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1 2 3 4 5 The Dalradian rocks of the north-east Grampian 6 7 Highlands of Scotland 8 9 D. Stephenson, J.R. Mendum, D.J. Fettes, C.G. Smith, D. Gould, 10 11 P.W.G. Tanner and R.A. Smith 12 13 * David Stephenson British Geological Survey, Murchison House, 14 West Mains Road, Edinburgh EH9 3LA. 15 [email protected] 16 0131 650 0323 17 John R. Mendum British Geological Survey, Murchison House, West 18 Mains Road, Edinburgh EH9 3LA. 19 Douglas J. Fettes British Geological Survey, Murchison House, West 20 Mains Road, Edinburgh EH9 3LA. 21 C. Graham Smith Border Geo-Science, 1 Caplaw Way, Penicuik, 22 Midlothian EH26 9JE; formerly British Geological Survey, Edinburgh. 23 David Gould formerly British Geological Survey, Edinburgh. 24 P.W. Geoff Tanner Department of Geographical and Earth Sciences, 25 University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow 26 27 G12 8QQ. 28 Richard A. Smith formerly British Geological Survey, Edinburgh. 29 30 * Corresponding author 31 32 Keywords: 33 Geological Conservation Review 34 North-east Grampian Highlands 35 Dalradian Supergroup 36 Lithostratigraphy 37 Structural geology 38 Metamorphism 39 40 41 ABSTRACT 42 43 The North-east Grampian Highlands, as described here, are bounded 44 to the north-west by the Grampian Group outcrop of the Northern 45 Grampian Highlands and to the south by the Southern Highland Group 46 outcrop in the Highland Border region. The Dalradian succession 47 therefore encompasses the whole of the Appin and Argyll groups, but 48 also includes an extensive outlier of Southern Highland Group 49 strata in the north of the region. The succession includes 50 shallow-marine sequences, glacigenic deposits at two 51 stratigraphical levels, the earliest evidence for volcanism in the 52 Dalradian, a later major development of basaltic and picritic sub- 53 marine lavas, and thick turbiditic sequences. 54 In the south, the Grampian-Appin group boundary is a high-strain 55 zone, with no obvious dislocation or stratigraphical excision, 56 which was formerly termed the Boundary Slide. Shear-zones at 57 higher structural levels are associated with pre-tectonic granites, 58 such as the Ben Vuirich Granite, which have been dated at c. 600 Ma 59 and hence place limits on the timing of sedimentation, deformation 60 61 62 63 64 65 1 2 3 4 and metamorphism. The region is divided from north to south by a 5 major zone of shearing and dislocation with associated igneous 6 intrusions, termed the Portsoy Lineament. To the west of the 7 lineament, the stratigraphy is more-or-less continuous along strike 8 with that of the Central Grampian Highlands. D1, D2 and D3 9 structures extend from the Tummel Steep Belt north-eastwards 10 throughout this area. The stratigraphical succession is broadly 11 continuous across the Portsoy Lineament but to the east, in the 12 Buchan Block, correlations are more tenuous and do not extend below 13 subgroup level. High-grade migmatitic paragneisses were once 14 interpreted as pre-Dalradian basement but they are now assigned to 15 the Crinan Subgroup, within the Dalradian succession. Within the 16 Buchan block the outcrop pattern is controlled by two broad, open, 17 post-metamorphic folds, the Turriff Syncline and the Buchan 18 Anticline. 19 The Buchan Block is the international type area for the high- 20 temperature/low-pressure Buchan-type regional metamorphism. To the 21 south and west, this passes into higher pressure Barrovian-type 22 metamorphism. South of Deeside, metamorphic conditions reached 23 820oC and over 8 kbar, well into granulite facies and the highest 24 recorded in the Grampian Terrane. The detailed relationship 25 between the high heat-flow and the emplacement of large bodies of 26 basic and silicic magma is a matter of ongoing research. Plutons 27 of the North-east Grampian Basic Suite, emplaced at c. 470 Ma, 28 during or shortly after the peak of metamorphism and the D3 29 deformation, provide key evidence for the timing of the Grampian 30 orogenic event. 31 32 33 1 INTRODUCTION 34 35 D. Stephenson, J.R. Mendum and D.J. Fettes 36 37 The North-east Grampian Highlands are defined here largely by two 38 geological boundaries (Figure 1). To the north-west, the boundary 39 40 with the Northern Grampian Highlands is taken at the Grampian–Appin 41 group junction, which to the south of the Cairngorm Pluton is 42 marked by the Boundary Slide or the Loch Tay Fault; farther north, 43 a rapid stratigraphical transition is present, albeit with some 44 local shearing. To the south, the boundary with the Highland 45 Border region is taken at the top of the Loch Tay Limestone 46 Formation between Pitlochry and the Mount Battock Pluton, and then 47 along the projected continuation of the Argyll–Southern Highland 48 group junction on Deeside, east to Aberdeen. The short south- 49 western boundary with the Central Grampian Highlands is the valley 50 of the rivers Garry and Tummel, as followed by the railway and A9 51 road, between Pitlochry and Blair Atholl. 52 The region is divided into three distinct geological areas by two 53 of the major lineaments of the Grampian Highlands (see Stephenson 54 et al., 2013a). The east–west Deeside Lineament, to the north of 55 the River Dee, is marked by a line of large granitic plutons with 56 only narrow intervening outcrops of Dalradian strata. The 57 58 Dalradian succession is generally coherent across this essentially 59 late-Caledonian lineament, although some facies changes have been 60 recognized and many of the formation names change. Hence it is a 61 62 63 64 65 1 2 3 4 useful boundary for descriptive purposes, at least in upper 5 Deeside. The north–south Portsoy–Duchray Hill Lineament is an 6 older structure that was active during Dalradian sedimentation and 7 was a locus for basic magmatism and major tectonic dislocation 8 during the Caledonian Orogeny (Fettes et al., 1986; Goodman, 1994). 9 It forms a fundamental stratigraphical and structural boundary 10 stretching from the north coast at Portsoy to Glen Muick on south 11 12 Deeside. To the south-west of the Lochnagar Pluton, it is less 13 well defined but marks changes in stratigraphy that were recognized 14 by Barrow (1912). It is coincident with the later, brittle Glen 15 Doll Fault for some distance but turns towards the south-west, 16 along strike, and peters out beyond Duchray Hill in Glen Shee. 17 To the west of the Portsoy–Duchray Hill Lineament, Dalradian 18 successions and structures can be traced into those of the Northern 19 and Central Grampian Highlands with little difficulty. A generally 20 eastward-younging stratigraphical succession does seem to continue, 21 albeit with some attenuation and disruption, across the lineament 22 and elements of the structural history are common to both sides. 23 However, to the east of the lineament only tentative 24 stratigraphical and structural correlations can be made with higher 25 parts of the Dalradian succession elsewhere and this area seems to 26 have had, to some extent, a distinctly different sedimentological, 27 structural and metamorphic history. It is commonly referred to as 28 the ‗Buchan Block‘ and has been regarded by several authors as a 29 30 tectonically juxtaposed separate subterrane. Regional gravity and 31 magnetic anomalies, which show steep gradients coincident with the 32 Portsoy–Duchray Hill Lineament, suggest that there are fundamental 33 differences in the sub-Dalradian basement (Trewin and Rollin, 34 2002). The southern margin of the Buchan Block is difficult to 35 define either geologically or geophysically. It extends southwards 36 at least to the Deeside Lineament, where the geophysical anomalies 37 are subsumed by the plethora of large granitic plutons. To the 38 south of Deeside the lithologies and structures gradually merge 39 with those of the Highland Border region. 40 Early interpretations divided the succession in the Buchan Block 41 into a ‗Banff division‘, restricted to a ‗Banff Nappe‘, separated 42 by a slide from an underlying, more typical Dalradian sequence, 43 termed the ‗Keith division‘ (Read, 1923, 1955; Read and Farquhar, 44 1956). Although some authors have also suggested that parts of the 45 area are allochthonous (Sturt et al., 1977; Ramsay and Sturt, 46 1979), most current interpretations have attempted to correlate the 47 48 stratigraphical succession in broad terms (i.e. at subgroup level) 49 with Argyll and Southern Highland group successions farther to the 50 south-west (Harris and Pitcher, 1975; Ashworth, 1975; Harte, 1979; 51 Treagus and Roberts, 1981; Ashcroft et al., 1984; Fettes et al., 52 1991; Harris et al., 1994; Stephenson and Gould, 1995). 53 Several other lineaments and dislocations in the North-east 54 Grampian Highlands have been recognized as being of more than just 55 local significance. Some have contributed significantly to debates 56 over the timing of Caledonian and earlier deformation, and their 57 associated intrusions have provided material for precise 58 radiometric age determinations that now define magmatic events both 59 at 600 Ma and 470 Ma. 60 61 62 63 64 65 1 2 3 4 1.1 Stratigraphy 5 6 7 1.1.1 Pitlochry–Blair Atholl area to Deeside 8 9 In this area, most of the Appin and Argyll group succession can be 10 correlated precisely with that of the adjoining Central Grampian 11 12 Highlands, certainly down to formation level and in many cases also 13 down to member level (Figure 2). However, facies changes do occur, 14 most notably the disappearance of the Schiehallion Quartzite 15 between Glen Tilt and Glen Shee, partly resulting from structural 16 excision but most probably due to non deposition or a local 17 unconformity. The absence of this familiar marker, together with 18 its basal tillite sequence, does cause problems in that it results 19 in the juxtaposition of the Killiecrankie Schist (Easdale Subgroup) 20 upon the lithologically similar upper part of the Blair Atholl 21 Subgroup. As the Cairn Mairg Quartzite of the Central Grampian 22 Highlands is also absent from this area, correlations of some key 23 units between the lower part of the Blair Atholl Subgroup and the 24 distinctive graphitic pelites and calcareous schists in the upper 25 Easdale Subgroup are rather uncertain (Goodman et al., 1997; Crane 26 et al., 2002). This in turn makes it difficult to estimate the 27 magnitude and significance of ductile displacements on some high- 28 strain zones. Local names for stratigraphical units are a 29 30 particular problem owing to the large number of workers who have 31 worked here at different times, with varying success in attempts to 32 correlate with adjoining areas. Unfortunately, many of these names 33 have been adopted on BGS maps, the publication of which which has 34 spanned a significant time period. Only recently has an attempt 35 been made to rationalize the nomenclature in the BGS Lexicon of 36 named rock units and those names have been used in this special 37 issue wherever possible. 38 Throughout much of this area the junction of the Appin Group with 39 the underlying Grampian Group is coincident with a zone of high 40 strain that is a continuation of the Boundary Slide-zone from the 41 Central Grampian Highlands. However, at the Gilbert’s Bridge GCR 42 site in Glen Tilt there does appear to be a continuous 43 stratigraphical transition from the Struan Flags of the Grampian 44 Group into calcareous semipelite lithologies of the Glen Banvie 45 Formation that have been assigned to the Lochaber Subgroup. A 46 similar situation occurs south-west of Braemar at the Glen Ey Gorge 47 48 GCR site, where flaggy psammites of the Grampian Group are overlain 49 by highly strained pelites and semipelites of the Tom Anthon Mica 50 Schist Formation (Upton, 1986). 51 All the constituent formations of the Ballachulish Subgroup can be 52 traced from the Blair Atholl area (Smith and Harris, 1976) 53 north-eastwards to Braemar (Upton, 1986), where they can still be 54 matched almost bed for bed with those in the type areas of Lochaber 55 and Appin. The distinctive basal dolomitic metalimestone, the 56 graphitic pelites passing via a striped transition into the An 57 Socach Quartzite, and the topmost limestone and phyllite formation 58 with its crystalline white limestone and striped ‗tiger rock‘, can 59 all be found throughout this area (Figure 2). 60 61 62 63 64 65 1 2 3 4 The base of the Blair Atholl Subgroup is well marked throughout 5 the area by a change from a background lithology dominated by 6 semipelites and psammites to one in which dark schistose pelites 7 predominate. Thick units of dark bluish-grey graphitic 8 metalimestone in the lower part of the subgroup are a distinctive 9 feature and several have been quarried extensively. Higher parts 10 of the subgroup tend to be paler and more semipelitic. Although 11 12 some of the metalimestones are laterally persistent, some units 13 have been recognized only locally, and it is the broad 14 characteristics of the subgroup that enable it to be traced from 15 Blair Atholl through the Glen Shee area almost to Braemar (Bailey, 16 1925; Pantin, 1961; Smith and Harris, 1976; Upton, 1986; Goodman et 17 al., 1997; Crane et al., 2002). 18 The Schiehallion Quartzite is the only unit of the Islay Subgroup 19 represented in the Blair Atholl area. In the lower part, locally 20 developed conglomeratic beds contain scattered clasts of granite 21 and quartzite and are considered to be equivalent to the tillites 22 of the Schiehallion Boulder Bed; dolomitic beds are also present 23 locally. The quartzite thins considerably north-eastwards and is 24 eventually excised by a slide in Glen Tilt. It re-appears, with 25 basal boulder beds, farther north-east in the Glen Shee area, where 26 it is termed the Creag Leacach Quartzite. This passes up through a 27 transition formation of interbedded quartzite and black graphitic 28 pelite into the graphitic Glas Maol Schist Formation, followed by 29 30 calcareous semipelite and schistose calcsilicate rocks (the Glen 31 Girnock Calcareous Formation), a sequence typical of the upper part 32 of the Easdale Subgroup in the Central Grampian Highlands (i.e. Ben 33 Eagach Schist and Ben Lawers Schist equivalents). At Coire Loch 34 Kander, stratabound syn-sedimentary barium deposits, similar to 35 those at the Craig an Chanaich to Frenich Burn GCR site in the 36 Central Grampian Highlands and at the same stratigraphical level, 37 occur in the Glas Maol Schist Formation. A 15 m-thick band of 38 barian quartzite contains sphalerite, galena and iron sulphides, 39 and bedded baryte-quartz rock, 4.5 m thick, has been proved by 40 drilling over a strike length of 700 m (Fortey et al., 1993). The 41 upper part of the Easdale Subgroup is dominated in places by basic 42 meta-igneous rocks. On Ben Vrackie, near Pitlochry, these have 43 been interpreted as intrusions, but in the Ballater area a sequence 44 of banded amphibolites at the top of the Easdale Subgroup 45 represents basic lavas in an equivalent position to the tuffaceous 46 Farragon Beds (Goodman and Winchester, 1993; Fettes et al., 2011) 47 48 (Figure 2). 49 In the Pitlochry area, the Crinan Subgroup is represented by the 50 dominantly semipelitic Ben Lui Schist Formation, as is the case 51 throughout the Central Grampian Highlands. However, 52 north-eastwards from Ben Vrackie, the metamorphic grade of the Ben 53 Lui Schist increases. The unit becomes migmatitic with abundant 54 concordant quartzofeldspathic segregations and thick pegmatite 55 veins and pods. In the Glen Shee area and through to the 56 headwaters of Glen Isla and Glen Clova, the Ben Lui Schist 57 Formation (known locally as the Caenlochan Schist) grades into 58 lit-par-lit migmatites that constitute the Duchray Hill Gneiss 59 Member (Williamson, 1935). The equivalent Queen‘s Hill Formation 60 can be traced north-eastwards into Glen Muick and to the eponymous 61 62 63 64 65 1 2 3 4 Queen‘s Hill, near Aboyne (Read, 1927, 1928). The dominant 5 migmatitic semipelites and pelites, well seen at the Balnacraig, 6 Dinnet GCR site, are interbedded with psammites, rare quartzites 7 and thin bands of calcsilicate rock. The formation also includes 8 numerous bands of gneissose amphibolite, implying that it was a 9 preferred horizon for the intrusion of basic sheets, which dominate 10 the Cairn Leuchan to Pannanich Hill GCR site. Similar migmatitic 11 12 and gneissose lithologies occur in lower Deeside between the Hill 13 of Fare and Mount Battock granites. 14 The Tayvallich Subgroup is represented by the Loch Tay Limestone 15 Formation, which can be traced from Pitlochry to Glen Doll. 16 Farther to the north-north-east, calcareous semipelites around the 17 head of Glen Mark pass into ribbed calcsilicate rocks and 18 metalimestones of the Water of Tanar Limestone Formation. In 19 middle Deeside, this becomes the Deeside Limestone Formation (Read 20 1927, 1928), which consists mainly of calcsilicate rocks with 21 calcareous psammite, amphbolite and thin layers of impure 22 metalimestone. These calcareous rocks are overlain by a diverse 23 but dominantly psammitic unit, the Tarfside Psammite Formation, 24 consisting of quartzites, psammites, semipelites and pelites but 25 with locally abundant calcsilicate and amphibolite bands (Harte, 26 1979). Parts of this unit are gneissose. 27 28 29 1.1.2 Deeside to the north coast, west of the 30 Portsoy–Duchray Hill Lineament 31 32 To the north of the Cairngorm, Glen Gairn, Ballater and Mount 33 Battock granitic plutons that mark the enigmatic ‘Deeside Lineament’, 34 many elements of the Appin Group and lower Argyll Group 35 stratigraphy of the Central Grampian Highlands can still be 36 recognized. This is particularly true of the Lochaber and 37 Ballachulish subgroups, in which correlations are possible at 38 formation level. The Blair Atholl, Islay and Easdale subgroups can 39 also be defined with confidence from their overall lithological 40 characteristics. The succession is terminated to the east by the 41 shear-zone that marks the Portsoy Lineament (Fettes et al., 1991). 42 The continuous section along the north coast that forms the western 43 part of the Cullen to Troup Head GCR site has been well known since 44 the work of H.H. Read (1923, 1936), but its connection with the 45 established succession of Perthshire was not known until the 46 detailed resurvey of the North-east Grampian Highlands by the 47 48 British Geological Survey in the 1980s and 1990s. 49 A poorly defined association of micaceous psammites with thin 50 lenticular quartzite units overlies Grampian Group psammites 51 conformably south of Tomintoul. However, farther north, psammites 52 grade upwards into slaty and calcareous semipelites, which 53 represent a thin development of the Lochaber Subgroup, well seen at 54 the Bridge of Brown GCR site. Close to the contact the psammites 55 and semipelites are commonly flaggy and highly strained, recalling 56 the features seen along the Boundary Slide-zone farther to the 57 south-west. 58 Still farther north, the Lochaber Subgroup thickens markedly 59 between Dufftown and the north coast (Read, 1923, 1936; Peacock et 60 al., 1968). Here a thick sequence of flaggy, micaceous psammites 61 62 63 64 65 1 2 3 4 and semipelites, the Findlater Flag Formation, forms the lower part 5 of the subgroup, whereas the upper part contains calcareous 6 lithologies, recalling the division in the Lochaber type area. The 7 calcareous rocks are lithologically variable but are characterized 8 locally by abundant tremolitic amphibole (Stephenson, 1993). They 9 are dominated by thinly banded grey, cream and pale-green 10 calcareous psammites and semipelites termed the Pitlurg Calcareous 11 12 Flag Formation, which grades laterally into the Cairnfield 13 Calcareous Flag Formation towards the coast. Beds and lenses of 14 metalimestone occur locally in the upper parts of this formation, 15 presaging the limestone development in the Ballachulish Subgroup. 16 The Ballachulish Subgroup is well developed and can be traced 17 northwards as far as the Keith area. The Mortlach Graphitic Schist 18 Formation is several hundred metres thick in Glen Livet but thins 19 locally to 5 or 6 m. A dark metalimestone, the Dufftown Limestone 20 Member, is commonly present at or near the base of the formation, 21 and other metalimestones occur in the lower part, notably in Glen 22 Rinnes. The formation appears to thicken markedly on the north- 23 east side of the NW-trending Rothes Fault and, although much of 24 this thickening is probably due to fold repetition, the fault could 25 coincide with an earlier synsedimentary structure or lineament 26 (Fettes et al., 1986). At the base of the Corryhabbie Quartzite 27 Formation there is a transitional unit of interlaminated pelite and 28 psammite, which is thick in the south but is reduced to a few 29 30 metres farther north, where the formation is made up of a lower 31 thick-bedded psammite unit, a clean, cross-bedded quartzite, and an 32 upper psammite unit. The succeeding Ailnack Phyllite and Limestone 33 Formation consists of phyllitic semipelite, with several 34 distinctive thin white metalimestones, calcsilicate beds and one 35 more-persistent banded metalimestone member. The Dufftown 36 Limestone and Mortlach Graphitic Schist are particularly well seen 37 at the Auchindoun Castle GCR site and most of the succeeding units 38 are exposed at the Bridge of Avon GCR site. 39 Farther to the north-east, marked facies changes, probably 40 associated with NW-trending growth faults, and increased structural 41 complexity, make individual units difficult to trace so that 42 formations become ill defined. The graphitic character of the 43 lower part of the subgroup is locally much reduced and thick, 44 persistent metalimestones are absent. A condensed sequence of 45 metalimestone and graphite-schist is seen around Deskford, but in 46 boreholes on the coast at Sandend Bay over 300 m of kyanite-rich 47 48 schistose graphitic pelite has been proved. The pelite is overlain 49 directly by phyllitic semipelite and metalimestone with no 50 intervening quartzite. 51 The Blair Atholl Subgroup consists mainly of schistose 52 semipelites, which are locally pelitic, graphitic and calcareous. 53 A thick bluish grey metalimestone formation, the Inchrory 54 Limestone, occurs in its central part (see the Bridge of Avon GCR 55 site report) and minor metalimestones occur locally in the lower 56 part. Maximum development of the subgroup occurs in the upper 57 Donside–Braes of Glenlivet area. Farther north, around Edingight, 58 black graphitic pelites with staurolite are interbedded with thin 59 beds of blue-grey metalimestone. From there the metalimestones 60 thicken considerably northwards and become dominant in the Fordyce 61 62 63 64 65 1 2 3 4 Limestone Formation that constitutes the only part of the subgroup 5 exposed on the coast in the Cullen to Troup Head GCR site. 6 The lowest units of the Argyll Group can be traced intermittently 7 to the coast. The lower part of the Islay Subgroup consists of two 8 interdigitating and diachronous formations, both of which are 9 represented in the Kymah Burn GCR site. On Donside, semipelites 10 and pelites with thin metalimestones and metadolomites comprise the 11 12 Nochty Semipelite and Limestone Formation. These lithologies pass 13 laterally northwards and westwards into thinly interbedded, locally 14 graded, psammites, semipelites and minor pelites, which comprise 15 the Ladder Hills Formation. This formation is several kilometres 16 thick in its type area, but is absent or only thinly developed 17 elsewhere. Boulder beds, typically associated with thin 18 metadolomite beds, occur locally towards the top of the Ladder 19 Hills Formation. The best section of these boulder beds is found 20 in the Muckle Fergie Burn GCR site, south of Tomintoul, where a 21 metadolostone unit is succeeded by a 10 m-thick boulder bed 22 containing clasts of dolostone in its lower, calcareous part and of 23 granite above. Minor beds of basic metatuff also occur locally and 24 in the Muckle Fergie Burn basaltic pillow lavas have been 25 recognized a short distance below the boulder beds. In some areas, 26 for example in upper Donside near Corgarff, boulder beds occur 27 within the lower units of the overlying Kymah Quartzite Formation 28 and in the Kymah Burn GCR site thin basic lavas and tuff lenses are 29 30 found near the base of the quartzite. The quartzite varies 31 considerably in thickness, from only 10 m over fault-controlled 32 structural highs (e.g. Lecht–Cockbridge) to a more-typical 33 development of 300 to 500 m in adjacent basins. To the west of 34 Huntly the Islay Subgroup is cut out structurally by the Portsoy 35 Shear-zone. However, north of the River Isla, the Durn Hill 36 Quartzite is confidently assigned to this subgroup on account of 37 loose blocks of metadiamictite (boulder bed) that have been found 38 within the outcrop of the underlying Arnbath Psammite Formation in 39 several locations around Fordyce and Edingight (Spencer and 40 Pitcher, 1968). 41 A typical Easdale Subgroup sequence crops out from east of the 42 Glen Gairn Pluton to north of Glenbuchat. There, the Kymah 43 Quartzite is overlain by the Culchavie Striped Formation, a thick 44 sequence of striped semipelites and psammites with a distinctive 45 pebbly quartzite. These are succeeded in turn by the Glenbuchat 46 Graphitic Schist Formation, followed by calcareous semipelites and 47 48 minor psammites with metalimestone and calcsilicate beds that 49 constitute the Badenyon Schist and Limestone Formation. Graphitic 50 pelites and semipelites are also present around the headwaters of 51 the River Don and in the eastern part of the Muckle Fergie Burn GCR 52 site, where they include the Delnadamph Volcanic Member, consisting 53 of basic pillow lavas and volcaniclastic beds. 54 To the north of these outcrops, the Easdale Subgroup is cut out 55 completely by the Portsoy Shear-zone. However, on the coast, in 56 the Cullen to Troup Head GCR site, a thin development of the Durn 57 Hill Quartzite is succeeded eastwards by a sequence of graphitic 58 pelites and semipelites with metalimestone and quartzite in its 59 upper part, which are intruded by gabbroic and ultramafic rocks 60 within the Portsoy Shear-zone. This Easdale Subgroup sequence, 61 62 63 64 65 1 2 3 4 consisting of the Castle Point Pelite and Portsoy Limestone 5 formations, is highly strained and very much attenuated as it 6 effectively lies within the shear-zone. 7 8 1.1.3 The Buchan Block 9 10 Within and immediately to the east of the Portsoy Shear-zone are a 11 12 number of formations, which are commonly bounded by ductile shears 13 and are interspersed with mafic and ultramafic intrusive rocks of 14 the 470 Ma North-east Grampians Basic Suite. They are difficult to 15 correlate with any established successions, but their overall 16 lithological character, consisting largely of semipelites and 17 graphitic pelites with gritty psammites and a few minor 18 metalimestones, is typical of the Argyll Group (Fettes et al., 19 1991). On published maps they are mostly designated as ‗Argyll 20 Group, subgroup unassigned‘, and they probably belong in the 21 Easdale, Crinan and Tayvallich subgroups. 22 Most notably, in the Cabrach area, a turbiditic sequence of black 23 pelites, semipelites, psammites, pebbly psammites and metavolcanic 24 rocks is termed the Blackwater Formation. The metavolcanic rocks 25 dominate the lower part of the formation, which is well exposed in 26 the Black Water GCR site. They are composed of aphyric, 27 pyroxene-phyric and pillowed tholeiitic metabasalts and both 28 massive and autobrecciated metapicrite lavas (MacGregor and 29 30 Roberts, 1963; Macdonald et al., 2005). Since the formation 31 appears to pass upwards into Southern Highland Group lithologies, 32 the volcanic rocks have been tentatively correlated with the 600 Ma 33 Tayvallich Volcanic Formation of the South-west Grampian Highlands, 34 with which they share some geochemical characteristics such as 35 unusually strong Fe and Ti enrichment and some evidence for crustal 36 contamination (Fettes et al. 2011). 37 The higher parts of the Argyll Group form a broad horseshoe 38 outcrop of generally gneissose semipelitic rocks around the Turriff 39 Syncline, from mid-Donside to Fraserburgh and in a narrower zone 40 from Huntly to Portsoy. Within these poorly exposed areas, thick, 41 mixed sequences of semipelite, psammite and pelite show little 42 mappable variation and no consistent detailed stratigraphy has been 43 established. The metamorphic grade is generally high and most of 44 the rocks are gneissose with local migmatization. The gneissose 45 and migmatitic textures clearly transgress primary lithological 46 boundaries but, by analogy with the development of the Queen‘s Hill 47 48 and Duchray Hill gneisses to the south, they have been assigned to 49 the Crinan Subgroup (Read, 1955; Harris and Pitcher, 1975; Harris 50 et al., 1994; Stephenson and Gould, 1995). However, some probably 51 belong to the Tayvallich Subgroup and it is possible that minor 52 units of the Easdale Subgroup are included in some areas (e.g. near 53 Portsoy). Hornfelsing and partial melting have further complicated 54 relationships close to the major basic intrusions. 55 In mid-Donside, between the eastern margin of the Morven–Cabrach 56 Intrusion and the Tillyfourie area, lies the Craigievar Formation, 57 which consists mainly of finely interlayered, schistose and 58 gneissose psammites and pelites. Major developments of pelitic 59 gneiss, concordant amphibolite and thin developments of 60 metalimestone and calcsilicate-bearing rock occur locally. East 61 62 63 64 65 1 2 3 4 and north-east of the Bennachie Granite Pluton, equivalent 5 Crinan/Tayvallich subgroup rocks are known as the Aberdeen 6 Formation (Munro, 1986). The dominant lithologies are less pelitic 7 than those to the west, consisting mainly of psammites and 8 semipelites and characterized by small-scale compositional banding. 9 The gneisses of the Ellon Formation crop out around the lower 10 Ythan valley (Read, 1952; Munro, 1986). They are derived mainly 11 12 from semipelitic and psammitic metasedimentary rocks, although 13 amphibolites are abundant locally. Calcsilicate rocks are rare. 14 The gneisses are distinguished from those of the Aberdeen Formation 15 by their lack of regular lithological banding, their poor fissility 16 and a foliated, streaky appearance. Bodies of migmatitic ‗granite‘ 17 are widespread. The boundary with the Aberdeen Formation is 18 transitional in places but elsewhere it is marked by shear-zones. 19 To the north and east of Ellon, the Ellon gneisses grade into the 20 structurally overlying Stuartfield ‗division‘ of semipelites, 21 pelites, psammites and metagreywackes. The upper part of this 22 ‗division‘ has a more coherent stratigraphy and is termed the 23 Strichen Formation. To the north this may be further divided into 24 a lower part containing massive channel quartzites up to 500 m 25 thick (e.g. the Mormond Hill Quartzite Member) and an upper part 26 containing calcareous beds; the latter have been taken to indicate 27 that the Strichen Formation spans the boundary between the Crinan 28 and Tayvallich subgroups (Kneller, 1988). 29 30 To the north of Peterhead is the Inzie Head Gneiss Formation (see 31 Read and Farquhar, 1956). This mixed assemblage of rocks, exposed 32 in the Cairnbulg to St Combs GCR site, has a general migmatitic 33 appearance due to more-homogeneous granitic gneisses alternating 34 with schollen and schlieren gneisses. The schollen show a wide 35 range of metasedimentary lithologies, including calcsilicate rock 36 and psammite, and can be discerned locally in trails resembling 37 dismembered sedimentary units. More-coherent bands of amphibolite, 38 psammite and calcareous schist with impure metacarbonate rock have 39 been mapped in places. On the west side of the Turriff Syncline, 40 the Cowhythe Psammite Formation crops out along the coast east of 41 Portsoy (see the Cullen to Troup Head GCR site report), and extends 42 southwards to near Huntly (Read, 1923). It is composed essentially 43 of schistose psammite and semipelite with rare metalimestone and 44 pelite beds. Streaky lit-par-lit migmatites and feldspathized 45 rocks occur, particularly in the semipelitic units, but for the 46 most part the original compositional banding can still be 47 48 discerned. 49 Most of the dominantly gneissose units described above probably 50 include some Tayvallich Subgroup rocks, as indicated by the 51 presence of calcsilicate and metalimestone beds. Notable examples 52 are the calcareous parts of the Strichen Formation and its lateral 53 equivalent, the Kinnairds Head Formation, which is well exposed on 54 the north coast in the Fraserburgh to Rosehearty GCR site. 55 Although metalimestone beds up to 20 m thick do occur in the 56 Strichen Formation, calcareous units are restricted in general to 57 thin-banded calcsilicate beds in an overall sequence of pelite, 58 semipelite and psammite. 59 On the west side of the Turriff Syncline, the Tayvallich Subgroup 60 comprises a 1200 m-thick sequence of semipelite, calcsilicate rock 61 62 63 64 65 1 2 3 4 and metalimestone, termed the Boyne Limestone Formation (Read, 5 1923; Sutton and Watson, 1955). It includes the Boyne Castle 6 Limestone Member, a thickly bedded but finely banded metalimestone, 7 some 200 m thick (Figure 2) (see the Cullen to Troup Head GCR site 8 report). The metalimestones can only be traced inland for some 9 2.5 km through poorly exposed ground. 10 The Southern Highland Group occupies the broad core of the Turriff 11 12 Syncline, represented by the Fraserburgh to Roseharty GCR site and 13 the eastern part of the Cullen to Troup Head GCR site, and a small 14 outlier on the east coast around the Collieston to Whinnyfold GCR 15 site. 16 In the Turriff Syncline a sedimentological transition from the 17 Argyll Group into the Southern Highland Group is well seen. On its 18 western limb, the base of the Southern Highland Group is drawn in 19 the coast section at the base of the first gritty psammite that 20 marks the change from lagoonal deposition of calcareous silts and 21 muds to turbiditic sedimentation. The overlying succession 22 consists of some 2000 m of psammite, with subordinate semipelite 23 and pelite, referred to as the Whitehills Grit Formation. On the 24 eastern limb a similiar transition is observed from the calcareous 25 successions of the Kinnairds Head Formation and the Strichen 26 Formation into the non-calcareous psammites and pelitic lithologies 27 of the Rosehearty Formation and Methlick Formation (Read and 28 Farquhar, 1956). In the core of the syncline the Southern Highland 29 30 Group is represented by the Macduff Formation (1700 m), a finer 31 grained, more-distal turbidite facies with slump deposits, clean 32 channel sandstones and subsidiary greywackes (Sutton and Watson, 33 1955). A more-persistent semipelitic facies to the south-west has 34 been termed the Clashindarroch Formation, and this unit has been 35 quarried extensively in the past for roofing slate in an E–W- 36 trending belt to the north of the Insch and Bogancloch intrusions. 37 The closure of the Turriff Syncline can be traced to the south of 38 the Insch Intrusion, in the Correen Hills. There, the Southern 39 Highland Group is represented entirely by the Suie Hill Formation, 40 which consists dominantly of semipelite and gritty psammite with 41 prominent pelite units. The base is taken at a magnetite-bearing 42 schistose pelite, which forms a regional magnetic anomaly. Similar 43 magnetic units occur on the western limb of the syncline and 44 elsewhere in the basal part of the group; the influx of detrital 45 magnetite could indicate a change in provenance caused by the 46 unroofing of a new source or a mafic volcanic input. 47 48 On the east coast, low-grade turbiditic rocks occur in an 49 eastward-younging sequence, represented almost in its entirety by 50 the Collieston to Whinnyfold GCR site but traceable for only a few 51 kilometres inland (Read and Farquhar, 1956; Munro, 1986). These 52 rocks, termed the Collieston Formation, are assigned to the 53 Southern Highland Group and form a predominantly psammitic graded 54 sequence with characteristic ‗knotted‗ pelites containing 55 andalusite and cordierite. Contacts with adjoining units are not 56 exposed but south of Collieston lenses and beds of calcsilicate 57 rock are common and thin impure metalimestones also occur, possibly 58 indicating a transition downwards into the Argyll Group. 59 Boulders and pebbles of igneous and metamorphic rocks, some of 60 extrabasinal origin, occur in the higher exposed part of the 61 62 63 64 65 1 2 3 4 Macduff Formation in the coastal section at Macduff (see the Cullen 5 to Troup Head GCR site report). These deposits have been 6 interpreted as the products of ice-rafting or as debris flows 7 linked to marine tills (Sutton and Watson, 1954; Hambrey and 8 Waddams, 1981; Stoker et al., 1999). Some poorly preserved 9 microfossils have also been found in the adjacent rocks and 10 correlations with various glacial periods, some as young as 11 12 Ordovician, have been suggested (see Stephenson et al., 2013a). 13 14 1.2 Structure 15 16 17 1.2.1 Major dislocations 18 19 The development of the concept of a ‘Boundary Slide’ separating the 20 Grampian Group from higher stratigraphical units of the Dalradian 21 throughout much of the Grampian Highlands has been fully discussed 22 by Stephenson et al. (2013a). The Boundary Slide can be traced 23 north-eastwards from Glen Tilt, where it is well exposed in a 24 continuous section at the historic Gilbert’s Bridge GCR site, 25 through the Glen Ey Gorge GCR site in upper Deeside to the eastern 26 end of the Cairngorm Granite Pluton (Upton, 1986). However, there 27 its overall effect might be much reduced. Farther north, zones of 28 high strain, accompanied locally by slides, are common at or below 29 30 the Grampian– Appin group transition, e.g. around Strath Avon, at 31 the Bridge of Brown GCR site, and in the upper part of Glen Rinnes. 