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Geology of the Aboyne district. Memoir for 1:50 000 geological sheet 66W (Scotland)

By D. Gould

Bibliographical reference: Gould, D. 2001. Geology of the Aboyne district. Memoir of the British Geological Survev, Sheet 66W (Scotland)

London: The Stationery Office 2001. NERC copyright 2001 First published 2001. ISBN 0 11 884553 5. Printed in the UK for The Stationery Office. Tj5001 C6 03/01

The grid used on the figures in this memoir is the National Grid taken from the Ordnance Survey map. (Figure 2) is based on material from Ordnance Survey 1:50 000 scale maps, numbers 44 and 45. © Crown copyright reserved. Ordnance Survey licence no. GD272191/2001

(Front cover) Cover photograph Coarse clast-supported orthoconglomerate, Gannochy Formation, Strathmore Group, Old Red Sandstone, River North Esk, Loups Bridge [NO 5948 7169] (D5347). Photographer: Tom Bain

(Rear cover)

Notes

Acknowledgements

The surveyors were C A Auton, S Carroll, D Gould and S Robertson (BGS) and B Harte (University of Cambridge, now at the University of Edinburgh).

The memoir has been written by Dr D Gould. Chapter 3 was written by K E Rollin. Some structural information from the North Esk Gorge has been taken from work by J E Booth and B Harte (University of Edinburgh). Chapters 4, 8 and 9 have been critically reviewed by J R Mendum. Chapter 6 has been critically reviewed by I B Cameron, and incorporates information from S Carroll's survey on Sheet 66E Banchory. Chapter 7 has been edited by A J Highton. Chapter 10 (Cainozoic) incorporates relevant. information obtained during work by C A Amon and C W Thomas in the Banchory and Stonehaven districts (Sheets 66E and 67) (Anton et al., 1990) and has been edited by C A Anton. The advice of M T Dean in compiling the Appendix is acknowledged. The memoir has been edited by A D McAdam. The figures were produed by BGS Cartography, Murchison House.

The co-operation of landowners in the district, especially of Glen Tanar, Fasque and Invermark estates, is gratefully acknowledged.

The surveyors of the component 1:10 000 sheets included wholly or partly in Sheet 66W are listed in the section entitled 'Information sources' together with the date of survey.

Preface

Understanding the geological framework of Britain is important to the future of the UK, whether in relation to exploration for resources, the avoidance of hazards, land-use planning, or conservation of geological and biological sites. Accordingly, central Government allocates funding to BGS to improve our knowledge of the three-dimensional geology of the UK national domain through a programme of data collection. interpretation and research, publication and archiving. One aim of this programme is to ensure by the year 2005 comprehensive coverage of the UK land mass by modern 1:50 000 scale geological maps accompanied by written explanations of the geology. This memoir on the Aboyne district of the East Grampians region of Scotland is part of the output from that programme.

The memoir is based on a resurvey of the solid geology of the district by the BGS. together with university research on the structural and metamorphic history of the Dalradian rocks around Invermark. It provides the first comprehensive account of tilt solid geology of this part of the eastern Grampian Highlands, and incorporates information from geophysical surveys conducted for metalliferous mineral exploration Recent surveys of the superficial deposits cover only a very small part of the district. and only a brief summary of the Quaternary geology is given, based largely on work done a century ago. This is in no way a reflection on the importance of developing 2. better understanding of the Quaternary, but rather is a reflection of current priorities in the light of limited budgets at the present time.

The Dalradian rocks which underlie most of the distict originated in late Precambriar, times as sediments deposited on the edge of the expanding and deepening Iapetus Ocean. During the Caledonian Orogenv, mainly in Ordovician to end-Silurian times, the Dalradian rocks suffered polyphase folding, and were regionally metamorphosed, the intensity of metamorphism increasing rapidly northwards from the Highland Border.

After the late Silurian to early Devonian mountain-building movements, the voluminous East Grampians Batholith, including the Mount Battock pluton, most of which lies within this district, was intruded at a high level in the crust.

Also during the Ordovician, deposition took place in a back-arc basin several hundred kilometres to the south of the site where the Dalradian rocks were being folded, metamorphosed and intruded. Basaltic pillow lavas interbedded with chert and black mudstone were unconformably overlain by coarse sandstones with beds of mudstone and rare limestone; these now form the Highland Border Complex, which was folded and weakly metamorphosed in the latest Ordovician before being juxtaposed against the Dalradian rocks in the Silurian by transcurrent faulting related to the closure of the Iapetus ocean.

South of the Highland Boundary Fault, lower Devonian rocks, derived from fluviatile sediments and minor volcanic rocks, record early stages in the uplift and erosion of the rocks of the Grampian Highlands.

During the Quaternary, repeated glaciation affected the landscape, and several conspicuous rock-cut meltwater channels were excavated. The end of the latest, Devensian, glaciation in the area was marked by the deposition of extensive glaciofluvial sands and gravels in Deeside and close to the Highland Boundary Fault in Strathmore.

Old, small-scale workings for silver, associated with lead in narrow veins, occur in Glen Esk. Pyrite veins along some faults may he associated with auriferous mineralisation. Very little exploitation of the hard rock or sand and gravel resources of the district has taken place, due to distance from urban centres, and little development has occurred in the district. The reality is that the primary resource of this area is of course its scenic beauty and its unspoilt character.

David A Falvey, PhD Director, British Geological Survey, Sir Kingsley Dunham, Keyworth Nottingham NG12 5GG

Geology of the Aboyne district—summary

This memoir describes the solid geology of a mountainous area at the southern edge of the Grampian Highlands; a brief summary of the drift geology of the district included. Apart from a small area of Silurian and Devonian sedimentary and volcanic rocks in the south-east, it is entirely underlain by metamorphic and igneous :-ocks.

The Dalradian sediments were deposited on the margin of the spreading Iapetus Ocean in late Precambrian times. Sands, calcareous silts and limestones deposited in shallow water were overlain by poorly sorted sands, silts and muds deposited in deep water by turbidity currents Dolerite sheets were intruded into the Dalradian sediments shortly after deposition.

The Dalradian rocks were deformed and metamorphosed during the Caledonian Orogeny. Large-scale nappe smctures were formed in the late Precambrian. In the Ordovician, three episodes of folding were accompanied by regional metamorphism. The northward increase in the maximum metamorphic grade is marked in pclitic rocks by the incoming of certain index minerals in sequence. The classic Barrovian sequence of index minerals was first described from the Aboyne district. During the last main told episode, brittle nmonoclinal folding close to the Highland Border accompanied retrogressive metamorphism. A few small bodies of muscovite granite were intruded at this time.

Also during Ordovician times, the rocks of the Highland Border Complex were formed in an island arc and a later back-arc basin, probably several hundred kilometres away from the orogenic belt. Basaltic pillow-lavas are intercalated with deep-water chert and dark mudstone. These are overlain unconformably by coarse sands and muds with thin limy layers. The strata were folded and weakly metamorphosed before being obducted on to the Dalradian block during the Silurian.

In the late Silurian and early Devonian, many postorogenic granitic plutons and associated minor intrusions were emplaced into the Dalradian block, coeval with uplift. Much of the northern part of the district is underlain by the Mount Battock granite pluton.

In latest Silurian and early Devonian times, red, fluviatile silts, sands and gravels, with some dacitic tuffs, were laid down in the Strathmore Basin, adjacent to a fault scarp bounding a rising mountain belt to the north. Progressive transcurrent faulting brought the Highland and Midland Valley terranes together during the Devonian. Quartz-dolerite dykes were intruded along east-northeast-trending fractures in late Carboniferous times.

Following extensive Cainozoic weathering, Quaternary glaciation removed the loose superficial material from the higher ground. Meltwater from the decay of ice-sheets cut large channels in bedrock. During the Late Devensian glaciation, which ended about 13 000 years ago, ice movement was to the north-east in the southeastern part of the district. An ice stream that flowed along Strathmore laid down deposits of red till, which can be distinguished from the yellow-brown till deposited by ice flowing over the Grampian Highlands. When the ice-sheet melted, large quantities of sand and gravel were deposited and then reworked by large river systems. In postglacial times, diatomite formed locally in shallow lakes in moundy terrain. Peat formed both in waterlogged lowland areas and as a blanket on hill tops.

Chapter 1 Introduction

The nature of the underlying rock and the history of erosion, uplift and glaciation have had a strong influence on the form of the land throughout Scotland, and hence have controlled patterns of settlement and land use for several thousand years. Where deposits of workable building materials, industrial minerals or metallic ores occurred. their exploitation has influenced human settlement and infrastructure development. Sites for damming surface water, or permeable rocks from which water could be pumped, have also had an important influence on the economic development of the country. As development becomes more intensive, the need for geological input into the planning process becomes more important, in order to optimise use of scarce resources, minimise the impact of hazards such as subsidence and landslips, and allow for conservation of areas which, through the interaction of geology, topography, climate and previous land use, contain unique associations of plants and animals, or unique geological features.

This memoir describes the solid geology of the Aboyne district, and is designed to complement the 1:50 000 Series geological Sheet 66W Aboyne, which covers the same area (Figure 1). The solid geology of all of the district, except for the part of Harte's (1966) PhD thesis area lying in the Aboyne district, was resurveyed between 1987 and 1990, and all relevant previous geological work in the district was assessed and incorporated into the geological interpretation. A directory of other BGS geological information sources covering the district, including maps, reports and databases, forms a section at the end of this memoir. A summary description of the geology is given in this chapter. Chapter 2 describes the applied geology of the district, with particular reference to geological resources and constraints of importance in planning and development. The deep structure of the district is discussed in Chapter 3. Detailed descriptions of the solid geology at surface or rockhead follow in Chapters 4 to 9. A brief summary of the Cainozoic geological history of the district, based on the original (1890s) survey, BGS work in neighbouring districts, and work done outside BGS, is given in Chapter 10.

In this memoir, the geological timescale of Harland et al. (1990 is used to correlate chronostratigraphy with radiometric ages; although significant revisions have recently been proposed to several critical boundaries (e.g. Silurian–Devonian) (see Gradstein and Ogg, 1996 for a summary).

Location and physical features

The district lies mostly in the new Aberdeenshire council area, but about 30 per cent lies in Angus council area (Figure 2). The main settlement is Aboyne (population 1529 in 1981) and the small villages of Kincardine O'Neil, Dinnet, Finzean, and Tarfside also lie within the district. The district is mostly moorland, with substantial areas of forest in lower Glen Tanar. Improved pasture with minor arable land occurs in the Dee valley, lower Glen Esk and part of Glen Lethnot. There is very little employment in the district, except from the tourist/leisure industry in the Dee valley; deer stalking and fishing are important activities.

Apart from the floodplains and river terraces of the Dee, Feugh and North Esk, there is no flat land in the district. The northern half of the district lies within the catchment of the River Dee, partly by way of its tributaries, the Water of Tartar, Water of Feugh, Water of Dye and Cattie Burn. The southern half is drained by the River North Esk and its tributary the West Water. The altitude varies from just below 60 m near Gannochy Bridge in Glen Esk and 80 m near Potarch in Deeside to 887 m on Braid Cairn. Mount Battock (778 m), Hill of Cat (742 m), Cruys (741 m), Hill of Wirren (678 m), Peter Hill (617 m) and Clachnaben (589 m) are other prominent hills in the district. The roughly concordant hill summits, between 700 and 900 m above OD or slightly lower, and declining steadily from west to east, arc regarded as remnants of a 'Grampian Surface', which was probably formed during a period of relative stability during the Cainozoic uplift of a landscape with a subdued topography. The tipper slopes of the hills are generally rounded, but the lower slopes are generally steeper, due to glacial erosion of the valleys. Many of the natural exposures occur in the beds of the smaller streams but, in addition, the steep sides of the glacially eroded valleys contain abundant rock exposures, especially in parts of Glen Esk, in Glen Effock and in lower Glen Mark. The Muir of Dinnet, lying partly in the district, is a large amphitheatre-like hollow which contains the rock basin occupied by Loch Kinord. The only other sizeable area of surface water in the district is Loch Lee in the floor of the steep-sided Glen Lee, now used as a water-supply reservoir.

Summary of the geology of the district

Over most of the Aboyne district, the bedrock is within a few metres of the land surface. Superficial deposits, formed during or since the Pleistocene glaciations, are thin, except in or close to the main river valleys but, nevertheless, hide the bedrock from view in the more gently sloping ground. Man-made excavations, such as quarries, pits, drainage ditches and boreholes, are few in number and largely confined to the valleys of the Dee and North Esk. Man-made deposits are absent, except in the floors of old quarries and pits.

Almost all of the Aboyne district lies within the Grampian Highlands, which are made of rocks of the Caledonian Orogenic Belt. These represent the roots of a large mountain belt, produced by continental collision 400 to 500 million years ago, when subduction in the earth's mantle caused the Iapetus Ocean, formed 700 to 590 million years ago, to be consumed. Due to the subsequent opening of the North Atlantic in Mesozoic times, remnants of this belt are now preserved in Greenland, Norway, Scotland, Ireland and eastern North America.

The Grampian Highlands are bounded to the northwest by the sinistral Great Glen Fault and to the southeast by the Highland Boundary Fault, which has a large downthrow to the south-east. The rocks exposed at surface include metasedimentary rocks of late Proterozoic age, together with later igneous and sedimentary rock:5 (Figure 1), (Figure 3). The late Proterozoic Dalradian Supergroup has a total thickness of about 20 km, as measured from exposure, although the total thickness preserved in any one place is probably considerably less.

The Highland Boundary Fault and related faults traverse the south-eastern corner of the Aboyne district. To the south-east of these faults lies the Midland Valley of Scotland, where almost all of the rocks which formed prior to the closure of the Iapetus ocean are covered by sedimentary and volcanic rocks formed between 415 and 285 million years ago, in the Silurian to Permian eras.

Dalradian

This name was applied by Geikie (1891) to the rocks derived from sediments laid down at the margin of a continent during the opening of the Iapetus Ocean, between about 700 and 590 million years ago. Both because of their Precambrian age and because of the metamorphism to which they have been subjected, the rocks are unfossiliferous. They have been divided into four groups. The oldest (Grampian) group represents sandy sediments with rare silts and muds laid down in a delta or shallow sea. The Appin Group represents rapidly alternating deposits on the floor of a shallow sea, on which muds, clean-washed sands and limestones here deposited. The lowest part of the overlying Argyll Group contains rocks containing boulders deposited from a floating ice sheet, recording a glaciation which is known to have occurred in many parts of the world about 650 million years ago. The upper part of the Argyll Group contains beds of quartzite and gritty, turbiditic psammite, alternating with dark muds and volcaniclastic deposits, recording the formation of small basins close to the Laurentian continental margin. There is a prominent limestone formation at the top of the group in much of the Grampian Highlands. In the succeeding Southern Highland Group calcareous rocks are scarce; apart from a few beds of volcaniclastic material, the rocks of this group were deposited as deep-water turbiditic sediments with a grain size ranging from pebbly grit to mudstone.

The oldest rocks exposed in the Aboyne district belong to the upper part of the Argyll Group. In Deeside, feldspathic psammites and gneissose semipelites and pelites 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.

The Tayvallich Subgroup is represented by the Deeside Limestone Formation, which underlies the Queen's Hill Formation in an inverted succession, and by the Tarfside Psammite Formation, which is dominantly psammitic but contains several thin beds of calcsilicate rock and calcareous psammite. In Glen Esk, significant amounts of semipelite are interbedded with psammite at the top of the formation. This heralds a transition to the situation in the Stonehaven, Aberdeen and Inverurie districts, where the absence of a mappable calcareous unit at the top of the Argyll Group makes it difficult to identify the base of the Southern Highland Group.

The Southern Highland Group rocks represent an overall pattern of turbiditic sedimentation, with beds of gritty psammite, best developed close to the Highland Border, becoming important. The distinction between a dominantly semipelitic lower unit (Glen Fffock Schist Formation) and a dominantly psammitic upper unit (Glenlethnot Grit Formation) is analogous to that in Perthshire and Glen Clova, except that no volcaniclastic unit equivalent to the Green Beds is recognised here or in areas to the east. Amphibolite sills and sheets, representing metamorphosed dolerite intrusions, are widely scattered in the Dalradian rocks of the district, but are most abundant in the Tarfside Psammite Formation. They are believed to have been intruded shortly after deposition of the Dalradian sediments, and may he coeval with the Green Beds extrusives of the Ballater district.

Highland Border Complex

The Highland Border Complex consists of rocks which were probably deposited several hundred kilometres away from the Dalradian rocks against which they are now juxtaposed. They now crop out sporadically along the south-eastern edge of the Grampian Highlands from Arran to Stonehaven, and also in Ireland. At least five rock units, possibly ranging from Lower Cambrian (about 550 million years) to Upper Ordovician (about 435 million wears), are represented within the complex. The rocks of the complex record the development of a hack-arc hasir on the northern side of the Iapetus Ocean. Only two units of the complex occur in the Aboyne district. The North Esk Formation consists of basaltic pillow lavas interbedded with cherts and dark mudstones, and is of Llanvirn to Llandeilo age (Curry et al., 1984). The Margie Formation, possibly of Caradoc to Ashgill age (but see Chapter 5), consists of coarse-grained psammite and dark pelite, with thin beds of limestone. There is evidence of a possible unconformitv between the two formations in the North Esk river section.

Old Red Sandstone

To the south of the Highland Boundary Fault, Upper Silurian and Lower Devonian rocks of Old Red Sandstone facies crop out. Only a small portion of the upper part of the 9 km-thick succession exposed in the Banchory and Stonehaven districts occurs in the Aboyne district. The narrow outcrop of the Lintrathen Tuff (Crawton Group), of latest Silurian age, is hounded to the north-west by the Highland Boundary Fault., and to the south-east it is faulted against rocks of the Pragian (Lower Devonian) Strathmore Group. The latter is represented by a fluviatile red-bed succession. The lowest formation exposed in the district, against the fault bounding the Lintrathen Tuff, is the red, silty Cromlix Mudstone, which is succeeded by the Gannochy Formation, consisting of conglomerate containing rounded quartzite cobbles. This is succeeded by the Edzell Sandstone Formation, a thick red pebbly sandstone unit with thin beds of matrix-supported conglomerate. The Strathmore Group rocks dip gently south-east at the south-east corner of the district, but within 1 km of the Highland Boundary Fault they are tilted to become vertical, and are even overturned in places. Vertical movement continued along the Highland Boundary Fault during the Lower Devonian but, by analogy with the Blairgowrie district to the south-west, a thin cover of Lower Devonian rocks was probably also deposited to the north of the Highland Boundary Fault. No Middle or Upper Devonian rocks occur in the district.

Igneous intrusions

A late-tectonic episode of granitic intrusion marks the first period of major uplift of the Caledonian orogenic belt in Scotland at about 460 to 440 Ma (Dempster, 1985). No large bodies of granite were emplaced in the district at this time, but some sheets of muscovite granite up to 100 m wide are correlated with Cairn Trench Granite of the Ballater district (Robertson, 1991), which has been dated by the Rb/Sr whole-rock isochron method at 453 ± 3 Ma (S Robertson, verbal communication).

The bulk of the granite in the district was intruded after the deformation and metamorphism of the Dalradian rocks. A number of intrusions ranging from quartz-diorite to granite has been grouped together as the Cradles Suite. The limited outcrop of these rocks in the district includes parts of the Torphins Diorite and Kincardine O'Neil Granodiorite, plus a small mass of diorite at Boat of Kincardine. The slightly later Cairngorm Suite consists of intrusions of biotite granite which are marked by close petrological and geochemical similarities. The Mount Battock pluton is at 360 km2 the second largest granite complex in the United Kingdom (after Cairngorm), and about 60 per cent of its outcrop lies in the Aboyne district. A small portion of the Ballater pluton also lies in the district, as does part of the small Ord Fundlie Granite. The name East Grampians Batholith was given by Plant et al. (1990) to the assemblage of granitic plutons which is probably continuous at depth over an extent of 150 km front Monadhliath to the eastern extremity of the Mount Battock pluton. The Mount Battock Granite was intruded in at least eight phases of varying grain size and porphyritic character. Extensive hydrothermal activity has caused reddening and loss of magnetic susceptibility in much of the granite exposed at surface. The constituent plutons of the East Grampians Batholith give rise to complex aeromagnetic anomalies, and the batholith is marked by a large and deep negative Bouguer gravity anomaly. The granites of the batholith are cut by veins and sheets of pegmatitic and aplitic granite, and also by north–south zones of brecciated and silicified aplitic microgranite.

Post-tectonic minor intrusions related to both suites of granitic intrusions, are common in the district; they are mostly dykes with some sills. Felsite, quartz-porphyry and microdiorite are the commonest lithologies, but lamprophyre (spessartite and vogesite) dykes are present.

Structure

Evidence of four deformation episodes can he recognised in the eastern Grampian Highlands. The earliest episode, which produced large scale recumbent SE-directed nappes, is currently believed to postdate the 590 ± 2 Ma Ben Vuirich Granite (Tanner, 1996). This episode is responsible for the regional inversion of the stratigraphy to the north of the Mount Battock Granite. The second fold episode predates emplacement of the 'Younger Basic' masses of adjacent districts. It resulted in intensification of D1 structures and was responsible for the pervasive cleavage of the Southern Highland Group rocks; however, no major folds of this age have been identified in the Aboyne district.

Kilometre-scale D3 folds and associated metre-scale minor folds with gently dipping axial planes are developed in the southern part of the eastern Grampian Highlands, though they are generally absent within 5 km of the Highland Boundary Fault. The third deformational episode occurred shortly after the peak of regional metamorphism and is approximately coeval with the intrusion of the 'Younger Basic' suite. A period of uplift occurred at about 440 to 460 Ma (Dempster, 1985), and this may be coeval with the Highland Border Downbend, a monoclinal fold which occurs a few kilometres north of the Highland Boundary Fault. The fourth fold episode, which produced kinks and crenulations of all earlier structures, probably postdates the Highland Border Downbend and may have been roughly contemporaneous with the retrogressive metamorphism which affects parts of the eastern Grampian Highlands. The intrusion of the Mount Battock Granite at about 416 Ma produced local updotning of the early nappe structures; this has resulted in erosion of the inverted limb of the Tay Nappe south of the Mount Battock pluton.

The Highland Border Complex rocks were obducted against the south-east margin of the Dalradian terrane, probably in late Ordovician times (Hutton, 1987), marking the closure of the Iapetus Ocean (Figure 4). The Highland Border Complex rocks are weakly metamorphosed. The pillow lavas are now largely converted to chlorite schists, and the pelites have a strong slaty cleavage. The chests and some of the pelites and psammites are strongly sheared and, in places, mylonitised. Much of the shearing and mylonitisation of the Highland Border Complex may have occurred during the obduction process; mylonitisation extends into the Dalradian rocks of the Banchory district (Sheet 66E). The line of the transcurrent faulting during the late Silurian and early Devonian which brought together the Highland Border and the Midland Valley terrane is marked by the Highland Boundary Fault system. Vertical movement along the Highland Boundary Fault was roughly contemporaneous with the deposition of the Silurian and Devonian sedimentary rocks in the Strathmore syncline.

Metamorphism

Deep burial, to 20 km or more, during the deformation of the Dalradian rocks was followed by a rise in temperature to 400–650°C; this caused many of the rock-forming minerals, particularly in the pelitic and calcareous rocks, to react with each other to produce mineral assemblages stable under the conditions of burial, While more startle minerals were largely recrystallised. This process is known as regional metamorphism. The breakdown of clay minerals in the more argillaceous sediments releases water which accelerates deformation and recrystallisation. Regional metamorphism is characteristic of the rocks of fold belts.

Glen Esk is the type area for the Barrovian sequence of regional metamorphic zones, characterised by the successive development of chlorite, biotite, almandine garnet, staurolite and kyanite in pelitic rocks (Figure 22); this sequence typically occurs under medium-pressure conditions (c.5–8? kb). The maximum temperature experienced by the rocks increases northwards. The metamorphic zones are unusually narrow in lower Glen Esk. The occurrence of librolitic sillimanite around Invermark and in Deeside has been regarded as a later overprint (Chinticr, 1966), due to the proximity to the 'Younger Basic' masses. However, it has been shown in adjacent districts (Robertson, 1994) to be only slightly younger than the staurolite and kyanite (syn-D3 at latest), and may be part of the main metamorphic episode. The granitic intrusions produce only limited contact metamorphic effects, being in part masked by the previous high-grade regional metamorphism.

Carboniferous to Pliocene

A major swarm of east- to ENE-trending quartz dolerite dykes was intruded during the late Carboniferous, notably in the Midland Valley of Scotland and the southwest Highlands, though a few members of the suite occur in the eastern Grampian Highlands. Several discontinuous dykes traverse the Aboyne 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 occur in the Pliocene Buchan Ridge gravels in the Dion district (Sheet 87W) and the Peterhead district (Sheet 87E), within 50 km of the northeast corner of the Aboyne district, and suggest that Upper Cretaceous chalk may have covered at least part of the Grampian Highlands.

At some time during the early Tertiary, the Scottish landmass was uplifted, with several kilometres of uplift near the present west coast of mainland Scotland, and possibly only a few hundred metres in the east. During this period, some features of the present landscape started to evolve. The landscape is dominated by two erosion surfaces, typically separated by an abrupt step. Th ese surfaces were formed by subaerial erosion in a warmer climate than the present. They have been modified by differential upli.:1 and by the effects of the Pleistocene glaciations. The upper Grampian Surface is represented by a gently rolling landscape at an altitude of 700 to 900 m (Sissons, 1967). The hill summits become progressively lower from west to east. The flat summit of the Hill of Wirren (600 m) may be an isolated remnant of the peneplain surface. Concordant summit levels of hills in the eastern Grampian Highlands record a gently undulating landscape subjected to intermittent uplift. The Grampian and Buchan surfaces are separated from each other by relatively steep slopes. The lower Buchan Surface is poorly represented in the district, but may be represented by some hill summits between the Dee and Feugh valleys which rise to about 300 m. The climate in Neogene times was warm and humid (JUL A M, 1985), and patchy deep weathering, with the formation of grosses, occurred widely in the eastern Grampian Highlands.

Pleistocene to Recent

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 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 Aboyne district, 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. Large quantities of meltwater were released, often suddenly when an ice or gravel barrier was breached, and the high flow rates caused the erosion of several narrow, deep channels, such as the Clash of Wirren, which are unrelated to present-day drainages.

The large amphitheatre-like hollow of the Muir of Dinnet was probably formed before the glaciation of the area, but glacial erosion has greatly enlarged it. Since then, hollows and river valleys have been partly tilled 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 BP.

Chapter 2 Applied geology

In this chapter the geological factors relevant to land-use planning and development within the Aboyne district are reviewed. Key issues are identified; some are considered in detail, and readers are directed to sources of further information.

Key issues

Geological factors have influenced settlement and land-use patterns from the earliest days of human settlement but, with increased urbanisation and infrastructure development, they have become increasingly important. Not only are cities and their immediate environs subjected to more intensive development, but rural areas are affected by the development of communications and by the requirements of the cities for commodities such as water, building materials arid metals. For this reason, rural areas are attracting increasing attention from land-use planners, not only to maximise availability of resources but also to ensure that areas of outstanding geological and biological importance are conserved.

Planning and conservation

Very little economic activity apart from agriculture arid tourism currently takes place in the Aboyne district, and this state of affairs is likely to persist for the foreseeable feature. The part of the Dee valley lying within the district is 35 to 55 km from the centre of Aberdeen, and only a small number of residents in the district commute to Aberdeen. Deeside is an important tourist area, and Aboyne is a tourist centre, though subsidiary to Ballater and Banchory. The A93 road is a major tourist route, but the Aberdeen–Ballater railway was abandoned in 1965. Concern for the visual impact of any development, particularly close to the A93, has combined with distance from the main market in Aberdeen to restrict development of hard roci: and sand and gravel resources, Glen Esk is popular with day visitors, but few people go far from the road and the main hill paths to Deeside, and there is no development of accommodation in the glen.

Residential and tourist development occurs at a modest level in the district. Most development has occurred and will continue to occur in the Dec valley. The alluvial deposits close to river level are unsuitable for development due to the flooding risk, and the edges of river and glaciofluvial terraces may be at risk of erosion by undercutting of river banks, but these terraces and the ground immediately overlooking them have been the most favoured sites for building. Due to the distance from Aberdeen, only the portion of the district lying in 10 km square NC 69 was included in a survey of the environmental aspects of the geology of the Aberdeen hinterland (Smith, 1986).

In view of the generally undeveloped nature of the district and its importance to the tourist industry, conservation is an important issue. Seven areas have been designated as Sites of Special Scientific Interest (SSSIs). Two of these areas contain geological features worthy of conservation, while all seven contain plant, and in some cases, animal communities which have become rare elsewhere due to agricultural and other development.

The two geological SSSIs are the Muir of Dinnet in the north-west corner of the district (extending into adjoining districts) and Gannochy Gorge (River North Esk) near the south-east corner. The former area is of interest for the Quaternary features such as glacial meltwater channels and glaciofluvial ridges (Chapter 10). The latter is listed for the Devonian Strathmore Group sedimentary rocks, but also contains good exposures of the Highland Border Complex (Chapters 5 and 6).

Areas of biological conservation interest include the Muir of Dinnet (hog vegetation), Dinnet, Quithel and Shannel (oak woods, some on base-rich soil derived from the Deeside Limestone Formation), Glen Tanar (native pinewood) and Potarch [NO 608 973] (riparian vegetation along the Dee). Apart from the base-rich soils at Dinnet [NO 462 980], Quithel [NO 575 975] and Shannel [NO 600 955], the conservation value of the biological SSSIs in the district is unrelated to the bedrock geology, although the Quaternary geology has a strong influence upon the drainage of the areas and the soil type, and hence the types of plant growing there.

