single chemical analysis by O'Brien (1985) from Tomnaverie Quarry is very similar to analyses of the Crathes and Clinterty granodiorites, except that Sr (520 ppm) and Ba (1000 ppm) are about 20% higher than in those intrusions, suggesting that the Tomnaverie magma is slightly less evolved.

Logie Coldstone Tonalite

This intrusion occupies much of the low ground between the Morven–Cabrach and Tarland basic masses (Figure 2). In the south-west it is intruded by the Ballater Granite. Exposure is very poor, especially in the area around Loch Davan. The contacts with the basic rocks are drawn with assistance from the EVL aeromagnetic survey. The Logie Goldstone and Tomnaverie intrusions may be joined at a shallow depth beneath the Tarland basic mass. Exposures occur near Pitline [NJ 434 060], and the Dalradian rocks in Pitline Quarry form an enclave within the tonalite. Exposures of tonalite have been recorded in a quarry near Cairnmore [NJ 440 048], near Corblelack [NJ 447 041] and in a silage pit at Kinaldie [NJ 423 052] in the Glenbuchat district (Sheet 75E). At Braehead of Easter Coynach [NJ 4429 0573], exposures of the tonalite are shattered at its contact with the brecciated pegmatite which separates it from the Tarland intrusion. Dalradian metasedimentary rocks crop out on the hill [NJ 444 000] between Loch Davan and Loch Kinord in the Aboyne district (Sheet 66W), but the location of the boundary between these rocks and the Logie Coldstone tonalite is uncertain. Joints are widely spaced, and reddening occurs along them.

Petrography

The Logie Coldstone intrusion consists of an equigranular grey tonalite, with grain size about 4 mm. Biotite and hornblende each form 5–10% of the rock, while quartz represents 20–25%, K-feldspar 2–3%, and plagioclase, zoned from An₃₂ to An₁₂, forms the remainder.

Cairngorm Suite

This suite comprises a number of granitic intrusions within the northern Grampian Highlands which share a number of distinctive characteristics. The suite postdates all other granitic suites in the eastern Grampian Highlands, including the Crathes Suite. Members of the suite from Monadhliath to Mount Battock are associated with a large regional negative Bouguer gravity anomaly, 100 km by 40 km, indicating that they are probably the surface expression of an extensive, largely buried, granitic body, the East Grampians Batholith (Plant et al., 1990). The Cairngorm Suite consists mostly of pink biotitegranites and leucogranites with a limited range of composition. However, some members, e.g. Lochnagar, are associated with granodiorites to quartz-diorites, and the annular magnetic anomalies associated with a number of them suggest that more basic phases may be associated with them at depth. The Cairngorm Suite granites are largely hornblende free; muscovite is mostly secondary, occuring principally in areas of greisening and hydrothermal alteration. Microgranite, aplite and pegmatite sheets and dykes are widespread. The potash feldspar is dominantly orthoclase perthite, but microcline is developed in the aplitic, pegmatitic and microgranitic phases. The granites have low initial ⁸⁷Sr/⁸⁶Sr ratios, close to 0.706, indicating evolution by differentiation from a largely mantle or lower crustal source, with little or no metasedimentary input (Harrison and Hutchison, 1987).

Members of the suite have narrow (c. 300 m) contact metamorphic aureoles (see Chapter 4); however, many of the plutons are partly fault bounded, and there the aureoles are absent. The main granite phases often grade into two-phase granites (Cobbing et al., 1986). The bimodal grain-size distribution is ascribed to sudden increase in the rate of crystallisation, due to decompression during crystallisation of the last portion of the magma. These features indicate emplacement into relatively cool country rock at a high level in the crust, probably no more than 5 to 8 km (Harrison and Hutchison, 1987).

The suite is represented in the Inverurie–Alford district by three sizeable plutons, the Bennachie, Hill of Fare, and Cromar granites, together with part of the Ballater pluton. In addition, the smaller Middleton, Cushnie and Ord Fundlie intrusions are probably cupolas from the regional underlying batholith and may join up with the larger plutons at a relatively shallow depth. Aplite–pegmatite vein complexes extend from the Bennachie Granite eastwards as far as the environs of the Middleton body, and westwards as far as Millockie Hill [NJ 565 205]. An aplite vein complex extends west and south-west from the Hill of Fare pluton as far as the Ord Fundlie mass. These vein complexes are described in Chapter 8.

The granites of the Cairngorm Suite show a highly evolved geochemistry, with phases enriched in Li, Be, B, Rb, REE, Th and U; veins associated with the granites contain Mo, Sn, W, As and Sb (Chapter 12). The high K, Th and U content of the suite prompted studies of the geochemistry and heat flow of four of the plutons including Ballater and Bennachie.

Isotope geology

Darbyshire and Beer (1988) obtained a Rb-Sr whole-rock isochron age of 400 ± 4 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70620 ± 0.00035 on the Bennachie Granite. Attempts to date the Middleton Granite failed to produce an isochron due to the extensive alteration, but an age of 398 ± 19 Ma was indicated. The Hill of Fare Granite has yielded a Rb-Sr whole rock isochron of 413 ± 3 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7057 ± 0.0003 (Halliday et al., 1979). These values are taken to date the emplacement of the plutons. Harrison and Hutchison (1987) suggest that the Cairngorm Suite can be subdivided on age into early (c. 415 Ma) and late (c. 404 Ma) groups; on this basis Hill of Fare would be early and Bennachie late. However, this distinction is suspect, as it is at the limit of resolution of the analytical methods.

Geophysical expression

Magnetic data

The granites of the Cairngorm Suite are marked by conspicuous aeromagnetic anomaly patterns (Figure 4). Typically, an annular positive anomaly surrounds an area of neutral or slightly elevated magnetic intensity.

The Bennachie Granite shows a flat magnetic pattern in the north, with a positive anomaly in the south. The eastern and western marginal faults have no magnetic expression. The positive anomaly in the southern part of the granite is probably related to the less geochemically evolved nature of this part of the granite, which has a higher magnetite content. This anomaly can be distingguished on the EVL survey from the anomaly over the Dalradian rocks on Kist Hill [NJ 638 162] where a magnetite-rich bed can clearly be recognised. A circular low occurs 4 km to the east of the centre of the Hill of Fare pluton; it may reflect the low susceptibilities of both the Hill of Fare and Crathes plutons. The surrounding annular high lies over rocks of the Crathes Suite, and may be due to more extensive diorites of Gask/Torphins type at depth. The Ballater Granite is also marked by a magnetic low, but it is largely surrounded by rocks with high magnetic susceptibilities (Morven–Cabrach Intrusion, Dalradian amphibolites), so this says little about the Ballater Granite itself. There is no obvious magnetic pattern over the Cromar, Ord Fundlie or Cushnie granites, which were not covered by the detailed EVL survey.

Gravity data

A large negative Bouguer anomaly covers much of the eastern Grampian Highlands; it encompasses the outcrop of the Cairngorm Suite granites (Figure 7). Two-dimensional profiles across four of the plutons, centred on the heat-flow borehole sites, and a three-dimensional model covering the whole area of the Cairngorm Suite granites were calculated by Rollin (1984). Assuming a density contrast of –0.10 to –0.15 g/cm³ compared to the Dalradian country rock, the granites of the Cairngorm Suite are believed to be linked at depth in the East Grampians Batholith (Plant et al., 1990). A depth of 13 km to the base of the batholith was generated by the model of Rollin (1984), but more recent work (Rollin, 1993) suggests that the East Grampians Batholith can be modelled with a maximum depth of 6–8 km below surface. McGregor and Wilson (1967) produced a gravity model of the Bennachie Granite, showing that it continues at shallow depth to the east and west of the boundary faults. More detailed work over the Middleton Granite (Kimbell, 1991) has helped to refine the outcrop boundary, and has shown that it is linked to the East Grampians Batholith at depth.

Heat flow

The heat production, heat flow, and thermal conductivity of the Cairngorm Suite granites were investigated as part of an assessment of the potential for hot, dry rock geothermal power (Webb and Brown, 1984; Lee et al., 1984; Wheildon et al., 1984). The results are summarised in Chapter 12.

Bennachie Granite

This pluton crops out over 55 km², including the Bennachie range of hills and also Cairn William and Pitfichie Hill to the south of the River Don (Figure 20). The contacts are generally poorly exposed, but good exposures within the pluton occur on the tors of Bennachie and the glaciated pavements of Cairn William. The northern contact, which runs approximately from south of Kirkton of Premnay [NJ 643 248] to north of Maiden Castle [NJ 695 245], is mostly unexposed. A boundary against norites of the Insch mass was reported by Wilson and Hinxman (1890) about 1 km south of Daies [NJ 650 251], although this is no longer visible. The southern margin of the intrusion runs from near Gateside [NJ 622 166] to near Pitfichie [NJ 684 166]. Good exposure of this contact occurs on Kist Hill [NJ 638 162], where the granite cuts across the foliation in sillimanite–K-feldspar gneisses. A few veins and pods of granite occur outside the main mass of granite. Slight retrogression of the high grade gneisses occurs within 500 m of the contact.

The western contact of the granite is fault bounded. At the southern end of this north–south-trending structure, exposures in a disused quarry (now backfilled) at Roadside [NJ 622 170] revealed brecciated granite (Michie, 1968). A sharp, possibly faulted contact between greisened granite and andalusite-schist is exposed in a ditch on the western slopes of Black Hill [NJ 628 222]. The eastern contact is marked by a 150 m-wide zone of brecciated aplite from just north of Maiden Castle [NJ 695 245] to Craignathunder [NJ 694 222]. To the south of this, the contact is unexposed, until the breccia reappears, only 20 m wide, at Mount Jane [NJ 684 170]. To the east of the breccia, two small areas of granite outcrop occur, one at Tullos [NJ 701 216] and one near Blairdaff [NJ 700 180]. Within the main granite outcrop, two small north–south-trending bodies of breccia, no more than 20 m wide, occur at Bruntwood Tap [NJ 668 216] and on the eastern slopes of the Mither Tap [NJ 686 232].

Most of the pluton consists of a pale pink coarse-grained (3–5 mm) sparsely to abundantly porphyritic biotite-granite. Quartz crystals are typically smoky or brownish in colour, and euhedral in form. Orthoclase perthite macrocrysts are generally 10–20 mm long, and roughly rectangular in shape, though becoming interstitial to other minerals at their margins. Biotite forms 2–5% of the rock, with zircon and magnetite as the chief accessory minerals. In places, for example 300 m northeast of the summit of Millstone Hill [NJ 680 206], this granite grades into an intensely porphyritic microgranite, 50–70% of which consists of sub- to euhedral quartz, plagioclase and orthoclase 3–5 mm in size and biotite up to 2 mm across, set in a microgranitic matrix, grain size 0.5–1 mm. This microgranite typically contains small drusy cavities, 1–2 mm in size. The microgranite appears to represent the last part of the main body of granite to crystallise, and the fine-grained matrix probably indicates rapid crystallisation of the magma.

In the north-western part of the intrusion, a zone up to 1 km wide consists of fine-grained granite, grain size 1–2 mm, with scattered 5–10 mm macrocrysts of K-feldspar. The contact with the main granite facies is not exposed. The main granite is intruded by several sheets and irregular bodies of aplite, for example on Little Oxen Craig [NJ 664 232], Watch Craig [NJ 655 222], and Little John's Length North [NJ 685 235]. Aplite and pegmatite veins up to 1 m wide are widely scattered in the main granite. A few veins have aplitic margins which grade into pegmatitic centres. In some cases the centres of the veins are hollow, and bordered by crystals of quartz and microcline up to 50 mm long; this is well seen on Hermit Seat [NJ 645 229].

Most of the Bennachie Granite is relatively fresh, with only slight reddening of feldspars. However, near the margins, and especially in the Blairdaff outlier, the granite is strongly reddened, with feldspars becoming brick red, quartz opalescent, and biotite either altered to chlorite or bleached. On the north-west slopes of Bennachie [NJ 6286 2180], the granite has been greisened in proximity to a suite of NNE-trending quartz veins. The quartz here is milky, and the feldspars are white to pale green, due to formation of small white mica flakes within the feldspar crystals (cf. alteration of Kemnay Granite at Tom's Forest Quarry).

The Tullos and Blairdaff masses consist of medium-grained (2 mm) non-porphyritic granite, rather finer grained than the main Bennachie Granite.

Breccias

The Maiden Castle and other breccias consist largely of subangular clasts up to 0.1 m in size of bleached silicified aplite set in a red matrix of cherty silica containing much haematite after sulphides. Similar breccias, also trending north–south, occur within the vein complexes to the east and west of Bennachie. They probably exploit pre-existing fractures, and represent the conduits for the rise of mineralised fluids.

Bennachil Vein Complex

This vein complex covers about 29 km² to the west of the Bennachie Granite outcrop. The members of the vein complex include both pegmatites and aplites. There are also some scattered aplite and pegmatite veins within the outcrop of the Bennachie Granite. In a few cases, the outer part of the vein consists of aplite, while the centre is pegmatite; the pegmatite commonly does not fill the whole of the central part of the vein, but euhedral crystals of quartz and feldspars grow into a central cavity. The veins are 0.1–20 m wide, and can be traced from loose blocks for up to 500 m. The most common trend is between NNW and NNE, but there arc some NE- to ENE-trending veins as well. A rather more diffuse spread of aplite veins, but with few or no pegmatites, occurs to the east of the Bennachie Granite between Blairdaff [NJ 695 177] and Middleton [NJ 730 225], where they are associated with small outcrops of microgranite. Both the Bennachie vein complex to the west of the Bennachie Granite, and the veins between Blairdaff and Middleton are most abundant where gravity data show downfaulted extensions of the Bennachie Granite. The veins described above may be related to the north-trending breccias described above.

Hill of Fare Granite

This subcircular pluton with an outcrop area of 37 km² was emplaced into granodiorite of the Crathes and Bah blair intrusions, except in the south-west where it cuts semipelitic gneisses of the Queen's Hill Formation (Figure 16). The metasedimentary host rocks show evidence of contact metamorphism, with andalusite overgrowing pre-existing muscovite and fibrolite, for up to 200 m from the contact. The contact is nowhere exposed, but can be constrained to within 50 m at Kennerty [NJ 673 000]. At Mill of Hole [NJ 717 051], a fine-grained marginal facies of the granite is developed. The south-eastern part of the contact is possibly faulted. The main granite facies in the intrusion is pink, medium grained (3 mm) and non-porphyritic. Within the area mapped as main granite, diffuse patches up to 400 m across consist of a slightly vuggy granite, with 50–70% of quartz, feldspar and biotite crystals up to 3 mm across, set in a microgranite ground-mass with a grain size of 0.5–1 mm. These textural features, similar to those around Millstone Hill in the Bennachie pluton, indicate that the roof of the intrusion was only a short distance above the present land surface.

Sheets of microgranite cut the main granite at Blackyduds [NJ 689 031], Queen's Chair/Berry Hill [NJ 720 015], and at the summit of the Hill of Fare [NJ 672 028]. Aplite and pegmatite sheets, though abundant in the vein complex to the north and west of the granite, are virtually absent within the pluton itself.

Hill Of Fare Vein Complex

This vein complex occurs over a 25 km² area lying north and west of the Hill of Fare Granite between Kincardine House [NO 603 998] and Upper Balblair [NJ 700 067] (Figure 2). The veins are up to 50 m wide and consist almost entirely of aplite. The aplite is pink, sugary textured, and very leucocratic, with grain size 0.5–1.5 mm. The strike length of most of the veins cannot be estimated due to poor exposure. Some of the aplites of the vein complex have been mapped as rounded to elongated patches, rather than veins. With the exception of a patch of aplite near Blairhead [NJ 660 020], there are no aplites within the Hill of Fare intrusion.

Cromar Granite

The Cromar Granite has an arcuate outcrop extending over 17 km² to the north of Aboyne, and includes the prominent hills of Mortlich [NJ 536 016], Craiglich [NJ 534 055], Corse Hill [NJ 551 060] and Mill Maud [NJ 572 066] (Figure 2). Much of the outcrop is now forested, and exposure is confined to forestry tracks and occasional crags. Most of the contacts appear to be intrusive, but a few short sections are clearly fault controlled. The contact is displaced by over 200 m along the line of a brecciated pegmatite 800 m east of Queen's Hill [NJ 540 005]. A faulted contact against Dalradian metasedimentary rocks is exposed in a small quarry to the east of Mill Maud [NJ 579 068]. The Dalradian country rocks here are shattered, and the contact is displaced by about 400 m. In the Glen of Peat Lochies, the contact is displaced by over 1 km [NJ 525 035] to [NJ 528 045], with a probable downthrow to the east. The Cromar Granite and Tomnaverie Granodiorite are reddened for about 50 m on either side of the fault line. Both rocks are cataclased, and large crystals of K-feldspar are visible; rocks of both intrusions have been-transformed to syenite by loss of quartz. The fault may have been a conduit for late-stage magmatic undersaturated fluids similar to those causing desilication of the Barnesmore Granite, County Donegal (Dempsey et al., 1990). Veins of pink granite occur close to the contact of the Cromar Granite at Chraad [NJ 540 066] and at Coull Castle [NJ 513 022], while a small body of pink granite occurs 200 m east of Whitehouse [NJ 555 047]. The Cromar granite is similar to the main facies of the Hill of Fare, being mostly uniform, pink, non-porphyritic, with grain size 3–5 mm. It is strongly reddened on Mill Maud and near the contacts in places, but otherwise fairly fresh. No geochemical data are available.

Ballater Granite

Only a very small part of the Ballater Granite, about 1.5 km², lies within the district (Figure 2). The northern part of the eastern contact lies under the Quaternary deposits of the Muir of Dinnet depression and is not exposed. The only exposure within the district is on the hill to the north of the Burn o' Vat Visitor Centre [NJ 426 002]. The granite is pink, leucocratic, porphyritic and coarse grained (10 mm). Orthoclase megacrysts reach 40 mm. A fuller account of this intrusion will appear in the memoir for the Ballater district (Sheet 65E).

Cushnie Granite

This granite crops out over an area of 0.5 km² to the north of Badychark [NJ 504 097] (Figure 2). The area is now forested, and the extent of the granite has been mapped from spoil thrown up from ploughing, together with exposures in forestry tracks. The granite is pink, fine to medium grained (2 mm) and non-porphyritic, closely resembling the Ord Fundlie Granite. Patches have been partly greisened. No chemical analyses are available, but stream sediment geochemistry shows that local Mo, As and Sb anomalies are related to the granite (Chapter 12) .

Ord Fundlie Granite

This granite crops out over an area of 0.6 km² on the southern slopes of Ord Fundlie [NJ 610 000] (Figure 16). It consists of pink non-porphyritic granite with a grain size of 1.5–2 mm. A satellite body crops out over 0.1 km² to the north of Kincardine House [NJ 603 003]. The larger outcrop is now almost entirely forested, but a small exposure of granite on the south side of Ord Fundlie [NO 610 995] carries xenoliths of diorite up to 0.3 m. Barrow (unpublished field notes, c. 1896), working when the ground was unforested, noted that relations between the Torphins Diorite, Kincardine O'Neil Granodiorite and Ord Fundlie Granite on Ord Fundlie are very complex. The Ord Fundlie granite is cut by a felsite dyke near its southern contact. In this it differs from all other granites of the Cairngorm Suite except Mount Battock.