32 Between Glen Rinnes and the north coast, at the western end of the 33 Cullen to Troup Head GCR site, the Grampian–Appin group boundary 34 appears to represent a relatively undisturbed, rapid 35 stratigraphical passage. 36 In this northern part of the region, ductile dislocations occur at 37 both higher and lower stratigraphical levels than the Grampian– 38 Appin group boundary, although it is unclear whether any of these 39 relate specifically to the Boundary Slide. The zones of shearing 40 in Glen Rinnes can be projected north-eastwards towards a major NE- 41 to NNE-trending shear-zone that passes through Keith and can be 42 traced for some 30 km to reach the coast between Sandend and 43 Portsoy in the Cullen to Troup Head GCR site. The effect of this 44 Keith Shear-zone upon the succession is difficult to determine 45 owing to poor exposure and uncertainties about the stratigraphical 46 affinities of some of the units involved but it appears to have 47 48 excised parts of the Ballachulish Subgroup in places. The shear- 49 zone consists of multiple branches, each dipping at a low to 50 moderate angle towards the south-east quadrant and commonly showing 51 a very strong down-dip stretching lineation. Shear-sense 52 indicators suggest a thrust (top to north-west) sense of movement. 53 Between the branches are several pods and lenses of deformed 54 muscovite-biotite granite, and zircons from two separate lenses of 55 this Keith–Portsoy Granite have yielded a precise U-Pb intrusion 56 age of c. 600 Ma (Barreiro, 1998). Although the granite pods and 57 adjacent metasomatic country rocks were deformed and metamorphosed 58 during the Grampian Event, which was undoubtedly a time of major 59 movement on the shear-zone, the sites of the individual shears were 60 clearly a locus for the intrusion of granite sheets. Hence they 61 62 63 64 65 1 2 3 4 must follow earlier lineaments that were in existence at around 600 5 Ma, possibly soon after sedimentation as the youngest rocks 6 affected are lower Islay Subgroup. 7 The best known and most extensively studied of the pre-Caledonian 8 c. 600 Ma intrusions is the Ben Vuirich Granite, between Pitlochry 9 and Glen Shee, which has yielded much vital information about the 10 timing of Caledonian deformation and metamorphism. Determinations 11 12 of the age of the intrusion and interpretations of its structural 13 setting have changed considerably since its significance was first 14 recognized by Bradbury et al. (1976) and these make the Ben Vuirich 15 GCR site one of the most significant in this special issue. The 16 granite was intruded into Blair Atholl Subgroup strata at around 17 590 Ma and hence provides a minimum age for Appin Group deposition 18 (Rogers et al., 1989; Pidgeon and Compston, 1992). It is now 19 considered to have been deformed by the D2 phase of deformation of 20 the Grampian Event (Tanner, 1996). The age and significance of an 21 earlier, D1 fabric affecting the granite is still a matter of 22 debate (e.g. Dempster et al., 2002; Tanner, 1996; Tanner et al., 23 2006). The intrusion does crop out between major slides to the 24 north-west and south-east, but there is no evidence that these are 25 earlier structures that might have controlled granite emplacement. 26 However, smaller nearby bodies of foliated granite, at Glach Ghlas 27 in Glen Tilt, and near Fealar Lodge, are either located between 28 major slides or within ductile shear-zones. 29 30 A number of major zones of shearing and dislocation occur on the 31 western margin of the Buchan Block and each of these has been used 32 at some time to define its limit. Thrust-related fabrics at the 33 western margin of the Cowhythe Psammite Formation were attributed 34 by Elles (1931) to a Portsoy Thrust, which is now regarded as the 35 eastern limit of the 1 km-wide Portsoy Shear-zone, described in 36 detail in the Cullen to Troup Head GCR site report. Earlier fold 37 axes and lineaments have been rotated so that they plunge down-dip 38 adjacent to and within the zone and a down-dip stretching lineation 39 is present locally. Highly sheared mafic and ultramafic igneous 40 rocks occupy the centre of the zone and cross-cutting, but 41 lineated, sheet-like granite bodies are present near the margins. 42 This zone can be traced inland to the south-south-west as a 43 narrower zone of dislocation that forms the western boundaries of 44 the Huntly-Knock and Morven-Cabrach mafic-ultramafic intrusions 45 (Munro and Gallagher, 1984; Ashcroft et al., 1984) and defines the 46 northern part of the Portsoy–Duchray Hill Lineament (Fettes et al., 47 48 1986, Goodman 1994). Major stratigraphical and structural 49 discontinuities occur across the shear-zone and marked differences 50 in metamorphic history on opposite sides indicate major westward 51 overthrusting during the regional D3 event (Baker, 1987; 52 Beddoe-Stephens, 1990). The margins of major syn-D3 mafic– 53 ultramafic intrusions of the North-east Grampian Basic Suite are 54 severely affected by this and by other related shear-zones, and 55 their aureoles have been displaced by several kilometres in places, 56 suggesting significant lateral movement (Ashcroft et al., 1984). 57 Kneller and Leslie (1984) demonstrated that the shearing occurred 58 whilst the adjacent rocks were at or close to their peak 59 metamorphic conditions. Farther south, the Coyles of Muick Shear- 60 zone, to the west of the Cairn Leuchan to Pannanich Hill GCR site, 61 62 63 64 65 1 2 3 4 lies on the same lineament, although there the discontinuities are 5 less marked (Goodman, 1994). 6 On the eastern edge of the Cowhythe Psammite Formation outcrop is 7 another zone of highly deformed rocks with some thin mylonites. 8 This zone marks the position of the Boyne Line of Read (1955), 9 which was interpreted as a major slide underlying his proposed 10 allochthonous Banff Nappe. In Read‘s model, movement on the Boyne 11 12 Line was held responsible for the excision of Tayvallich Subgroup 13 calcareous lithologies, which are absent over much of the 14 North-east Grampian Highlands, apart from the Boyne Limestone 15 Formation, which is seen only in the coast section. 16 Some structural and Rb-Sr geochronological evidence has been 17 interpreted to infer that the Cowhythe Psammite Formation, along 18 with all the other gneissose units of the North-east Grampian 19 Highlands, represents a pre-Caledonian Neoproterozoic basement 20 gneiss complex (Sturt et al., 1977). Ramsay and Sturt (1979) 21 suggested that all the rocks above the Portsoy Thrust constitute an 22 allochthonous block, and that this consists of a gneissose basement 23 separated from a Dalradian metasedimentary cover by a décollement 24 along Read‘s Boyne Line. However, subsequent detailed mapping and 25 advances in the reliability of radiometric dating techniques now 26 suggest that the gneissose parts of the succession can be explained 27 as part of the Dalradian stratigraphy, albeit heavily deformed, 28 thrust and metamorphosed during the mid Ordovician Grampian Event 29 30 (see Ashcroft et al., 1984; Stephenson and Gould, 1995). 31 32 1.2.2 Folding 33 34 Between the Tay Nappe and the Boundary Slide there is a progressive 35 change in dips from the flat-lying strata and pervasive S2 cleavage 36 of the Flat Belt, north-westwards into a 10 km-wide zone of steep 37 to vertical folded strata known as the Tummel Steep Belt (Bradbury 38 et al., 1979) (Stephenson et al., 2013a, fig. 7). The tight, 39 upright folds characteristic of the steep belt have been documented 40 in the Central Grampian Highlands east of the Loch Tay Fault by 41 Treagus (1999, 2000) and a similar structural pattern can be traced 42 north-eastwards into the Kirkmichael–Glen Shee area (Crane et al., 43 2002) and on towards Braemar, where it is known as the Cairnwell 44 Steep Belt (Upton, 1986). Throughout these areas, the steepening 45 has been interpreted as at least partly the result of late, D3 to 46 D4, ENE-trending flexuring and corrugation of pre-existing flat- 47 48 lying, recumbent, SE-facing D1 and D2 structures linked to the Tay 49 Nappe and its complementary underlying syncline, formerly referred 50 to in this area as the Kirkmichael Fold (Bailey, 1925; Read, 1935, 51 1955). Tight F3 folds are commonly co-axial with the earlier folds 52 and hence the F1, F2 and F3 fold closures can be difficult to 53 distinguish. In some areas, large-scale F2 and F3 folds trend 54 north-west, most notably in the complex 5 km-wide NW-trending Carn 55 Dallaig Transfer Zone that effectively links the Tummel Steep Belt 56 and the offset Cairnwell Steep Belt (Crane et al., 2002, fig. 19). 57 This transfer zone has a marked effect upon the outcrop pattern in 58 the Gleann Fearnach area, but as the F2 fold axes are not re- 59 orientated within it, Crane et al. interpreted it as a steep 60 61 62 63 64 65 1 2 3 4 transpressive D2 structure, analogous to a lateral ramp in thrust 5 terrains. 6 To the north of the Deeside Lineament and west of the Portsoy– 7 Duchray Hill Lineament, Appin and Argyll group rocks are disposed 8 in a series of large-scale NW- to SW-facing early tight folds, 9 which can be traced downwards into the underlying Grampian Group. A 10 related fine-scale penetrative cleavage (S2) is also developed. 11 12 Later folds, which fold the S2 fabric and post-date the primary 13 metamorphic assemblages, are commonly co-axial with the early 14 folds. The related S3 penetrative cleavage, typically a finely 15 spaced or a tight crenulation, is best developed in the more- 16 pelitic lithologies (well seen in the Auchindoun Castle GCR site). 17 These F3 folds are typically close to tight and upright to NW- 18 facing. Their axes trend north or north-east and they exert a 19 strong control on the outcrop pattern both locally and regionally, 20 as is well demonstrated by the Ardonald Fold in the Dufftown area. 21 Post-D3 minor chevron folds and kink bands are widely developed but 22 usually only local in extent. In part they are related to late 23 uplift, faulting and basement block movement. 24 Within the main part of the Buchan Block, along the north coast 25 that forms the eastern part of the Cullen to Troup Head GCR site 26 and the Fraserburgh to Rosehearty GCR site, the rocks of the 27 Macduff Formation exhibit locally complex open to tight upright F1 28 folding. A related S1 spaced cleavage, formed by pressure 29 30 solution, is well developed in the psammites and a slaty cleavage 31 occurs in the intervening pelites. The folding, is responsible for 32 the generally steep bedding dips over much of the section, although 33 regionally the dip of the overall stratigraphy is relatively 34 shallow (Figure 3a). In fact, the outcrop pattern is controlled 35 by two late (D3 or D4), open, broad, upright folds, the Turriff 36 Syncline and the Buchan Anticline, whose axes plunge gently to the 37 north-north-east. Read (1955) considered that the Dalradian 38 succession, which is generally the right way up across the section, 39 constitutes the upper limb of a major early SE-facing recumbent 40 anticline which he termed the Banff Nappe (Figure 3b). The overall 41 ‗nappe‘ has similarities to the Tay Nappe of the Highland Border 42 region and some authors have linked the two structures (e.g. 43 Treagus and Roberts, 1981; Ashcroft et al., 1984). In Read‘s 44 model, the high-grade migmatitic gneisses seen in the Cairnbulg to 45 St Combs GCR site form the core of the nappe and are exposed in the 46 hinge-zone of the later Buchan Anticline (Read and Farquhar, 1956). 47 48 To the west of Banff the beds are subvertical and form the steep 49 limb of a monoform, regarded as a major early (F1) fold closure by 50 Sutton and Watson (1956) who named it the Boyndie Syncline. 51 Subsequent workers have regarded this structure as a later (F3) 52 structure, devaluing its regional importance (Johnson and Stewart, 53 1960; Johnson, 1962; Fettes, 1970), although Treagus and Roberts 54 (1981) also assigned it to D1. 55 Recumbent, tight to isoclinal, east-facing F1 folds occur on the 56 east coast and are well exposed at the Collieston to Whinnyfold GCR 57 site. The fold geometry is in marked contrast to the north coast 58 section, where bedding is generally steep within the shallow 59 Turriff Syncline; here the generally flat-lying beds collectively 60 define steeply dipping overall stratigraphical boundaries (Figure 61 62 63 64 65 1 2 3 4 40), a point that was highlighted by Read and Farquhar (1956). The 5 beds are regionally inverted and the folds and cleavages face to 6 the east. A major early fold closure must occur between this 7 section and the coast sections around Fraserburgh, where the 8 succession is the right way up. The axial surface of this fold 9 cannot readily be traced; it lies in a poorly exposed complex 10 sheared zone between Ellon and Inverallochy that is characterized 11 12 by the presence of several mafic intrusions and high-grade 13 metamorphism. It was regarded as the hinge-zone of the Banff Nappe 14 by Read (1955) and Read and Farquhar (1956) and some later workers 15 have regarded it as equivalent, at least in part, to the Tay Nappe 16 (Stephenson et al., 2013a, fig. 7). Such a correlation must however 17 remain highly speculative given the considerable distance between 18 the traces of the structures and the intervention of the Deeside 19 Lineament. 20 21 1.3 Metamorphism 22 23 The North-east Grampian Highlands include an area of Buchan 24 metamorphism, characterized by low P/T, an area of typical 25 Barrovian intermediate-P/T metamorphism and a transitional zone 26 between the two focussed on the Portsoy–Duchray Hill Lineament. 27 This pattern can be attributed to high heat flow in the Buchan 28 area, falling off to the west and south. In general, the 29 30 metamorphic grade increases with structural and 31 lithostratigraphical depth, the lowest grade rocks occurring in the 32 core of the Turriff Syncline (Stephenson et al., 2013a, fig. 12). 33 The highest grade rocks are associated with late-metamorphic 34 intrusions (see below). The porphyroblast growth was broadly 35 synchronous across the region and occurred from syn-D2 to syn- to 36 post-D3 (Johnson, 1962, 1963; Crane et al., 2002; Strachan et al., 37 2002). 38 In the area west of the Portsoy–Duchray Hill Lineament, 39 metamorphic mineral assemblages are characteristic of the epidote- 40 amphibolite facies in the south but mostly fall within the lower 41 amphibolite facies. Mineral assemblages in pelitic lithologies are 42 typical of Barrovian zones (biotite → garnet → staurolite → 43 kyanite → sillimanite). In general, progressive increases in 44 pressure are assumed to have taken place during the main phases of 45 deformation, along a simple curve on a pressure-temperature plot, 46 to reach a metamorphic peak in D3. However, in the Tummel Steep 47 48 Belt the Barrovian zones are poorly developed and Dempster and 49 Harte (1986) documented a significant post-D3 increase in pressure, 50 with the replacement of chloritoid + biotite by garnet + chlorite, 51 as well as the localized growth of kyanite- and staurolite-bearing 52 assemblages. They ascribed the pressure increase, of c. 2–3 kbar, 53 to rotation and burial of originally flat-lying strata (i.e. in the 54 Flat Belt) associated with the development of the D3 steep belt. 55 In the north, there is good evidence that the line defining the 56 inversion of regional andalusite to kyanite lay to the west of the 57 Portsoy–Duchray Hill Lineament. In a well-defined zone up to 10 km 58 wide, immediately to the west of the lineament, original andalusite 59 is overprinted by later kyanite as a result of an increase in 60 pressure due to westward overthrusting during D3 (Chinner and 61 62 63 64 65 1 2 3 4 Heseltine, 1979; Baker, 1985; Beddoe-Stephens, 1990). Pseudomorphs 5 of kyanite after andalusite are well seen in the Auchindoun Castle 6 GCR site and immediately west of Portsoy in the Cullen to Troup 7 Head GCR site. 8 To the east of the Portsoy–Duchray Hill Lineament, the metamorphic 9 conditions were characterized by low pressures (2–4 kbar) and by a 10 high temperature gradient. This is the type area for the Buchan 11 12 zones (biotite → cordierite → andalusite → sillimanite → 13 sillimanite+K-feldspar). The lowest grade rocks (greenschist 14 facies) occur at the highest structural levels, in the core of the 15 Turriff Syncline, and the metamorphic grade increases structurally 16 downwards. The regional high geothermal gradients were closely 17 associated with the emplacement of large volumes of basic and 18 silicic magma, during or shortly after the peak of metamorphism at 19 c. 470 Ma (Fettes, 1970; Pankhurst, 1970; Ashworth, 1975, 1976). 20 Consequently the highest grade rocks are found in close contact 21 with these igneous bodies, with local pressures and temperatures of 22 over 8 kbar and c. 820°C, characteristic of granulite-facies 23 conditions (Baker and Droop, 1983; Baker, 1985). Granulite-facies 24 hornfelses and migmatites, characterized by garnet-orthopyroxene- 25 cordierite assemblages, are found in the roof-zones, inner aureoles 26 and in screens within the mafic and ultramafic intrusions, notably 27 the Huntly–Knock Pluton (Fletcher and Rice, 1989). The assemblages 28 imply that anatectic melting occurred at temperatures of 800–900°C 29 30 under pressures of 4.5–5 kbar (Droop and Charnley, 1985; Johnson et 31 al., 2001a; Droop et al., 2003). The link to the large early- 32 Ordovician granite intrusions is not clear. However, Johnson et 33 al. (2003) showed that granulite-facies metamorphism and 34 emplacement of mafic rocks into the host Dalradian metasedimentary 35 rocks is a feasible mechanism to have derived granitic melts, which 36 might have coalesced to form larger bodies such as the Strichen and 37 Aberdeen plutons (c.f. Oliver et al., 2008). The higher grade 38 rocks have been subjected to widespread migmatization, as is well 39 seen in the Cairn Leuchan to Pannanich Hill, the Balnacraig, Dinnet 40 and the Cairnbulg to St Combs GCR sites. 41 The background cause of metamorphism across the Grampian Highlands 42 was thermal relaxation of an overthickened crust. In addition, in 43 the North-east Grampian Highlands there was a very significant 44 advective heat input, leading to the low-P/T style of metamorphism 45 (e.g. Vorhies and Ague, 2011). How far the various syn- to late- 46 metamorphic igneous intrusions are the underlying cause of this 47 48 advective heat and how far they are an expression of it is 49 uncertain. 50 Initial workers believed that the thermal effects of the igneous 51 bodies were imposed on a regional metamorphic pattern. For 52 example, Chinner (1961, 1966) argued that sillimanite formed in 53 response to a thermal overprint on an original depth-controlled 54 metamorphism. However, Fettes (1970) demonstrated that the 55 ‗regional‘ porphyroblast growth was also directly related to the 56 effects of the igneous bodies. Harte and Hudson (1979) recognized 57 two phases of sillimanite growth, closely linked in time, one 58 ‗regional‘ and the other related to the basic intrusions. On this 59 basis, they delineated a ‗regional‘ sillimanite isograd within the 60 overall sillimanite zone (Stephenson et al., 2013a, fig. 12), 61 62 63 64 65 1 2 3 4 although they agreed that both phases might relate to a general 5 high heat input. However, in the high-grade areas of the south the 6 distinction is problematical and more-recent work, for example in 7 the area around the Cairn Leuchan to Pannanich Hill GCR site, has 8 regarded the sillimanite growth as the climax of a single prograde 9 event (Smith et al., 2002). 10 Current models for metamorphism clearly identify the advective 11 12 heat input (including that from the igneous bodies) as the primary 13 cause of the higher grades of metamorphism. Thus the growth of 14 sillimanite must be seen as the culmanating phase of the 15 progressive metamorphism in the areas of greatest heat input. As 16 such, any separation of growth phases might relate to a pulsed heat 17 input (Ague and Baxter, 2007; Vorhies and Ague, 2011). 18 The cause of the high heat input and associated magmatism in 19 Buchan is uncertain; it might relate to lithospheric stretching 20 and/or slab drop-off beneath the Buchan area immediately following 21 the main arc–continent collision that resulted in the Grampian 22 orogenic event (Kneller, 1985; Oliver, 2002). 23 24 1.3.1 The North-east Grampian Basic Suite 25 26 The large intrusions of mafic and ultramafic rock are entirely 27 confined to the Buchan Block and comprise the North-east Grampian 28 Basic Suite. They are described in some detail in the Caledonian 29 30 Igneous Rocks of Great Britain GCR volume (Stephenson et al., 1999) 31 but the relationships between the intrusions, the Buchan 32 metamorphism and the D3 deformation make the suite a vital time 33 marker for the peak of the Grampian Event. A number of U-Pb 34 mineral ages are now available from these intrusions, which imply 35 that basic and silicic magmatism was focussed in a short time 36 interval at around 470 Ma. It seems clear that the Grampian Event 37 in the North-east Grampian Highlands was well under way by 480–475 38 Ma and was completed by c. 460 Ma (Oliver et al., 2000; Oliver 39 2001; Carty 2001; Dempster et al., 2002). 40 41 2 BEN VUIRICH 42 (NO 008 686–NO 012 700 AND NN 990 703) 43 44 P.W.G. Tanner 45 46 47 48 2.1 Introduction 49 50 Ben Vuirich (2903 m) is a prominent feature in the Perthshire 51 landscape, 13 km north-east of Pitlochry. It provides some 52 excellent exposures of the deformed and foliated Ben Vuirich 53 Granite Intrusion, which was emplaced before the main deformation 54 and regional metamorphism that has affected the Appin Group country 55 rocks. Its importance has therefore long been recognized as a 56 target for radiometric dating to determine a minimum age for the 57 Dalradian succession. More recently, with its radiometric age well 58 established as 590 Ma, attention has turned to its field 59 relationships, which reveal crucial evidence for the relative 60 61 62 63 64 65 1 2 3 4 timing of structural events that have affected the granite and its 5 host rocks. 6 The Ben Vuirich Granite is a member of the small, but geologically 7 important, suite of pre-Caledonian intrusions that has commonly 8 been referred to as the ‗Older Granites‘ (e.g. Barrow et al., 9 1905). Such granites are uncommon but are scattered widely 10 throughout both the Northern Highlands and the Grampian Highlands. 11 12 Several smaller bodies occur near Ben Vuirich in Glen Tilt (e.g. 13 within the Gilbert’s Bridge GCR site). Recent radiometric dating 14 has added several new intrusions to this suite, which is now 15 recognized as representing a major 600 Ma magmatic event (Strachan 16 et al., 2002; Kinny et al., 2003b). Hence any deductions regarding 17 the structural relationships and timing of the Ben Vuirich 18 intrusion have profound implications for the tectonic history of 19 the whole Grampian Terrane. 20 The Ben Vuirich Granite Intrusion was first described by Barrow et 21 al. (1905) and is included in the British Geological Survey‘s 1:50 22 000 Sheet 55E (Pitlochry, 1981). Research into the structural 23 significance and age of the granite has aroused much controversy, 24 which was generated initially by a large difference between the 25 apparent ages given by early Rb-Sr (whole-rock) and U-Pb (zircon) 26 dating methods (Giletti et al., 1961; Bell, 1968; Pankhurst and 27 Pidgeon, 1976), and later by disagreement over structural 28 correlations. A precise U-Pb age on abraded zircons from the Ben 29 30 Vuirich Granite of 590 ± 2 Ma, obtained by Rogers et al. (1989), 31 combined with the existing structural interpretation of Bradbury et 32 al. (1976), was thought to show that the Dalradian block had been 33 affected by both a Neoproterozoic orogeny (D1 and D2) and an Early 34 Palaeozoic orogeny (D3 and D4). The 590 Ma age was subsequently 35 confirmed by Pidgeon and Compston (1992) using the SHRIMP ion- 36 microprobe. However, the structural interpretation was challenged 37 by Tanner and Leslie (1994) who concluded that: 38 39 (1) the foliation in the granite is correlated with S2 in the 40 country rocks, and 41 (2) the granite is pre-D2 in age and only post-dates a fabric which 42 is possibly of regional D1 age. 43 44 The current dispute is between those workers who consider that the 45 intrusion was most likely intruded, during a pre-orogenic rifting 46 episode, into previously undeformed sedimentary rocks at c. 590 Ma 47 48 (Soper and England, 1994; Tanner, 1996; Soper et al., 1999; Tanner 49 et al., 2006) and those who favour emplacement into a sequence that 50 had already been affected by a pre-590 Ma Neoproterozoic orogenic 51 event (Rogers et al., 1989; Bluck and Dempster, 1997; Dempster et 52 al., 2002). In short, whether or not a pre-Grampian orogenic event 53 has affected that part of the Dalradian Supergroup that lies below 54 the base of the Southern Highland Group (dated at c. 600 Ma; 55 Dempster et al., 2002), the origin of the earliest fabric in the 56 hornfels and xenoliths found at this GCR site is pivotal. Two 57 critical localities, one within hornfels of the contact metamorphic 58 aureole and the other featuring xenoliths within the marginal part 59 of the granite, preserve evidence of the undeformed nature of the 60 61 62 63 64 65 1 2 3 4 Dalradian host rocks immediately prior to the intrusion of the 5 granite. 6 7 2.2 Description 8 9 The Ben Vuirich Granite Intrusion is a small (6 × 2 km) sheet-like 10 body of pink or grey peraluminous monzogranite containing 11 12 megacrysts of oligoclase and K-feldspar, up to 7 mm in length, 13 together with quartz, muscovite, biotite, titanite, zircon and 14 almandine-grossular garnet. It cuts poorly exposed metacarbonate 15 rocks, quartzites, psammites, semipelites and pelites belonging to 16 the Blair Atholl Subgroup of the Appin Group (Crane et al., 2002). 17 The Dalradian country rocks have been affected by four phases of 18 deformation (D1–D4), the first three of which comprise the Grampian 19 Event. The granite was variably deformed during D2, resulting in 20 NE-trending zones of strongly foliated rock transecting the main 21 body of weakly foliated to granoblastic granite (Figure 4). The 22 intrusion lies within the Tummel Steep Belt and is contained 23 between two tectonic slides (ductile faults) of D2 age (Crane et 24 al., 2002) (Figure 4). The Killiecrankie or Glen Loch Slide is 25 inferred to follow the western margin of the intrusion, where 26 metacarbonate rocks belonging to the Sron nan Dias Pelite and 27 Limestone Formation strike towards this contact, and there is clear 28 excision of part of the stratigraphical sequence. The Creag 29 30 Uisge Slide to the east of the intrusion is seen locally as a 31 prominent zone of mylonitic rock, and its position farther south is 32 taken as the western margin of the Ben Lawers Schist. 33 The Ben Vuirich GCR site consists of two groups of exposures 34 (localities A and B on Figure 4). An exposure of hornfels found at 35 locality A on the north-west flank of the mountain is the only 36 place in the whole Dalradian outcrop where rocks from the contact 37 metamorphic aureole of one of the ‗Older Granites‘ are exposed. At 38 this locality, spotted hornfels have developed in finely banded 39 semipelite typical of the Tulaichean Schist Formation (Crane et 40 al., 2002, plate 4). At locality B on the north-east side of the 41 body, xenoliths of locally-derived quartzose psammite, caught up in 42 the granite magma before it was fully crystalline, preserve the 43 lithological layering but show no sign of pre-intrusion minor 44 folding. There, the granite locally cuts across bedding in the 45 Tulaichean Schist Formation. 46 The main feature of the GCR site is the hornfels that occurs in 47 48 metre-scale exposures and small patches of scree on the north-west 49 side of the hill. The early workers reported hornfels-like rocks 50 (Barrow et al., 1905; Pantin, 1961) but it was not until 1990 that 51 the spotted hornfels was discovered at locality A, 750 m west- 52 north-west of the summit of Ben Vuirich (Tanner and Leslie, 1994). 53 Examination of the material in the screes shows that the hornfels 54 grades from finely laminated, non-spotted rock to coarser grained, 55 spotted hornfels that originally contained andalusite (of the 56 chiastolite variety) and cordierite. The andalusite-bearing types 57 are inferred to have come from the innermost parts of the aureole. 58 Porphyroblasts of andalusite are now pseudomorphed by feathery 59 intergrowths of kyanite and are identical to those found in the 60 contact metamorphic aureole of the Carn Chuinneag intrusion, an 61 62 63 64 65 1 2 3 4 ‗Older Granite‘ in the Moine Supergroup rocks of Sutherland 5 (Tanner, 1996). The largest and most abundant spots were 6 originally of cordierite; they reached 2 cm across and grew across 7 a pre-existing, fine-grained fabric. That fabric is still 8 preserved within the pseudomorphs after cordierite, although the 9 original mineral has been altered to an aggregate of minute flakes 10 of biotite and muscovite, with small grains of almandine garnet 11 12 0.4–0.8 mm across (Figure 5) and less-obvious kyanite needles. 13 Petrological studies have shown that the original overall contact 14 metamorphic assemblage, which contained cordierite ± andalusite, 15 has been overprinted and converted to an equilibrium assemblage of 16 muscovite + biotite + garnet + kyanite + plagioclase + quartz 17 during the D2 regional metamorphism (Tanner and Leslie, 1994; 18 Tanner, 1996). A scanning electron-microscope and electron- 19 microprobe study of the small regional metamorphic D2 garnets has 20 shown that they preserve an extremely unusual chemical zonation 21 with, for example, Ca increasing from the core to the rim of the 22 garnet (Ahmed-Said and Tanner, 2000). 23 At locality B, an irregular contact of the granite with quartzite 24 and quartzose psammite is well exposed locally, with apophyses of 25 granite cutting the country rock. Angular to sub-rounded xenoliths 26 of country rock are common locally in the marginal facies of the 27 granite; they commonly preserve a finely spaced alternation of 28 light and dark layers, which on microscope examination is seen to 29 30 be bedding with some mimetic growth of micas (Tanner and Leslie, 31 1994; Tanner, 1996; Tanner et al., 2006). Along the contact 32 farther to the south-west, schistose pelites containing garnet over 33 1 cm across are in direct contact with the granite. 34 35 2.3 Interpretation 36 37 Now that both the radiometric age for the granite intrusion of 590 38 Ma, and the D2 structural age of the main fabric that affects it, 39 are generally accepted, the only contentious issue at present 40 concerns the origin of the early fabric in the Ben Vuirich 41 hornfels. It is developed in rocks with a grain size of only 0.1– 42 0.4 mm and, in all but the highest grade (andalusite-cordierite) 43 hornfels, represents a very low-strain deformation (Figure 6). 44 This fabric could have resulted from: 45

46 (1) Neoproterozoic, pre-D1, tectonism; 47 48 (2) D1 deformation at an early stage in the development of the Tay 49 Nappe; or 50 (3) Deformation of the country rocks synchronous with emplacement 51 of the granite. 52 53 There are problems with both (1) and (2) above. If the fabric is 54 of Neoproterozoic (pre-590 Ma) age, it would be absent from all 55 rocks younger than the 600 Ma Tayvallich Lavas, and would 56 necessitate the presence of a so-far undiscovered orogenic 57 unconformity within the Argyll Group in Scotland. Alternatively, 58 if the fabric is of D1 age, this would restrict D1 to 600–590 Ma 59 and separate it by 120 Ma from D2 at 470 Ma. However, from work at 60 Callander, Tanner (1995) has demonstrated that D1 in Southern 61 62 63 64 65 1 2 3 4 Highland Group rocks of the Highland Border is post-515 Ma in age 5 (see the Keltie Water GCR site report). As D1 and the D2 can be 6 correlated between the Tummel Steep Belt, including the rocks 7 around Ben Vuirich, and the Highland Border (Crane et al., 2002), 8 and an orogenic unconformity has not been demonstrated in this 9 ground, (1) is not a viable option. The most likely interpretation 10 is that the early fabric is of syn-emplacement origin (Tanner, 11 12 1996). This conclusion was based on evidence that the pre-hornfels 13 fabric increases in intensity toward the granite margin, and that 14 microveinlets of granite are seen in thin section to have been 15 deformed by the early fabric-forming event. 16 Further evidence of the nature of the pre-intrusion fabric is 17 given by the xenoliths in the granite at locality B. The 18 compositional layering in the population is randomly orientated, 19 with some xenoliths exhibiting folds. However, it is clear that 20 the S2 schistosity in the granite is axial planar to the folds in 21 the xenoliths. In addition, long, thin blocks are parallel with, 22 or at a small angle to, this external S2 fabric, whereas the 23 shorter, more equidimensional, blocks contain folds. This 24 situation is directly analogous to that described from the Port 25 Selma GCR site (Treagus et al., 2013), where a population of clasts 26 originally showing randomly orientated planar bedding, has been 27 deformed. The result is that clasts with bedding initially at a 28 low angle to the resultant cleavage plane are stretched and show 29 30 boudinaged layers, whereas those with bedding initially at a high 31 angle to the resultant cleavage are buckled internally. The 32 conclusion to be drawn from the Ben Vuirich xenoliths is that the 33 bedding laminations in these were all planar before the D2 34 deformation, and that the folds seen in some of them are the result 35 of the D2 deformation of the internal foliation of the bedding in 36 suitably orientated xenoliths. 37 An intriguing feature of the hornfelsed rocks at this GCR site is 38 that they not only preserve a very fine-grained early fabric, but 39 also show little sign of having been affected by the D2 40 deformation, despite the rocks to either side of them (granite 41 intrusion and country rock, respectively) having been strongly 42 deformed during D2. At that time, the country-rock pelites were 43 being transformed into coarse-grained schists with garnets up to 2 44 cm across (see Figure 4 for their distribution). The contact 45 metamorphic assemblage may be used to give an estimate of the 46 lithostatic pressure at which the assemblage crystallized, and 47 48 hence the depth of emplacement of the granite. Comparison with 49 published data on hornfelsed rocks gave an estimated pressure of 2– 50 4 kbar, representing a depth of 7–14 km (Tanner, 1996). This 51 conclusion is important when considering the origin and tectonic 52 setting of the granite, and has been the cause of some debate (see 53 Dempster et al., 2002). In addition, the chemical zonation 54 patterns in D2 regional metamorphic garnets reported by Ahmed-Said 55 and Tanner (2000) were interpreted by them to indicate that there 56 was a marked increase in confining pressure as the garnets grew 57 during D2, caused by the major crustal-thickening event that was 58 taking place at the time. 59 It has been proposed that the granite was emplaced during 60 extensional tectonism approximately synchronous with the eruption 61 62 63 64 65 1 2 3 4 of the Tayvallich volcanic rocks (e.g. Soper, 1994), and 100 Ma 5 before the Early Palaeozoic Grampian Event. This hypothesis 6 involves neither a separate Neoproterozoic orogenic event, nor a 7 splitting of the D1–D4 deformation episodes in the Grampian Terrane 8 into two packages separated by a long, ill-defined time break. It 9 is supported by the results of a recent geochemical study of the 10 granite (Tanner et al., 2006). The granite samples have a 11 12 restricted compositional range and occupy the within-plate granite 13 field of Pearce et al. (1984), the A-type granitoid field (on Ga/Al 14 plots) of Whalen et al. (1987), and lie wholly within the field for 15 A2-group granites, as defined by Eby (1992). Such granitoids are 16 characteristically found in rift-related environments. The Ben 17 Vuirich Granite is one of an increasing number of c. 600 Ma pre- 18 Caledonian intrusions that are now being recognized throughout the 19 Northern and Grampian highlands of Scotland (Strachan et al., 2002; 20 Kinny et al., 2003b). These are regarded as being related to a 21 swarm of diverse A-type magmatic bodies intruded at around 700–600 22 Ma which, in a Neoproterozoic reconstruction, can be traced across 23 the Appalachian Fold Belt (Bingen et al., 1998; Tanner et al., 24 2006). 25 26 2.4 Conclusions 27 28 The Ben Vuirich Granite was one of the first members of an 29 30 important suite of c. 600-million-year-old intrusions to be 31 recognized and dated radiometrically. Members of this suite were 32 intruded into the Neoproterozoic rocks of the Scottish Highlands 33 before they were deformed during the Caledonian Orogeny. Hence the 34 Ben Vuirich granite is of great national importance. Its 35 radiometric age has been the subject of numerous investigations, 36 initially to establish a minimum age for the Dalradian succession, 37 and increasingly as a means of dating the early phases of 38 Caledonian deformation. 