Stream sediment geochemistry

The presence of trace amounts of certain chemical elements in soils and water can influence the health of animals, crops and people, and hence is of importance to agriculture and medicine. Trace quantities of metals of economic interest can be used to prospect for metalliferous mineral deposits and to characterise different rock types; results also help to establish relationships between different rock units and to aid geological mapping where bedrock is poorly exposed. Stream sediments are used as the preferred sampling medium in order to provide a representative selection of all of the soil, and hence the bedrock, in the catchment area upstream from the sample site. In mineral exploration, stream sediment geochemistry is a preliminary wide-ranging, reliable sampling method commonly used prior to detailed exploration by soil geochemistry, geophysics and drilling.

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 per cent of 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 be related readily to the geochemical signatures of the different rock types in the district. Others are related to metalliferous mineralisation, while some anomalies have been traced to enrichment by adsorption on organic material or contamination from man-made sources.

The distinction in geochemical signatures between stream-sediment samples derived from Argyll and Southern Highland Group rocks is much less clear cut than in the Inverurie and Alford districts. This is partly due to masking by the effects of sediment derived from the Mount Battock Granite, and partly to the lower concentration of trace elements derived from detrital minerals in the largely psammitic rocks of the Tarfside Formation. Stream sediments from areas immediately underlain by or adjacent to the Ballater and Mount Battock granites contain higher levels of the elements in which the granites are enriched, namely Be, Bi, Ga, La, Pb, Li (only parts of Mount Battock are anomalous), Mo (localised, relatively intense anomalies), K, Rb, U, Y. Different parts of the Mount Battock Granite have different geochemical signatures, and they also differ from that of the Ballater Granite. Strongly chalcophile elements, especially those regarded as 'pathfinders' for rare metals such as Au, have distributions which reflect concentrations in fault zones. Coincident As and Sb anomalies occur in the Southern Highland Group outcrop, and there is a Bi anomaly along the Highland Boundary Fault near the North Esk gorge. A small Ag anomaly occurs over the Tarfside Formation rocks to the south of the Mount Battock granite; this may be related to the Craig Soales mineral occurrence.

Mineral resources

Metalliferous minerals

Apart from the small silver/lead deposit at Lochlee (Wilson, 1921), no metalliferous mineralisation worthy of exploitation has been discovered to date in the district. Exploration for several different kinds of mineralisation has been conducted over the centuries, but results have been generally unpromising. The post-tectonic Cairngorm Suite granites are associated with multi-element geochemical anomalies, and there are indications of vein-type Mo mineralisation associated with their roof zones. Gold has been reported from quartz veins along NW-trending fault zones in adjacent districts, but the occurrences have not vet been fully explored and evaluated.

Lead and silver

In a cleft in the hill below the adit of the abandoned Lochlee Mine, a sufwertical quartz vein containing cubes of galena up to 10 mm across is exposed. The vein is 0.2 in wide at the base of the cliff, but appears to vary considerably with height.

The spoil of small pits up to 2 m across is visible at the south-west side of Craig Soales [NO 505 802]. Galena and calcite occur on a joint plane about 30 m away from the old pits. This occurrence lies close to the postulated intersection of two faults.

Copper

Despite records of discovery of two copper 'mines' in the 1590s (see below), there is no reliable record of copper mineralisation in the district. However, the area of the 1592 concession includes the outcrop of the Highland Border Complex, which contains beds of sulphide-rich chert. There is thus the possibility that there may be copper mineralisation in the complex.

Gold

The East Grampians regional geochemical atlas shows strong, coincident arsenic and antimony anomalies, with minor bismuth, over the Southern Highland Group rocks in the south of the district. These elements are commonly associated with gold. One anomaly is centred near Minden [NO 540 790] and the other near Waterhead [NO 466 715]. Both may he related to mineralisation along NW-trending faults.

The Crown Estates granted prospecting licences to two companies to prospect for gold in the district in 1089, one in Southern Highland Group rocks and one in Highland Border Complex rocks. Both licences have now lapsed.

Reconnaissance drainage sampling in the area from Pitlochry to Glen Clova (Coats et al., 1993), followed by geochemical analysis of stream sediments and panned concentrates for a wide range of elements, yielded anomalies which were followed up by more detailed drainage sampling. As and Cu anomalies are located along the lines of NW-trending faults between Glen Cloya and Noranside in the Ballater, Kirriemuir and Forfar districts; Sb and Bi are not anomalous. Occurrences of gold in panned concentrates are associated with the presence of quartz–limonite veins in the fault breccias. The analogous NW-trending faults cutting the Southern Highland Group rocks in the southern half of the ;Moyne district may also be auriferous.

Limestone

Resources of limestone occur in the Deeside limestone Formation and to a lesser extent in the 'Tarfside Psammite Formation and Margie Formation. Beds of relatively pure limestone occur at Deecastle [NO 441 969], Ballogie [NO 576 961] and Mains of Midstrath [NO 590 955], at all of which they have been quarried, the workings at the last of these being the most extensive, These occurrences are described by Robertson et al. (1949), and petrographcal descriptions and chemical analyses are given by Muir and Hardie (1956). Small pits have been dug in the 'Tarfside Psammite Formation rocks at Quarry Hill [NO 495 805], near Tarfside village, but the beds of calcsilicate rock are very thin and impure, and no major working occurred south of the Mount Battock Granite. The northern outcrop of the Margie Limestone in the banks of the North Esk [NO 577 733] yielded a small lenticular body of pure limestone.

Hard-rock aggregate

Most of the Dalradian metamorphic rocks and plutonic igneous rocks of the district are suitable for hard-rock aggregate where fresh, although some of the semipelitic and pelitic rocks south of the Mount Battock Granite may he too fissile, due to abundant mica. Also, felsites make good aggregate. However, deep weathering and hydrothermal alteration in the Mount Battock Granite occur over extensive areas, making much of the granite unsuitable for high-grade aggregate, but useful for subbase material. Small quarries were developed in the 19th and early 20th centuries at localities dictated largely by considerations of access and transport.

Sand and gravel

The distribution and origin of glaciofluvial sand and gravel is described in Chapter 10. 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. Sedimer is 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 Dee, Feugh and North Esk (Figure 5), though most of this material occurs below the water table, Where glacial or glaciofluvial sand and gravel deposits have been reworked, the resulting alluvial deposits constitute a resource of sand and gravel. Deeply weathered bedrock is in places sufficiently soft to be removed without blasting and used as low-grade aggregate (see above).

The only area of sand and gravel resources in the district which has been assessed in any detail is the part of the Feugh valley to the east of grid line 60 (Auton et al., 1990). Reconnaissance studies of the sand and gravel resources (Paterson, 1977; Peacock et al., 1977) have shown that there are potentially workable deposits along the Dee and North Esk valleys.

Glaciofluvial sand and gravel deposits in the Muir of Dinnet consist mainly of unsorted gravel with cobbles up to 0.2 m diameter in a silty, and sandy matrix. They are partly overlain by lacustrine deposits and peat, and, as they lie within a National Nature Reserve, are unlikely to he exploitable.

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 hoard, fillers, extenders, and bulking agents, and in filter aids and adsorbents.

In the north-western part of the district, in the Muir of Dinnet and adjacent areas, diatomite formed on the beds of postglacial lakes (Figure 6). The average thickness of the deposits within the district which have been evaluated varies between 0.36 m and 1,14 m. The contacts with the overlying peat and with the underlying deposits are generally sharp. The deposits are mostly underlain by sand, but in places by lacustrine silt and clay. Further details of these deposits, which lie partly in the Alford district (Sheet 76W), may be found in the memoir for that district (Gould, 1997).

Two deposits are known to lie within the Aboyne district. Over an area of 0.75 km2 at the edge of Braeroddach Loch, diatomite was estimated to average 0.77 m in thickness, giving a volume of 57 800 m3, with an equivalent dry weight of 12 800 tonnes. At Bogingore on the western side of Loch Kinord, the equivalent figures are: area 0.63 km2, thickness 0.81 m, volume 50 800 m3, and equivalent dry weight 11 300 tonnes. The diatomite obtainable from, most of the deposits in the district contains too much organic matter to be usable without calcination and, therefore, it could not be considered competitive against imported material for most purposes. However, 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. Development is very unlikely because of the National Nature Reserve status of the Muir of Dinnet.

Peat

Hill peat

Much of the more gently sloping ground between 400 and 600 m OD is covered by peat up to 2 m in thickness, probably formed during the last 5000 year. (Chapter 10). Due to remoteness and low population density, very little of this peat has been exploited as fuel. and working is unlikely in the foreseeable -future.

Basin peat

Small deposits of peat occur in the Muir of Dinnet basin. At. Bogingore, on the west side of Loch Kinord, the peat overlies diatomite and glaciofluvial sand and gravel. Small hollows around Powlair [NO 604 910] in the Feugh valley are filled with peat, but resources are very small. Extraction appears to have been minimal.

Mineral extraction

No mineral extraction is presently (1995) occurring in the district, and the abandoned workings are all quite small. The largest, at 0.03 km2, is the recently abandoned Sluie gravel pit [NO 614 968]; for details see Harris et al. (1994). Infill commenced in 1994. Small, disused pits elsewhere in the district exploited both glaciofluvial deposits and weathered and hydrothermally altered granite for sand and gravel. The sandy facies of the Banchcry Till Formation was also exploited, e.g. the type locality at Finzean [NO 603 922].

Compared with the surrounding districts, there has been little exploitation of the rocks of the district for aggregate. There are no currently working hard-rock quarries in the district. The largest disused hard-rock quarry is at Birsemore [NO 533 970], in the Mount Battock Granite. Numerous small quarries witness extraction in the 19th and early 20th centuries, largely for local use as road metal. Limited extraction for forestry and estate roads may continue intermittently in the future.

There are disused limestone quarries at Deecastle [NO 440 969], Ballogie [NO 576 961] and Mains of Midstrath [NO 588 953]. Most, if not all, of the output was burnt for agricultural lime, and the remains of kilns occur at Mains of Midstrath. A small disused limestone quarry also occurs on the east bank of the North Esk at the outcrop of the Margie Limestone [NO 587 733]. Small pits in impure limestone occur on Quarry Hill, Tarfside [NO 495 805].

Metalliferous mineral extraction occurred from the 16th to 18th centuries, but on a very small scale. The only working recorded by Wilson (1921) is the Lochlee silver/lead mine. The adit is situated approximately 40 in up a steep cliff on Gilfumman [NO 422 816], and is inaccessible without ropes. There are signs that the surface outcrop of the vein in the cliff under the adit has been worked in places. There are no records of production, but Wilson says that attempts were made to work the vein in 1728. At Craig Soales [NO 505 802], there is another galena occurrence; only a few small hollows representing pits and mounds of spoil give evidence of working.

A concession to prospect for metals and minerals over the banks of the North Esk from the 'closure of Glen Esk' (presumably the gorge at the mouth of the glen) as far as the confluence of the Burn of Mooran was granted by Sir David Lindsay to Henry Lok in 1592 (Cochran-Patrick, 1876). As a result of this, two copper 'mines' were discosered, and the Scottish Crown appointed Eustache Roche to develop them. There is no record of any production, or any sign of workings on the ground.

Water resources

Most of the water resources of the district consist of surface water in the rivers Dee and North Esk. Water from the River Dee is abstracted [at NO 6448 9635] near Trustach Cottage (Sheet 66E) and treated at Invercanny water works (Sheet 66E). Loch Lee [NO 42 79] is used as a reservoir to supply Dundee; a low dam has been built at its mouth. Most of the catchment is in the Ballater district, although the dam and half of the loch lie in the Aboyne district. The River Dee passes the villages of Braemar, Ballater and Aboyne before reaching the abstraction point, but there are very few other potential sources of contamination of the water. The catchment of Loch Lee is almost entirely uninhabited, and contains no known potential sources of contamination, although the pH of the water may be low due to the extensive hill peat in the catchment.

Due to the very limited porosity of the bedrock of the district, little use is made of groundwater, whose occurrence is largely fracture controlled in solid formations. Limited quantities of water suitable for domestic use by individual farms are obtained from wells sunk in fractured igneous and metamorphic rocks and glaciofluvial sands and gravels. Two springs on the hill slopes to the south of Aboyne, at Allt Roy [NO 507 958] and Birse [NO 560 965], have been used for many years as sources for public supply. One yields 600 m3 per day. The Devonian sedimentary rocks of the south-eastern corner of the district have a higher potential for groundwater. Very promising results were obtained from trial drilling to 100 m below ground level. The rocks form part of the extensive Devonian aquifer of Strathmore and are considered to have good potential for groundwater exploitation. No investigation has been made of the potential of the glacial and glaciofluvial sands and gravels of the district as aquifers.

Geothermal power

A combined BGS–Open University team carried out the Hot Dry Rock (HDR) programme to investigate the geothermal potential of the post-tectonic granites of the eastern Cairngorm Suite during the early 1980s. 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 only borehole put down in the Aboyne district was that to sample the Mount Battock pluton, located near Burnfoot [NO 543 906]. The geochemical analyses of the Ballater and Mount Battock specimens are discussed in Chapter 7.

The total depth of the Burnfoot Borehole was 261 m. Cores were taken at 94 to 99 m, 184 to 190 m and 255 to 261 m. All cored sections consist of grey biotite microgranite with a grain size of 1 to 2 mm and scattered feldspar and quartz phenocrysts. Reddening along joints is widespread, and jointing is intense, especially in the middle cored section. A calcitic breccia was noticed in the lowest cored section. The mean saturated density used for geothermal calculations was 2.61 Mg m−3.

The known distribution of U, Th and K in the four granites was used to calculate their heat-producing capacity. The heat production values from the three cored sections of the Burnfoot Borehole showed a down-hole increase from 4.0 µW m−3 at 94 to 99 m to 5.5 µW m−3 at 255 to 261 m. The eastern Grampians granites proved to be the most highly radiothermal granites known in the United Kingdom. Ballater was estimated to have a heat production value of 6.8 µWm−3 and Mount Battock 4.8 µWm−3, the latter being the lowest of the four plutons studied. 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 Mount Battock pluton is shown in (Figure 7)a, and the corresponding thermal model in (Figure 7)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 mWm2 and the Mount Battock Borehole 59 mWm2. The heat-flow and heat production information for granitic rocks within the United Kingdom was summarised by Lee and others (1984). The eastern Grampians granites and metasedimentary country rocks have high thermal conductivities, with the result that temperatures suitably for HDR geothermal power are not present at sufficienty shallow depths to be of interest anywhere within the region (Figure 7)c.

Chapter 3 Concealed geology

The greater part of the Aboyne district lies within the Grampian Highlands, while the south-eastern corner lies within the Midland Valley. The Grampian Highlands lie within the orthotectonic Caledonides, defined as that pan of the belt where pre-Caledonian basement and late-Precambrian sedimentary rocks have suffered deep burial, polyphase folding and thrusting, regional metamorphism, and intrusion of plutonic igneous rocks, The Grampian Highlands are hounded 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.

Deep crustal structure

The LISPB seismic survey (Bamford, 1979) indicated 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 into three layers by P-wave velocity. The uppermost laver, 12 to 15 km thick, is interpreted front the surface geology to consist largely of Dalradian metasedimentary rocks and granitic plutons. The second layer, from 12 to 15 km to about 20 km depth, is interpreted as granulite facies basement gneisses; the physical properties are similar to those of Lewisian rocks exposed in the North-west Highlands. The third layer, from 20 km to 28 to 35 km depth, might be basic granulites which have partly regressed to amphibolites (Hall, J, 1985). A second analysis of the EISPB data (Barton, 1992) included a density inversion and suggested that the three-layer model of the crust north of the Highland Boundary Fault is less clear. A distinct zone of low P-wave velocity (5.8 km s−1, compared with an average 6.2 km s−1 for the upper 20 km) at depths of 2 to 4 km, was modelled in the region north of the Highland Boundary.

BGS has over the years conducted regional aeromagnetic and gravity surveys of the UK landmass and continental shelf, The aeromagnetic data were collected at 303 m above terrain, along east–west flight lines spaced approximately 2 km apart. The data were collected in 1963 and subsequently digitised, gridded at 0.5 km, and smoothed with a simple 9-point filter. The gravity data have been collected at various dates, and the average station spacing used in the current compilation is about 1 per 2.5 km2.

(Figure 8) is a smoothed version of the aeromagnetic map of the Aboyne district and neighbouring areas, and (Figure 9) is part of the similarly smoothed Bouguer gravity anomaly map covering the same area. Lineaments picked from images of the regional geophysical data are also displayed in (Figure 8) and (Figure 9). The most prominent feature is the strong gravity lineament running WNW to the south of the Dee valley, part of a structure traceable across most of Scotland and into the central North Sea.

In Perthshire the low-velocity zone in the upper crust beneath the southern Grampians (Barton, 1992) is associated with a linear positive gravity and magnetic anomaly. The gravity anomaly is probably caused by the high-density Dalradian (Aberfoyle Slates) in the downbend of the Tay Nappe and partly by high-density components of the Midland Valley Terrane. The Highland Border Complex is not especially anomalous in terms of density, but profile models suggest that it may extend for several kilometres north of the mapped Highland Boundary Fault (Dentith et al., 1992). The associated magnetic anomaly has two components: one is due to shallow basic and ultrabasic rocks within the Highland Border Complex; the deeper source is possibly clue to ophiolitic rocks beneath the overthrust Dalradian.

Aeromagnetic anomalies

The most conspicuous features of the aeromagnetic map are the highs which occur over parts of the Mount Battock Granite. The regional aeromagnetic map shows a large positive anomaly occupying the margins and north-central part of the Mount Battock Granite, with a roughly circular low in the south-central part, around Mount Battock itself. The magnetic susceptibility of hand specimens of the Mount Battock Granite ranges from 0.01 x 10−3 to 10.7 x 10−3 SI units, with the main plutonic phases in the Aboyne district averaging between 2.09 x 10−3 SI units (Clachnaben Granite) and 6.17 x 10−3 SI units (Fungle Granite) (Figure 1). Microgranites, felsites and porphyries have lower, thought varied susceptibilitites, and aplites have effectively zero susceptibility. This is partly clue to variation in original magnetite content, and partly to hydrothermal alteration. The most highly altered granitic rocks, with brick red feldspars and milky quartz, typically have very low susceptibilities (0 to 0.1 x 10−3 SI), but several exceptions to this rule have been noted in the Mount Battock pluton.

The pattern of magnetic anomalies over the granite outcrop is related to the polyphase nature of the pluton. The largest positive anomaly corresponds with the outcrop of the Water of Feugh Granite (Figure 1), and reflects its relatively undifferentiated composition. Smaller positive anomalies lying close to the southern contact near [NO 500 850] and [NO 550 820] may he caused by small satellitic diorite intrusions at depth, although the Main Porphyritic Granite which crops out over these anomalies is one of the geochcmically less-differentiated phases of the pluton, and is associated with an area of raised aeromagnetic anomaly in the valley of the Water of Aven around [NO 600 900].

The Ballater Granite is marked by a magnetic low, but it is largely surrounded by rocks with high magnetic susceptibilities (Morven–Cabrach Intrusion, Dalradian amphibolites), sc this says little about the Ballater Granite itself. There is a small, fairly steep positive magnetic anomaly a short distance east of the Ord Fundlie Granite.

The lew magnetic anomalies over the Dalradian outcrop are circular rather than linear in shape. They may be related to magnetite-rich beds within the Glen Effock and Glen Lethnot formations, which have been displaced by faulting, thus truncating otherwise linear anomalies. There is no evidence of magnetic anomalies lying over the major faults in the district, which might be related to mineralisation along the faults. It is notable that the rocks of the North Esk Formation do not give rise to a significant magnetic anomaly.

Bouguer gravity anomalies

The regional gravity map of the eastern Grampian Highlands is dominated by a group of negative Bouguer gravity anomalies with values down to −55 mGal, which encompasses the outcrop of the Cairngorm Suite granites. Two-dimensional. profiles across four of the plutcms„ centred on the heat-flow borehole sites, and a three-dimensional model covering the whole area of the anomaly, 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 he linked at depth in an East Grampians Batholith (Plant et al., 1990). A depth of 13 km t.-) the base of the hatholith was generated by the model of Rollin (1984); the present model (Figure 10) is based on a depth of 10 to 12 km for the base of the granitic plutons.

The western portion of the Mount Battock pluton is marked by a Bouguer anomaly low of −48 mGal centred on [NO 410 920] (Ballater district), close to the western contact. The Bouguer anomaly map (Figure 9) shows that the most intense part of the anomaly related to the Mount Battock pluton underlies the area around Mount Keen and gradually becomes less intense eastwards. A shallow ridge close to the tongue of metasedimentary rocks between the Mount Battock and Ballater plutons separates the Mount Keen anomaly from an even more intense negative anomaly extending westwards from the southern part of the Ballater pluton to the Glen Gairn pluton. The steady, steep gradient over the south-eastern part of the pluton and the adjacent country rock implies that the south-east. margin of the East Grampians

Batholith is steep for several kilometres. however, the gravity gradient across the Tayvallich Subgroup south of Mount Keen is gentler and discordant to the granite contact. This implies relatively shallow granite beneath the Tarfside Culmination. The magnetic data support this.

The positive gravity anomaly in the southern part of the district runs parallel to the Highland Boundary Fault and may reflect a slab of Highland Border Complex rocks. These could he either metabasalts like the North Esk Formation or tiltramafic rocks like those occurring near Lintrathen, underlying the Dalradian rocks of lower Glen Lethnot and extending into the Forfar (Sheet 57W) district. The implication of this would be that the North Esk Fault is a relatively low-angle reverse fault, whose outcrop is cut out by the Highland Boundary Fault northeast of Clatterin' Brig in the Banchory district and south west of Cortachy in the Kirriemuir (Sheet 56E) district. At the surface, the North Esk Fault was recorded by Barrow (1901) as dipping north-west at 35°, while Dentith et al„ (1992) have postulated a dip of 20° at depth between Loch Lomond and Callander. The negative Bouguer anomaly to the south-east of the Highland Border rocks is related to the thick pile of Old Red Sandstone sedimentary rocks in Strathmore.

Geophysical cross-section

(Figure 10) shows part of a crustal model (to 40 km depth) of the geophysical data along the section GG′ shown or (Figure 8) and (Figure 9). The profile crosses the area of (Figure 8) and (Figure 9) from [NO 370 050] to [NO 640 650] and is part of a longer line extending from the Inner Moray Firth to the Forth approaches. interactive modelling of polygon shape and properties (Table 2) with simultaneous calculation of gravity and magnetic fields has approximately matched the observed anomalies.

The high-grade Dalradian metalimestones, gneissose psammites and semipelites between the Ballater and Mount Battock granites are modelled as a thin wedge less than 1 km thick. The aeromagnetic data indicate several phases of igneous intrusion. The Mount Battock Granite is modelled to a depth of about 10 km below OD with bulk mean susceptibilities of the more dioritic phases up to about 25 x 10−3 SI. These values are significantly large: than those observed at surface outcrops. The Argyll and Southern Highland group strata exposed south of the Mount Battock pluton are not inverted but form part of the Tay Nappe which is shown as overthrust over a Midland Valley basement extending on the north side of the Highland Boundary Fault in a structural model similar to that proposed by Dentith et al. (1992).

Chapter 4 Dalradian

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 margin of the Laurentian continent during the late Precambrian, with the younger parts of the succession rocks being of latest Precambrian to early Cambrian age (Halliday et al., 1989; Tanner, 1995), The rocks of the supergroup have been subjected to at least tour major tectonic episodes, some of which were accompanied by regional metamorphism up to amphibolite facies, and are intruded by several suites of plutonic igneous rocks.

Barrow published several papers (1898; 1901; 1912) describing the metamorphism of the Dalradian, and the stratigraphy and structural relations of the Highland Border Complex. Most of the type area for Barrovian regional metamorphism lies within the Aboyne district. Read (1927; 1928) re-examined the Dalradian rocks of mid-Deeside, paying particular attention to the 'xenolithic' gneisses to the north of the Dee and to the stratigraphical position of the Deeside Limestone. Harte (1966; 1979) mapped the area From lower Glen Mark to the Water of Saughs, and postulated the presence of the Glen Mark Slide to explain the change in younging direction between the right-way-up rocks of Glen Esk and the inverted rocks of Glen Mark and upper Glen Tartar,

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 Aboyne district. The formations cropping out are listed in (Table l) (inside front cover). Detailed stratigraphical correlation between Deeside and Glen Esk is hindered by the large Mount Battock pluton.

The Dalradian rocks in Deeside, between the Muir of Linnet and Potarch, can be correlated with the corresponding Crinan and 'Fayvallich subgroup (Argyll Group) rocks in the Ballater district, and continue for several kilometres into the Alford and Banchory districts to the north and east respectively, The Tarfside Psarnmite Formation can he traced from Deeside through the gap between the Ballater and Mount Battock granites to its type area around Tarfside, where it forms a gently dipping dome structure. The Southern Highland Group rocks dip away from this dome to the south and east, and are represented here by the Glen Effock Schist Formation and Glen Lethnot Grit Formation.

Most of the formations in the district consist of varying proportions of psammite, semipelite and petite, 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 per cent 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 per cent total micas. A few chemical analyses of the gneissose rocks of the Queen's Hill Formation are given by Read (1927), and analyses of limestones from the district are quoted by Muir and Hardie (1956). The only modern geochemical analyses of metasedimentary rocks from the district are of psatnmites to pelites from the Southern Highland Group rocks of Glen Esk (Mohamad, 1980).

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 Rh, but relatively depleted in Be, B, Li, Zn, Ba and Ga with respect to the Southern Highland Group. The higher levels of Fe, Co, Ni, Cr, Ca, Si, Ti, and V in the Argyll Group rocks reflect a higher contribution of tholehtic volcanic material to the sedimentary protolith of the Argyll Group than to that of the Southern Highland Group rocks. The B and Li enrichment in the Southern Highland Group rocks reflects a high concentration of illitic clay in the original sediments, indicating a saline depositional environment and hydrothermal circulation.

The base of the Southern Highland Group is taken as the first incoming of beds of magnetic semipelite within a mixed psammite/semipelite succession. There is a gradual increase in the proportion of semipelite from the Glen Turret Member of the Tarfside Formation to the Glen Effock Formation. The use of a portable magnetic susceptibility meter to identify magnetite-bearing layers has enabled the boundary to he mapped with reasonable confidence in the area around Glen Mark. Similar magnetic units occur at or near the Argyll–Southern Highland Group boundary elsewhere in the East Grampians (Smith et al., in press).

The depositional age of the Dalradian rocks exposed in the district is constrained by radiometric work in critter parts of the Grampian Highlands. Halliday et al. (1989) obtained a Cl/Pb zircon age of 395 ± 4 Ma from a keratophyre sill associated with the metabasalts of the Tayvallich Volcanic Formation close to the base of the Southern Highland Group. A high-precision U/Pb zircon age of 590 ± 2 Ma for tire intrusion of the Ben Vuirich granite (Rogers et al., 1989), which was intruded into meta-sedimentary rocks of the Argyll Group in the Pitlochry district (Sheet 55E), but probably predates the first deformation episode in the district (Tanner, 1996), implies that the Southern Highland Group sediments were probably deposited. not long after 600 Ma. In the absence of an obvious itratigraphical break within the Southern Highland Group, Tanner's (1995) assertion that the upper part of the Southern Highland Group is in stratigraphical continuity with the Leny Limestone and thus of early Cambrian age (now estimated at 550 to 535 Ma) is difficult to sustain.

Argyll Group

Queen's Hill Formation

The Queen's Hill gneisses were named by Read (1927) in the area north of the River Dee between the Muir of Dinnet and the Muir of Dess. Within this area, which lies partly in the Aboyne district and partly in the Alford district, a regionally inverted, ENE-striking sequence of amphibolite-facies metasedimentary rocks, intruded by metahasic sills, was recognised (Figure 11). The total thickness is at least 2 km. The Queen's Hill Formation in the area between Dinnet and Queen's Hill can be divided into three lithostratigraphical units, the oldest in the north-west.

The structurally highest, and thus, by implication, the stratigraphically lowest unit of the formation in this area consist of mixed psammite, semipelite and pelite. The rocks are all gneisses, typically with feldspar porphyroblasts, in which lithological distinctions have become markedly blurred 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], the rocks have a well-developed foliation and a degree of compositional layering, with psammitic and pelitic layers being distinguishable (Plate 1)a. West of Dinnet House [NO 432 975], the rock is a uniform, weakly layered gneiss with large subhedral porphyroblasts of plagioclase up to 5 mm and garnet up to 3 mm, set in a matrix of quartz, biotite, sillimanite and cordierite. The gneiss contains rounded 'xenoliths' of more refractory lithologies, principally quartzite, calcsilicate rock, and aluminous pelitic material, with rare amphibolite. On Tomachallich [NO 474 997], 'xenolithic' gneisses have been sheared and partially retrogressed. The rocks exposed on the hill north. of New Kinord [NJ 444 001] are less massive and are coarsely layered, migmatitic semipelites.

Along strike to the north-east, on Mulloch Hill [NJ 470 006] and Scar Hill [NJ 482 014] in the Alford district (Sheet 76W), the stratigraphically equivalent rocks consist of uniform, poorly layered heterogeneous gneisses with porphyroblasts of plagioclase up to 5 mm and garnet up to 3 mm, set in a matrix of quartz, biotite, sillimanite and cordierite. Xenoliths of refractory lithologies are abundant, and the rock shows signs of partial melting adjacent to the contact of the Tarland basic intrusion. The xenolithic rocks of Mulloch have been sheared and partly retrogressed.

The second unit, cropping out to the south-east of Balnagowan Hill, [NJ 505 006] and near Coull Home Farm [NJ 513 012] consists of very coarse-grained, gneissose, biotite-rich pelite and semipelite with euhedral plagioclase porphyroblasts up to 10 mm south-east. This unit has an estimated apparent thickness of 500 m.

The third, and youngest unit consists of very coarse-grained gneissose feldspathic psammite with less than 5 per cent muscovite + biotite which crops out extensively on Queen's Hill [NJ 530 005]. The foliation is defined by flattened and stretched quartz crystals. In thin section (S78522), ribbon quartz is abundant, and the grain boundaries do not represent original (lasts. 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 [NO 511 999] to [NJ 516 000]. The unit is estimated to be 500 to 600 m thick. On the southern slopes of Court Hill [NO 514 997], the psammites are less massive. and are interbedded with semipelite.