Middleton Granite

This granite crops out over an area of only 0.1 km² around Upper Middleton [NJ 7296 2202] (Figure 2). The contacts of the intrusion are not exposed, but geophysical studies and drilling during the search for Mo mineralisation have enabled the boundaries to be established with reasonable accuracy (Colman et al., 1989). A disused quarry shows small exposures of greisened granite, and three boreholes intersected similar material. The bulk of the granite is a medium- to coarse-grained (34 mm) equigranular biotite-granite. It is strongly reddened, and shows signs of extensive hydrothermal alteration, particularly the formation of greisens. It is cut by abundant veins up to 3 m thick of aplite and quartz breccia, the latter bearing Mo and rare W mineralisation. The core from the borehole closest to the contact consists largely of a quartz-feldspar-phyric microgranite, which is, like the coarser-grained granites, intensely greisened and veined by aplite and quartz.

Petrography

The granites of the Cairngorm Suite are mostly biotite-granites, but are texturally heterogeneous. In the larger plutons, there is at least one phase of coarse-grained (> 3 mm) primary-textured granite. Within the district, only one phase of primary-textured granite occurs per pluton. In the case of Cromar and Hill of Fare, the granite is equigranular, non-porphyritic and has a grain-size of 5 mm in the centre of the pluton, reducing gradually to 3 mm towards the intrusive contacts. The Bennachie Granite has a grain size of 3–5 mm, and is typically porphyritic, with crystals of perthitic orthoclase up to 20 mm. The proportion of large orthoclase crystals ('macrocrysts') varies irregularly from less than 5% to nearly 20%. By contrast, the portion of the Ballater Granite cropping out within Sheet 76W has an average grain size of 10 mm, with scattered alkali feldspar megacrysts up to 40 mm. All of these rocks have a typically magmatic hypidiomorphic texture. Plagioclase is euhedral, zoned from An_24–16 to An_10–4. The cores are more or less uniform in composition, but are enclosed by thin, highly zoned rims. Plagioclase compositions in the main phases of the Bennachie, Hill of Fare and Cromar plutons are similar. Quartz shows idiomorphic faces against alkali feldspar and occurs in aggregates of fractured grains up to 6 mm across. Particularly in Bennachie, it is dark brown to smoky in hand specimen. The alkali feldspar is a perthitic orthoclase, which has rare Carlsbad twinning, and poikilitically encloses small plagioclase and quartz crystals. Biotite is the principal mafic mineral, normally dark brown, with rare pleochroic haloes. The crystals are commonly rather ragged, with some late-stage overgrowths of, or partial replacement by, slightly paler biotite, especially the more evolved granites from Bennachie, where enrichment of the biotite in Li may have occurred. Muscovite is rarely developed as an interstitial, late-magmatic phase, but is common as a secondary mineral, especially where the primary-textured granite has been invaded by fluids and greisened near aplites, pegmatites or quartz veins. Magnetite, ilmenite, apatite, monazite and zircon are the principal accessories. Sphene is much rarer than in the granitic rocks of the Crathes Suite.

Granites with a bimodal grain-size pattern occur as diffuse patches in the Bennachie and Hill of Fare plutons, and comprise the whole of the Cushnie mass. The textures, particularly the euhedral form of the quartz and feldspar crystals at the margins of the tiny vugs, indicate the local presence of a vapour phase. The diffuse patches of bimodal granite in the Bennachie and Hill of Fare plutons consist largely of an intimate mixture of coarse-grained granite and aplitic microgranite. Dihedral crystals of quartz, plagioclase and alkali feldspar up to 3–4 mm in size form 50–90% of the rock. In places they form aggregates up to 15 mm in size. The matrix consists of fine-grained (0.5–1 mm) leucocratic aplitic microgranite, with a saccharoidal texture. Forceful disruption of the original texture is less evident in the plutons of the district compared with others in the Cairngorm Suite, e.g. Cairngorm, Monadhliath, Mount Battock, and the South-east Asian tin granites by Gobbing et al. (1986).

Greisens are associated with the Middleton and Cushnie bodies (Colman et al., 1989). They are extensively recrystallised microgranites or aplites in which the alkali feldspar has been largely replaced by a mosaic of ragged muscovite crystals, particularly near planes of fluid transfer. Quartz is typically sutured, and plagioclase crystals contain disseminated sericite. The sparse original biotite is completely chloritised.

Almost all the breccias which occur along northsouth-trending fractures within or at the margins of the granites consist of subangular clasts up to 0.1 m in size of aplite set in a matrix of reddened jasper. The clasts are fine grained, white in hand specimen, and have a saccharoidal quartz-feldspar texture with a grain size of 0.10.2 mm. Mafic minerals and muscovite are absent. Some of the original feldspar has been partly replaced by microcrystalline quartz. The matrix consists of microcrystalline quartz, flecked with tiny opaque flakes, probably haematite. Against rare vugs in the breccias, the grain size of the microcrystalline quartz increases to about 0.1 rum. Boxworks of haematite and limonite after pyrite and other sulphides are common. Along the western margin of the Bennachie pluton, the coarse-grained granite has been brecciated at Roadside [NJ 622 170] (Michie, 1968).

Geochemistry

Whole rock chemical analyses by Walsworth-Bell (1974) are available for several components of the Crathes Suite. In addition a few analyses for major and trace elements are provided by O'Brien (1985). The Crathes, Balblair and Torphins plutons have been adequately sampled, and there are reasonable data for Clinterty, but only one to two analyses each exist for Tomnaverie and Gask, while no data exist for Logic Coldstone, Lumphanan, and Kincardine O'Neil.

Each of the major sampled intrusions varies within a discrete composition range. The composition of the members of the suite ranges from diorite to a granite close to the minimum melt in the system SiO₂–KAlSi₃O₈–NaAlSi₃O₈ at 4Kb P_H2O (Figure 17b). This cannot be used to deduce the depth of intrusion. An AFM plot (Figure 18)b shows that whereas the Crathes, Balblair and Clinterty granodiorites lie along a typical talc-alkaline trend, the Torphins and Gask diorites show a wide spread; this indicates the probability that they have incorporated a significant proportion of either mafic cumulate material, possibly precipitated from a tonalitic magma, or restite from assimilated metasedimentary material. The trend defined by the Crathes, Balblair and Clinterty granodiorites is a continuation of that shown by the Tillyfourie tonalites; the Corrennie granite lies at the extreme mafic-poor end of the same trend (Figure 18)a. This brings into question the allocation of the Tillyfourie and Corrennie plutons to the late-tectonic group, despite the tectonic fabrics displayed by the rocks of these intrusions. The trace element data indicate a pattern of fractional crystallisation, but with significant differences between Torphins and Gask diorites on one hand and the Balblair, Crathes and Clinterty granodiorites on the other (Figure 21). Rb shows a fairly steady increase with decrease in TiO₂, whereas Sr shows very little pattern except that it is generally higher in Balblair than in Crathes or Clinterty. Y decreases steadily with decrease in TiO₂, but there is one sample from Gask and one from Torphins which are anomalously high in Y, Zr and TiO₂ show an antipathetic relationship in Torphins and Gask, with a wide scatter, but in Balblair, Crathes and Clinterty the two elements are closely associated.

The granites of the Cairngorm Suite have an evolved bulk composition with SiO₂ in the range 71–77%, CaO and FeO (total) less than 2% and MgO less than 1%. Major element compositions show little variation, except that fine-grained variants are mostly particularly low in Fe, Mg and Ca. Plotted on the SiO₂–NaAlSi₃O₈–KAlSi₃O₈ diagram (Figure 17)c, the Bennachie and Hill of Fare compositions lie close to the ternary eutectic composition at P_H2O = 1 kb, confirming that magmatic evolution was complete, as far as bulk composition was concerned, by the time the magmas responsible for the bulk of the plutons had been formed. However, the minor phases (aplites, microgranites and pegmatites) of these plutons show enrichment in the elements associated with volatile-rich fluid phases (Li, Be, B, Rb, Y, Nb and U). In addition, quartz veins bearing As, Sb, Mo, Sn and W are developed, especially in cupolas. Plant et al. (1990) point out that these characteristics are typical of tin-uranium granites worldwide.

Analyses of the Bennachie Granite are available from Webb and Brown (1984) and O'Brien (1985). Although apparently uniform petrographically, the northern and southern halves of the main granite can be distinguished geochemically. The northern half is more geochemically evolved, being enriched in Li, Be, Rb and U, and depleted in Ti, Zr, Sr, Ba, compared to the southern half (Figure 20). This difference is confirmed by the aeromagnetic anomaly over the southern part of the pluton (Figure 4). Aplites and pegmatites are still more evolved than the northern granites. The stream sediment geochemistry (BGS, 1991) confirms this trend, and also shows As, Sb, Mo, Sn and W anomalies around the southwestern and northern contacts, although a little molybdenite near the south-west contact (Anderson, 1971) and the specular haematite in Henley's Quarry [NJ 663 170] represent the only known mineralisation associated with the body.

Samples from the Hill of Fare Granite were analysed by Walsworth-Bell and O'Brien. Walsworth-Bell's results indicate that the Hill of Fare Granite is more evolved than the plutons of the Crathes Suite, and belongs to the Cairngorm Suite. However, trace element data show that it is less evolved than the Bennachie pluton. The two intrusions show different evolutionary trends when Rb, Sr, Y and Nb are plotted against Zr (Figure 22). The Hill of Fare pluton has undergone much less differentiation than Bennachie; this is most easily seen in the case of Rb. The absence of highly evolved differentiates from Hill of Fare is noteworthy.

Geochemical analyses of the Ballater Granite are provided by Webb and Brown (1984), Harrison (1987) and O'Brien (1985). The geochemistry of the Ballater pluton is discussed in the memoir for Sheet 65E, currently in preparation.

Geochemical data on the Middleton Granite from Colman et al. (1989) show that the fine-grained granites and greisens are all enriched in Rb, Bi, Nb, Zn, U and Th, and depleted in Mg, Fe, Ti, Sr, Ba and P. They are geochemically similar to the more evolved specimens from the Bennachie pluton. Mo, As, Sb and W are only slightly enriched in the granitic rocks. They are concentrated in the quartz veins which cut the greisens, probably due to leaching of these elements by the fluids which greisened the granite and from which the veins were formed.

Chapter 8 Minor intrusions

Late-tectonic minor intrusions

Granitic intrusions into the late-tectonic basic masses

A suite of aplite, granite and pegmatite veins and sheets is confined to the late-tectonic basic and ultramafic rocks. Granite pegmatites cutting the late-tectonic Belhelvie intrusion were dated at 462 ± 5 Ma (Rb-Sr whole rock–mineral isochron) by van Breemen and Boyd (1972). A large sheet of granite pegmatite is well exposed in the west face of Pitscurry Quarry [NJ 7290 2672] (Plate 6). It is 20 m thick in its central portion, but thins northwards and southwards along the face. It gives off subvertical veins, 10 cm to 1 m thick, of granite and aplite. The chief minerals in the pegmatite are quartz, microcline and plagioclase, in crystals up to 10 cm in size. Books of muscovite and euhedral crystals of black tourmaline (schorl) are conspicuous, and beryl is also present in 2–3 cm pale green, cracked crystals (Gallagher, 1959). A 5 m-thick sill-like sheet of granite pegmatite also occurs in Pitmachie Quarry [NJ 666 284]. This dips at 15°, and maintains a roughly constant thickness. It contains schorl, but beryl is not recorded from this locality. Most of the other minor intrusions of this group in the district are sheets and dykes of aplite and granite, ranging in width from 0.3 m to at least 100 m. The aplites are pink, leucocratic rocks, grain size 0.5–1 mm, with muscovite almost as abundant as biotite. The granites lack the saccharoidal texture of the aplites and are generally coarser-grained (1–2 mm), but otherwise similar. A few quartz veins, up to 5 m wide, possibly related to the pegmatites, also cut the Insch intrusion.

Post-tectonic minor intrusions

The post-tectonic minor intrusions of the district are of several types. A few plugs of appinitic diorite occur in the Torphins area. Dykes and sheets of microdiorite occur widely throughout the district. These cross-cut the late-tectonic granites and the Crathes Suite, but predate the Cairngorm Suite. Dykes of felsite and felsic porphyry have a similar distribution. An aplite vein complex surrounds the Hill of Fare Granite, and aplite, pegmatite and quartz veins form a vein complex to the west of the Bennachie Granite, as well as occurring more sporadically within it and between it and the Middleton Granite.

Appinitic diorite

Three small rounded plug-like bodies occur in the Lumphanan–Torphins area. Several small exposures and numerous boulders of a coarse-grained (2–5 mm) dioritic rock occur 300 m north of Tillyneckle [NJ 610 014].

Euhedral hornblende crystals are abundant, and are enclosed by interstitial poikilitic andesine, and minor microcline and quartz. The circular form of the intrusion can be made out fairly well. Abundant angular blocks of a similar mafic diorite occur 600 m south of Glenmillan House [NJ 594 047]. A few blocks of appinitic diorite similar to the Tillyneckle occurrence occur in poorly exposed ground 1.2 km WSW of Learney House [NJ 624 042]. Another plug is postulated to occur here, although there is a possibility that the blocks could be erratics.

Lamprophyre

Only three lamprophyre dykes have been recognised in the district: on Queen's Hill [NJ 5261 0050]; 500 m south of Logie [NJ 505 188]; and on the north side of the Hill of Flinder [NJ 584 283], where a 1 m dyke lies along the line of an ENE-trending fault. None of these dykes can be traced for more than a few hundred metres. Owing to alteration, lamprophyres are difficult to distinguish from microdiorites and the more mafic porphyries, except for the lamprophyres' lack of feldspar phenocrysts. The euhedral biotite and hornblende phenocrysts are chloritised, and the groundmass is altered to a sericite-chlorite mass, in which the form of the original plagioclase laths is sometimes visible. The lamprophyres are believed to have originally been spessartites or minettes.

Microdiorite

Microdiorite dykes are moderately abundant within the district, and there are also a few low-angle sheets or sills. The abundance of microdiorite dykes in the larger quarries, e.g. Corrennie, Tom's Forest, indicates that poor exposure is probably concealing many dykes additional to those mapped. Microdiorites are only slightly less abundant than felsites and felsic porphyries. The dykes are typically less than 1 m thick, but may reach 5 m; sills are typically 1–5 m thick but may reach 10 m. Dyke margins are frequently irregular, and their similarity on opposite sides of a dyke indicates dilational opening. The larger dykes, over 0.5 m thick, frequently have chilled margins. Where microdiorite and felsite dykes occur together, the felsite in most cases cuts the microdiorite.

Where fresh, the microdiorites are dark grey to black rocks, with andesine (An₃₆ to An₃₀ zoned to c. An₂₀) phenocrysts up to 3 mm, but generally 1–1.5 mm long, set in a matrix with a grain size of 0.1–0.4 mm. The plagioclase phenocrysts show oscillatory zoning, but are in many cases broken. Hornblende and biotite phenocrysts are less abundant than plagioclase, and are frequently chloritised. The groundmass consists of laths of andesine plagioclase and subhedral crystals of biotite, together with minor hornblende, quartz and iron oxides.

Felsite and felsic porphyry

These are the most abundant dykes in the district. They cut all rocks except the Cairngorm Suite granites and associated veins, the Devonian sedimentary rocks and the late Carboniferous dykes. Grain size is variable, and correlates only in part with dyke width. The commonest trends are north to NNE and ENE to east. The traceable strike length of the dykes varies from a few metres to 3 km. A 25 in-wide dyke of felsic porphyry can be traced for 2 km from just above Upper Broomhill [NJ 609 080] to 500 m north-west of Denwell [NJ 625 094], and possibly continues as far as Green Hill [NJ 645 145], where several dykes parallel its 40° trend near Denwell. A felsite dyke has been traced for 3 km from Stot Hill [NJ 593 031] to near Leyton [NJ 599 006].

The felsites are pink to brick red, with a very fine-grained matrix of which quartz forms approximately one third. The relative proportion of plagioclase to potash feldspar is usually not determinable, due to the fine grain-size and sericitisation. Chloritised biotite and rare iron oxides are the only mafic minerals. Some felsites are non-porphyritic, but many are sparsely porphyritic, with phenocrysts of perthitic orthoclase up to 5 mm and rare corroded quartz phenocrysts or xenocrysts up to 3 mm. Phenocryst abundance rarely exceeds 15%.

Felsic porphyries are distinguished from porphyritic felsites by coarser groundmass grain-size (generally 0.20.5 mm). Groundmass textures are more variable than in felsites, and in many porphyries the groundmass has a patchy or uniformly granophyric texture. Euhedral perthitic orthoclase phenocrysts range up to 30 mm in size, and form up to 25% of the rock. Quartz phenocrysts are typically 3–5 mm, rounded and often corroded. Plagioclase phenocrysts (cores An_34–26 zoned to An_20–14) rarely exceed 5 mm; some show oscillatory zoning as in microdiorites, whilst others, enclosed in orthoclase phenocrysts, show arrested zoning. Biotite forms rare phenocrysts up to 2 mm long, but is ubiquitous in the groundmass, forming 3–5% of the rock. Chilled margins are well developed in the larger dykes, in places grading from a marginal felsite to a central porphyry.

Late Carboniferous

A suite of late Carboniferous quartz-dolerite dykes occurs abundantly in the Midland Valley of Scotland and northern England, where intrusion demonstrably postdates Duckmantian sedimentary rocks. K-Ar whole-rock dating gives an age of 290–295 Ma (Fitch et al., 1970). This suite is considerably less abundant north of the Highland Boundary Fault, but a few examples occur as far north as Peterhead. Two groups of quartz-dolerite dyke segments cross the Inverurie–Alford district, trending ENE, one extending from near Wheedlemont [NJ 476 258] to Sunside [NJ 576 288], and the other extending from Milltown of Kildrummy [NJ 467 162] to the Middleton Granite [NJ 729 221]. An isolated quartz-dolerite dyke was recorded from a gas pipeline trench between North Coldstream and South Coldstream [NJ 786 003], but its extent is unknown.

Due to a high level of reversed remanent magnetisation, the dykes show up as distinctive linear aeromagnetic anomalies, with the high to the north, which enables them to be traced across unexposed ground. The lines of the dykes interpreted from the Exploration Ventures Ltd aeromagnetic survey by Gallagher (1983) are shown in (Figure 23).

Petrographically, the quartz-dolerites are characterised by the ophitic clinopyroxene–plagioclase intergrowth which forms the groundmass. Ilmenite forms 2–5% of the rock, and there is a devitrified mesostasis associated with rare interstitial quartz. Rare labradorite (An₆₂) phenocrysts are the only ones present.

Rhynie dyke

This dyke extends from near the western boundary of the Rhynie outlier at Wheedlemont to the northern boundary of the district near Sunside. The same dyke can be traced intermittently via Rothienorman and Arnage to the east coast at [NK 132 418], near Boddam. Within the Afford district, it is exposed at Mains of Rhynie [NJ 490 263], Druminnor [NJ 508 264], Seggieden [NJ 540 276] and Sunside [NJ 576 288]. Aeromagnetic surveys supplemented in places by ground magnetic traverses have shown that the dyke is not continuously present at surface, but that eight sections of dyke 1–4 km long are separated by breaks of 0.2–2 km. In some places the pattern of dyke outcrops shows an en échelon pattern. The thickness of the dyke at outcrop varies considerably at different exposures, but is generally about 5 to 10 m; a thickness of 13 m was recorded by Read (1923) at Auchinbradie [NJ 621 300], in the Huntly district. The dyke usually has well-developed chilled margins, 0.5 to 1 m wide. For much of its length in the Alford district, the dyke follows the line, postulated as a fault, separating the Insch quartz-biotite-norites from the Kennethmont diorite.