39 The granite magma was intruded as a sheet-like body into a 40 sequence of limestones, mudrocks, and quartzose sands belonging to 41 the Appin Group. The heat that was released transformed the 42 adjoining sedimentary rocks into a baked rock, or hornfels, of 43 which only a small area remains and is currently exposed at ground 44 level. Because of having been heated to c. 600°C, and losing much 45 of their fluid during this process, the hornfelsed rocks became 46 resistant to later deformation and hence much of their early 47 48 history has been preserved intact. Their most significant feature 49 is that minerals such as cordierite and andalusite, which grew in 50 response to the heat, preserve a weak tectonic fabric within them. 51 Unfortunately, it is not possible to be absolutely certain whether 52 this fabric formed due to the forceful emplacement of the granite, 53 or during an earlier, pre-Caledonian deformation event. However, 54 it is clear that the granite was emplaced before the first 55 deformation to affect the Dalradian Supergroup (D1). Fragments of 56 the country rock that were torn off and encapsulated in the magma 57 when it was emplaced, preserve significant evidence of the non- 58 folded state of the rocks at that time. 59 From a study of the mineralogy of the hornfels it has been 60 concluded that the granite stopped rising and had crystallized 61 62 63 64 65 1 2 3 4 fully at an estimated depth of 7–14 km beneath the surface of the 5 Earth. The granite and enclosing country rocks were then strongly 6 deformed, metamorphosed, and recrystallized, during the main 7 progressive deformation (D2) and associated climax of regional 8 metamorphism of the Grampian Event at about 470 million years ago. 9 This process had a dramatic effect upon the mineralogy of the 10 hornfels, as the low- to medium- pressure cordierite and andalusite 11 12 were replaced by new high-pressure minerals such as kyanite and 13 garnet that reflect the greatly increased depth of burial reached 14 during the orogeny. 15 Geochemical and isotopic analyses of the Ben Vuirich Granite show 16 that it has the chemical fingerprint characteristic of granites 17 found in rifted portions of the Earth‘s crust. This finding 18 supports an earlier hypothesis that the granite formed in the same 19 extensional tectonic setting that enabled the extrusion of the 600 20 Ma Tayvallich Lavas to take place (see the West Tayvallich 21 Peninsula GCR site in Tanner et al., 2013a). The granite was 22 possibly derived from partial melting of the Dalradian sedimentary 23 rocks, or their basement, and belongs to a swarm of rift-related 24 granitoids that originally stretched along the Caledonian orogenic 25 belt from the Appalachians to Scotland and heralded the break-up of 26 the supercontinent Rodinia. Hence its age and structural 27 relationships have attracted considerable international interest. 28 29 30 3 GILBERT’S BRIDGE, GLEN TILT 31 (NN 881 699–NN 903 719) 32 33 R.A. Smith 34 35 36 3.1 Introduction 37 38 The river section around Gilbert’s Bridge in Glen Tilt, Perthshire 39 is a classic historical GCR site that provides good sections 40 through the tectonized junction between the Grampian and Appin 41 groups of the Dalradian. This junction, between the psammitic 42 Struan Flags to the north-west of Glen Tilt and a pelite, 43 semipelite, metalimestone and quartzite succession to the south- 44 east, was formerly considered to be the Moine–Dalradian boundary. 45 It is currently regarded as part of the Boundary Slide-zone, but 46 its nature is a matter of considerable debate (see 1.2.1 in 47 48 Introduction). 49 The first geological appraisal of Glen Tilt was made during the 50 historic visit by James Hutton and Sir John Clerk of Eldin in 1785 51 in search of evidence for the intrusive nature of granite (see the 52 Forest Lodge GCR site report in the Caledonian Igneous rocks of 53 Great Britain GCR volume; Stephenson et al., 1999). A plan of the 54 central part of the glen, drawn by Clerk, clearly shows the 55 orientation of the strata and three concordant sills of ‗porphyry‘ 56 immediately downstream from Gilbert‘s Bridge (Craig et al., 1978). 57 The results of the primary geological survey were published as 1‖ 58 Sheet 55 (Blair Atholl, 1902) with an accompanying memoir (Barrow 59 et al., 1905). Barrow‘s interpretation (Barrow, 1904, figure 9) 60 emphasized the importance of concertina folding and metamorphic 61 62 63 64 65 1 2 3 4 recrystallization of part of the Dalradian to form the ‗Moine 5 gneiss‘. But it was Bailey (1925) who established that this is the 6 site of a major junction between a very thick succession of Struan 7 Flags and the structurally overlying Dalradian. He described the 8 Dalradian succession, which is separated from the bulk of the 9 Dalradian to the south-east by the Loch Tay Fault, as the ‗Blair 10 Atholl Series of the Banvie Burn Belt‘. He indicated a possible 11 12 slide between the Banvie Burn Belt and the Struan Flags on his map 13 but was unable to confirm its existence due to ‗lack of local 14 evidence‘. Its continuation in the Schiehallion area was later 15 established as the Boundary Slide by Bailey and McCallien (1937), 16 who described a fundamental structure, or decollement, separating 17 complex folding in the Dalradian from simpler structures in the 18 Struan Flags below. Pantin (1961), who introduced the simpler term 19 Glen Banvie ‗Series‘ for the ‗Blair Atholl Series of the Banvie 20 Burn Belt‘, found no evidence for sliding at Gilbert‘s Bridge. 21 However, Harris (1963) interpreted the junction to be a thrust 22 slide carrying the Dalradian rocks to the north-west, and Thomas 23 (1965, 1980) concluded that the Boundary Slide is associated with a 24 tectonic schist equivalent to the Beoil Schist of the Schiehallion 25 area (Rast, 1958) (see the Strath Fionan GCR site report in Treagus 26 et al., 2013). 27 The Struan Flags have since been re-allocated to the Grampian 28 Group and the Glen Banvie ‗series‘ to the Appin Group, both of the 29 30 Dalradian Supergroup (Harris et al., 1978). Although the junction 31 is recognized as a zone of high strain (Smith, 1980), the amount of 32 excision could be relatively small because a through succession 33 from the Grampian Group into the Appin Group can be demonstrated 34 nearby in the area around Schiehallion (Treagus and King, 1978; 35 Treagus, 1999). The fact that the Ballachulish Subgroup is present 36 on the southern side of Glen Tilt (Smith and Harris, 1976) also 37 makes the tectonic break less important stratigraphically and the 38 Glen Banvie ‗Series‘ was correlated tentatively with the Lochaber 39 Subgroup by Smith (1980). It has now been formally designated the 40 Glen Banvie Formation. 41 42 3.2 Description 43 44 The section in Glen Tilt between Gilbert’s Bridge and Marble Lodge 45 shows a clear and consistent relationship between the Struan Flags, 46 now part of the Glen Spean Subgroup of the Grampian Group, and the 47 48 younger Dalradian (Figure 7). The Grampian Group psammites, which 49 are perfectly exposed in the along-strike river section, have a 50 characteristic parallel flaggy banding which is largely tectonic; 51 their junction with rocks of the overlying Glen Banvie Formation is 52 strongly attenuated. Most of the attenuation appears to be 53 flattening, although a weak down-dip lineation is evident locally 54 and the axial planes of early (F2) folds have been drawn into 55 parallelism with the main foliation. 56 Sections across the Boundary Slide are exposed in the River Tilt 57 immediately below Gilbert‘s Bridge (NN 8812 7005), at the junction 58 of the Tilt with the Allt Fas-charaidh (NN 8832 7035), near its 59 junction with the Allt Mhairc (NN 8902 7118) and at Coille Sron an 60 Duine (NN 8950 7164). At all of these localities, approaching the 61 62 63 64 65 1 2 3 4 Boundary Slide from the Grampian Group, the flagginess of the 5 psammites increases and the banding in them becomes closer spaced 6 and parallel. Coarse white mica is concentrated on the flaggy 7 surfaces. At the slide itself, about 1 m of coarse muscovite- 8 biotite schist has a platy aspect with parallel quartz lenticles. 9 However, similar bands of schist occur within the Glen Banvie 10 Formation above, together with mylonitized metalimestones and 11 12 strongly deformed quartzites, so that the formation as a whole 13 appears to constitute a high-strain zone. The strong fabric in the 14 rocks near the slide is best appreciated where small quartz pebbles 15 with elongation ratios of 6:1 occur in the quartzites. The 16 elongation lineation is fairly uniform, plunging at about 30o to the 17 south-east. 18 On the south-east side of Glen Tilt, the Glen Banvie Formation 19 comprises intercalated metalimestones, calcsilicate rocks, 20 schistose pelites and semipelites, psammites and quartzites. A 21 strong foliation has effectively destroyed any sedimentary 22 structures. Alumina-rich calcsilicate rocks with pink microcline 23 are particularly characteristic of the lower part of the formation. 24 Epidote, actinolite and diopside are present locally in these 25 rocks; plagioclase is less common, as is quartz. Downstream from 26 Gilbert‘s Bridge at NN 881 698, pale-grey crystalline 27 metalimestones are interbedded with quartz-plagioclase-mica 28 schists. The grey metalimestones contain sparse graphite dust and 29 30 impure types have tremolite and clinozoisite-epidote or zoisite. 31 Some of the very fine-grained metacarbonate bands are mylonitic. 32 Elsewhere, white metalimestones, up to 2 m thick, are fairly common 33 and several contain calcsilicate minerals. A well-known example of 34 the latter type is the ‗Glen Tilt Marble‘, which was quarried for a 35 short time at NN 9027 7186. This decorative marble has pale and 36 darker green blotches of serpentine mineral (antigorite) elongated 37 within the foliation. Antigorite is a common alteration product of 38 tremolite, diopside and forsterite. White mica (?talc) lies in the 39 foliation but pyrite is disseminated within the marble. Dolomitic 40 metalimestones are also present and are composed of up to 90% 41 fibrous tremolite. 42 In the central part of the Glen Banvie Formation, a quartzite 43 unit, up to 300 m thick, forms a persistent marker (Figure 7). 44 Within the quartzite are local feldspathic bands and thin beds of 45 dark pyritic semipelite. Above and below the quartzite are pelites 46 composed of quartz-muscovite-biotite-plagioclase, with or without 47 48 garnet and kyanite. Some of the pelites are intercalated with 49 fine-grained quartzites on a centimetre scale; others are graphitic 50 and many are retrogressed to chloritic schists. Slightly 51 calcareous pelites contain zoisite, clinozoisite and minor calcite. 52 Near the Sron a‘ Chro Granite, andalusite porphyoblasts overprint 53 the earlier pelitic assemblages. 54 The Glen Banvie Formation contains two types of amphibolite that 55 are both considered to have been intrusions; thin sheets of dark- 56 green hornblende schist and larger amphibolitic bodies. The 57 hornblende schists contain up to 75% hornblende, minor plagioclase, 58 quartz, garnet, titanite and iron oxides; the larger bodies 59 additionally contain thin leucocratic bands rich in pinkish 60 sericitized plagioclase, quartz and epidote. 61 62 63 64 65 1 2 3 4 Two sheets of foliated granite are exposed in Glen Tilt about 30 m 5 above the Boundary Slide. They are up to 2 m thick and are roughly 6 concordant with the foliation/banding in the Glen Banvie Formation. 7 The reddish biotite granites show a granoblastic texture of quartz, 8 microcline and albite, and a parallel alignment of sparse biotite 9 laths. Minor secondary muscovite is present and biotite is locally 10 retrogressed to chlorite. At NN 895 716 by the River Tilt, 11 12 xenoliths of biotite semipelite (0.15 to 1.2 m long) possess a 13 foliation that is oblique to the one in the granite. 14 The Glen Banvie Formation has been subjected to three major phases 15 of deformation (Smith, 1980). It typically has a strong flaggy 16 foliation dipping at about 40o to the south-east, which is a 17 composite S1+S2 fabric. Boudinage of the more-competent bands 18 within the main foliation is common and is accompanied by flexing 19 in the less-competent units. Most of the long axes of boudins 20 trend north-east perpendicular to a stretching lineation and both 21 are considered to relate to D2. It is likely that the foliation of 22 the granite sheets is also related to D2, so that the oblique 23 fabric preserved in the xenoliths is probably S1 (c.f. Bradbury et 24 al., 1976; Tanner and Leslie, 1994; Tanner, 1996). A poorly 25 developed local crenulation cleavage is assigned to D3. Local 26 brittle deformation, fracturing and kink bands are related to 27 sinistral movement on the Loch Tay Fault, a branch of which lies 28 about 0.5 km south-east of Gilbert‘s Bridge (Treagus, 1991). 29 30 The peak regional metamorphism is of amphibolite facies, as is 31 indicated by the presence of kyanite-grade assemblages in the 32 pelites. Estimates of pressure and temperature on garnet rims from o 33 the Glen Tilt area indicate 9–12 kbar and 550–620 C (Wells and 34 Richardson, 1979). Recrystallization of minerals such as the 35 amphiboles in the hornblende schists (post D2) and later partial 36 retrogression of the rocks is common. 37 38 3.3 Interpretation 39 40 There are two issues concerning the geology of the central section 41 of Glen Tilt that have generated much debate in the past and are 42 not as yet fully resolved. The first is the stratigraphical 43 affinity of the Glen Banvie Formation and the second is the nature 44 and regional significance of the high-strain zone, the so-called 45 Boundary Slide, that lies between this and the structurally 46 underlying Struan Flags‘. 47 ‘ 48 The varied lithological assemblage that comprises the Glen Banvie 49 Formation, with its distinctive calcsilicate and metacarbonate 50 rocks, resembles parts of the Appin Group. However, it is difficult 51 to correlate with the Dalradian succession because it lies between 52 the Boundary Slide and the Loch Tay Fault. A calcsilicate-bearing 53 metalimestone, about 3 m thick, downstream from Gilbert‘s Bridge 54 was formerly correlated with the unit now known as the ‗Dark 55 Limestone‘ of the Blair Atholl Subgroup (Barrow, 1904). Further 56 metalimestone intercalations were considered by Barrow to be folded 57 repetitions of this metalimestone, and in places it was seen to be 58 in contact with the Struan Flags and other rock types such as dark 59 schist and red microcline-rich rock. This was explained as a 60 tightly folded local unconformity of the metalimestone on the Moine 61 62 63 64 65 1 2 3 4 rocks (Barrow, 1904). However, Bailey (1925) measured the section 5 downstream from Gilbert‘s Bridge and concluded that the 6 intercalations are not the Struan Flags or the ‗Dark Limestone‘, 7 but are all part of his ‗Blair Atholl Series of the Banvie Burn 8 Belt‘. He suggested that the rocks of the Banvie Burn Belt could 9 be equated with his ‗Pale Group‘, i.e. the upper part of what is 10 now the Blair Atholl Subgroup, but he was not certain because of 11 12 the structural complexity in Glen Tilt. Pantin (1961) made a 13 similar correlation, although the presence of local graphitic bands 14 and a thick quartzite unit are not consistent with this 15 interpretation. A suggestion by Thomas (1965) that the central 16 quartzite unit occupies a synclinal core, and hence a higher 17 stratigraphical level, was refuted by Smith (1980), who found no 18 structural or stratigraphical evidence for this hypothesis. 19 Although the outcrop of the Glen Banvie Formation is entirely 20 bounded by faults, its position between the Grampian Group 21 psammites and the upper part of the Appin Group, together with its 22 overall lithology, suggests that it could be part of the Lochaber 23 Subgroup (Smith, 1980) e.g. equivalent to the calcareous upper 24 parts of the Leven Schist. It could alternatively represent the 25 Lochaber Subgroup and the lower part of the Ballachulish Subgroup 26 in a condensed sequence (c.f. Treagus, 1999, 2000 in the 27 Schiehallion area). 28 The importance of the Boundary Slide was stressed by Thomas 29 30 (1980), who interpreted the presence of a muscovite-rich schist to 31 be a result of the sliding (c.f. the Beoil Schist of the 32 Schiehallion area). According to Thomas, the slide probably 33 developed during the evolution of the primary F1 nappes but was re- 34 activated locally during later deformation. He described it as 35 a dislocation between the Appin Group succession of the Tay Nappe 36 and the Grampian Group rocks beneath, which form a primary south- 37 east- and downward-facing Atholl Nappe. However, Smith (1980) 38 inferred that, because tight to isoclinal F2 folds had their limbs 39 cut out and were sheared locally along their axial planes during 40 D2, the age of the major sliding is D2. The Glen Banvie Formation 41 as a whole is strongly deformed and there are numerous minor slides 42 between the contrasting lithologies, such as the one that Barrow 43 (1904) interpreted as an unconformity. This fact, coupled with the 44 sharp contrast in stratigraphy between the formation and the 45 Dalradian succession to the south-east, led Smith (1980) to the 46 conclusion that the Loch Tay Fault might obscure another major 47 48 slide forming the south-eastern boundary to the Glen Banvie 49 Formation. He also suggested that the Grampian Group psammites 50 acted as a competent block during deformation and might have 51 influenced the location of the Loch Tay Fault close to the Boundary 52 Slide. 53 On a regional scale, the age, nature and importance of the 54 Boundary Slide are still debatable issues. Within the northern 55 Grampian Highlands, Appin Group rocks rest on various levels of the 56 Grampian Group (Smith et al., 1999, Highton et al., 1999), and in 57 the Central Grampian Highlands, large parts of the Appin and Argyll 58 group succession are absent where high-strain zones are present 59 just above the Grampian Group (Roberts and Treagus, 1979). So it 60 is even possible that the Boundary Slide and/or related structures 61 62 63 64 65 1 2 3 4 could conceal orogenic unconformities at various stratigraphical 5 levels, which could eventually help to explain such outstanding 6 problems as the apparently wide age span between older parts of the 7 Grampian Group at c. 730 Ma and the top of the Dalradian, which 8 contains an Early Cambrian (c. 515 Ma) fauna (e.g. Prave, 1999; 9 Dempster et al., 2002; Hutton and Alsop, 2004). See Stephenson et 10 al., 2013a for further discussion. 11 12 13 3.4 Conclusions 14 15 The River Tilt at Gilbert’s Bridge provides a classic section 16 through the junction between the Grampian Group and the Appin Group 17 successions of the Dalradian Supergroup. This junction, usually 18 referred to here as the Boundary Slide, was originally regarded as 19 a major tectonic dislocation or decollement between the highly 20 variable lithologies of the Dalradian to the south-east and the 21 structurally underlying, dominantly psammitic succession to the 22 north-west that was formerly regarded as part of the Moine. 23 Subsequent work has failed to find specific evidence for a major 24 dislocation or for significant excision of strata. However, the 25 junction does occur within highly deformed rocks and the Boundary 26 Slide, at least in this area, is presently interpreted as a high- 27 strain zone related to the D2 phase of deformation. 28 The problem is compounded in Glen Tilt by uncertainty over the 29 30 stratigraphical affinities of the strata that lie structurally 31 above the Boundary Slide but separated from the main Dalradian 32 succession to the south-east by the Loch Tay Fault. These 33 lithologically variable strata, termed the Glen Banvie Formation 34 and including some distinctive metacarbonate and calcsilicate 35 rocks, are currently assigned tentatively to part of the Lochaber 36 Subgroup. If this correlation is correct, there could be no 37 significant stratigraphical break here at the Boundary Slide 38 between the Grampian Group and the lower part of the Appin Group. 39 No matter whether the Boundary Slide is a major dislocation or 40 merely a high-strain zone focused upon the junction between two 41 successions of markedly different competence, it does constitute a 42 major boundary throughout much of the Grampian Highlands. The 43 section at Gilbert‘s Bridge is one of the best exposed and the 44 conclusions drawn from this GCR site, however tentative, have 45 broader implications for the possible presence or absence of 46 tectonic and/or stratigraphical breaks elsewhere in the Dalradian 47 48 succession. It is therefore of national importance. 49 50 4 GLEN EY GORGE 51 (NO 0867 8834–NO 0884 8630) 52 53 C.G. Smith and D. Stephenson 54 55 56 4.1 Introduction 57 58 Glen Ey in upper Deeside provides one of the best-exposed sections 59 through the Boundary Slide, a zone of highly strained rocks, which 60 separates the Grampian Group from other, more lithologically 61 62 63 64 65 1 2 3 4 variable, parts of the Dalradian throughout much of the Grampian 5 Highlands. At one time the slide was regarded as a fundamental 6 tectonostratigraphical boundary separating the Moine and Dalradian 7 successions, but the re-assignation of most of the Moine south-east 8 of the Great Glen Fault to the Dalradian has lessened its potential 9 importance. Discussion now centres upon whether or not the high- 10 strain zone coincides with a major dislocation and whether any 11 12 stratigraphical units or major structures have been excised (see 13 1.2.1 in Introduction). The Glen Ey gorge section is particularly 14 valuable since, unlike some sections to the south-west, there is 15 continuity from the top of the Grampian Group, through increasingly 16 strained rocks, into highly deformed schistose rocks assigned to 17 the Lochaber Subgroup at the base of the Appin Group. Here, at 18 least, there would appear to be no reason to suggest that large 19 parts of the succession are missing. 20 The area around Glen Ey was first mapped by the Geological Survey 21 in 1898–99 and the results were incorporated into the one-inch 22 Sheet 65 (Balmoral, 1904) and the accompanying memoir (Barrow and 23 Cunningham Craig, 1912). This area of the present BGS 1:50 000 24 Sheet 65W (Braemar, 1989), is for the most part based on the 25 mapping and re-interpretation of Upton (1983, 1986), who was the 26 first to recognize the Boundary Slide in the Braemar area. Much of 27 the ensuing geological description and interpretation is based on 28 Upton‘s work. 29 30 31 4.2 Description 32 33 The Ey Burn is a major tributary on the south side of the River 34 Dee, approximately 7 km south-west of Braemar. To the north of the 35 ruins of Auchelie (NO 0875 8630), as far as the bridge at NO 0867 36 8834, the burn flows for about 2 km through a narrow gorge, 37 generally less than 10 m deep but with steep to vertical sides. 38 The best-known part of the gorge, between NO 0872 8718 and NO 0875 39 8695, includes a historical curiosity known as the Colonel’s Bed. 40 This rocky recess in the western wall of the gorge is where John 41 Farquharson of Inverey, the ‘Black Colonel’, reputedly hid while 42 being pursued after the Battle of Killiecrankie in 1689. 43 The Boundary Slide in the Ey Gorge separates essentially 44 arenaceous rocks of the Grampian Group from the more-varied pelitic 45 and calcareous lithologies of the Lochaber and Ballachulish 46 subgroups of the Appin Group (Figure 8). 47 48 The northernmost 600 m or so of the section are entirely within 49 Grampian Group rocks, which dip consistently at low angles (less o 50 than 20 ) to the south-east or east. These uppermost units of the 51 Grampian Group are probably equivalent to the Struan Flags of the 52 Blair Atholl area (see the Gilbert’s Bridge GCR site report). In 53 Glen Ey they have been termed the Deeside Quartzites and Psammites 54 by Upton (1983, 1986). These comprise units of pale slabby 55 quartzite and quartzose psammite, 5–15 cm thick, interbedded with 56 pale-green and buff colour-banded psammite and thin beds of 57 semipelite (up to 5 cm thick). The semipelites contain 58 poikiloblastic garnet aligned parallel to the schistosity, 59 indicating syn- to late-D2 growth. Heavy-mineral laminae are 60 commonly seen in the quartzites and Upton (1983) recorded south- 61 62 63 64 65 1 2 3 4 easterly younging evidence in the psammites, confirming that the 5 succession is the right way up. 6 The quartzite and psammite contain bedding-parallel pelitic 7 laminae dominated by thick felts of porphyroblastic muscovite and 8 titaniferous biotite, which define the S2 fabric and whose spacing 9 reflects proximity to the Boundary Slide. Thus, by the bridge at 10 the lowest point in the section, these partings are widely spaced 11 12 with an interval of 20–30 cm, whereas at NO 0880 8800, where a 13 small tributary enters from the east, the spacing reduces from 10– 14 20 cm at river level to 3–7 cm at the lip of the gorge. 15 Approximately 20 m upstream on the east bank, at NO 0879 8797, 16 psammite becomes increasingly flaggy upwards and is overlain by 17 quartz schist, marking a transition into the Tom Anthon Mica Schist 18 Formation, the lowest unit of the Appin Group. South of this, the 19 contact between the two formations descends gradually to reach 20 river level at NO 0885 8776. 21 Roughly 200 m to the south, the gorge turns to the west and the 22 rocks dip gently to the east or, more rarely, to the north-east. 23 As a result of this slight change in overall strike, the gorge cuts 24 down the succession upstream and Grampian Group rocks crop out to 25 form an elongate ‗window‘ up to 100 m wide and 500 m long. As in 26 the lower part of the gorge, the quartzites and psammites 27 demonstrate an upward increase in flagginess and pass up into the 28 Tom Anthon Mica Schist Formation at stream level just to south of 29 30 the Colonel‘s Bed. 31 The Tom Anthon Mica Schist, which in this area is estimated to be 32 40–70 m thick, is a very distinctive platy silvery grey rock. It 33 is essentially a quartz-feldspar-muscovite-biotite schist dominated 34 by thick porphyroblastic muscovite felts, which give the rock its 35 particularly distinctive appearance. Isolated garnets crystallized 36 late, during the D2 deformation. The thick mica foliae are 37 compressed around augens of recrystallized porphyroblastic 38 plagioclase, the lamellar twinning of which has been accentuated by 39 the high strain. Also present are augen-shaped lenses of calcite 40 and calcsilicate-rich hornblendic amphibolites. The lithology at 41 the base of the formation is dominated by mica foliae with only 42 rare lenses of quartz, feldspar and amphibolite. The mica foliae 43 contain heavy minerals such as tourmaline, zircon, apatite and rare 44 small poikiloblastic garnets. There is an upward increase in the 45 quartz-feldspar content of the matrix and at the top of the 46 formation the mica felts are separated by quartzofeldspathic 47 48 microlithons. This partly reflects a more-arenaceous protolith and 49 partly the effects of high D2 strain producing a pressure-solution 50 striping in the rock. 51 The top of the Tom Anthon Mica Schist is not exposed in the gorge, 52 but small exposures of epidote-tremolite-dolomite-bearing 53 calcsilicate rock occur in an eastern tributary at around NO 0900 54 8782 and mark the location of the overlying Baddoch Burn Dolomite. 55 These rocks are easily recognizable and form an important marker 56 horizon. Here they consist of green calcsilicate rocks, dominated 57 by tremolite laths with subordinate amounts of quartz, feldspar and 58 muscovite or phlogopite. The tremolite laths are preferentially 59 aligned, forming a rough schistosity. 60 61 62 63 64 65 1 2 3 4 The succeeding Glen Clunie Graphitic Schist Formation crops out in 5 the gorge section to the south of the major NE-trending Tom Anthon 6 Fault (see below). This formation is dominated by dark-grey 7 graphitic pelite with porphyroblasts of garnet, staurolite and 8 kyanite. It is evident from these rocks that garnet and staurolite 9 formed early during D2, whereas kyanite porphyroblasts, which 10 marked growth at the peak of metamorphism, developed statically 11 12 after D2 but before D4. The formation also includes minor beds of 13 calcsilicate rock and rare psammite units and is cut by hornblendic 14 sheets thought to represent metamorphosed basic intrusions. 15 Close to the Tom Anthon Fault, the dip of the foliation in the Tom 16 Anthon Mica Schist steepens and is predominantly to the north. At 17 around NO 087 869, the Ey Burn swings sharply to the north-east and 18 for about 80 m follows the trace of the fault, which is well seen 19 in this section. The Tom Anthon Mica Schist is exposed on the 20 north bank of the burn, whereas to the south there are good 21 exposures of intensely tectonized and strongly folded pale-green 22 and grey calcsilicate rocks. The two contrasting lithologies are 23 separated by a breccia zone, over 2 m wide, of pale-green rock cut 24 by anastomosing calcite veins. A further 30 m or so to the south 25 another breccia zone separates the calcsilicate rocks from the Glen 26 Clunie Graphitic Schist, which crops out upstream for a further 27 kilometre. 28 29 30 4.3 Interpretation 31 32 In common with Dalradian rocks elsewhere, at least three of the 33 regional episodes of deformation (D1, D2 and D4) have been 34 recognized in the rocks of the Glen Ey area. They were also 35 affected by two major dislocations of contrasting style and age, 36 the ductile Boundary Slide, which is equated with D2 and the later, 37 more-brittle Tom Anthon Fault. The main fabric/schistosity is 38 recognized as being S2, although evidence of an earlier, S1 fabric 39 is present locally. There is little evidence of early folds in the 40 section, but from the regional synthesis of Upton (1986; Figure 9) 41 it can be established that the rocks all lie on the lower, inverted 42 limb of a major F1 isoclinal anticline, possibly equivalent to the 43 Tay Nappe, or on a parasitic fold on the lower limb of the nappe. 44 The present disposition and attitude of the rocks is largely 45 attributable to D2, when a stack of tight recumbent downward- and 46 SE-facing folds was created. These F2 folds are broadly equivalent 47 48 to the Ben Lui folds below the Tay Nappe in the South-west Grampian 49 Highlands. Thus the right-way-up Grampian Group rocks of Glen Ey 50 lie on the upper limb of a major F2 synformal anticline that faces 51 downwards to the south-east. The rocks overlying the Boundary 52 Slide are also right way up, being on the lower limb of the F2 53 Morrone Antiform, which also faces downwards to the south-east. 54 Only minor, near upright F4 folds occur in this area. 55 The nature of the Boundary Slide at this GCR site is ambiguous. 56 There is a well-exposed, gradual, possibly sedimentary, transition 57 from the Deeside Quartzites and Psammites of the Grampian Group 58 into the Tom Anthon Mica Schist of the Lochaber Subgroup, implying 59 little or no dislocation or excision of strata. However, elsewhere 60 in upper Deeside, Upton (1983) has recognized a higher unit of the 61 62 63 64 65 1 2 3 4 Grampian Group, the Linn of Dee Banded Pelites and Psammites, which 5 is absent from this section. Has it been excised due to truncation 6 of the upper limb of the F2 fold by the Boundary Slide or is it 7 absent as a result of facies changes or an unconformity? Above the 8 inferred position of the slide, the Tom Anthon Mica Schist appears 9 to be analogous to the Beoil Schist of the Schiehallion area (see 10 the Strath Fionan GCR site report in Treagus et al., 2013). These 11 12 highly schistose lithologies are thought to have developed as a 13 result of high strain on pelitic rocks in the area of the Boundary 14 Slide. As is the case elsewhere along its trace, there is 15 undoubtedly high strain focussed upon the marked contrast in 16 lithology and competence at the Grampian–Appin group boundary, but 17 it is not possible to prove any dislocation or excision. 18 The Tom Anthon Fault is a major dislocation, trending north-east– 19 south-west, which can be traced for over 15 km from Braemar on 20 Deeside to Fealar Lodge in the upper Glen Tilt area. It might be 21 considered as one of several splays of the Loch Tay Fault, which to 22 the north-east of upper Glen Tilt departs from its usual single 23 straight course. The fault plane is considered to be near 24 vertical, as its straight trace is unaffected by considerable 25 topography throughout the area. The movement on the fault is hard 26 to estimate, but it almost certainly includes both strike-slip and 27 dip-slip components. However, the evidence regarding the sense of 28 movement in the Glen Ey area is conflicting. The stratigraphy 29 30 suggests a downthrow to the south-east. However, Upton (1983) 31 proposed that the rocks to the south-east of the Tom Anthon Fault 32 are at a lower structural level, below the Morrone Antiform. 33 There, the dips of the S2 fabric are much steeper, and the outcrop 34 thickness of the Glen Clunie Graphitic Schist is increased 35 significantly as a result of repetition about the F2 An Socach Fold 36 trace and an F1 fold-pair. This structural interpretation would 37 imply a downthrow to the north-west on the Tom Anthon Fault. 38 39 4.4 Conclusions 40 41 The Glen Ey Gorge encompasses one of the most complete sections 42 through the Boundary Slide, in terms of degree of exposure, 43 accessibility and continuity of Dalradian lithostratigraphy. The 44 perceived importance of the slide has diminished in recent years, 45 as it no longer represents a fundamental tectonostratigraphical 46 boundary between the Moine and Dalradian successions. However, it 47 48 does comprise a zone of intense ductile deformation that, 49 throughout much of its length, is focussed upon the boundary 50 between arenaceous rocks of the Grampian Group and the more-varied 51 lithologies found in higher parts of the Dalradian succession. 52 In Glen Ey there is a continuous section, in a right-way-up 53 sequence, from increasingly flaggy quartzites and psammites of the 54 Grampian Group, into the highly sheared Tom Anthon Mica Schist 55 Formation of the Lochaber Subgroup (Appin Group). Large-scale, 56 isoclinal F2 folds have been identified in the area, and the main 57 deformation fabric is S2, but it is not possible to establish 58 whether any stratigraphical units or any fold limbs have been 59 excised by movement on the shear-zone. The so-called ‗slide‘ could 60 merely be a zone of high strain with no significant displacement. 61 62 63 64 65 1 2 3 4 A later NE-trending brittle fault, possibly related to the Loch 5 Tay Fault, is well exposed in the gorge and is marked by breccia 6 zones cut by calcite veins. 7 8 5 CAIRN LEUCHAN 9 (NO 380 908–NO 393 941) 10 11 12 C.G. Smith 13 14 15 5.1 Introduction 16 17 The hill of Cairn Leuchan (679 m) is located 5 km south-east of 18 Ballater in upper Deeside, on the broad NE-trending ridge that 19 separates Glen Muick to the west from Glen Tanar to the east. The 20 GCR site is representative of a variety of rocks that have 21 contributed considerably to the understanding of conditions of 22 high-grade regional metamorphism in the Grampian Terrane. Firstly 23 it lies within the area where Barrow established the concept of 24 metamorphic zones and index minerals. Secondly, it lies close to 25 the trace of the kyanite–andalusite isograd, which in simple terms 26 has been taken to define the boundary between the Barrovian and 27 Buchan styles of metamorphism. Thirdly, and perhaps most 28 importantly, geothermometric and geobarometric studies on gneisses 29 30 from Cairn Leuchan have established that peak metamorphic 31 temperatures and pressures were among the highest recorded in the 32 Grampian Terrane, and that this was one of the few areas in the 33 Scottish Highlands where metamorphic conditions during the 34 Caledonian Orogeny approached granulite facies. 35 The primary survey of the Glen Muick–Glen Tanar area was carried 36 out by Barrow in 1896 and the results were incorporated into the 37 one-inch Sheet 65 (Balmoral, 1904) and the accompanying memoir 38 (Barrow and Cunningham Craig, 1912). The area lies close to the 39 boundary between the Dalradian of Perthshire and that of the North- 40 east Grampian Highlands and, as such, forms an important link 41 between these two contrasting successions, as was highlighted by 42 Read (1928). Cairn Leuchan was the focus of detailed studies of 43 metamorphic conditions by Baker and Droop (1983) and Baker (1985). 44 The area was resurveyed by staff of Aberdeen University and Queen‘s 45 University, Belfast under contract to the British Geological 46 Survey, resulting in a new edition of the 1:50 000 Sheet 65E 47 48 (Ballater, 1995) and an accompanying memoir (Smith et al., 2002). 49 Bedrock at the GCR site consists dominantly of the Queen‘s Hill 50 Formation, which in this area constitutes the entire thickness of 51 the Crinan Subgroup and comprises two main gneissose lithologies, 52 one semipelitic to pelitic and one hornblendic. The hornblendic 53 rocks are metamorphosed basic sheets, which were mostly intruded 54 before deformation, although some fine-grained rocks exposed near 55 the base of the formation are regarded as metavolcanic. 