Deeside Limestone Formation

This formation has an extensive outcrop along the Dee valley, but is very poorly exposed, being overlain mainly by thick abundant glaciofluvial deposits, alluvium and river terraces along the Dee valley. The formation consists dominantly of calcsilicate rock with minor psammitic layers and several beds of limestone. It has a thickness of about 250 m in the Aboyne area. The Deeside Limestone Formation extends south-west from Deeside into the narrow gap between the Ballater and Mount Battock plutons. Small exposures of calcsilicate rock occur on Craigrae Beg [NO 424 943]. At the contact with microgranite of the Mount Battock pluton at [NO 423 940], the calcsilicate rock contains grossular and idocrase and is cut by 2 to 5 mm wide fluorite veins.

The calcsilicate rocks range from bluish rocks similar in appearance to the limestones, to hornblende- and clinozoisite-rich rocks, in part resembling amphibolites (Plate 2). They typically contain plagioclase, clinozoisite, and diopside; amphibole occurs in the originally impure, muddy lithologies, and where retrogression has taken place.

Within the Aboyne district, several limestone beds have been exposed by quarrying. Their economic potential for agricultural use or lime burning was investigated by Robertson et al. (1949, p.47), but all contained significant proportions of silicate minerals, and tonnages were small. At Deecastle [NO 4404 9691], a 3 m-thick bed of fine-grained pale blue limestone with abundant interstitial pyrrhotite is exposed in the old, overgrown quarry. At Mains of Midstrath [NO 5878 9526], several pits and one quarry with a face 4.5 m high expose a bed of blue-grey sugary crystalline limestone within calcsilicate rocks (Plate 1)b. The limestone contains a smaller proportion of silicate minerals than elsewhere in the district, and was burnt for agricultural purposes in the limekiln beside the farmhouse. Scapolite recorded from this locality may be an indicator of local metasomatism. A disused quarry about 700 m NNE of Ballogie House [NO 5757 9607] exposes 12 to 15 m of soft, white to pale blue limestone. Limestone and calcsilicate rocks exposed about 500 m south of Ballogie House [NO 5747 9502], adjacent to the north-west contact of the Mount Battock pluton, have suffered contact metamorphism, with the formation of large crystals of grossularite and idocrase and sheaves of wollastonite; they are intruded by a felsite dyke. A small exposure of limestone occurs about 300 m WNW of Wreaton [NO 4978 9940], close to the margin of a quartz dolerite dyke.

Exposures of mixed psammite and migmatitic semipelite with only minor calcareous psammite occur within the formation 300 m south of Wreaton [NO 5036 9909] and in a disused railway cutting on the western outskirts of Aboyne [NO 519 987].

Tarfside Psammite Formation

This formation lies stratigraphically above the Deeside Limestone Formation. In Deeside subdivision has not been possible, but around Tarfside, it has been divided into two members, a lower, dominantly psammitic Glen Tanar Psammite Member, and an upper Glen Turret Psammite and Semipelite Member. The overall thickness of the formation is about 1.5 km (Figure 12).

North of the Mount Battock granite

In this area, the formation extends from the western margin of the district to near Bogieshiel Lodge [NO 559 951], where its boundary with the Deeside Limestone Formation to the north and east is cut off by the Mount Battock Granite. Exposure is poor except in the banks of the Water of Tanar around the Brig o'Ess [NO 5045 9722] and around Knockie Bridge [NO 478 953], and on a few of the hills, e.g. Creag na Slice [NO 461 959] and Craigendinnie [NO 515 962].

About 300 m north-east of the Brig o'Ess, psammite with micaceous partings is seen in contact with calcsilicate rocks of the Deeside Limestone Formation. At the Brig o'Ess itself, the psammites are quartzose, but contain beds 1 to 10 cm thick of calcareous psammite and psammite with micaceous partings. The rocks on the north-west slopes of Craigendinnie are sparsely feldspathic psammites with a distinctive, slightly carious weathering surface, due to the preferential weathering out of feldspar. The psammites are interbedded with a few layers of feldspar-porphyroblast gneiss, containing plagioclase crystals up to 4 mm across. Semipelitic rocks occur only as thin partings a few millimetres to centimetres thick. A roughly bedding-parallel foliation, defined by alignment of biotite flakes, occurs in the psammites, which also have a poorly developed alternation of more- and less-feldspathic layers. Psammites similar to those on Craigendinnie also occur along the axis of a gently east-plunging antiform from near Netherton [NO 4565 9789] to Belrorie Hill [NO 4818 9775]; a prominent east- to ESE-plunging mineral lineation is prominent on many of these exposures.

Exposures of calcsilicate rock, mostly containing diopside, zoisite, dark green amphibole and disseminated pyrrhotite, occur at Knockie Bridge [NO 487 953]. The exposed thickness of these rocks is only 30 m, due to the gent; e northward dip, and they are succeeded to the south by semipelite with foxy red biotite.

Between Newton [NO 4603 9703] and the summit of Creag na Slice [NO 4614 9592], a southward-dipping, probably inverted, succession is moderately well exposed. Psammite with calcsilicate layers is intruded by two 30 m-thick amphibolite sills, and is succeeded to the south by a 80 m-thick bed of pelite containing garnet and sillimanite. Continuing south, there is a return to slightly feldspathic psammite, with a 30 m-thick bed of pelite and semipelite occurring just north of the summit of the hill.

In this area, the Tarfside Psammite Formation can be divided into a lower Glen Tanar Member, composed dominantly of psammite, and an upper Glen Turret Member containing roughly equal proportions of psammite and semipelite, closely interbedded.

Glen Tanar Psammite Member

This member crops out in the core of the Tarfside Dome, over an area of 64 km2 centred around Milton [NO 481 807]. A thickness of about 700 m is exposed, but the base is not seen. Exposures are good, both in streams and on hillsides such as Craig Soales [NO 509 812]. The psammites are thickly bedded, with beds up to 2 m thick of almost pure quartzite separated by well foliated flaggy psammite to semipelite in units up to 0.2 m thick (Plate 3). Beds of calcareous psammite and calcsilicate rock are picked out on hillsides by areas of green grass and rabbit burrows exposing yellow-brown soil.

Good sections through the rocks of the Glen Tarim- Member are exposed n the North Esk and all of its northern tributaries from the Water of Tarf up to the Burn of Branny. With the exception of Craig Soales [NO 509 812] and Badadarrach [NO 411 823] exposures on most of the hills are relatively poor.

Beds of calcsilicate rocks are reasonably abundant, and widely scattered over the outcrop of the member, but beds of limestone with enough calcite to be of use for agricultural purposes are few. Small disused workings in very impure limestone occur on Quarry Hill [NO 495 805].

Exposures of the Glen Tanar Member in the North Esk occur upstream of Dykeneuk [NO 473 786]. They consist of highly quartzose psammite, thickly bedded, with rare micaceous partings. Upstream from [NO 4545 7978], calcsilicate beds up to a few metres in thickness can be distinguished by their pale blue or green colour and different weathering characteristics.

The Water of Tarf exposes only a small part of the succession, as the stream runs along strike in many places. Thickly bedded psammites with rare micaceous partings are dominant (Plate 3). Near Creag na h'lolaire [NO 472 845], the psammite contains a few coarse-grained, slightly gritty beds. Calcsilicate beds are absent from this section.

The rocks in the Burn of Branny are almost horizontally disposed, and only a small part of the member is exposed Calcsilicate rock is more abundant here, and one bed a few tens of metres in thickness is exposed [NO 4435 8315] to [NO 4385 8477]. On Badadarrach, to the west of the Burn of Branny, the psammite becomes more feldspathic, and layers of micaceous psammite up to 20 cm thick, with coarse muscovite along foliation planes, are noticeable. This may be a transition towards rocks of the Glen Turret Member.

In the Burn of Laurie [NO 427 803] to [NO 420 809] and the adjacent hillsides to east and west, Harte (1966) described several thin, impersistent beds of calcsilicate rock and impure limestone interbedded with quartzose and slightly feldspathic psammite and minor hiotite-clor semipelite. The transition to the Glen Turret Member is gradual, and records a gradual upward increase in the proportion of semipelitic rocks.

On the south side of the North Esk, the psammites and minor calcareous rocks of the Glen Tanar Member are directly overlain by rocks of the Glen Effock Schist Formation. The exposures on Tod Craig [NO 437 798] and Craig Lour [NO 444 797] consist almost entirely of quartzitic psammite. The lower slopes of Craig Dullet [NO 425 794] (below the cliff face) and Hare Craig [NO 455 783] expose psammite with impersistent beds of impure limestone and calcsilicate rock.

Glen Turret Psammite and Semipelite Member

This member stratigraphically overlies the Glen Tanar Psammite Member and is the topmost member of the Argyll Group. It is about 1000 m thick in Glen Turret and about 500 m thick in lower Glen Mark, but thins gradually southwards and is absent south of a line through Loch Lee from [NO 419 795] to [NO 471 775], where the Glen Tanar Member is directly overlain by the Glen Effock Schist Formation. The Glen Turret Member is probably a lateral facies variant of the upper part of the Glen Tanar Member and its attenuation does not imply an unconformity at the base of the Southern Highland Group.

It is characterised by the interbedding of psammite and biotite-rich schistose semipelite on a scale of 0.01 to 0.1 m. Some of the semipelitic beds are coarsely crystalline and muscovite rich. Weathering of scattered iron oxide crystals gives some semipelites a rusty colour along partings. Migmatisation is developed in the more feldspathic semipelites, and in closely interbedded semipelite and feldspathic psammite. A greater proportion of the semipelite and feldspathic psammite sequence in Glen Mark is migmatitic than in Glen Turret. The alternating psammitic and semipelitic lithology is more susceptible to deformation than the underlying massive psammite of the Glen Tanar Member, and has been folded during D3 into stacks of folds with horizontal axial planes and east–west axes (Plate 9).

Exposures in the Burn of Turret and the Burn of Blackhills consist of psammite and semipelite interbedded on a scale of 10 to 50 cm. The psammite is fairly thickly bedded, feldspathic and commonly quartzitic, while the semipelite is biotite rich, flaggy and fissile. Calcareous beds are less abundant than in the Tarfside Member. Beds of blue-green calcsilicate rock 5 to 10 cm thick occur near [NO 5447 8048] in the Burn of 'Turret, and in an 80 m-section at [NO 5391 8035] in the Burn of Blackhills where the calcsilicate rock contains amphibole-rich units.

There are few exposures in the Water of Mark downstream of the Queen's Well [NO 1199 8285], but the steep slopes on both sides of the valley contain many exposures, On the east side (Skeir Craig, Greenbush) feldspathic psammite and biotitic fissile thinly bedded semipelite are interbedded in roughly equal proportions. A tine lamination at 5 to 10 mm scale is developed. On the west side of Glen Mark, the cliffs of GilMillman give a complete section through the Glen Turret Member. The lithologies are Very similar to those on the east side of Glen Mark, but the succession has been intruded by a 40 m-thick sill of amphibolite [NO 4281 8118], just south of the old mine adit [NO 4212 8164], a 30 m bed of friable impure blue-grey limestone occurs. It is cut off to the north by a mineralised fault.

Southern Highland Group

Glen Effock Schist Formation

'This unit was named the Glen Effock Schist by I Earle (1966), and the name has now been formalised. As presently defined it includes both Harte's 'Green-feldspathic-mica-schist Formation' and his Glen Effock Formation, following the definition of 'Glen Effock Schist given in Norte (1979). Within the Aboyne district, the formation occurs only to the south of the Mount Battock granite, where the succession is right way up. It conformably overlies the Glen Turret Psammite and Sernipelite Member in lower Glen Mark and from Drumgreen [NO 476 783] to Hill of Fingray [NO 570 815]; however, from Loch Lee to Auld Craig [NO 455 783] it rests directly on the Glen Tamar Member, due to the lensing out of the Glen Turret Member. It is well exposed in crags on the south side of Glen Effock and to the south of Loch Lee, in sections in the North Esk from Dalhastnie [NO 538 778] to its junction with the Water of Tarf [NO 494 790], on crags on the lower slopes of Craigangowan [NO 585 788], and in the Water of Chaff [NO 590 784] to [NO 612 801]. Its stratigraphical equivalent in the Balloter district to the west is the Longshank Gneiss Formation (Smith et al., in press).

In the field, rocks of the lower part of the Glen Effock Formation can he distinguished from those of the Glen Turret Member by the abundance of uniform, dark green-grey feldspathic, weakly fissile, fine-grained semipelites and by the absence of calcareous rocks. Also, within the Glen Effock Formation there are beds up to 0.5 m thick of fine-grained magnetite-bearing semipelite with green or dark brown hiotite which have magnetic susceptibilities ranging from 25 x 10−3 to 150 x 10−3 SI, in contrast with the uniformly low magnetic susceptibilities (<0.5 x 10−3 SI) of the rocks of the Glen Turret Member. The base of the Glen Effock Formation in the Aboyne district is taken as the lowest of these magnetic beds. This green feldspathic semipelite (Craig Dullet lithology of Harte, 1979) mapped separately by Harte (1966), who showed that, within the area from Auld Craig [NO 455 783] to the head of Loch Lee, it forms the basal 150 m of the Glen Effock Formation. To the east of Auld Craig, this lithology is not always present at the base of the formation, but occurs sporadically at different levels within it.

The formation consists dorrtinandv of semipelite with beds of psammite, and minor petite. Many of the semipelites are finely speckled and rusty weathering, with roughly equal proportions of hiotite and muscovite, and moderate proportions of garnet and stanrolite, but little or no kyanite or sillirnanite. The psammites are typically medium grained and moderately well sorted, and are generally micaceous, with some 2 to 10 per cent. of biotite. The textures are generally schistose, though with the coarser fabric characteristic of the staurolite and kyanite zones. Gneissose layering and incipient migmatisation are developed in the northern part of the outcrop in feldsapthic psammites and some of the grey-brown semipelites. The green feldspathic semipelites are not migmatitic. Possibly due to their 'higher metamorphic grade, the Glen Effock Formation rocks mostly lack the spaced cleavage so prominent in the overlying Glen Lethnot Formation. Beds of gritty psammite are rare, except near the top of the formation, and graded bedding has not been recorded. Beds of almost pure quartzite up to 20 m thick with specks of bright green chlorite are developed locally about one third of' the way up the formation and within 100 m of its top.

The base of the formation is well exposed on Auld Craig [NO 455 784] (Harte, 1987) where it immediately overlies the Glen Tanar Member. Greenish feldspathic and micaceous semipelite is dominant. The magnetic susceptibility of the lowest semipelite bed is 47 x 10−3 SI, but the susceptibility decreases rapidly upwards. The biotite is dark green to dark brown, garnets rarely exceed 2 runt, and staurolite is moderately abundant. Kyanite is rare and is largely replaced by muscovite. Fibrolitic sillimanite is sparsely developed; it partly replaces biotite and, i-areh], kyanite. A very similar lithology is described from the crags of Craig Dullet [NO 426 792] by Harte (1966) plate 1c. The lowest part of the formation is also well exposed. in the River North Esk between Dalhastnie Bridge [NO 5345 7823] and 350 rim west of Minden [NO 5370 7867] where greenish semipelite predominates, and on the western part of the Craig of Dalhastnie [NO 543 778] where a higher proportion of the exposures consists of micaceous psammite, probably due to preferential weathering of the intervening semipelite. The lower part of the formation is also exposed on the Craig of Greenhorn [NO 555 790], where again the sudden increase in magnetic susceptibility compared with the Glen Turret. Member is readily observed, Here, the semipelites are darker coloured and finer grained, with more hiotite (largely chlorifised) and less muscovite than those of the Glen Turret N4ember.

Higher parts of the formation are well seen in the River North Esk section for 2 km upstream from the confluence with the Burn of Ronnoch [NO 5330 7775], and in its tributaries the Burn of Beag and the Burn of Keenie. Semipelite is dominant but grey, micaceous psammite, varying from poorly bedded to well foliated, forms about 30 per cent of the succession. The semipelites typically carry biotite, garnet, and staurolite, but kyanite and sillimanite ;:,re rare. The green colour of the rocks is largely due to chloritisation of the biotite and, locally, garnet. The same lithologies are also exposed on Tom Darrach [NO 567 793], on the lower slopes of Craigangower around [NO 572 782], and on the south side of Craigancash [NO 586 776]; at these localities the retrogression is even more widespread, with biotite and garnet chloritised and with any staurolite and kyanite completely replaced by shimmer aggregate.

White quartzite with rare specks of green chlorne (Barrow and Craig, 1912, p.21) forms only about one per cent of the rocks of the Glen Effock Formation, but it is very cur spicuous. Discontinuous beds crop out, mostly near the top of the Glen Effock Formation, throughout the Aboyne district from Cairn Caidloch [NO 4283 7805] in the west to the Short Gormack Bunt [NO 6077 7860] in the east, and into the Banchory district as far as the Cairn o'Mount. The quartzite beds occur between 600 in above the base, and the top of the formation, In places, for example near the Clash of Wirren [NO 491 750], two beds of quartzite occur, separated by about 40 m of semipelite. Bed thickness ranges from 10 in to 95 m. The quartzite is typically poorly bedded and cut by orthogonal joints at 0.5 to 2 m intervals. The relative persistence of the quartzite bed about 80 m below the top of the Glen Effbck Schist Formation makes it potentially useful as a stratigraphical marker.

Glen Leithnot Grit Formation

This formation crops out in the southern part of the Aboyne district and is cut off to the south-east by the North Esk Fault. The unit was called the 'Water of Saughs Formation' by Harte (1966), but was referred to as 'Glen Lethnot Grits' by Harte (1979). It is stratigraphically equivalent to the lower part of the Rottal Schist Formation of the adjacent Ballater district (Smith et al., in press t. It is distinguished front the Glen Effock Schist Formation by the higher proportion of psammite, much of it coarse grained and poorly sorted, and the occurrence within it of beck of silvery pelite, rich in muscovite and haematite, and kyanite-bearing in places. The formation is well exposed in the West Water and its tributaries in the south-west of the district, and on the steeper slopes nearby, especially where corries are developed. Good exposure also occurs along the banks of the North Esk from the North Esk Fault as far upstream as Auchmull. Most of the outcrop of the formation in the Aboyne district is right way up, but several reversals of younging occur within 2 km of the North Fsk Fault, and many of the rocks in this area are inverted (Chapter 8). The gritty psammites and schistose dark semipelites to pelites of the Glen Lethnot Formation are very similar to the metasedimentaty rocks of the Ronal Schist in the extreme south-east corner of the Ballater district, but the volcaniclastic rocks and amphibolites of the Rottal Schist do not occur in the Ahoyne district.

The transition from the Glen Effock to the Glen Lethnot Formation is marked by the first conspicuous development of graded bedding in the psammites and the incoming of abundant gritty material in the psammite units, although a few gritty beds do occur in the Glen Effock Formation. The boundary occurs about 80 m above the most persistent bed of the quartzite with specks of green chlorite.

The Glen Lethnot Formation occurs within all of the Barro ian metamorphic zones except the sillimanite zone. Grey to blue kyanite is developed principally in the silvery, muscovite-rich pelites, where it forms crystals up to 50 nun long. These occur for up to I km south of the 'kyanite' isograd (see Chapter 9 for definition of the kyanite isograd), due to the high oxidation state of the iron in these rocks. This restricts the amount of Fee' available to enter minerals such as biotite, staurolite and garnet, which therefore become very magnesian in composition. Under these conditions, the reaction

biotite1+ muscovite1+ chlorite + quartz→biotite2 +muscovite2 + staurolite + kyanite

proceeds at lower temperatures than when the Mg/Fe2+ ratio of the rocks is low (Chinner, 1965; Atherton and Brotherton, 1972; Flarte, 1973). Staurolite is abundant and garnet moderately abundant in all semipelitic and pelitic rocks of the staurolite and kyanite. zones. Garnet is rare, and is present as small crystals in rocks of garnet grade. Biotite is developed in all semipelitic and pelitic rocks from the biotite zone upwards. In biotite and some garnet zone rocks, it is poikilitic in habit, but in staurolite and kyanite grade rocks it occurs with muscovite in felted aggregates which define the foliation.

The lowest part of the formation is exposed on Craigangowan [NO 5389 7538] and on the south-east slopes of Bulg [NO 5433 7623]. Here, exposures are mostly of semipelite and pelite containing beds of psammite with a well-developed spaced cleavage typically with 3 to 5 mm spacing. On the ridge joining Craigangowan to the Hill of Wirren, beds of feldspathic psammite with flattened clasts up to 4 mm across are exposed. Similar stratigraphical levels are also exposed at the south end of the crags on the east side of the Clash of Wirren [NO 491 747], from where almost continuous exposure extends down the Burn of Tillybardine as far as the sheepfold at Tillybardine farm [NO 490 733]. Here, psammite is dominant; it is micaceous and feldspathic, with a well-developed spaced cleavage, and is interbedded with flaggy, micaceous semipelite and coarse gritty psammite, both in beds of 0.1 to 0.5 m thickness. In the Smithy Burn, the basal beds of the formation are flaggy, fissile semipelites with thin beds of gritty psammite. Beds of silvery pelite are rarely found in the lowest 300 m of the formation, with the exception the section at Craigoshina [NO 572 765], where several beds of silvery muscovite-kyanite schist are present.

The middle portion of the formation is probably best exposed in the West Water from its confluence with the Mill Burn [NO 4825 7335] to the point where it crosses the southern boundary of the district [NO 5125 7105]. From the Mill Burn to 500 m downstream of Stonyford, the rocks are dominantly micaceous psammites with thin gritty lavers increasing in thickness and abundance up the succession, interbedded with fissile fine-grained semipelites. In a few places graded units up to 1 m thick are present (Plate 4). About 500 m downstream of Stonyford [NO 5083 7219], there is a 50 m-thick bed of silvery muscovite-rich pelite with kyanite porphyroblasts up to 2 cm long (Plate 1)d. From this bed downstream to the southern boundary of the district, the proportion of psammite is less than to the north-west, but the psammite beds are more massive and the spaced cleavage is not everywhere developed, while the semipelites and pelites are dark, greenish, and generally coarser grained.

In the western part of the district, the middle portion of the formation is distinguished by the occurrence of several units of silvery, muscovite-rich pelite, which is typically associated with beds of dark haematite-rich semipelite. The best exposures of this association occur on the southern slopes of Broom Craig [NO 466 720]. Pods of massive haematite up to 2 m long are enclosed in muscovite-rich pelite containing brownish staurolitc crystals up to 3 mm and rosettes of grey, rarely bluish kyanite porphyroblasts up to 3 cm long. The muscovite-rich pelite in places contains o'ter 10 per cent haematite but magnetite and ilmenite are absent, and the magnetic susceptibility is low (<1 x 10−3 SI — comparable with Farfside Psammite Formation rocks).

Between Corrie na Berran [NO 441 720] and Hill of Berran [NO 453 717]. two units of this muscovite-rich pelite with large kyanite porphyroblasts, 10 to 25 m thick, can he traced for up to 2 km along strike. This lithology is also exposed in the West Water 500 m downstream of Stonyford and on the Craig of Stomford [NO 502 723].

North-east of the West Water, the muscovite-rich pelite is hardet to identify. but Barrow traced a bed of iron-ore schist' shown on the old edition of 1:63 360 sheet 66 (Geological Stine. of Scotland. 1897). with rather less muscovite than the examples around Broom Craig, from neat Stonyford to the southeastern slopes of Bulg. This rock differs from the more silken pelite in being finer grained (rarel. over 1.5 mm). and in containing a small proportion of magnetite as well as the dominant haematite. The gritty psammites are less pure than the psammites of the Glen Effock Formation, and are mostly coloured green by chlorite, which typically forms 5 per cent of the rock. The proportion of gritty psammite in the succession increases southwards, rising to over 60 per cent in the North Esk south of Auchmull.

The banks of the North Esk from the North Esk Fault [NO 5863 7332] to its confluence with the Burn of Auchmull [NO 5809 7421] expose a total of about 600 m of the highest Dalradian stratigraphical levels within the Aboyne district. There is a major reversal of younging direction at an antiformal axis 200 m downstream from the Burn of Auchmull [NO 5806 7404] (Figure 13); from here downstream to the North Esk Fault most of the succession youngs northwards, although there are further reversals of younging caused by a series of folds within 550 m of the North Esk Fault. Immediately north of the North Esk Fault, there is a 80 m-thick unit consisting dominantly of fissile, slaty pelite and semipelite with thin psammitic beds. The rest of the succession to the Burn of Auchmull is dominated by coarse-grained gritty psammite with thin pelitic partings, but two 50 m-thick semipelitic units crop out near the confluence with the Burn of Mooran. Graded units up to 2 m thick are common, The coarsest grained gritty psammites are poorly foliated. but finer-grained psammites contain a well-developed St spaced cleavage. which has obliterated any cross-bedding. The spacing of the cleavage is widest where the percentage of phyllosilicates in the rock is least.

For 750 m upstream from its confluence with the North Esk at [NO 5835 7348]. the Burn of Mooran exposes gritty psammites which are. if anything. even coarser grained and more massive than those in the North Esk itself: the are interbedded with micaceous psammite and fissile semipelite in thin beds. Graded bedding is well developed. The Burn of Auchmull, from its confluence with the North Esk [NO 5809 7421] to the Angus/Aberdeenshire boundary [NO 5955 7475]. exposes a dominant semipelitic succession with minor micaceous psammite.

Chapter 5 Highland Border Complex

Correlation and history of research

The Highland Border Complex is a group of rocks of Cambrian to Ordovician age, which has a narrow, discontinuous outcrop on the northern margin of the Midland Valley of Scotland from Arran to near Stonehaven. Similar rocks occur in an analogous position in Ireland from Clew Bay to County Tyrone. The rocks of the complex record a history of sedimentation, volcanism and erosion which seems largely incompatible with the tectonometamorphic history of the Dalradian rocks to the north, although it has been argued that there is structural and stratigraphical continuity between the Dalradian rocks and some elements of the Highland Border Complex (Tanner. 1995). The Highland Border Complex is fault-bounded to the north and is itself transected by faults; the rocks of the complex are mostly sheared, and in places mylonitised. The structural model proposed by Henderson and Robertson (1982) for the Abeifoyle area involves the thrusting of ultrabasic rocks with overlying Arenig sedimentary rocks on to the mid-to late Ordovician volcanic and sedimentary rocks (North Esk and Margie Formations and their equivalents) in the late Ordovician or early Silurian, followed by later (probably early Silurian) docking of the complex as a whole against the Dalradian block. The outcrop of the complex is in most places bounded to the south-east by a fault with a large downthrow to the south, but at Stonehaven the complex is overlain by Upper Silurian sedimentary rocks. Therefore, the extent to which Highland Border Complex rocks underlie the Midland Valley of Scotland is unknown.

Fettes (in Stephenson and Gould, 1995; Chapter 9) describes the evolution of ideas on the relations of the various components of the Highland Border Complex to each other and to the Dalradian rocks. Five components have been identified in the complex, of which only (1) and (5) occur in the Aboyne district.

  1. Margie Formation and equivalents: psammite and grey mudstone with minor limestone (Caradoc to ?Ashgill) (Curry et al., 1984).
  2. North Esk Formation and equivalents: basalt pillow-lava, chert and black mudstone (Llanvirn to Llandeilo) (Campbell, 1913; Downie et al., 1971: Curry et al., 1984).
  3. Dounans Limestone basal breccia with serpentinite fragments overlain by limestone (Arenig) (Curt et al., 1982).
  4. Leny Limestone. limestone and mudstone (late Lower Cambrian, c.540 Ma) (Pringle, 1939).
  5. Oceanic basement: dunite, harzburgite, amphibolite (537 ± 11 Ma, Lower or Middle Cambrian) (Dempster and Bluck, 1991).

Limits to the timing of the juxtaposition of the Highland Border Complex with the Dalradian block are set by the end of deposition of the Margie Formation (?Ashgill, about 443 to 439 Ma) and the earliest sedimentation to overstep the Ordovician–Dalradian boundary. This is the Telychian Loch Mask Fornration of western Ireland (about 432 to 430 Ma). however, the convergence of the Dalradian rocks with the Highland Border Complex was almost certainly oblique, and may well have been later in the Silurian in eastern Scotland.

The geological setting of the Highland Border Complex in the Aboyne district and surrounding areas is shown in (Figure 14). The North Esk and Margie Formations are well exposed in a 1.2 km section along the banks of the North Esk, and there are a few other exposures in the district.

The North Esk section has been described by several workers. Imrie ( 1812) described a section from near Edzell to Mount Battock, and noted many sudden changes of rock type, also the folding of the Margie Formation and the Dalradian rocks. Barrow (1901) differentiated the rocks of the complex from the 'Highland Schists' to the north, and divided them into a 'jasper and Green Rock Series' (now North Esk Formation) and a 'Margie Series' (now Margie Formation) and assigned a Silurian age to the latter. He interpreted the 'Green Conglomerate' to the north of the North Esk Formation as a basal conglomerate, lying above an unconformitv, and implying that the North Esk Formation is older than the Margie Formation. Pringle (1942) re-examined the section, and concluded that the southern boundary of the 'Green Conglomerate' is tectonic, and the Margie Formation rocks young towards the contact.. Anderson (1946) accepted this information, and proposed that the North Esk Formation is younger than the Margie Formation, which he correlated with the Dounans Limestone.

The North Esk Formation contains acritarchs of Tremadoc to early Llanvirn age (Downie, 1971). Fossil assemblages from the black mudstones associated with the pillow lavas at Stonehaven indicate a Llanvirn to Llandeilo age of these rocks, which are also assigned to the North Esk formation (Campbell, 1913; Curry et. al., 1984). The Margie Formation rocks were stated by Burton et al. (1983), to contain chitinozoa of Caradoc to Ashgill age. The present account is based on this correlation, but more recently conodonts suggesting an Arenig age have been reported (C.J. Burton, personal communication in Tanner, 1995).