Kildrummy–Bennachie–Middleton dykes

A series of parallel, discontinuous, sometimes en échelon dyke segments, similar to the Rhynie dyke, occurs between Kildrummy and Middleton, mostly known only from magnetic anomalies (Gallagher, 1983). The dykes show very clearly as a series of aeromagnetic anomalies running through the Howe of Alford, crossing the southern part of the Bennachie Granite, and passing through the Middleton Granite, where several quartz-dolerite dykes have been located during ground magnetic surveys, pitting and drilling. Further dykes occur in the Aberdeen district (Munro, 1986b), along a continuation of this trend, as far as the mouth of the River Ythan [NJ 999 280]. The dyke exposed at Milltown of Kildrummy [NJ 467 162] is the westernmost occurrence along this lineament. This dyke is at least 20 m thick, while those intersected by drilling at Upper Middleton (Colman et al., 1989) range from a few centimetres to 13 m in thickness. The variation of strike direction within the dyke swarm, and the occasional divergence from the main swarm direction, are typical of the swarm in the Aberdeen district as well as the Inverurie–Alford district.

Chapter 9 Devonian

Introduction

Devonian sedimentary rocks of continental Old Red Sandstone facies occur in the Rhynie and Towie outliers, which are partly fault-bounded. Minor intercalated volcanic rocks occur in the Rhynie Outlier. Exposure is poor, except where the rocks have been worked for building stone. The rocks were first described in detail by Geikie (1878), who set up a stratigraphical succession which has survived with only slight modification, and is now formalised (see Geological Sequence–inside front cover). The Rhynie Group includes all of the Devonian rocks of the district and their extension into the Huntly district (Sheet 86W), together with the Devonian rocks of the Cabrach outlier in Sheet 75E, 10 km west of Rhynie. The present account includes not only information obtained during the original survey and current BGS resurvey, but also the results of sedimentological studies by Archer (1978) and geophysical work and drilling in the period 1988–1991 (W A Ashcroft, written communicaton, 1990; Rice and Trewin, 1988; Trewin and Rice, 1992).

Since its discovery (Mackie, 1914), the Rhynie Chert has been the principal focus of attention in the Rhynie outlier. At first this interest was palaeontological, but latterly the hot spring environment in which the chert was laid down has been seen as being favourable to metalliferous mineralisation (Rice and Trewin, 1988). Recent drilling has enabled the stratigraphy of the chert-bearing succession to be worked out and compared with that of the succession in the rest of the outlier (Trewin, 1994). Due to the unusual nature of the Rhynie Chert environment, the flora and fauna of the chert are for the most part of little use for stratigraphical correlation. However, the spores indicate a Lower Devonian (Pragian) age, and the Rhynie Group as a whole is considered to be of Lochkovian to Pragian age. Turner (quoted in Lyon and Edwards, 1991, p.331) has obtained a radiometric age of 395 ± 6 Ma for the Rhynie Chart.

Form of the outliers

The Rhynie outlier extends from Glaschul Hill, Kildrummy [NJ 458 148] to north of Gartly [NJ 522 324], but only a very thin development is present to the north of the Glen of Cults Fault (Figure 24)a. The main part of the outlier is divided into roughly equal parts by the Auchinleith Fault, and the stratigraphy and structure differ markedly on either side of the fault. Between the Auchinleith and Glen of Cults faults, the outlier has the form of a gentle syncline, with a partially faulted western margin and a number of small NW–SE cross faults

(Figure 24)b. The stratigraphical development is asymmetric, because the Quarry Hill Sandstone Formation is developed only to the south-east of the synclinal axis; however, it is likely that the Dryden Flags Formation is, at least in part, the lateral equivalent of the Quarry Hill Formation. The north-west margin of the outlier from Wheedlemont [NJ 475 261] to Newnoth [NJ 517 303] is an unconformity in some places, but it is locally faulted. In view of the disparate thickness of strata on the north-west and south-east limbs of the syncline, it is likely that the north-western unconformity oversteps a sizeable penecontemporaneous fault. Ashcroft's geophysical interpretation suggests that the eastern boundary of the outlier from near Mains of Cairndard [NJ 504 256] to near Bogend [NJ 524 273] is also faulted, though the displacement is generally small. To the south-east of the synclinal axis, as on Quarry Hill and in the Kearn Burn [NJ 516 278], the strata dip west to north-west at 20°–40°, but the angle of dip decreases to 8°–15° in the Dryden–Den of Wheedlemont [NJ 480 262] area. To the west of the synclinal axis, near the Rhynie hot spring locality, dips are mostly 30°–45° to the south-east.

To the south of the Auchinleith fault, the basin is a half-graben with a fault-bounded western margin (Figure 24)c. Strata dip westwards at 30°–35°, decreasing to 25° in the extreme west of the outlier, as at Broadley [NJ 454 184]. In the southern part of the outlier, a thick development of the Tillybrachty Sandstone Formation overlies the Carlinden and Corbie's Tongue formations. Only the lower part of the overlying Quarry Hill Formation is seen. To the north of the Auchinleith Fault, the Corbie's Tongue and Carlinden formations are not recognised. Occasional thin developments of conglomerate and shale occur near the base of the Tillybrachty Formation in its eastern outcrop, but they are too thin and impersistent to be mapped. The Quarry Hill Formation is developed only southwards from near Bogend [NJ 523 282]. To the north of Bogend, and on the western limb of the syncline, a thin development of Tillybrachty Formation passes up directly into the Dryden Formation. Volcanic rocks in the outlier are confined to the Tillybrachty Formation. A thin development of lava occurs in several places, and tuffaceous deposits have been found in the vicinity of the Rhynie Chert locality.

The small Towie Outlier is at present unexposed. The western boundary is believed to be faulted. The northern continuation of this fault forms a topographic feature between the Glenkindie Arms Hotel [NJ 442 139] and the Den of Kildrummy [NJ 446 147]. The extent of the outlier is marked by an area of red soil. Equivalents of the Corbie's Tongue, Carlinden and Tillybrachty formations were recognised during the primary survey.

Stratigraphy

Corbie's Tongue Conglomerate Formation

Wilson and Hinxman (1890) recorded several exposures of the basal conglomerate: Milltown of Kildrummy [NJ 471 166], Linthaugh Burn [NJ 4791 2062], Carlinden Burn [NJ 4845 2264], Slughallan Burn [NJ 4945 2433]. The rock is described as a very compact breccia or conglomerate, with small, sub-angular fragments enclosed in a hard, commonly calcareous, matrix. None of these exposures currently exists, but Archer (1978, 114) confirmed the Linthaugh and Carlin-den Burn exposures by angering. An exposure of conglomerate recorded on old field maps on the River Don near Towie [NJ 4392 1302] has been concreted over. The only present-day exposures attributed (doubtfully) to this formation are the conglomerates exposed on either bank of the Corbie's Tongue gully [NJ 4923 2433] (Plate 7a). Here, a 3–4 m succession contains 0.5 m beds of coarse conglomerate interbedded with 1 m beds of finer conglomerate and pebbly sandstone. The clasts consist of aplite and vein quartz, with minor diorite and gabbro. The matrix of the conglomerate is coarse sand, which does not wholly fill the space between the cobbles, and the conglomerate appears to be clast supported. The basal unconformity is not seen, so the stratigraphical position with respect to the shale exposures described by previous workers is uncertain. With the dip of the strata (280–400 to the west) being only slightly steeper than the slope of the Bullied hillside above the Water of Bogie, the total thickness of the Formation is difficult to estimate, but probably does not exceed 10 m.

The summit of Glaschul Hill [NJ 4585 1485] is littered with rounded cobbles of quartzite and aplite, with minor pegmatite and migmatitic gneiss. Many of the cobbles have strongly reddened skins. The roadbed of tracks in the forest on the northern slopes of the hill consists of red mud with abundant rounded quartzite and aplite cobbles, indicating that there is up to 20 m of conglomerate on the northern slopes of Glaschul Hill. Dalradian rocks at 'The Ship', in the Den of Kildrummy [NJ 4490 1513] are strongly reddened. This may indicate either a northward continuation of the fault bounding the Towle outlier or a small patch of deep weathering underlying Devonian sedimentary rocks, now eroded.

Carlinden Shale Formation

The only exposures of this formation discovered during the present resurvey are in the Carlinden Burn [NJ 4823 2276]–[NJ 4830 2275], where dark red mudstones with paler silty layers are intermittently exposed. The total thickness of the formation in this section is estimated at 50 m. An exposure in the Corbie's Tongue near [NJ 493 243] recorded by Wilson and Hinxman (1890) is described as 'red, greenish and purple sandy shales intercalated with calcareous sandstone, and layers of flattened oval concretions of finely crystalline grey limestone'. A record of fish remains by Malcolmson (1859, p.350) is attributed by Wilson and Hinxman (1890, p.28) to this locality. Archer (1978) made small excavations in this area, exposing red and purple sandy shale intercalated with unlaminated red-brown calcareous nodules. These calcareous nodules were interpreted as calcrete horizons, and thus terrestrial deposits, causing Archer (1978, pp.115–116) to query Malcolmson's record, as it would be the only known occurrence of lacustrine deposits in the Lower Old Red Sandstone of the Grampian Highlands. A water well at Lumsden [NJ 476 223] (Robins, 1990) encountered mudtstone at 49–50 m depth under red sandstones of the Tillybrachty Formation.

Tillybrachty Sandstone Formation

At its base, this formation is characterised by soft reddish sandstone with thin, impersistent conglomeratic beds of pebble to cobble grade; the frequency of pebbly conglomerate reduces rapidly up the succession, and cross-bedding becomes dominant. Siltstone films are present at the top of some upward-fining units, and mud clasts occur rarely on foreset laminae. The lower part of the formation is well exposed at the Craigs of Tillybrachty [NJ 493 246], but due to the softness of the sandstone the rock is partly obscured by sand washed out from the top part of the exposure.

The upper part of the formation is exposed in Kildrummy Castle Gardens [NJ 455 165], where the lithologies are transitional to those of the overlying Quarry Hill Formation. Approximately 20 m of succession are exposed in the face of an old quarry (now an ornamental garden). Four lithologies are recognised: cross-stratified sandstone; massive sandstone; thinly-bedded fine sandstone and siltstone; thin pebble- to cobble-conglomerate. These occur in thick, indivisible units, comparable with units lower in the formation. Mudflakes and rootlet horizons are absent. The occurrence of both conglomerates and silty sandstone layers interrupting the massive sandstone units implies that this exposure is near the boundary between the Tillybrachty and Quarry Hill formations. These are the only exposures where a significant thickness of the succession is exposed. Other exposures are patchy and tend to be very soft and weathered.

Exposures occur sporadically in the Burn of Craig [NJ 4776 2447]–[NJ 4724 2489]; all except the most westerly are assigned to the Tillybrachty Formation. The rocks are red sandstones with a few thin paler beds, layered on a millimetre to centimetre scale. Some leaching has occurred along joint planes. The sandstone is flaggy in places, and there is at least 1.5 m of fissile red finely bedded siltstone at a locality 200 m east of the western boundary fault [NJ 4739 2470]. Similar siltstones with pale laminae occur beside the road 400 m south of Auchenleith [NJ 4643 2305]. Whitish pebbly sandstone, cross-bedded in places, occurs in Reid's Burn [NJ 486 235]. Red and white fine-grained sandstones, bedded on centimetre scale, with dark silty layers, occur in the Carlinden Burn at [NJ 4813 2285]. Possible calcareous concretions occur along the bedding planes here. Pink, weathered sandstone occurs in the stream beside The Den, Kildrummy [NJ 4585 1593]; exposures in the nearby road consist of conglomerate with cobbles up to 0.1 m in a fawn sandy matrix. Exposures near Bareflat [NJ 5022 2580]; [NJ 5026 2587] consist of highly weathered soft medium- to coarse-grained red sandstone with pebbly beds up to 0.1 m thick. A water borehole at Lumsden [NJ 476 223] penetrated 49 m of hard red sandstone, well sorted but grading from fine to medium, before passing into shales of the Carlinden Formation (Robins, 1990).

The basal beds along the north-west boundary of the outlier between Wheedlemont and Newnoth are lithic sandstones with conglomeratic beds. Exposures are confined to the area between Nether Ord and Windyfield, where the rock has been silicified. Up to 35 m of sandstone have been proved below the lavas in boreholes (Trewin and Rice, 1992). Most of the clasts are locally derived quartz-biotite-norite of the Boganclogh Intrusion. Sorting in the sandstones is moderate to poor. The first 40 m of sedimentary rocks overlying the lavas exposed near the Burn of Easaiche [NJ 492 277] are described as tuffaceous sandstones and are assigned to the Tillybrachty Formation. They are fine-grained sandstones and shales, laminated and bedded on a millimetre to centimetre scale and containing variable amounts of volcanic debris. The volcanic clasts are highly rounded and vesicular, occurring as oversize grains in fine sandstones. These are interpreted as water-transported tuffs.

The thickness of the Tillybrachty Formation increases steadily south-westwards from 100 m to 400 m along the south-eastern crop in the fault block between the Glen of Cults Fault and the Auchinleith Fault, whereas the thickness of the Tillybrachty Formation in the north-west crop out between Wheedlemont and Newnoth remains more constant at 60–100 m. On the south side of the Auchenleith Fault it is approximately 1400 m, and decreases gradually southwards to about 800 m between the Den of Kildrummy and Kildrummy Castle Hotel.

Volcanic rocks

The volcanic rocks in the Tillybrachty Formation are only recorded to the north of the Auchinleith Fault. Mostly they consist of lava, with pyroclastic rocks above the lava in the Rhynie area. The lavas vary from fresh to moderately altered, except near the Rhynie hot spring system, where they are strongly altered. Brown to purple, slightly amygdaloidal lava is exposed in a pit near Contlach [NJ 4740 2428]. This occurrence is close to the boundary between the Tillybrachty and Quarry Hill Formations. Moderately altered, fine-grained lava with 1–2 mm feldspar phenocrysts is exposed 300 m and 400 m north-east of Boghead [NJ 5237 2748]; [NJ 5243 2753]. The lava of the latter exposure is highly vesicular, with amygdales of calcite and possibly zeolites. The amygdales are flattened in the plane of extrusion. In this area, the lava appears to be in the middle of the Tillybrachty Formation, but the formation is much thinner than near Contlach, so the two flows could be roughly contemporaneous. Similar vesicular lava has been recorded from the Glen of Cults and near Gartly in the Huntly district (Sheet 86) (Read, 1923, pp.179–182). The Glen of Cults occurrence, which is no longer exposed, is in a similar stratigraphical position to that at Boghead, while to the north of the Glen of Cults fault only the lavas and a thin underlying development of Tillybrachty Formation sedimentary rocks are preserved. Lava occurs on a small ridge above the Easaiche Burn at [NJ 4925 2785] (Mackie, 1914; Trewin and Rice, 1992) (Figure 25). The flow is up to 20 m thick, and can be traced along strike for 350 m.

Petrography

The lava exposed at Contlach is an ophitic pyroxeneandesite with 1–2 mm-long plagioclase microphenocrysts (S19480). The phenocrysts are well zoned, with cores of approximately An₄₈. A corroded core is surrounded by a zone from which specks of dark iron oxide has exsolved, and this zone is surrounded in turn by clear plagioclase, which in places shows oscillatory zoning. The ground-mass plagioclase forms 0.1–0.5 mm crystals zoned from An₄₅ to An₂₅. Subhedral to euhedral augite forms crystals 0.3–0.5 mm long, but much of the ferromagnesian material (originally augite or hornblende) is altered to actinolite. Rare quartz, 2–5% of the rock, forms 0.05 mm crystals interstitial to plagioclase. Widely scattered euhedral magnetite crystals form about 5% of the rock. Fresh specimens from the Gartly area in the Huntly district (Sheet 86W) range from hornblende-andesite (S19470) to olivine-basalt, (S19871)–(S19872) though the latter may be from a sill (Read, 1923, pp.181–182). The Burn of Easaiche andesite is strongly altered and crystals of K-feldspar have replaced some of the primary minerals while the mafic minerals are completely chloritised, leading Mackie (1914) and Horne et al. (1917) to call the rock a rhyolite.

Quarry Hill Sandstone Formation

This formation is distinguished from the underlying Tillybrachty Sandstone Formation by the common occurrence of siltstone interbeds between the massive sandstone units; these are rare in the Tillybrachty Formation. Mudclasts and rootlets are also common. Pebbly layers are generally absent, though a few occur in the basal beds of the formation, particularly in the Den of Craig section. The Quarry Hill Sandstone is generally paler pink, better cemented and harder than the Tillybrachty Sandstone, and has been preferred as a building stone.

The total thickness of this formation is 440 m on Quarry Hill, but it thins rapidly northwards and disappears to the north of Bogend [NJ 523 282]; there is probably a lateral transition to the Dryden Flags Formation. The formation occurs only on the south-east limb of the syncline which is developed between the Auchinleith and Glen of Cults faults; this implies that it thins from 400 m to nil between Contlach [NJ 470 243] and Wheedlemont [NJ 475 262], a distance of only 2 km. To the south of the Auchinleith Fault, the maximum thickness of the formation is 400 m, but the top is not seen. Good exposures of the formation are available in the disused stone quarries near Quarryfield [NJ 454 178] and on Quarry Hill [NJ 487 254], while small exposures occur in streams and disused pits.

At Quarryfield, two quarries have exposures of white to pale yellow sugary coarse-grained sandstone, with 12 mm layers showing a concentration of dark minerals. A few red shaly bands up to 1 cm thick occur only in the southern quarry. The sandstone contains massive and cross-bedded units. Mudflakes on foresets are rare. Exposures of similar whitish to pink sandstone occur in the Mossat Burn at Wester Clova [NJ 4518 1938], in the Murchie Burn 400 m east of Mid-Clova [NJ 4528 2091], and in a roadside cutting south of the Burn of Corchinnan [NJ 4502 2275].

The sandstone exposed in the Burn of Craig immediately east of the western boundary fault [NJ 4724 2489] is paler and more massive than downstream, with thin argillaceous partings, and is assigned to the Quarry Hill Formation. Beside the Rhynie to Bareflat road [NJ 5015 2666], red medium-grained rather flaggy sandstone, with slight ripple marks, is exposed. A small quarry on the ridge to the west of the kennels at Druminnor Home Farm [NJ 508 264] exposes finely bedded dark red shales and thin beds of flaggy sandstone, cut by a quartzdolerite dyke. This exposure, although low in the formation, is more reminiscent of Dryden Flags lithologies.

Water borehole Rhynie No.2 [NJ 499 265], total depth 54.5 m, passed through interbedded grey, brown and red sandstone and mudstone (Robins, 1990).