56 The Dalradian rocks were affected by four phases of folding as a 57 result of which the strata are inverted and dip steeply to the 58 north-west. At the same time the rocks underwent high-grade 59 metamorphism and were extensively migmatized. They were intruded 60 subsequently by small late-tectonic ultrabasic to intermediate 61 62 63 64 65 1 2 3 4 bodies and, to the north and east of Cairn Leuchan and Pannanich 5 Hill, by post-tectonic silicic rocks of the Ballater and Mount 6 Battock granite plutons. Later faults trend north, north-west and 7 east-north-east. 8 9 5.2 Description 10 11 12 The following account is based on descriptions by Baker and Droop 13 (1983), Goodman et al. (1990) and Smith et al. (2002). Exposures 14 in the immediate area of Cairn Leuchan are dominated by gneissose 15 basic meta-igneous rock with subordinate interlayered 16 metasedimentary gneiss (Figure 10). The meta-igneous rocks show a 17 wide range of magnetic susceptibilities, but generally record 18 higher values than the surrounding metasedimentary rocks, and hence 19 ground magnetic surveys were employed extensively in the last 20 resurvey of the area. 21 The intrusive meta-igneous rock is part of a sheet-like unit that 22 can be traced along strike for nearly 12 km. To the south of Am 23 Mullach (NO 375 904) the outcrop width of this sheet is generally 24 less than 200 m but in the vicinity of Cairn Leuchan, largely as 25 the result of repetition in the core of a major F2 fold, it reaches 26 a width of around 1 km. In common with other meta-igneous sheets 27 in the area it consists of coarse-grained hornblendic gneiss, which 28 is locally agmatitic. 29 30 Semipelitic to pelitic metasedimentary gneisses are exposed c. 400 31 m to the north-west of Cairn Leuchan and are inferred from magnetic 32 mapping to underlie the unexposed ground at a comparable distance 33 to the south-east. The outcrop width of these units ranges from 34 nearly 500 m to less than 30 m. 35 Three episodes of folding, corresponding to the D1–D3 regional 36 events have been recognized in the Dalradian rocks of the Cairn 37 Leuchan to Pannanich Hill area. In common with the surrounding 38 ground there is no evidence of any large-scale F1 folds. However, 39 the earliest event produced a number of small-scale (5–10 cm 40 wavelength or less) rootless isoclines that are intrafolial to the 41 regional fabric and are particularly evident in the hinge-zones of 42 F2 folds. These early structures fold an evenly spaced fabric 43 marked by quartzofeldspathic material, reminiscent of tectonic 44 striping or spaced cleavage. This suggests that these are not 45 truly F1 folds, but could be an early D2 phase. The main 46 deformation occurred during D2 with the development of large 47 48 asymmetrical folds, and the axial surface trace of a major synform 49 passes directly through Cairn Leuchan. These F2 folds have long 50 NW-dipping and short horizontal or SSE-dipping limbs and plunge to 51 the south-west. It has been suggested that they verge to the 52 north-west, but the outcrop pattern suggests that they verge to the 53 south-east. The main regional fabric (S2) is aligned parallel to 54 the long limbs and hence, in common with the sheet dip, strikes 55 north-east–south-west and is inclined steeply to the north-west. 56 Few examples of F3 folds are to be seen in the area, but the 57 orientation of the S2 fabric is largely attributed to that phase of 58 folding. 59 The Dalradian rocks are all coarse-grained gneisses or migmatites, 60 whose mineral assemblages and textures largely reflect the peak 61 62 63 64 65 1 2 3 4 metamorphic conditions that prevailed during the D2 deformation. 5 The meta-igneous rocks vary from those with a planar gneissose 6 foliation to heavily agmatized types. The latter are characterized 7 by lenticular clots rich in ferromagnesian minerals, surrounded by 8 anastomosing stringers of leucocratic material that can form up to 9 40% of the rock; the subparallel alignment of these clots gives 10 rise to an indistinct foliation. Compositional banding is not 11 12 present. They have the mineral assemblage garnet-clinopyroxene- 13 hornblende-plagioclase-quartz-ilmenite-apatite-titanite, although 14 there is considerable variation in the proportions of minerals 15 present; e.g. hornblende content can range from 10–70%. Garnets 16 are common, particularly in the more-mafic examples, and are of 17 almandine composition, typically Alm56Gr30Py7Sp7, little zoned, and 18 stable within the metamorphic assemblage. In the Cairn Leuchan– 19 Drum Cholzie area, diopside occurs as large idioblastic 20 porphyroblasts, several centimetres in diameter, with a typical 21 composition of Ca1Mg0.5Fe0.5Si2O6. They too are stable within the 22 regional metamorphic assemblage. 23 The metasedimentry gneisses also exhibit a wide range of mineral 24 assemblages. Simplest are those that consist of quartz, andesine, 25 K-feldspar and dark brown biotite, with accessory opaques, apatite, 26 zircon and titanite. The foliation in the rocks is defined by the 27 orientation of biotite laths and by elongation of quartz grains, 28 which commonly show considerable strain. Almandine garnets are 29 30 common in pelitic rocks, often being full of biotite and quartz 31 inclusions. They typically have the composition Alm73Py16Gr7Sp3, 32 although they can show some zoning, the rims being slightly more Fe 33 rich and Mg poor than the cores. In places the garnets have atoll 34 shapes, the central parts having been replaced by a fine-grained 35 biotite symplectite. Sillimanite is present in pelites as 36 fibrolite, and is also found at the centre of atoll garnets; the 37 sillimanite sworls seen in some specimens could be pseudomorphs 38 after garnet. 39 Two main episodes of migmitization in the metasedimentary rocks 40 are recognized in the Ballater district; an early generation, which 41 is widely developed and is characterized by stromatic leucosomes of 42 leucotonalitic composition, and a later episode from which the 43 ultimate product is a massive coarse-grained rock of igneous 44 aspect, consisting of biotite, oligoclase and quartz. The later 45 episode has a much more localized development than the earlier. 46 Although it has not been recognized in the immediate area of Cairn 47 48 Leuchan, there is an extensive development 2–3 km to the north, 49 where it is known as the Pannanich Hill Complex (Goodman, 1991). 50 (See also the Balnacraig GCR site report.) 51 52 5.3 Interpretation 53 54 Major- and trace-element analyses of the basic meta-igneous rocks 55 show they have tholeiitic affinities, typical of the volcanic rocks 56 that were erupted during the extensional tectonic regime that 57 prevailed during deposition of the Argyll and Southern Highland 58 groups (Fettes et al., 2011). It has been suggested that such 59 meta-igneous rocks, which occur throughout the Crinan and 60 Tayvallich subgroups in various parts of the Grampian Highlands, 61 62 63 64 65 1 2 3 4 were high-level intrusions associated with the volcanic activity. 5 However, those in the area of Craig Leuchan and Pannanich Hill 6 intruded rocks that post-date the nearest metavolcanic rocks (the 7 Meall Dubh Metabasite Formation and the Balnacraig Metabasite 8 Member, close to the Easdale–Crinan subgroup boundary). Hence they 9 were most likely to have been associated with the tuffaceous Green 10 Beds that occur within the Southern Highland Group and crop out 11 12 extensively in the Glen Clova area, some 15–20 km to the south 13 (Smith et al., 2002). 14 The coexisting mineral phases present in the rocks of the Cairn 15 Leuchan to Pannanich Hill area are considered to represent 16 equilibrium assemblages and, other than some minor patchy 17 retrograde alteration, these are the products of peak metamorphic 18 conditions. The assemblage garnet-clinopyroxene-hornblende in the 19 basic meta-igneous rocks suggests conditions that approach 20 granulite facies, although Smith et al. (2002) have suggested that 21 the presence of diopside might reflect the protolith composition 22 rather than metamorphic grade. But, using the garnet-clinopyroxene 23 (Ganguly, 1979; Ellis and Green, 1979) and garnet-biotite (Ferry 24 and Spear, 1978) Fe-Mg exchange thermometers, Baker and Droop 25 (1983) calculated peak pressure in the area to have been close to 8 26 kbar at 820oC. Baker (1985) subsequently suggested that pressures 27 might even have exceeded 9 kbar, and these are undoubtedly the most 28 extreme metamorphic conditions so far recognized in the Grampian 29 30 Terrane. 31 The mineral assemblages, including abundant sillimanite in the 32 regional foliation, are intimately associated with F2 folds, and 33 hence the peak of metamorphism and the early migmitization were 34 syn-D2. Although there is some evidence in the area (e.g. near 35 Creag Dearg, NO 360 876) that some of the sillimanite developed 36 from kyanite, this is considered to be a continuous prograde 37 regional event and there is no evidence of it being a separate 38 later overprint as was suggested by Chinner (1961, 1966). In rocks 39 of the nearby Pannanich Hill Complex, reaction rims between peak 40 metamorphic assemblages preserved in inclusions of refractory 41 material and the migmatized matrix indicate that the later 42 migmatites developed some time after peak temperatures, almost 43 certainly between D2 and D3 (Goodman, 1991). Formation of these 44 later migmatites was aided by the presence of aqueous fluids, 45 shearing and high heat flow, the latter attributed by Goodman to 46 intrusions of the 470 Ma North-east Grampian Basic Suite that crop 47 48 out nearby (e.g. the Coyles of Muick Intrusion). There are no 49 estimates of the pressures and temperatures under which the later 50 migmatization occurred, but the minerals present in the Pannanich 51 Hill Complex inclusions, particularly andalusite and sillimanite, 52 indicate that their re-equilibriation occurred at lower pressure 53 than the regional maximum, though possibly still at high 54 temperature, suggesting some post-D2 uplift. 55 56 5.4 Conclusions 57 58 Mineral assemblages in both metasedimentary and basic meta-igneous 59 gneisses of Cairn Leuchan, Pannanich Hill and adjacent areas 60 clearly demonstrate that the peak metamorphic conditions here were 61 62 63 64 65 1 2 3 4 the most extreme so far recorded in the Scottish Dalradian, with 5 pressures in excess of 8 kbar and temperatures approaching 820oC. 6 The metasedimentary rocks, together with those of the nearby 7 Pannanich Hill Complex, clearly show two generations of migmatites, 8 the first coinciding with D2 deformation and peak metamorphism, the 9 second occurring somewhat later, after some regional uplift and 10 probably between the D2 and D3 events. The rocks also demonstrate 11 12 that the growth of sillimanite was the climax of a single prograde 13 metamorphic event and does not represent a later overprint as has 14 been previously suggested. The metamorphic rocks of this GCR site 15 therefore provide a wealth of information on the nature and timing 16 of high-grade regional metamorphism in the Grampian Terrane. 17 18 6 BALNACRAIG, DINNET 19 (NJ 4755 0045–NJ 4855 0160) 20 21 D. Gould 22 23 24 6.1 Introduction 25 26 The Balnacraig GCR site in northern Deeside provides a spectacular 27 illustration of the effects of intrusion of basic magma into 28 metasedimentary rocks that were already undergoing amphibolite- 29 30 facies regional metamorphism. Here, xenolithic gneisses containing 31 large, prismatic crystals of sillimanite have yielded valuable 32 information on the variation of pressure and temperature with time 33 during metamorphism. Adjacent exposures of amphibolite display 34 polyphase folding and are intruded by veins of leucotonalite. 35 Pelitic, semipelitic and psammitic metasedimentary rocks, 36 belonging to the Queen‘s Hill Formation of the Crinan Subgroup, 37 were intruded shortly after deposition by sills of tholeiitic 38 dolerite, which are now amphibolite sheets. During the Grampian 39 Event (c. 470 Ma), amphibolite-facies metamorphism and 40 migmatization occurred, and at the time of peak metamorphism, 41 norites of the Tarland Intrusion were emplaced, resulting in 42 partial melting of the adjacent metasedimentary rocks. 43 The first geological description of the area followed primary 44 mapping by the Geological Survey (Grant Wilson and Hinxman, 1890). 45 Work by Read (1927) highlighted the evidence for migmatization and 46 ‗injection‘ phenomena, as well as the presence of xenoliths of 47 48 more-refractory lithologies in the gneissose pelites. Read‘s 49 detailed petrography still forms the main descriptive work on the 50 rocks, although the interpretation has changed. Read described the 51 rocks between the outcrop of the Deeside Limestone Formation to the 52 south and the gabbroic and granitic intrusions to the north as a 53 ‗injection complex‘, which he considered to be caused by the 54 intimate admixture of silicic igneous materials with sedimentary 55 and igneous materials of earlier date. Depending on the proportion 56 of igneous material, the result ranged from largely metasedimentary 57 ‗oligoclase-porphyroblast-schist‘ through ‗oligoclase-biotite- 58 gneiss‘ to ‗orthoclase-oligoclase-biotite-gneiss‘, which Read 59 classed as largely igneous in origin. However, even the latter 60 61 62 63 64 65 1 2 3 4 contains fine-grained wisps of quartz, feldspar and biotite, and 5 isolated xenoliths, representing an original metasedimentary host. 6 Subsequently, Baker (1985) undertook a detailed petrological 7 study, including geothermometry and geobarometry, on these and 8 other selected rocks within the Dalradian of the North-east 9 Grampian Highlands to try to unravel the timing and localization of 10 metamorphic peaks. Then, following a resurvey of the area, Gould 11 12 (1997) re-interpreted the textural features described by Read as 13 being caused by anatexis. He also recognized the Tarland Intrusion 14 as a syn-D3 pluton of the North-east Grampian Basic Suite, and 15 identified the heat of the basic magma as the cause of the intense 16 local metamorphism and partial melting of the country rocks. 17 18 6.2 Description 19 20 This GCR site is located on the northern side of the valley of the 21 River Dee in Aberdeenshire, about 2 km north of Dinnet, and is 22 centred upon the cottage at Balnacraig. It lies within a belt of 23 coarse-grained, gneissose metasedimentary rock, 0.3 – 1.3 km wide 24 (Figure 11), which includes the rocky hills of Creag Ferrar, 25 Mullloch, Craigie and Scar Hill (300 m), and extends as far west as 26 the contact with the Tarland Intrusion at NJ 475 015. In places 27 the rocks are recognizably psammitic, semipelitic or pelitic, but 28 in most places the protolith is not recognizable. From south of 29 30 Craigie to Mulloch, the rocks have suffered shearing and 31 retrogressive metamorphism. However, a traverse from the eastern 32 side of Craigie at NJ 477 008 to the summit of Scar Hill (NJ 482 33 013) enables the full range of partial melting phenomena to be 34 examined and forms the nucleus of the site. 35 On the eastern slopes of Craigie there is a transition from 36 coarse-grained pelite with scattered porphyroblasts of oligoclase 37 up to 20 mm across, to gneisses in which pelitic material is cut by 38 irregular veins of granitic material carrying large garnet and 39 biotite crystals. In thin section, the pelites consist of 40 plagioclase (An23), biotite, garnet, sillimanite, and magnetite. 41 Minor cordierite and orthoclase are also present. The sillimanite 42 is coarsely crystalline, forming stumpy prisms aligned within the 43 main foliation. Later retrogressive metamorphism has formed 44 sericitic aggregates and, in places, andalusite replaces 45 sillimanite and plagioclase. 46 The gneisses display a single, coarsely developed foliation, 47 48 striking north-east and dipping steeply to the north-west. As the 49 traverse is continued from Craigie to Scar Hill, the proportion of 50 granitic material increases and the leucosome of the gneiss 51 includes orthoclase as well as plagioclase feldspar. The feldspar 52 porphyroblasts are mostly well crystallized, and contain few 53 inclusions. The proportion of pelitic material decreases until all 54 that is left is a number of large xenoliths, in which the foliation 55 is no longer parallel to that in the host, and a few streaks of 56 biotite and fibrolitic sillimanite. 57 The most noteworthy feature of the rocks is the presence of 58 sporadic xenoliths within the porphyroblastic gneisses of more- 59 refractory lithologies (quartzite, calcsilicate rock, silica-poor 60 61 62 63 64 65 1 2 3 4 pelitic hornfels) (Read, 1927). These xenoliths vary in shape from 5 elongated to almost spherical (Figure 12). 6 The siliceous xenoliths are banded feldspathic quartzites with 7 sutured quartz grains. They contain small grains of basic 8 plagioclase and flakes of colourless to red-brown biotite. The 9 pelitic xenoliths contain fractured crystals of garnet and 10 prismatic sillimanite and streaks of biotite lying within 11 12 aggregates of sericite. Biotite and garnet are in many cases 13 altered to chlorite. One pelitic xenolith contains staurolite, 14 forming large prisms concentrated within certain layers and 15 streaked out into films of pale mica with fibrolite needles. The 16 intervening layers consist largely of large flakes of muscovite and 17 biotite. 18 Xenoliths of calcsilicate rock show a poikiloblastic granular 19 texture and consist of diopside, amphibole, labradorite plagioclase 20 and quartz. Amphibole-rich xenoliths are abundant; compared with 21 the orthoamphibolites in the intrusive sheets of Craig Dhu and 22 Balnacraig Cottage (see below), they contain a higher proportion of 23 amphibole, and the plagioclase is more calcic, reaching anorthite 24 in some cases. Quartz occurs only as small pellets or tubules in 25 the plagioclase and amphibole. Some specimens contain large sieve- 26 like porphyroblasts of garnet. 27 A sheet of amphibolite, about 100 m thick, is well exposed at 28 Balnacraig Cottage (NJ 479 006). The rocks of the sheet have a 29 30 grain size of 0.5–1 mm, with a well-developed planar fabric and 31 consist of prismatic hornblende and granoblastic labradorite, with 32 minor clinopyroxene in places. The sheet is traversed by veins of 33 leucocratic material, both parallel to and cutting across the 34 foliation of the amphibolite. The veins consist of plagioclase 35 (generally oligoclase, in contrast to the labradorite of the 36 amphibolite), with minor quartz and hornblende. In places the 37 veins expand into large, diffuse patches within the amphibolite 38 (Figure 13). 39 40 6.3 Interpretation 41 42 Modern interpretations of the xenolithic gneisses at this GCR site, 43 and of other examples nearby within Crinan Subgroup semipelitic 44 rocks, suggest that the least-refractory metasedimentary rocks have 45 been partially melted during the peak of regional metamorphism 46 (syn-D3). This was contemporaneous with the intrusion of magmas, 47 48 now represented by mafic and ultramafic rocks of the Tarland, 49 Coyles of Muick and other plutons, which produced a large heat 50 flux. A granitic melt was formed first in semipelitic rocks, then 51 in pelites and feldspathic psammites. As the proportion of partial 52 melt of the pelitic rocks increased, to include some ferromagnesian 53 material, the resulting melt crystallized as quartz, orthoclase and 54 andesine, with minor garnet and biotite. The residue after this 55 more-intense partial melting was highly aluminous, and 56 recrystallized as knots of biotite, sillimanite, cordierite, 57 magnetite and, locally, spinel. Later metamorphism, producing 58 andalusite and sericitic aggregates, was post D3, but pre-dated the 59 local contact metamorphism associated with the Mount Battock and 60 Cromar granite plutons. Where partial melting of the amphibolite 61 62 63 64 65 1 2 3 4 occurred, the first melt to form was tonalitic, crystallizing as 5 quartz, oligoclase, and minor hornblende, reflecting its 6 undersaturation in alumina. 7 Migmatites resembling those in the Balnacraig GCR site also occur 8 within the Queen‘s Hill Formation to the south-west of the Ballater 9 Pluton, where they have been described as the Pannanich Hill 10 Complex (Goodman, 1994; see the Cairn Leuchan to Pannanich Hill GCR 11 12 site report). There, garnetiferous oligoclase-biotite gneisses 13 contain refractory inclusions showing peak regional metamorphic 14 assemblages. The gneissose matrix is considered to have formed by 15 reconstitution of semipelitic lithologies, aided by the presence of 16 abundant aqueous fluids and high temperatures. Peak metamorphic 17 conditions were estimated at 820°C and 8 kbar (Baker and Droop, 18 1983). Partial melting was considered to be likely under those 19 conditions. More conclusive evidence of partial melting of similar 20 metasedimentary rocks was obtained by Goodman and Lappin (1996) 21 from the aureole of the Lochnagar Pluton, where intrusion of 22 dioritic magma followed by granite magma into high-grade regional 23 metamorphic rocks produced temperatures of up to 750° C at 2.5 kbar 24 pressure in the aureole. Temperatures in the Balnacraig rocks 25 would have been comparable to those in the Pannanich Hill Complex 26 during the D3 metamorphic peak due to the proximity to the Tarland 27 basic intrusion, so at least some partial melting could be 28 expected. 29 30 The polyphase nature of the metamorphism was demonstrated by Baker 31 (1985), who found that andalusite, where present, always post-dated 32 sillimanite, indicating that pressures were significantly lower 33 during the later metamorphic episodes. Two specimens of gneissose 34 pelite from Balnacraig gave results of 762°C at 7.6 kbar, and 799°C 35 at 9.2 kbar for the main metamorphic event, similar to those at 36 Pannanich Hill. 37 38 6.4 Conclusions 39 40 The Balnacraig GCR site is of national and possibly international 41 importance as an exceptional example of the enhancement of the 42 effects of regional metamorphism by the emplacement of basic magma 43 shortly after the regional peak. Many classical features of 44 migmatization of metasedimentary rocks are displayed, including 45 veining by a granitic leucosome and the formation of xenoliths of 46 refractory compositions within irregularly layered gneisses. The 47 48 migmatization was a result of partial melting of the rocks during 49 the local D3 phase of deformation, which occurred shortly after the 50 regional metamorphism had reached its peak in the upper amphibolite 51 facies. This deformation was contemporaneous with the intrusion of 52 basic magma (the Tarland Intrusion), which might have increased the 53 heat flux sufficiently to cause a local increase in the proportion 54 of partial melt. 55 A striking feature of the gneisses is the presence of large 56 prismatic crystals of sillimanite that formed during the peak of 57 metamorphism. Later retrograde metamorphism at lower pressure 58 resulted in the growth of andalusite, and the well-preserved 59 mineral assemblages from this GCR site have contributed 60 significantly to a study of variations in temperature and pressure 61 62 63 64 65 1 2 3 4 during metamorphism in this part of the Grampian Highlands. 5 Possible further research could include an investigation of 6 variations in mineral compositions between the xenoliths and their 7 host rocks, and a comparison with similar metasedimentary rocks 8 near the eastern contact of the Morven–Cabrach Pluton. 9 10 11 7 MUCKLE FERGIE BURN 12 (NJ 164 140–NJ 167 139) 13 14 J.R. Mendum 15 16 17 7.1 Introduction 18 19 The lower part of the Muckle Fergie Burn, a tributary of the River 20 Avon 5 km south of Tomintoul, cuts through the basal units of the 21 Islay Subgroup, which here include several metamorphosed diamictite 22 beds that are readily correlated with the Port Askaig Tillite of 23 Islay and the Schiehallion Boulder Bed of Perthshire. This 24 section, together with other occurences in Upper Donside and 25 farther north to within sight of the Banffshire coast, shows that 26 the tillite unit recognized at the base of the Argyll Group in 27 Connemara, Donegal and Islay can also be traced through the 28 Grampian Highlands. The tillite unit was probably deposited over a 29 30 relatively short period of geological time; hence, it is 31 effectively considered to be a chronostratigraphical marker unit 32 within the Dalradian succession. In addition, the section below 33 the tillites in the Muckle Fergie Burn preserves the earliest 34 record of mafic volcanism in the Argyll Group, whilst farther east 35 in the section tuffs and lavas are also found higher in the Islay 36 Subgroup and in the Easdale Subgroup (Figure 14). 37 The Muckle Fergie Burn section was first described by L.W. Hinxman 38 during primary mapping for the Geological Survey in 1888-9. He 39 recognized the existence of a ‗boulder bed‘ akin to that described 40 near Schiehallion and recorded the presence of granitic and 41 dolomitic clasts in a greenish grey, sandy to silty matrix in the 42 brief memoir for Sheet 75 (Hinxman, 1896). Gregory (1931) also 43 included a description of the unit in his book on Dalradian 44 Geology. Both authors failed to attribute a glacial origin to the 45 beds, interpreting them as pebbly calcareous psammites. Morgan 46 (1966) carried out more-detailed mapping over a wide part of the 47 48 Muckle Fergie–Inchrory area and provided detailed descriptions of 49 the stratigraphy and structure. Then, following the recognition of 50 the glaciomarine origin of the Port Askaig boulder bed (Kilburn et 51 al., 1965), Spencer and Pitcher (1968) extended the correlation and 52 interpretation to the Muckle Fergie Burn section and published a 53 stratigraphical log, as well as noting metadiamictite occurrences 54 farther north-east near Fordyce. The Geological Survey remapped 55 the area in the 1980‘s during the revision of 1:50 000 Sheet 75W 56 (Glenlivet, 1996) and this description uses material gathered 57 during that work. 58 Spencer and Pitcher (1968) recognized the wider importance of 59 metadiamictite units with regard to correlation along the whole 60 Dalradian outcrop and more widely in the North Atlantic region, a 61 62 63 64 65 1 2 3 4 concept that was extended to a worldwide scale by Hambrey and 5 Harland (1981). A tentative correlation has long been made between 6 the Port Askaig Tillite and the Varanger tillites of northern 7 Norway (Spencer, 1971). Recent Rb-Sr illite age dating has 8 bracketed deposition of the Varanger glacial deposits between 620 9 and 590 Ma (Gorokhov et al., 2001; Bingen et al., 2005), which is 10 more comparable with the top of the Argyll Group than the bottom. 11 12 Hence, more-recent suggested correlations are with the older 13 Marinoan (c. 635 Ma) or Sturtian (c. 720 Ma) glaciations. (See 14 Stephenson et al., 2013a for a full discussion.) 15 16 7.2 Description 17 18 The lower part of the Muckle Fergie Burn flows through a wooded and 19 shrubby gorge, some 240 m south of Auchnahyle. The burn provides a 20 reference section for the Auchnahyle and overlying Kymah Quartzite 21 formations (Islay Subgroup) and in its upper part passes through 22 the overlying semipelitic, pelitic and basic metavolcanic units of 23 the Easdale Subgroup. Generally the sequence dips at 30–70° to the 24 east and is the right way up, although some overturned sections are 25 present locally. Medium-scale, close to tight folding repeats some 26 of the ‘boulder bed’ units. 27 The metadiamictite beds lie within the Auchnahyle Formation, an 28 interbedded sequence of psammites and semipelites containing 29 30 amphibolites in the lower part and beds of metalimestone and 31 metadolostone in the upper part. The formation maps out as a 32 lensoid unit, faulted out on its northern margin, whose strike 33 length is about 2 km and maximum thickness is of the order of 150 34 to 200 m (Figure 14). The contact of the Auchnahyle Formation with 35 the underlying Glenfiddich Pelite Formation of the Blair Atholl 36 Subgroup is not exposed in the Muckle Fergie Burn. However a rapid 37 transition from graphitic pelites and semipelites conformably up 38 into amphibolites is seen on a rocky bluff at NJ 163 143, some 300 39 m to the north. 40 The exposed burn section (Figure 14) starts with 7 m of dark grey- 41 green amphibolite that contains two cleavages and prominent quartz 42 and minor calcite veining. Tightly folded, thin- to medium-bedded 43 psammites and semipelites overlie the amphibolite and within these 44 beds (at NJ 1647 1401) is a prominent coarse- to medium-grained 45 amphibolitic unit with an internal structure resembling pillows. 46 No vesicles are seen but the ‗pillow structures‘ do contain radial 47 48 cracks and show a crude textural zonation; features indicative of 49 lavas. The micaceous and feldspathic psammite and semipelite 50 sequence immediately upstream from the pillowed amphibolites 51 contains some lenticular siliceous psammite units. Several 52 psammite beds show cut-offs and grading implying younging to the 53 east. A c. 4 m-thick sheet of foliated metadiorite intrudes the 54 psammite-semipelite sequence here. 55 At NJ 1655 1399 a c. 4 m-thick, thinly banded, grey, fine-grained 56 crystalline metalimestone with thin biotite-rich pelitic interbeds 57 is seen. Upstream for some 15 m, graphitic pelite and minor thin 58 metalimestones are present in the thinly bedded psammite-semipelite 59 sequence. They are succeeded by a c. 6 m-thick cream- to fawn- 60 weathering, mottled white and pink, fine- to medium-grained 61 62 63 64 65 1 2 3 4 metadolostone, which has a rubbly fragmental zone at its top. It 5 passes up into a c. 5 m-thick, unbedded, grey-green, amphibole- 6 bearing, highly micaceous psammite containing subangular to 7 subrounded rusty brown-stained metadolostone clasts in its upper 8 part. The metadolostone clasts are deformed with elongations as 9 high as 7:1. This metadiamictite unit and others upstream are 10 notably pyritic. A thin psammite bed separates the lower 11 12 metadiamictite from the succeeding 5–7 m-thick metadiamictite that 13 contains moderately abundant granitoid and metadolostone clasts in 14 a slightly purplish grey matrix of highly micaceous psammite. 15 These units are succeeded by psammite and a further 6 m-thick 16 metadiamictite with quartz and rare granitoid pebbles and cobbles. 17 A thicker sequence of psammites follows eastwards. The lowest 18 unit is an indurated recrystallized quartzite with pyrite and minor 19 chlorite giving it an unusual translucent dark bluish green-grey 20 colour, but the higher parts vary from pink siliceous psammite to 21 feldspathic and micaceous psammite. Thin amphibolite sheets are 22 present. A further 4–5 m-thick metadiamictite unit is exposed at 23 the second waterfall (at NJ 1657 1396). It contains scattered 24 granitoid clasts in a grey-green amphibole-bearing semipelitic 25 matrix grading up to a grey semipelite with small metadolostone 26 clasts (Figure 15). A 4 m-thick sheet of metadiorite could be the 27 same intrusion as that seen in the lower part of the section. An 28 upper metadiamictite, some 5m thick and consisting of green-grey 29 30 highly micaceous psammite with scattered small granitoid and rare 31 metadolostone pebbles, occurs at NJ 1661 1395. 32 There is a lack of exposure for some 300 m in the burn section 33 above this lower part until the more-massive cross-bedded siliceous 34 psammite of the Kymah Quartzite Formation is reached. This forms a 35 major scarp feature with the prominent small crags of Sìdh Beag and 36 Sìdh Mòr above the burn (Figure 14). 37 The metadiamictite units are invariably matrix supported. The 38 matrix is normally a highly micaceous psammite, which in thin 39 section is seen to be markedly inequigranular with angular to 40 subrounded clasts of quartz, plagioclase and potash feldspar, and 41 clots of chlorite, partly overgrown by biotite. Apatite is a 42 common accessory and zircon is also present. The matrix is 43 dominated by chlorite but locally it is rich in carbonate or 44 amphibole, the latter now mainly altered to chlorite and biotite. 45 The matrix is normally structureless, although a crude ill-defined 46 lamination can be discerned in places. 47 48 The majority of clasts are of pebble size with a few cobbles up to 49 26 cm and a white granite boulder some 41 cm by 26 cm was recorded 50 by Morgan (1966). White to pale-yellow metadolostone is the 51 predominant sedimentary variety of clast, although grey 52 metalimestone (Blair Atholl Subgroup), quartzite, gritty psammite 53 and slate have also been recorded (Gregory, 1931; Morgan, 1966). 54 The igneous clasts are largely granitic and range from white to 55 pink granite to quartz-syenite and granodiorite, with diorite less 56 abundant. 57 Although the exposure in the Muckle Fergie Burn does not allow 58 full documentation of the structure, open to tight minor folds are 59 seen at various points and bedding dips range from near horizontal 60 to near vertical, implying the presence of small- and medium-scale 61 62 63 64 65 1 2 3 4 folds. Minor fold axes plunge gently to the south-east, 5 corresponding approximately with a lineation (L2). A crenulation 6 cleavage (S3) that dips steeply to the north-east is developed 7 locally in the more-pelitic units (Morgan, 1966). Two major fold 8 phases (F2 and F3) are well displayed in Blair Atholl Subgroup 9 metalimestones in the Little Fergie Burn 3 km to the south-south- 10 east, where F2-F3 fold interference patterns are present. There, 11 12 F2 and F3 fold axes mainly plunge gently to the east-south-east, 13 but some F2 axes plunge gently to the north-west. 14 15 7.3 Interpretation 16 17 The true thickness of the Auchnahyle Formation in the Muckle Fergie 18 Burn is unclear. Morgan (1966) alluded to repetition by folding, 19 but confusingly included the repetition in his stratigraphical 20 sequence. Similarly, the stratigraphical log given by Spencer and 21 Pitcher (1968) shows nine metadiamictite beds, but does not allow 22 for or even reflect the fold repetition. Allowing for dips varying 23 from 30° to near vertical, the sequence would be 250 m to 280 m 24 thick, but fold repetition probably reduces this to nearer 150 m. 25 The metadiamictite beds are interpreted as marine glacial tillites 26 (Spencer and Pitcher, 1968) and are correlated with the more- 27 extensive and less-deformed Port Askaig Tillite Formation 28 documented by Spencer (1971) from Islay and the Garvellach Islands 29 30 (see the Garvellach Isles GCR site report in Tanner et al., 2013a). 31 The lowest fragmental metadolostone unit represents the start of 32 glacial deposition in this area, recording the scouring of the 33 immediately underlying unit, presumeably by ice and/or meltwater. 34 The overlying tillite bed contains some metadolostone clasts but 35 also includes granitoid cobbles and represents input from a wider 36 area. 37 Abundant amphibolite sheets and some amphibole-rich metadiamictite 38 units occur in the upper part of the succession, whereas thicker 39 amphibolite sheets and basic pillow lavas are found below the 40 metadiamictite beds. These features, together with the ubiquitous 41 presence of pyrite, suggest that input from a mafic volcanic source 42 coincided with the glacial episode. Much of this altered 43 amphibolitic material could be derived either from erosion of basic 44 volcanic units or relate directly to volcanic activity. In the 45 Ladder Hills area to the north-east, the lowest Islay Subgroup 46 sedimentary sequence is considerably thicker than in the Muckle 47 48 Fergie Burn and tuffaceous and lava units occur within the 49 turbiditic psammite-semipelite succession (see the Kymah Burn GCR 50 report). This volcanic association is prominent only in the 51 tillite units of the North-east Grampian Highlands (Harris et al., 52 1994; Fettes et al., 2011). 53 There is also a close association of metadiamictite with 54 metadolostone units in the Muckle Fergie Burn, in Upper Donside and 55 in the Ladder Hills, suggesting that the dolomitization and 56 glaciation are closely related. 57 The tillite unit is generally accepted to be a 58 chronostratigraphical marker and could have formed at either c. 635 59 Ma (Marinoan tillites) or at c. 723 Ma (Sturtian tillites). 60 Adoption of either age creates problems in trying to understand the 61 62 63 64 65 1 2 3 4 depositional history and palaeogeography of the Dalradian 5 succession. The older age allows little time, possibly only 30 Ma, 6 for the deposition of the Grampian and Appin groups but gives a 7 period of c. 120 Ma for the deposition of the Argyll Group. The 8 more likely younger age, however, allows 120 Ma for deposition of 9 the Grampian and Appin groups and c. 30 Ma for deposition of the 10 Argyll Group (see Stephenson et al., 2013a). 11 12 13 7.4 Conclusions 14 15 The lower part of the Muckle Fergie Burn section contains several 16 poorly sorted, matrix-supported ‘boulder beds’, similar to those 17 found near the base of the Argyll Group at Schiehallion, the 18 Garvellach Isles and on the Isle of Islay. The ‘boulder beds’, more- 19 precisely termed ‘metadiamictites’, represent marine glacial tills 20 and form a small relict fragment of a unit that can be traced 21 intermittently from Connemara in the west of Ireland to the 22 Banffshire coast. They record the presence of glacial conditions 23 during a finite time period in the late Neoproterozoic and hence 24 form a chronostratitigraphical marker unit that is possibly the 25 most reliable and the most widespread in the whole Dalradian 26 succession; the Muckle Fergie Burn provides the most north-easterly 27 detailed section through this crucial unit. 28 In the Muckle Fergie Burn section, the lowest metadiamictite rests 29 30 upon an erosion surface and is dominated by clasts of metalimestone 31 and metadolostone that were most likely derived quite locally from 32 the underlying Appin Group rocks. But, as is the case elsewhere, 33 higher metadiamictites contain an increasing proportion of granitic 34 clasts reflecting a much more-widespread source area. 35 The section also contains poorly preserved pillow lavas, which 36 provide evidence of basic volcanism coeval with the glaciation. 