Booth and Harte (Booth, 1984) surveyed the North Esk section in detail, especially the northern outcrop of the Margie Formation, and this work was used by Harte et al. (1984) in a structural synthesis of the Highland Border Complex. Robertson and Henderson (1984) studied the geochemistry of the meta-igneous rocks of the complex.

North Esk Formation

These deposits form a typically oceanic association, consisting of metabasalt, chert and dark blue slaty pelite. The metabasalts have a mid-ocean ridge basalt (MORB) geochemistry, while the metasedimentary rocks are geoche mically similar to hydrothermal precipitates with a minor terrigenous component (Robertson and Henderson, 1984). These authors envisaged deposition of the North Esk formation in a small oceanic basin at the margin of the Iapetus Ocean. The outcrop of the formation within the district forms a belt which reaches a maximum width of 650 m in the North Esk, thins to 350 m at the eastern boundary of the district, and is cut out near the southern boundary of the district. Its boundary with the northern outcrop of the Margie Formation in the North Esk section is probably tectonic, but the dislocation is probably of little more significance than the dislocation at the internal sheared boundaries within the formation; it appears to cut out the unconformity between the formations together with a few metres of the succession.

Within the district, the formation is well exposed in the banks of the North Esk; some exposures also occur in the Kirkton Burn. The succession in the North Esk section is summaried below (1–9). Due to the shearing, most of the lithological boundaries in the formation are now tectonic.

North Esk section from south to north [NO 5931 7233] to [NO 5863 7332]; no younging direction is implied.

  1. Dark blue fissile slaty pelite with a few green, chloritic layers and rare pods of yellow-weathering chert. Possible fault at boundary with 2.
  2. Dark green to black metabasalt, including at least three units of pillow lava separated by more schistose, green, chloritic metabasalt (Plate 5). The pillows indicate that the succession here youngs southwards. The metabasalt becomes greener and more chloritic northwards.
  3. Layer of chert, 10 m thick, showing a shear foliation and close-spaced jointing. The chert is dark grey, but weathers brown and smells of sulphide when hammered. It is veined by quartz and dolomite. At the margins of the layer, chert and metabasalt are interlayered on 10 cm scale; this is probably a sheared contact.
  4. Dark green, schistose, chloritic metabasalt with pods and layers of red jaspery chert up to 3 m thick. The foliation in the basic rocks wraps around the chert pods. Much sulphide occurs at chert/metabasalt contacts. At one locality metabasalt becomes interlayered with dark purple slaty rock.
  5. Purple, slaty, fissile rock; thin sections show that it contains much mylonitised chert. It contains green mottlings and pods of possible dolomite up to 1 cm by 4 cm, stretched parallel to the foliation.
  6. Dark green, schistose, chloritic metabasalt with a good planar foliation, rather soft; a few purplish patches near southern contact.
  7. Chert, dark grey where fresh, brown weathering, closely jointed.
  8. Metabasalt, dark green, schistose, but slightly less well foliated than unit (6). In places a second foliation is observed.
  9. 'Green Conglomerate'. See Margie Formation.

Exposures in and on the slopes east and west of the Kirkton Burn [NO 6025 7340] to [NO 6001 7377] show grey-green schistose, chloritic metabasalt for 50 m upstream from the faulted contact with the Margie Formation, succeeded for 200 m to the north-west by poorly exposed grey to grey-green phyllites with thin beds of gritty psammite. To the north-west schistose metabasalt is exposed for 150 m before present-day exposures cease. However, Barrow (1901) recorded an exposure of the 'Green Conglomerate' [NO 6001 7382]. Dark blue, fissile, papery slate is exposed in the Burn of Balfour [NO 6107 7430] to [NO 6103 7434]. Red weathering along joints and partings suggests that some of this material may be mylonitised chert.

Metabasalt

In thin section, the metabasalts range from rocks where the primary texture is preserved to completely recrystallised chlorite schists. Where the primary texture is preserved, the original plagioclase has been replaced by albite, and the pyroxene has been replaced by fine-grained aggregates of actinolite and chlorite.

Chert

Where least tectonised, the chert is a mosaic of very fine-grained (0.01 mm) quartz crystals with specks of interstitial pyrite, which is in many places oxidised to haematite or limonite. Patches of clear quartz up to 0.2 mm across may be the recrystallised tests of radiolaria. Where the chert has been sheared, haematite forms along the planes of movement which, in the case of the most sheared examples, are spaced as closely as 0.5 mm apart. The proportion of opaque material in these sheared cherts appears greater than in the untectonised cherts. Where the chert has been oxidised but only slightly sheared, the rock becomes a red to buff jasper.

Pelite

Thin beds of dark blue-grey slaty pelite are difficult to distinguish in hand specimen from mylonitised chert.

Margie Formation

This Formation is dominated by psammite, generally coarse grained, buff weathering and largely carbonate cemented. It crops out in two belts: one between the Highland Boundary Fault and the North Esk Formation, and the other between the North Esk Formation and the North Esk Fault.

The southern belt has an outcrop width of 550 m in the Margie Burn, just south of the district boundary (Figure 14), decreasing to 480 m in the North Esk and 360 m at the eastern edge of the district. Exposure is almost continuous along the banks of the North Esk when the water is low. The southern part of the outcrop is dominated by coarse-grained brown-weathering psammite with gritty, in part conglomeratic, beds, whereas the northern part consists largely of semipelite to pelite. Limestone beds, 1 to 1.5 m thick, occur near the south-east and north-west boundaries of the outcrop. The southern one lies 150 m north-west of the northern boundary of the Lintrathen Tuff and the northern one immediately north of the larger of the two quartz-dolerite dykes near the northern boundary of the outcrop, where it has been quarried and is no longer exposed. Other exposures of the southern belt occur near Dalbog farmhouse [NO 5862 7180] to [NO 5840 7180], in the Kirkton Burn [NO 6043 7314] to [NO 6044 7321], and in the Margie Burn, just south of the district [NO 5657 7017] to [NO 5627 7053]. The Dalbog and Kirkton exposures are mostly grins psammite with thin pelitic lasers: the Kirkton rocks are intruded by a sheet of docile porphyrs which occupies most of the space between the Highland Boundary Fault and the fault separating the Margie and North Esk formations. The Margie Burn exposures contain a higher proportion of pelitic material than those in the North Esk.

The northern belt has an outcrop width of 80 m in the North Esk section (Figure 16). and is not exposed elsewhere in the district, except for the exposure of 'Green Conglomerate' in the Burn of Balfour recorded by Barrow (1901). This part of the North Esk section was mapped in detail by Booth and Harte (Booth 1984), who described it as a tectonic mélange made tip of at least six components.

The boundaries between the components are faults which have been subjected to later folding to give cursed outcrops. The lithologies involved are gritty psammites, grey brown and purple slaty pelites. a bed of limestone. and the 'Green Conglomerate'. The limestone occurs within 10 m of the North Esk Fault and has an outcrop width of 15 to 20 in. Small quarries in the limestone, now overgrown. occur on the eastern bank of the North Esk.

The beds dip to the north at angles of 60° to 70°. The principal cleavage is. as in the southern outcrop, subparallel to bedding. and Harte and Booth recognised four phases of deformation (Chapter 8). The sense of younging could be determined at only two places. where it was to the south.

Psammite

The ( lasts are quartz and oligoclase, generally less than 5 mm across and are only slightly flattened. The amount of fine-grained schistose matrix is less titan in the Southern Highland Group grim psammites. Most of the psammites are cemented by quartz, but in many places the cement is brown-weathering calcite and ankerite with rare dolomite. In thin section. the gritty psammites show a much lesser degree of grain flattening than the Dalradian rocks of similar grain site.

Pelite

The slats pelites are cream to red in the southern part of the southern outcrop, but elsewhere are mostly dark grey or dark brown. They have a well-developed cleavage marked by the flattening of small quartz and feldspar clasts, and by alignment of sericite and chlorite flakes. Barrow (1901) claimed to recognise grains of elastic mica (brown and white) in these rocks, and for this reason considered them to be of lower metamorphic grade than the Dalradian rocks immediately north of the Highland Boundary Fault. Chlorite and sericite in the slates are less completely recrystallised and less well aligned than in Dalradian or North Esk Formation pelites.

Margie Limestone

Two exposures of limestone were recorded in the soufaern outcrop and one in the northern outcrop. In all cases the limestone is recorded as being coarse grained or pebbly. The pebbles are mostly slaty pelite, and are set in a carbonate matrix. The limestone in the northern belt where still exposed, is blue-grey, fine grained and poorly foliated. It consists largely of calcite, probably with a little ankerite. It is cut by veins of glassy quartz and white calcite.

'Green Conglomerate'

This lithology was named by Barrow (1901). It occurs at the southern boundary of the northern outcrop of the Margie Formation in the North Esk section, where it forms a unit approximately 15 to 20 m thick. The rock consists of subrounded clasts of metabasalt with chert and rare amphibolite and granite up to 5 cm across set in a schistose, chloritic matrix (Plate 6). Clasts form a small proportion of the rock, which is matrix-supported. Many of the pebbles are crushed, and the matrix of the conglomerate is identical in colour and lithology to the schistose metabasalt of the North Esk Formation to the south, from which it is separated by a small fault. Near the northern boundary of the unit, there is an intensely sheared zone 1 to 1.3 m thick, in which the pebbles are elongated or flattened, and there is a 0.5 m-wide zone of mylonitised material drawn equally from the chloritic material to the south and the arenaceous material to the north. As both contacts are tectonic, it is not clear whether the boundary between the North Esk and Margie formations should be placed at the southern (Barrow. 1901) or northern (Pringle, 1942) boundary of the conglomerate. If the Margie Formation is of Caradoc to Ashgill age, the 'Green Conglomerate' probably represents a period during the deposition of the Margie Formation when large quantities of North Esk Formation material became available, together with amphibolite and granite from an unknown source. The younging evidence indicates that a considerable tectonic break must occur at the formation boundary. If, however, the Margie Formation is of Arenig age, then the 'Green Conglomerate' is a locally derived basal unit of the North Esk Formation, and the boundary between the formations is a sheared unconformity.

The Kirkton Burn exposure [NO 5997 7375] recorded by Barrow (1901) lies to the north of the North Esk Formation outcrop and is separated from the Dalradian rocks to the north by 100 m of unexposed ground.

Chapter 6 Old Red Sandstone

Silurian and. Devonian sedimentary and volcanic rocks of continental Old Red Sandstone (ORS) facies were deposited over the Midland Valley of Scotland as a molasse sequence at the margin of the Caledonian mountain belt. The deposition was contemporaneous with the intrusion of the post-tectonic granite plutons in the Grampian Highlands. The northern margin of the Midland Valley is faulted, but a few outliers show that at least the upper part of the ORS sedimentary pile transgressed on to the rocks of the Grampian Highlands. The most complete Lower Old Red Sandstone sequence in Scotland occurs in the Stonehaven district (Sheet 67). Only a small corner of the Aboyne district is underlain by rocks of this age. Most of the outcrop belongs to the uppermost, Devonian, Strathmore Group, but a fault-bounded outcrop of the Silurian Crawton Group also occurs. The Silurian and Devonian rocks of the district were included in Armstrong and Paterson's work on the Old Red Sandstone of Strathmore (1970), and the area east of grid line 60 has been described by Carroll (1995), No fossils have been recorded from the Silurian or Devonian rocks of the district.

Crawton Group

Lintrathen Tuff

The Lintrathen Tuff (Porphyry on map) is a distinctive welded dacitic tuff which is recognised from Lintrathen, in the Kirriemuir district (Sheet 56E). to Glenbervie, in the Banchory district (Sheet 66E). It is regarded as a member of the Crawton Volcanic Formation of the coast section in the Stonehaven district, and is probably of latest Silurian age, i.e. Přídolí , as the succeeding Arbuthnot Group has yielded fossils of earliest Devonian age. The Silurian–Devonian boundary in the northern Midland Valley is conventionally taken at the base of the Arbuthnott Group. Within the Aboyne district, its outcrop is 140 m wide in the North Esk section, and wedges out to the south-west near Dalhog and to the north-east about 500 m north-east of the Kirkton Burn [NO 610 734]. To the south it is faulted against rocks of the Strathmore Group. The northern contact, against rocks of the Margie Formation, was believed to be faulted by Anderson (1946), who considered this fault to be the local expression of the Highland Boundary Fault. However, Henderson and Robertson (1982) considered that the Lintrathen Tuff in the North Esk lies unconformably on Margie Formation rocks. The exposures of the Lintrathen Tuff in the North Esk section are very poor, and no evidence to support the latter interpretation was seen in the present survey. To the east of the

Glen Esk road, its outcrop is marked by a flat valley with scarp features to the north and south, both of which appear to be fault controlled. The only exposures of the ignimbrite occur in the River North Esk [NO 5935 7233], and near Kirkton of Balfour [NO 6020 7286]. The North Esk exposures are only accessible when the river level is low.

The Lintrathen Tuff is a heavily fractured, friable, pale brick red rock, becoming yellow where most strongly weathered. More coarsely crystalline patches derived from lapilli weather out on the surface. The flow structure is defined by flattening of lapilli and their elongation in the direction of flow, A planar structure, formed by parallel alignment of biotite phenocrysts, may be that shown as dipping steeply south on old field slips. It is more readily visible in weathered specimens than the flow structure.

Petrography

In thin section, the matrix, consisting of devitrified glass, has a well-developed flow structure. Crescentic patches of very coarsely crystalline silica may represent collapsed shards of pumice. The enhedral quartz and biotite phenocrysts show signs of fracturing and shearing, due to flow of the ignimbrite. Thirlwall (1988) obtained a Rb/Sr whole-rock/biotite age of 415.5 ± 5.8 Ma from the LintrathenTuff (locality unstated) which is compatible with the latest Silurian age deduced from its position at the top of the Crawton Group. This is taken to date the top of the Crawton Group. The sill of dacite porphyry which intrudes the Margie Formation rocks south of Kirkton of Balfour is petrographically similar to the Lintrathen Tuff and is probably genetically related to it.

Strathmore Group

A succession of sedimentary rocks belonging to the Strathmore Group is exposed along the banks of the River North Esk, from the faulted contact with the Lintrathen Tuff south-eastwards to the southern margin of the district, 20 m downstream from Gannochy Bridge (Figure 17). Exposure is poor in the lowest formation, the Cromlix Mudstone, but the Gannochy and Edzell formations are well exposed where the North Esk flows though a narrow gorge. The total thickness of the group on the northern limb of the Strathmore syncline is highly variable, due to large lenses of coarse conglomerate up to 1400 m thick; where these are absent it is about 2100 m. About two-third of this succession crops out in the Aboyne district. The Strathmore Group has been dated from plant remains as late Pragian to early or middle Emsian age by Weston (1977).

Cromlix Mudstone Formation

The rocks lying immediately south-east of the faulted contact with the Lintrathen Ignimbrite in the North Esk section were assigned by Armstrong and Paterson (1970) to their Edzell Mudstone Formation. Carroll (1995) has abolished this formation name and incorporated these rocks into the Cromlix Formation, which has an extensive outcrop on the south-east limb of the Strathmore syncline. Cromlix Formation rocks are only exposed along the banks of the North Esk at periods of very low river flow. The stratigraphical thickness is estimated at 200 m by Armstrong and Paterson (1970), increasing to 280 to 350 m to the east (Carroll, 1995). The formation typically consists of red and green mottled sandy mudstone with intercalations of locally micaceous sandstone and finely cross-laminated siltstone, but the exposures in the North Esk section are mainly poorly sorted fine- to medium-grained muddy sandstones. Exposures near Mid Mains of Balfour [NO 6247 7402] in the Banchory district show that the uppermost part of the formation becomes more arenaceous to the north-east, with poorly sorted fine-grained arkosic sandstone dominant over very poorly sorted arkosic wacke.

Gannochy Formation

Carroll (1995) has redefined this formation from that initially proposed by Armstrong and Paterson (1970), so that it now comprises only a 250 m-thick succession lying immediately above the Cromlix Formation in the North Esk section. It dies out along strike to the north-east and is not exposed in the Banchory district.

The redefined Gannochy Formation consists of coarse-grained, dark red, clast-supported orthoconglomerate cropping out from 300 m upstream [NO 5947 7198] to immediately downstream [NO 5947 7165] of Loups Bridge (cover photo). The typically well-rounded, ovoid clasts are mostly 0.2 to 0.3 m in size, but range up to 1 m. They consist largely of psammite, quartzite and vein quartz, with lesser amounts of semipelite, and are set in a matrix of poorly sorted arkosic sandstone cover.

Edzell Sandstone Formation

This formation, as redefined by Carroll, now includes the upper part of Armstrong and Paterson's Gannochy Formation. The total thickness in the core of the Strathmore Syncline is approximately 1350 m, of which 950 m is exposed within the Aboyne district. The lower part of the formation, upstream from Gannochy Bridge, consists of dark red sandstone with pebbly beds. The pebbly beds are up to 2 m thick and are unlaminated and poorly sorted, with irregular, poorly rounded clasts of vein quartz, quartzite and schistose psammite up to 0.1 m across, set in a quartzose to lithic sandy matrix. The intervening layers consist of darker red, finely laminated sandstone.

South of Gannochy Bridge, there is a transition to thickly planar-bedded fine-grained sandstone and silty sandstone with very thin beds of laminated mudstone. These rocks are exposed in the Forfar district (Sheet 57W) to the south, but are not exposed in their outcrop in the extreme south-east corner of the Aboyne district.

Sedimentology and depositional environment

Based on the North Esk exposures, rapid changes in depositional environment occurred during deposition of the Strathmore Group.

The Cromlix Mudstone Formation probably represents a relatively stable piedmont floodplain, occasionally inundated by ephemeral sheet floods producing isolated lenses of pebbly sandstone, but with long periods when the floodplain dried out, with the production of desiccation cracks.

The Gannochy Formation is an alluvial fan deposit, laid clown by a large, fast-flowing river. Its impersistence along strike is evidence of local influx of coarse material. The rounded nature of the clasts suggests transport for several kilometres. The source was to the north, as the clasts are all derived from the Dalradian terrane. The sudden onset of coarse sedimentation is more likely to reflect change of the course of the river than renewed movement along the Highland Boundary Fault.

The pebbly beds within the Edzell Sandstone Formation indicate a relatively high-energy environment, possibly with flash flooding of a plain or broad valley and finer-grained deposition in flood plains. By the time the upper part of the formation was deposited, the supply of sand-grade sediment. was more regular, and there was less washing-in of coarse material from the valley sides.

Chapter 7 Igneous rocks

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. Although no evidence of cross-cutting relationships is available within the Aboyne district, their uniformity of composition and lack of metasedimentary laminae suggest that they, like those in the Alford and Inverurie districts to the north, represent. basic sheets intruded into the Dalradian rocks shortly after deposition. They were probably intruded contemporaneously with the extrusion of the Green Beds pyroclastic rocks which occur in the Southern Highland Group rocks of Glen Clova in the Ballater district (Sheet 65E) and areas further west. Amphibolite sheets are most abundant in Argyll Group rocks close to the outcrop of the Deeside Limestone Formation, and in rocks of the Tarfside Psammite Formation north and west of Tarfside village. They are thin and scarce within the Glen Effock Formation, and no examples have been recorded within the Glen Lethnot Formation rocks of the district.

Deeside

Several sheets of amphibolite occur on the south bank of the River Dee between Dinnet Bridge and the western margin of the district. They are roughly concordant with the layering of the surrounding Dalradian rocks. Several of these sheets give rise to sizeable magnetic anomalies but others are almost non-magnetic, the difference reflecting variations in the proportion of magnetite in the rocks. The mapping of the area between the Mount Battock and Ballater plutons on the border between the Ballater and Aboyne districts was accomplished with the aid of a. ground magnetic survey by A G Leslie (Queen's University, Belfast). The largest sheet attains a maximum thickness of 200 m; the sheets thin eastwards along strike. Ord Hill [NO 438 986], a small isolated hill in the Muir of Dinnet, is made of amphibolite, but the extent of this intrusion is unknown. Smaller sheets occur at Tomachallich [NO 474 999], Woodcroft Cottage [NO 580 983] and Shannel [NO 600 955]. The Deeside sheets form an extension of the group in the Alford district (Sheet 76W) to the north, where the thickest is over 400 m thick. The internal foliation within each sheet is generally concordant with that in the country rocks. The contacts of the bodies with the host metasedimentary rocks are sharp, but subsequent deformation and metamorphism have obliterated any primary intrusive relationships and fabrics.

Glen Esk

The largest amphibolite bodies in this area crop out on Cairn Robie [NO 468 804], and on the hill behind Invermark Lodge [NO 427 813], forming concordant sheets up to 100 m thick. Amphibolite sheets up to 30 m thick are widely scattered in the Tarfside Formation; they are not concentrated at. any particular stratigraphical level. Amphibolites in the Glen Effock Formation are confined to the area north and east of the North Esk. None can he traced further than 200 in along strike and most examples are seen only in a single stream section. Two sheets, both less than 2 m thick, are exposed in the Water of Charr [NO 597 791] to [NO 602 793], and an 8 m-thick sheet occurs near Colmeallie [NO 572 778].

Petrography

To the north of the Mount Battock pluton, the amphibolites typically consist of roughly equant hornblende crystals 1 to 2 mm across forming a mosaic with plagioclase crystals of similar size and smaller crystals of iron oxdes and sphene. The hornblende, where fresh, is moderately pleochroic in shades of brown, and unzoned. In the freshest example (S80452), the plagioclase composition is An56. Most of the amphibolites are moderately to strongly altered; the plagioclase is saussuritised, and the hornblende replaced by fibrous aggregates of pale green actinolite. In a few examples, red-brown biotite also grows across the primary minerals.

To the south of the Mount Battock pluton, the amphibolites are finer grained, and have a better-developed planar foliation and a good linear alignment of the long axes of the amphibole crystals (S82777). They are hornblende–plagioclase rocks with traces of ilmenite, sphene and quartz. Nearly all of them lie within the kyanite and sillimanite zones of regional metamorphism. The amphibole has a pleochroic scheme: α = pale yellow; β = dark olive green; γ = greyish- or bluiush green; and is unzoned. Plagioclase has a composition of about An43. Alteration is less widespread than to the north of the Mount Battock pluton, but some specimens have saussuritised plagioclase; alteration of amphibole is rare, but in one specimen bright. green chlorite partly replaces the amphibole. A specimen from the thick amphibolite sheet in lower Glen Mark [NO 4281 8118] has a slightly more brownish amphibole (S92688), transitional to that developed north of the Mount Battock pluton, and is characterised by elongated pods, 10 mm by 3 mm, consisting almost entirely of saussuritised plagioclase. This body may have been coarsely feldsparphyric prior to regional metamorphism.

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 granitic intrusions

The granitic intrusions of the eastern Grampian Highlands have been divided into late-tectonic and a post-tectonic groups. Members of the late-tectonic group were intruded shortly after the climax of regional metamorphism and the deformational episode at about 470 Ma. They are typically foliated muscovite–biotite granites but with some diorites, tonalites and granodiornes. C/Pb zircon and monazite analyses from the Aberdeen and Strichen granites give ages of 470 ± 1 Ma and 475 ± 5 Ma respectively, although Rb/Sr ages for the group are typically in the range 450 to 470 Ma (Kneller and Aftalion, 1987; Pidgeon and Aftalion, 1978: Parkhurst, 1974). The only late-tectonic intrusions in the Aboyne district are a number of veins, sheets and small pods of muscovite–biotite granite with pegmatitic patches, which are considered analogous to the Cairn Trench Granite of the Ballater district (Robertson, 1991). This is peraluminous S-type granite, which has yielded an Rb/Sr whole-rock isochron of 453 ± 4 Ma and an initial 87Sr/86Sr ratio of 0.7194 ± 0.0002, confirming its similarity to the Aberdeen Granite (Munro, 1986). Geochemical analyses (Smith et al., in press) show that despite low Fe, Mg, Mn and P contents, the Cairn Trench Granite has not undergone extreme fractionation.

The late-tectonic granite veins and sheets; between 5 and 50 in wide, intrude the Dalradian rocks to the south of the Mount Battock pluton. Examples occur east of Craig Crane [NO 528 790], north of Tarfside village [NO 495 502], on Cowie Hill [NO 492 773] and on Cairn Robie [NO 466 806]. The sheets typically have near-vertical contacts and strike NNE, The veins cut the foliation of the Dalradian country rock, and are un foliated.

Petrography

The late-tectonic granite of the Aboyne district is a pink to grey muscovite granite, with a grain size of 3 to 5 mm, increasing to 10 mm in irregular pegmatitic patches up to 1 m in size. In the typical granite (S82799), plagioclase Forms irregular crystals up to 5 nun with cores An20–17, zoned out to rims of An11. The larger crystals are lightly to moderately sericitised. Quartz forms anhedral crystals, nuns of which have broad cracks running through them. Some of the large quartz crystals have recrystallised to mosaics of smaller crystals with sunned boundaries. The potassium feldspar is microcline, with good cross-hatch twinning. Myrinekite is developed in places at plagioclase–microcline boundaries, while the microcline crystals contain irregular blebs of exsolved albite. Biotite, originally forming 1 to 2 per cent of the rock, is largely chloritised and has been partly replaced by Muscovite in crystals growing across the biotite. Muscovite forms 3 to 5 per cent of the rock and appears to be mostly coeval with the quartz and microcline. Rare rounded garnet crystals up to 0.15 mm across are associated with altered

Post-tectonic granitic intrusions

This grouping includes the majority of the intermediate to acid intrusions in the eastern Grampian Highlands. They were intruded in the later Silurian to early Devonian (420 to 395 Ma) (Stephenson and Gould, 1995, table 2; Gould, 1997), compared with 475 to 470 Ma (early Ordovician) for the late-tectonic intrusions. They are unfoliated or only patchily foliated, and have been divided (Gould, 1997) into an earlier, largely granodioritic, Crathes Suite and a later Cairngorm Suite consisting almost entirely of biotite granites.

Little work was done on the granites of the district prior to 1980. Walsworth-Bell's (1974) study of granitic rocks west of Aberdeen barely extended to the edge of the Aboyne district. Webb and Brown (1984), in the course of evaluation of the geothermal potential of the Mount Battock and Ballater granites, analysed samples for major and selected trace elements. Harrison (1987) identified various phases within the Ballater and Mount Battock granites, and studied the geochemistry and mineralogy of the plutons, but did little detailed mapping. O'Brien (1983) included the Mount Battock and Ballater granites in a geochemical stitch of Caledonian granites.

The granitic intrusions lying up to 30 km west of Aberdeen (Aberdeen and Inverurie districts) were studied by Walsworth-Bell (1971), who realised that they belong to several different groups, divided by age. field relations and geochemistry. About half of the intrusions share a certain coherence, and comprise recognisable phases within a suite whose members extend outside the area studied by Walsworth-Bell at least as far as Logie Goldstone (Alford district). This suite, named the Cradles Suite by Gould (1997), extends only a short distance into the Aboyne district, where it includes parts of the Torphins Diorite and the Kincardine O'Neil Granodiorite. A small diorite body at Boat of Kincardine is also assigned to this suite.

The Cairngorm Suite is widely represented in the eastern Grampian Highlands, extending from the Monadhliath to the Mount Battock, and probably Peterhead, plutons. These intrusions form the major components of the east–west-trending East Grampians Batholith (Plant et al., 1900). The Cairngorm Suite is represented in the Aboyne district by the Mount Buttock, Ballater and Ord Fundlie plutons.

Crathes Suite

These intrusions were emplaced after the cessation of tectonothermal activity related to the Grampian Orogenv, but predate the intrusion of the Cairngorm Suite and probably the onset of early Devonian faulting and volcanism in the district. Contacts with the country rocks are sharp on outcrop scale, though in a few cases vein complexes are developed. There is no sign of chilling at the margin of any intrusion or of contact metamorphism of country rocks. The granodicwites display a typical calc-alkali differentiation trend, but the diorites show geochemical indications of enrichment in cumulus phases. Individual members of the suite show only a limited range of compositional variation. The finer-grained, more-basic parts of the Torphins Diorite show complex relationships within slightly differing dioritic phases. A limited vein complex extends for a short distance beyond the Kincardine O'Neil Granodiorite, but otherwise veining of the host rocks enclosing plutons is rare. Chemical analyses by Walsworth-Bell (1974) are available for the Torphins Diorite, Brown et al. (1965) obtained a K/Ar biotite age of 420 ± 2 Ma from a specimen of Crathes Granodiorite from Craigenlow Quarry in the lnverurie district [NJ 732 093]. It is likely that this represents a cooling age, but suggests that the Crathes Suite is probably older than the Cairngorm Suite, if only by a few million years.

Torphins Diorite

A very small part of this pluton occurs near the northeast corner of the Aboyne district. Coarse-grained (4 mm) diorite with patches of tonalite and granodiorite is exposed in a disused pit at the foot of Ord Fundlie [NO 6070 9965], and blocks of similar lithology occur higher up the slope from this pit. About 150 m to the east [NO 6086 9962], Nenoliths of diorite occur within the Ord Fundlie granite close to its contact with the diorite.

Petrography

The coarser facies of the Torphins Diorite e.g. (S77499) consists of light grey, coarse- grained (4 to 5 mm), unfoliated diorite with about 10 to 15 per cent each of hornblende and biotite, and up to 5 per cent quartz. The sub-to euhedral plagioclase crystals have cores of An46, zoned to An18. Sphene forms tip to 5 per cent of the rock, opaque minerals form 1 to 2 per cent and apatite forms 1 per cent of the rock. Quartz, and potassium feldspar where present, are interstitial to the other minerals,

Geochemistry

The geochemistry of the Torphins Diorite was studied by Walsworth-Bell (1974), and is summarised by Gould (1997). It shows a wide range of chemical composition, indicating the probability that it has incorporated a significant proportion of hornblendic cumulus material, possibly precipitated from a little-evolved dioritic magma.

Kincardine O'Neil Granodiorite

This granodiorite crops out in several small scattered areas, none larger than 0.5 km2. Most of these are in the Alford district, but an outcrop no more than 150 m across is exposed in the stream at Mill of Dess [NO 571 998].