At Quarry Hill, several beds have been extensively worked in a discontinuous series of quarries which provide by far the best exposures of the upper part of the formation; these are assumed to be typical (Plate 7)b. Archer (1978, pp.121–125) measured sections of 20.88 m in the lower quarry [NJ 4865 2524], 7.36 m in the middle quarry [NJ 4875 2533], and 10.08 m and 10.38 m in the upper quarry [NJ 4880 2541]. Ripples, load casts, cross-bedding and graded bedding are all well developed. Exposures in the lower quarry consist of large units of flat-bedded and massive sandstone, but with significant siltstone and mudflake conglomerate (Figure 26). The middle quarry exposures are also generally of massive and flat-bedded sandstone, but cross-stratification is absent, while mudflake conglomerates, occurring as large channel infills, are abundant. Rootlet horizons are a common feature in the upper portions of sandstone units overlain by siltstone. The upper quarry contains abundant massive sandstone but less flat-bedded sandstone and more cross-stratified sandstone than the lower quarry. Mudflakes occur on bedding planes, but mud-flake conglomerates are not developed. Rootlet horizons, burrows and locomotion trails all occur.

Dryden Flags Formation

This formation occurs only between the Auchinleith and Glen of Cults faults, where it occupies the core of an asymmetric syncline. The section in the Kearn Burn from Castlehill [NJ 5178 2818] for 800 m to a point 1.1 km north of Bridge of Kearn [NJ 5158 2752] , shows the Dryden Formation overlying a very attenuated Quarry Hill Formation, and dipping north-west at 20°–36°. The occurrences at Mains of Rhynie [NJ 4904 2626], Dryden [NJ 4822 2626] and the Den of Wheedlemont [NJ 4798 2624], where dips are mostly to the WNW to west, indicate that the Dryden Formation overlies the Quarry Hill Formation. Hence it is likely that there is a lateral transition from the Quarry Hill Formation to the Dryden Formation to the north-west and north-east of Quarry Hill, rather than the Dryden Formation underlying the Quarry Hill Formation as suggested by Trewin and Rice (1992).

The formation has a maximum thickness of 300 m along the Kearn Burn section, decreasing to about 200 m, both to the south-west in the Den of Wheedlemont and to the north-east along the Glen of Cults Fault. However, the top of the formation is nowhere seen, so the succession is incomplete.

Typical lithologies are fawn to greenish grey flaggy siltstones with rare sandstone layers. Poorly preserved plant remains are quite common in the grey mudstone and poorly preserved spores have been extracted from the mudstones and micaceous siltstones. Exposures occur at Mains of Rhynie, where a sandstone bed is cut by a quartz-dolerite dyke but little induration has occurred, at Dryden and at the Den of Wheedlemont. The rocks in the Kearn Burn stream section are siltstones and fine- to medium-grained micaceous sandstones, in which grey and fawn colours dominate. A pebble conglomerate occurs at the top of one of the exposures [NJ 5154 2790]. Rhynie No.1 water borehole [NJ 502 276], penetrated hard, dark grey to brown shaly mudstone, from rockhead to the base of the hole at 53 m.

Drilling at Windyfield [NJ 4950 2795] has proved over 30 m of 'shale and thin sandstone' (Trewin and Rice, 1992), assigned to the Dryden Formation. This succession consists dominantly of grey-green micaceous shaly mudtstone with millimetre to centimetre scale sandstone and siltstone laminae, but also includes some 10 m of chertbearing strata which consitute the Rhynie Chert Member. Rip-up clasts and desiccation cracks occur and U-shaped burrows disrupt the siltstone and sandstone beds.

Rhyme Chert Member

This member is restricted to the area around Windyfield to the north-west of Rhynie (Figure 25) where it occurs as boulders and float in the field, and has been incorporated into dry stone walls. This distinctive lithology was first noticed by Mackie (1914) and was recorded in pits 1 to 3 of Horne et al. (1917). More recently it has been proved in borehole 19c (Trewin and Rice, 1992, fig. 7) in which it is largely confined to a 10 m section close to the base of the Dryden Formation. The blue-black chert occurs as beds up to 0.65 in thick interbedded in a sequence dominated by grey cherty and carbonaceous and micaceous sandstone. The chert shows laminated, brecciated, vuggy and geopetal textures typical of siliceous sinters. Most of the cherts are fossiliferous, containing the common genera Rhynia and Aglaaphyton as well as rarer forms (Plate 8). At depth, fractures and vugs are filled with calcite and minor barite. The plants in the cherts are generally preserved in positions of growth, and were deposited on a sandy substrate. The chert is extremely impersistent laterally, dying out between borehole 19c and borehole 3150 m to the north-west, where it is replaced by tuffaceous sandstones.

Loose blocks of brownish chert are also concentrated near Windyfield (eastern house) [NJ 498 282]. It is associated with thin-bedded cherty sandstone and white sticky clay (probably hydrothermally altered shale).

Sedimentology and depositional environment

Nearly all of the exposures good enough for serious sedimentological study come from the upper part of the Tillybrachty Sandstone Formation and from the Quarry Hill Sandstone Formation (Archer, 1978). These are supplemented by pitting and drilling in the Windyfield area (Horne et al., 1917; Trewin and Rice, 1992), but this area is atypical due to the presence of the Rhynie hot spring system.

The basal conglomerate is an alluvial fan deposit, laid down by sheetflood processes. It is cemented with calcareous material and indicates a semi-arid terrestrial environment. The Carlinden Shale Formation probably represents a relatively stable piedmont floodplain, occasionally inundated by ephemeral sheetfloods, but with a low accretion rate which allowed the formation of red-brown calcrete-type nodules.

The lower part of the Tillybrachty Formation is marked by a predominance of coarse-grained sediments, frequent erosion surfaces and a lack of distinct stratification. Deposition probably occurred in a broad valley in an intermontane basin where small, possibly ephemeral channels crossed a distal alluvial fan. The coarse sediments accumulated in interchannel areas during sheet-flood episodes when the fan surface was submerged. A high-energy environment was maintained during deposition of the upper parts of the formation, although the frequency of ephemeral sheet flooding decreased. The increasing abundance of cross-stratified sandstone units indicates that large quantities of sand were transported as small subaqueous dunes. This, together with evidence of channelling, indicates that a shallow, braided, fluvial system was being established. The variation in thickness of the formation indicates that the valley was becoming progressively wider as deposition of the rocks overstepped the original valley margins after the lower part had been filled by coarser debris. The Tillybrachty Formation rocks along the north-western crop of the Rhynie Outlier from Wheedlemont to Newnoth and those along the south-eastern crop to the north of Bogend are thin and contain much more coarse lithic debris than the rocks from the Craigs of Tillybrachty southwards. This might have been partly due to the availability of volcanic debris from the Bogend, Glen of Cults and Windyfield areas. Although it is not certain whether the lava at Contlach was erupted at the same time as the lavas at these three localities, this would be possible if Contlach was near the centre of the basin of deposition and the other localities were near the edges. Three cross-stratified units in the Tillybrachty Formation indicate sediment derivation from the south-east.

The depositional environment of the Quarry Hill Sandstone Formation represents an evolution of the conditions developing during the deposition of the Tillybrachty Formation (Figure 26). The lowest deposits are dominated by thick, massive sandstones, with rare cross-stratified sandstones beds and occasional fine sandstone and thin conglomerates; only large-scale channelling is apparent. In the middle part of the formation, distinct persistent siltstone beds with rootlet horizons and burrows are common. In the upper part there is a return to massive sandstones, albeit thinner bedded than in the lower part of the formation. Archer (1978) postulates that the channel ceased to be braided, except possibly in its central portions, accounting for the domination of massive sandstone beds. The plant stems recorded by Murchison (1859) are probably derived from plants growing on the floodplain which were washed away by floods. If the eurypterid remains (see below) come from this formation, they may reflect the animal's movement up a river system from more permanent water to the north. Archer (1978) recorded channel axes, symmetrical and asymmetric ripple-marks, cross-stratification, parting lineation, drag or tool marks, mudclast imbrication and mudcrack orientation from rocks of the Quarry Hill Formation, but could only conclude that sediment transport was from a generally southern direction, towards the Orcadian Basin. (Plate 7)b illustrates some of the sole markings visible on bedding planes at the base of sandstone beds within the formation. The record of Ptetygotus sp. indicates that the basin of deposition was not a basin of internal drainage; this view is supported by the absence of evidence of desiccation.

The Dryden Flags Formation is dominated by siltstones and thin sandstones. The siltstones are reddish brown to grey, micaceous and generally internally massive, with occasional plant fragments on bedding surfaces. The thin sandstones are mostly of fine sand grade, and generally pale grey to buff and highly micaceous. Scouring occurs rarely at the base of ripple-laminated units. The sequence shows no evidence of desiccation or the development of burrows or rootlet horizons. Archer considered that the sediments were deposited in a flood-plain environment, accumulation being predominantly by vertical accretion of low-energy bedforms. Overbank flooding of fluvial channels provides a likely mechanism of sedimentation. If, as now seems likely, the Dryden Formation is partly the lateral equivalent of the Quarry Hill Formation, it implies that the northern part of the Rhynie outlier was downstream from the Quarry Hill area, and received considerably less, and finer-grained, material during flooding episodes.

The depositional environment of the Rhynie Chert Member is discussed by Trewin and Rice (1992) and Trewin (1994). The sandstones, siltstones and cherts contain abundant organic material, and the plant material in the chert is often preserved in a growth position. In most cases, the plants were rooted in fine-grained sand. The source of the silica in the cherts was hydro-thermal, and Kidston and Lang (1921b) interpreted the deposit as a peat bog which had repeatedly been flooded by silica-rich water from a hot spring, which had become colloidal, and finally solid, on cooling. This environment was probably located towards the edge of a floodplain, where the silt and fine sand was deposited during occasional floods on a thin sequence dominated by volcanic rocks and lithic sandstone. The Rhynie Chert Member is overlain by typical Dryden Flags Formation lithologies.

Palaeontology

Apart from the Rhynie area, there are few fossil records from the Rhynie Outlier, and none from the Towie Outlier. A fragment of eurypterid (Pterygotus sp.), preserved in sandstone which may belong to the Quarry Hill Formation, was obtained from Quarryfield, Kildrummy [NJ 4545 1782] some time in the 19th century (GSE 1177). Crustacean or annelid tracks have been recorded from the same locality. Plant stems are recorded from the Quarry Hill Sandstone Formation in the quarries on Quarry Hill by Murchison (1859, p.432; Hinxman, 1888, p.421). Read (1923, p.180) records fragmentary plants including cf. Pachytheca from shales of the Tillybrachty Formation immediately overlying the 0.6 m basal conglomerate in the Glen of Cults (Sheet 86W).

The discovery of the Rhynie chert by Mackie (1914) and the subsequent pitting by Horne and et al. (1917) yielded material which has been the subject of study ever since. The unusual depositional conditions preserved not only the most completely preserved specimens of early land plants, but also a selection of arthropods. The original pitting has been complemented by collection from loose blocks, by pitting by A G Lyon in the 1960s and 1970s, and by drilling (Rice and Trewin, 1988; Trewin and Rice, 1992). Trewin (1994) briefly lists the flora and fauna of the Rhynie Chert and comments on the status of some of the taxa and on the floral and faunal associations. Tasch (1957) attempted to assess the palaeoecology of the Rhynie biota. Chaloner and Macdonald (1980) give a clear, popular, account of the Rhynie flora and its significance in the evolution of land plants. The assemblage of animals preserved in the Rhyriie chert, and their relations with each other and with the plants, are discussed by Rolfe (1980).

Plants from Rhynie

Kidston and Lang (1917; 1920a; b; 1921a; b) described Rhynia gwynne-vaughani, R. major, Horneophyton (Hornea) lignieri and Asteroxylon mackiei, which are among the earliest known vascular plants. Rhynia and Horneophyton are simple branching axes with no differentiation between root and stem, and are assigned to the Rhyniophyta, a group which may be ancestral to the ferns. Asteroxylon is a more complex plant, with veinless 'leaves' covering the stems, and is related to the lycopods (club-mosses). Sporangia, originally attributed to Asteroxylon, are now known to belong to Nothia, another rhyniophyte (El Saadawy and Lacey, 1979). Rhynia major has been renamed Aglaophyton major by Edwards (1986), who has shown that it lacks true xylem in its axes. Its affinity is in doubt, but it shares more features with the bryophytes (mosses) than with any other group. Lyon and Edwards (1991) have recently recorded Trichopherophyton, a zosterophyll. These plants are all sporophytes, diploid plants which disperse airborne spores. The haploid spores, on germination, produced game tophytes, which release waterborne gametes, which then fuse to give rise to the sporophyte. For many years, the gametophyte phase of the Rhynie plants was unknown. Edwards (1980) confirmed the sporophytic status of Rhynia gorynne-vaughani, while Remy and Remy (1980a; b) described Lyonophyton rhyniensis, a vascular plant of similar size to Rhynia, believed to be the gametophyte corresponding to one of the sporophytic plants.

The spores Retusotriletes and Apiculiretusispora, with rare spores of Emphanisporites, occur in other sedimentary rocks of the Dryden Flags Formation (Richardson, 1967). They indicate a Pragian (early Devonian) age for the formation. These spores are similar to ones found in the sporangia of Rhynia, but the poor preservation of the spores, some of which were already germinating, makes exact correlation with spore genera impossible.

Blue-green algae have been identified in the chert (Croft and George, 1959), and an ascomycete fungus, Mycokidstonia, has also been recognised (Pons and Locquin, 1981). The problematical green alga Nematophyton taiti was described by Kidston and Lang (1921b), and the related Nematoplexus rhyniensis by Lyon (1962). Several additional green algae were described by Edwards and Lyon (1983). The Rhynia, Horneophyton and Asteroxylon flora shows signs of partial decomposition, with many fungal hyphae of Palaeomyces sp. and Mycokidstonia. The plants also have lesions on their axes, possibly caused by sap-sucking animals; Rhyniella is likely to have been a sap-feeder.

Arthropods from Rhynie

The animals discovered so far in the Rhynie Chert are all arthropods. They comprise the crustacean Lepidocaris (Scourfield, 1926), the mite Protocarus, the trigonotarbid arachnid Palaeocharinus (Hirst, 1923, Shear et al., 1987) the eurypterid Heterocrania (Hirst and Maulik, 1926), at least one insect, the collembolid Rhyniella, and the possible insect remains Rhyniognatha. Rhyniella is very similar to modern springtails, wingless insects with powerful rear limbs which enable them to jump from place to place (Scourfield, 1940; Whalley and Jarzembowski, 1981). The mite may have fed on plant material, while the insects are likely to have been sap-suckers. The trigonotarbids are the largest of the arthropods, and are believed to have been carnivores which fed on the other, smaller arthropods (Rolfe, 1980). The form Palaeocteniza (Hirst, 1923), originally classified as a spider, is now believed to be a moult of a juvenile trigonotarbid (Selden et al., 1991).

Chapter 10 Post-Caledonian faulting

Throughout the district there is evidence that the youngest post-tectonic granite intrusions have been affected by faulting, which was in most cases associated with minor intrusions probably cogenetic with the plutonic rocks. In addition, the Devonian sedimentary rocks are affected by faulting along trends which in many cases are parallel to the faults affecting the post-tectonic intrusions. The age of the Devonian sedimentary rocks (Lochkovian to Pragian) is 408–390 Ma (Harland et al., 1990), very similar to that of the youngest post-tectonic intrusions in the district (Bennachie Granite 400 ± 4 Ma, Darbyshire and Beer, 1988). It is therefore reasonable to regard the faulting affecting the Devonian rocks and that affecting the late granites as a single episode. The principal post-Caledonian faults in the district are shown in (Figure 23.)

Fault pattern in the Devonian outliers

Three dominant fault trends affect the Devonian rocks (Figure 24)a. The most important one is north to NNE. The major fault which forms the western boundary of the southern part of the Rhynie outlier runs from Glaschul Hill [NJ 453 150] to the Burn of Corchinnan [NJ 450 228]. It has a downthrow to the east of at least 1500 m (Figure 24)c. It is for the most part concave on the downthrow side, but near Mid Clova [NJ 4529 2091] it is convex on the downthrow side. Exposure is poor, but brecciation of rocks within 20–30 m of the fault is visible near Culsh [NJ 452 174] and near Mid Clova. A similarly arcuate fault bounds the Towie outlier on the west. The valley which runs from the Glenkindie Arms Hotel [NJ 442 139] to near Goryhill [NJ 445 146] is probably a northward continuation of this fault. The NNE-trending fault which bounds the northern part of the Rhynie outlier between south of Contlach [NJ 470 237] and the eastern foot of Cnoc Cailliche [NJ 474 262] is almost straight. In the Den of Craig [NJ 473 247] it is a sharp break, with slight induration of sandstone for about 10 m east of the fault, and uralitisation of quartz-biotite-norite for about 50 m to the west of the fault. On the eastern slopes of Cnoc Cailliche, the fault is marked by a 50 m-wide belt of brecciated granite lying between serpentinite to the west and Dryden Flags Formation rocks to the east. A NNE-trending fault lies close to the eastern margin of the southern portion of the Rhynie outlier in the Lumsden area (W A Ashcroft, written communication, 1990). It accounts for the absence of the lowest part of the succession to the northeast of North Deskie [NJ 476 213]. A downthrow to the west of about 150 m is postulated.

The second important direction of early Devonian faulting is ENE. In particular, it is the orientation of the major Auchinleith Fault across the Rhynie outlier (Figure 24)a. This fault has a component of downthrow to the south, but the amount of throw increases rapidly WSW, and, in addition, there is a marked change in the thickness of the Tillybrachty Sandstone Formation on either side of the fault, from about 400 m north of the fault to 1400 m south of the fault. Hence it is likely that the fault was active during the deposition of the Rhynie Group sediments. There is probably no more than 1 km of dextral movement along the fault, judged by the displacement of the Devonian sedimentary rocks at the eastern side of the outlier; supporting this view is the continuity southwards of the aeromagnetic anomalies caused by the serpentinite of the Hill of Towanreef [NJ 457 242] under the Devonian rocks near Govals [NJ 476 231], though at a lower amplitude due to the increased cover thickness. However, this raises questions as to why there is a large difference in the position of the western boundary fault across the Auchinleith fault. A smaller ENE-trending fault occurs at Kildrummy [NJ 450 168]–[NJ 475 177]. The eastern boundary of the Rhynie outlier from near Mains of Cairndard [NJ 503 255] to Boghead [NJ 525 274] is marked by an ENE- to NE-trending fault, which has a small downthrow to the NW.

The third principal fault direction is north-west. Cross-faulting in this direction occurs in the northern part of the Rhynie outlier where it displaces the above NE-trending fault, and appears to be the latest fault episode. Throws along these faults are small. In the Windyfield [NJ 495 280] area, one NW-trending fault lies about 300 m north-east of the main Rhynie Chert occurrence, but there is no proven connection between the faulting and the hot spring system.

Faults affecting post-tectonic intrusions

The principal fault direction affecting the Bennachie granite and neighbouring rocks is north to NNE (Figure 23). Fractures with this trend cut the main plutonic phase, and have acted as conduits for later fluids. Both the eastern and western margins of the intrusion are faulted, but the granite continues at a shallow depth to the east and west (McGregor and Wilson, 1967). The only exposure of the faulting along the western contact (Michie, 1968) has been covered over. The eastern marginal fault zone is occupied by an intrusive breccia which varies in width from 20 m at Mount Jane [NJ 684 170] to 150 m at Craignathunder [NJ 695 228] and at least 50 m at Maiden Castle [NJ 695 245]. The breccia probably exploited the fracture developed along the original fault plane. These breccias are described in Chapter 7.