37 Not only is this the only area in the whole strike length of the 38 tillite unit where volcanic rocks are found but it is also the 39 earliest evidence for basic igneous activity anywhere in the 40 Dalradian succession; volcanism subsequently continued throughout 41 Argyll Group and much of Southern Highland Group times. 42 These tillites and related lithologies clearly consititute vital 43 evidence for a major glacial period in the Earth‘s history and 44 hence are of great international importance. Much interest is 45 currently focussed upon correlation with other, dated, glacial 46 deposits in the North Atlantic region and worldwide. Possibilities 47 48 include the Marinoan tillites at c. 635 Ma or even the Sturtian 49 tillites at c. 723 Ma. The correlation has vital implications for 50 the history of Dalradian sedimentation and for global 51 reconstructions of the Neoproterozoic Era. 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 8 BRIDGE OF BROWN 5 (NJ 1200 2117–NJ 1269 2019) 6 7 J.R. Mendum 8 9 10 11 8.1 Introduction 12 13 The river section at Bridge of Brown, on the A939 between Tomintoul 14 and Grantown-on-Spey, provides one of the few coherent sections 15 through the transition from the Grampian Group up into the Appin 16 Group. The section spans the uppermost psammite and quartzite units 17 of the Grampian Group, the interbedded psammites, semipelites and 18 highly calcareous semipelites of the Lochaber Subgroup, and lower 19 Ballachulish Subgroup metalimestone, calcareous semipelite, and 20 graphitic pelite–semipelite units. Within the Lochaber Subgroup is 21 a distinctive gneissose kyanite-garnet-muscovite-biotite semipelite 22 unit that can be traced northwards as far as the Banffshire Coast. 23 The sequence is deformed but no evidence is seen for a major slide, 24 such as occurs at this stratigraphical level in the Schiehallion 25 and Glen Tilt areas of Perthshire to the south-west. 26 The Bridge of Brown GCR site is complementary to the adjacent 27 Bridge of Avon GCR site in that it extends the stratigraphical 28 section down through the lower part of the Appin Group and into the 29 30 Grampian Group. It also continues the structural cross-section to a 31 lower level. The bedding dips moderately to the south-south-east 32 throughout the section and, although minor folds are seen locally, 33 there is no evidence for significant fold repetition or inverted 34 bedding. Similarly, although the bedding appears to be somewhat 35 attenuated, lineations and strong planar fabrics are conspicuously 36 absent. The transition from the thick, competent, lithologically 37 relatively uniform, psammite-dominated Grampian Group to the mixed 38 pelite–metalimestone–quartzite Appin Group sequence must act as a 39 focus for enhanced deformation. An early slide is interpreted to 40 lie at or near the base of the Ballachulish Subgroup over much of 41 the Glenlivet district to the south but in the Burn of Brown 42 section only a small part of the stratigraphy appears to be 43 excised. South of the Cairngorm Granite Pluton, ductile sliding is 44 focussed along the Grampian–Appin group boundary (see the Glen Ey 45 Gorge and Gilbert’s Bridge GCR site reports) and farther north 46 around Ben Rinnes mylonitic rocks are also developed at this level. 47 48 On the Banffshire Coast section, west of Sandend a stratigraphical 49 transition is well seen (see the Cullen to Troup Head GCR site 50 report), but there the Lochaber Subgroup is abnormally thick. 51 The Bridge of Brown area was originally mapped by L.W. Hinxman 52 during the primary geological survey but was not deemed worthy of 53 particular mention in the Sheet 75 memoir (Hinxman, 1896). The 54 area was remapped by the Geological Survey as part of the revision 55 of the bedrock geology of the Glenlivet district (1:50 000 Sheet 56 75W, Glenlivet, 1996) and that work forms the basis for this 57 account. 58 59 60 61 62 63 64 65 1 2 3 4 8.2 Description 5 6 The Burn of Brown flows through a narrow incised gorge, where the 7 original, General Wade road crossed the burn; the more-recent road 8 bridges occur a few hundred metres downstream (Figure 16). The 9 incised section probably represents capture of the Glen Brown 10 drainage basin by the Burn of Lochy, with the former drainage 11 12 flowing eastwards to the River Avon via Fodderletter. 13 14 8.2.1 Stratigraphy 15 16 The Grampian Group rocks are seen near the ruin of Blàr an Lochain 17 (NJ 1213 2110), where they comprise feldspathic, siliceous and 18 micaceous psammite with semipelite interbeds with rare 19 calcsilicate-rock lenses and lenticular bands. These mixed 20 lithologies form part of the Strathavon Psammite Member of the 21 Tormore Psammite Formation. Individual semipelite units reach 2 to 22 3 m in thickness and show evidence of two penetrative cleavages. 23 The semipelites contain garnet, muscovite and biotite and the 24 calcsilicate-rock lenses contain garnet and hornblende. These 25 metamorphic mineralogies are characteristic of the lower 26 amphibolite facies. Upstream, at around NJ 1207 2100, thin- to 27 medium-bedded psammites are dominant with bed thicknesses normally 28 ranging from 2 cm to 30 cm, but reaching up to 50 cm in the coarser 29 30 grained units. Good examples of cross-bedding and slump folds with 31 prominent cut-offs show that the beds are right way up. Slump fold 32 axes plunge to the south-east. Upstream, thinly bedded feldspathic 33 and micaceous psammites with minor semipelite beds and partings are 34 dominant with thin quartzite beds present locally. 35 Just below Bridge of Brown, flaggy, thinly bedded to laminated 36 semipelite and micaceous psammite become dominant and constitute 37 the basal Lochaber Subgroup unit, the Dalvrecht Slate Formation. 38 These rocks have a strong penetrative planar fabric and a slaty 39 parting. In thin section they contain quartz, plagioclase 40 feldspar, muscovite and biotite, with garnet common in parts. The 41 abundant micas define three separate cleavages in some of the 42 specimens. Thin bands of calcsilicate rock are also present. 43 Between the present bridge and the Wade bridge greenish grey 44 micaceous and highly micaceous psammites with minor calcsilicate- 45 rock bands crop out. They pass upstream into flaggy, sparsely 46 garnetiferous, micaceous psammites with thin siliceous psammite 47 48 interbeds. Calcsilicate-rock bands are common and quartz veins are 49 present. The Dalvrecht Slate Formation is some 120 m thick here. 50 Upstream the beds become grey-green in colour and consist of 51 thinly bedded but lithologically more-uniform, calcareous 52 semipelite and highly micaceous psammite with abundant lenses of 53 white and green calcareous quartzite. Locally the calcsilicate- 54 rock lenses overgrow bedding features showing that they were formed 55 during diagenesis or perhaps even later. In parts darker green 56 amphibolitic beds (originally marls) are seen. These calcareous 57 units constitute the Fodderletter Calcareous Flag Formation. In 58 thin section the calcsilicate rocks contain much tremolite but 59 relict diopside is present locally. At NJ 1266 2034, a massive 60 gneissose kyanite-muscovite-biotite-(garnet) semipelite bed, some 5 61 62 63 64 65 1 2 3 4 m thick, forms a small waterfall. This is the Fireach Beag Kyanite 5 Gneiss Member and contains elongate laths of blue-grey kyanite up 6 to 2 cm long. Under the microscope the gneiss consists of coarse- 7 grained, well-formed muscovite laths intergrown with more-ragged 8 biotite enclosing coarse-grained aggregates of quartz and 9 plagioclase (oligoclase). Kyanite forms laths up to 5 mm long and 10 is partly altered to fine-grained muscovite with chlorite common 11 12 locally. Garnets are altered to quartz-feldspar-biotite-ilmenite 13 aggregates and fractured staurolites up to 1 mm long are present. 14 Ilmenite is common and rutile, apatite and tourmaline are also 15 present. Above the gneiss the formation is lithologically more 16 variable and consists of thinly bedded calcareous and non- 17 calcareous micaceous psammites, semipelites and biotitic pelite 18 units. Calcsilicate-rock units remain abundant and are locally 19 pyritic. At NJ 1267 2027, graded units are seen and possible slump 20 folds occur at NJ 1268 2023. The highest lithology exposed in the 21 GCR site consists of slaty micaceous psammite and semipelite, which 22 show tight folding locally. The overlying Mortlach Graphitic 23 Schist Formation is not exposed in the burn section but a block of 24 silicified metalimestone has been recorded in the nearby till and 25 graphitic pelite that characterizes the formation is seen widely in 26 the float, together with dark-grey tremolitic material representing 27 graphitic calcareous mudstone. 28 29 30 8.2.2 Structure 31 32 The Grampian and Appin group units form an ordered succession that 33 dips between 20 and 45° to the south-east. Although minor folding 34 has duplicated the succession locally, the sequence is essentially 35 right way up and youngs to the south-east (Figure 17). The best 36 examples of tight F2 folds and open to tight F3 folds are seen in 37 the interbanded psammites and semipelites of the Grampian Group at 38 the northern end of the section. Above the Allt an Doruis, at NJ 39 168 2139, gritty quartzite beds and intervening semipelites are 40 tightly folded, with the axes of tight recumbent F2 folds plunging 41 gently to the east-north-east and axial planes dipping moderately 42 to the south-east. A quartz mineral lineation also plunges gently 43 to the east-north-east, near-coincident with the F2 axes. The fine 44 S2 cleavage is best seen in semipelite units in the F2 hinges. 45 Higher in the stratigraphy, very little evidence is seen for minor 46 F2 folds, with only isolated examples reported. The F3 folds vary 47 48 from open to tight and are also best seen in the mixed Grampian 49 Group lithologies. Examples are recorded at the Allt an Doruis and 50 around NJ 1198 2109, where layers of calcsilicate rock show 51 excellent NW-verging close F3 folds, whose axes plunge gently to 52 the north-east. A penetrative axial plane crenulation cleavage 53 that dips steeply to the south-east is developed in the adjacent 54 semipelite layers. The F3 folds refold an earlier easterly 55 plunging L2 lineation. 56 Although minor folds are rare in the Lochaber Subgroup rocks, 57 several cleavage generations can be recognized in hand specimen and 58 in thin section these show discordant relationships. In the 59 laminated psammite-semipelite units of the Dalvrecht Slate 60 Formation, an early fine-scale mica cleavage, S1, is preserved 61 62 63 64 65 1 2 3 4 locally in the 0.5 to 2 mm microlithons between the dominant spaced 5 muscovite-rich lamellae that form the main S2 spaced/crenulation 6 cleavage. In some specimens a later cleavage, defined by muscovite 7 laths, lies markedly discordant to the earlier fabrics. This S3 8 cleavage can relate to open to close minor folds. Generally all 9 three cleavages dip more steeply than bedding. Garnets, where 10 present, contain inclusion trails of the S1 cleavage and apparently 11 12 pre-date the S2 cleavage. In the more-slaty units, by Bridge of 13 Brown, minor F4 kink folds are also sparsely developed. Recorded 14 F4 axes plunge gently east and south-west. 15 Peak metamorphic conditions were attained during the D2 16 deformation, and in this area they reached temperatures of 620 to 17 650°C and pressures of 8 to 8.5 kbar (Beddoe-Stephens, 1990). 18 These conditions lie close to the upper limits of the lower 19 amphibolite facies. 20 21 8.3 Interpretation 22 23 The Grampian Group rocks represent shallow-marine shelf-sands and 24 subsidiary silts with material being repeatedly reworked. The 25 presence of cross-bedding and slump structures attests to the 26 presence of strong currents and at least locally, relatively rapid 27 deposition. The transition to Appin Group rocks is marked by the 28 incoming of more-intermixed psammite and semipelite and calcareous 29 30 lithologies that make up the Lochaber Subgroup. It signifies basin 31 shallowing and regression in this area with some possible emergent 32 areas, although in the Northern Grampian Highlands, Banks (2005) 33 has suggested that the semipelitic and pelitic elements represented 34 more-distal deposition during a moderate transgression. The 35 quartzitic units are interpreted as a product of reworking of the 36 underlying succession, rather than input of additional sand 37 material. The Fireach Beag Kyanite Gneiss, which forms a marker 38 unit, represents aluminous mud and silt, possibly representing 39 input of tropically weathered material derived from the nearby 40 source area. 41 Elsewhere in the Dalradian succession, block uplift appears to 42 have created local unconformities in the Lochaber Subgroup 43 succession, and in extreme cases the entire subgroup is absent (see 44 Treagus et al., 2013). Although the presence of gaps in the 45 succession at Bridge of Brown cannot be ruled out, there is no 46 evidence for significant gaps in the lithostratigraphy. The 47 48 Fodderletter Calcareous Flag Formation occupies a similar position 49 in the stratigraphy to the Leven Schist Formation of Glen Spean and 50 Appin and the Baddoch Burn Striped Pelite of Glen Shee. It 51 presages the incoming of the Ballachulish Subgroup, indicative of a 52 more-widespread transgression that covered most of the upstanding 53 blocks (Banks, 2005). 54 The Grampian–Appin group boundary throughout much of Perthshire is 55 marked by a zone of very highly attenuated Appin and Argyll group 56 rocks that form a major NNW-verging D2 shear-zone, termed the 57 'Boundary Slide' (see the Allt Druidhe, Strath Fionan, Gilbert’s 58 Bridge and Glen Ey Gorge GCR site reports). In those areas, the 59 position of the slide might also reflect an original unconformity 60 or a basement lineament. The tectonics of the North-east Grampian 61 62 63 64 65 1 2 3 4 Highlands are somewhat different and shear-zones are found within 5 several parts of the Grampian, Appin and Argyll group succession, 6 mostly reflecting thrusting to the north-west during the major 7 Grampian D2 event. Although the Grampian–Appin group boundary does 8 represent a major lithological competence contrast, it is not 9 coincident with a single, laterally continuous major shear-zone and 10 in many places, particularly towards the north coast, there is no 11 12 shearing or dislocation at all. At Bridge of Brown, although there 13 is evidence of increased strain and even localized shearing in the 14 Lochaber Subgroup rocks, no specific Boundary Slide-type structure 15 is present. 16 17 8.4 Conclusions 18 19 The Bridge of Brown GCR site demonstrates the transitional nature 20 of the contact between Grampian Group and Appin Group strata in the 21 North-east Grampian Highlands. Here, there is no major shear-zone 22 or dislocation at this junction, in marked contrast to the 23 situation in Perthshire, where the Boundary Slide is recognized. 24 Structurally the rocks are relatively simple in that the 25 succession dips moderately to the south-east and the beds become 26 younger in that direction. The Grampian Group rocks are psammites 27 and subsidiary semipelites that show cross-bedding and slump 28 structures indicative of shallow-marine shelf deposition. Where 29 30 lithologies are mixed, two sets of folds and related cleavages are 31 developed. They pass upwards into thinly interbedded psammites and 32 semipelites with thin quartzites that mark the lowest beds of the 33 Lochaber Subgroup. These beds are attenuated and show evidence of 34 three cleavages and increased strain. They are succeeded upwards 35 by calcareous semipelites and micaceous psammites with abundant 36 bands and lenses of calcsilicate rock, minor graphitic pelite and 37 some thin quartzite beds. These lithologies show very little 38 internal structural complication but they do contain a prominent 39 massive gneissose kyanite-muscovite-biotite-garnet semipelite unit 40 that can be recognized as a marker bed in several parts of the 41 North-east Grampian Highlands. 42 This site is complementary to the Bridge of Avon GCR site, which 43 effectively extends the cross-section to the south-east. It also 44 provides an important reference site between the complex geometry 45 of the Boundary Slide in Perthshire and the enhanced 46 stratigraphical sequence of the Banffshire Coast. 47 48 49 9 BRIDGE OF AVON 50 (NJ 1497 2032–NJ 1541 1915) 51 52 J.R. Mendum 53 54 55 9.1 Introduction 56 57 The River Avon and adjacent areas around Bridge of Avon, 2 km 58 north-west of Tomintoul, expose sections through the Ballachulish 59 and Blair Atholl subgroup rocks that here are disposed in 60 kilometre-scale fold patterns. The sequence includes several very 61 62 63 64 65 1 2 3 4 distinctive units that can be recognized not only throughout the 5 North-east Grampian Highlands but also along most of the Dalradian 6 outcrop elsewhere and hence are valuable stratigraphical markers. 7 Here, the stratigraphy is condensed and probably represents 8 deposition over an original basin high. The Ballachulish Subgroup 9 sequence ranges from graphitic pelite of the Mortlach Graphitic 10 Schist Formation up through the Corriehabbie Quartzite Formation to 11 12 the mixed Ailnack Phyllite and Limestone Formation. The Inchrory 13 Limestone Formation represents the overlying Blair Atholl Subgroup 14 rocks. Although the lithologies are deformed and metamorphosed to 15 kyanite grade (lower amphibolite facies), they clearly show 16 internal bedding features in parts e.g. cross-bedding in the 17 quartzite and grading in some of the semipelitic units. 18 The structure of the Dalradian rocks is complex and four 19 deformation phases can be recognized. Bedding generally dips 20 moderately to the south and south-east but dips vary from 25° to 21 vertical locally. Cleavages are best seen in the more-pelitic 22 units, notably in the graphitic pelite. Minor folding is well 23 displayed in the thinner bedded units, notably in the calcareous 24 semipelite and psammite. A set of faults that mainly trend north– 25 south disrupt the structural pattern. These faults are of Devonian 26 and later age, as several of them affect the conglomerates and 27 sandstones of the late Silurian to Early Devonian Tomintoul 28 Outlier, which overlie the Dalradian rocks about a kilometre south 29 30 of the GCR site. 31 The Bridge of Avon area was mapped by L.W. Hinxman during the 32 primary geological survey and brief descriptions were given in the 33 Sheet 75 memoir (Hinxman, 1896). Hinxman mapped the area solely on 34 the basis of lithology and in some areas his map linked together 35 several stratigraphically disparate units. The area was remapped 36 by the British Geological Survey between 1982 and 1988 (1:50 000 37 Sheet 75W, Glenlivet, 1996), and that work forms the basis for this 38 account. A detailed geochemical study of Dalradian metacarbonate 39 units by Thomas (1989, 1999) included several samples from the 40 Bridge of Avon area. 41 42 9.2 Description 43 44 The Bridge of Avon GCR site encompasses some 1.4 km of the River 45 Avon section, extending from downstream of the old General Wade 46 bridge, up river almost to below the abandoned lime quarry at Creag 47 48 Chalcaidh. It also includes the lowermost part of the Allt na 49 Cluaine and some small crags marginal to alluvial terraces. 50 Exposure is not continuous but amalgamation of all the information 51 available gives a moderately comprehensive picture of the geology 52 (Figure 18). 53 54 9.2.1 Stratigraphy 55 56 The Mortlach Graphitic Schist Formation forms the core of an F2 57 anticline that is mapped mainly from the float on Tom Beag, west- 58 south-west of Bridge of Avon. Poor exposures are seen in the river 59 section on its west bank. It consists mainly of dark-grey, to 60 nearly black, schistose graphitic pelite and semipelite, with small 61 62 63 64 65 1 2 3 4 garnets abundant in parts. Pyrite and quartz veins are also 5 abundant locally and in thin section kyanite and staurolite 6 porphyroblasts are common. The dominant parting is a millimetre- 7 scale penetrative spaced cleavage (S2). The earlier fine-grained 8 S1 fabric, which generally lies near-parallel to bedding, is seen 9 only in thin section. The formation shows a rapid transition 10 upstream into the Corriehabbie Quartzite Formation, a white to 11 12 fawn, blocky, commonly indurated, fine- to coarse-grained 13 quartzite, with minor siliceous and feldspathic psammite. It is 14 thin to medium bedded with some gritty feldspathic basal zones in 15 individual sand units. Cross-bedding is present locally, defined 16 by heavy-mineral streaks (magnetite/haematite). Around Bridge of 17 Avon the quartzite formation is only about 55 m thick but it 18 thickens to over 250 m a few kilometres to the north-east and 19 farther south in the Water of Ailnack section. The outcrop pattern 20 of the quartzite is convolute in the area north-west of Tomintoul 21 as a result of both F2 and F3 kilometre-scale folding. These give 22 rise to the interference structures seen in plan on Figure 18, 23 which together with the initial thickness variations and later 24 faulting, result in a complex structural pattern. Some of the 25 step-like offsets of the quartzite outcrop around Bridge of Avon 26 are a product of local F4 folding. 27 The overlying Ailnack Phyllite and Limestone Formation consists of 28 psammite, semipelite, metalimestone and minor pelite lithologies. 29 30 Calcsilicate rocks are characteristic of parts of the formation and 31 thin quartzite beds commonly occur near its base. An across-strike 32 section occurs beneath the Bridge of Avon itself (NJ 1495 2015) but 33 the unit forms a complex outcrop pattern in this area. It contains 34 some prominent members that can be recognized widely on Sheet 75W 35 (Glenlivet). At the base of the formation lies the Torulian 36 Limestone Member, which here forms a prominent, almost pure white 37 unit c. 5 m thick. Its outcrop can be traced from the bank into 38 the rocky bed of the river, where it is cut out against a fault. 39 The metalimestone consists dominantly of calcite with pyrite-rich 40 laminae. It shows a poorly defined, thin banding that has been 41 etched out by the river to show tight to isoclinal folds with 42 amplitudes of up to 2 m. Their axial planes lie subparallel to the 43 bedding, and the folds are confined to individual layers, in parts 44 truncated by overlying thin beds. They are interpreted as slump 45 folds but could be F1 structures. 46 The metalimestone member is succeeded by phyllitic semipelite and 47 48 pelite, in parts graphitic, which is followed by thinly banded 49 calcareous semipelite, calcsilicate rock and impure metalimestones 50 that show good examples of F3 minor folding. These latter 51 lithologies are exposed directly beneath the Wade bridge. They 52 pass upwards into the more-semipelite-dominated upper part of the 53 Ailnack Phyllite and Limestone Formation, which here is distinctive 54 enough to be termed the Kylnadrochit Semipelite Member. This unit 55 is about 65 m thick, and consists of purplish and greenish mid- 56 grey, flaggy to blocky, calcareous, highly micaceous psammite and 57 semipelite. The beds are laminated to thinly banded, commonly with 58 retrogressed garnet porphyroblasts. They are folded by open to 59 tight F3 folds that have attenuated limbs and a related penetrative 60 S3 planar cleavage; they verge mainly to the south-west. In the 61 62 63 64 65 1 2 3 4 lower part of the Allt na Cluaine section, between NJ 1475 1932 and 5 NJ 1494 1937, more-pelitic and graphitic lithologies at the top of 6 the member are exposed. These are charcoal grey to blue-grey, 7 flaggy to fissile, calcareous semipelite and pelite with abundant 8 small garnet porphyroblasts. Tight folding is present, but in the 9 semipelitic units good fine-scale grading has been recorded. These 10 uppermost lithologies are probably equivalent to parts of the 11 12 Clashnoir Semipelite Formation, which is mapped as a separate unit 13 in the Blair Atholl Subgroup to the north-east in the Braes of 14 Glenlivet, where the sequence is considerably thicker. 15 In the Bridge of Avon area, the Kynadrochit Semipelite Member 16 passes up by rapid transition into the Inchrory Limestone 17 Formation, the main metalimestone unit of the Blair Atholl 18 Subgroup. Transitional lithologies consisting of interbedded 19 calcareous semipelite, graphitic pelite and grey crystalline 20 metalimestone are seen below Urlamore at NJ 1507 1998 and in crags 21 east of the A 939 road at NJ 1506 1967. In the Campdalmore area, 22 north-west of Tomintoul, the Inchrory Limestone Formation appears 23 to be notably thick (c. 430 m), but where seen in the Creag 24 Chalcaidh Lime Quarry (NJ 156 194), it contains numerous tight to 25 isoclinal minor and medium-scale folds. The overall outcrop 26 pattern also implies fold repetition and its true stratigraphical 27 thickness is estimated to be closer to 150 m. The unit is composed 28 of blue-grey, flaggy to massive, fine- to coarse-grained (typically 29 30 2 mm grain size), crystalline metalimestone, normally thinly to 31 thickly bedded or banded. The finer grained variants are mid to 32 dark grey, whereas the coarser grained metalimestones are pale 33 bluish grey and commonly almost translucent. Laminae and thin 34 interbeds of graphitic pelite are common and pyrite is also 35 abundant. Minor thin siliceous, cherty bands are present locally 36 in the metalimestone, and thicker calcareous semipelite interbeds 37 are also seen. Calcite veining is common, and adjacent to faults 38 the metalimestone is recrystallized. Good exposures are seen in 39 the River Avon section at NJ 1535 1917, where metalimestones with 40 thin graphitic semipelite interbeds exhibit abundant tight F3 41 folding. 42 Dark-green amphibolite lenses and pods are seen in parts of the 43 sequence, notably in the Inchrory Limestone Formation and the 44 underlying Kylnadrochit Semipelite Member. They appear to cross- 45 cut bedding locally but are strongly deformed and commonly 46 boudinaged. They represent metadolerite or metabasalt intrusions 47 48 and many show metamorphic reaction rims with the adjacent 49 metalimestone. It is unclear as to whether they were intruded 50 early in the geological history, possibly coeval with volcanic 51 units in the Argyll Group, or whether they are linked to the 52 Morven–Cabrach Pluton, which is mid Ordovician in age. A 0.5 to 1 53 metre-thick boudinaged sheet is seen in semipelites in the Allt na 54 Cluaine at NJ 1488 198 and amphibolite pods are abundant in the 55 Inchrory Limestone Formation at NJ 1529 1917 and at NN 1506 1966. 56 Larger pods are seen on the north-west face of the Creag Chalcaidh 57 Lime Quarry (NJ 1554 1944). 58 59 60 61 62 63 64 65 1 2 3 4 9.2.2 Structure 5 6 The distinctive lithologies in the Bridge of Avon area define a 7 basic refolded F2-F3 fold pattern that is further complicated by 8 the presence of late-stage steeply plunging open folds (F4) and the 9 abundant faulting (Figure 17). The area lies close to the marked 10 ‘knee-bend’ in the Dalradian succession, defined by the strike of 11 12 bedding that swings from north-east around the Bridge of Avon and 13 Bridge of Brown GCR sites to south-east farther south (see 1.2.2 in 14 Introduction, Figure 1 and Stephenson et al., 2013a, fig. 1). The 15 vergence of the main folds changes from north-west to south-west 16 respectively around this major structure. 17 The D1 structure is normally expressed as a bedding-parallel 18 cleavage or schistosity, best seen in thin sections of the more- 19 semipelitic or pelitic rocks. However, tight to isoclinal folds, 20 invariably confined to individual beds or packages of beds, are 21 seen locally, particularly in some of the metalimestone units. It 22 is unclear whether they represent slump folding, convolute bedding 23 or early tectonic deformation. Examples are seen in the Torulian 24 Limestone Member and in the Inchrory Limestone Formation. Where 25 differential weathering has occurred, such folds stand out in the 26 metalimestones, but in quarries and in clean river sections the 27 folds are more difficult to recognize, unless pelitic interbeds are 28 present. In some clean-washed sections stylolites are 29 30 preferentially seen, e.g. in the River Avon at NJ 1573 1926 where 31 they are folded. Early formed extensional slides are present in 32 the sequence and a major south-easterly dipping slide does underlie 33 the Bridge of Avon area at shallow depth. The metamorphic grade 34 that accompanied D1 deformation is not known but was probably 35 either greenschist or lower amphibolite facies. 36 The D2 deformation was penetrative and resulted in the generation 37 of a widespread cleavage and tight folding on both small and medium 38 scales. It was accompanied by middle amphibolite-facies 39 metamorphism, and samples from pelites 1 to 2 km north-north-west 40 of Bridge of Avon have yielded consistent pressure estimates of 8 41 to 8.5 kbar and temperature estimates of 620 to 650°C (Beddoe- 42 Stephens, 1990). Minor folds are typically tight but vary from 43 close to isoclinal. F2 fold axes plunge gently to moderately to 44 the south-east and south-south-east, and their axial planes dip 45 moderately to the south-east. 46 The D3 deformation has resulted in abundant medium- to small-scale 47 48 folding and local generation of a penetrative or spaced S3 cleavage 49 dependent on the intensity of deformation and lithology. F3 folds 50 refold earlier D2 features but small-scale examples of fold 51 interference structures are only rarely seen. Where the succession 52 is thinly banded, F3 minor folds are abundant (Figure 19). F3 axes 53 plunge gently to moderately to the south-east and south-west and 54 fold axial planes mostly dip moderately to steeply to the south- 55 east, although locally they do show considerable variation in 56 orientation. The folds are typically asymmetrical and generally 57 verge to the north-west and south-west. Accompanying metamorphism 58 attained lower to middle amphibolite-facies conditions. 59 The D4 deformation was a relatively local event, manifested as 60 steeply plunging medium-scale open folds that affect the outcrop 61 62 63 64 65 1 2 3 4 pattern around Bridge of Avon and a late-stage steeply dipping 5 crenulation cleavage in the Mortlach Graphitic Schist Formation. 6 Such structures might reflect the unusual structural position of 7 the area, which lies close to the northern termination of a fault 8 system that extends southwards along the valley of the Avon to join 9 the Loch Tay Fault. Alternatively they could reflect an earlier 10 lineament, e.g. the NW-trending Lecht Lineament (Fettes et al., 11 12 1986) or the structural high that appears to have controlled 13 sedimentation patterns in this area. The D4 structural event 14 occurred under greenschist-facies conditions, possibly linked to 15 retrogression of the earlier higher grade metamorphic assemblages. 16 Faulting is abundant in the Bridge of Avon area, which lies at the 17 junction of the roughly N–S-trending fault system that tracks Glen 18 Avon and the NW-trending Lecht Fault-system. The faults appear to 19 be steeply dipping, commonly subvertical, and have associated 20 localized brecciation and alteration. They are Early Devonian or 21 later in age. There is no evidence that these late faults mimic or 22 even reflect the earlier lineaments that controlled sedimentation. 23 24 9.3 Interpretation 25 26 The Dalradian rocks in the Bridge of Avon area show a condensed 27 stratigraphical sequence. Most elements of the Ballachulish and 28 Blair Atholl subgroup successions are represented but there seem to 29 30 be gaps in the sequence. For example, it is clear that the 31 Torulian Limestone Member overlies the Corriehabbie Quartzite 32 Formation almost directly, whereas in adjacent areas some tens of 33 metres of semipelite and micaceous psammite are present between the 34 two units. As Appin Group units were deposited under shallow 35 marine conditions and can be traced over much of the overall 36 Dalradian outcrop, uniform conditions obviously prevailed over a 37 wide area. Hence, in the Bridge of Avon area, deposition could be 38 interpreted as having taken place over a basin high, albeit with 39 some transitional lithologies being absent, and/or in part of the 40 basin where sediment supply was deficient. The area lies close to 41 the major strike swing of the Dalradian outcrop (the ‘knee bend’) 42 that is now thought to reflect a lineament that stretched from 43 Deeside to Dulnain Bridge and to have separated parts of the 44 depositional basin. The localized occurrence of quartzites and 45 metalimestones in the Lochaber Subgroup rocks some 5 km to the west 46 and early-formed slides in the succession all suggest that 47 48 structural activity occurred during and following deposition and 49 that the basin geometry controlled the local patterns of 50 sedimentation. 51 Deformation has been focussed on the Bridge of Avon area from 52 early in the geological history. The presence of a slide, whose 53 trace crops out just to the north-west, shows that early 54 extensional movements occurred either during sedimentation of 55 younger Dalradian rocks or early in the tectonic history of this 56 area. The superimposition of F2 and F3 folds in this area has 57 generated a kilometre-scale fold pattern that probably dates from 58 the mid Ordovician and formed part of the Grampian Event of the 59 Caledonian Orogeny. Minor fold orientations and vergence are 60 variable, particularly for the F2 folds, but F2 and F3 vergence is 61 62 63 64 65 1 2 3 4 mainly towards the north-west. The lower Ballachulish Subgroup 5 units, the Mortlach Graphitic Schist and Corriehabbie Quartzite 6 formations, form an anticlinal outcrop that closes both to the east 7 and the west (a pericline). The major fold closures are defined by 8 F2 anticlines and synclines with the F3 folds effectively 9 corrugating the earlier pattern. The folding results from Grampian 10 deformation of the thin, yet lithologically variable sequence, 11 12 focussed on a pre-existing lineament. The major change in strike 13 of the Dalradian succession that forms a ‗knee-bend‘ just to the 14 south of the Bridge of Avon area could also reflect the original 15 basin geometry and presence of lineaments (Fettes et al., 1986). 16 17 9.4 Conclusions 18 19 The Bridge of Avon GCR site provides an excellent stratigraphical 20 cross-section through a condensed Appin Group succession that is 21 interpreted as having formed on a high in the original offshore 22 sedimentary basin. Although reduced in thickness, the 23 characteristic lithologies of this Ballachulish and Blair Atholl 24 subgroup sequence are still eminently recognizable and are 25 representative of a large area of the North-east Grampian 26 Highlands. The Ballachulish Subgroup comprises the Mortlach 27 Graphitic Schist Formation, succeeded by the Corriehabbbie 28 Quartzite Formation and the mixed Ailnack Phyllite and Limestone 29 30 Formation. The overlying Blair Atholl Subgroup is represented in 31 the site area only by the Inchrory Limestone Formation. These 32 units represent alternating deeper and shallower water parts of the 33 succession, providing a record of transgression and regression that 34 is typical of many shallow marine sedimentary sequences. 35 The sequence has undergone early tectonic sliding and subsequently 36 has been deformed and metamorphosed under lower amphibolite-facies 37 conditions during the Grampian orogenic event in the mid 38 Ordovician. Although four phases of deformation can be recognized, 39 only two main sets of folds and related cleavages are widely 40 developed and they provide a good example of a kilometre-scale fold 41 interference pattern. The overall geometry is of NW-verging 42 folding and moderate to steep south-easterly dipping cleavages. 43 However, the area lies close to a regional swing of strike (the so- 44 called ‗knee-bend‘) and change in fold vergence, with the fold 45 structures in the rocks a few kilometres to the south generally 46 verging to the south-west. The Bridge of Avon area forms a natural 47 48 focal point in the Dalradian outcrop of the North-east Grampian 49 Highlands. 50 51 10 KYMAH BURN 52 (NJ 2881 2304–NJ 3008 2236) 53 54 J.R. Mendum 55 56 57 10.1 Introduction 58 59 The Kymah Burn is a major headwater tributary of the River Livet, 60 which cuts through the Ladder Hills providing a cross-section 61 62 63 64 65 1 2 3 4 through the Ladder Hills Formation and overlying Kymah Quartzite 5 Formation of the Islay Subgroup. The section illustrates the varied 6 nature of the stratigraphy at the base of the Argyll Group and the 7 overall fold structure. It is the type section for the Kymah 8 Quartzite Formation. Thin pillow-lava units show that periodic 9 basic volcanism took place during sedimentation. 