Petrography

The rock at Mill of Kincardine (S80460) is a medium-grained (3 to 4 mm) white granodiorite to monzogranite, with rare K-feldspar macrocrysts up to 10 turn. Plagioclase is zoned from An28 to An16. The rock contains 5 to 8 per cent biotite and 2 to 3 per cent sphene, while hornblende is absent.

Boat of Kincardine Diorite

A small exposure of quartz diorite occurs on the south bank of the River Dee at Boat of Kincardine [NO 585 993]. The exposure extends for no more than 50 m, and the quartz diorite is cross-cut by granite veins up to 0.3 thick. Contacts with the country rock are not exposed.

Petrography

The sectioned specimen (S80501) is a hornblende-biotite-quartz diorite with about 70 per cent plagioclase, 10 per cent hornblende, 5 per cent biotite, 5 per cent quartz, 5 per cent microcline and 5 per cent spheric. The plagioclase Forms subhedral crystals up to 4 nun across, which commonly contain ragged inclusions of green hornblende up to 1 mm across. The plagioclase has rounded cores of An39 surrounded by relatively thick rims zoned to about An18. Biotite forms mid-brown euhedral crystals to ragged flakes, commonly chloritised. Cross-hatch twinning is well developed in microcline which, like the quartz, is interstitial to plagioclase and hornblende,

Cairngorm Suite

This suite comprises a number of granitic intrusions within the northern Grampian Highlands. The suite coincides with a large east–west-trending regional negative Bouguer gravity anomaly (Rollin, 1984). This suggests that they are probably the surface expression of a very large buried granitic body, the East Grampians Batholith (Plant et al., 1990), which has an extent of 100 km by 40 km. The component plutons consist mostly of leucocratic biotite granite with a limited range of composition. Some members, for example Lochnagar, are associated with granodiorite to quartz diorite intrusions, and are associated with annular magnetic anomalies. This suggests the presence of more basic phases at depth. The granites of the Cairngorm Suite are largely hornblende-free. Biotite is universallly present; in some more-evolved facies it is lithium-bearing. Muscovite is secondary, and present principally in areas of greisening and hydrothermal alteration. Magnetite, ilmenite, monazite, apatite and zircon are normally present, but in most facies total less than 1 per cent of the rock. In a few of the granites, there is sufficient Ca for sphene and allanite to form instead of ilmenite and monazite. Microgranite, aplite and pegmatite sheets and dykes are widespread. These essentially I-type 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 metasedimentaty input (Harrison and Hutchison, 1987).

The early facies of the plutons are mostly primary-textured granites, but are in many cases cut by, or partially made over into two-phase granites (compare with Cobbing et, al., 1986). The bimodal grain-size distribution is ascribed the separation of a hydrous fluid phase, due to decompression during crystallisation of the last portion of the magma.

The granites of the Cairngorm Suite have generall peraluminous compositions with SiO2 in the range 71 to 77 per cent, CaO and FeO (total;) less than 2 per cent and MgO less than 1 per cent. Major element compositions show little variation, except that fine-grained variants are mostly particularly low in Fe, Mg and Ca. The minor phases (opines, microgranites and pegmatites) of these plutons show enrichment in the elements associated with volatile-rich fluid phases (Li, Be, B, Rb, Y, Nb, REE, Th and U). In addition, quartz veins bearing As, Sb, Mo, Sn and W are developed, especially in cupolas. the high K, Th and U content of the suite prompted studies of the geochemistry and heat flow of four of the plutons including Mount Battock and Ballater (Webb and Brown, 1984; Lee et al., 1984; Wheildon et al., 1984). Plant et al. (1990) pointed out that these characteristics are typical of tin–uranium granites worldwide.

Members of the suite have narrow (c. 300 m) contact metamorphic aureoles (Chapter 9); however, many of the plutons are partly fault-bounded, and there the aureoles are absent. 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 Aboyne district by parts of two large plutons, Ballater and Mount Battock. In addition, part of the small Ord Fundlie intrusion is included. It is probably a cupola from the regional underlying batholith and may join up with the larger plutons at a relatively shallow depth. The geophysical expression of these plutons in the Aboyne district and its immediate vicinity is discussed in Chapter 3. Geochemical analyses of the Mount Battock and Ballater granites are provided by Webb and Brown (1984), Harrison (1987) and O'Brien (1985).

Mount Battock Pluton

This is the second largest pluton of the Cairngorm Suite, with an outcrop of over 370 km2 (Figure 18). Over half of the pluton lies within the Aboyne district, extending a short distance into the Ballater district (Sheet 65E) and almost to the eastern margin of the Banchory district (Sheet 66E).

The Mount Battock Granite consists of several facies which can be distinguished by grain sire and texture. Due to poor exposure, cross-cutting relationships are rarely seen. Hence an interpretation of the emplacement order has been based to a considerable extent on the outcrop pattern. The position of many contacts within the western part of the district can be defined to within 25 to 150 in by means of actual exposure and block fields, which are developed on the highest hills. Elsewhere, stream sections provide the main exposures. Howevet, large-scale relationships are commonly partly obscured by mierogranite and pegmatite sheets. The outer Coitact of the pluton is partly faulted, but over half of the boundary is interpreted as an intrusive contact. The Mount Battock Granite is unusual among the Cairngorm Suite plutons in that its earlier phases are cut by a suite of north–south-trending acid dykes, varying in texture from fine-grained, spherulitic felsite to micro-granite and microgranodiorite. The inference that it is one of the earliest members of the Cairngorm Suite is corroborated by the Rb/Sr whole rock isochron age of 416 ± 4 Ma (Harrison and Hutchinson, 1987). The presumed order of intrusion and distinguishing characteristics of the various phases of the granite are listed in (Table 3). The Water of Dye and Mongour granites do not crop out in the Aboyne district and will not be described in detail.

All of the phases are granites or microgranites, and all but the Cock Cairn Granite are leucocratic, though less so than the Ballater Granite.

Main Non-porphyritic Granite (GM1)

This facies has the largest area of outcrop, and is best developed in the western part of the pluton. Together with the Main Porphyritic Granite, it was one of the earliest facies to be intruded. it consists of pale pink, equigranidar granite with a grain size of 2 to 3 mm, though in places it becomes slightly coarser grained, exceptionally reaching 5 mm. It is a primary-textured granite, with a hypidiomorphic granular texture. Quartz crystals and aggregates are cracked but rarely strained. Biotite forms 2 to 3 per cent of the rock, and is dark brown and unzoned. Within the Main Non-porphyritic Granite, the central parts have a more calcic plagioclase than the marginal parts.

Main Porphyritic Granite (GM2)

This phase crops out to the north-east and south-east of the Main Non-porphyritic Granite, in two areas separated by the Clachnaben Granite. Contacts with the Main Nonporphyritic Granite are poorly exposed, and may be gradational. It is a primary-textured granite, with a ground-mass grain size of 4 to 5 mm, rising to 8 mm in a few places. K-feldspar megacrysts are widely scattered, generally reaching 15 to 20 mm. Within the Aboyne district, this primary texture is preserved, but in the Banchory district, areas of post-crystallisation disruption occur, marked by the recrystallisation of the matrix quartz and feldspars to a finer-grained ( 1 to 2 mm) mosaic of anhedral interlocking crystals. Where the primary texture is preserved, biotite is dark brown and unzoned, but in and close to areas of post-crystallisation disruption, the primary dark biotite is overgrown and partly replaced by pale brown to colourless Li-rich mica.

Water of Feugh Granite (GM3)

Confined to an area of 17 km' between Birse Castle and Finzean, this phase is noteworthy for the large size (30 mm) of the K-feldspar megacrysts relative to the I to 1.5 mm groundmass. Its extent is difficult to map, due to the high level of hydrothermal reddening, which makes the large phenocrysts difficult to see. In the area of The Gwaves [NO 522 915], the contact with the Main Nonporphyritic Granite is faulted, arid a zone of bleaching and greisening about 20 m wide runs along the contact. Shattering along east–west lines running close to the Water of Feugh occurs near [NO 560 908], and veins of fluorite up to 5 mm wide follow these fractures.

Cock Cairn Granite (GM4)

This phase is the most mafic in the pluton, but still has a K-feldspar/plagioclase ratio of about 1:1. It is relatively line grained with phenocrysts of white K-feldspar tip to 10 mm long. It occurs only in the south-western part of the pluton. The largest body occurs on Braid Cairn [NO 426 872] and extends for at least 1 km from the southern contact of the pluton to the Water of Gairney [NO 417 900] Outcrops also occur on Cock Cairn [NO 1462 886] and as far east as the south slopes of Gannoch [NO 495 878]. These are probably irregular sheets, but could he stock-like in form. They cut the Main Non-porphyritic Granite and probably the Main porphyritic Granite. The Cock Cairn Granite is priinary textured and similar to the Main Porphyritic Granite, except for the finer grain size of the matrix and the larger size and greater abundance of K-feldspar megacrysts.

Fungle Granite (GM5)

The Fungle Granite has an outcrop bounded largely by faults and the northern margin of the pluton. It also forms a small outlier of granite near Auchnafoy [NO 565 960]. It is strongly porphyritic, with abundant megacrysts up to 40 mm across set in a matrix with a grain size of 3 to 5 mm. Unlike the Clachnaben Granite, the matrix appears to be primary textured throughout.

Clachcnaben Granite (GM6)

This roughly circular body mostly lies in the Banchory district, and only the western fifth is in the Aboyne district. It is characterised by abundant K-feldspar megacrysts set in a fine-grained matrix which shows evidence of post-crystallisation disruption, though less so in the western part of the outcrop.

Microgranites

Dykes, sheets and larger bodies of microgranite are widely developed throughout the pluton. They range in size from veins 10 mm in width to masses at least 1 km wide and 2 to 3 km long. Texturally, they range from sugary, aplitic to xenomorphic, with mutually interfering grains. The grain size ranges from 0.3 mm to 1.5 mm, and about a quarter of the bodies are porphyritic with K-feldspar phenocrysts up to 5 mm. Granophyric textures are rare. The aplitic varieties are all very leucocratic, while the others have up to 3 per cent dark minerals, largely biotite but with some opaque iron oxides. Many of the microgranites have been affected by hydrothermal reddening of the feldspars. At the contact of a micro-granite body with calcsilicate rocks of the Deeside Limestone Formation at Glendui [NO 425 941], late crystals of fluorite up to 1.5 mm across are developed in the granite.

Felsites and porphyritic microgranites

These grade into the microgranites. They occur mainly as dykes, typically trending north–south, and up to 50 m wide. 'They form part of a suite which extends from the Mount Battock Granite northwards through country rock at least as far as the outcrop of the Ord Fundlie Granite, which they also cut. They are described along with the other minor intrusions at the end of this chapter.

Zones of hydrothermal alteration

The Mount Battock pluton has been affected by two distinct types of hydrothermal alteration. The most widespread, affecting areas of several square kilometres, particularly south of the Water of Feugh and around the microgranite of The Strone [NO 472 935], causes exsolution of minute flakes of haematite from K-feldspar and plagioclase, causing them to become brick red in hand specimen, while quartz becomes milky and biotite is completely chloritised.

The other type results in bleaching of the rock, with K-feldpar and plagioclase becoming white to palest yellow-green, due to formation of microcrystalline sericite and epidote; it is related to greisening. This alteration is confined to linear zones within 20 M of a fault or contact between different facies of the pluton. The best example is at The Gwaves [NO 522 915]; a similar effect occurs near the Shiel of Glentanar, in the Ballater district.

Petrography

The main textural characteristics of the different fades of the pluton are listed in (Table 3). Many textural features are common to all phases of the pluton, and the rocks show many similarities to the other members of the Cairngorm Suite. However, a major textural distinction exists between the earlier-crystallised phases, which have 'primary' textures, as defined by Gobbing et al. (1992, pp.4–8), and the later facies, mostly in the Banchory district, e.g. the Clachnaben and Water of Dye granites, which have 'two-phase' textures.

The original primary-textured granites typically have magmatic hypidiomorphic textures. Plagioclase forms sub-to euhedral crystals which show normal zoning, with rare oscillatory zoned crystals present in the Water of Feugh Granite. The most calcic plagioclase occurs in the Fungle and Water of Feugh granites, which have more or less uniform cores of An24–16, enclosed by thin, highly zoned rims of An10–4. Quartz has idiomorphic faces against alkali feldspar and occurs in aggregates 3 to 5 mm across of fractured grains; crystals are dark grey to smoky in hand specimen. The alkali feldspar is a perthitic orthoclase, with occasional Carlsbad twinning, and with broad stringers of exsolved albite running through the crystals. It is the last phase to complete crystallisation, and in thin section is seen to be interstitial to the other minerals even where it forms large crystals. The porphyritic nature of the Fungle, Water of Feugh and Main Porphyritic granites is attributed to low nucleation rates for K-feldspar compared with the other minerals. Harrison (1987) stated that oscillatory zoning of high-Ba/low-Na and low-Ba/high-Na zones is prevalent. Biotite is the only major mafic mineral. It is dark brown, subhedral, early crystallised, with abundant pleochroic haloes. In the Fungle and Water of Feugh granites Ti and rare earth elements are accommodated in spheric and allanite respectively, whereas in the other facies they are accommodated in ilmenite and monazite. Magnetite, apatite and zircon occur in all primary-textured phases as early formed, euhedral crystals. Harrison (1987) has interpreted aggregations of biotite, Fe-Ti oxide, apatite, sphene and zircon crystals, occurring in the Fungle, Water of Feugh and, more rarely, Main Non-porphyritic granites, as restite clots.

Two-phase granites, characterised by a bimodal grain-size, occur as diffuse patches in the Clachnaben Granite and in the easternmost parts of the Main Porphyritic Granite (in the Banchory district), as well as making up almost the whole of the Water of Dye Granite (also in the Banchory district). The texture is characterised by a fine-grained matrix with an association of strained and fragmented original grains and clusters of grains set in a fine-grained matrix of freshly crystallised material, usually of a more geochemically evolved composition. The matrix contains tiny vugs at whose margins the quartz and feldspar crystals are typically euhedral. Whereas the Water of Dye Granite is entirely a two-phase granite, the eastern parts of the Main Porphyritic and Clachnaben granites largely retain their primary texture, although patches and irregular veins of fine grained material are widespread. Possible patchy replacement of the Main Non-porphyritic Granite by the Cock Cairn Granite occurs in the west of the pluton.

These textures are similar to those described from the South-east Asian tin granites by Cobbing et al. (1986), in which the original magmatic texture was modified by intrusion of microgranite into an almost-crystallised granite. It is likely that similar textures can be produced by meatsomatic processes, which may involve the presence of a hydrothermal/pegmatitic fluid phase; such an origin has been proposed for the Coilacreich Li-granite in the Balllater district (Smith et al., in press).

Where primary-textured granite has been partly disrupted by later intrusion or metasomatism, euhedral crystals of quartz, plagioclase and alkali feldspar up to 3 to 4 mm in size form 50 to 90 per cent of the rock. In places they form aggregates up to 15 mm in size. The matrix consists of fine-grained (0.5 to 1 mm), leucocratic, aplitic granite, with a saccharoidal texture. In many of the two-phase granites, crystals or aggregates of quartz 3 to 5 mm in size form abundant xenocrysts set in a finer-grained matrix. The plagioclase in the Water of Dye and Mongour granites (Banchory district) has more sodic cores than the primary granite phases (An15–10) . The potassium feldspar is microcline, and considerably less perthitic than that in the primary-textured granites. The original biotite flakes are commonly rather ragged, with late-stage overgrowths of paler brown mica, and in some specimens there is a later generation of small, euhedral crystals of pale brown 11-rich biotite. In the Clachnaben Granite and the eastern part of the Main Porphyritic Granite, the occurrence of Li-rich overgrowths on the biotite is an indication of incipient infiltration by Li-rich hydrothermal fluids. Primary muscovite is rare, except in microgranites and aplites where it occurs as an interstitial, late-magmatic phase. Secondary muscovite is common in two-phase granites, and near aplite, pegmatite, greisen and quartz veins.

Mineral analyses

Biotites from the Mount Battock Granite (Harrison, 1987) have Fe/ (Fe + Mg) ratios of around 0.4, lower than in any of the other Caringonn Suite plutons. An Al-Fe(Mg + Mn) triangular plot of biotite compositions showed that the Fungle and Water of Feugh granites were the least-evolved phases, while the Main Non-porphyritic Granite showed a considerable range of biotite compositions. Biotites from the more-evolved minor intrusive phases tend to be richer in Alvi than those from the plutonic phases. There is no correlation between Alvi and Fe/ (Fe + Mg) in the analysed biotites. Harrison's mineral analyses showed a shortfall in the total of ions filling octahedral (6-coordinated) sites. This shortfall, partly due to the presence of unanalysed elements and partly due to vacant sites in the crystal lattice, was denoted □vi. The plot of Alvi against □vi shows that, whereas for most biotites there is a linear correlation corresponding to the substitution 2Alvi ⇄ □vi biotites from microgranites and aplites contain excess □vi. From the appearance of these biotites, is seems likely that the □vi recorded in the microprobe analyses represents lithium, which the probe cannot detect.

Geochemistry

Geochemical analyses of rocks from the Mount Battock pluton were carried out by Webb and Brown (1984), O'Brien (1985) and Harrison (1987). The largest data set is that of Webb and Brown. However, the range of trace elements analysed by them was limited, and coverage of the part of the pluton lying in the Banchory district was poor; for these reasons O'Brien's data are also of value. Harrison's data include analyses of biotites; the other workers did not analyse individual minerals. The amalgamation of analyses from more than one dataset is not considered to be a problem in interpretation of the Rb, Sr, Zr, U and TiO2 analyses, at least in the Aboyne district. A greater problem is attribution of analysed samples to the correct facies. Webb and Brown supplied grid references and one-line lithological discripdons for each sample, but the attribution to phases was done by the author. Geochemical analyses of sediments from streams draining the pluton were used in compiling the East Grampians geochemical atlas (BGS, 1991).

Whole rock data

(Figure 19) shows the spatial distribution of Rh, U, Zr and TiO2 within the pluton. Despite the unsatisfactory distribution of sample locations, the maps show that the Main Non-porphyritic Granite is most highly differentiated in Glen Tanar, where Rb and U are relatively high and TiO2 and Zr are relatively low. The Clachnaben Granite and the part of the Water of Dye Granite lying near the Cairn o'Mount road are also high in Rb and U and low in TiO, and Zr. Most of the microgranites are geocheinically similar to the nearby granites, but the aplites form a separate population with much more evolved compositions. The samples of Water of Dye Granite from the Slug road (A957, Banchory district) are anomalous in having a low U content despite being otherwise evolved.

Trends of magmatic evolution within certain phases of the pluton can he deduced from plots of Rb and Sr against Zr, and Rb against Sr (Figure 20). The most coherent trends are shown by the Main Non-Porphyritic Granite and the Water of Feugh Granite. By contrast, the Main Porphyritic and Water of Dye granites show poor coherence, which may be attributed to post-crystallisation metasomatism in parts of their outcrop. The microgranites include a few samples lying on the main trend, but mostly show a wide and rather erratic spread. The aplite samples also fail to show a trend and are widely scattered in the highly evolved parts of the respective diagrams.

Only O'Brien (1985) reported analyses of Li, Be, B, V, Cr, Co, Ni, Sn, Ba, La, Ce and W. Li ranged from 33 to 131 ppm, with the highest values being in the Water of Dye Granite, consistent with the Li-rich overgrowths on the biotite in this phase. The highest Be value was 13 ppm, and the highest W was 5 ppm. All except Harrison's samples were analysed for Cu and Mo. One sample of Main Non-porphyritic Granite contained 314 ppm Cu, although no other samples exceeded 14 ppm. Mo was generally 2 ppm or less, but one sample of Fungle Granite contained 16 ppm, while 3 out of 5 samples from the heat flow borehole exceeded 10 ppm.

Stream sediment data

Sb, As, Pb, Mo, and U are all enriched in the area between the Cairn o'Mount and Glen Dye (Water of Dye Granite), while elements associated with the least differentiated granitic rocks (Ca, Pb, Ba, Zr, Ti, Sr, Zn, V) are highest in the Fungle and Water of Feugh granites. Elements characteristic of highly differentiated granites (Li, Be, Rb, U) are concentrated in the northern and western parts of the Main Non-porphyritic Granite (Glen Tanar to Mount Keen) and in the Water of Dye Granite, especially between Glen Dye and the Slug road. However, the patterns for each element differ slightly. A localised but intense (> 100 ppm) Cu anomaly over part of the Fungle Granite may be due to contamination.

Isotope geology

Harrison and Hutchinson (1987) obtained a Rb/Sr whole-rock isochron age of 416 ± 4 Ma with an initial 87Sr/86Sr ratio of 0.7058 ± 0.0004 from the Mount Battock Granite. The sample distribution is very uneven over the pluton, and only a few of the facies of the pluton were sampled. Separate isochrons were calculated for the Water of Feugh Granite and for scattered samples from the remainder of the pluton. The two isochrons showed no significant difference, and the quoted age is taken from a combined isochron using all samples from the pluton. This is one of the earliest Rb/Sr ages obtained from the Cairngorm Suite. By comparison, U/Pb zircon analyses of the Lochnagar pluton have yielded U/Pb zircon ages of 426 to 423 Ma for the Abergeldie Diorite and L2 granite facies and 417 Ma for the L2 granite facies (Smith et al., in press). Harrison and Hutchison (1987) suggested that the Cairngorm Suite can be subdivided on age into early (c.415 Ma) and late (c.404 Ma) groups, but this distinction is suspect, as it is at the limit of resolution of the analytical methods.

Ballater Pluton

Only a very small part of the Ballater pluton, about 1.5 km2, lies within the district (Figure 1). 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 [NO 426 002]. The granite is pink, leucocratic, porphyritic, and very coarse grained (5 to 20 mm). Orthoclase megacrysts reach 50 mm. Two generations of biotite are recognised, an early medium-brown one associated with apatite, opaques and zircon, and a later interstitial very dark brown biotite. A fuller account of this intrusion will appear in the memoir for the Ballater district (Sheet 65E) (Smith et al., in press).

Ord Fundlie Intrusion

This intrusion crops out over an area of 0.6 km2 on the southern slopes of Ord Fundlie [NJ 610 000]; most of the outcrop lies in the Alford district (Sheet 76W). Most of the exposures recorded during the primary survey are now obscured by forest. The rock is a pink, non-porphyritic granite with a grain size of 1.5 to 2 mm. A small exposure of granite on the south side of Ord Fundlie [NO 610 995] carries xenoliths of diorite up to 0.3 m. 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.

Late Silurian to early Devonian dykes and sheets

The post-tectonic minor intrusions of the Aboyne district are of several types. Their spatial distribution is shown in (Figure 21). Dykes and sheets of lamprophyre and microdiorite occur widely throughout the district, except in the Mount Battock, Ballater and Ord Fundlie granites. Dykes of felsite and porphyritic microgranite are particularly abundant in the north-eastern part of the district, and intrude the Mount Battock and Ord Fundlie granites as well as earlier rocks. Aplite, pegmatite and quartz veins related to the Cairngorm Suite granites occur within and close to these intrusions.

Lamprophyre

Only seven lamprophyre dykes have been recognised in the district. Owing to alteration, they can be difficult to distinguish from microdiorite and the more mafic porphyritic microgranodiorites, except that the lamprophyres lack feldspar phenocrysts. Examples occur on Tomachallich [NO 473 996], at Milton Cottage [NO 477 810], and Craig Duchrey [NO 500 719] and [NO 502 721] while there are three in the vicinity of Auchmull [NO 586 747]. None of these dykes can be traced for more than a few hundred metres. 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 feldspar laths is sometimes visible. The lamprophyres are believed to have originally been spessartites. possibly with some xogesites and 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 indicates that poor exposure is concealing mam dykes additional to those mapped. Microdiorites are only slighth less abundant than felsites and porphyritic microgranites in the district. The dykes are typically less than 1 m thick. but may reach 5 m: sills are typically 1 to 5 m thick and may reach 10 m. Dyke margins are frequently irregular. their similarity on opposite sides of a dyke indicating that they are of dilational origin. The larger dykes, over 0.5 m thick, commonh have chilled margins. Where microdiorite and felsite dykes occur together. the felsite in most cases cuts the microdiorite.

Where fresh. the microdiorite is a dark grey to black rock, with andesine (An36–30 zoned to c.An20 phenocrysts up to 3 mm. but generally 1 to 1.5 aim long, set in a matrix with a grain size of 0.1 to 0.1 mm. The plagioclase phenocrysts show oscillatory zoning, but are in many cases broken. Hornblende and biotite phenocrysts are less abundant than plagioclase. and are commonly chloritised. The groundmass has a granular texture: it consists of laths of andesine plagioclase with biotite and minor hornblende. Quartz and iron oxides are minor components.

Dacite sill

The fine- to medium-grained felsic igneous rock exposed in the stream below Kirkion of Balfour [NO 605 733] is interpreted as a sill of late Silurian age, possibly coeval with the Lintralen Tuff. intruding the rocks of the Margie Formation. In hand specimen it is a dark reddish purple fine-grained quartz- and feldsparphyric rock. with no planar fabric. In thin section (S82709). the rock is seen to be strongly altered. Phenocrysts of plagioclase and rare K-feldspar are sericitised, or replaced by zeolites or calcite. Many of the quartz phenocrysts have been replaced by sutured microcrystalline quartz aggregates. The ground-mass feldspar has largely been altered to sericite and calcite, and the mafic minerals have been replaced by iron oxides.

Felsite and porphyritic microgranite to microgranodiorite

These are the most abundant dykes in the district. They cut all rocks except the Ballater Granite and associated veins, the Devonian sedimentary rocks and the late Carboniferous dykes (Plate 7). Grain size is variable, and correlates only partly 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. Chilled margins are well developed in the larger dykes, with 0.5 to 1 in of marginal felsite passing by coarsening of the groundmass into a central porphyritic micro-granite or microgranodiorite.

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 potassium feldspar is normally 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 orthoclase perthite phenocrysts up to 5 mm and rare corroded quartz phenocrysts or xenocrysts up to 3 mm. Phenocryst abundance rarely exceeds 15 per cent.

Porphyritic microgranites are distinguished from porphyritic felsites by coarser groundmass grain-size (generally 0.2 to 0.5 nun). Groundmass textures are more variable than in felsites, and in many microgranites the groundmass has a patchy or uniformly granophyric texture. Euhedral orthoclase perthite phenocrysts range up to 30 mm in size, and form up to 25 per cent of the rock. Quartz phenocrysts are typically 3 to 5 mm, rounded and often corroded. Plagioclase phenocrysts (cores An34–26 zoned to An20–14) rarely exceed 5 mm; some show oscillatory zoning as in the microdiorites; others, enclosed in the orthoclase megactysts, show arrested zoning. Biotite forms rare phenocrysts up to 2 mm across, but is ubiquitous in the groundmass forming 3 to 5 per cent of the rock.

Aplitic and pegmatitic rocks

Veins and sheets of aplitic microgranite are widely distribuecl in the Mourn Battock pluton, where they cut all facies of the pluton. They range in width from a few centimetres to 30 m. They are distinguished from other microgranites by their leucocratic nature, saccharoidal texture, evolved geochemistry, and in a few cases. association with pegm: tine veins. Sheets of aplitic microgranite are relatively rare away from the Mount Battock pluton, and are commonest near the contacts. They typically cross-cut the foliation of the country rocks. One example near Balnacraig House [NO 581 985] intrudes an amphibolite sheet and is itself cut by a dyke of feldsparphyric felsite.

Sheets and pods of pegmatitic granite are less abundant than aplitc microgranites. They are most widely developed in the Tarfside Psammite Formation south of the Mount Battock pluton. an area where sheets of fine-grained granite up to 5 m wide are common. Some pegmatitic sheets e.g. that on the south slopes of Hill of Rowan [NO 474 792], follow the line of faults. The bodies of pegmatitic granite can he traced for up to 400 m by means of float. One sheet of aplitic microgranite between Mount Een and Bennygray [NO 526 824] contains abundant patches of granite pegmatite. The granite pegmatites typicallly have a grain size of 50 to 200 mm. Quartz varies from cleat to turbid; microcline is pale pink; muscovite and bioitite are of equal abundance.

Quartz veins

Several of the faults and other major fractures in the Aboyne district are locally filled by quartz veins, commonly containing pyrite. In many cases, the vein quartz is brecciated due to post-intrusion movement along the plane of the fracture. The NNE-trending quartz vein which crops out in the headwaters of the Water of Charr [NO 592 786] is about 2 m thick and contains a little disseminated pyrite. A brecciated quartz vein with abundant pyrite occurs along the line of a major WNW-trending fault in the Burn of Ranoch [NO 564 765]. Many blocks of vein quartz up to 1 m across occur in the Whitestane Burn [NO 4325 7170], and are probably derived from a vein filling the fault which runs along the side of the valley.

Late Carboniferous quartz dolerite dykes

A suite of late Carboniferous dykes occurs abundantly in the Midland Valley of Scotland and northern England. where it demonstrably postdates Duckmantian sedimentary rocks. K–Ar whole-rock dating gives an age of 290 to 295 Ma (Fitch et al., 1970). This suite becomes considerably less abundant north of the Highland Boundary Fault, but a few examples occur as far north as Peterhead.

Within the Aboyne district, examples are widely scattered (Figure 21), nearly all in the southern half of the district. They are typically 5 to 10 m thick, but a few examples reach 20 m. Where measurable, the dykes trend 060°–075°, and have subvertical contacts. Chilled margins up to 0.5 m wide are common. Apart from spheroidal weathering, the rocks are generally fresh at outcrop.

Two large dykes cut the Margie Formation rocks in the North Esk section [NO 5907 7260] and [NO 5897 7263], the more easterly being 10 m thick and the more westerly, which is shattered along its northern contact, being 15 to 20 m thick. The dyke in the Burn of Boyach and Burn of Beag [NO 5323 7650] to [NO 5349 7676] is notable because of its somewhat sinuous outcrop with a trend of 030°–045°. The only quartz dolerite dyke in the northern half of the district occurs WNW of Wreaton [NO 4983 9934] (Plate 8). It is at least 5 in wide and is intruded close to the contact between limestone of the Deeside Limestone Formation and quartzose psammite of the Queen's Hill Formation. It forms a ridge at least 100 m long trending 075°.