The Cromar Granite is partly bounded by a NNE-trending fault in the Glen of Peat Lochies, where the contact is displaced by over 1 km [NJ 525 035]–[NJ 528 045]; the amount of vertical displacement is not known, but is probably much less than this. The south-eastern contact of the Hill of Fare Granite is more linear than the rest, and microgranite is in contact with granodiorite country rock; this part of the contact is believed to be faulted.

Several north-trending faults affect the western end of the Corrennie Granite in the vicinity of Kirkton of Tough [NJ 615 130]. The westernmost part of the granite is displaced 400 m northward by faulting, and the portion of the granite between this fault and the one which truncates the granite is sheared and, in places, mylonitised.

The alignment of the larger felsic porphyry and felsite dykes is also NNE, especially between Strone Hill [NJ 579 135] and Green Hill [NJ 643 145], implying that a tensional regime was operative at this time.

Some of the NNE-trending faults in other parts of the district may also be of Lower Devonian age, especially where they are associated with breccias. For example the fault which displaces the Insch Complex Upper Zone cumulates from Hill of Newleslie [NJ 585 255] to Gallow Hill [NJ 600 263], continues north to Cunrie Craig [NJ 593 306], where a breccia with many gabbro fragments occurs along the fault.

Chapter 11 Cainozoic

The Quaternary deposits of the eastern part of the district were surveyed between 1986 and 1988 with particular emphasis on the potentially economic sand and gravel deposits. Otherwise no recent work has been done specifically on the Cainozoic geology of the district. The summary presented here relies heavily on Auton and Crofts (1986) and Auton et al. (1988) for the Inverurie, Kemnay and Dunecht areas; for the rest of the district, information has been taken from various sources, mostly outside the BGS, except for Peacock et al. (1977).

In the absence of a recent comprehensive survey, large parts of the generalised map of the Quaternary deposits of the district (Figure 27) are based on the original survey (Geological Survey of Scotland, 1886). Consequences of this are: areas shown as till include hummocky glacial deposits; kames, kame-terraces and deltas occur within areas shown as glaciofluvial sand and gravel; glaciolacustrine deposits are present in places, concealed beneath mapped alluvium; and some peat deposits are included within the alluvium. Only a few of the more important glacial meltwater channels and eskers in the district are shown, where they have been recorded in later studies.

The district was last glaciated during the late Devensian, when an ice sheet, which reached its maximum extent about 18 000 BP (years before present), advanced from the western parts of the Grampian Highlands to cover even the highest ground.

After complete, or nearly complete, deglaciation in the Windermere Interstadial (c. 13000–11000 BP), glaciers readvanced within the western parts of the Grampian Highlands during the Loch Lomond Stadial (c. 11000–10000 BP). Although there is no evidence that glaciers developed within the Inverurie–Alford district, corrie glaciers developed in the Cairngorms and the district experienced a rigorous climate.

As the ice sheet began to melt, the district was subjected to intense glaciofluvial erosion, as indicated by the numerous glacial meltwater channels that occur in it. The valleys occupied by the principal rivers today were also primary routes of meltwater discharge. Secondary drainage channels are very common, ranging in size from linear depressions a few metres deep and a few hundreds of metres in length, to steep-sided valleys, tens of metres deep and several kilometres in length. The orientation of the secondary channels shows that much of the meltwater followed routes which diverge widely from the present drainage pattern.

Weathering of bedrock

Deep chemical and physical weathering of the bedrock took place throughout the district during the Cainozoic, but much of this weathered material was removed during the Pleistocene glaciations. Relics of deep weathering believed to date from the late Miocene are preserved in the northern part of the district. Some 20 m of weathered anorthositic gabbro was proved in a borehole near Oldmeldrum [NJ 8200 2617]. Decomposed Bennachie Granite exposed in a road cutting near Pittodrie [NJ 693 245] has undergone relatively intensive weathering, with the formation of clayey gruss (Hall, 1993). The clay mineral macaulayite has been identified from rubefied zones in the granite.

Conditions during the late Miocene are believed to have been subtropical (Hall, A M, 1985), but the weathering products were modified under more temperate conditions during the Pliocene (Fitzpatrick, 1963). The susceptibility of bedrock to deep weathering is thought to be related to closeness of jointing and the intensity of any previous hydrothermal alteration.

Glacial erosion

North-east Scotland was glaciated on several occasions during the Pleistocene (2.5 Ma–0.01 Ma), but the extent of ice cover during each cold phase remains a matter of debate. During the glacial episodes, considerable erosion of the regolith occurred, but the amount of erosion was highly variable and, especially in the north of the district between Bennachie and Oldmeldrum, the amount of material removed may have been quite small. The glacial episodes modified the pre-Quaternary land surface by widening, straightening and deepening river valleys, breaching watersheds, and polishing and striating outcrops of resistant bedrock. Erosion associated with each ice advance largely removed deposits laid down during previous glaciations.

Glacial striae and trails of erratic blocks provide evidence of the direction of ice travel during the last (late Devensian) glaciation. Some of the best striated pavements occur on hill tops, especially Cairn William [NJ 656 168], Pitfichie Hill [NJ 665 171] and Strone Hill [NJ 579 135]. Although glacial erosion generally produced smooth rock surfaces, in places the lee sides of crags were plucked to produce roches moutonnées, as on Ord Fundlie [NJ 612 001]. Crag-and-tail features are well developed in the Correen Hills. Ice movement was generally from west to east. However as the ice sheet began to melt its flow was deflected to some extent by upstanding masses such as Bennachie, Cairn William, Pressendye and Benaquhallie, causing much of the flow to be funnelled through the Tillyfourie gap [NJ 637 130]–[NJ 650 122]. On the lower ground, several basins were hollowed out, largely in softer or more weathered rock. Many basins, such as those occupied by the Loch of Skene and Loch Davan, were first scoured in rock, and then partially filled by deposits of glaciofluvial sand and gravel. In others, such as Moss Maud [NO 628 997], any glaciofluvial deposits are covered by peat. The Howe of Alford may conceal a rock basin which has been partly filled with glaciolacustrine deposits and alluvium.

Erratic blocks give an indication of ice-flow direction. A trail of erratics of basic rocks derived from the Kildrummy mass [NJ 47 14] extends eastwards over Langgadlie Hill [NJ 514 137], where erratics are so abundant as almost to outnumber the boulders of the local andalusite-schist. A very large erratic of fibrolite-hornfels, possibly derived from around Kist Hill [NJ 638 162] , occurs near The Homer [NJ 748 156]. Serpentinite boulders, possibly from Lynturk [NJ 599 117], occur on the western slopes of Benaquhallie [NJ 606 086]. Rounded cobbles of quartzite derived from the conglomerate at the base of the Devonian succession, such as on Glaschul Hill [NJ 458 149], occur as erratics on the slopes of Coiliochbhar Hill [NJ 510 155].

Glacial meltwater channels

During deglaciation, the highest ground became ice free before the major valleys, which still contained valley glaciers. Kame-terraces were laid down along these valley glacier margins by subaerial meltwater streams. In general, the ice front receded from east to west, but in many topographical hollows pockets of stagnant ice remained. These ice masses caused the diversion of many of the meltwater streams and hence controlled the disposition of many ice-marginal glaciofluvial deposits.

Many of the larger meltwater channel systems are cut through the drift into bedrock and form gorges for part of their courses, e.g. the Queen's Chair [NJ 718 017]. These large channels may have been partially formed beneath the ice sheet, by meltwaters under hydrostatic pressure. This interpretation is confirmed where the channels show an up-and-down longitudinal profile.

The meltwater channels lying within the catchment of the River Don have been mapped and described by Aitken (1991); the largest are indicated on (Figure 27). They include large valleys cut in bedrock now filled by small misfit streams, e.g. the Den of Kildrummy [NJ 453 155], the Tom's Forest channel' [NJ 764 172] and the Tillyfourie–Cluny meltwater channel. This last extends from the Tillyfourie Gap [NJ 650 122], a notch in the Benaquhallie–Cairn William watershed, almost to the confluence of the Ton Burn with the River Don [NJ 709 136]. The western part of the channel is excavated in bedrock, but the lower, eastern part is excavated in drift and now partly infilled by peat. Its eastern extremity abuts the western end of the Kemnay Esker, indicating that g this point the regime of the meltwater stream changed from net erosion to net deposition. A complex assemblage of meltwater channels occurs in the Howe of Alford to the north of the present course of the River Don between Montgarrie and Keig (Bremner, 1921, pp.112–118). These channels formed when the present course of the Don was blocked by stagnant ice.

Glacial deposits

Till

The form and internal structure of the drift deposits around Inverurie and Alford show that most were laid down from ice sheets, either while the ice was still active or during its stagnation and retreat. The retreat was generally orderly from east to west, but pockets of stagnant ice were left in valleys lying parallel to the ice sheet margin and in the lee of bedrock highs. Evidence of deposition from beneath the ice sheet is ubiquitous in the district, with most of the lower-lying ground being covered by till.

Two lithologically distinct till types have been recognised:

• Lodgement till: an overconsolidated clayey diamicton with a strong fabric, containing angular erratics of local and more distant derivation, dark grey or olive-grey in colour where fresh, but in many places weathered to brown.
• Melt-out or flow till: a sandy diamicton with a weaker fabric, containing angular and rounded erratics, mainly of local provenance, typically dark brown to yellowish brown in colour.

In general the clayey till is stiff, with a high clast content. The clasts, which range up to boulder size, are mainly of metamorphic and igneous rock types, notably schist, granite and diorite. The overconsolidated character of the matrix, the strong orientation of the clasts, and the fact that it is overlain in places by melt-out till are all typical attributes of a lodgement till, deposited beneath an active ice sheet. Firm, clayey lodgement till was recorded from many exposures, trial pits and assessment boreholes by Auton and Crofts (1986). In several instances it rests directly on bedrock and contains a high proportion of clasts derived from the immediate vicinity. For example, in borehole (NJ71NW/12) [NJ 7236 1636], clasts of the underlying grey granite are incorporated in the basal till and in trial pit (NJ61SE/1) [NJ 6792 1136] the till contains abundant fragments of the granodiorite bedrock.

The colour of the till ranges from grey to brown; colour differences can be attributed in part to variation in the groundwater levels between sites, the grey till being the gleyed 'unoxidised' equivalent of the brown till, which generally occurs above the permanent water table. The prominent orange mottling seen in some occurrences of the grey till indicates that the till has been alternately gleyed and oxidised, probably as a result of seasonal changes in the height of the water table. The proportions and types of bedrock debris incorporated in the till also affect its colour and texture. For example, till deposited from ice that has passed over an outcrop of dark pelitic schist may be clayey and is characteristically dark grey in colour, whereas till deposited from ice that has passed over weathered granitic rock will tend to be gritty and lighter in colour. Some tills appear to be weathered, but on close inspection they are seen to contain rock that was deeply weathered before being incorporated into the till. Little or no sand and gravel seems to have been deposited in association with the clayey tills.

Dark brown, yellowish brown and greyish brown, sandy, less cohesive melt-out or flow till is the most widespread drift deposit in the district. The fabric is highly variable, but broadly shows an east–west preferred orientation of clasts. Unlike the clayey till, the sandy till commonly contains lenses of sand and gravel. Beds of brown sandy till are locally intercalated with thick sequences of sand and gravel and often separate material of ice-contact and glaciofluvial origin. More commonly brown sandy till rests directly on brown clayey till or on bedrock, e.g. pit (NJ61NE/1) [NJ 6881 1728].

Moundy glacial deposits

Moundy deposits, formed of poorly sorted glacial debris, were laid down at the margins of the ice sheet in the valleys or beside stranded, stagnant ice masses during the deglaciation. These moundy deposits are included within the areas of till shown on (Figure 27). As the ice melted, sandy flow till was deposited on top of previously deposited lodgement till and also on areas of newly exhumed bedrock. Where little or no reworking by melt-waters has taken place, these deposits form irregular morainic hummocks and ridges that stand up to 20 m above the surrounding ground surface. They are formed of a complex mixture of clayey and sandy diamictons and intercalated lenses of poorly stratified, often clay-bound, sand and gravel. The irregular mounds and hummocks reflect the in-situ decay of stagnant ice. An extensive complex of such hummocky morainic deposits was laid down in the Muir of Dinnet (Clapperton and Sugden, 1972; Gordon, 1993), where a large mass of dead ice became detached from the main ice sheet during deglaciation. However, most of the morainic material was subsequently reworked into kame and outwash terraces.

Elongate ridges on the flanks of valleys (lateral moraines) and transverse ridges lying across valley-floors (terminal moraines and retreat moraines) were formed locally during the retreat of the ice sheet within some east–west-oriented valleys. Aitken (1991) describes crossvalley-oriented hummocky moraines at Mill of Brux [NJ 481 146], south of Kildrummy, composed of large angular blocks of local bedrock intermixed with sand, silt and clay.

Glaciofluvial and glaciolacustrine deposits

Eskers

As the ice-sheet decayed, sediment-laden meltwaters issued from subglacial or englacial tunnels to form moundy spreads of water-sorted glacial ice-contact deposits. Sand and gravel which was deposited within the sub-glacial and englacial tunnels now forms sinuous, steep-sided esker ridges. These deposits were often let down on to the exhumed land surface as the ice, in which they were contained, melted. As a result, the eskers commonly cut obliquely across the present topography and their internal stratification often shows evidence of post-depositional collapse.

At Kemnay, a large esker extends from near the confluence of the Ton Burn and the River Don [NJ 710 140] to about 200 m west of Kemnay Quarry [NJ 735 167]. It forms a steep-sided discontinuous ridge over 3 km in length, rising between 8 m and 13 m above the adjacent ground surface. At its south-western end it emerges from the Tillyfourie–Cluny meltwater channel. The channel and esker together mark a route of north-easterly directed subglacial drainage. A section and pit in this esker, (NJ71NW/13) of Auton and Crofts (1986) [NJ 7240 1510], shows it to be composed, at least in part, of poorly sorted cobble gravel. Small eskers are present on the floor of the valley occupied by the Ton Burn to the west; they may represent a western extension of the Kemnay esker system.

Eames and kame-terraces

Sediment-laden meltwater, issuing from the ice sheet during periods of still-stand or slow ice-retreat, left large accumulations of glaciofluvial sand and gravel at or near the ice margin. This material forms complexes of hemispherical mounds (kames) and flat-topped terraces (kame-terraces) of sand and gravel. Kame-terraces are typically developed along the sides of the main valleys where orderly retreat of the valley glacier took place and glacial debris deposited from the retreating ice was reworked by powerful meltwater streams. The meltwaters in places disgorged into temporary ice-marginal lakes, where they built deltas of sand and gravel; laminated clay and silt-grade material was deposited contemporaneously in the distal parts of the lakes.

Don catchment

Kame-terraces occur on the flanks the valley of the Don to the east of Monymusk [NJ 685 153], and also flank the valleys of other large rivers in the district. They form extensive spreads of glaciofluvial sand and gravel, but individual terraces in the Don catchment cannot generally be traced for more than 2 km. Although up to three successively higher kame-terraces may be present at any point along the river valley, precise correlation of fragmentary staircases of kame-terraces within each river catchment is dubious; their altitude and slope are governed by the disposition of bedrock highs, rather than an orderly pattern of glacier retreat.

Many terraces, e.g. those at Tavelty [NJ 790 172], grade into and overlie fans developed at the mouths of meltwater channels (the Tom's Forest channel in the case of the Tavelty terraces). Many show coarsening-upwards sequences, suggesting deltaic deposition into ice-marginal lakes. Others are composed of poorly sorted matrix-rich deposits, suggesting that they may have been deposited as glaciofluvial fans.

The sands and gravels which underlie the kame-terraces flanking the valleys of the Don and Urie constitute a major resource of sand and gravel within the district. The terrace surfaces are often uneven, being pitted with kettleholes, some of which contain peat and silt. For example, an extensive kame-terrace on the southern side of the Don valley to the north and east of Kintore includes many kettleholes, some of which penetrate down to the underlying till or bedrock.

There are no extensive terraced glaciofluvial deposits in the valley of the Don between Ardhuncart [NJ 480 175] and Rothens [NJ 686 172]. The absence of kames and kameterraces from the Howe of Alford is surprising, and was commented upon by Bremner (1921, p.44), who supposed that it resulted from the slow in-situ melting of a large mass of stagnant ice.

Extensive glaciofluvial deposits occur to the west and north of the Don valley from Kildrummy to Lumsden and beyond, covering much of the low ground underlain by Devonian sedimentary rocks. These deposits are extensively kettled, but flat-topped terraces are also widely developed and extend across the watershed into the Bogie valley. The spreads of sand and gravel may have been laid down when the relatively narrow valley between Ardhuncart [NJ 480 175] and Kirkton [NJ 519 170] was still blocked by ice after the Kildrummy area had become ice free. This ice blockage probably ponded up meltwater around Lumsden, which overflowed into the valley of the Water of Bogie. Upstream of the Kildrummy area, kame-terraces occur on the northern side of the Don valley between Glenkindie [NJ 435 138] and Kinclune [NJ 458 133].

Dee catchment

Kame-terraces on the northern side of the Dee valley between Dinnet and Dess lie within the district. In the vicinity of the Muir of Dinnet, abundant kames and a number of kame-terraces were formed by glacial meltwater reworking hummocky glacial deposits whilst the basins now occupied by Loch Davan and Loch Kinord were filled by stagnant ice. The Dee valley to the south may also have been blocked by ice at this time.

Wide terraces of sand and gravel are developed to the south and east of the Hill of Fare, between the Loch of Skene and Drumoak (Auton et al., 1988).

Buried glaciofluvial deposits

Glaciofluvial deposits occur beneath the alluvium of several broad river valleys, particularly that of the Don from Glenkindie to Ardhuncart and downstream of Monymusk. They are also present within the valleys of the Beltie Burn near Torphins, and the Tarland Burn in the Howe of Cromar. Most are gravels, laid down by braided rivers. Individual deposits of sand and gravel of possible economic interest are listed and briefly described in Chapter 12.

Glaciolacustrine deposits

Laminated clays and silts of glaciolacustrine origin occur in several parts of the Don valley, where they overlie ice-contact deposits and are typically overlain by alluvial sand and gravel (Aitken, 1991). These fine-grained sediments indicate that lakes existed in the Don valley in the period immediately following deglaciation. The largest lake extended eastwards from Kintore [NJ 790 166] beyond the eastern margin of the district as far as Dyce [NJ 889 141], and was partly infilled by deltaic deposits to the west of Kintore. Smaller lakes occurred around Alehousewells [NJ 725 167], at the eastern end of the Howe of Alford [NJ 620 180], and near Drumallachie [NJ 477 145]. The Drumallachie lake may have owed its existence to ponding by the morainic deposits at Mill of Brux [NJ 481 146].

Glaciolacustrine deposits also occur in the lower Urie valley and the valley of its eastern tributary, the Lochter Burn. The lake (Glacial Lake Urie) [NJ 773 230]–[NJ 770 260] was probably ponded up by an ice dam at Inverurie. It stood at about 63 m above OD, and was rapidly infilled with deltaic sand and gravel and finely interlaminated silt and clay (Auton et al., 1988).