10 The Ladder Hills Formation is laterally equivalent to the 11 12 metadiamictite-bearing Auchnahyle Formation, which lies some 16 km 13 to the south-west and is described in the Muckle Fergie Burn GCR 14 site report. It consists mainly of interbedded psammite, 15 semipelite and pelite, much of it showing grading and other bedding 16 features typical of turbidite deposition. It also includes some 17 mottled grey, green and cream ‗fragmental‘ units rich in chlorite 18 and epidote, possibly representing tuffaceous units, and rare cream 19 metadolostone beds. Along strike to the south, similar lenticular 20 metadolostone beds are associated with metadiamictite units but no 21 metadiamictites have been reported from the Kymah Burn section. To 22 the south-east, towards Glen Buchat, the upper part of the Ladder 23 Hills Formation passes laterally into a more-pelitic and calcareous 24 unit, the Nochty Semipelite and Limestone Formation. The overlying 25 Kymah Quartzite Formation consists mainly of quartzite and 26 psammite, but includes thin amphibolitic metavolcanic units, which 27 locally show vesicular textures and pillow structures. Basic 28 sheets also intrude the quartzite, and although they are now 29 30 foliated and metamorphosed to amphibolite, discordant relationships 31 are still visible in places. 32 The succession has been folded into a kilometre-scale refolded 33 fold, repeating the stratigraphy. As the beds themselves show only 34 low to moderate strain, the structure can be determined from 35 bedding and cleavage measurements and observations, taking 36 cognizance of relatively abundant way-up indicators (grading, 37 cross-bedding, pillow lavas). 38 The area was first mapped by L.W. Hinxman for the Geological 39 Survey in 1892-3 and it was he who recognized the presence of basic 40 sheets in the quartzite. No further work was done until the area 41 was resurveyed by the British Geological Survey in the mid 1980‘s 42 as part of the revision of 1:50 000 Sheet 75E (Glenbuchat, 1995). 43 44 10.2 Description 45 46 The Kymah Burn runs through an incised gorge in the Ladder Hills, 47 48 where relief ranges from 150–230 m in the lower part, overlooked by 49 the crags of The Eachrach, to c. 100 m in the higher parts of the 50 section. Although there is a reasonable coherent bedrock section 51 along the burn, the sides of the gorge are mostly scree covered or 52 are obscured by slipped material. To the north of the gorge 53 section, the outcrop of the Kymah Quartzite Formation terminates 54 where it has been intruded and hornfelsed by a small microgranite 55 body. This is part of the immediately adjacent Glenlivet Granite 56 Pluton, which crops out in the lower part of the burn. 57 The burn section traverses obliquely across the overall strike of 58 the Ladder Hills and Kymah Quartzite formations, which here are 59 disposed in a complex, large-scale, refolded fold pattern (Figure 60 20). The Ladder Hills Formation consists of cream to fawn and 61 62 63 64 65 1 2 3 4 pale-grey, thin- to medium-bedded, typically flaggy to blocky, 5 micaceous and feldspathic psammites interbedded with dark-grey 6 pelites and mid-grey semipelites. The psammites commonly show 7 grading, in parts from gritty psammite bases up to semipelitic or 8 even pelitic tops. Graded bedding, load structures, cut-offs and 9 cross-bedding are all seen, indicative of turbiditic conditions 10 during deposition. Locally, deformed thin psammite dykelets occur, 11 12 indicating dewatering during compaction, and implying rapid 13 deposition of the sand and silt sequence. Beds of graphitic pelite 14 are thin in this section, but elsewhere such units can attain 20 m 15 in thickness. The base of the Ladder Hills Formation is not 16 exposed in the Kymah Burn but is seen some 3 km to the south-west 17 by Ladderfoot (at NJ 2663 2066), where there is a transition from 18 the graphitic pelites and semipelites of the underlying Glenfiddich 19 Pelite Formation up into thinly interbedded semipelites and 20 psammites. 21 Good exposures of the typical Ladder Hills Formation occur in the 22 Kymah Burn between the tributary burns of Caochan Domhain and 23 Caochan Ranaich (Figure 20a). Although cleavages are developed in 24 the pelitic lithologies the beds are not schistose. Here, 25 chloritic material occurs in the basal parts of some psammite beds 26 and amphibole + plagioclase feldspar-bearing ‗fragmental‘ units up 27 to 2 m thick are also present. These mottled grey-green-fawn 28 amphibole-bearing units weather irregularly to a pale-brown colour 29 30 and are invariably altered to chloritic and biotitic material. 31 Examples of these probable volcaniclastic ‗green beds‘ are seen at 32 NJ 2988 2245 and, adjacent to a 10–25 cm-thick lenticular 33 metadolostone bed, at NJ 2994 2244. The upper parts of the 34 formation are seen between NJ 2936 2263 and NJ 2958 2257; there, 35 amphibolite units are interbedded with micaceous and feldspathic 36 psammite, semipelite, minor quartzite and amphibole-bearing 37 psammite. In one instance, exposures of a variably bluish to 38 purplish grey, fine-grained, knobbly weathering amphibolite are 39 seen. This amphibolite unit is several metres thick, apparently 40 concordant, finely cleaved, and notably pyritic. 41 The transition up into the Kymah Quartzite Formation is marked by 42 the incoming of thicker quartzite units, commonly with gritty 43 bases. The boundary is faulted in the Kymah Burn section, but as 44 the beds dip moderately to steeply to the east-south-east, probably 45 only a small part of the succession is missing. The lowest 46 exposures stratigraphically (at NJ 2942 2262) consist of blocky to 47 48 massive, thick-bedded, gritty quartzites and feldspathic quartzites 49 with thin pelitic interbeds and laminae. Grading and bottom 50 structures show that the beds are inverted and young to the west. 51 The succeeding lithologies are interbedded quartzites and 5–20 m- 52 thick units of grey-green to purplish grey amphibolite. More- 53 indurated, massive to blocky, cream to white quartzite beds, 54 locally showing gritty bases, are present downstream. 55 Where the Dry Stripe enters the main gorge at NJ 2927 2268, a c. 56 10 m-thick, greenish grey metabasalt sheet passes laterally into a 57 c. 4 m-wide dyke, which transects the quartzite beds adjacent to a 58 small SE-trending fault. This intrusive sheet is massive, sparsely 59 feldspar-phyric and is a member of a suite of early-Caledonian 60 metadolerite intrusions (see Interpretation) 61 62 63 64 65 1 2 3 4 The middle and higher parts of the Kymah Quartzite Formation are 5 folded into a tight downward-facing antiform (where the beds young 6 downwards). The white to cream quartzites are well bedded, locally 7 gritty and show good cross-bedding and some grading. On the 8 eastern limb of the antiform, the beds dip very steeply to the 9 south-east and the change in dip that marks the antiformal hinge- 10 zone corresponds with the change in burn orientation from north– 11 12 south to ESE–WNW at around NJ 2912 2312. Downstream, the beds 13 first dip very steeply to the west and young eastwards, but west of 14 The Eachrach they dip steeply eastwards and young westwards, i.e. 15 they define a downward-facing synform (Figure 20b). The bedding is 16 difficult to discern in these massive to blocky quartzites. Where 17 the burn turns northwards, several amphibolitic beds are 18 interbanded with the quartzites. At NJ 2882 2324 a several metre- 19 thick unit of purplish and greenish grey, pyritic amphibolite shows 20 vesicular textures and pillow structures. Tuffaceous units are 21 also present. In thin section these basic metavolcanic rocks 22 consist of plagioclase feldspar, chlorite, hornblende, biotite, 23 quartz, ilmenite, and titanite. Epidote is locally abundant in 24 some units. Two fine but penetrative cleavages can be discerned in 25 parts of the metavolcanic outcrops. The quartzite downstream dips 26 steeply eastwards and shows cross-beds in parts. It is generally 27 indurated and becomes hornfelsed with chlorite and pyrite veining 28 adjacent to the microgranite intrusion. 29 30 The outcrop of the Kymah Quartzite Formation diminishes in width 31 to the south-south-west where it lies in the core of the earlier 32 tight syncline that is refolded by the later upright antiforms and 33 synforms in the Kymah Burn section (Figure 20a, b). The Ladder 34 Hills Formation and its lateral equivalent, the Nochty Semipelite 35 Formation, underlie most of the Ladder Hills area, where their c. 36 3–5 km-wide outcrop is a result of a similar refold structure to 37 that defined by the main quartzite unit in the Kymah Burn section. 38 39 40 10.3 Interpretation 41 42 The distinctive stratigraphy of the Islay Subgroup rocks in the 43 Ladder Hills shows that a several kilometre-thick turbiditic sand- 44 silt-mud succession developed locally in this area. The resultant 45 Ladder Hills Formation and Nochty Semipelite Formation were 46 probably deposited relatively rapidly from density currents in a 47 48 small fault-bounded marine basin, accompanied by periodic basic 49 volcanicity. Metadiamictite units and associated metadolostones 50 occur within the Ladder Hills Formation farther south around NJ 242 51 177. The occurrence of thin metadolostones and tuffaceous units in 52 the Kymah Burn section suggests that either diamictites were not 53 deposited here, or that they were deposited and eroded prior to 54 deposition of the overlying sediments. However, it is clear that 55 the deposition and volcanicity here were coeval with the occurrence 56 of shallow marine ice sheets elsewhere in Scotland and Ireland (see 57 the Muckle Fergie Burn, Tempar Burn and Garvellach Isles GCR site 58 reports). 59 The concordant and interbedded nature and pillow-lava features of 60 several of the amphibolitic units show that they were originally 61 62 63 64 65 1 2 3 4 deposited as sub-marine basaltic lavas. Unequivocal metavolcanic 5 rocks occur in the uppermost parts of the Ladder Hills Formation 6 and at the lower and middle levels of the Kymah Quartzite 7 Formation. Tuffaceous units occur close to lavas in the Kymah 8 Quartzite Formation and scattered through the Ladder Hills 9 Formation. The metavolcanic rocks show two penetrative cleavages 10 locally, suggesting that they have undergone the same structural 11 12 history as the adjacent metasedimentary rocks. Other mafic units 13 might represent subvolcanic basic sheets, but the thicker sheets 14 with relict metadolerite textures were probably intruded during a 15 later period of basic intrusion approximately coeval with the 16 emplacement of the nearby Morven–Cabrach Pluton at c. 470 Ma 17 (Dempster et al., 2002). These early-Caledonian basic sheets and 18 related dykes are very abundant on upper Donside around Corgarff 19 and in the eastern part of the Ladder Hills. 20 The structure of the area can be readily interpreted as the 21 product of two phases of ductile deformation. Two distinct 22 penetrative cleavages are seen in pelitic and mafic lithologies and 23 a later crenulation cleavage is present locally in the some of the 24 pelitic units. The Kymah Quartzite Formation is little deformed 25 internally, thus preserving original sedimentary features. 26 However, the formation acted as a more-competent layer and has 27 provided the focus for the large-scale refold structure. The 28 quartzite was originally disposed in a tight, anticline-syncline 29 30 fold-pair, overfolded to the north-west and with an axial plane 31 dipping gently to moderately south-east. The mapping of bedding 32 and way up, combined with limited cleavage observations, shows that 33 upright tight folds have refolded this primary structure, with an 34 antiform prominent in the Kymah Burn section. It is difficult to 35 know whether the original basin architecture has had any control on 36 the subsequent structural development, but such phenomena are 37 common in more-recent basins subject to tectonic compression and 38 inversion. 39 The rocks of the Kymah Burn and surrounding area do not preserve 40 their full metamorphic history. Here, later retrogression to 41 greenschist facies has altered many of the earlier amphibolite- 42 facies assemblages in both the metasedimentary and meta-igneous 43 rocks. In particular, the high-pressure kyanite assemblages 44 recorded to the north and west from nearby Glen Fiddich and Glen 45 Livet by Beddoe-Stephens (1990) are absent (see the Auchindoun 46 Castle GCR site report). Additional, later fluid-related 47 48 alteration is linked to the presence of NNE-trending faults and 49 related breccia zones that are present in this area. 50 51 10.4 Conclusions 52 53 The Kymah Burn GCR site provides a spectacular cross-section 54 through a large-scale refolded fold affecting the lowest units of 55 the Argyll Group. By careful observation of bedding and cleavage 56 orientations, combined with sedimentological way-up evidence, it is 57 possible to recognize an early near-isoclinal syncline-anticline 58 fold-pair whose axial plane originally dipped gently to the south- 59 east. These NW-verging folds have been refolded by more-obvious 60 61 62 63 64 65 1 2 3 4 tight upright folds, giving rise to a large-scale interference 5 structure that is transected by the Kymah Burn. 6 The site is representative of three major formations and is a type 7 section for one. The Ladder Hills Formation is a sequence of 8 turbiditic psammites, semipelites and pelites over a kilometre 9 thick that formed in a small local basin coeval with the deposition 10 of tillites and related glacial deposits over a wider area of 11 12 Scotland and Ireland. It includes minor metabasalt units with 13 pillow structures, altered basic volcaniclastic units and thin 14 lenticular metadolostone beds. Laterally it passes south-eastwards 15 into the Nochty Semipelite and Limestone Formation. Both 16 formations are overlain by the Kymah Quartzite Formation, marked by 17 the incoming of thicker purer sands, typically showing cross- 18 bedding and gritty bases to the beds. Basaltic lavas and possible 19 tuffaceous units are also recognized in this formation. 20 Metadolerite sheets and rare dykes have also intruded the whole 21 sequence during the Caledonian Orogeny. Although the beds have 22 been metamorphosed to amphibolite facies, later retrogression has 23 partly altered the original peak metamorphic mineral assemblages to 24 greenschist facies. 25 The site is of national importance as it documents some unique 26 stratigraphical variations in the Islay Subgroup, displays good 27 evidence of basic volcanism coeval with the widespread mid- 28 Dalradian glaciation, and provides a valuable insight into the 29 30 overall structure of this part of the North-east Grampian 31 Highlands. 32 33 11 BLACK WATER 34 (NJ 355 303–NJ 378 308) 35 36 D. Stephenson and D.J. Fettes 37 38 39 11.1 Introduction 40 41 The lower part of the Black Water, a major tributary of the River 42 Deveron in the Cabrach area, south-west of Huntly, provides a 43 continuous section through the most-extensive sequence of 44 metavolcanic rocks in the Dalradian of the North-east Grampian 45 Highlands. 46 The metavolcanic rocks occur within a varied succession of gritty 47 48 psammites and pelites of turbiditic character that crop out 49 immediately to the east of the Portsoy Lineament (see 1.1.3 in 50 Introduction). Together, these metasedimentary and metavolcanic 51 rocks form the Blackwater Formation. As with most stratigraphical 52 units to the east of the Portsoy Lineament, direct correlation at 53 formation level with Dalradian outcrops to the west and south is 54 not possible (Fettes et al. 1991; Stephenson and Gould 1995). 55 However, the rocks have lithological characteristics that are 56 typical of the Argyll Group and pass upwards and south-eastwards 57 into Southern Highland Group strata. This would seem to be 58 consistent with a stratigraphical position near the top of the 59 Argyll Group, probably equivalent to the Crinan and/or Tayvallich 60 subgroups elsewhere. 61 62 63 64 65 1 2 3 4 The lower part of the Blackwater Formation has been divided into 5 three members, based upon the compositions of the metavolcanic 6 rocks. In ascending stratigraphical order these are the Lynebain 7 Basic Volcanic Member, the Kelman Hill Ultrabasic Volcanic Member 8 and the Ardwell Bridge Basic Volcanic Member. The Lynebain and 9 Ardwell Bridge members consist mainly of metabasaltic rocks, which 10 locally exhibit complete or fragmental pillow structures. The 11 12 Kelman Hill Member contains some metabasalts but is dominated by a 13 variety of ultrabasic rocks (metapicrites), some of which are 14 highly fragmented with a fine-grained hyaloclastite appearance. 15 Above, the formation consists mainly of dark-grey pelites, 16 graphitic in parts, with conspicuous andalusite schists and a 17 number of persistent beds of gritty psammite (the Corinacy Pelite 18 Member). 19 The Blackwater Formation is poorly exposed over much of its 20 outcrop but high-amplitude magnetic anomalies, mostly restricted to 21 the part of the formation that is known to contain metavolcanic 22 rocks, have greatly assisted in its mapping. Measurements of 23 magnetic susceptibility on pelites from the Black Water section 24 indicate that some of the anomalies result from high magnetite 25 contents in metasedimentary rocks (Fettes et al., 1991). An 26 igneous source for this magnetite seems likely and this strengthens 27 the case for the meta-igneous rocks being penecontemporaneous with 28 their host sediments in a volcanic setting. Magnetic evidence is 29 30 unable to differentiate between the basic and ultrabasic types. 31 The first Geological Survey map of this area was published as one- 32 inch Sheet 85 (Rothes, 1898), with an accompanying memoir (Hinxman 33 and Grant Wilson, 1902). Mackie (1908) was the first to suggest 34 that some of the basic meta-igneous rocks in the area had a 35 volcanic origin and Dewey and Flett (1911) identified pillow lavas 36 at this GCR site. The pillow structures were described in some 37 detail by MacGregor and Roberts (1963), together with an account of 38 their petrography and metamorphic history. A detailed resurvey, 39 incorporating the results of ground magnetic traverses at 200 m 40 spacing, was undertaken by the British Geological Survey and 41 published as 1:50 000 Sheet 85E (Glenfiddich, 1996). This work was 42 incorporated in a regional synthesis (Fettes et al., 1991) and 43 formed the basis for a programme of geochemical sampling and 44 drilling that targeted the igneous rocks as potential hosts of gold 45 and platinum-group elements (Gunn et al., 1990). However, the 46 mineral investigations were not encouraging, with uniformly low PGE 47 48 (maxima 11 ppb Pt, 10 ppb Pd, 5 ppb Rh), only sporadic slight 49 enrichment in gold (maximum 150 ppb Au) and no attendant enrichment 50 in base metals or chalcophile elements. Mineralogical and 51 geochemical aspects of the volcanic rocks have been described and 52 discussed by Macdonald et al. (2005) and by Fettes et al. (2011), 53 and there is a brief field guide to the eastern end of the section 54 by Gillen (1987). 55 56 11.2 Description 57 58 The Blackwater Formation is bounded to the north-west throughout 59 its outcrop by the Portsoy Shear-zone. In this area, the shearing 60 is concentrated in a 1 km-wide zone, in which lie many pods of 61 62 63 64 65 1 2 3 4 sheared serpentinite and metagabbroic rocks between the larger 5 Succoth–Brown Hill and Blackwater intrusions of the North-east 6 Grampian Basic Suite (Fettes et al., 1991). To the south-east of 7 the shear-zone, for a cross-strike width of at least 2 km, and 8 certainly extending across the entire outcrop width of the 9 metavolcanic rocks, all lithologies have a single, possibly 10 composite, planar fabric sub-parallel to the bedding. The whole 11 12 sequence strikes north-east–south-west with a generalized steep dip 13 to the south-east. No folds of any scale are seen, and instances 14 where the fabric in pelitic beds is slightly oblique to bedding are 15 rare. 16 The stream section that is the GCR site extends from the faulted 17 contact of the Blackwater Formation with the sheared margin of the 18 Blackwater Intrusion at NJ 355 303, downstream to Blackwater Bridge 19 (NJ 378 308), which was formerly known as Ardwell Bridge (Figure 20 21). From this section, the outcrop of the metavolcanic rocks 21 extends north-eastwards, averaging c. 2 km in width, for some 8 km. 22 To the south-west lava exposures become impersistent in poorly 23 exposed ground and their magnetic anomalies cannot be traced for 24 more than 5 km. 25 The metavolcanic rocks are, for the most part, interbedded with 26 gritty psammites and mica schists, locally with black, graphitic, 27 schistose or phyllitic pelites. Some gritty psammites have been 28 mapped out separately and commonly have distinctive blue-grey 29 30 quartz clasts. A basal predominantly pelitic unit is present in 31 places. Excellent graded units at the western end of the Black 32 Water section (for example, at NJ 3596 3040 and NJ 3621 3040) 33 indicate younging to the south-east. 34 The metavolcanic units range from a few metres to 50 m in 35 thickness and are concordent with the metasedimentary rocks. 36 Contacts with the metasedimentary rocks are variable; in some cases 37 these are relatively sharp, in others the metavolcanic rock is 38 rather nodular with carbonate veining and in some cases the edge of 39 the metavolcanic unit is brecciated with metasedimentary infilling. 40 In excellent examples at NJ 3714 3058 and NJ 3734 3087 a 41 metavolcanic unit has a carbonated nodular margin passing into a 42 relatively massive centre, the opposite margin being brecciated 43 with a metasedimentary matrix. This asymmetry is consistent with 44 an origin as a lava, although it is not clear which margin marks 45 the base and which the top. Vesiculation is common in all three 46 members. 47 48 Metabasaltic pillows are well exposed near Blackwater Bridge, at 49 NJ 3776 3083 and NJ 3751 3082 (MacGregor and Roberts, 1963). 50 Fragmental pillows also occur in places and are particularly well 51 developed in the River Deveron at Lynebain (NJ 412 351), some 7 km 52 north-east of the Black Water section. The pillows at Blackwater 53 Bridge are ellipsoidal, with horizontal cross-sections of some 60 x 54 15 cm and vertical dimensions of up to 150 cm. Small, originally 55 spherical vesicles within the pillows show concentric banding in 56 places and, rarely, elongate vesicles radiate around the noses of 57 individual pillows. The pillows are bordered by fine-grained, non- 58 vesicular selvedges and small volumes of altered basaltic material 59 occur between the pillows. Both pillows and amygdales are 60 flattened within the regional fabric, which is most strongly 61 62 63 64 65 1 2 3 4 developed at the margins of and between the pillows. 5 Interpretation of the pillow orientations is equivocal but better 6 evidence from the metasedimentary rocks confirms that the Black 7 Water section traverses a continuous south-east-younging sequence. 8 In terms of their whole-rock chemistry, the metavolcanic rocks 9 range from ultrabasic (metapicrites) to basic (metabasalts and 10 meta-basaltic andesites), with some intermediate compositions 11 12 (meta-andesites) (Macdonald et al., 2005). As a result of 13 amphibolite-facies regional metamorphism, they have mineral 14 assemblages dominated by amphiboles. 15 The metabasalts and meta-andesites are mainly aphyric. They 16 consist of aggregates of dark green clinoamphibole (actinolite to 17 magnesiohornblende to pargasitic magnesiohastingsite in 18 composition), with lesser amounts of plagioclase, quartz and 19 ilmenite, the latter commonly rimmed or replaced by titanite. 20 Aggregates of epidote and quartz could represent pseudomorphs after 21 plagioclase phenocrysts. Distinctive pyroxene-phyric types crop 22 out near Shenval (NJ 368 308) and have been found as float on 23 Kelman Hill (NJ 396 334), some 3 km to the north-east. In these 24 rocks, amphibole pseudomorphs after phenocrysts of original, 25 igneous clinopyroxene contain rare relict cores of ferroan 26 diopside. 27 The metapicrites are variable, ranging from massive to highly 28 fragmented with sharp fine-grained shards, giving the appearance of 29 30 a hyaloclastite. The more-massive forms consist almost entirely of 31 felted intergrowths of colourless to pale green magnesian 32 clinoamphibole (tremolite to magnesiohornblende in composition) 33 with chlorite and sparse small rounded grains of chromian 34 magnetite. Excellent examples of brecciated ultrabasic rocks are 35 found as float to the east of Shenval. These consist of ultramafic 36 clasts set in an ultramafic matrix (Figure 22). The fragments are 37 of varying type, up to several centimetres in size and constitute 38 60–80% of the rock; they are generally flattened into alignment 39 with the regional fabric. The matrix to the fragments is highly 40 sheared, streaky and chlorite rich. At the microscopic scale, so 41 few original features are preserved that it is difficult to 42 determine whether the rocks are of extrusive or intrusive origin. 43 Some sections contain highly elongate grains of ilmenite, which 44 might indicate rapid cooling and therefore a volcanic origin. In 45 others a variolitic texture is preserved, while some of the 46 fragments were originally glassy and now have a grain size less 47 48 than 10 microns. 49 Overall, there is little doubt of the predominantly volcanic 50 origin of most of the meta-igneous rocks, although some of the 51 more-massive sheets could have been shallow, subvolcanic sills. An 52 undoubtedly intrusive metabasaltic unit occurs near Torr of 53 Shenwell (NJ 3746 3083), where a c. 20 m-thick sheet shows a sharp, 54 non-vesiculated contact against psammites and andalusite schists. 55 The intrusion is geochemically similar to extrusive rocks of the 56 Ardwell Bridge Member and it is assumed that they were broadly 57 coeval. The most evolved, and finest grained, rock occurs at the 58 eastern margin of the intrusion and there is a gradational increase 59 in grain size towards a metagabbroic central facies, which is less 60 61 62 63 64 65 1 2 3 4 evolved. The intrusion seems, therefore, to have formed from a 5 magma column that had become differentiated at greater depth. 6 The ENE-trending faults that are prominent on Figure 21 are part 7 of a regional set, which has been particularly well delineated in 8 this area by the ground magnetic survey (Fettes et al., 1991). The 9 linear magnetic anomalies are clearly displaced, the inferred 10 dislocations commonly coincide with topographical features and some 11 12 are readily seen on air photographs. This is one of the youngest 13 sets of regional faults, which elsewhere in the North-east Grampian 14 Highlands are associated with late-Carboniferous quartz-dolerite 15 dykes. 16 17 11.3 Interpretation 18 19 The geochemical studies of Macdonald et al. (2005) and Fettes et 20 al. (2011) have shown that the Blackwater metavolcanic rocks as a 21 whole are of tholeiitic affinity and are broadly similar to 22 metavolcanic rocks elsewhere in the Dalradian succession. Their 23 inferred parental magmas were relatively Ti- and Fe-rich high- 24 magnesia basalts with total iron oxides c. 14 % and MgO c. 10 %. 25 Fractionation of iron-titanium oxide minerals, olivine and 26 clinopyroxene from the parental magmas generated a range of 27 daughter magmas extending to tholeiitic andesite composition. Some 28 of the more-evolved rocks show evidence of minor accumulation of 29 30 iron-titanium oxides. A continuous enrichment in Al2O3 indicates 31 that plagioclase fractionation must have been absent or muted, 32 which is consistent with an absence of Eu anomalies in rare-earth 33 patterns. Crystallization of plagioclase can be significantly 34 delayed under conditions of high PH2O and hence Macdonald et al. 35 (2005) suggested that the Blackwater magmas might have been 36 relatively hydrous. The picritic rocks formed by accumulation of 37 olivine and minor chrome-spinel within the parental basalts, 38 probably at deep crustal levels. Their high MgO content (over 18% 39 and ranging up to 35%) had originally led to speculation that they 40 might reflect primary, high-temperature (possibly komatiitic) 41 magmas, which to some extent prompted the investigations for gold 42 and platinum-group mineralization (Gunn et al., 1990; Fettes et 43 al., 1991). However, this was not considered likely by Macdonald 44 et al. (2005). 45 Concentrations of incompatible trace elements such as Zr, Nb and Y 46 suggest that the primary magmas of the Blackwater metavolcanic 47 48 rocks were generated from a mantle source that was relatively 49 enriched compared to a Mid-Ocean Ridge Basalt (MORB) source. This 50 is a feature that they share with other late-Argyll Group 51 metavolcanic rocks such as the Tayvallich lavas (Fettes et al., 52 2011). Other metavolcanic rocks, from lower in the Dalradian 53 succession, have geochemical characteristics more typical of a 54 depleted, MORB-like, mantle source (e.g. Goodman and Winchester, 55 1993). Hence Macdonald et al. (2005) and Fettes et al. (2011) have 56 speculated that Dalradian metavolcanic rocks represent varying 57 degrees of mixing of magmas from these two mantle sources. 58 It would appear that there was an overall trend in the Dalradian 59 from basalts generated in more-depleted mantle sources, which were 60 erupted earlier, to ‗enriched‘ types, which were erupted later. 61 62 63 64 65 1 2 3 4 The latter, including the Blackwater metavolcanic rocks, can be 5 classed as Fe-Ti basalts, which are developed typically at 6 propagating rifts that are progressively breaking through rigid 7 lithosphere, and Macdonald et al. (2005) suggested that progressive 8 rupturing along the margin of Laurentia, resulted in the more- 9 enriched source rising to higher levels and tending to mix less 10 with the depleted source. 11 12 The Blackwater metavolcanic rocks are interbedded with 13 metasedimentary lithologies, characterized by coarse turbidites 14 that originated as deep-water basin sediments. Together they 15 record a crucial stage in the break-up of the supercontinent of 16 Rodinia, as lithospheric thinning, crustal instability and 17 continental rifting led into the formation of the Iapetus Ocean 18 during Argyll Group times (Fettes et al., 2011). The siting of 19 this sub-marine volcanism, along the Portsoy Lineament, emphasises 20 the importance of the lineament as a tectonothermal boundary and 21 suggests that its origins might lie in the basin architecture that 22 evolved as a result of the initial continental rupture (Ashcroft et 23 al., 1984; Fettes et al., 1986). 24 25 11.4 Conclusions 26 27 The Black Water provides a continuous river section through the 28 thickest and most extensive sequence of metavolcanic rocks in the 29 30 Dalradian of the North-east Grampian Highlands. The presence of 31 metabasaltic pillow lavas in this section has long been known but 32 even more remarkable are the wide range of fragmented high- 33 magnesium ultrabasic lavas (metapicrites) that originated by the 34 accumulation of olivine from the basaltic magmas in deep-crustal 35 magma chambers. The formation of pillows and the fragmentation of 36 the metapicrites are the results of sub-marine eruption in deep 37 unstable basins, characterized by turbibitic sedimentation. 38 The Blackwater metavolcanic rocks, together with the near- 39 contemporaneous Tayvallich lavas in the South-west Grampian 40 Highlands, are typical chemically of volcanic rocks in propagating 41 rift basins, and provide vital information about the 42 tectonomagmatic conditions that resulted from the break-up of 43 Rodinia and the initial formation of the Iapetus Ocean, some 600 44 million years ago. The basin in which the Blackwater rocks were 45 erupted might have been related in some way to the initial 46 formation and location of the Portsoy Lineament, which was to 47 48 influence sedimentation, magmatism and tectonics for the following 49 140 million years or more. 50 51 12 AUCHINDOUN CASTLE 52 (NJ 345 368–NJ 362 375) 53 54 D. Stephenson 55 56 57 12.1 Introduction 58 59 The ruins of Auchindoun Castle stand on a knoll of metalimestone 60 above the River Fiddich, 3.5 km south-east of Dufftown. Exposures 61 62 63 64 65 1 2 3 4 below the castle, in the river banks, are of dark graphitic pelite 5 of the Mortlach Graphitic Schist Formation, and it is the regional 6 metamorphic minerals in the pelite that are the main feature of 7 interest at this GCR site. Square cross-sections of chiastolite (a 8 variety of andalusite), clearly seen in hand specimen, are seen in 9 thin section to have been replaced by kyanite, indicating a 10 significant increase in regional pressure. The metalimestone is 11 12 the Dufftown Limestone Member at the base of the Mortlach 13 Formation, which in this area marks the base of the Ballachulish 14 Subgroup. 15 The primary survey of the area was published as one-inch Sheet 85 16 (Rothes) in 1898, together with a brief memoir (Hinxman and Grant 17 Wilson, 1902). The area was not revisited until it was remapped by 18 the British Geological Survey for 1:50 000 Sheet 85E (Glenfiddich, 19 1996), which was when the interesting relationships between the 20 regional metamorphic minerals were discovered. This led to a 21 detailed investigation of the pressure–temperature conditions of 22 metamorphism by Beddoe-Stephens (1990) that formed part of a wider 23 study of metamorphic conditions on either side of the Portsoy– 24 Duchray Hill Lineament. The rocks at Auchindoun lie on the west 25 side of the lineament, where peak pressures were up to 4 kbar 26 higher than they were immediately to the east of the lineament, due 27 to near-isothermal compression beneath westerly directed thrusting 28 along the line of the Portsoy-Duchray Hill Lineament/Shear-zone. 29 30 Samples from this locality were also used in a regional geochemical 31 study of Dalradian metacarbonate rocks, which proved to be of 32 significant value in stratigraphical correlation (Thomas, 1989), 33 and calcsilicate beds within the pelites provided material for a 34 study of amphibole geochemistry that revealed implications for the 35 original depositional environment (Stephenson, 1993). 36 37 12.2 Description 38 39 The area around Auchindoun lies on the south-eastern limb of the 40 Ardonald Anticline, a NW-verging, tight, regional-scale fold of 41 possible D3 age. The right-way-up succession extends from the 42 Pitlurg Calcareous Flag Formation of the Lochaber Subgroup, here 43 poorly exposed, through the Mortlach Graphitic Schist Formation, to 44 the Corriehabbie Quartzite Formation of the Ballachulish Subgroup, 45 which forms a continuous ridge to the south-east of Glen Fiddich 46 (Figure 23). Bedding dips to the south-east at between 30o and 65o 47 48 and the dominant cleavage in the pelites (?S2) commonly dips at a 49 lower angle, indicating local inversions possibly due to 50 intermediate-scale folds. 51 The Dufftown Limestone Member in this area is of variable 52 thickness, up to about 3 m, but at Auchindoun Castle the outcrop is 53 thickened by a series of tight, SSW-plunging folds. The member is 54 composed typically of banded, grey, crystalline metalimestone, but 55 in places thin beds of metalimestone and pinkish brown-weathering 56 calcsilicate rock are interbedded with phyllitic pelites. At the 57 castle, thin pelitic partings in the metalimestone have a strong 58 spaced cleavage (?S2) as is seen in the overlying pelites. South- 59 west of the castle the metalimestone outcrop is terminated by a 60 fault. 61 62 63 64 65 1 2 3 4 The main part of the Mortlach Graphitic Schist Formation, above 5 the Dufftown Limestone Member, is well exposed in the banks of the 6 River Fiddich and in tributaries on the south-east side of Glen 7 Fiddich, in particular the Allt a‘ Choileachain, the Red Burn and 8 the Small Burn, where the outcrop width is greatly increased by 9 tight folding. In this area, it is composed predominantly of dark- 10 grey, fine-grained, finely banded pelite. Banding takes the form 11 12 of thin, 1–5 mm-wide bands of pale semipelite or psammite, which 13 enable the orientation of the original bedding to be seen in most 14 exposures. The pelites are usually hard and blocky, but are 15 phyllitic to schistose in places, with a strong S2 16 spaced/crenulation cleavage. Where the dominant cleavage is near 17 coincident with the bedding, the rock becomes very hard and slaty. 18 Such slates have been quarried at several places on the hill slopes 19 to the east (e.g. at NJ 358 370 and NJ 375 386). In most exposures 20 the pelites contain prominent, square-sectioned chiastolite and 21 many are garnetiferous. Most are graphitic and some are quite 22 pyritiferous. Bands and pods of tremolitic amphibole, with or 23 without subsidiary carbonate-rich laminae, within the pelites have 24 been interpreted as para-amphibolites and indicate a continuation 25 of the calcareous facies above the basal metalimestone member 26 (Stephenson, 1993). 27 Throughout the exposures of pelitic rocks, there is good evidence 28 in thin section of replacement of chiastolite by kyanite in what 29 30 appears to be a direct pseudomorph relationship. Squarish to 31 rectangular porphyroblasts of chiastolite with preserved inclusion 32 ‗crosses‘ of graphite have been replaced by radial fan-like sheaves 33 of kyanite (Figure 24), which commonly show varying degrees of 34 later replacement by muscovite. A fine-grained crenulated 35 micaceous fabric can be seen to post-date the chiastolite and 36 slight strain effects in the kyanite suggest that this fabric might 37 also post-date kyanite growth. Other regional metamorphic minerals 38 present are garnet and biotite. 39 40 12.3 Interpretation 41 42 It has long been recognized that metamorphism in the Buchan region, 43 to the east of the Portsoy–Duchray Hill Lineament, is distinct from 44 that elsewhere in the Grampian Terrane, being characterized by 45 relatively low-pressure/high-temperature mineral assemblages (see 46 1.3 in Introduction). The western limit of this low-pressure 47 48 metamorphism is broadly coincident with the shear-zones that define 49 the Portsoy–Duchray Hill Lineament and also mark the western margin 50 of the structurally and stratigraphically distinct ‘Buchan Block’ 51 (Baker, 1985; Fettes et al., 1986; Harte and Dempster, 1987). To 52 the west of the lineament, lower structural levels and older 53 Dalradian rocks are exposed and the metamorphic mineral assemblages 54 are characteristic of a higher pressure. 