The average grain size of the central parts of the dykes is 1 to 1.5 mm. The quartz dolerites are characterised by the ophitic clinopyroxene–plagioclase inter-growth which forms the groundmass. Ilmenite forms 2 to 5 per cent of the rock, and there is a &vitrified mesostasis associated with rare interstitial quartz. Labradorite (An62) phenocrysts are very rare, and there are no other phenocrysts.

Chapter 8 Structure

Regional setting

The Aboyne district lies at the south-eastern margin of the Grampian Highlands, and extends north as far as the River Dee (Figure 11). The major structural feature of the district is the Highland Boundary Fault Zone, which runs roughly SW–NE close to the south-east corner of the district. It separates the orthotectonic Caledonides to the north-west from the Midland Valley to the south-east, which is covered by post-Caledonian sedimentary and volcanic rocks, The Highland Boundary Fault Zone brings to the surface rocks of the Highland Border Complex, which record parts of the evolution and closure of the Iapetus Ocean. To the north-west of the zone, the Aboyne district is dominated by earlier Caledonian structures running roughly parallel to the fault zone, some of which can he traced from Kintyre to the coast north of Stonehaven.

The Tay Nappe is a large recumbent structure recognised in the southern Grampian Highlands from Kintyre as far north-east as the Aboyne district; its extension further north-east is problematical. The generally accepted view of its development (Stephenson and Gould, 1P)5) is that a recumbent anticline formed during the D1 episode was flattened, tightened, locally sheared and folded during D., and D3 to produce a large modified nappe-like structure whose amplitude appears to decrease north-eastwards (Figure 22). However, studies in the Glen Shee area (Krabbendam et al., 1997) raise the possibility that originally upright F1 folds were modified by a D2 shearing event to produce the shallow NW-dipping foliation in the 'flat belt'. At the present level of exposure, the lower, inverted limb of the Tay Nappe craps out over a wide belt in the Southern Highlands, The modified bedding and composite foliation ntat result from the D1 and D2 events, and in part the D3 event, are essentially parallel. Later NE–SW-trending secondary folds, such as the sharply defined Highland Border Downbend, were subsequently imposed on the nappe. In the Glen Esk area, a broad late antiform, :he Tarfside Culmination, exposes a wide zone of non-inverted strata, assigned by Harte (1979) to the Tarfside Nappe, which lies below the Tay Nappe.

The status of the Deeside Lineament proposed by Harte et al. (1984) is uncertain. If it exists, it would form the southern boundary of the Buchan Block, which has a distinctive structural and metamorphic history. There is no obvious shear zone or east–west fault close to the Dee valley, but. a zone of weakness in this area may have provided a conduit for the large number of plutonic intrusions which occur along it. 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) regarded the Deeside and Portsoy–Duchray Hill lineaments as reflecting pre- to syndepositional boundaries which controlled both the pattern of Dalradian sedimentation and the later tectonic and intrusive events.

Dalradian

Folding

Four principal deformational episodes ( D1 to D1) 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, and is a recumbent antiform/synform pair with an inverted limb extending about 20 km across strike. The D2 deformation produced small-scale folds which deform the Tay Nappe. The outcrop pattern is strongly influenced by the regional inversion of the northern part of the district by the F1 Loch Tay Nappe, kilometre-scale folds of the F3 episode and regional folds of the D4 episode. The D4 episode produced the Highland Border Downbend and a large number of associated minor folds, and is best developed near the Highland Boundary Fault Zone.

Major folds

Tay and Tarfside nappes

The regional structure of the Dalradian rocks within 30 km of the Highland Boundary Fault is dominated by the Tay Nappe. It is generally believed to have formed during D1 and been modified during D2 (Shackleton, 1958; Harte et al., 1984), but Krabbendam et al. (1997) proposed that it developed by shearing during D2. The Oueen's Hill and Deeside Limestone formations to the north of the Mount. Battock pluton are inverted and dip gently northwards; they are regarded as part of the inverted limb of the Tay Nappe. The rocks of the Tarfside Psammite Formation to the south of the Mount Battock pluton are subhorizontal and uninverted, and form part of Harte's (1979) Tarfside Nappe, which he regarded as a nappe underlying the Tay Nappe at a deeper structural level. The axial plane trace of the fold responsible for this change in attitude was identified by Harte (1979) in Glen Mark within the outcrop of the Glen Effock Formation near the south-west corner of the Mount Battock pluton [NO 400 838] (Ballater district), where, due to the effect of the post- D4 Tarfside culmination, it closes to the west. This axial plane trace is cut off by the Mount Battock pluton at [NO 404 848], still within the Ballater District.

Harte (1979) proposed that the axial plane of the SE-verging synformal fold that separates the Tay and Tarfside nappes is cut off near the western end of Loch Lee [NO 420 794] (Ballater district) by the Glen Mark Slide, a planar structure developed during D2 and D3 by tightening of the D1 Tay Nappe; the slide was considered to cut out the hinge of the D1 fold separating the lower limb of the Tay Nappe from the upper limb of the Tarfside Nappe. He extrapolated the trace of this slide outside ground which he had surveyed to pass through the south-west corner of the Aboyne district and turn east to meet the Highland Boundary Fault close to the northern margin of the Forfar district (Sheet 57W). The resurvey of the Ballater district (Smith et al., in press), has found no evidence for the existence of the Glen Mark Slide. Instead, the boundary between the inverted and right-way-up successions is considered to mark the closure of major tight-to-isoclinal F1 fold, separating the Tay and Tarfside nappes. In addition, a certain amount of high-strain deformation has occurred at the boundary between the Tarfside Psammite and the Glen Effock Schist due to the contrasting ductility of the dominant lithologies in the two formations.

Booth (1984) discussed the possible extension of the Tarfside Nappe to the north-east of Tarfside. Based or his interpretation of the stratigraphy of the coast section north of Stonehaven as largely uninverted and upward facing, he extended the Glen Mark Slide through the Mount Battock Pluton and through the Maryculter area to reach the coast near Findon; this would make the whole of the Aboyne district between the Mount Battock Pluton and the North Esk Fault part of the Tarfside Nappe.

Recent work by C W Thomas (verbal communication, 1995) has shown that the coast section is downward facing and largely inverted, and hence, probably, part of the Tay Nappe. The North Esk section from 200 m downstream of the confluence with the Burn of Auchmull as far as the North Esk Fault is also largely inverted. This implies the presence of a downward-facing antiform consitituting the boundary betweeen the Tarfside Nappe to the north-west and the Tay Nappe to the south-east. In the lower part of the River North Esk section, a major antiformal structure crops out about 200 m downstream of the confluence of the Burn of Auchmull with the North Esk [NO 5806 7404]; south of this, most of the Dalradian rocks in the North Esk young northwards (Figure 13). This is the most likely candidate for a major D1 fold hinge, although it may have been modified during D4. The rocks exposed in the West Water from west of Stonyford [NO 502 725] downstream to the southern boundary of the Aboyne district dip steeply to the southeast, young northwards and are largely inverted; the minor D3 and D4 folds in this area are downward facing. Southward-younging, uninverted strata occur near Tillybardine [NO 486 732], and it is likely that the Tarfside Nappe is exposed only within a roughly circular window lying largely within the Aboyne district (Figure 23). The nature of the exposure and the prevalence of D4 minor folds have made it impossible to trace the boundary of the Tarfside Nappe on the ground. There is no field evidence of a high-strain zone along the postulated outcrop of the fold hinge. The trace of the fold hinge probably passes from the Longshank Gneiss (equivalent to the Glen Effock Schist) to the Rottal Schist (equivalent to the Glen Lethnot Formation) within the Ballater district, and lies within the outcrop of the Glen Lethnot Formation within the Aboyne district, possibly crossing the North Esk 200 m downstream of its junction with the Burn of Auchmull [NO 5806 7404]. Booth (1984) postulated this hypothesis but rejected it on the basis of his equivocal stratigraphical conclusions from the Stone-haven coast section.

D3 folds

The D3 episode was broadly synchronous with the emplacement of late-tectonic basic to ultramafic intrusions north of the Dee, and continued after intrusion was complete. Intrusion of the basic rocks was accompanied by ductile shearing, during which temperatures remained high (Kneller and Leslie, 1984), and sillimanite-grade metamorphism occurred over a wide area extending several tens of kilometres away from the intrusions themselves.

Within the Aboyne district, few kilometre-scale or larger folds attributable to the D3 episode occur, although minor D3 folds are abundant in certain parts of the district. However, the pair of flat-lying folds which bends the boundary between the Tarfside Psammite Formation and the Deeside Limestone Formation around Netherton [NO 157 977] may well be of D3 age, or at least a D3 tightening of an earlier structure, as it is associated with a prominent mineral lineation plunging ESE at 10°–20°, similar to the axes of the minor D3 folds elsewhere in the district.

Highland Border Downbend

The post D3–pre D4 Highland Border Downbend structure is well developed in the Glen Clova area to the south-west and in the coast. section north of Stonehaven (Harte et al., 1987), where it runs roughly parallel to, and about 4 to 5 km north-west of the Highland Boundary Fault Zone.

The Downbend is not so readily identifiable in the Aboyne district. The steeply dipping, SE-younging succession in lower Glen Esk contrasts with the extensive 'flat belt' of gently dipping overturned strata which is developed between about 5 and about 25 km north-west of the Highland Boundary Fault Zone both to the southwest in Perthshire and to the north-east on the coast north of Stonehaven. The dip of the uninverted succession steepens at the south-eastern side of the Tarfside Dome, running from near Auchowrie [NO 502 733] to Allrey [NO 540 825]. This is 6 to 10 km north-west of the Highland Boundary Fault Zone. The trend of the axis of steepening is oblique to the trend of the Highland Border Downbend; its status is uncertain (Figure 23), but it may be the effect of the refolding of the Highland Border Downbend by the Tarfside Culmination.

D4 structures

The major antiformal structure about 200 m downstream of the confluence of the Burn of Attchmull with the River North Esk [NO 5806 7401] was probably tightened during D4. Most of the more open folds which occur within 500 m of the North Esk Fault are of D4 age. Due to the infrequency of younging evidence away from the major stream sections, it is likely that similar F1 folds may he present within 1 to 2 km of the Highland Boundary Fault Zone away from the North Esk.

Tarfside Culmination

'This has been shown by Harte (1979) to postdate the D1 deformation by the fact that D4 minor folds, as well as all other structures, have curved traces round the core of the dome-like structure. It is possible that the formation of the Tarfside Dome is related to the intrusion of the Mount Battock pluton, especially as A G Leslie (verbal communication, 1990) has suggested that some of the faulting and folding in the area between the Ballater and Mount Battock plutons (Ballater district) was produced by forceful intrusion of the pluton.

The Tarfside Culmination is a broad dome-like warp centred about 1 km north-east of the summit of Badadarrach [NO 445 832], where the bedding is virtually horizontal. From this point, the roughly bedding-parallel foliation dips away at angles of 15°–20° to the west, south and east, while to the north the Dalradian rocks are cut by the Mount Battock pluton. As mentioned above, there is a sudden steepening of the dip at the south-east side of the dome, from Auchowrie to Allrey. This may be part of the domal structure, or may be the due to the domal folding acting on the warping of the Highland Border Down bend.

Minor folds

Within the Dalradion rocks of the district, minor folding is widespread. The style of folding varies with the grade of regional metamorphism, and later tightening and extension have notably modified the earlier generations of folds.

Within the district, minor D1 and D2 Folds can only be distinguished from each other by their effect on the vergence of larger D3 structures, due to the absence of any recognisable D2 major folds. D1 folds are scarce except near the Highland Boundary Fault where the later deformations are absent or only mildly developed. They are preserved principally as tight folds and isoclines within lithological units (bedding) in the Tarfside Formation, and as centimetre-size folds with axial planes parallel to the main foliation in the main outcrop of the formation. Minor D2 folds and other deformational features are not widely developed. They have been recorded from the North Esk section by Harte and Booth (Booth, 1984); J R Mendum (verbal communication, 1991) considered that Harte and Booth underestimated their importance in Glen Esk. Elsewhere in the southern part of the district, the evidence for F3 minor 'biding consists chiefly of rotation of early cleavages within F3 minor folds.

Irregular, almost ptygmatic folds are widely developed in the migmatitic rocks north of the Mount Battock pluton, although major (kilometre and smaller) F3 folds with similar orientations are relatively scarce. They were largely developed during or after the migmatisation, which rendered the rocks ductile, and obscured folds and cleavages related to the earlier episodes, which were highly modified. South of the Mount Battock pluton, close to tight minor D3 folds are well developed in and close to lower Glen Mark, where the alternation of psammite and semipelite is particularly susceptible to folding. Metre-scale folds with horizontal axial planes and east–west axes are ubiquitous (Plate 9). Typically, a S3 axial plane foliation is developed only in the semipelitic layers, and may show refraction. Minor F3 folds are less abundant in the thickly bedded psammites of the Glen Tanar Member, but are well developed in the semipelites and tnicaceous psammites of the Glen Effock Formation. Minor F3 folds become less conspicuous south-eastwards, and have not been recognised southeast of a line from the Burn of Freoch [NO 512 712] to the Water of Charr [NO 610 800].

Most of the minor folds in the southern half of the district belong to the F4 generation; they are less easy to recognise north of the Mount Battock pluton due to the coarse migmatitic fabric of the rocks. Crenulations with steep to vertical axial planes striking NE–SW, mostly dipping south-east, and with roughly horizontal axes are abundant in the Southern Highland Group rocks, where some of them are seen to refold F3 folds (Plate 10). 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 50 cm, and the wavelength from about a centimetre to about a metre. The retrogressive metamorphism which affects much of the area is in places focused on the axial planes of F4 folds, and was undoubtedly associated with the D4 deformation.

Planar and linear fabrics

The Datradian rocks of the Aboyne district all display at least one planar fabric. In the lower-grade rocks a penetrative cleavage is developed in pelites, semipelites and the more micaceous psammitic lithologies. This takes the form of a slaty cleavage in the greenschist-facies rocks near the Highland Boundary Fault, but even a short distance away from the fault the slaty cleavage becomes coarser and develops into a foliation defined by the alignment of chlorite and mica flakes. A prominent spaced cleavage is developed in psammitic rocks. At about the staurolite isograd, the foliation in semipelitic and pelitic rocks becomes sharply coarser, while in many psammitic rocks the original clastic grains can no longer be recognised. Migmatitic effects are first seen slightly below the sillimanite isograd, where a coarse gneissose layering is developed in semipelitic rocks, with segregation of leucosomes and formation of biotitic selvedges. The timing of mineral growth with respect to the fold episodes was studied by Harte and Johnson (1969). The foliation developed during D1 and D2 was coarsened with extensive recrystallisation syn- to post- D2 (Robertson, 1999), when the porphyroblasts of biotite, garnet, staurolite, and kyanite were largely formed. In sillimanite-grade rocks the formation of fibrolite is syn- to post-D3. Secondary biotite crystallised where primary biotite reacted to form fibrolite.

The primary foliation is typically subparallel to the original bedding, but in places it cuts across mainly bedding-parallel compositional variations, and is axial-planar to D1 and D2 minor folds. Growth of garnet, staurolite and kyanite porphyroblasts during the syn- to post-D2 coarsening of the metamorphic fabric is demonstrated by the preservation of a finer grained fabric as inclusions within some porphyroblasts. In the unmigmatised rocks, the dominant foliation, with good alignment of micas, is generally taken to be a composite S S S surface, possibly including S3 in places. In areas where D3 minor folds are well developed, an axial-planar D3 cleavage is superimposed on the composite S0–S1–S2 fabric near the fold hinges. Only rarely, in the cores of the tightest D3 folds, is the S0–S1–S2 fabric overprinted by the S3 fabric. Despite the abundance of D1 minor folds and crenulations near the North Esk Fault, only a weak axial-planar S4 cleavage is developed within the district. In the rocks of the Glen Lethnot Formation immediately north of the North Esk Fault, Booth (1984) recognised a fine crenulation or weak folding plunging gently to moderately to the west, with a southward-dipping axial-planar fabric, and verging to an antiform to the south, which he correlated with the latest structural elements developed in the rocks of the Highland Border Complex to the south. This folding postdates the D4 regional episode. It could possibly be related to the formation of the Tarfside Culmination, which also postdates D4.

Lineations are well developed in all lithologies except massive psammites and quartzites, and the feldspar-porphyroblast gneisses of the Queen's Hill Formation. South of the Mount Battock Granite, the most commonly developed lineation is related to D2. It is expressed as a grooving and rodding of quartz veins, a mineral striping or striation on bedding and schistosity planes in psammites, an alignment of micaceous aggregates or a fine mica crenulation in pelitic and semipelitic rocks, and a coarse alignment of hornblende crystals in amphibolites (Harte and Johnson, 1969). In this area, D3 lineations are associated with the tighter D3 minor folds, and take the form of mineral striping and S2/S3 intersections. On Deeside, a prominent D3 mineral lineation plunging ESE at 10°–20° is present in rocks of the Tarfside Formation near Netherton [NO 456 977], where it is parallel to the axis of a major fold.

In unmigmatised rocks of the Queen's Hill and Tarfside 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 to isoclinal, commonly '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 hinge zones of F1 folds. The migmatitic rocks of these formations show a foliation marked by alignment of micas and variation in the relative proportions of micas and quartzofeldspathic material, on a 1 to 5 mm scale. Superimposed upon this layering, and roughly parallel to it, is a coarse gneissose layering, marked by the segregation of granitic veins, mostly with biotitic selvedges. However, the gneisses around Dinnet House [NO 432 975] are poorly foliated 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 is probably a composite S1–S2–S3 foliation, and 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.

Highland Border Complex

Based on detailed remapping of the exposures in the North Esk section, Booth (1984) erected structural histories for the northern and southern outcrops of the Margie Formation, and the North Esk Formation. He postulated five deformational episodes in all. The first three episodes were unrelated to any recognised in the Dalradian rocks to the north, while the fourth episode, recognised only in the northern outcrop of the Margie Formation, was tentatively correlated with the regional D4 of the Dalradian rocks, and the fifth episode was correlated with post- D4 minor structures which he had recognised in the Dalradian rocks to the north.

First deformation

This episode is marked by very tight to isoclinal folds with a strong axial-planar schistosity sub-parallel to bedding. The plunge of these folds, marked by the S0/S1 intersection and S1 stretching lineations, is near vertical or very steeply to the west.

Second deformation

In the North Esk Formation and the northern outcrop of the Margie Formation, open to close and sharp-crested folds with a near-vertical plunge and a weak, sporadically developed axial planar cleavage are developed. These folds generally have a wavelength of around 0.5 m, but the change in vergence along the section suggests the presence of much larger folds.

The second deformation in the southern outcrop of the Margie Formation produced open rounded folds plunging gently east, with a poor axial-planar fabric. These folds are different in character to those in the North Esk formation, and may represent a different deformation.

Third deformation

The southern crop of the Margie Formation and the southern part of the North Esk Formation outcrop are affected by a fine crenulation plunging very steeply north-west, in places accompanied by a north-west-dipping fracture or crenulation cleavage.

Fourth deformation

A sporadically developed crenulation with subhorizontal axes and a northward-clipping axial-planar cleavage occurs in the northern outcrop of the Margie Formation. The axes trend in the same direction as those of the folds in the adjacent.
Dalradian rocks, and Booth (1984) correlated the fold episodes on either side of the North Esk Fault despite the obvious difference in the scale of folding across the fault.

Fifth deformation

A set of open crenulations with gently east- or west-plunging axes and a vergence implying an antiforn to the south is developed throughout the Highland Border Complex outcrop in the North Esk section, This is correlated with a fine crenulation or weak folding, plunging gently to moderately to the west, with a southward-dipping axial-planar fabric, and verging on an antiforin to the south, which is developed in the Dalradian rocks to the north.

Old Red Sandstone

The nature of the north-western boundary of the outcrop of the Lintrathen Tuff has been disputed (Chapter 6), but it is now believed to be a normal fault throwing down to the south-east, although the amount of throw is not determinable. The angle of clip of the igneous layering in the Lintrathen Tuff could not be determined due to the water level in the North Esk, but thickness estimates have been made on the assumption of a steep dip.

The rocks of the Strathmore Group young consistently south-east. From the faulted boundary with the Lintrathen Tuff as far as Gannochy Bridge, the dips are steep and in places overturned. The average dip is about 70° to the south-east, but about 200 m north-west of Gannochy Bridge, the dip shallows to 55–60°. Downstream of Gannochy Bridge, the dip continues to shallow to become 15–20° about 400 m downstream of Gannochy Bridge [NO 605 704], in the Forfar district. The bedding remains undulating and subhorizontal across the core of the Strathmore Syncline to the south-east.

Faulting

North of Highland Boundary Fault Zone

The Dalradian metasedimemary rocks and the granite intrusions have been affected by faulting, most of which is believed to be of late Silurian to early Devonian age. Some of these faults may be related to obduction of the Highland Border Complex on to the Dalradian block. However, similar faults in the Banchory district are truncated by the North Esk Fault, showing that there has been some later movement along that fault. The faults around the margins of the Mount Battock Granite are probably related to its emplacement and uplift, and are associated with pegmatite and aplite veins and sheets.

Relatively few faults have been recognised cutting the Dalradian rocks of Deeside. Some ENE-trending faults that have been recognised in the area between the Ballater and Mount Battock plutons (Ballater district) from magnetic surveys, continue into the Aboyne district. A NW-trending fault is interpolated near Aboyne to account for displacement of the units within the Queen's Hill Formation.

The Mount Battock Granite has been affected by faulting. This is manifested by displacement of the contact, zones of hydrothermal alteration, and, rarely, by shattering of the granite. Many of the pegmatite, aplite and quartz veins follow fractures where displacement has occurred. Several of the contacts between differen phases of the granite appear to be faulted. The western part of the Fungle Granite is faulted, as is part of the western boundary of the Water of Feugh Granite. A line of boulders of brecciated aplitic microgranite with scattered sulphide spots, and veined by quartz, occur; north of Birse Castle [NO 520 917]–[NO 523 908].

In the southern part of the district, a succession of dextral NW- to WNW-trending wrench faults occurs at I to 3 km intervals, and they continue into the Forfar, Ballater and Banchory districts to the south, west and east. These faults are typically picked out by the drainage and stretches of the Water of Saughs and River North Esk follow such faults. Quartz veins, locally with pyrite, have been intruded along some of these fault planes. A set of NNW-trending high-angle faults cuts the Tarfside Psammite Formation rocks north and west of Tarfside village, and some of these displace the contact of the Mount Battock Granite by up to 2 km.

Highland Boundary Fault Zone

The system of ENE- to NE-trending faults along the northern margin of the Midland Valley of Scotlart1 extends from the Island of Arran to Stonehaven. Late Ordovician transcurrent faulting and possibly overthrusting caused the juxtaposition of the Highland Bordcr Complex against the Dalradian rocks. In the late Silurian and early Devonian, renewed transcurrent faulting caused juxtaposition of the Midland Valley and Highland Border terranes (Hutton, 1987). Penecomemporaneous vertical movements caused downfaulting of the Lower Old Red Sandstone rocks of Strathmore against the Highland Border Complex.

The Aboyne district contains only a short segment of the Highland Boundary Fault Zone. In this segment, the zone of disruption is unusually wide, and four separate faults are recognised. The North Esk Fault juxtaposes Glen Lethnot Formation rocks against Margie Formation psammite and limestone in the North Esk river section, but elsewhere the rocks to the south of the fault belong to the North Esk Formation. The southern boundary of the North Esk Formation against the Margie Formation is faulted throughout. The third fault separates Margie Formation rocks to the north from Lintrathen Tuff to the south. The fourth fault, generally taken to be the main Highland Boundary Fault here, throws down Strathmore Group sedimentary rocks against the Lintrathen Tuff.

The direction and amount of dip on each fault plane is difficult to ascertain from the available exposures, however, the North Esk Fault was recorded by Barrow (1901) as dipping north-west at 35°, whereas the other faults are believed to be near vertical, probably clipping north-west at over 75°. Movement on the North Esk Fault is assumed to be of late Ordovician age, representing the docking of the Highland Border Complex terrane against the 'Dalradian terrane. The amount and sense of movement on the North Esk Fault cannot be calculated with any confidence, It has been suggested that the Highland Border Complex rocks represent a back arc basin within the Iapetus Ocean, which was then thrust under the Dalradian rocks. This implies several tens of kilometres of north-westward movement, to which may be added an uncertain amount of strike-slip movement, which may be required to account for the presence of a south-easterly source for some of the conglomerates in the Crawton Group in the Stonehaven district.

The Highland Boundary Fault clearly moved in Devonian times, though there may have been an earlier history of movement on this fault as well as those to the north-west. Over much of the country between Helensburgh and Stonehaven, the Highland Boundary Fault has cut out exposure of the Highland Border Complex, juxtaposing Silurian and Devonian sedimentary rocks against Dalradian metamorphic rocks.

The Highland Boundary Fault throws sedimentary rocks of the Strathmore Group down to the south-east. The amount of displacement is probably of the order to 2 to 5 km. This is less than the total thickness of Lower Old Red Sandstone rocks in the Strathmore basin. The discrepancy is partly due to tilting of the succession within 2.5 km of the fault. Also, the coarse conglomerates of the Gannochy and Strathfinella Hill formations show that there was high land a short distance to the north-west of the Highland Boundary Fault during deposition of the Strathmore Group (Carroll, 1995); thus, vertical movement on the fault was probably contemporaneous with deposition of the Old Red Sandstone of Strathmore. Vertical movement on the fault probably continued during the formation of the Strathmore Syncline, causing the steepening of its north-west limb.

Chapter 9 Metamorphism

The Dalradian rocks, together with the pre-orogenic intrusions, have all been regionally metamorphosed during the tectonic episodes described in the previous chapter. It was in the southern part of the Aboyne district, and the adjoining Banchory district to the east, that Barrow (1893) first demonstrated the use of index minerals in pelitic rocks to characterise zones of increasing grade within the green-schist and lower amphibolite facies of regional metamorphism. The characteristic metamorphic minerals are chlorite, biotite, garnet, staurolite, kyanite and sillimanite (Figure 24). This sequence of index minerals is known as the Barrovian scheme and has been recognised in many orogenic belts. To the north and east, in the Alford, Inverurie and Aberdeen districts, the geothermal gradien was steeper, with high temperatures being reached at lower pressures. As a result, staurolite and kyanite are not developed and garnet is rare, and the sequence of index minerals includes andalusite and cordierite. This is known as the Buchan metamorphic scheme. At intermediate geothermal gradients, staurolite still occurs but andalusite replaces kyanite. Such conditions occurred along the coast north of Stonehaven. The position of the andalusite/kyanite inversion is difficult to trace because of the extensive development of sillimanite and the large, areas of granite outcrop on Deeside, where it would he expected. The timing of the peak temperatures reached during regional metamorphism showed a systematic variation, with the highest-grade rocks displaying the latest metamorphic peak. In rocks of kyanite grade and below, the period of the peak temperatures occurred immediately before the intrusion of the late-tectonic basic-ultrahasic masses elsewhere in the eastern Grampians, whereas in sillimanitegrade rocks it postdated the intrusions and continued through the D3 deformation episode. Migmatisation is associated with the regional metamorphism over the northern half of the Aboyne district, being most intense on Deeside. The increased thermal gradient during the emplacement of the basic–ultrabasic masses caused increased intensity of migmatisation, culminating in partial melting, particularly in the north-west corner of the district, closest to the Tarland and Morven–Cabrach intrusions. Much later, narrow metamorphic aureoles were developed around the granites of the Cairngorm Suite, where pelitic rocks display characteristic low-pressure thermal assemblages. The metamorphic isograds and the boundary of migmatisation within the Aboyne district are shown in (Figure 24).

Regional metamorphism

Dalradian

Pelitic and semipelitic rocks

The metamorphic zonation in the pelitic and semipelitic rocks of almost all of the district follows the Barrovian scheme:

  1. Sillimanite a staurolite ± garnet ± biotite
  2. Kyanite ±staurolite a garnet ± biotite
  3. Staurolite ± garnet ± biotite
  4. Garnet ± biotite ± chlorite
  5. Biotite ± chlorite
  6. Chlorite

Rocks of zones ( 1 ) to (3) occur in narrow strips north of the North Esk Fault (Figure 24),The staurolite zone is narrow in the east of the district, but widens progressively westwards. The limit of regional migmatisation occurs within the kyanite zone, and all sillimanite-zone pelites and semipelites are migmatitic.

The development of the metamorphic fabric of the Dalradian rocks ninth of the Mount Battock Granite has been described in detail by Harte and Johnson (1969). They showed that four phases of mineral growth occurred:

  1. syn-D1 to syn-D2: fine-grained fabric developed with growth of quartz, chlorite, micas and ore minerals (greenschist facies)
  2. post-D2, pre-D3: coarsening of mica fabric; growth of quartz, micas, ore minerals and porphyroblasts of plagioclase, garnet, staurolite and kyanite in rocks of staurolite grade and above; growth of hioitite and garnet porphyroblasts: without coarsening of fabric in rocks of garnet grade and below (peak metamorphic temperatures in rocks of kyanite grade and below)
  3. post-D3, pre-D4: continuing growth of quartz, plagioclase and micas, followed by growth of sillimanite, nucleated on biotite, and associated muscovite, with reddening and bleaching of biotite — confined to areas above sillimanite isograd, where peak temperatures developed during this episode
  4. syn-post-D4: retrogression with growth of chlorite, shimmer aggregate, vermiculite and epidote and sericitisation of plagioclase — sporadically developed, but most intense near axial planes of F1 folds

The fibrolitic sillimanite which occurs in the northern half of the district has been regarded as an 'overprint' related to the intrusion of the basic masses to the north of the Dee (Chinner, 1966), although the sillimanite isograd extends many kilometres from outcrops of the syn- D3 basic–ultrabasic intrusions in neighbouring districts, In general the sillimanite does not nucleate on kyanite or andalusite, but grows within crystals of biotite, which is partly reddened, and partly replaced by later muscovite. The fibrolite needles grow with their long axis within the basal cleavage plane of the original biotite. A new generation of foxy red biotite crystals grows across the foliation. In the highest grade rocks, magnetite in octahedral crystals up to 1 mm is developed.