Extensive spreads of glaciolacustine sediment were laid down in one or more lakes which occupied the large ice-scoured depression lying between Berry Hill [NJ 718 013] and Drumoak [NO 790 987], approximately 0.5 km to the south of the district. Glaciolacustrine silts and clays probably occur beneath the waterlogged peat of Red Moss [NJ 74 01], and resistivity soundings at a site on Black Moss [NJ 7451 0003] indicate about 5.9 m of sandy silt beneath alluvium. Dating of organic remains within sandy silts and clays from the lacustrine sequence within Loch of Park, [NO 772 988] 0.6 km to the south of the district, gave 14C ages of 11 900 ± 260 BP (HEL-417) and 10 280 ± 220 BP (HEL-416) (Vasari, 1977). The dated organic sediments also yielded a full late-glacial pollen sequence dominated by herbs and grasses. If the dating of the organic sediments is correct, the lake was in existence during both the Windermere Interstadial and the subsequent Loch Lomond Stadial, clear evidence that the district remained ice free when glaciers had returned to parts of the Scottish Highlands during the Loch Lomond Stadial between about 11 000 and 10 000 BP.

Resistivity soundings by Aitken (1991) indicate the existence of glaciolacustrine deposits beneath the post-glacial alluvium in the Howe of Alford. The lake was probably ponded by ice which blocked the narrow gorge between Bridge of Keig and Pitfichie [NJ 622 189]–[NJ 681 165], formed where the valley of the present River Don cuts through the Bennachie Granite.

Postglacial deposits

Pollen and plant macrofossil sequences spanning the early postglacial period have been described by Vasari (1977) from radiocarbon-dated sequences of organic-rich lake muds and silts deposited in Loch Kinord [NO 435 997] and in Loch of Park [NO 772 988] (both just south of the district). The pollen and plant macrofossil assemblages show that during the early part of the Flandrian stage the district experienced a milder and drier climate than occurs today, but wetter, colder conditions set in about 5000 BP and lasted until about 200 BP (i.e. around 1800 AD).

Diatomite

After the ice sheet had melted, several small rock basins with outlets blocked by a rock step or by sand and gravel deposits were exhumed. In most of these basins fine-grained sediments were deposited, but in a few noteworthy examples, biogenic material also accumulated. The clear water, with very little clastic input, was an ideal breeding ground for minute unicellular algae with siliceous tests (diatoms). On death, these tests were deposited on the lake floors, and over time the biogenic material became slightly consolidated to form deposits of diatomite. The principal occurrences of diatomite in the district are in the Muir of Dinnet–Logie Coldstone area (Figure 28), but it has also been recorded from around Premnay (Wilson and Hinxman, 1890, p.37) and from Red Moss [NJ 74 01]. All of the diatomite deposits, except possibly those in the centre of Loch Davan, are overlain by 0.5–3 m of peat (Plate 9).

The principal period of diatom growth was probably from 10 000 to 5000 BP, after which the cooler and wetter climate encouraged the spread of sphagnum mosses, reducing the area of clear water. This process was aided by shallowing of the lakes due to silting up by the deposition of diatomite and other fine-grained sediment. A species list of Diatomaceae found in diatomite from Loch Kinord is given in Wilson and Hinxman (1890, pp.40–42). The economic aspects of the diatomite deposits are discussed in detail in Chapter 12.

Peat

Mapped spreads of peat are less abundant in the Inverurie–Alford district than in the areas to the west and south. This is partly due to the lower altitude of the hills and partly to the greater density of human settlement, which has caused more of the lowland peat to be worked out. Many of the thin spreads of valley peat are not distinguished on (Figure 27), as they were mapped as alluvium during the primary survey.

Most of the surviving peat deposits of the district are raised mosses developed in hollows or poorly drained valleys where the supply of sediment is small and the flow of water low. Although most of the peat deposits of the district are on low ground, small peat-filled basins do occur on some of the hills, e.g. between the Mither Tap and Oxen Craig on Bennachie [NJ 672 225], and the Mosses of Essie [NJ 438 280] and Tolophin [NJ 435 260] to the west of Rhynie. Upland peat hags are absent, except on the plateau of the Hill of Fare [NJ 685 040], where they are generally less than 1 m thick.

Lacustrine and fluviatile alluvium

The floodplains of the major rivers vary considerably in width, and are underlain by alluvium which is still accumulating (Figure 27). In many places, the floodplains are flanked by older postglacial alluvial terraces, which do not contain the kettle-holes characteristic of glaciofluvial kame-terraces. Narrow alluvial plains occur along some of the minor streams, though where there is little flow these have become choked by boggy vegetation and peat is accumulating.

A large alluvial spread occurs in the Howe of Alford, between Bridge of Alford [NJ 561 171] and Bridge of Keig [NJ 618 187]. The River Don flows close to the northern margin of the alluvium which extends southwards almost to Kirkton of Tough [NJ 615 130]. Bremner (1921) suggests that the alluvial deposits in the Howe of Alford are considerably less extensive than indicated by the primary survey of the district and shown in (Figure 27). Nevertheless, alluvial terraces over 1.6 km wide are present upstream of Bridge of Keig. These terraces narrow steadily westwards, to become only about 500 m wide at Bridge of Alford.

A wide alluvial spread also occurs in the Howe of Cromar along the margins of the Tarland Burn from Netherton [NJ 470 047] to Coull Home Farm [NJ 510 015]. The Howe of Cromar is the site of a former lake, which extended from near Mill of Kincraigie [NJ 501 035] to Bridgend [NJ 510 025], and was drained in historic times (Bremner, 1912, p.30); the Tarland Burn now flows in a trench, up to 15 m deep, for 100 to 200 m near the remains of Coull Castle [NJ 513 023]. Another lake probably occupied the valley of the Beltie Burn from Sundays-wells [NJ 611 023] to Nether Mains [NJ 630 005], near Torphins.

The glacial and glaciofluvial deposits of the Drumoak–Loch of Skene and Muir of Dinnet areas are overlain by thin, patchy deposits of postglacial lacustrine and fluviatile alluvium. The only lochs of any size in the district not to have been artificially drained for peat-cutting or agriculture are Loch Davan and the Loch of Skene.

Chapter 12 Economic geology

Metalliferous minerals

No metalliferous mineralisation worthy of exploitation has been discovered to date in the district, but exploration for several different kinds of mineralisation has been conducted, particularly over the last 25 years, and the results have been sufficiently promising to justify further work. The late-tectonic basic and ultramafic rocks of the north-east Grampian Highlands have excited considerable interest as possible hosts of Cu-Ni and platinum group element (PGE) mineralisation. Although most of the work was conducted outside the district, some surveys were conducted on the Insch and Boganclogh intrusions. The post-tectonic Cairngorm Suite granites are asso-ciated with multi-element geochemical anomalies, and there are indications of vein-type Mo mineralisation associated with their roof zones. Sulphide-bearing breccias associated with these granites have the potential for gold mineralisation. Gold has also been reported from the Lower Devonian Rhynie Chert hot spring deposit and from fault breccias in the associated hydrothermal system.

Stream sediment geochemistry

The BGS regional geochemical mapping programme has involved stream sediment sampling at a density of 1 sample per 4 km², and analysis of the samples for 27 major and trace elements by direct reading emission spectrograph. In addition, U was determined by neutron activation and As and Sb by atomic absorption spectrophotometry on 60% of the samples. The East Grampians regional geochemical atlas (BGS, 1991) discusses the geochemical signatures of the major rock units within the region, and presents 30 single-element anomaly maps, each with a brief explanation. Many of the single-and multi-element anomalies which are evident from the maps can readily be related to the geochemical signatures of the different rock types in the region. Other anomalies are related to occurrences of metalliferous mineralisation, either known or at present undiscovered, while some anomalies have been traced to enrichment by adsorption on organic material or contamination from manmade sources.

Stream sediment samples derived from Argyll and Southern Highland Group rocks have different major and trace element signatures, reflecting differences in the geochemistry of the parent rocks. The Argyll Group–Southern Highland Group boundary is easily recognisable on many of the single-element maps, implying that a change in provenance and/or depositional environment occurred at this boundary. Stream sediments from areas immediately adjacent to the late-tectonic basic intrusions and Cairngorm Suite granites contain higher levels of elements dispersed from these bodies, but the main geochemical signatures of the metasedimentary rocks are still discernible. The area within 3 km of Inverurie was not sampled during the geochemical mapping programme because of the likelihood of contamination from urban and industrial sources.

The Argyll Group metasedimentary rocks of the district show enrichment in Mg, Fe, Co, Ni, Cr, Ca, Sr, Ti, V, Y, La, K, Rb and, to a slight extent, Zr. The Southern Highland Group rocks are enriched in Be, B, Li, Zn, Ba and Ga, while contents of As, Sb, Bi, Sn, Pb and Ag are low, and close to detection limits in both groups. The enrichment of elements characteristic of basic igneous rocks (Mg, Fe, Co, Ni, Cr, Ca, Sr) reflects a continuing input of basic igneous material into the Argyll Group rocks. Some of this is in part due to intrusive, volcanic or volcaniclastic material within the Group, but some may reflect detrital material in the metasedimentary rocks derived from basic igneous rocks which crop out outwith the district. A source of basic igneous material would also account for the low Ga in the Argyll Group rocks. The enrichment in Ti, V, Y and La may indicate a higher content of detrital heavy minerals in the parent sediments of the Argyll Group, although there is only a slight enrichment in Zr in the Argyll Group stream sediments. The slight enrichment in K and Rb in the Argyll Group may reflect original K-feldspar in some of the parent sediments. The higher levels of Li, Be and B in the Southern Highland Group are believed to reflect the deposition of clays in sea water of increased salinity, concomitant with the development of an extensional basin in the late Precambrian (Fettes et al., 1986). If this is so, it is surprising that K and Rb have lower values in stream sediments derived from the Southern Highland Group rather than those derived from the Argyll Group.

Cu-Ni and PGE mineralisation associated with the Insch and Boganclogh intrusions

Worldwide, layered basic and ultramafic rocks are the most economically important source of platinum-group elements, and also yield significant amounts of Cu, Ni, Cr and Co. The Cu, Ni and Co occur as sulphides and the PGE as a mixture of native alloys and complex arsenides and tellurides. Chromium occurs as chromite. Both Cu-Ni and PGE are concentrated in particular layers within layered intrusions, frequently associated with magma mixing or wall-rock assimilation; both of these can result in a sudden decrease in the solubility of sulphides in the magma, and the separation of an immiscible sulphide melt, which sinks to the floor of the magma chamber. There is also a possibility of the precipitated sulphides, with attendant PGE minerals, being remobilised and carried into veins, which frequently occur near the contact of the intrusion.

From 1968 to 1973, Exploration Ventures Ltd, a company jointly owned by Consolidated Goldfields and Rio Tinto Zinc, conducted exploration for Cu-Ni sulphide mineralisation over the outcrop of the late-tectonic basic and ultramafic intrusions of the north-east Grampian Highlands. A helicopter-borne aeromagnetic survey was flown, and soil and stream sediment sampling were conducted over the intrusions and some of the surrounding areas. The most interesting anomalies were found ouwith the Inverurie–Alford district, and only limited follow-up work was done within the district. Minor Ni mineralisation occurs just outside the district at Old Merdrum [NJ 470 298], where disseminated pentlandite occurs in hornfelses at the contact with serpentinised ultramafic rocks. In 1972–73, Noranda-Kerr carried out additional soil sampling and shallow drilling over anomalies underlain by ultramafic rocks of Hill of Barra [NJ 803 256] (Insch intrusion, Lower Zone) and the Hill of Towanreef [NJ 456 242] (Boganclogh intrusion), but concluded that the Ni was held in silicate minerals and thus of no economic interest.

More recently, PGE have attracted more interest than Cu-Ni, and BGS has examined the Insch Lower Zone rocks in the Hill of Barra area (Gunn and Shaw, 1991). Overburden samples were collected with a percussion drill from the till–bedrock interface, and bedrock samples were also collected. Samples were analysed for Pt, Pd, Rh, Au, and a standard suite of major and trace elements, including Cr, Cu, Co Ni and As. A 10 m x 50 m total magnetic intensity survey was also conducted. The results showed that limited Cu-Ni mineralisation, with probable structural control, exists along the faulted southern margin of the intrusion. PGE and Au levels, however, were uniformly low, with maximum sample values of 6 ppb Pt, 6 ppb Pd, 2 ppb Rh and 12 ppb Au.

Other copper mineralisation

Wilson and Hinxman (1890) report disseminated copper oxide mineralisation within 'granite' (actually tonalite and quartz-diorite) from Syllavethy [NJ 567 177]. No further information is available. Heddle (1901, Vol. 1, p.31) records chalcopyrite disseminated in gneiss at 'Dobston' (= Dubston) quarry, Inverurie [NJ 749 220].

Gold

Kidston and Lang (1921b) proposed that the Rhynie Chert formed by silicification of peat in a hot spring system, but this model of chert formation has subsequently been refined. Rice and Trewin (1988), realising the association of many gold deposits with hydrothermal systems, surveyed an area of approximately 1 km² immediately surrounding the chert locality. They found abundant evidence of silicification and potash metasomatism in the basal Devonian rocks, and along faults. Six specimens of the chert and its silicified carbonaceous sandstone margin contained 50–180 ppb Au, 15–300 ppm As and < 5–71 ppm Sb, whereas four specimens of silicified fault breccia contained < 4–89 ppm As and < 5–8 ppm Sb. The distribution of Au within these samples was too erratic for reliable estimates of average gold content to be made. Each breccia sample was split into between 3 and 10 portions for gold analysis; the richest sample gave splits with 40–1720 ppb Au. Ag was below the limit of detection (2 ppm) in all of the chert and sandstone sample, but two of the fault breccia samples contained detectable silver (3.0 and 3.5 ppm).

The East Grampians regional geochemical atlas shows a strong Sb anomaly and a rather weak Ag anomaly centred on the Rhynie chert locality, but there is no corresponding As anomaly. Paired As and Sb anomalies occur near the southern end of the Rhynie Devonian outlier [NJ 45 15], and over the ultramafic rocks in the south-western part of the Insch intrusion. The source of these anomalies is at present unknown, but traces of sulphide mineralisation were discovered during the present survey at Drumgowan [NJ 581 241], where pyrrhotite occurs in fine-grained quartz rock very close to the southern contact of the Insch intrusion, and north of Newtonhill [NJ 555 243], where gossanous material occurs close to the syenite/serpentinite contact within the Insch intrusion. Adamson (1988) refers to a record by Atkinson (1619) of gold occurrences at 'Drumgowan' ? = Drumgowan [NJ 581 241] and 'Boggs of New Leslie' = Bogs [NJ 589 240].

A number of N–S-trending breccias are associated with the post-tectonic granite intrusions, especially the Bennachie Granite. In many places, such as near Southbog [NJ 605 218], they contain red and yellow ochreous material, commonly in boxworks, obviously a secondary development after sulphides. In a few rare cases, some pyrite is preserved. This environment appears favourable for Au mineralisation, although the lack of As/Sb anomalies in the regional geochemical pattern may be a counter indication.

Molybdenum, tin and tungsten

Molybdenite was recorded as being disseminated in psammites near Middleton of Balquhain [NJ 7340 2245] by Heddle (1901, Vol. 1, p.16). Exploration Ventures Ltd followed up anomalous Mo levels in soils discovered by the soil survey (Glentworth and Muir, 1963), and located a float of molybdenite-bearing vein-quartz. Despite soil sampling, pitting and trenching, no vein material was found in situ. More detailed investigations by BGS (Colman et al., 1989) have located the source and defined the environment of the mineralisation. A molybdenite-bearing quartz vein was discovered cutting greisened granite at the margins of the Middleton Granite. Specimens of vein-quartz bearing beryl and wolframite were discovered during pitting, and scheelite was found in spoil from a gas pipeline trench. The overburden was sampled by auger drill and seven boreholes were drilled into bedrock in the search for further mineralisation. A gravity survey (Kimbell, 1991) was conducted to define the shape of the Middleton Granite cupola. The veining was shown to be closely associated with sericitisation of granite at the margins of the Middleton Granite intrusion. The East Grampians geochemical atlas shows Mo and U anomalies over the Middleton Granite; unfortunately the samples were not analysed for W.

Soil sampling by Exploration Ventures Ltd identified a Mo anomaly near the Glen of Cushnie [NJ 500 105]. This anomaly is situated close to the northern contact of the Cushnie Granite, a small partly greisened granite pluton, which shows similarities with the Middleton Granite. Coincident Mo, Sn and Ag stream sediment anomalies related to the Cushnie Granite are shown in the East Grampians geochemical atlas. Samples of mineralised bedrock yielded up to 12 ppm Sn, 62 ppm Cu and 120 ppm Mo, but the anomalies of different trace elements were not coincident. It was concluded that scanty Mo mineralisation had been scavenged by oxidising pyrite.

Anderson (1971) records molybdenite at the southwest contact of the Bennachie Granite on Kist Hill [NJ 631 163]. The East Grampians geochemical atlas shows no Mo anomaly at this location, but there is a weak Sn anomaly. Mo and Bi anomalies occur along the western contact of the Bennachie Granite, while Mo and Sb anomalies occur near the south-east contact of the intrusion, suggesting that the eastern and western contacts, which are both faulted, may be mineralised. An unexplained Sn anomaly also occurs near the late-tectonic Syllavethy Intrusion.

Iron

A large quartz vein cuts the Bennachie Granite at Henley's Quarry, near the summit of Pitfichie Hill [NJ 663 170]. Specular haematite was found and trial excavations made, but the haematite was seen to be a small pocket within the vein, and work was soon abandoned. Heddle (1901, Vol. 1, p.89) provides an analysis of this material.

Titanium

Stream sediments and panned concentrates derived from the Middle and Upper zones of the Insch, Boganclogh, Morven–Cabrach and Tarland intrusions have high titanium contents (typically 2–10% TiO₂ in stream sediments and 20–30% TiO₂ in panned concentrates). A study by Colman (1983) established that the titanium was almost entirely in the form of ilmenite. Samples of till over the basic masses have uniformly low TiO₂ contents (< 1%). Glaciofluvial sand and gravel in areas peripheral to the basic masses, e.g. between Inverurie and Kintore, contain appreciable quantities of ilmenite (0.4–3.5% TiO₂), which could readily be extracted from the loose material, possibly as a by-product from sand and gravel production.

Beryllium

Large crystals of pale green, fractured beryl are sparsely disseminated in a large granitic pegmatite body intruding norite of the Insch intrusion in Pitscurry Quarry [NJ 729 267] (Gallagher, 1959). Small quantities of beryl are reported from veins near the contact of the Middleton Granite [NJ 730 220] (see above). Beryl is also recorded by Heddle (1901, Vol. 2, p.45) from near Bridge of Keig [NJ 618 187]. None of these occurrences is of economic importance.

Limestone

There are very few sources of this material in the district. A bed of relatively pure limestone, about 10 m thick, occurs in the mill lade, 100 m north of the waterfall in the Dess Burn [NJ 5661 0050]. It forms the northern most occurrence of the Deeside Limestone Formation, which has a large outcrop to the south-west in the Aboyne district (Sheet 66W). Lime has been burnt at the Limer Shank [NJ 514 213] in the Correen Hills, but the material is a calcsilicate rock with less than 30% calcium carbonate and little was worked. A very small, fault-bounded outcrop of impure limestone including much grossular and idocrase has been worked at Loanend [NJ 606 241], near the southern margin of the Insch intrusion; it is faulted against olivine-ferrogabbro to the south and serpentinite to the north. Other beds of calc-silicate rock within the Dalradian rocks of the district are too thin and have too little free carbonate to be suitable for any known use.