55 The low- and high-pressure assemblages are characterized 56 essentially by andalusite and kyanite respectively, and D.J. Fettes 57 (on the BGS 1:250 000 Sheet 57N 04W, Moray–Buchan, 1977) and 58 Chinner and Heseltine (1979) each plotted andalusite–kyanite 59 isograds, parallel and close to the Portsoy–Duchray Hill Lineament. 60 However, there is a well-defined zone, up to 10 km wide on the 61 62 63 64 65 1 2 3 4 western side of the lineament, where original andalusite has 5 inverted to kyanite indicating a pressure increase after the 6 initial metamorphism (Chinner and Heseltine, 1979; Baker, 1985). 7 On the Banffshire coast, this zone is narrow but is beautifully 8 illustrated in the well-known chiastolite-bearing pelites at the 9 swimming pool west of Portsoy (see the Cullen to Troup Head GCR 10 site report). There, the chiastolite is clearly seen in thin 11 12 section to be pseudomorphed by kyanite and muscovite, but the 13 relationships are complicated by the presence of sillimanite, which 14 pre-dates the kyanite, and by later overgrowths of kyanite and 15 muscovite that post-date the main fabric. Inland, and especially in 16 the area around Auchindoun Castle, there is a much simpler 17 replacement of the original chiastolite porphyroblasts. 18 The regional study of Beddoe-Stephens (1990) placed quantitative 19 pressure and temperature constraints on the observed metamorphic 20 reactions on both sides of the Portsoy–Duchray Hill Lineament. 21 Values were calculated from various thermodynamic calibrations 22 based upon reactions between commonly occurring minerals. Both 23 thermal and barometric ‗breaks‘ are clearly seen at the lineament. 24 East of the lineament, where only andalusite and sillimanite occur, 25 pressures never exceeded 4.5 kbar and temperatures of up to 660oC 26 are recorded. West of the lineament, where kyanite occurs either 27 as the sole aluminosilicate phase or as a replacement of 28 andalusite, pressures of 7.5 to 8.5 kbar are recorded close to the 29 lineament and these increase to over 9 kbar farther west. There is 30 o o 31 also a corresponding temperature increase from 500 C to 600 C 32 westwards from the lineament. A sample from close to Auchindoun 33 Castle, some 4.5 km to the north-west of the Portsoy–Duchray Hill 34 Lineament, gave values of 8.5 kbar and 605oC. Compositional zoning 35 in garnet crystals enables the pressure–temperature path that the 36 rock has experienced during the growth of the garnet to be 37 modelled. Using this method, samples from immediately west of the 38 lineament have shown an increase in pressure of about 2 kbar, 39 associated with only minor heating, which was sufficient to account 40 for the observed inversion of andalusite to kyanite. 41 From detailed studies such as that of Beddoe-Stephens (1990), 42 associated with previous work based largely on the coastal sections 43 (e.g. Harte and Hudson, 1979; Hudson, 1985; Baker, 1987), it has 44 been possible to deduce a sequence of structural and metamorphic 45 events to account for all of the features described above. The 46 development of andalusite, characteristic of high-temperature, low- 47 48 pressure metamorphism, clearly extended westwards from the Buchan 49 area, across the position of the Portsoy–Duchray Hill Lineament as 50 is shown by the relics of andalusite, for example in the Auchindoun 51 area. The andalusite–kyanite isograd of Chinner and Heseltine 52 (1979) marks the western limit of this original andalusite, which 53 might have developed at least in part in response to high heatflow 54 associated with the emplacement of basic magma in the Buchan Block 55 at around 470 Ma. 56 Subsequent to the development of andalusite, the rocks immediately 57 to the west of the lineament underwent a pressure increase of up to 58 2 kbar that transformed the andalusite to kyanite and it is these 59 peak metamorphic conditions that are recorded by the calculated 60 pressure and temperature values of Beddoe-Stephens (1990). 61 62 63 64 65 1 2 3 4 Ashcroft et al. (1984) suggested that it was subvertical shear 5 movements along the Portsoy–Duchray Hill Lineament after the 6 emplacement of the basic magmas, with relative uplift to the west, 7 that brought up the higher grade rocks. However, Baker (1987) and 8 Beddoe-Stephens (1990) refined this to suggest that westerly or 9 north-westerly directed thrusting across the lineament emplaced a 10 thick upper Dalradian sequence of the Buchan Block above older 11 12 rocks to the west, which were hence subjected to increased 13 overburden pressure and near-isothermal compression. Although this 14 explanation has been generally accepted, there is little 15 stratigraphical evidence for overthrusting and the Dalradian 16 stratigraphy seems to young consistently from west to east across 17 the Portsoy–Duchray Hill Lineament with no repetition (Fettes et 18 al., 1991). Hence Dempster et al. (1995) have offered the 19 alternative suggestion that the pressure increase was due to 20 magmatic loading caused by emplacement of the basic magmas. 21 The present steep attitude of the shear-zones along the Portsoy– 22 Duchray Hill Lineament is probably attributable to subsequent late 23 folding and crustal warping, during D3 and later events, resulting 24 in exhumation of strata from deeper levels on the western side. 25 This uplift must have been relatively rapid in order to have 26 preserved the mineral relationships without further retrograde 27 reactions taking place, and this is nowhere more true than at 28 Auchindoun Castle. 29 30 31 12.4 Conclusions 32 33 Metamudstones (pelites) in the banks of the River Fiddich, below 34 Auchindoun Castle, contain prominent minerals that provide a 35 fascinating insight into the history of deformation and regional 36 metamorphism in Dalradian rocks of the North-east Grampian 37 Highlands. Rectangular white cross-sections are easily visible in 38 hand specimen. Some have a dark ‘cross’ due to inclusions of 39 graphite and their overall appearance is characteristic of 40 chiastolite (a variety of the aluminium silicate, andalusite). 41 However, thin sections reveal that the original andalusite has been 42 replaced by kyanite, identical in composition to andalusite but 43 stable under higher pressure conditions. These are exceptionally 44 clear examples of a feature that has great significance in the 45 understanding of metamorphic terranes and hence could be said to 46 have international importance. 47 48 Detailed mineralogical studies have enabled the temperature and 49 pressure at the peak of metamorphism to be calculated and, when 50 combined with similar determinations throughout the region, these 51 data reveal significant differences in metamorphic history between 52 rocks on either side of the N–S-trending Portsoy–Duchray Hill 53 Lineament. It has been suggested that this is due to a 54 considerable thickness of low-pressure–high-temperature rocks from 55 the Buchan Block in the east having been overthrust westwards, 56 increasing the overburden pressure on the rocks below and hence 57 causing the low-pressure andalusite to recrystallize as the high- 58 pressure form of aluminium silicate, kyanite. 59 60 61 62 63 64 65 1 2 3 4 13 CULLEN TO TROUP HEAD 5 (NJ 511 673–NJ 828 669) 6 7 D. Stephenson, J.R. Mendum and D. Gould 8 9 10 11 13.1 Introduction 12 13 This very large GCR site extends for 32 km along the north coast of 14 the North-east Grampian Highlands from Cullen Harbour in the west 15 to Troup Head in the east. Apart from a 1 km-wide outcrop of 16 Caledonian igneous rocks at Portsoy and a 2 km-wide outcrop of 17 Devonian sedimentary rocks at Gardenstown, the bedrock is entirely 18 Dalradian (Figure 25). It comprises a near-complete succession 19 from the Cullen Quartzite Formation at the top of the Grampian 20 Group to the highest parts of the Macduff Formation of the Southern 21 Highland Group, although the Portsoy–Duchray Hill Lineament forms a 22 major stratigraphical, structural and metamorphic break in the 23 middle of the section (see 1.2.1 in Introduction). Exposure is 24 generally excellent, notably in the intertidal zone, in marked 25 contrast to the drift-covered inland areas where it is largely 26 confined to generally poor stream sections. The coast therefore 27 provides an invaluable and unique type section for the Dalradian 28 succession that can be compared and contrasted with that of the 29 30 Central Grampian Highlands. To the west of the Portsoy–Duchray Hill 31 Lineament, many elements of the stratigraphy are common to both 32 successions, and correlations are possible to subgroup and in some 33 cases to formation level. However, east of the lineament 34 correlations, even at subgroup level, are more tenuous. 35 The strata young overall from west to east, with only minor local 36 reversals. To the west of the Portsoy–Duchray Hill Lineament, 37 structures are comparable to those of the Central Grampian 38 Highlands and are dominated by tight NW-verging folds. Here, there 39 is no high-strain zone comparable to the Boundary Slide at the 40 Grampian–Appin group boundary, but shear-zones do occur at higher 41 stratigraphical levels. The lowest, stratigraphically and 42 structurally, is the Keith Shear-zone, which encloses pods of 600 43 Ma granite and whose trace intersects the coast just west of 44 Portsoy. Deformation increases dramatically eastwards towards the 45 Portsoy–Duchray Hill Lineament, with most structures becoming 46 largely coplanar and colinear. The wide shear-zone that here marks 47 48 the lineament contains variably deformed mafic and ultramafic 49 intrusive igneous rocks of the 470 Ma North-east Grampian Basic 50 Suite. East of the lineament, in the so-called ‘Buchan Block’, the 51 rocks show small- and medium-scale folding, in parts with 52 interference patterns, but the regional outcrop pattern is 53 dominated by the broad, open Turriff Syncline, whose axis plunges 54 gently to the north-north-east. 55 The coast section also provides a continuous section across the 56 low-pressure regional metamorphic Buchan zones that characterize 57 this part of the North-east Grampian Highlands (Stephenson et al., 58 2013a, fig. 12). The overall grade of metamorphism increases from 59 low greenschist facies (biotite zone) in the centre of the Turriff 60 Syncline in the east, to upper amphibolite facies (sillimanite-K 61 62 63 64 65 1 2 3 4 feldspar zone) adjacent to the Portsoy Lineament farther west. 5 Immediately to the west of the lineament the Buchan mineral 6 assemblages have been overprinted by higher pressure metamorphism 7 of Barrovian type (lower amphibolite facies; kyanite zone), and 8 there is evidence that pressures increased to the west. 9 The area was first mapped for the Geological Survey by J. Horne, 10 and one-inch Sheet 96 (Banff) was published in 1895. The area was 11 12 subsequently remapped in greater detail by H.H. Read who published 13 the first full account of the district (Read, 1923) together with a 14 revised one-inch map (also published in 1923). Read‘s work 15 established the lithostratigraphy of the coast section and led to 16 numerous important publications on the magmatism, structure and 17 metamorphism of the region (Read, 1919, 1936, 1955; Read and 18 Farquhar, 1956). These in turn prompted and inspired other 19 investigations, such that this coast section became one of the most 20 intensively studied parts of the whole Dalradian outcrop. It has 21 certainly been studied by the greatest number and variety of 22 workers. The sedimentation was studied by Sutton and Watson (1955) 23 and Loudon (1963), but most of the subsequent investigations 24 concentrated upon the structure and metamorphism (Sutton and 25 Watson, 1956; Johnson and Stewart, 1960; Johnson, 1962; Loudon, 26 1963; Fettes, 1970, 1971; Treagus and Roberts, 1981; Moig, 1986). 27 Several papers have concentrated upon the position and nature of 28 the western margin of the Buchan Block and its relationship to the 29 30 ‗main‘ part of the Grampian Terrane (Elles, 1931; Sturt et al., 31 1977; Ramsay and Sturt, 1979) leading to speculation on the nature 32 and significance of the Portsoy–Duchray Hill Lineament (Ashcroft et 33 al., 1984; Fettes et al., 1986, 1991). More-detailed studies of 34 the metamorphism have been undertaken by a number of workers 35 (Chinner, 1966; Ashworth, 1975, 1976; Hudson, 1980, 1985; Hudson 36 and Harte, 1985; Baker, 1985, 1987; Beddoe-Stephens, 1990; Dempster 37 et al., 1995). The presence of glacigenic boulder beds at two 38 separate stratigraphical levels has generated much interest and 39 discussion on the age of the succession (Sutton and Watson, 1954; 40 Spencer and Pitcher, 1968; Hambrey and Waddams, 1981; Stoker et 41 al., 1999) but the palaeontological studies of the higher parts of 42 the succession are now viewed as inconclusive (Skevington, 1971; 43 Downie et al., 1971; Bliss, 1977; Downie, 1984; Molyneux, 1998). 44 The area has been remapped by the British Geological Survey, 45 resulting in the publication of two 1:50 000 sheets, 96W (Portsoy, 46 2002) and 96E (Banff, 2002). As part of this remapping programme, 47 48 samples from the coast section contributed to a regional 49 geochemical study of metacarbonate rocks that proved to be of value 50 in stratigraphical correlation (Thomas, 1989). 51 Much of the coastline is relatively easily accessible and it is 52 very popular with field parties. Excursions have been described by 53 Read (1960) and by C. Gillen, N.H. Trewin and N.F.C. Hudson (in 54 Trewin et al., 1987). 55 56 13.2 Description 57 58 The coastal section is described here from west to east, moving up 59 the stratigraphical succession to originally higher structural 60 61 62 63 64 65 1 2 3 4 levels and correspondingly lower metamorphic grades (Figures 25, 5 26). 6 7 13.2.1 Grampian Group: Cullen Quartzite Formation 8 9 The GCR site includes only the upper three members of the Cullen 10 Quartzite Formation, the lowest, the Findochty Quartzite Member is 11 12 exposed between Buckie and the western side of Cullen Bay. 13 The section from Cullen Harbour (NJ 5110 6733) to Logie Head (NJ 14 5310 6810) exposes the upper 1000 m of the Logie Head Quartzite 15 Member, some 70% of its total thickness. The strata are 16 overturned, and dip at between 55° and 90° to the north-west; 17 apparently shallower dips are mostly due to landslips. They are 18 largely psammites with thin pelitic and semipelitic interbeds. 19 Planar-laminated beds with flat bases, generally less than 0.5 m 20 thick, together with tabular cross-bedded units, are 21 characteristic. Some psammite units show aligned mud clasts and a 22 poorly defined wispy lamination. Fold axes of prominent slump 23 folds imply an original palaeoslope towards the north-north-west. 24 At the western side of Portlong Hythe (NJ 5215 6765), some 70 m of 25 pale-brown thicker bedded quartzites are present, but along its 26 eastern side psammites are interbedded with semipelite beds ranging 27 from a few centimetres up to 7 m thick. Pale-green garnet- 28 clinozoisite-bearing calcsilicate pods and lenses are developed 29 30 locally (e.g. at NJ 5254 6770). Thick lenticular quartzite and 31 psammite beds form the western and eastern promontories of Logie 32 Head, separated by thinly bedded micaceous and feldspathic 33 psammites and semipelites with calcsilicate lenticles. 34 The Dicky Hare Semipelite Member, c. 200 m thick, is exposed on 35 the foreshore east of Logie Head from NJ 5310 6810 to NJ 5325 6755. 36 It consists of thinly bedded, flaggy micaceous and feldpathic 37 psammites, and garnetiferous semipelites and pelites with scattered 38 calcsilicate lenses and rare centimetre-thick garnet-rich bands. 39 In the upper part of the member, sedimentary structures include 40 fine-scale and ripple-drift cross-bedding, flame structures and 41 slump structures. Minor and medium-scale, close to tight and 42 rarely isoclinal tectonic folds are also abundant in the mixed 43 lithologies. The folds are generally asymmetrical with a Z- 44 profile, and their axes mainly plunge gently to the south-west. In 45 parts a folded lineation (L1) is seen and in the more-pelitic units 46 a penetrative crenulation cleavage (S3) is developed. Read (1923) 47 48 considered this unit to be a faulted repetition of the lowest part 49 of the West Sands Member of the succeeding Findlater Flag 50 Formation. The contact with the overlying Sunnyside Psammite 51 Member is defined by a strike-fault, marked by several metres of 52 shattered rock, but this does not result in any significant 53 repetition of the near-vertical strata. 54 The Sunnyside Psammite Member, some 500 m-thick, consists of 55 typically grey to fawn, planar bedded quartzite and psammite, with 56 tabular cross-bedding seen in parts. Individual beds are normally 57 less than 1 m thick and semipelitic interbeds are present near the 58 top of the member. Open to tight folding (F3 and F1) is present, 59 particularly in its lower part. 60 61 62 63 64 65 1 2 3 4 13.2.2 Appin Group: Lochaber Subgroup: Findlater Flag 5 Formation 6 7 The Findlater Flag Formation consists of grey to brown, planar, 8 thin-bedded to laminated, micaceous and feldspathic psammites with 9 some thin, schistose, locally garnetiferous semipelite units. 10 Flaggy partings rarely exceed 15 cm in thickness. They strike 11 12 north-east, generally dip steeply south-east and show few outward 13 signs of early large-scale fold structures. However, in parts 14 minor folds with axial planes roughly parallel to bedding and 15 gently NNE- or SSW-plunging axes are developed. The psammitic beds 16 are not conspicuously deformed, but the more-pelitic lithologies 17 have a well-developed schistosity commonly at a low angle to 18 bedding. 19 Two members are recognized within the formation, which is 20 otherwise undivided. The basal West Sands Mica Schist Member is 21 about 125 m thick and consists of dark-grey to green-grey schistose 22 garnetiferous semipelites, with psammitic ribs up to 1 cm thick. 23 The bedding and well-developed schistosity are both subvertical, 24 but centimetre-scale minor folds have axes that plunge gently to 25 the south. The c. 7.5 m-thick Findlater Castle Quartzite Member 26 forms the spine of a conspicuous promontory, topped by the ruins of 27 Findlater Castle at NJ 5418 6720. The white to fawn quartzite is 28 thickly bedded and steep bedding surfaces show conspicuous large- 29 30 scale ripple marking (Figure 27). Cross-bedding is visible on 31 clean-washed exposures at the base of the promontory. At low water 32 the quartzite can be traced into the bay to the east where it is 33 tightly folded. 34 35 13.2.3 Cairnfield Calcareous Flag Formation 36 37 This formation consists mainly of micaceous psammite and semipelite 38 but is characterized by the presence of calcsilicate- and 39 carbonate-bearing units. Most lithologies contain amphibole in the 40 compositional range tremolite–magnesio-hornblende but its mode of 41 occurrence varies (Stephenson, 1993). The dominant flaggy, finely 42 bedded calcareous psammites and semipelites, range in colour from 43 striped dark-grey and cream to pale-green and bluish grey. 44 Amphibole in these beds is mainly fine grained and disseminated but 45 some of the more-schistose lithologies contain radiating coarse- 46 grained aggregates (c.f. garbenschiefer ). The beds strike north- 47 ‘ ’ 48 north-east and dip very steeply to the east-south-east. Small- 49 scale tight folding is common locally and fold interference 50 patterns are seen in places. Fold axes typically plunge gently to 51 the north-north-east or south-south-west. 52 In the coast section, two distinctive members can be recognized. 53 The lower, Crathie Point Calcsilicate Member consists of about 300 54 m of predominantly pale-fawn and grey calcsilicate-bearing rocks 55 with beds of impure cream metacarbonate rock, possibly dolomitic; 56 the latter weather typically with a pale brownish and honeycombed 57 surface. Some 250 m south of Crathie Point (around NJ 5490 6716), 58 thin bands of gneissose muscovite-biotite semipelite contain 59 kyanite and staurolite, indicative of lower amphibolite-facies 60 metamorphism. This lithology can be correlated southwards as far 61 62 63 64 65 1 2 3 4 as Glenlivet and the Cairngorm Pluton (see the Bridge of Brown GCR 5 site report). The upper, Garron Point Tremolitic Flag Member 6 consists of about 350 m of generally dark-grey, thin-bedded, flaggy 7 muscovitic psammite and semipelite with abundant disseminated 8 amphibole. Thin beds of dark green amphibole-rich rock (para- 9 amphibolite) are characteristic and thin beds or lenses of impure 10 metacarbonate rock occur rarely. 11 12 13 13.2.4 Ballachulish Subgroup: Mortlach Graphitic 14 Schist Formation 15 16 About 250 m east of Garron Point (at NJ 5565 6685), rocks of the 17 Garron Point Member are stratigraphically overlain by black, 18 pyritic, highly graphitic pelite interbedded with metalimestone. 19 These lithologies pass upwards at Sandend Harbour into grey 20 calcareous psammite and semipelite with subsidiary graphitic pelite 21 and metalimestone units. These lithologies constitute the Sandend 22 Harbour Limestone Member, the basal unit of the Mortlach Graphitic 23 Schist Formation. The beds of white to pale-grey coarsely 24 crystalline metalimestone are up to several metres thick. They are 25 prominent among the intertidal exposures on account of their creamy 26 yellow weathering and well-developed banding, with many pelitic 27 partings. Some are finely laminated and it has been suggested that 28 they might be stromatolitic. Thin beds rich in amphibole, similar 29 30 to those in the Garron Point Member, are also present in places. 31 Abundant tight to isoclinal minor folds plunge at low angles to the 32 north-north-east. 33 The main part of the Mortlach Graphitic Schist Formation lies 34 beneath the beach and dune sands of Sandend Bay. It has been 35 proved by a series of BGS boreholes (1992) to be c. 325 m thick and 36 to be composed of dark-grey, graphitic, schistose to slaty pelite, 37 with pale-grey kyanite porphyroblasts abundant in places. The 38 boreholes found no trace of an overlying quartzite, the 39 Corriehabbie Quartzite, one of the most persistent markers in the 40 Appin Group of the Central and North-east Grampian Highlands. 41 However, the quartzite does crop out inland, some 10 km to the 42 south-west of Sandend Bay. 43 44 13.2.5 Tarnash Phyllite and Limestone Formation 45 46 A disused and flooded quarry behind the dunes of Sandend Bay at NJ 47 48 5578 6595 exposes centimetre-scale, banded, flaggy, pale-grey 49 metalimestone with phyllitic semipelite partings and some thin beds 50 of micaceous psammite. This is the Linkbrae Limestone Member that 51 occurs at the base of the Tarnash Phyllite and Limestone Formation. 52 Its lower contact was penetrated in a BGS borehole, in which the 53 Mortlach Graphitic Schists pass upwards into banded greenish grey 54 psammites and semipelites, which are calcareous in parts. The 55 remainder of the formation (the major part) that consists of grey 56 schistose to phyllitic semipelite, is very poorly exposed 57 ephemerally on the beach at Sandend. 58 59 60 61 62 63 64 65 1 2 3 4 13.2.6 Blair Atholl Subgroup: Fordyce Limestone 5 Formation 6 7 Exposures on the east side of Sandend Bay are of blocky to massive, 8 thin- to thick-banded, generally blueish grey crystalline 9 metalimestones with subsidiary interbedded phyllitic to schistose 10 pelites and semipelites. Cherty lenses and calcite veining are 11 12 common. In contrast to the exposures on the west side of the bay, 13 the axes of the numerous tight minor folds here plunge steeply (45– o 14 60 ) to the east-north-east. Fold interference patterns are seen in 15 parts. The metalimestones are overlain unconformably by Old Red 16 Sandstone breccia and sandstone. 17 18 13.2.7 Argyll Group: Islay Subgroup: Arnbath Psammite 19 Formation 20 21 The upper part of the Fordyce Limestone Formation is cut out by a 22 NW-trending fault that passes through Red Haven (NJ 5640 6630). To 23 the north-east of this fault, and beyond a complex zone of 24 shearing, most of the headland that terminates in Redhythe Point 25 (NJ 5760 6715) is composed of quartzites, micaceous and feldspathic 26 psammites and minor semipelites that constitute the Arnbath 27 Psammite Formation. Calcsilicate pods are locally abundant. Small 28 shear-zones are developed and the quartzite beds are commonly 29 30 boudinaged giving a complex structurally disharmonic appearance to 31 the rocks. Close upright F3 folds that plunge southwards at 30 to o 32 45 are prominent in the cliff exposures, which are composed 33 dominantly of thinly banded and laminated grey quartzites of the 34 basal Redhythe Quartzite Member. The upper parts of the formation 35 crop out inland and are very poorly exposed. However, on the 36 north-west flank of Durn Hill (at NJ 567 642), some 2 km from the 37 coast, numerous blocks of metadiamictite in field walls, almost 38 certainly derived from the upper parts of this formation, have been 39 interpreted as tillites (Spencer and Pitcher, 1968). 40 From about 300 m south-east of Redhythe Point, at NJ 5765 6695, 41 the strata dip steeply and consistently to the south-east and 42 display a strong bedding-parallel fabric; fold axes plunge more 43 steeply (45 to 70o) to the south-east and east-south-east. At the 44 inlet of Foul Hole (NJ 5775 6675) there is a sharp contact of 45 quartzites with gneissose and locally migmatitic psammites and 46 semipelites, which is interpreted as the position of the Keith 47 48 Shear-zone. About 1500 m south of here, at Boggierow Quarry (NJ 49 5747 6516), a highly foliated and xenolithic pale- to mid-grey 50 muscovite-biotite granite, the Portsoy Granite, is inferred to mark 51 the position of the shear-zone. U-Pb dating of zircons from the 52 granite has given an emplacement age of 599.9 ± 2.5 Ma (Barreiro, 53 1998). Farther east, at NJ 5780 6679, folded migmatitic and 54 gneissose psammites, semipelites and quartzites are cross-cut by a 55 12 m-thick amphibolite body with foliated margins, which is cross- 56 cut in turn by a c. 60 cm-wide irregular sheet of foliated 57 muscovite-biotite granite. 58 59 60 61 62 63 64 65 1 2 3 4 13.2.8 Durn Hill Quartzite Formation 5 6 East of Foul Hole, in a rocky semicircular bay at NJ 5790 6659, a 7 fault divides amphibolite to the west from micaceous psammites 8 overlain by c. 13 m of finely banded cream and grey metalimestones 9 and metadolostones. Amphibolite lenses and pods are common and a 10 2.2 m-wide talc-magnesite pod occurs at the base. The 11 12 metacarbonate rocks are succeeded by thick-bedded quartzites that 13 contain a few quartz and granite pebbles, and pass up into poorly 14 bedded, blocky to massive quartzites with subsidiary siliceous and 15 micaceous psammites. The ‘beds’ dip steeply eastwards but are 16 characterized by a strong foliation and steep down-dip lineation. 17 Although traces of tight to isoclinal folds with steeply plunging 18 axes can be distinguished in parts, in other areas the quartzites 19 are rodded to form mullions. Deformation features dominate here 20 and the lithologies are not typical of the Durn Hill Quartzite as 21 seen inland. 22 23 13.2.9 Easdale Subgroup: Castle Point Pelite 24 Formation 25 26 At St John’s Well, the rapid transition from the Durn Hill Quartzite 27 into the Castle Point Pelite Formation is well exposed on the 28 foreshore. Graphitic pelite and semipelite are dominant at the 29 30 base but pass eastwards into mid- to dark-grey, schistose, partly 31 graphitic semipelite and pelite with minor thin beds of micaceous 32 psammite. Calcsilicate pods and metacarbonate beds are present but 33 only become abundant near the top of the formation at the outdoor 34 swimming pool west of Portsoy. The formation typically shows 35 abundant small-scale chevron-style folding and crenulation 36 cleavages, but in its upper part close to tight, small- to medium- 37 scale folding is well seen. Fold axis orientations are variable 38 with refold patterns present. The rocks contain kyanite locally, 39 both as blades and as pseudomorphs of chiastolite (andalusite); 40 notable examples can be seen in the uppermost part of the formation 41 at NJ 5846 6638. Staurolite is also present in parts and 42 sillimanite has been recorded from muscovite-bearing pelites at 43 Sandy Pots (NJ 5844 6654). 44 45 13.2.10 Portsoy Limestone Formation 46 47 48 The Portsoy Limestone Formation encompasses all of the 49 metasedimentary rocks within the Portsoy Shear-zone on the coast 50 section. It cannot be traced inland where thick superficial 51 deposits mantle the bedrock, but many of its elements are certainly 52 lenticular and its lateral continuity southwards is questionable. 53 The variable lithologies are tightly folded and generally very 54 strongly deformed, in parts having almost mylonitic fabrics. 55 The basal part of the formation, well exposed at Legg Moon (NJ 56 5851 6645), consists of tight to isoclinally folded pale to dark 57 and bluish grey metalimestones with thin pelitic laminae. These 58 thick metalimestones mark the transitional change up into more- 59 varied lithologies. Semipelites and quartzites succeeding the 60 metalimestones are intruded by gabbros and a large serpentinized 61 62 63 64 65 1 2 3 4 lherzolite or harzburgite body, known locally as the ‘Portsoy 5 Marble’. This is succeeded eastwards by graphitic pelite, 6 quartzite, semipelite and micaceous psammite. Pebbly and gritty 7 quartzites show strong rodding and folding about steep, commonly 8 near-vertical axes on a distinctive promontory at NJ 5864 6632 9 (Figure 28); they are succeeded by tightly folded and refolded 10 metadolostones, graphitic pelite and psammite, and white to pale- 11 12 grey, tight folded metalimestone. The metalimestone also contains 13 small veins of sphalerite, pyrite and chalcopyrite. 14 15 13.2.11 Intrusive igneous rocks of Portsoy 16 17 The metasedimentary rock outcrop is terminated against a vertical 18 pod of anorthosite and to the east, beneath the town of Portsoy, 19 variably deformed and amphibolitized ultrabmafic and mafic 20 intrusive rocks, cut by sheets and pegmatitic veins of granite, 21 dominate an across-strike section of 750 m. However, substantial 22 screens and rafts of metasedimentary rocks are present in the Old 23 Harbour area. The mafic and ultramafic rocks are interpreted as 24 being truncated against a major fault on the eastern side of Links 25 Bay, which probably coincides with a steep shear-zone on the 26 eastern edge of the Portsoy Lineament. Almost all of the intrusive 27 rocks have been shown by U-Pb zircon dating to have been emplaced 28 at c. 470 Ma. Their complex field relationships and wider age 29 30 constraints imply that emplacement was coeval with thrusting and 31 penetrative deformation. 32 33 13.2.12 Crinan Subgroup: Cowhythe Psammite Formation 34 35 The majority of this formation consists of micaceous and 36 feldspathic psammite with subsidiary biotitic semipelite and some 37 minor calcareous units, notably the distinctive Rosehall Croft 38 Limestone and calcareous semipelites on Cowhythe Head. At the 39 western edge of the outcrop, on the east side of Links Bay, are 40 tightly folded calcsilicate rocks, metalimestones and semipelites 41 with lenticular quartzite and psammites bodies. Although those 42 beds are shown as part of the Cowhythe Psammite Formation on the 43 BGS 1:50 000 Sheet 96W (Portsoy, 2002), some authors have 44 attributed them to the Portsoy Limestone Formation. 45 The rocks have been metamorphosed to sillimanite grade and 46 migmatization is variably developed, with the more-migmatitic units 47 48 found in the western part of the outcrop. The first coherent unit 49 of sheared and migmatitic semipelite that marks the eastern extent 50 of the mixed calcareous rocks at Links Bay has been interpreted as 51 marking the Portsoy Thrust (Elles, 1931; Ramsay and Sturt, 1979; 52 see Interpretation below). Although deformation is high and small- 53 scale dislocations are present, no major dislocation is now 54 recognized in the sequence. Indeed, as Ramsay and Sturt (1979) 55 showed, there are ‘enclaves’ of unmigmatized rocks within the 56 migmatitic semipelitic units. Tight folding on very steeply 57 dipping axes, large-scale cuspate structures and shear-zones can 58 readily account for the complex detailed distribution of 59 lithologies. 60 61 62 63 64 65 1 2 3 4 Anastomosing shear-zones are found near the south-eastern margin 5 of the Cowhythe Psammite Formation, which is marked on the north- 6 western side the bay of Old Hythe by a steep narrow zone of 7 mylonitic biotite-rich semipelite and tightly folded recrystallized 8 white metalimestones. This structure, termed the Boyne Line by 9 Read (1923), was thought to represent a fundamental regional 10 dislocation in the Dalradian succession, but later work has shown 11 12 that it does not correspond to a major stratigraphical, structural 13 or metamorphic break (see Interpretation). The structure is 14 further complicated due to the presence of variably orientated, 15 commonly steeply plunging F2 and F3 folds, a later monoform, and 16 faulting (see Read, 1923, figure 5). 17 Although there are steep dips in parts, the overall bedding and 18 structural profile of the Cowhythe Psammite Formation are only 19 gently inclined. In some areas the bedding is notably lenticular 20 and chaotic zones are present; this seems to be a primary 21 depositional feature. Some idea of the structural complexity is 22 given by the outcrop of the Rosehall Croft Limestone on the 23 foreshore and cliffs around NJ 5978 6640, which shows a refolded 24 fold pattern that can be traced over a few hundred metres. 25 Two main fold generations can be distinguished, an earlier tight 26 folding (F2) with a penetrative cleavage (S2), and a dominant late 27 open to tight folding (F3) with a variably developed crenulation or 28 penetrative cleavage (S3). Fold axes show a wide range of plunge 29 30 orientations. A moderately plunging rodding lineation (L2) that 31 appears to define the maximum finite extension direction during the 32 D2 deformation episode, traces an arc between south-south-west at 33 Links Bay and west at Kings’s Head. Late-stage crenulation 34 cleavages are developed locally in the more-semipelitic lithologies 35 and monoformal structures also deform the earlier D2 and D3 fabrics 36 and folds. 37 On the foreshore some 250–500 m south-west of East Head, around NJ 38 667 600, are several small circular to ovoid bodies of ultramafic 39 rock, which hornfels the adjacent metasedimentary rocks. By East 40 Head, the gneises are cut by 3–4 m-thick NNE-trending veins of pink 41 to orange muscovite and tourmaline-bearing pegmatitic granite. 42 43 13.2.13 Tayvallich Subgroup: Boyne Limestone Formation 44 45 The lowest unit of the formation, the Old Hythe Semipelite Member 46 consists of purple-grey semipelites interbedded with white to grey 47 48 metalimestones and calcsilicate-rock bands and lenses. Immediately 49 east of the ‘Boyne Line’ ductile dislocation, the beds are tightly 50 folded, contain small-scale shears, and show widespread development 51 of a later fracture cleavage. Quartz pods are common. Semipelite 52 outcrops in the central part of Old Hythe bay show more-typical 53 structural patterns and sillimanite (fibrolite) has been recorded 54 from these beds. 55 On the south-eastern side of Old Hythe, spectacularly refolded 56 interbedded metalimestones and calcsilicate rocks of the succeeding 57 Boyne Castle Limestone Member are exposed in the cliffs. F1 and F3 58 folding is abundant in these white, cream, pale-purple and greenish 59 grey, laminated and thinly banded metalimestones and subsidiary 60 calcsilicate rocks, which are very well exposed on the Craig of 61 62 63 64 65 1 2 3 4 Boyne (NJ 6163 6612). The distinctive fine-scale lamination, thin 5 cherty layers and pelitic laminae of this unit might relate to an 6 algal, partly stromatolitic origin. The unit has been extensively 7 quarried immediately inland at Boyne Bay Quarry. 8 On the south-east side of Boyne Bay are thinly bedded, pale-grey 9 to pink-purple, greenish grey and cream, calcsilicate rocks, impure 10 metalimestones and calcareous semipelites. These candy-striped 11 12 rocks mark the base of the Whyntie Brae Calcsilicate Member and 13 show excellent F3 and earlier F1 folding with interference patterns 14 common. F1 folds are tight and their axes typically plunge 15 moderately to the north-east or south-west. The superimposed F3 16 folds vary from open to tight and verge to the north-west, with 17 their axes mainly plunging gently to moderately to the south-west 18 and south-south-west. Overall the beds dip gently to the south- 19 east, but the folding gives rise to steeper dips locally. 