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 northern boundary of the district. Kyanite was recorded from one specimen collected during the primary survey of the part of the district north of the Mount Battock Granite (S6989), but re-examination of the thin section failed to reveal any kyanite. Regional andalusite was recorded by Porteous (1973) from Mill of Cammie [NO 695 919] in the Banchory district (Sheet 66E), and andalusite occurs in the Alford district on Scar Hill [NJ 4813 0150] (S78539), and 600 in to the north-west, near the contact with the Tarland Intrusion [NJ 4766 0134] (S71813). It forms small, irregular crystals, and in a thin section described by Read (1927) it is surrounded by shimmer aggregate. Staurolite occurs in a pelitic xenolith from Craig Ferrar [NO 493 995], partly replaced by prismatic sillimanite (Read, 1927, p.330). However, later andalusite occurs in the thermal aureoles of the Mount Battock and Hill of Fare granites on Deeside, and care is required in interpretation of the textures of the andalusite-bearing rocks.

The first appearance of kyanite in metamorphosed pelites and semipelites under Barrovian metamorphism is controlled by the effective Mg/Fe2+ ratio (excluding Fe2+ tied up in minerals such as magnetite) of the rocks (Atherton and Brotherton, 1972: Harte, 1973), hence in the muscovite-haematite schists of the Glen Lethnot Formation, where most of the iron has been oxidised to Fe3+, kyanite crystallises under conditions characteristic of the staurolite zone. The kyanite isograd for the Aboyne district has been drawn for rocks with a Mg/ (Mg Fe2+) ratio of 0.51.

The iron-ore schists, containing magnetite as well as haematite, are developed within the garnet zone (S82757). They contain abundant muscovite flakes and rare primary chlorite flakes, which define the foliation, together with equant crystals of quartz up to 0.3 mm across and opaque iron oxides up to 0.1 mm across, and a few rare plagioclase porphyroblasts. The absence of biotite and garnet is attributed to the iron being tied up in the opaque phases, causing a very Mg-rich chlorite, stable to higher temperatures, to crystallise. The rock shows no sign of retrogression.

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.

In the lowest grades of metamorphism, the gritty psammites of the Glen Lethnot Formation have a fabric which is dominated by only 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. At higher grades, the fine-grained matrix is replaced by a more coarsely crystalline muscovite, biotite, and chlorite, and the original quartz and chlorite clasts show more extensive recrystallisation. The phyllosilicate layers which define the prominent spaced cleavage in many psammites of the Glen Lethnot Formation consist of chlorite in the chlorite-zone rocks and of biotite, with or without chlorite (depending on Mg/Fe ratio) at higher grades. The psammites of the Queen's Hill, Tarfside and, to a lesser extent, Glen Effock formations have a fabric consisting of a mosaic of quartz and plagioclase with a weak alignment of long axes, and a foliation is defined by the alignment of the minor proportion of biotite and muscovite. The textura differences observed are largely due to the variation in the degree of sorting and clay content of the original sediments. However, the more pervasive recrystallisation of the higher grade rocks in the north may in places have obliterated any original gritty texture. The higher-grade psammites also lack the segregations of mafic minerals in micaceous layers which characterise rocks of lower grades. Many psammites, especially those of Court Hill [NO 521 999], have suffered extensive shearing, as evidenced by the ribbon texture of the quartz.

Limestone and calcsilicate rocks

North of the Mount Battock Granite calcareous rocks are principally found in the Deeside Limestone and Tarfside Psammite formations, all within the sillimanite zone. The most typical assemblage is quartz + plagioclase + calcite icliopside clinozoisite + sphene e.g. (S80504). The plagioclase is generally strongly sericitised and partly replaced by clinozoisite and, in some specimens, epidotc. Opaque minerals are common in crystalline limestones, and, at least at Deecastle [NO 4404 9693], are predominantly pyrrhotite (S80465), In many of the limestones and call-silicate rocks, diopside is partly replaced by blue-green secondary amphibole. In calcsilicate rocks derived from marly sediments, a dark green-brown primary amphibole is present. Wollastonite was recorded from Deecastle and scapolite from Mains of Midstrath [NO 5878 9526] by Robertson et al. (1949). The scapolite is probably metasomatic, being formed by fluids derived from the nearby Mount Battock Granite.

To the south of the Mount Battock Granite, most meta-limestones and calcsilicate rocks lie within the Tarfside Psammite Formation and within the sillimanite isograd of pelitic and semipelitic rocks. The mineral assemblages are largely the same as to the north of the granite (quartz + calcite + plagioclase + clinozoisite + diopside + sphene). K-feldspar may also be present (e.g. (S82790). Amphibole, where present, is invariably secondary, replacing diopside, and is tremolitic. Rare calcareous layers in the Southern Highland Group consist largely of calcite, quartz, plagioclase, and chlorite or tremolitic amphibole.

Metabasic rocks within Dalradian

The amphibolites which intrude the rocks north of the Mount Battock Granite consist almost entirely of dark hornblendic amphibole and andesine plagioclase; garnet is rare to absent, and sphene and ilmenite present in accessory amounts. In contrast to the calcsilicate rocks, the texture is polygonal in fresh rocks, for example thin section (S80452), with a grain size of 0.5 to 1.5 mm, but many rocks have been retrograded, and the dark brown-green amphibole is replaced by paler green actinolitic amphibole, and the edges of the original amphiboles are ragged (S80480). The amphibolites intruding the Tarfside Psammite Formation and Glen Effock Schist Formation in Tarfside and Glen Esk, largely in the sillimanite zone with a few in the kyanite zone, are generally finer grained and less sheared than those to the north of the Mount Battock Granite. The amphibole crystals are more elongate and show a better alignment in the foliation plane; the amphibole is mostly a brighter, less brownish green colour. No amphibolites have been recorded intruding the Glen Lethnot Formation.

Highland Border Complex

All the rocks of the Highland Border Complex were metamorphosed to chlorite grade, although the rocks of the North Esk Formation are more extensively recrystallised than those of the Margie Formation. If the depositional ages for the complex quoted in Chapter 5 are correct, the metamorphism of the complex must postdate the main metamorphism of the Dalradian rocks, although it may he close in age to D1 and the retrogressive metamorphism in the Dalradian.

North Esk Formation

Due to the extensive shearing of the metabasalts, the original texture is preserved only in a few places, e.g. around the Rocks of Solitude [NO 589 727]. Here the original labradorite has been replaced by albite and the mafic minerals by chlorite and calcite. In a few specimens adjacent to beds of haematised chert, the albite and chlorite have been replaced by microcrystalline quartz and haematite, while preserving the original ophitic texture of the metabasalt. Where the texture of the metabasalts has been obliterated by shearing, the rocks have recrystallised as fine-grained (0.1 to 0.2 mm) schist displaying the assemblage quartz + plagioclase (probably albite) + chlorite + calcite + epidote + sphene + haematite. Actinolite is present in only a few specimens. The cherts consist almost entirely of microcrystalline quartz and haematite with minor calcite and saussuritised plagioclase. The dark slaty pelites in the formation are so fine grained and heavily stained with haematite that mineral identification is uncertain; however, they appear to contain fine sericitic material interbedded with micro-crystalline quartz and minor calcite.

Margie Formation

The gritty psammites of this formation consist of quartz grains up to 2 mm across set in a matrix of fine-grained sericite, chlorite and minor calcite (S82712). Barrow (1901) noted remnants of digested clastic mica, both brown and white, in some specimens, and distinguished them from the completely recrvstallised Dalradian on this basis. This distinction is no longer accepted, and all of the mica in the Margie Formation is now believed to be metamorphic. The matrix of the psammites in the northern outcrop of the Margie Formation is more coarsely recrystallised than in the southern outcrop, and muscovite crystals up to 0.2 mm across occur (S82728). The Margie Limestone is a fairly pure calcite limestone, with about 10 per cent quartz (S82729); a few wisps of opaque material and a few flakes of sericite define the bedding. The limestone is partly recn,stallised to a mosaic of 0.2 to 0.5 mm calcite crystals, but patches of finer grained (0.05 to 0.1 mm) calcite are preserved.

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. The quartzofeldspathic component of the migmatite is known as the leucosome, and the mafic selvedge along the margins of the leucosome is known as the mclanosome. Some of the migmatites in the district are believed to have formed by diffusion of elements via a water-rich fluid phase post- D2 pre-D3, whereas others record partial melting of the host rock (anatectic migmatites), and occurred post-D3, coincident with the intrusion of the basic–ultrabasic masses to the north of the district. 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 24) lies between the kyanite and sillimanite isograds. Migmatisation is first manifest in the semipelites, where quartzofeldspathic len tides and veinlets 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 themselves become granitic in composition and coarser grained than the rest of the rock. The appearance of the most intensely migmatised rocks along the northern 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 east of the district boundary. Here granite and pegmatite veins, which amount to 50 to 65 per cent of the rock, form a ramifying stockwork. About half of the veins are parallel to the foliation in the semipelitic gneisses. The veins are mostly 5 to 10 mm wide, but a few reach 50 mm. The grain size in the veins is typically 2 to 3 mm, but pegmatitic patches with a 10 mm grain size occur as pods and in the wider veins. They comprise quartz, plagioclase, K-feldspar and biotite, while muscovite occurs in the pegmatitic patches. The host semipelite/pelite has a grain size of 1 to 2 mm, and is well segregated into biotite-rich and quartzofeldspathic lavers 2 to 20 mm thick. This lithologv is essentially similar to the 'injection gneiss' described by Barrow (1912) from Cairnshee Quarry [NO 739 939] in the Banchory district.

On the north side of the River Dee, from the western boundary of the district to Craig Ferrar [NO 493 995], evidence of probable anatexis occurs, with xenoliths of quartzite, amphibolite, limestone and silica-poor pelitic hornfels set in a poorly foliated porphyroblastic quartz-plagioclase-biotite-sillimanite-garnet-cordierite gneiss. The psammites between Court Hill [NO 510 999] and the lower slopes of Queen's Hill [NO 526 000] 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 Ord Hill [NO 438 986] have suffered no migmatisation, but a layer of amphibolite at Balnacraig [NJ 479 005], just north of the district boundary, is cut by numerous trondhjemitic veins, mostly discordant to the foliation, but ptygmatically folded in places.

The Queen's Hill Formation migmatites on Creag Ferrar [NO 493 995] and Tomachallich [NO 474 997] are relatively weakly layered gneisses with a good alignment of the biotite flakes and with porphyroblasts of plagioclase and garnet up to 5 mm and 3 mm respectively. However, clots and segregations of restite material are rare. Similar rocks, though slightly better foliated, occur to the north of New Kinord, between lochs Davan and Kinord [NJ 444 001]. They are transitional to the heterogeneous xenolithic gneisses which occur from the River Dee west of Dinnet Lodge [NO 447 978], to Mulloch Hill [NJ 469 005].

The partial melting is attributed to proximity to the Morven–Cabrach and Tarland basic masses, which probably extended further south before intrusion of the Ballater and Logie Coldstone plutons.

Contact metamorphism

There is little evidence of contact metamorphism in the district, but members of the Cairngorm Suite of granites have produced contact aureoles in adjacent districts (Figure 24). The poor development of the thermal aureole of the Mount Battock Granite in the Aboyne district is due partly to faulting, and partly to the psammitic nature of the rocks along much of the contact. Where contact metamorphic effects occur, they extend no more than 300 m from the granite contact. In pelitic and semipelitic lithologies, cordierite and foxy red biotite are developed, and garnet is retrograded to corderite and fibrolite to muscovite. The rocks in the innermost part of the aureole in places develop fresh, clear andalusite. The best examples in the Aboyne district occur in the Kettock Burn [NO 594 807] (S82686). Calcsilicate rocks and impure limestones develop grossular, wollastonite and idocrase adjacent to the granite contact in the Burn of Cattie near Ballogie [NO 576 951] (S77197). On the southern slopes of Craigrae Beg [NO 425 911] the grain-size of the calcsilicate rocks is greatly increased, and there is evidence of metasomatism in the greisening and development of disseminated fluorite in the marginal 10 m of microgranite.

Contact effects related to the Ballater Granite are visible only outside the district, for example in the Pollagach Burn (Ballater district) [NO 400 925] to [NO 420 955].

Chapter 10 Cainozoic

No work was done specifically on the Cainozoic geology of the district during the present survey, but a small portion of the district was included in an assessment of sand and gravel resources in the area south of Aberdeen (Autos et al., 1990), and the survey of Quaternary deposits of the Banchory district was extended westwards to complete the 1:10 000 map sheets covering the border between the two districts. A summary of the Cainozoic geology of the district is presented here, based on the work of Barrow and others during the primary survey of the district (Geological Survey of Scotland, 1897) and information noted by the author during the survey of the solid geology, with additional information and reinterpretation from Peacock et al. (1977) and sources outside BUS,

In the absence of recent surveys, most of the generalised map of the Quaternary deposits of the district (Figure 5) is based on the original survey (Geological Survey of Scotland, 1897). Moundy glacial deposits are shown separately from till in parts of the district only; the distinction between ice-contact and sheet glaciofluvial sand and gravel deposits is suspect; and the boundary of the hill peat areas has largely been sketched in from the author's observations. Only a minor proportion of the more important glacial meltwater channels and eskers in the district are shown; they have been added from miscellaneous later studies.

The area was last glaciated during the Late Devensian, when an ice sheet, which reached its maximum extent about 18 000 years ago, advanced from the Grampian Highlands to cover the district.

After complete, or nearly complete, deglaciation during the Windermere Interstadial (c.13 000 to 11 000 BP), glaciers readvanced to the western parts of the Grampian Highlands during the Loch Lomond Stadial (c.11 000 to 10 000 BP). Although there is no evidence that glaciers developed within the Aboyne 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 glaciolluvial erosion, as indicated by the glacial meltwater channels that cross 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

During and after the largely Palaeogene uplift of Scotland associated with the formation of the North Atlantic, erosion of the landmass took place, largely under warm and humid conditions. Deep chemical and physical weathering of the bedrock took place throughout the Grampian Highlands, and a number of erosion surfaces were formed, of which the 700 to 900 m Grampian Surface and the approximately 300 m Buchan Surface are the most persistent (Chapter 1).

The Pleistocene glaciations removed most of the weathered material from the higher ground, but pockets of deep weathering persist in parts of the eastern Grampian Highlands (Hall, A M. 1985). Glacial erosion was deeper in the Aboyne district than in Buchan to the north, and few remnants of bedrock weathered in pre-Pleistocene times are preserved in the district,

Glacial erosion

North-east Scotland was glaciated on several occasions during the Pleistocene (2.5 to 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, and over much of the region all of the superficial deposits and weathered bedrock were removed. 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 has largely removed deposits laid down during previous glaciations.

During the Dimlington Stadial (Main Late Devensian) glaciation, almost the whole of the Aboyne district was covered by ice which moved over the Grampian Highlands, but the extreme south-east corner was covered by a stream of ice which passed over the low ground of Strathmore, and picked up clasts of Silurian and Devonian rocks as well as some metamorphic and igneous clasts. The confluence of the two ice streams lies close to the Highland Boundary Fault.

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 shoulders overlooking the Dee and North Esk valleys. Although glacial erosion generally produced smooth rock surfaces, in places the lee sides of crags were plucked to produce roches moutonnées, for example on Craig Ferrar [NO 493 995]. Ice movement was generally from west to east. On the lower ground, several basins were hollowed out, largely in softer or more weathered rock. Many basins, such as the Muir of Dinnet and the valley of the Feugh south of Birse Castle, were scoured in rock, and were subsequently partially filled by deposit; of glaciofluvial sand and gravel, commonly overlain by peat.

Tors of well-jointed granite occur at or near the summits of several hills, showing that there were periods when the hill summits lay outside the ice sheets and were subjected to severe conditions with many freeze–thaw cycles. The best example is Clachnaben [NO 615 866], which has a small tor to the west of the summit, and a much larger tor on the eastern slopes. Several small tors also occur on Clachan Yell [NO 446 911].

Glacial meltwater channels

During the main Late Devensian deglaciation, the higher ground became ice free before the valleys of the Dee and North Esk and their main tributaries, which still contained valley glaciers, name-terraces were laid down along the glacier margins by subaerial meltwater streams, and many pockets of stagnant ice remained in hollows. These ice masses caused the diversion of meltwater streams and hence controlled the deposition of ice-marginal glaciofluvial deposits.

A few meltwater channels breach major watersheds, for example the Clash of Wirren [NO 492 751], which forms a 100 in-deep notch in the watershed separating Glen Esk from Glen Lethnot. Many of the larger meltwater channel systems are cut through the drift deposits into bedrock and form gorges for part of their course, for example the Burn of Turret from [NO 541 798] to [NO 544 802]. Narrow, overdeepened valleys, such that of the Water of Aven from [NO 568 873] to [NO 607 892], have probably been at least partly excavated by meltwater, rather than by ice flow. Some of these channels may have been partially formed beneath the ice sheet, by meltwaters under hydrostatic pressure.

Glacial and glaciofluvial deposits

East Grampian Drift Group

This group includes all of the deposits laid down during the latter part of the late-Devensian glaciation by the 'Grampian ice sheet', which flowed eastwards from accumulation centres in the Grampian Highlands, together with glaciofluvial and glaciolacustrine deposits derived by reworking the glacial deposits. The tills of the group are assigned to the Banchory Till Formation, but moundy glacial deposits are not allocated to a formation. The Glen Dye Silts Formation comprises glaciolacustrine sediments derived from deposits of the 'Grampian ice-sheet', while the glaciofluvial deposits of the group are assigned to the Lochton Sand and Gravel Formation.

Banchory Till Formation

Almost All of the glacial deposits of the district belong to this formation, which includes lodgement and flow tills laid down by an ice sheet moving from west to east over the eastern Grampian Highlands. The form and internal structure of the till deposits of the Aboyne district show that most were laid down, either beneath active ice or as debris flows 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.

Two lithologically distinct till types have been recognised in adjacent districts, but have not been mapped separately in the Aboyne district.

  1. Lodgement an overconsolidated clayey till with a strong fabric, containing angular erratics of metamorphic and igneous rocks of both local and more distant derivation, dark grey or olive grey in colour where fresh, but in many places weathered to brown.
  2. Melt-out or flow till: a sandy till with a weaker fabric, containing angular and rounded metamorphic and igneous clasts, 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 mostly of granite and psammite. Where the base of the deposit is seen in natural exposures, it rests directly on bedrock. 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.

The colour of the till ranges from grey to brown; colour differences can be attributed in part to variation in the groundwater level 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 sonic occurrences 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 proportion 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. Little or no sand and gravel seems to have been deposited in association with the clayey tills.

The melt-out or flow till is dark yellowish to greyish brown, sandy, and less cohesive than lodgement till. The fabric is highly variable, and, unlike the clayey tills, the sandy till rarely contains lenses of sand and gravel. Indeed, beds of brown sandy till are locally intercalated with thick sequences of sand and gravel and commonly separate material of ice-contact and glaciofluvial origin. More commonly, brown sandy till rests directly on brown clayey till or on bedrock.

Moundy glacial deposits

Moundy deposits, formed of poorly sorted glacial debris, were laid down at the margins of the ice sheet in valley; or beside stranded, stagnant ice masses during deglaciation. The areas of moundy deposits shown on (Figure 5) are those recognised during the resurvey of the solid geology; other moundy glacial deposits may occur within the areas shown as till. Where little or no reworking by meltwaters 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 boulder-rich dianticton and intercalated lenses of poorly stratified, commonly clay-bound, sand and gravel. The irregular mounds and hummocks reflect 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.

Elongate ridges on the flanks of valleys (lateral moraines) and transverse ridges lying across valley floors (retreat moraines) were formed locally during decay of the ice sheet within some east–west-oriented valleys. A large complex spread of hummocky glacial deposits occurs in the valley of the Water of Feugh, south and east of Midclune [NO 608 909]. The mounds to the south of Midclune take the form of a retreat moraine which was laid down by ice that impeded glaciolluvial drainage in the upper reaches of time valley of the Water of Feugh to form an ice-dammed lake between Wester Chine [NO 596 913] and Midclune. The elongate ridges to the east of Midclune are lateral moraines which are associated with a complex array of esker ridges and intervening peat-filled hollows.

Small moraines are developed at the mouths of small corries in the hill areas. Examples occur in Glen Effock at the mouth of Corrie Doune [NO 435 766], in Glen Lethnot at the mouth of Corrie na Berran [NO 443 726], and to the north [NO 442 836] where a small comic drains into the Burn of Branny.

Lochton Sand and Gravel Formation and Glen Dye Silts Formation

Deposits of the Lochton Sand and Gravel Formation are characterised by containing clasts derived exclusively from the Grampian Highlands, and lack clasts of Silurian or Devonian sedimentary or igneous rocks. In the Aboyne district, the clasts consist of Dalradian inetasedimentary rocks and Caledonian granitic rocks, with rare clasts of basic igneous rocks and felsite.

Ice-contact deposits
Eskers

As the ice sheet decayed, sediment-laden melt-waters issued from subglacial or englacial tunnels and laid down stratified sands and gravels which now form sinuous, steep-sided esker ridges. These deposits were commonly 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 commonly shows evidence of post-depositional collapse. A well-developed esker system occurs along the south side of the valley of the Water of Feugh, starting near Blackhole [NO 606 915] and extending well into the Banchory district. Another prominent esker occurs in the valley of the Water of Tanar from near Glen Tanar House [NO 476958] to beyond Millfield [NO 491 967]. Other esker systems occur near Blackcraigs [NO 532 802], near the Burn of Blackpots [NO 565 800], and near Whitehillocks [NO 460 790].

Moundy sand and gravel deposits

During deglaciation, stratified deposits of sand and gravel were laid down within large crevasses or small englacial lakes, close to the ice margin. This material forms complexes of convex mounds (Lanes) of sand and gravel. Such kames are well developed around the Loch of Aboyne [NO 535 995], near Ballogie House [NO 572 955], and between Gannochy and Dalbog [NO 593 713].

Sheet deposits

Muchof the mounds sand and gravel was worked by meltwater streams and laid down, mostly at higher levels along the present major valleys as flat-topped terraces. Kettled kame-terraces are typically developed in an ice-marginal setting, 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 which laid down the sands and gravels of the Lochton Formation in places disgorged into temporary ice-marginal lakes, where distal deposits of laminated clay and silt-grade material (Glen Dye Silts Formation) were deposited contemporaneously.

Dee catchment

Extensive terraced deposits of sand and gravel lie on both banks of the Dee west of Dinnet, and extend eastwards along the northern side of the Dee valley as Far as Aboyne. In the vicinity of the Muir of Dinnet, abundant kames and a number of kame terraces were formed while 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. Terraces of glaciofluvial sand and gravel also occur along the south hank of the Dee near Northbrae [NO 578 971], and in the valley of the Burn of Cattie from Shannel [NO 600 956] eastwards to the boundary of the :Moyne district.

North Esk catchment

Glaciofluvial terraces are less well developed than along the Dee, but some examples are noteworthy. Spreads of sand and gravel occur in lower Glen Mark, forming terraces from the Queen's Well to near Auchronic [NO 420 828] to [NO 440 810], Downstream of the bridge [at NO 430 815], the terraces rise to 10 to 20 m above the level of the stream bed, which is incised into bedrock. Large terraced spreads of sand and gravel occur around the confluence of the Water of Tarf and the North Esk. The surface of the terraces is irregular due to kettling, and the deposits pass into moundy sand and gravel away from the rivers. Downstream From this area, terraces are poorly developed on the north bank and almost entirely absent on the south bank of the North Esk [as far as 589 730] near the Rocks of Solitude. Downstream from the Rocks of Solitude, wide spreads of terraced sand and gravel occur 20 to 30 m above the incised gorge of the North Esk. These spreads extend well beyond the southern and eastern margins of the Aboyne district.

Mearns Drift Group

These are deposits laid down front the ice which occupied Strathmore and moved north-eastwards towards the coast near Stonehaven. They are distinguished frorn the deposits of the East Grampians Drift Group by the types of clasts which they contain and, in most cases, by the colour of the matrix.

Mill of Forest Till Formation

This till is generally redder and more clayey than the Band-wry Till, and contains clasts mostly of Silurian and Devonian sedimentary and Volcanic rocks, although sonic-Caledonian igneous and Dalradian metamorphic dusts are also present, especially near the Highland Border. The formation has only a very small outcrop in the district around Burn Farm [NO 6085 7225] and west of Gannochy Lodge [NO 590 710].

Drumlithie Sand and Gravel Formation

Ice-contact sand and gravel deposits assigned to this formatirm occur around Lochodlev Cottage [NO 6075 7295] and are exposed in a gravel pit near Flatnadreich [NO 6110 7315], on the district boundary. These deposits occur hellcat) glaciofluvial sheet deposits of the Lochton Formation deposited by meltwatet-s which flowed down the North Esk

Late- and postglacial deposits

Pollen sequences spanning the Loch Lomond Stadial and early Flandrian have been described from organic-rich lake muds and silts deposited in Loch Kinord [NO 435 997]. These sequences have yielded radiocarbon dates (Vasari, 1977) of between 11 520 ± 220 14C years BP (HEL-418) and 9820 ± 250 14C years 1W (HEL-421). The dated pollen assemblage shows that sedimentation commenced during the Late glacial, and that the early part of the Flandrian stage experienced a milder and drier climate than occurs today.

Diatomite

After the ice sheet had melted, several small rock basins with outlets blocked by a rock step or by glaciofluvial sand and gravel deposits were uncovered. In most of these basins fine-grained sediments were deposited, but in a few noteworthy examples, hiogenic material also accumulated. The clear-water, with very little elastic 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 Aboyne district are in the Muir of Dinnet–Logie Coldstone area (Figure 6). All of these diatomite deposits, except possibly those in the centre of Loch Davan, are overlain by 0.5 to 3 m of peat.

The principal period of diatom growth was probably from 10 000 to 5000 years BP, after which the cooler and wetter climate encouraged the spread of sphagnum mosses which reduced the area of clear water. This process was aided by shallowing of the lake water due to diatomite and other sediment deposition.

A species list of Diatomaceae found in diatomite from Loch Kinord is given by Wilson and Hinxman (1890, pp.40–42). The economic aspects of the diatomite deposits are discussed in Chapter 2.

Peat

Large areas of hill peat occur in the mountainous parts of the district. During the early to mid Flandrian, they fat one time formed a blanket over much of the Grampian Surface at about 600 to 700 in altitude. However, they have been considerably eroded, especially in the southern part of the district. In addition, basin peat was deposited as raised mosses in rock basins and in poorly drained valleys.

Hill peat covers approximate 77 km2 within the district. Most lies at altitudes between 400 and 650 and rests on weathered bedrock on ground with slopes cf less than 15°. In the northern half of the district the peat cover is only slightly dissected, but in the south, and especially on the southern slopes of Cruys [NO 417 748] to [NO 440 739], much of the vegetation cover has been stripped off the peat and it is eroding rapidly. On the Hill of Wirren [NO 52 73], and along the southern edge of the peat moss that blankets the upland between Mount Battock and the Cairn o'Mount [NO 560 814] to [NO 649 807], the hill peat cover ends abruptly, forming a bench 1 to 2 m in height.

Basin peat is developed in hollows in the glaciofluvial and lacustrine deposits in the Muir of Dinner. The wester shore of Loch Davan is underlain by peat, and sizeable spreads also occur west of Ord Hill [NO 433 985] and around both Bracroddach Loch [NJ 477 000] and the Loch of Dinnet [NO 455 988]. Basin peat also occurs in the valley of the Feugh, in kettleholes within the moundy glacial deposits near Blackhole from [NO 599 913] to [NO 611 911].

Lacustrine deposits

Much of the low-lying ground in the Muir of Dinnet is underlain by lacustrine silts and clays, commonly with a high organic content, These deposits grade into the diatomite which formed in the deeper parts of the large lake which occupied the Muir of Dinnet depression in early Flandrian times. The deposits mostly overlie glaciolluvial sand and gravel. The only other area of lacustrine deposits in the district occurs in the valley of the Water of Feugh, near Blackhole. This lake basin originally formed behind ice which laid clown hummocky glacial deposits and blocked the drainage of the upper reaches of the Water of Feugh catchment. The lake probably existed throughout the Late-glacial and into the Flandrian, when the uppermost deposits of fine-grained sand and laminated silt were laid down. Sedimentation ceased when the deposits blocking the drainage of the valley were breached by the Water of Feugh. The deposits consist of fine-grained sand and laminated silt.

Alluvial deposits

The floodplains of the major rivers vary considerably in width along then length, and are underlain by alluvium which is still accumulating (Figure 5). Along the Dee valley, the floodplain is flanked in many places by older postglacial alluvial terraces, which do not contain the kettle-holes characteristic of glaciothivial kame-terraces. Such terraces are rare along the North Esk and the smaller rivers of the district, although a Aside terrace occurs at The Burn [NO 598 717]. Narrow alluvial plains occur along some of the minor streams, though where there is little flow these become choked by boggy vegetation and peat accumulates. Alluvial fans are developed where a small stream flows into a larger stream. A cone-shaped deposit, typically consisting largely of silt and fine sand, spreads across the floodplain of the larger stream. Within the Aboyne district, most of the alluvial fans are less than 100 m across.

Information sources

Further geological information held by the British Geological Survey relevant to the Aboyne district is listed below. It includes published material in the form of maps, memoirs and reports and unpublished maps and reports. Also included are other sources of data held by BGS in a number of collections, including borehole records and core, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index System in BGS libraries. This is a developing computer-based system which carries out searches of indexes to collections and digital databases for specified geographical areas. It is based on a geographical information system linked to a relational database management sytem. Results of the searches are displayed on maps on the screen. At the present time (1997) the datasets are limited and not all are complete. The indexes which are available are listed below:

Details of geological information available from BGS can be accessed on the BGS Web Home Page at: http://www.bgs.ac.uk

BGS maps

Geology maps

The Aboyne district is the western half of the the area which was formerly covered by Sheet 66 (Banchory) of the Geological Map of Scotland.