Dimension stone

Several quarries in the district have been worked for dimension stone, but all have now been abandoned or converted to quarrying hard rock for aggregate. However, some granite blocks are still extracted at Kemnay Quarry [NJ 737 168], as a sideline to the aggregate business. If the demand for dimension stone were to revive, abundant reserves could probably be readily located, although the use of blasting for the production of aggregate may have made parts of the original quarries unsuitable. The main rock types exploited in the past are as follows:

Andalusite-cordierite schist was worked at Correen Quarry [NJ 522 213]. It was widely used for large flagstones, lintels, and mantelpieces. Although its fissility is generally poor, some roofing slates were also made from this material.

Sandstone from the Devonian Rhynie Outlier was worked at Kildrummy [NJ 454 165], Quarryfield [NJ 453178] and in several quarries on Quarry Hill [NJ 489 255]. Sandstone from Quarryfield was white, sugary and rather soft, but the pink Kildrummy and Quarry Hill sandstones were better cemented and slightly harder. The sandstones were all freestones, which were easy to work and were used extensively for building in the western part of the district.

Several different 'granites' were exploited. The principal quarries were at Kemnay [NJ 737 168] (Kemnay Granite: Front Cover), Tillyfourie [NJ 646 127] (Tillyfourie Tonalite), Corrennie [NJ 643 119] (Corrennie Granite), and Raemoir [NJ 701 003] to Craigton [NJ 712 006] (Hill of Fare Granite). Other workings were at Anguston [NJ 804 020] (Balblair Granodiorite) and Syllavethy [NJ 567 177] (diorite of Syllavethy intrusion). Attempts were made to quarry the Bennachie Granite (coarse-grained granite at Pitgaveny

[NJ 675 215] and microgranite at Little Oxen Craig [NJ 665 232], where discarded blocks are strewn on the quarry floor), but were abandoned at an early stage. The Kemnay Granite was exported far afield from the district. It is an almost white, slightly foliated, biotite-muscovite-granite which is very similar in appearance to the Aberdeen Granite from Rubislaw and other quarries. Blocks are still in demand for restoration and conservation work on buildings made of Aberdeen and Kemnay granites, and for cladding.

Hard-rock aggregate and reconstituted stone

The five main working quarries in the district produce crushed hard-rock aggregate in varying sizes. This material is used as coarse aggregate for road bases and the surfacing of minor roads; it is also used as general-purpose concreting aggregate. In addition, Kemnay Quarry is a centre for the manufacture of 'Fyfestone', a reconstituted building material made by binding finely crushed rock (either white granite from Kemnay Quarry or red granite from Corrennie Quarry) with cement. The principal working quarries in the district are:

• Craigenlow, Dunecht [NJ 732 094]: pink and grey Crathes and Balblair granodiorites (Plate 10) Kemnay [NJ 737 168]: white Kemnay Granite (cover photograph)
• Corrennie, Monymusk [NJ 643 119]: red Corrennie Granite (only quarried on an intermittent basis for Fyfestone)
• Pitscurry, Pitcaple [NJ 729 267]: norite of the Insch intrusion
• Tom's Forest, Kintore [NJ 761 170]: Kemnay Granite, much of it hydrothermally altered

The district contains several sizeable disused quarries, believed to have been used principally for aggregate production. Those at Pitmachie, Old Rayne [NJ 666 283], Govals, Pitcaple [NJ 732 256], and Harthill, Oyne [NJ 683 258] exploited rocks of the Insch intrusion and marginal hornfelses. Whitestones [NJ 750 147] and Tuach Hill, Kintore [NJ 796156] worked the Kemnay Granite. Gask [NJ 794 064] exploited the Gask Diorite, and Invermossat [NJ 491 187] worked gritty psammite of the Southern Highland Group. Smaller quarries, used locally for road metal, building foundations, etc., are all disused, except for Suiefoot Quarry [NJ 553 244], where syenite and serpentinite are still extracted by the Forestry Commission and the owners of Knockespock Estate, and East Aquhorthies Quarry, Kemnay [NJ 725 206], where psammite from the Aberdeen Formation is used by the Forestry Commission. Most of the small disused quarries are in igneous rocks, with the wider felsite and porphyry dykes having been particularly preferred. In places, decomposed granitic rocks have been worked for sand.

Abundant reserves of hard-rock aggregate exist within the district. Most of the bodies of plutonic igneous rock (except where they are highly decomposed) contain material suitable for aggregate production, as do the more massive Argyll Group psammites in the area around Inverurie and the gritty psammites of the Southern Highland Group. The more massive migmatitic semipelites in the Dee catchment, e.g. Craiglash Quarry, Torphins, just to the south of the district [NO 621 987], are also useable as hard core for road making.

Sand and gravel

The distribution and origin of sand and gravel are described in detail in Chapter 11, but some general comments on the suitability of various types of deposit for economic exploitation are given below.

Sand and gravel are widely used as aggregate for production of general purpose concrete, usually after screening. Finer-grained deposits are used as building sand provided the proportion of fines is not too high. The sand and gravel resources of the district consist mostly of glaciofluvial sand and gravel, but some sandy or gravelly tills derived from granites are suitable for a limited range of uses. Moundy glacial deposits which form moraines are in general too poorly sorted, and contain too many very large boulders, to be attractive sources of aggregate. Decomposed rock, mostly occurring as relict patches of Neogene deep weathering and often covered by till, is in places sufficiently soft to be extracted by digging and used as low-grade gravel without crushing.

Sediments forming eskers commonly contain a high proportion of coarse, poorly sorted gravel, and other ice-contact glaciofluvial deposits, such as kames, though finer-grained, are also less well sorted than the sediments that form kame-terraces and deltas. Well-sorted gravels of glaciofluvial origin are also commonly present beneath the alluvium of the major rivers, though most of this material occurs below the water table. Glaciolacustrine sands have been exploited in only a few places; most have too high a content of fines to be considered workable. Where glacial or glaciofluvial sand and gravel deposits have been reworked by rivers, the resulting alluvial deposits constitute a resource of sand and gravel.

The sand and gravel resources in the eastern part of the district have been assessed by Auton and Crofts (1986) and Auton et al. (1988) as part of a study of bulk minerals occurring within 30 km of Aberdeen (Figure 29). The resources were classified as:

i. sand and gravel in continuous spreads at the ground surface

ii. sand and gravel in continuous spreads, but overlain by overburden (peat and/or alluvium)

iii. discontinuous spreads of sand and gravel at surface or buried at shallow depth

The criteria used to delimit potentially workable deposits within each resource category were refined in a subsequent study by Merritt et al. (1988). This involved subdivision into dominantly sandy and dominantly gravelly deposits, classifying them according to their disposition above or below the water table, and exclusion of the smaller and thinner spreads. This produced a list of those deposits considered to be most attractive as resources. Twenty of these deposits lie within the Inverurie district; details of these are given in (Table 4) and their location is shown on (Figure 29).

In 1993, there were only four active commercial sand and gravel operations within the part of the district surveyed by Auton and Crofts (1986) and Auton et al. (1988), namely Backward Croft [NJ 744 174] and Mill Farm [NJ 740 176], near Kemnay, and Tavelty/Townhead [NJ 782 173] and Castle Farm [NJ 781 156], near Kintore. However, due to the inherently thin and impersistent nature of most Quaternary sequences, operations move site more frequently than hard-rock quarries, and some of the other deposits listed by Merritt et al. (1988) may be opened up in the near future.

In the remainder of the Inverurie–Alford district, only reconnaissance studies of the sand and gravel resources have been made (Peacock et al., 1977). The principal occurrences noted during that survey are shown in (Figure 27) and briefly described below.

A. Lumsden

Much of the Devonian outcrop between Mossat and Auchindoir, in an area 8 km by 2 km, is covered by ridges and mounds formed of highly variable Quaternary sediments. Coarse gravel containing boulders up to 0.25 m in diameter is present near South Deskie [NJ 474 207] for example, whereas fine-grained sand occurs at Birkenbrewl [NJ 464 209]. Although the deposits around Lumsden average only 5 m in thickness, mounds of sand and gravel up to 15 m high are present locally. Several small disused workings occur south of Govals [NJ 478 228].

B. Logie Goldstone

Extensive spreads of sand and gravel occur along the lower eastern slopes of Culblean Hill [NJ 422 027] straddling the boundary with the Glenbuchat district (Sheet 75E). They continue southwards towards Loch Davan, where they are in places covered by lacustrine deposits and peat. A ridge of sand and gravel approximately 100 m long and 20 m wide is currently worked at Mill of Newton [NJ 430 048], and a kame 8 m high formed of poorly sorted sand and gravel occurs near Blelack House [NJ 440 032].

C. Muir of Dinnet

This is the southern continuation of the Logie Coldstone area, and extends into the Aboyne district (Sheet 66W). The deposits are partly overlain by lacustrine deposits and peat, and, as they lie within a National Nature Reserve, are unlikely to be exploited. They appear to consist mainly of unsorted gravel with cobbles up to 0.2 m diameter in a silty and sandy matrix.

D. Howe of Cromar

The sand and gravel deposits cover a small area, but include a few isolated mounds which may contain a considerable thickness of deposit. A ridge of glaciofluvial sand at Mains of Kincraigie [NJ 494 044] forms a ridge 100 m long and 10 m high. A small area of low mounds 1–2 m high occurs at Wester Coull [NJ 480 025], and a mound of very coarse morainic or glaciofluvial gravel occurring near Coull Home Farm [NJ 513 011] is currently worked. The sand and gravel deposits within the valley of the Tarland Burn pass southwards into terraced spreads along the north bank of the Dee, which are almost entirely in the Aboyne district except for some of the deposits between Aboyne Castle [NO 526 995] and Mill of Dess [NO 569 994].

E. Torphins

Mounds of sand and gravel cover an area of about 1.5 km by 0.5 km to the south-west of Torphins [NJ 620 015], but there are no exposures.

Peat

Peat deposits are less abundant in the Inverurie–Alford district than to the south and west, due to the lower altitude of the hills and intensive exploitation in the 19th and earlier centuries, when it was the main fuel in agricultural areas. The peat deposits occur principally in basins where drainage has been impeded since the last glaciation. There is very little blanket peat left on the hills, due to agricultural improvement, especially drainage, and its use as fuel. The peat deposits of the district were described by Fraser (1943; 1948). The location of the deposits evaluated by Fraser (1948) are shown on (Figure 27), and the resource estimates in (Table 5). Extensive deposits occur in the Moss of Tolophin [NJ 435 260], Red Moss [NJ 74 01] and the Muir of Dinnet [NJ 45 00]; the last named lies within a National Nature Reserve, reducing its potential for peat extraction. Considerable areas of peat still remain in the south-eastern part of the district around the Loch of Skene and the small basins between that loch and the Dee valley.

Diatomite

Diatomite is composed of the fossil remains of diatoms, unicellular green algae with siliceous tests. On the death of the algae, the tests become hollow spheres which, even when packed together, form a light, porous material with a high absorbency. Diatomite was used as a filler for explosives, especially dynamite, until the 1930s, but is now principally used in insulation board, fillers, extenders and bulking agents, and as a filter aid and adsorbent.

In the south-western part of the district, in the Muir of Dinnet and adjacent areas, diatomite formed on the beds of postglacial lakes (Figure 28). The maximum thickness of diatomite recorded at any locality is 3.20 m in the centre of Black Moss. The average thickness of the deposits within the district which have been evaluated varies between 0.36 m and 1.14 m. The content of organic matter varies from 22.3% (Kinord) to 50.57% (Ordie), calculated on a moisture-free basis (Haldane et al., 1940). In general the diatomite contains little admixed sand or clay, apart from where it reaches its thickest development, in the centre of Black Moss. There, the diatomite becomes impure towards the base and grades downwards into blue-grey clay. Otherwise, the contacts with the overlying peat and with the underlying deposits are sharp. The deposits are mostly underlain by sand, but in places they are underlain by lacustrine silt and clay. The diatomite deposits are overlain by up to 4 m of peat, which developed when the lakes partially dried out.

The diatomite deposits were first investigated by Macadam (1882; 1883). In 1885, 200 tons were removed from Black Moss and 200 tons from Ordie Moss. From 1900 to 1918, the deposits in Black Moss and Ordie Moss were worked intermittently, and small-scale working also took place in Bogingore (Figure 28), the material being used in the explosives industry. The diatomite, as mined, contained approximately 87% water by weight. On drying, the product contained 87–95% SiO₂, the residue being mainly CaO, Al₂O₃ and alkalies (derived from feldspar). (Plate 9) shows the trenching required to drain and excavate the deposit. Work stopped at the end of the war in 1918, and the installations were all removed. The deposits were described in detail by Haldane and others (1940) , who quote analyses of the calcined material.

The only subsequent work is that of Gardiner and Taylor (1950), who investigated all of the depressions in the Muir of Dinnet and surrounding areas, and categorised the diatomite deposits into: (i) deposits occurring around the margins of existing lochs; and (ii) occurrences within isolated peat mosses. The following basins were found to contain diatomite: Black Moss, Ordie Moss, Braeroddach Loch, Auchnarran Moss, Newton Dam, Loch Davan (western shores), Loch Kin ord (western shores around Bogingore), and the Loch of Dinnet.

Gardiner and Taylor report reserves of 584 600 m³ of in-situ diatomite, excluding the areas worked prior to 1918 and the areas permanently under water. Assuming a density of 0.214 Mg m⁻³ of dried material (equivalent to the 6 cubic yards per ton quoted by Wilson and Hinxman (1890)), resources of approximately 130 000 tonnes of in-situ diatomite are present in the district. (Figure 28) shows the location of the diatomite deposits investigated by Gardiner and Taylor (1950); tonnages and thickness are listed in (Table 6). The Auchnarran Moss deposit, though thinner than the others, is characterised by a seam of exceptionally pure diatomite, virtually free of organic matter. However, according to P S Bide (unpublished report for BGS, 1982), the diatomite obtainable from most of the deposits in the district contains too much organic matter to be usable without calcination, and it could therefore not be considered competitive against imported material for most purposes. It should be noted, however, that several authors suggest that the floor of the central part of Loch Kinord is directly underlain by soft diatomaceous ooze without an intervening peat layer. Hence diatomaceous ooze could possibly be pumped directly to the shores of Loch Kinord, as is done at Myvatn, Iceland.

However, development is very unlikely because of the National Nature Reserve status of the Muir of Dinnet.

Groundwater

Limited quantities of water suitable for domestic use by individual farms may be obtained from wells sunk in the fractured igneous and metamorphic rocks of the district. However, the most significant groundwater resource in the district lies within the Devonian sedimentary rocks of the Rhynie outlier, though even here the permeability of the aquifer is principally due to fractures in the rocks. Drilling for village supplies for Rhynie and Lumsden took place in 1989 (Robins, 1990). Rhynie No. 1 borehole [NJ 502 276] in the Dryden Flags Formation was abandoned because the water was too rich in Fe and Mn for public supply. However, Rhynie No. 2 borehole [NJ 499 265] in the Quarry Hill Sandstone Formation and Lumsden borehole [NJ 476 223] in the Tillybrachty Sandstone Formation yielded adequate supplies of acceptable quality for the respective villages. The test pumping was carried out before the wells had been properly developed, so only a rough indication of the sustainable yield was obtained. Suggested maximum yields were 20 l/s for Rhynie No. 1 and Lumsden, and 10 1/s for Rhynie No. 2. bicarbonate-type waters. No investigation has been made of the potential of the glacial and glaciofluvial sands and gravels of the district as aquifers.

Geothermal power

The Hot Dry Rock (HDR) geothermal potential of the post-tectonic granites of the eastern Cairngorm Suite was investigated during the early 1980s by a combined BGS Open University team. The Cairngorm, Ballater, Bennachie and Mount Battock granites were investigated by geochemical analysis of surface and borehole samples. One borehole was drilled to a depth of about 300 m near the centre of each pluton (Webb and Brown, 1984). The geochemical analyses of the Ballater and Bennachie specimens are discussed in Chapter 7.

The known distribution of U, Th and K in the four granites was used to calculate their heat-producing capacity. The eastern Grampians granites proved to be the most highly radiothermal granites known in the UK.

Bennachie was estimated to have a heat production value of 7.0 μW m³ and Ballater 6.8 μW m⁻³. However, these values only relate to the topmost few hundred metres of the intrusions.

The granites of the eastern Grampian Highlands were thermally modelled on a two-dimensional basis (Wheildon et al., 1984). The crust was modelled to 30 km, but the base of the granites was taken as 13 km, as indicated by the gravity modelling of Rollin (1984). The heat flow profile across the Bennachie pluton is shown in (Figure 30)a, and the corresponding thermal model in (Figure 30)b. The model explains the combination of relatively high heat production values but relatively low heat flow in the granites of the eastern Grampian Highlands in terms of a relatively rapid decrease in heat production with depth and a relatively low background heat flow. Heat production is assumed to decrease exponentially downwards from the surface to the base of the granite.

Heat flows measured down the Ballater borehole gave 71 mW m⁻² and the Bennachie borehole 76 mW m⁻². The heat-flow and heat-production information for granitic rocks within the United Kingdom was summarised by Lee et al. (1984). The East Grampians granites and meta-sedimentary country rocks have high thermal conductivities, with the result that temperatures suitable for HDR geothermal power are not present at sufficiently shallow depths to be of interest anywhere within the region (Figure 30)c.

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Appendix 1 List of boreholes in 1:50 000 Sheets 76E and 76W

Sand and gravel assessment pits and boreholes

Full details of these will be found in Auton and Crofts (1986) (1988) and Auton et al. (1988).

Water supply boreholes

Heat flow investigation borehole

Full details may be found in Webb and Brown (1984).

Water supply boreholes at Rhynie and Lumsden

Details of pump tests may be found in Robins (1990)

Stratigraphical boreholes in Rhynie Chert area (Trewin and Rice. 1992)

Appendix 2 Geological Survey photographs

Copies of these photographs are available for reference in the library of the British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA. Colour or black and white prints and 35 mm slides may be supplied at a fixed tariff.

Photographs C2162–77 were taken about 1917 by R Lunn, C3750–61 and C3571–74 in June 1939 by W D Fisher and the D prefix photographs by F I MacTaggart in September 1989 and June and September 1990. The National Grid references are those of the viewpoints, where known.

Dalradian

Late-tectonic basic intrusions

Late- to post-tectonic granitic intrusions

Post-tectonic granitic intrusions

Quaternary

Economic geology

Views

Appendix 3 List of fossils recorded from the Devonian rocks of the district

No additional collection was carried out during the present survey. All of the forms listed below come from the Rhynie chert, except for a specimen of Pterygotus sp. from Quarryfield, Kildrummy [NJ 415 480], which is held by BGS at Murchison House, Edinburgh.

Tracks of a burrowing crustacean or annelid have been recorded from Quarry-field, Kildrummy, and obscure plant remains are abundant at Quarry Hill, Rhynie [NJ 485 252]–[NJ 491 255].