1:10000 and 1:10 560

The surveyors of the component 1:10 000 sheets included wholly or partly in Sheet 66W were C A Anton, S Carroll, D S Robertson and C G Smith (BGS), S Goodman (University of Aberdeen) and B Harte (University of Cambridge; now University of Edinburgh).

Maps which show the solid geology only are coded S below; those showing solid and drift geology are coded S+D. These maps are only available as dyeline prints from the British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA. Sheets coded N were not clean copied; only the solid geology was surveyed in these areas.

NJ 40 SW

Logie Goldstone

N

DC, SR

1987

NJ 40 SE

Tarland

S

DC,

1987

NJ 50 SW

Coull

N

DG

1987

NO 47 NW

Loch Lee

N

BH

1964

NO 47 NE

Tarfside

S

DC, BH

1964, 1989–1990

NO 47 SW

Ruragh

S

DG, BH, SR

1964, 1990

NO 47 SE

Hunthill

S

DG, BH

1964, 1990

NO 48 NW

Mount Keen

N

DC, SG

1988– 1989

NO 48 NE

Braid Cairn

N

DG

1988–1989

NO 18 SW

Inverrnark

S

DG, BH, SR

1964, 1989–1990

NO 48 SE

Baillies

S

DC;

1989–1990

NO 49 NW

Cambus o'May

S

DC, SR

1987–1988

NO 49 NE

Dinnet

S

DC

1987–1988

NO 49 SW

Etnach

S

DG, SG, CGS

1987–1988

NO 49 SE

Water of Allochy

N

DG

1988

NO 57 NW

Minden

S

DG, SR

1990

NO 57 NE

Craigoshina

S

DG

1990

NO 57 SW

Stonyford

S

DG

1989–1990

NO 57 SE

Gannochy

S

DC

1989

NO 58 NW

Mudlee Bracks

N

DG

1988–1989

NO 58 NE

Peter Hill

N

DC

1988–1989

NO 58 SW

Blackcraigs

S

DC

1989

NO 58 SE

Head of Glen Dye

N

DG

1989

NO 59 NW

Aboyne

S

DG

1987–1988

NO 59 NE

Kincardine o'Neil

S

DG

1986–1988

NO 59 SW

Birse Castle

N

DC

1989

NO 59 SE

Finzean House

N

DG

1988

NO 67 NW

Easque

S+D

DG

1989, 1993

NO 67 SW

Balbegno

S+D

DG, DC

1989, 1993–1994

NO 68 NW

Clachnaben

S+D

CAA, DG

1988–1989, 1993

NO 68 SW

Meluncart

S+D

DC

1989, 1993

NO 69 NW

Potarch

S+D

DG

1985, 1994

NO 69 SW

Fenghside

S+D

CAA, DG

1985, 1988–1989

Geophysical maps

BGS books

Memoirs, reports and papers relevant to the Aboyne district arranged by topic. Some are either out of print or are not widely available, but may be consulted at BGS and other libraries.

General geology

Economic geology

Documentary collections

Borehole record collection

BGS holds collections of records of boreholes which can be consulted at BGS, Edinburgh, where copies of most records may be purchased, For the Aboyne district the collection consists of the sites and logs of about 30 boreholes. The logs are either handwritten cr typed and many of the older records are drillers logs.

A borehole was drilled into the Mount Battock Granite at [NO 542 905] as part of the programme of investigation into the geothermal potential of the UK.

Azimuth: vertical. Total depth; 261.07 in. Cored: 90.0896.10 m; 184,60–190,85 m; 255.65–261.07 m, For full details refer to Webb and Brown, 1984.

Site exploration reports

This collection consists of site exploration reports carried out to investigate foundation conditions prior to construction. There is a digital index and the reports themselves are held on microfiche. For the .,thoyne district there are presently (1997) 2 reports.

Archival information

Records of progress in the primary survey of the district may be found in the Annual Reports of the Geological Survey of Scotland for the period 1880–1896.

Material collections

Geological Survey photographs

About 40 photographs illustrating aspects of the geology of the Aboyne district are deposited for reference in the libraries at BGS, Murchison House, Wrest Mains Road, Edinburgh EH9 3LA and BGS, Kevworth, Nottingham NG12 5GG; and in the BGS Information Office, Natural History Museum Earth Galleries, Exhibition Road, London SW7 2DE.

The photographs were taken at various times. The; depict details of the various rocks and sediments exposed either naturally or in excavations and also some general views. A list of titles cart be supplied on request. The photographs cats be supplied as black and white or colour prints and 2 X 2 colour transparencies, at a fixed tariff.

Photograph numbers from the Aboyne district are as follows:

C series (half plate black and white negatives): C2125–2130; C2141–2161.

D series (size colour negatives and 35 mm transparencies): D4314, D4544–D4547; D4920–D4926.

Petrological collections

All thin sections prepared from rocks collected by BGS personnel working on the survey of the district, including those made for Barrow's revision in the 1890s, are held at BUS Edinburgh, Hand specimens for all sliced rocks are also kept at BGS Murchison House. Localities of those made during the 1985–1990 resurvey are shown on the 1:10 000 maps, where published. Details of grid reference, name of locality, type of section and rock type may be obtained on application to BGS Mineralogy and Petrology Unit, Murchison House. The BGS specimen and thin section numbers for each 1:10 000 sheet in the Aboyne district are as follows:

Thin sections from rocks collected by Dr Harte are stored in the Harker Collection, Department of Mineralogy and Petrology, University of Cambridge; additional information should be requested from Dr Harte.

Specimens collected and analysed during the geothermal investigations are detailed by Webb and Brown (1981). Hand specimens, thin sections and powders are held at the Open University.

Bore core collection

The core from the geothermal borehole (see above) is held by the Open University/BGS Keyworth. There is no other registered core available from the district.

Palaeontological collections

The appendix lists all fossils recorded front the district. Fossils have only been found in rocks of the Highland Border Complex. No fossils have been collected from the Silurian or Devonian rocks of the district. The specimens described by Downie et al.(1971) are held at BGS, Keyworth, while those described by Curry, et al. (1984) are held at the Hunterian Museum, University of Glasgow.

Adresses for data sources

BGS hydrogeology enquiry service: wells, springs and water borehole records. British Geological Survey, Hydrogeology Group. Maclean Building, Crowmarsh Githird, Wallingford, Oxfordshire OXO 8BB. Telephone 01491 838800 Fax 01491 692345.

London information Office at the Natural History. Museum Earth Galleries, Exhibition Road, South Kensington London SW7 9DE Telephone 020 7589 4090 Fax 020 7584 8270

British Geological Survey (Headquarters] Keyworth, Nottingham NG12 SGG Telephone 0115 936 3100 Fax 01 15 936 3200

Web www.bgs.ac.uk

E-commerce www.british-geological-survey.co.uk

British Geological Survey Murchison House West Mains Road Edinburgh EH9 3LA Telephone 0131 667 1000 Fax 0131 668 2683

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation.

ANDERSON, J G C. 1946. The geology of the Highland Border: Stonehaven to Arran. Transactions of the Royal Society of Edinburgh, Vol. 61, 479–515.

ARMSTRONG, M, and PATERSON, I B. 1970. The Lower Old Red Sandstone of the Strathmore region. Institute of Geological Sciences, Report No. 70/12.

ASHWORTH, J R. 1985. Migmatites. (Glasgow: Blackie.)

ATHERTON, M P, and BROTHERTON, M S. 1972. The composition of some kyanite-bearing regionally metamorphosed rocks from the Dalradian. Scottish Journal of Geology, Vol. 8, 203–213.

AUTON, C A, THOMAS, C W, and MERRITT, J W. 1990. The sand and gravel resources of the country around Strachan and between Auchenblae and Catterline, Grampian Region: Description of parts of 1:25 000 sheets NO 68, 69, 77, 78, 87 and 88. British Geological Survey Technical Report, WF/90/7.

BAMFORD, D. 1979. Seismic constraints on the deep geology of the Caledonides of northern Britain. 93–96 in The Caledonides of the British Isles - reviewed. HARRIS, A L, HOLLAND, C H and LEAKE, B E (editors). Special Publication of the Geological Society of London, No. 8.

BARROW, G. 1893. On an intrusion of muscovite-biotite gneiss in the southeast Highlands of Scotland and its accompanying metamorphism. Quarterly Journal of the Geological Society of London, Volume 49, 33–58.

BARROW, G. 1898. On the occurrence of chloritoid in Kincardineshire. Quarterly Journal of the Geological Society of London, Vol. 54, 149–155.

BARROW, G. 1901. On the occurrence of ?Silurian rocks in Forfarshire and Kincardineshire along the eastern border of the Highlands. Quarterly Journal of the Geological Society of London, Vol. 57, 328–345.

BARROW, G. 1912. On the geology of lower Deeside and the southern Highland border. Proceedings of the Geologists' Association, Vol. 23, 274–290.

BARROW, G, and CRAIG, E H C. 1912. The geology of the districts of Braemar, Ballater and Glen Clova (Explanation of Sheet 65). Memoir of the Geological Survey, Scotland. Sheet 65 (Scotland).

BARTON, P J. 1992. LISPB revisited, a new look under the Caledonides of northern Britain. Geophysical Journal International, Vol. 110, 371–391.

BOOTH, J E. 1984. Structural, stratigraphic and metamorphic studies in the south-east Scottish Dalradian Highlands. Unpublished PhD thesis, University of Edinburgh.

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

BROWN, P E, MILLER J A, GRASTY R L, and FRASER, W E. 1965. Potassium-argon ages of some Aberdeenshire granites and gabbros. Nature, London, Vol. 207, 1287–1288.

BURTON, C J, HOCKEN, C, MACCALLUM, D, and YOUNG, M E. 1983. Chitinozoa and the age of the Margie Limestone of the North Esk. Proceedings of the Geological Society of Glasgow for 1982–83, 27–32.

CAMPBELL, R. 1913. The geology of south-eastern Kincardineshire. Transactions of the Royal Society of Edinburgh, Vol. 48, 923–960.

CARROLL, S. 1995. Geology of the Fettercairn area 1:10 0.00 sheets NO 67 NW, NO 67 NE, NO 67 SW, NO 67 SE and NO 68 SE (south of the Highland Boundary Fault). British Geological Survey Technical Report, WA/95/91.

CHINNER, G A. 1965. The kyanite isograd in Glen Clova, Angus, Scotland. Mineralogical Magazine, Vol. 34, 132–143.

CHINNER, G A. 1966. The distribution of pressure and temperature during Dalradian metamorphism. Quarterly Journal of the Geological Society of London, Vol. 122, 159–186.

CHINNER, G A, and HESELTINE, P J. 1979. The Grampide andalusite/kyanite isograd. Scottish Journal of Geology, Vol. 15, 117–127.

CLAPPERTON, C M, and SUGDEN, D E. 1987. The late Devensian glaciation of north-east Scotland. 1–13 in Studies in the Scottish lateglacial environment. GRAY, J M, and LOWE, J J (editors). (Oxford: Pergamon Press.)

COATS, J S, SHAW, M H, ROLLIN, K E, ROBERTSON, S, and REDWOOD, S D. 1993. Mineral exploration in the Pitlochry to Glen Clova area, Tayside Region, Scotland. British Geological Survey Technical Report, WF/93/1 (Mineral Reconnaissance Programme Report, No. 126).

COBBING, E J, MALLICK, D I J, PITFIELD, P J, and TEOH, L H. 1986. The granites of the South-east Asian tin belt. Journal of the Geological Society of London, Vol. 143, 537–550.

GOBBING, E J, PITFIELD, P J, DARBYSH1RE, D P F, and MALLICK, D I J. 1992. The granites of the South-east Asian tin belt. Overseas Memoir of the British Geological Survey, No. 10.

COCHRAN-PATRICK, R W. 1876. Early records relating to mining in Scotland. (Edinburgh: David Douglas.)

CURRY, G B, BLUCK, B J, BURTON, C J, INGHAM, J K, SIVETER, D J, and WILLIAMS, A. 1984. Age, evolution and tectonic history of the Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Volume 75, 113–133.

CURRY, G B, INGHAM, J K, BLUCK, B J, and WILLIAMS, A. 1982. The significance of a reliable Ordovician age for some Highland Border rocks in central Scotland. Journal of the Geological Society of London, Vol. 139, 451–454.

DEMPSTER, T J. 1985. Uplift patterns and orogenic evolution in the Scottish Dalradian. Journal of the Geological Society of London, Vol. 142, 111–128.

DEMPSTER, T J, and BLUCK, B J. 1991. The age and tectonic significance of the Bute amphibolite, Highland Border Complex, Scotland. Geological Magazine, Vol. 128, 77–80.

DENTITH, M C, TRENCH, A, and BLUCK, B J. 1992. Geophysical constraints on the nature of the Highland Boundary Fault Zone in western Scotland. Geological Magazine, Vol. 129, 411–419.

DOWNIE, C, LISTER, T R, HARRIS, A L, and FETTES, D J. 1971. A palynological investigation of the Dalradian of Scotland. Report of the Institute of Geological Sciences, No. 71/9.

FETTES, D J, GRAHAM, C M, HARTE, B, and PLANT, J A. 1986. Lineaments and basement domains; an alternative view of Dalradian evolution. Journal of the Geological Society of London, Vol. 143, 453–464.

FITCH, F J, MILLER, J A, and WILLIAMS, S C. 1970. Isotopic ages of British Carboniferous rocks. Compte rendue du 6me Congres International de Stratigraphie et de Géologie du Carbonifère, Sheffield 1967, Vol. 2, 771–789.

GARDINER, A, and TAYLOR, A K. 1950. The diatomite deposits of Dinnet, Aberdeenshire. Aberdeen University: Report to Scottish Council (Development and Industry), No. SCDI/MRP (50) 16.

GEIKIE, A. 1891. The younger schists: Dalradian. Quarterly Journal of the Geological Society of London, Vol. 47, 48–162.

GEOLOGICAL SURVEY OF SCOTLAND. 1897. Banchory. Scotland Sheet 66. Solid and Drift geology 1:63 360. (Southampton: Ordnance Survey for Geological Survey of Scotland.)

GORDON, J E. 1993. Muir of Dinnet. 246–251 in Geological Conservation Review No. 6: Quaternary of Scotland. Gomm, J E, and SUTHERLAND, D G (editors). (London: Chapman and Hall.)

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

GRADSTEIN, F M, and OGG, J. 1996. A Phanerozoic time scale. Episodes, Vol 19, Nos. 1 and 2, 3–5

HALL, A M. 1985. Cenozoic weathering covers in Buchan, Scotland and their significance. Nature, London, Vol. 315, 392–395.

HALL, J. 1985. Geophysical constraints on crustal structure in the Dalradian region of Scotland. Journal of the Geological Society of London, Vol. 142, 149–155.

HALLIDAY, A N, GRAHAM, C M, AFTALION, M, and DYMOKE, P. 1989. The depositional age of the Dalradian Supergroup: U-Pb and Sm-Nd isotopic studies of the Tayvallich volcanics, Scotland. Journal of the Geological Society of London, Vol. 146, 3–6.

HARLAND, W B, ARMSTRONG, R L, CRAIG, L E, SMITH, A G, and SMITH, D G. 1990. A geologic time-scale, 1989. (Cambridge: Cambridge University Press.)

HARRIS, P M, HIGHLEY, D E, HILLIER, J A, and WIIITWOOD, A (compilers). 1994. Directory of Mines and Quarries, 1994: 4th edition. (Keyworth, Nottingham: British Geological Survey.)

HARRISON, T N. 1987. The evolution of the eastern Grampians granites. Unpublished PhD thesis, University of Aberdeen.

HARRISON, T N, and HUTCHINSON, J. 1987. The age and origin of the Eastern Grampians Younger Granites. Scottish Journal of Geology, Vol. 23, 269–282.

HARTE, B. 1966. Stratigraphy, structure and metamorphism in the south-eastern Grampian Highlands of Scotland. Unpublished PhD thesis, University of Cambridge.

HARTE, B. 1973. The composition of some kyanite-bearing regionally-metamorposed rocks from the Dalradian. Scottish Journal of Geology, Vol. 9, 239–244.

HARTE, B. 1979. The Tarfside succession and the structure and stratigraphy of the eastern Scottish Dalradian rocks. 221–228 in The Caledonides of the British Isles - reviewed. HARRIS, A L, HOLLAND, C H, and LEAKE, B E (editors). Special Publication of the Geological Society of London, No. 8.

HARTE, B, BOOTH, J E, DEMPSTER, T J, FETTES, D J, MENDUM, J R, and WATTS, D. 1984. Aspects of the post-depositional evolution of Dalradian and Highland Border Complex rocks in the Southern Highlands of Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 75, 151–163.

HARTE, B, BOOTH, J E, and FETTES, D J. 1987. Stonehaven to Findon: Dalradian structure and metamorphism. 211–226 in An excursion guide to the geology of the Aberdeen area. (Edinburgh: Scottish Academic Press for Aberdeen Geological Society.)

HARTE, B, and HUDSON, N F C. 1979. Pelite facies series and the temperatures and pressures of Dalradian metamorphism in E Scotland. 323–337 in The Caledonides of the British Isles reviewed. HARRIS, A L, HOLLAND, C H, and LESLIE, B E (editors). Special Publication of the Geological Society of London, No. 8.

HARTE, B, and JOHNSON, M R W. 1969. Metamorphic history of Dalradian rocks in Glens Clova, Esk and L#hnot, Angus, Scotland. Scottish Journal of Geology, Vol. 5, 54–80.

HENDERSON, W G, and ROBERTSON, A H F. 1982. The Highland Border rocks and their relation to marginal basin development in the Scottish Caledonides. Journal of the Geological Society of London, Vol. 139, 433–450.

HUTTON, D H W. 1987. Strike-slip terranes and a model for the evolution of the British and Irish Caledonides. Geological Magazine, Vol. 124, 405–425.

IMRIE, N. 1812. A description of the strata which occur in ascending order from the plains of Kincardineshire to the summit of Mount Battoc, one of the most elevated points in the eastern district of the Grampian mountains. Transactions of the Royal Society of Edinburgh, Vol. 6, 3–19.

KNEELER, B C, and AFTALION, M. 1987. The isotopic and structural age of the Aberdeen Granite. Journal of the Geological Society of London, Vol. 144, 717–721.

KNEELER, B C, and LESLIE, A G. 1984. Amphibolite facies metamorphism in shear zones in the Buchan area of NE Scotland. Journal of Metamorphic Geology, Vol. 2, 83–94.

KRABBENDAM, M, LESLIE, A G, CRANE, A, and GOODMAN, S. 1997. Generation of the Tay Nappe, Scotland, by large-scale SE-directed shearing. Journal of the Geological Society of London, Vol. 154, 15–24.

LEE, M K, WHEILDON, J, WEBB, P C, BROWN, G C, ROLLIN, K, 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 (1981–1984). Investigation of the geothermal potential of the UK (Keyworth, Nottingham: British Geological Survey.)

MOHAMAD, H B. 1980. Geochemistry and petrology of the Southern Highland (Upper Dalradian) pelitic-psammitic schists of Glen Esk, Angus, Scotland. Unpublished PhD thesis, University of Strathclyde.

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O'BRIEN, C. 1985. The petrogenesis and geochemistry of the British Caledonian granites, with special reference to mineralized intrusions. Unpublished PhD thesis, University of Leicester.

PANKHURST, R J. 1974. Rb-Sr whole-rock chronology of Caledonian events in northeast Scotland. Bulletin of the Geological Society of America, Vol. 85, 345–350.

PATERSON, I B. 1977. Sand and gravel resources of the Tayside region. Report of the Institute of Geological Sciences, No. 77/6.

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

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POTEOUS, W G. 1973. Metamorphic index minerals in the eastern Dalradian. Scottish Journal of Geology, Vol. 9, 29–43.

PRINGLE, K. 1939. On the discovery of Cambrian trilobites in the Highland Border rocks near Callander, Pertthshire. Report of the British Association for the Advancement of Science, Vol. 6, 252.

PRINGLE, K. 1942. On the relationship of the green conglomerate to Margie Series in the North Esk, near Edzell; and on the probable age of the Margie Limestone. Transactions of the Geological Society of Glasgow, Vol. 20, 136–140.

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WILSON, G V. 1921. Special reports on the mineral resources of Great Britain. The lead, zinc, copper and nickel ores of Scotland. Memoir of the Geological Survey of Scotland. Vol. 17.

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Appendix—List of fossils recorded from the Aboyne district

No collection was possible during the present survey.

Figures, plates and tables

Figures

(Figure 1) Solid geological map of the Aboyne district.

(Figure 2) Topographical map of the district and environs.

(Figure 3) Geological cross-section across the district.

(Figure 4) Stages in the closure of Iapetus and the evolution of the Highland Border Complex (from Hutton, 1987).

(Figure 5) Quaternary geological map of the district.

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

(Figure 7) Preferred thermal model for the Mount Battock Granite (after Wheildon et al., 1984). a. Heat flow profile across the model b. Thermal model c. Predicted subsurface temperature profiles: 1. Maine–Dalradian basement with q0 = 40 mWm−2 2. Moine–Dalradian basement with q0 = 55 mWm−2 3. Mount Battock pluton 4. Ballater pluton

(Figure 8) Aeromagnetic map of the district and adjacent areas, with simplified geology.

(Figure 9) Bouguer gravity anomaly map of the district and adjacent areas, with simplified geology.

(Figure 10) Geophysical profiles and model across the district along line G–G′ (for location see (Figure 9)). a. Magnetic profile b. Bouguer gravity anomaly profile c. Geophysical model

(Figure 11) Solid geological map of the Dinnet–Queen's Hill area (dykes omitted for clarity).

(Figure 12) Generalised vertical section through the Dalradian metasedimentary rocks of the Aboyne district.

(Figure 13) Horizontal section along the line of the River North Esk from the Burn of Auchmull to the North Esk Fault.

(Figure 14) Highland Border Complex from Bridgend to Clatterin' Brig.

(Figure 15) Horizontal section along the line of the River North Esk from the North Esk Fault to the Highland Boundary Fault.

(Figure 16) Northern outcrop of Margie Formation in the River North Esk, geological exposure map by J E Booth and B Harte (from Booth, 1984).

(Figure 17) Horizontal section along the line of the River North Esk from the Highland Boundary Fault to 400 m downstream of Gannochy Bridge.

(Figure 18) Geological map of the Mount Battock pluton.

(Figure 19) Variation in Rb, U, Zr and TiO2 within the Mount Battock Granite.

(Figure 20) Plots of Rb and Sr against Zr, and Rb against Sr for samples of granitic rock from the Mount Battock Granite (data from Webb and Brown, 1984; O'Brien, 1985; Harrison, 1987).

(Figure 21) Principal minor intrusions of the district.

(Figure 22) Block diagram of major structures in the north-east Grampian Highlands (brittle faults not shown). Taken from Stephenson and Gould (1995) and based largely on Thomas (1979). Rectangle shows location of Aboyne district.

(Figure 23) Conjectural boundary of the Tarfside Nappe and trace of the Highland Border Downbend.

(Figure 24) Distribution of metamorphic index minerals in the Aboyne district and adjoining areas.

Plates

(Front cover) Cover photograph Coarse clast-supported orthoconglomerate, Gannochy Formation, Strathmore Group, Old Red Sandstone, River North Esk, Loups Bridge [NO 5948 7169] (D5347). Photographer: Tom Bain

(Rear cover)

(Plate 1)a Photomicrographs of Dalradian metasedimentary rocks. Feldspar-porphyroblast gneiss with coarseh crystalline sillimanite, garnet and biotite. Queen's Hill Formation, south side of disused railwayline, 250 m north-west of Dinnet House [NO 4475 9807] (S80453). Crossed polars. x 11.2 magnification. 1b Photomicrographs of Dalradian metasedimentary rocks. Impure metalimestone. containing diopside clinotoisite and sphene. Deeside Limestone Formation. Gallahill Quarry, Ballogie [NO 5758 9602] (S80506), Plane ploarised light. Magnification x 14.3. 1c Photomicrographs of Dalradian metasedimentary rocks. c Petite. coarse-grained. with large porphyroblasts of garnet. staurolite and plagioclase, the lattet containing abundant inclusions (top centre). Glen Lethnot Grit Formation. ‘Aater of Saughs [NO 4342 7359] (S92715). Crossed polars. magnification x 11.2. 1d Photomicrographs of Dalradian metasedimentary rocks. d Semipelite. laminated. Biotite porpfn roblasts cutting across foliation are bent and cracked b. brittle D4 crenulations. Glen Lethnoi Grit Formation. Burn of Mooran. 800 m south-west of Cornescorn [NO 5679 7356] (S82746). Plane polarised light, magnification x 17.8.

(Plate 2) Calcsilicate rock, Deeside Limestone Formation, Dinnet Bridge [NO 4615 9818J (D 4314). Length of hammer 40 cm.

(Plate 3) Thickly bedded psammite, Glen Tartar Member, Tarfside Psammite‘ Formation, Water of Tarf [NO 4888 8135] (GN 145).

(Plate 4) Graded bedding in grin. psammite. Glen Lethnot Formation. West Water. 250 m west of Stomford [NO 5033 7259]. Succession Youngs towards camera (GN 146). Length of hammer 41) cm.

(Plate 5) Massiemetabasalt, onls slightly sheared, with traces of pillow structure. North Esk Formation, Riser North Esk. 200 m west of Doolic Tower [NO 5886 7272] (GN147) Chequered scale is in cm.

(Plate 6) Green Conglomerate. Margie Formation, Highland Border Complex. Clasts of metabasalt. chert and rare amphibolite and granite. set in a schistose. chloritic matrix. Riser North Esk. 800 m north-west of Woodburn [NO 5872 7326] (C2155). Length of hammer 40 cm.

(Plate 7) Felsite dyke cutting psammite of Queen's Hill Formation. The dyke is about 3 m wide and forms the island in foreground and the palest rocks on the far bank. Potarch Bridge [NO 6075 9730] (D4546).

(Plate 8) A quartz dolerite dyke forms exposures beside figure and extends along ridge away from the camera. It cuts calcsilicate rocks of Deeside Limestone Formation and psammites of Queen's Hill Formation, 300 m west of Wreaton [NO 4980 9935] (D4547).

(Plate 9) D. folds with flat-lying axial planes and east-weq axes folding interbedded psammites and semipelites of the Glen Turret Member. Tarfside Psammite Formation. Greenbush, lower Glen Mark [NO 4270 8245] (GN148). Length of hammer 40 cm.

(Plate 10) Box-shaped F3 folds refolded by F4 crenulations in finely laminated psamrnite arid semipelite of Glen Lethnot Formation, Craig Duchrey [NO 4978 7135] (GN 149).Length of hammer 40 cm.

Tables

(Table 1) Geological sequence in the Aboyne district.

(Table 2) Measured formation magnetic susceptibilities and mean magnetic susceptibility and density of formations.

(Table 3) Relative ages and distinguishing features of the different phases of the Mount Battock pluton.

Tables

(Table 2) Measured formation magnetic susceptibilities of geological formations; mean magnetic susceptibility and density of formations as used in model of (Figure 10)c. a. Measured formation magnetic susceptibilities

No. of specimens

Range of susceptibilities (SI 10−3)

Mean susceptibility (SI 10−3)

DALRADIAN SUPERGROUP

Queen's Hill Formation

17

0.04–0.37

0.19

Deeside Limestone Formation

24

0.00–8.16

0.83

Tarfside Psammite Formation

Glen Turret Member

6

0.05–0.27

0.14

Glen Tanar Member (north of granite)

13

0.08–3.93

0.60

Glen Tanar Member (south of granite)

23

0.04–5.15

0.85

Glen Effock Schist Formation

47

0.16–72.1

6.31

Glen Eethnot Grit Formation

61

0.01–72.1

7.28

HIGHLAND AND BORDER COMPLEX

Margie Formation

7

0.04–0.21

0.14

North Esk Formation

27

0.14–53.6

4.26

MOUNT BATTOCK PLUTON

Main Non-porphyritic Granite

18

0.01–7.23

2.55

Main Porphyritic Granite

14

0.08–10.7

4.04

Water of Fengh Granite

6

0.61–7.80

3.49

Cock Cairn Granite

55

2.51–6.00

3.68

Fungle Granite

3

0.10–11.8

6.17

Clachnaben Granite

6

0.07–7.34

2.09

Water of Dye Granite

14

0.00–1.54

0.24

Mongour Granite

3

0.08–0.33

0.16

Microgranites

28

0.01–10.4

1.54

Felsites

9

0.10–8.43

1.42

Amphibolites

23

0.29–7.36

1.20

Caledonian rumor intrusions

23

0.01–28.3

2.64

Carboniferous quartz dolerites

5

19.9–53.4

37.8

(Table 2) Measured formation magnetic susceptibilities of geological formations; mean magnetic susceptibility and density of formations as used in model of (Figure 10)c. b. Average properties for polygons of each formation in 213 model

Formation

density (Mg m−3)

susceptibility (SI 10−3)

SILURIAN-DEVONLAN

Sedimentary rocks

2.64

0.0

Acid volcanic rocks

2.70

0.0

Intermediate-basic volcanic rocks

2.70

20.0

Highland Border Complex

2.78

15.0

DALRADIAN

Glen Lethnot Grit Formation

2.75

5.0

Glen Effock Schist Formation (upper)

2.75

5.0

Glen Effock Schist Formation (lower)

2.77

95.0

Tayvallich Subgroup

2.73

12.0

Appin and Argyll groups (undivided)

2.75

4.0

Granite

2.63

16.0

Granodiorite and tonalite

2.64

32.0

Gabhroic rocks*

2.88

22.0

Ultramafic rocks*

2.82

50.0

Midland Valley basement

2.85

0.0

Pre-Dalradian basement

2.77

20.0

* Natural remanent magnetisation (NRM) of 1.0–2.0 A/m with a declination of 170°, inclination +40°