Plantae

• Algae
• Archaeothrix contexta Kidston & Lang MS
• Archaeothrix oscillators Kidston & Lang
• Kidstoniella fritschi Croft & George
• Langiella scourfieldi Croft & George
• Mackiella rotundata Edwards & Lyon
• Nematophyton taiti Kidston & Lang
• Nematoplexus rhyniensis Lyon
• Palaeonitella cranii (Kidston & Lang) Pia
• Rhynichertia punctata Edwards & Lyon
• Rhyniella vermilormis Croft & George
• Rhyniococcus uniformis Edwards & Lyon
• Fungi
• Mycokidstonia spheroidales Pons & Locquin
• Palaeomyces agglomerate Kidston & Lang
• Palaeomyces asteroxyli Kidston & Lang
• Palaeomyces gordoni Kidston & Lang
• Palaeomyces gordoni major Kidston & Lang
• Palaeomyces horneae Kidston & Lang
• Palaeomyces simpsoni Kidston & Lang
• Palaeomyces vestita Kidston & Lang
• Incertae sedis cf. Bryophyta
• Aglaophyton major (Kidston & Lang) Edwards
• Rhyniophyta
• Horneophyton lignieri (Kidston & Lang)
• Lyonophyton rhyniensis Remy & Remy
• Nothia aphylla Lyon ex Høeg
• Rhynia gwynne-vaughani Kidston & Lang
• Lycophyta
• Asteroxylon mackiei Kidston & Lang
• Zosterophyllophytales
• Trichopherophyton teuchansii Lyon & Edwards
• Spores
• Apiculiretusispora sp.
• Emphanisporites sp.
• Retusotriletes sp.

Animalia

• Arthropoda
• Chelicerata: Eurypterida
• Heterocrania rhyniensis (Hirst & Maulik)
• Pterygotus sp.
• Arachnida
• Palaeocharinus rhyniensis (Hirst) Shear et al.
• Protocarus crani Hirst
• Insecta
• Rhyniella praecursor Hirst & Maulik
• Rhyniognatha hirsti Tillyard
• Crustacea
• Lepidocaris rhyniensis Scourfield

Appendix 4 List of BGS Mineral Assessment, Environmental Geology, Hydrogeology, Mineral Reconnaissance Programme and Geothermal Investigation reports

AUTON, C A, and CROFTS, R G. 1986. The sand and gravel resources of the country around Aberdeen, Grampian Region. Description of 1:25 000 resource sheets NJ71, 80, 81 and 91, with parts of NJ61, 90 and 92, and with parts of N089 and 99. Mineral Assessment Report, British Geological Survey, No. 146.

AUTON, C A, MERRITT, J W, and ROSS, D L. 1988. The sand and gravel resources of the country around Inverurie and Dunecht and between Banchory and Stonehaven. Description of 1:25 000 resource sheets NJ70 and 72 and N079 and parts of N088, 89 and 99. British Geological Survey Technical Report, WT/88/ 1.

BRITISH GEOLOGICAL SURVEY. 1991. Regional geochemistry of the East Grampians area. (Keyworth, Nottingham: British Geological Survey.)

COLMAN, T B, BEER, K E, CAMERON, D G, and KIMBELL, G S. 1989. Molybenum mineralisation near Chapel of Garioch, Inverurie, Aberdeenshire. British Geological Survey Technical Report, NAT/89/3. (Mineral Reconnaissance Report, No. 100.)

GUNN, A G, and SHAW, M A. 1991. Investigations for Cu-Ni and PGE in the Hill of Barra area, near Oldmeldrum, Aberdeenshire. British Geological Survey Technical Report, WF/91/5. (Mineral Reconnaissance Report, No. 119.)

KIMBELL, C S. 1991. A gravity survey of the Middleton Granite, near Inverurie, Aberdeenshire. British Geological Survey Technical Report, WF/91 /6. (Mineral Reconnaissance Report, No. 120.)

LEE, M K. 1984. Analysis of geophysical logs for the Shap, Skiddaw, Cairngorm, Balloter, Mount Battock and Bennachie heat flow boreholes. Investigation of the geothermal potential of the UK (Keyworth, Nottingham: British Geological Survey.)

LEE, M K, WHEILDON, J, WEBB, P C, BROWN, G C, ROLLIN, K E, CROOK, C N, SMITH, I F, KING, G, and THOMAS BETTS, A. 1984. HDR prospects in Caledonian granites: evaluation of results from the BGS-IC-OU research programme (1981–84). Investigation of the geothermal potential of the UK (Keyworth, Nottingham: British Geological Survey.)

MERRITT, J W, AUTON, C A, and ROSS, D L. 1988. Summary assessment of the sand and gravel resources of northeast Scotland. British Geological Survey Technical Report, A1'F/88/2.

MERRITT, J W, and PEACOCK, J D. 1983. A preliminary study of the sand and gravel deposits around Aberdeen. (Covers 1:25 000 sheets NJ71, 80, 81, 90, 91 and parts of 61 and 92, and N069 and 79 and parts of 89). (Keyworth: Institute of Geological Sciences.)

PEACOCK, J D, CLARK, G C, MAY, F, MENDUM, J R, ROSS, D L, and RUCKLEY, A E. 1977. Sand and gravel resources of Grampian Region. Report of the Institute of Geological Sciences, No. 77/2.

ROBINS, N S. 1990. Groundwater development in the Rhynie outlier. British Geological Survey Technical Report, WD/90/2.

ROLLIN, K E. 1984. Gravity modelling of the Eastern Highlands granites in relation to heat flow studies. Investigation of the geothermal potential of the UK. (Keyworth, Nottingham: British Geological Survey.)

SMITH, C G. 1986. Planning for development: Aberdeen Project. British Geological Survey Technical Report, WA/86/1.18.

WEBB, P C, and BROWN, G C. 1984. The Eastern Highlands granites: heat production and related geochemistry. Investigation of the geothermal potential of the UK. (Keyworth, Nottingham: British Geological Survey.)

WHEILDON, J, KING, G, CROOK, C N, and THOMAS-BETTS, A. 1984. The Eastern Highlands granites: heat flow, heat production and model studies. Investigation of the geothermal potential of the UK (Keyworth, Nottingham: British Geological Survey.)

Figure, plates and tables

Figures

(Figure 1) Topographical sketch map of the Inverurie–Alford district.

(Figure 2) Generalised Solid geology of the Inverurie–Alford district. List of intrusions. Late-tectonic basic-ultramafic intrusions: 1. Insch 2. Boganclogh 3. Morven–Cabrach 4. Tarland 5. Lawel Hill 6. Kildrummy 7. Lynturk Late-tectonic granitic intrusions: 8. Kennethmont 9. Kemnay 10. Tillyfourie 11. Syllavethy 12. Corrennie Post-tectonic granitic intrusions: (a) Crathes Suite 13. Gask 14. Torphins 15. Crathes 16. Balblair 17. Clinterty 18. Kincardine O'Neil 19. Lumphanan 20. Tomnaverie 21 . Logie Goldstone (b) Cairngorm Suite 22. Bennachie 23. Hill of Fare 24. Cromar 25. Ballater 26. Cushnie 27. Ord Fundlie 28. Middleton

(Figure 3) Geological map of the Scar Hill–Queen's Hill area (dykes omitted for clarity).

(Figure 4) Aeromagnetic anomaly map of the Inverurie–Alford district and surrounding areas, with simplified geology.

(Figure 5) Distribution of major shear belts in north-east Scotland (modified after Ashcroft et al., 1984).

(Figure 6) Block diagram illustrating the relations of the primary structures in the Dalradian rocks of the northeast Grampian Highlands (after Ashcroft et al., 1984).

(Figure 7) Bouguer gravity anomaly map of the Inverurie–Alford district and surrounding areas, with simplified geology.

(Figure 8) Geophysical profiles and model across the Inverurie–Alford district along line G–G9 (figures 4 and 7) (a) Magnetic profile (b) Bouguer gravity profile (c) Geophysical model. Aeromagnetic and Bouguer gravity anomalies modelled in 2.5D to a depth of 10 km along a section line from [NJ 000 525] to [NO 910 975]. Polygons have half-strike lengths between 10 km and 50 km. A list of polygon properties and formations is given in ((Table 1)).

(Figure 9) Ground magnetic survey of the Pitcaple area (survey carried out for the BGS by Aberdeen University under supervision of W A Ashcroft, 1986). (a) total magnetic field anomaly (b) geological interpretation

(Figure 10) Map showing distribution of certain metamorphic index minerals in the Dalradian rocks of the north-east Grampian Highlands (modified from Harte and Hudson, 1979; Porteous, 1973). Contact metamorphic effects removed, as far as possible.

(Figure 11) Late-tectonic basic/ultramafic intrusions of the north-east Grampian Highlands.

(Figure 12) Geological map of the Boganclogh and Kennethmont intrusions, and western part of the Insch intrusion. Symbols and colours as for ((Figure 13)).

(Figure 13) Geological map of the Insch intrusion.

(Figure 14) Plots of En in orthopyroxene against Mg# in clinopyroxene, for Insch, Boganclogh and Morven–Cabrach intrusions (data from Wadsworth, 1988 with additional unpublished analyses by the author and W J Wadsworth).

(Figure 15) Geological map of the Tarland and Kildrummy intrusions, and eastern part of Morven–Cabrach intrusion.

(Figure 16) Geological map of the granitic intrusions north of Banchory.

(Figure 17) Normative QAP plots for granitic rocks from the Inverurie–Alford district. (a) Late-tectonic granitic intrusions (data from Walsworth-Bell, 1974 and Busrewil et al., 1975). (b) Crathes Suite (data from Walsworth-Bell, 1974). (c) Cairngorm Suite (data from Walsworth-Bell, 1974 and O'Brien, 1985).

(Figure 18) AFM plots for granitic rocks from the Inverurie–Alford district. (a) Late-tectonic granitic intrusions (data from Walsworth-Bell, 1974 and Busrewil et al., 1975). (b) Crathes Suite (data from Walsworth-Bell, 1974) A = (Na₂O + K₂O); F = (Fe0 ( total) + MnO); M = MgO All oxides expressed as weight percentages.

(Figure 19) Plots of (a) Rb, (b) Sr, (c) Y and (d) Zr against TiO₂ and (e) Rb against Sr for late-tectonic granitic intrusions (data from Busrewil et al., 1975, and BGS analyses of Walsworth-Bell's samples).

(Figure 20) Variation in (a) Rb, (b) TiO₂, (c) Zr and (d) U within the Bennachie Granite (after Webb and Brown, 1984),

(Figure 21) Plots of (a) Rb, (b) Sr, (c) Y and (d) Zr against TiO₂ for the members of the Crathes Suite (data from BGS analyses of Walsworth-Bell's samples).

(Figure 22) Plots of (a) Rb, (b) Sr, (c) Y and (d) Nb against Zr for the Hill of Fare and Bennachie granites (Hill of Fare data from BGS analyses of Walsworth-Bell's samples and from O'Brien, 1985; Bennachie data from Webb and Brown, 1984 and O'Brien, 1985).

(Figure 23) Post-Caledonian faults and late Carboniferous dykes.

(Figure 24) The Rhynie and Towie outliers. (a) Geological map (b) Cross-section A–B (c) Cross-section C–D

(Figure 25) Geological map of the Rhynie Chert locality, with locations of pits and boreholes (redrawn from Trewin and Rice, 1992).

(Figure 26) Stratigraphical sections within the Quarry Hill Formation on Quarry Hill [NJ 485 252]–[NJ 488 255] (from Archer, 1978).

(Figure 27) Simplified map of the Quaternary geology of the Inverurie–Alford district.

(Figure 28) Diatomite occurrences in the Muir of Dinnet–Logie Coldstone area (after Gardiner and Taylor,.1950).

(Figure 29) Sand and gravel resources of the eastern part of the Inverurie district (after Auton and Crofts, 1986, and Auton et al., 1989). 1. Don floodplain, Pitfichie 2. Dalbraidie 3. Nether Mains 4. Kemnay north 5. Kemnay south 6 Monyroads 7. Seuchen 8. Lescha ogle 9. Don floodplain, north of Kintore 10. Tavelty–Fullerton 11. Nethermill–Toft Hills 12. Wester Fintray 13 Dunecht 14. Garlogie–Leuchar Moss 15. Woodside–Finnarcy 16. Horsewells–Hardgate 17. Whiteford 18. Inverurie 19. Burnhervie–Port Elphinstone 20. Dalmaik

(Figure 30) Preferred thermal model for the Bennachie Granite (after Wheildon et al., 1984). (a) Heat flow profile across the model (b) The thermal model (c) Predicted subsurface temperature profiles: (1) Moine–Dalradian basement with q₀ = 40m Wm⁻² (2) Moine–Dalradian basement with q₀ = 55m Wm⁻² (3) Bennachie Granite (4) Ballater Granite

Plates

(Front cover) Kenmay Quarry (Kenmay Granite) with Bennachie (Bennachie Granite) in the distance Most of the intervening ground is underlain by Dalradian metasedimentary rocks [NJ 7387 1685] (D4338).

(Rear cover)

(Frontispiece) Bennachie (Bennachie Granite) from Pitscurry Quarry, Pitcaple. Low ground in foreground is underlain by gabbroic rocks of the Insch intrusion [NJ 728 266] (D4331).

(Geological succession) Geological sequence in the Inverurie–Alford district

(Plate 1) Photomicrographs of Dalradian metasedimentary rocks. (a) Feldspar porphyroblast gneiss with sillimanite-rich restite xenolith mantled by biotite, Queen's Hill Formation. Scar Hill, Tarland, 250 m north-east of summit [NJ 4813 0150] (S78539), plane polarised light. (b) Gritty psammite with fine-grained matrix, Suie Hill Formation. Lower quarry beside A944, Invermossat 14903 1865] (S78656), crossed polars. (c) Andalusite-cordierite-magnetite-schist, Suie Hill Formation. Correen Quarry [NJ 5218 2133] (S78663), crossed polars. (d) Cordierite-fibrolite-K-feldspar-hornfels, Clashindarroch Formation. 1000 m south-east of summit of Tap o' Noth [NJ 4932 2886] (S78702), crossed polars.

(Plate 2) (a) Xenolithic gneiss with folded remnants of psammite dispersed in poorly-foliated feldspar-porphyroblast gneiss. Queen's Hill Formation, Craigie, 1.5 km north-east of Dinnet [NJ 476 007] (D4529). (b) Andalusite-cordierite-schist, coarsely porphyroblastic. Suie Hill Formation, Correen Quarry (disused), 5 km ESE of Lumsden [NJ 5225 21351 (D 4343).

(Plate 3) The 'Red Rock Hills': Hill of Christ's Kirk (left, distance) and Hill of Dunnideer (centre, distance, with ruined castle on summit), from B992 1.5 km north of Auchleven. The 'Red Rock Hills' are made of syenite and olivine-monzonite of the Insch intrusion, while the surrounding lower ground is underlain by olivine-ferrogabbro of the Insch intrusion. The high ground beyond the 'Red Rock Hills' is underlain by hornfelsed Dalradian metasedimentary rocks at the northern margin of the Insch intrusion [NJ 624 2611 (D 4543).

(Plate 4) Photomicrographs of rocks from the Insch intrusion. Olivine-norite (olivine-plagioclase-orthopyroxene cumulate), Insch intrusion, Lower Zone. 400 m south–east of Barra Castle [NJ 7955 2545] (S2782), plane polarised light. Norite (orthopyroxene-plagioclase cumulate), Insch intrusion, Middle Zone. 40 m east of Lady's Bridge, Logie Durno [NJ 6966 2655] (S 74730), crossed polars. Granular-textured clinopyroxene-norite, Insch intrusion, Middle Zone. 80 m south-east of Lady's Bridge, Logic Durno [NJ 6966 2649] (S 74729), crossed polars. Olivine-monzonite (oliyine-clinopyroxene-plagioclase-K-feldspar-ilmenomagnetite-apatite-zircon cumulate), Insch intrusion, Upper Zone. 30 m east of Bridge of Johnston [NJ 5699 2478] (S77439), crossed polars.

(Plate 5) (a) Foliated biotite-granite with rare biotitic schlieren, Kemnay Granite (quarried block), Kemnay Quarry [NJ 7383 1693] (D 4336). (b) Tillyfourie Tonalite (coarser-grained, more mafic) intruded by Balblair Granodiorite. Upper Kebbaty, 7.5 km north-east of Torphins [NJ 6575 0785] (D4344). (c) Corrennie Granite in contact with Tillyfourie Tonalite, Corrennie Quarry, Monymusk [NJ 6420 1188] (D4339). (d) Pink coarse-grained granodiorite in contact with diorite xenolith, which is veined by granodiorite. Pegmatitic development at contact. Both diorite and pink granodiorite are veined by fine-grained blue-grey granodiorite. Block, Craigenlow Quarry, Dunecht. [NJ 732 092] (D4519).

(Plate 6) Norite intruded by 10 m-thick sheet of pegmatite, with narrow pegmatite veins coming off main sheet, Pitscurry Quarry, Pitcaple [NJ 7290 2762] (D 4332).

(Plate 7) (a) Corbie's Tongue Conglomerate Formation; conglomerate, with rounded cobbles, mainly quartzite, set in a fine-grained red sandstone matrix. Corbie's Tongue, 2.3 km SSW of Rhynie [NJ 4926 2440] (D 4323). (h) Quarry Hill Sandstone Formation; sole markings on underside of loose block. Quarry Hill, Rhynie [NJ 486 254] (D 4539).

(Plate 8) Photomicrograph of chert with silicified plant stems, Rhynie Chert Member. Trench No. 3, Rhynie [NJ 4941 2767] (S19505), plane polarised light.

(Plate 9) Diatomite overlain by peat; vertical trenches show method of draining peat in 1917. Black Moss, 2.5 km north of Dinnet Station [NJ 468 015] (C2165).

(Plate 10) Crushing plant, Craigenlow Quarry, Dunecht, [NJ 732 092], working Crathes and Balblair granodiorites, in 1989 (D4519A).

Tables

(Table 1) Magnetic and gravity properties of the polygons used in calculating the model of ((Figure 8)) (c).

(Table 2) Variation of mineral composition in the Insch, Boganclogh and Belhelvie intrusions.

(Table 3) Summary of mineral composition from the Bogangloch. Tarland. Kildrummv and I.vntnrk intrusions

(Table 4) Potentially exploitable sand and gravel resources in the eastern part of the Inverurie district (after Merritt et al., 1988).

(Table 5) Peat resources of the Inverurie–Alford district (from Fraser, 1948).

(Table 6) Diatomite resources in the Muir of Dinnet–Logie Coldstone area (after Gardiner and Taylor, 1950).

Tables

Table 1 Magnetic and gravity properties of the polygons used in calculating the model of (Figure 8) (c).

Table 3 Summary of mineral composition from the Bogangloch. Tarland. Kildrummv and I.vntnrk intrusions

Table 4 Potentially exploitable sand and gravel resources in the eastern part of the Inverurie district (after Merritt et al., 1988).

Numbers correspond to those on (Figure 29). * Deposit crosses boundary of Inverurie district.

Deposits 1, 2, 5 and 10 were regarded as having the highest potential for extraction.

Table 5 Peat resources of the Inverurie–Alford district (from Fraser, 1948).

Table 6 Diatomite resources in the Muir of Dinnet–Logie Coldstone area (after Gardiner and Taylor, 1950)