20 At NJ 6215 6585 a c. 10 m-thick sheet of dark green-grey 21 metadolerite intrudes the calcareous rocks. It is locally 22 discordant to bedding, yet defines an upright, close, NW-verging 23 fold-pair some 70 m across, interpreted as an F3 structure. At 24 least four further metadolerite sills occur to the east, and an 25 unfoliated hornblende metagabbro body, some 50–60 m thick, crops 26 out around Whyntie Head (e.g. at NJ 6284 6583). These mafic bodies 27 probably relate to the large poorly exposed Boyndie Intrusion that 28 lies some 1.5 km to the south-south-east. 29 30 31 13.2.14 Southern Highland Group: Whitehills Grit 32 Formation 33 34 The metagabbro sheet at Whyntie Head marks the base of the 35 overlying Whitehills Grit Formation, which signals a change in the 36 overall depositional facies of the Dalradian succession from 37 shallow-marine shelf to deeper water turbiditic sedimentation. The 38 Southern Highland Group rocks appear to overlie the underlying 39 Argyll Group succession with slight unconformity. The basal 40 lithology is a notably thick gritty psammite, locally calcareous in 41 part. Farther east interbedded psammites, semipelites and pelites 42 are more typical of the formation. A single metalimestone bed 43 occurs near the base of this mixed succession but calcareous 44 psammites and semipelites, and calcsilicate lenses are relatively 45 abundant throughout. The thicker psammite beds are commonly gritty 46 and show coarse to fine grading. A prominent thick, relatively 47 48 planar bed of massive, cream to fawn calcareous gritty psammite is 49 well exposed on the foreshore at Stake Ness (NJ 6442 598). The bed 50 has an erosional lower contact and shows internal, tight 51 disharmonic folds, interpreted as slump folds. Similar intrafolial 52 folds occur in some of the other thick psammite beds. The lower 53 part of the formation is intruded by metadolerite sheets up to 4 m 54 thick. 55 The formation has low overall regional dips but exhibits small- 56 and medium-scale tight F1 folding, mainly along NE- and NNE- 57 plunging axes. However, the outcrop pattern is dominated by the 58 more-open, NW-verging F3 folds whose axes plunge gently to the 59 north-east. Fold interference patterns are clearly seen in places. 60 A penetrative S1 cleavage is normally present, but an S3 61 62 63 64 65 1 2 3 4 crenulation cleavage is only variably developed. Andalusite is 5 seen in the pelitic units and the tops of graded psammite–pelite 6 beds, and staurolite is present in some pelites (Hudson, 1980, 7 1985). 8 9 13.2.15 Macduff Formation 10 11 12 A thick gritty psammite unit forms the promontory and skerries of 13 Craig Neen (NJ 6517 6571) at the western margin of Whitehills 14 village; this effectively marks the end of the calcareous units and 15 the base of the Macduff Formation. A kilometre-thick lower unit, 16 the Knock Head Grit Member is dominated by pelite, semipelite and 17 psammite units with minor calcsilicate lenses in the lower part but 18 the frequency and thickness of intervening gritty psammites 19 increases eastwards until, at Knock Head, pebbly psammites are 20 dominant. From there, the pelitic element increases eastwards 21 towards the west side of Boyndie Bay, where psammite is again 22 subsidiary. The pelitic units commonly show concentrations of 23 large (average c. 1 cm long) grey elongate ‘slugs’ of andalusite, 24 and/or black rounded cordierite porphyroblasts, reflecting the 25 respective iron-rich and less common magnesium-rich nature of the 26 protolith muds (Figure 29). Porphyroblasts reflect the grading in 27 the turbiditic units, becoming larger and more abundant in the 28 pelitic tops. Small red manganiferous garnets and brown 29 30 staurolites up to 2 mm long are also present. The bedding in the 31 Knock Head Grit Member generally dips east at over 50°. Minor 32 folding is only rarely seen, except near Whitehills where close to 33 tight F1 folds with moderately north-plunging axes are present. D3 34 effects are mainly limited to development of a coarsely spaced 35 crenulation cleavage in some pelitic units. S3 clearly overprints 36 the porphyroblast growth, which overgrows the earlier S1 37 cleavage/schistosity (Johnson, 1962; Fettes, 1971; Hudson 1980). 38 This steep zone forms the western limb of the so-called Boyndie 39 Syncline, whose status is discussed below. 40 The Dalradian outcrops lie close to the Devonian land surface in 41 this area, and small patches of breccioconglomerate are present. 42 In parts the Dalradian rocks are heavily stained red-brown; in 43 other areas the andalusite is stained red, with the pelitic matrix 44 remaining mid to dark grey. 45 The higher parts of the Macduff Formation occupy the remainder of 46 the coast section from the eastern side of Boyndie Bay to More Head 47 48 and around Troup Head, which is north-east of the Devonian outlier 49 at Gardenstown. The metamorphism is greenschist facies (biotite 50 grade) and only a single deformation phase is present. Hence, the 51 sedimentary features of the interbedded psammites, semipelites and 52 pelites are clear. Bouma sequences are common, with flame 53 structures, grading, cross-bedding, ripple lamination and mud-flake 54 breccias, all indicative of a turbiditic, density current origin. 55 The psammites, which vary from quartz-rich to micaceous, have 56 gritty and even pebbly bases locally. Good examples can be seen in 57 Tarlair Bay, around NH 7188 6450. The more-quartzose lithologies 58 commonly form discrete lenticular units and probably represent 59 reworked channel-fill material in an offshore fan. 60 61 62 63 64 65 1 2 3 4 At Macduff thin metadiamictite units, with pebbles, cobbles and 5 rare boulders, are interspersed in a turbiditic succession that 6 occurs on the broad hinge of a syncline between NJ 7128 6491 and NJ 7 7144 6488. These beds were interpreted as glaciomarine deposits by 8 Sutton and Watson (1954). Stoker et al. (1999) have documented the 9 sequence in detail and confirmed that at least some of the larger 10 boulders are probable dropstones from floating ice (Figure 30). 11 12 Metamorphic grade increases from east to west. Rounded dark-grey 13 cordierite porphyroblasts, seen to be altered to pinite in thin 14 section, are first seen in the pelitic units west of Banff Harbour. 15 Farther west at Scotstown the cordierites are paler with only 16 partial or marginal dark-grey alteration to pinite. Zoned 17 calcsilicate pods are present locally (Hudson and Kearns, 2000). A 18 little farther west, at NJ 6817 6459, Hudson (1987) recorded the 19 first andalusite porphyroblasts as small whitish grey laths or 20 square-section crystals in cordierite-bearing pelitic units and 21 staurolite appears on the west side of Boyndie Bay in the Knock 22 Head Grit Member. 23 The rocks of the Macduff Formation are disposed in a series of 24 open to tight F1 folds typified by steeply dipping bedding, well 25 seen on the foreshore at Banff and Scotstown. However, somewhat 26 perversely, the overall or sheet dip is shallow, with the 27 succession defining a very open regional syncline, the Turriff 28 Syncline (Figures 3a, 26). Although it is difficult to correlate 29 30 the detailed stratigraphy, the upper part of the formation exposed 31 on this coast section could be only 2–3 km thick. 32 Farther east at the Howe of Tarlair (NJ 719 646), a thick gritty 33 psammite unit defines an upright F1 anticline-syncline pair with an 34 amplitude of some 130 m and a wavelength of c. 320 m. F1 axes 35 plunge gently northwards. The related S1 cleavage is a variably 36 developed pressure-solution cleavage in the psammites but a slaty 37 cleavage in the pelitic units. Cleavage-bedding relationships are 38 well seen and the abundant sedimentary features clearly show that 39 the structures are upward facing. The fold structure is controlled 40 by the thickness of psammite units and generally the folds have a 41 neutral vergence. However, at Stocked Head, the F1 fold vergence 42 changes from neutral to westerly and S-profile folds dominate 43 farther east. Late kink folds are common locally and the F1 44 structures are refolded by open monoforms. All of these structures 45 lie within the broad axial zone of the Turriff Syncline. 46 The Gardenstown outlier of Devonian sandstones, siltstones and 47 48 conglomerates has faulted boundaries against Dalradian rocks, 49 although an unconformity can be seen in a small exposure at Crovie. 50 The Macduff Formation rocks to the north-east that form the 51 peninsula of Troup Head are fault bounded to the south-east and 52 form an isolated enclave. They consist of psammites and pelitic 53 units and have gentle but variable dips. Their overall dip is 54 easterly and they show west-verging close F1 folds. 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 13.3 Interpretation 5 6 7 13.3.1 Stratigraphy and sedimentation 8 9 The generally clean, mature nature and ubiquitous cross-bedding of 10 the Cullen Quartzite Formation suggest that the sands were 11 12 deposited under shallow marine conditions where tidal, wave and 13 current action were important. Slumped units suggest that at times 14 deposition was rapid and local instabilities resulted. The 15 succeeding more-pelitic units probably represent quieter 16 sedimentation conditions dominated by mud deposition under inshore 17 intertidal, lagoonal or estuarine conditions. Similar conditions 18 prevailed during deposition of the overlying Findlater Flag 19 Formation. The clean quartzite units such as the ripple-marked and 20 cross-bedded Findlater Castle Quartzite probably represent short- 21 lived sandy channels (Figure 27). However, the upper parts of the 22 Lochaber Subgroup, as elsewhere in the Grampian Highlands, show a 23 progressive increase in calcareous material. Calcsilicate rocks 24 are abundant and dolomitic metacarbonate rocks, calcareous 25 semipelites, and minor kyanite-bearing rocks characterize parts of 26 the succession. A study of the tremolitic amphiboles that are a 27 major component of the Cairnfield Calcareous Flag Formation 28 revealed that the protolith muds were very high in Mg relative to 29 30 Ca, with high but variable Al (Stephenson, 1993). The aluminous, 31 magnesian and potassic nature of these lithologies imply input of 32 periodic weathered material, probably under very shallow water, 33 locally evaporitic, partially emergent conditions, with only 34 restricted circulation of seawater (Thomas, 1999). Detrital input 35 of silica and clay minerals was variable and possibly seasonal, 36 giving rise to the colour banding; marl beds with a low detrital 37 component gave rise to the essentially monomineralic para- 38 amphibolites. 39 The graphitic pelite and metalimestone succession of the 40 Ballachulish Subgroup is indicative of a widespread marine 41 transgression across the Dalradian basin, marked by deposition of 42 anoxic or reduced mudstones and chemical or derived limestones. In 43 this section, boreholes have revealed a continuous stratigraphical 44 passage from the graphitic pelites up into overlying thinly bedded 45 semipelites, metalimestones and calcsilicate rocks of the Tarnash 46 Phyllite and Limestone Formation. A return to very shallow water 47 48 conditions with possible local evaporates is indicated, though 49 without any major influx of detrital siliciclastic material, as 50 represented by the quartzite seen elsewhere at this stratigraphical 51 level. The deposition of graphitic semipelites and pelites and 52 metalimestones, typical lithologies of the Blair Atholl Subgroup, 53 signals a further transgression phase. 54 The Islay Subgroup has a locally developed metadiamictite at the 55 base that relates to a marine glacial event. This was accompanied 56 and followed by local basin formation and deposition of largely 57 siliciclastic sediments. Although the Arnbath Psammite Formation 58 contains a greater proportion of semipelite than is normal at this 59 stratigraphical level, it does contain much sandy material 60 including a quartzite member. The overlying Durnhill Quartzite 61 62 63 64 65 1 2 3 4 Formation is also reduced in thickness and purity by comparison 5 with its manifestation inland. 6 The rapid change to more-graphitic and aluminous pelitic rocks of 7 the Easdale Subgroup (Castle Point Pelite and Portsoy Limestone 8 formations) appears to mark a transgression, although this could 9 reflect formation of a local marine basin. The presence of 10 isolated quartzite lenses, a pebbly quartzite unit, and mixed 11 12 metalimestone–graphitic pelite–quartzite units suggests that the 13 palaeoenvironment and sediment provenance were locally varied. The 14 overlying Cowhythe Psammite Formation, assigned to the Crinan 15 Subgroup, is highly metametamorphosed, structurally complex and 16 partly migmatitic. However, in places in the psammite and 17 semipelite unit there are suggestions of grading, disrupted 18 bedding, and slumped or chaotic units. The beds might have been 19 turbiditic and if that was the case they would represent the infill 20 of a local deeper water basin. The rapid transition up into 21 semipelites and metalimestones of the Boyne Limestone Formation 22 (Tayvallich Subgroup) represents a shallowing, with marine shelf 23 conditions becoming dominant. The thick metalimestones in this 24 formation show laminae and small-scale structures suggestive of 25 algal reef growth, implying that warm shallow conditions prevailed. 26 The overlying calcareous semipelites and calcsilicate rocks were 27 originally marls and possible evaporitic deposits, suggesting 28 possible lagoonal conditions with some local emergence. 29 30 This quiet shallow marine-shelf deposition was interrupted by the 31 incoming of the first turbiditic units of the Southern Highland 32 Group and signalled the development of a deeper marine basin that 33 developed subsequently into the Iapetus Ocean. The lowest psammite 34 unit of the Whitehills Grit Formation appears to cross-cut the 35 underlying Argyll Group succession regionally, suggesting that it 36 might mark a slight angular unconformity. The sediments deposited 37 ranged from coarse siliciclastic to muddy but contained a large 38 component of derived carbonate material, probably from nearby 39 reefs. The succeeding Macduff Formation consists of psammites and 40 intervening pelitic units that show excellent features typical of 41 turbiditic fans. The rocks have been interpreted as channel, 42 overbank or outer-fan deposits, depending on the coarseness of the 43 units, Bouma features and lithological associations (Kneller, 44 1987). Hence, the Knock Head Grit Member represents a channel 45 environment, whereas the adjacent more-pelitic succession 46 represents an interchannel or perhaps outer-fan environment. 47 48 The Macduff Boulder Bed, which lies near the top of the 49 stratigraphical sequence in this GCR site, has been interpreted as 50 a glaciomarine deposit with slumped units and dropstones (Figure 51 30) (Sutton and Watson, 1954; Hambrey and Waddams, 1981; Stoker et 52 al., 1999). By analogy with Pleistocene deposits offshore to the 53 north-west of the British Isles, such glacigenic deposits are 54 characterized by debris-flow packages and the Macduff succession is 55 compatible with distal deposition in a base-of-slope or basin-plain 56 situation, with a water depth of over 1000 m. The deposit has been 57 correlated with Ordovician glacial episodes on the basis of a 58 single acritarch Veryhachium lairdii collected from adjacent 59 pelitic units (Molyneux, 1998). However, failure to replicate this 60 find and its atypical state of preservation despite metamorphism to 61 62 63 64 65 1 2 3 4 biotite grade cast doubt upon the acritarch evidence, which should 5 probably be discounted at the present time. Hence, the glacial 6 deposits more likely correlate with dropstone localities near the 7 base of the Southern Highland Group in Donegal (Condon and Prave, 8 2000), which would imply that only the lowermost parts of the group 9 are exposed in the North-east Grampian Highlands, albeit in a 10 relatively thick sequence. 11 12 13 13.3.2 Structure 14 15 Figure 26 shows the composite cross-section along the coast, based 16 upon recent British Geological Survey mapping and the overall 17 structure is illustrated in Figure 3a. Note that on these 18 sections, the areas of steeply dipping succession have been much 19 reduced, compared with previously published sections (e.g. Sutton 20 and Watson, 1956; Treagus and Roberts, 1981). This has several 21 consequences: the stratigraphical succession is thinner than 22 formerly estimated; the Boyndie Syncline is reduced to a smaller 23 scale steep zone with more-gently dipping, locally subhorizontal 24 beds on either side; the Cowhythe Head and Redhythe Point areas, 25 that lie respectively east and west of the Portsoy Shear-zone, also 26 show regionally shallow dips. In detail many parts of the coastal 27 section show alternating steep and shallow dipping zones and it is 28 unclear as to their age relative to main phases of folding and 29 30 related cleavage formation. The following discussion progresses 31 broadly from higher structural levels in the east of the section to 32 deeper levels as seen in the west. 33 The linked concepts of the Boyne Line and the SE-facing Banff 34 Nappe were introduced by Read (1955) to explain the major structure 35 of the North-east Grampian Highlands (see 1.2.2 in Introduction; 36 Figure 3b). Along much of the coast section east of Boyne Bay, the 37 rocks exhibit locally complex F1 folding with steep dips, but 38 regionally they are the right way up and are disposed in a broad, 39 gentle syncline, the Turriff Syncline, which would constitute the 40 upper limb of the nappe in Read‘s model. The overall structural 41 and metamorphic sequences of this section are quite well understood 42 due to detailed studies of the minor structures (Johnson, 1962; 43 Loudon, 1963; Fettes, 1971; Treagus and Roberts, 1981). Early (F1) 44 folds and related cleavages face consistently upwards and the style 45 of the folds varies according to structural level, being generally 46 upright and open to close at the highest levels in the centre of 47 48 the Turriff Syncline and generally recumbent and tight on the 49 limbs. Across the whole section F1 folds face to the north-west, 50 apparently contrary to Read‘s (1955) model of a SE-facing Banff 51 Nappe. However, Read attributed these folds to gravitational flow 52 on the western limb of the Buchan Anticline, which he envisaged as 53 having developed as an early structure accompanying nappe 54 formation. 55 Post-D1 structures are restricted to the lower structural levels 56 and hence are seen only on the western and eastern limbs of the 57 Turriff Syncline. There is some confusion, particularly in the 58 Portsoy area, as to the number of major fold phases that can be 59 identified, and their relationship to regional D2 and D3 defined 60 elsewhere in the Grampian Highlands. Both Johnson (1962) and 61 62 63 64 65 1 2 3 4 Treagus and Roberts (1981) have identified separate D2 and D3 5 phases but differ as to their distribution and interpretation. 6 Their conclusions are incorporated in the following summary, which 7 is based largely on that of Kneller (1987). D2 folds and cleavages 8 are recognized only to the west of the Boyne Limestone outcrop and 9 to the east of Fraserburgh Bay. In the area around Portsoy, F2 10 folds are locally dominant. Those folds have steeply plunging 11 12 axes, which are colinear with a very strong stretching lineation 13 attributed to thrusting and focussed along the Portsoy Shear-zone. 14 The D3 deformation, which post-dates the peak of metamorphism and 15 main growth of porphyroblasts east of Boyne Bay, has limits that 16 effectively coincide with the andalusite isograd. Large-scale F3 17 folds are characteristically open to close and monoclinal in form. 18 Associated finely spaced cleavages and tight crenulation cleavages 19 occur in the more-pelitic units and small-scale open to tight fold 20 structures are well seen in the more-banded units. The Turriff 21 Syncline and Buchan Anticline have been tentatively attributed to 22 the D3 phase by several authors. However, west of Whitehills, 23 smaller scale D3 structures have easterly to south-easterly dipping 24 axial planes and a westerly vergence. It is difficult to correlate 25 these D3 structures with the open and upright character of the 26 Turriff Syncline, which is more compatible with major D4 structures 27 elsewhere. Numerous sets of minor kink and brittle folds in the 28 area, notably in the pelitic lithologies, are attributed to later 29 30 events. 31 Steeply dipping beds on the western limb of the Turriff Syncline 32 are the result of a monoform, regarded as a major fold closure pre- 33 dating the metamorphic peak by Sutton and Watson (1956), who named 34 the structure the Boyndie Syncline. However, Read (1923, 1955), 35 who concentrated firstly on the stratigraphy and secondly on the 36 regional structural interpretation, portrayed the syncline merely 37 as a perturbation on the top limb of the Banff Nappe. Subsequently 38 Johnson (1962) and Fettes (1971) recognized two fold phases in the 39 Whitehills area and allocated the Boyndie Syncline to the secondary 40 phase, which they termed D3. They argued that the syncline folds 41 the metamorphic isograds and hence post-dates the metamorphic peak. 42 Treagus and Roberts (1981) argued that F1 folds consistently face 43 upwards along the coast section and favoured a D1 age for the 44 Boyndie Syncline. The structural evidence and metamorphic pattern 45 would be compatible with formation of the steep zone during or 46 prior to D1, followed by superimposition of the metamorphic pattern 47 48 and D3 deformation. This would accord with other coastal sections 49 where F3 folds appear to be superimposed on a stepped profile. 50 The Boyne Line as envisaged by Read (1955) underlies the Banff 51 Nappe and separates it from the Argyll and Appin group succession 52 to the west; it is a ‘lag’ rather than thrust structure, i.e. 53 younger rocks are juxtaposed over older rocks (Figures 3b, 26). 54 Ashworth (1975) argued strongly against Read’s model, preferring 55 Horne’s original idea of sedimentary facies variations to explain 56 the absence of the Boyne Limestone Formation to the south (Horne, 57 in Read, 1923, p.72). Most authors agree that later faulting is 58 also present. However, the presence of biotite-grade mylonitic 59 rocks at Old Hythe and the local deformation patterns do suggest 60 that it is the site of a steep shear-zone, possibly with lateral 61 62 63 64 65 1 2 3 4 movement. That said, there is no metamorphic hiatus and moderate 5 structural continuity is retained across the section. The 6 structure may well be linked to the Portsoy Shear-zone that crops 7 out to the west but there is no need to invoke a Boyne Line sensu 8 Read (1955). 9 Between the putative Boyne Line and the Portsoy Shear-zone, Sturt 10 et al. (1977) and Ramsay and Sturt (1979) recognized a complex 11 12 structural history in the Cowhythe Psammite Formation and allocated 13 the migmatitic gneissose rocks to a pre-Dalradian basement unit 14 formed in the Neoproterozoic. They maintained that the contact of 15 the migmatitic semipelite at the base of the Cowhythe Psammite is 16 visibly discordant with the structurally underlying calcareous 17 psammitic and semipelitic succession to the west along the Portsoy 18 Thrust, thus resurrecting Read’s concept of an allochthonous 19 succession in Buchan. However, most workers regard the gneisses as 20 migmatized equivalents of the upper parts of the Argyll Group (see 21 1.1.3 in Introduction). 22 The Portsoy Shear-zone shows evidence of a complex and lengthy 23 tectonic history (Goodman, 1994), as discussed on a regional basis 24 in section 1, Introduction. The main problems associated with the 25 structure are to reconcile its identities as an initially steep 26 lineament, a subsequent shallow dipping shear-zone, and a steep 27 fault/shear-zone along which preferential uplift later occurred. 28 Although evidence for these three manifestations is present on the 29 30 coast section, it is unclear as to how and where the different 31 elements might be projected at depth or farther south to Deeside 32 and beyond. Local and regional stratigraphical patterns imply that 33 a lineament could well have been active at the time of Argyll Group 34 sedimentation (Fettes et al., 1991). The main tectonic activity 35 was focussed in mid-Ordovician time at around 470 Ma when thrusting 36 to the west-north-west and associated deformation, metamorphism and 37 fluid flow, occurred roughly coeval with intrusion of basic and 38 ultrabasic bodies (Baker, 1987; Oliver, 2002). The overprint of 39 kyanite after andalusite to the west of the shear-zone reflects a 40 pressure increase of c. 2 kbar at this time, consistent with an 41 increase in overburden of 6–7 km (Beddoe Stephens, 1990). Later 42 steep shear-zones, along which granitic veins are intruded in 43 parts, are well seen around Portsoy and show foliations and 44 geometries indicative of dextral shearing. 45 The position of the Keith Shear-zone on the coast section is 46 marked by strong planar and linear fabrics and localized 47 48 migmatization of Islay Subgroup rocks around Foul Hole just west of 49 Portsoy. Shear-sense indicators imply top-to-west movement. The 50 shearing post-dated both lithification of these rocks and intrusion 51 of the Keith–Portsoy Granite and was most likely associated with 52 the Grampian Event in mid-Ordovician time. Just inland, at 53 Boggierow, the granite has been reliably dated at c. 600 Ma and is 54 interpreted as having been intruded into an early lineament that 55 was subsequently re-activated as the shear-zone, thereby providing 56 a minimum age for its initiation. A full appraisal of the shear- 57 zone and its associated intrusions, incorporating evidence from 58 outwith this GCR site, is given in section 1.2.1 in Introduction. 59 Farther west in the Appin Group rocks, two phases of penetrative 60 deformation are seen, manifested as folding and related planar and 61 62 63 64 65 1 2 3 4 linear fabrics. These structures are particularly clear in the 5 pelitic and metalimestone lithologies, but it is uncertain whether 6 they should be attributed to D1 and D2, D2 and D3, or D1 and D3. 7 By analogy with deformation chronologies farther south they are 8 generally attributed to D2 and D3. In fact, Peacock et al. (1968) 9 showed that to the south-west of Cullen, the penetrative early 10 fabrics and folds attributed to D2 post-date an earlier bedding- 11 12 parallel schistosity that they termed S1. 13 14 13.3.3 Regional Metamorphism 15 16 The Cullen–Troup Head section remains one of the classic areas for 17 studying the nature of the relatively low-pressure and high- 18 temperature Buchan-type metamorphism (Read, 1952). Many studies 19 have documented the transition from greenschist facies in the 20 Macduff–Gardenstown area to lower and upper amphibolite facies in 21 the Portsoy and Fraserburgh areas (Johnson, 1962; Chinner, 1966; 22 Fettes, 1971; Hudson, 1980, 1985; Hudson and Harte, 1985; Johnson 23 et al., 2001a, 2001b). The occurrence of upper amphibolite-facies 24 assemblages (sillimanite-potash feldspar) in the Cowhythe and East 25 Head area and granulite-facies assemblages east of Fraserburgh Bay 26 (see the Cairnbulg to St Combs GCR site report) was shown by 27 Ashworth (1975) and Johnson et al. (2001b) to relate to mafic 28 intrusions of the North-east Grampian Basic Suite. Pressure and 29 30 temperature conditions ranged from 1.2 kbar, 400°C in the lowest 31 biotite-grade rocks to 1.8 kbar, 490°C at the andalusite isograd and 32 3.3 kbar, 545°C at the sillimanite isograd (Hudson, 1985). The 33 higher grade rocks reached 4.2 kbar, 630°C, values consistent with 34 pressure–temperature estimates derived from the hornfels mineralogy 35 of larger mafic intrusions (Droop and Charnley, 1985). 36 The Buchan metamorphic peak occurred following the D1 deformation 37 but mainly prior to the later D3 event (Johnson, 1962, 1963). 38 Porphyroblasts overgrow generally planar S1 fabrics, but thin- 39 section studies from several rocks in the Inverboyndie area show 40 that cordierite overgrowths also occurred synchronous with D3 41 deformation (Phillips, 1996). A later phase of retrogressive 42 metamorphism characterized by chlorite and sericite (muscovite) is 43 found in many of the rocks, in parts associated with late-formed 44 structures. 45 The metamorphic pattern has influenced both the original 46 lithological divisions and former tectonic models for the Buchan 47 48 area. Read (1923) defined several lithological units on the basis 49 of their metamorphic features. Thus, the former Boyndie Bay 50 ‗group‗, west of Banff, which is characterized by andalusite 51 schists, is now regarded as merely the lower part of the Macduff 52 Formation. Similarly, to the south-east, the Macduff Formation 53 grades downward into the ‗Fyvie Schist‘, which is characterized by 54 andalusite and cordierite porphyroblasts giving a ‗knotted‗ 55 appearance and is now placed in the Methlick Formation. He also 56 erected the Boyne Line on the basis of a supposed metamorphic 57 hiatus between his overlying ‗Banff division‘ and underlying ‗Keith 58 division‘ rocks. However, Ashworth (1975) dismissed the evidence 59 for a metamorphic hiatus and showed that the metamorphic zones 60 appeared to be superimposed on the early-formed structural pattern. 61 62 63 64 65 1 2 3 4 Hence, he rejected the concept of both the Boyne Line and the Banff 5 Nappe. 6 Within the Portsoy Shear-zone the metamorphic mineralogy is 7 undoubtedly complex. By the Portsoy swimming pool (at NJ 5844 8 6648), graphitic pelite shows apparent andalusite (chiastolite) 9 porphyroblasts. However, in thin section a different picture 10 emerges. Large andalusites are partly pseudomorphed by blocky 11 12 kyanite, set in platy to felted muscovite. Ragged early staurolite 13 with sigmoidal inclusion trails and smaller, later, more-euhedral 14 porphyroblasts are also present. Garnets are ragged and show 15 evidence of partial dissolution. The biotite-rich fine-grained 16 schistose matrix is deflected around the andalusites but platy 17 muscovite and kyanite overgrow this fabric locally. Fibrolitic 18 sillimanite also occurs, which on textural evidence pre-dates 19 growth of small kyanite blades (Beddoe-Stephens, 1990). Although 20 the mineralogy gives us a record of at least parts of the lengthy 21 metamorphic history of this rock, it is difficult to unravel which 22 mineral associations were in equilibrium at each stage. 23 West of the Portsoy Shear-zone, porphyroblast growth occurred at 24 several times. Garnet and biotite formed both prior to and 25 synchronous with formation of the S2 fabric; the metamorphic 26 minerals are wrapped by the later amphibolite-facies schistosity 27 and show some resorption or alteration (Peacock et al., 1968). 28 Beddoe-Stephens (1990) showed that the Buchan-type assemblages were 29 30 present in this area but were subsequently overprinted under higher 31 pressure conditions during the D3 deformation (see the Auchindoun 32 Castle GCR site report). It is this event that deformed the Keith– 33 Portsoy Granite, implying thrusting to the west-north-west, 34 probably with movement focussed along the Portsoy and Keith shear- 35 zones. 36 37 13.4 Conclusions 38 39 The coast between Cullen Bay and Troup Head provides the longest 40 continuous section across the strike of the Dalradian succession in 41 Scotland. It extends from the top of the Grampian Group to the 42 highest preserved beds of the Southern Highland Group and is the 43 type section for the Argyll and Southern Highland groups of the 44 North-east Grampian Highlands. It records the progression from 45 deposition in a shallow marine gulf in an intracontinental rift 46 that became a series of fault-bounded deeper sedimentary basins on 47 48 a continental shelf heralding the opening of the Iapetus Ocean, to 49 deposition from turbidity currents in major sub-marine fans on the 50 subsiding shelf and continental slope. 51 The coast also provides a near-complete structural and metamorphic 52 transect across the Dalradian outcrop. The beds are basically the 53 right way up and range from steeply dipping in the west to 54 regionally shallow dipping in the core of the Turriff Syncline to 55 the east. The succession is interrupted by several major 56 dislocations, each of which probably marks the site of a 57 fundamental structure with a long history extending back to the 58 time of sedimentation. The Keith Shear-zone intersects the coast 59 west of Portsoy; it does not remove or repeat much of the 60 succession but it does include granite dated at 600 million years 61 62 63 64 65 1 2 3 4 old. At Portsoy, a 1 km-wide zone of shearing that involves 5 various 470 million year-old mafic and ultramafic intrusions marks 6 the position of the Portsoy Shear-zone and the Portsoy Lineament. 7 The latter structure seems to have acted as a controlling feature 8 at the time of sedimentation, and through the deformation, 9 metamorphism and magmatism of the Grampian Event. It separates the 10 so-called Buchan Block to the east, which exposes higher 11 12 stratigraphical levels and exhibits higher-level structures and 13 low-pressure regional Buchan-type metamorphism, from structurally 14 and stratigraphically lower level, higher pressure rocks to the 15 west that seem to be a continuation of the sequences and structures 16 typical of the Central Grampian Highlands. 17 The nature and origin of the Buchan Block and of its boundaries 18 have been a constant source of discussion. It has clearly been 19 decoupled at times from the adjacent terranes and possibly from the 20 lower parts of the originally underlying Dalradian rocks; it has 21 even been suggested that it might have been a separate 22 allochthonous terrane with an entirely different sedimentological, 23 structural and metamorphic history to the remainder of the Grampian 24 Terrane. However, within this GCR site it could be argued that 25 elements of both the stratigraphy and the structure are continuous 26 across the Portsoy Shear-zone and relics of the Buchan metamorphism 27 have been identified up to 10 km farther west, casting doubt on 28 suggestions of large-scale regional displacement. 29 30 The coast section has contributed to speculation on the age of the 31 Dalradian. At least the lower parts of the succession must be 32 older than the 600 million year-old Portsoy Granite, and the 33 highest parts have reportedly yielded some poorly preserved 34 microfossils. An Ordovician age has been suggested, both from the 35 microfossils and from the presence of glacigenic boulders that have 36 been attributed to a late-Ordovician glacial period, but this 37 interpretation cannot be reconciled with the dating of metamorphic 38 events or the Dalradian succession elsewhere. 39 Most of this GCR site is well exposed and readily accessible, 40 except for parts where the cliffs are particularly steep. With 41 thick superficial deposits characterizing the hinterland this 42 reference section is therefore extremely important in a national 43 and possibly an international context. It has been the subject of 44 more research and more publications than any other part of the 45 Dalradian outcrop and is used extensively for teaching purposes. 46 Yet there is so much about it that is still to be learned. 47 48 49 14 FRASERBURGH TO ROSEHEARTY 50 (NK 001 663–NJ 918 668) 51 52 D.J. Fettes 53 54 55 14.1 Introduction 56 57 This GCR site comprises the coastal section running westwards from 58 Fraserburgh harbour to Rosehearty. It is separated from the 59 Cairnbulg to St Comb GCR site by the wide, sandy expanse of 60 Fraserburgh Bay. Because the section runs for most of its length 61 62 63 64 65 1 2 3 4 at almost right angles to the regional strike and the main 5 structures, it provides an excellent geological profile. It is of 6 interest for three main reasons. Firstly, it exposes a transition 7 from the calcareous rocks and metagreywackes of the Tayvallich 8 Subgroup (Argyll Group) into the overlying andalusite schists and 9 metagreywackes of the Southern Highland Group. Secondly, it 10 provides a section through the western limb of the Buchan Anticline 11 12 (corresponding to the eastern limb of the Turriff Syncline) and 13 clearly demonstrates the nature of the first and second 14 deformational phases. Thirdly, the rocks display excellent 15 examples of Buchan metamorphism and the transition from the 16 andalusite schists to sillimanite-bearing schists and gneisses. 17 The geology of the area was first described by Grant Wilson (1882, 18 1886) but the basis of the modern interpretation lies in a series 19 of papers by H.H. Read on the Buchan area (Read, 1952, 1955) that 20 culminated in a paper on the Buchan Anticline (Read and Farquhar, 21 1956). The detailed sedimentology and structure of the area was 22 first interpreted by Loudon (1963) as part of a wider study of the 23 upper Dalradian. Subsequently important contributions were made on 24 the structural and metamorphic history by Fettes (1968, 1970), on 25 the nature of the metamorphism by Hudson (1975) and Harte and 26 Hudson (1979), and on the sedimentology and structure by Kneller 27 (1988). An excursion guide to the eastern part of the site was 28 provided by Kneller (1987). 29 30 31 14.2 Description 32 33 The coastal section (Figure 31) provides almost continuous exposure 34 along its length of c. 9 km, which comprises a series of low 35 cliffs, rocky foreshores, small sandy bays and the rugged headland 36 of Kinnairds Head. The stratigraphy youngs to the west away from 37 the core of the Buchan Anticline, whereas the metamorphic grade 38 rises to the east, towards the anticlinal core. 39 40 14.2.1 Stratigraphy and sedimentation 41 42 The eastern part of the section, from Fraserburgh harbour westwards 43 to Broadsea at around NJ 988 676, is dominated by calcareous 44 turbiditic units, including some partial Bouma sequences (Kneller, 45 1987). Kneller also reported a slump breccia around NJ 998 677. 46 The sequence is characterized by impure sandy metalimestones, 47 48 calcareous psammites and calcsilicate bands and nodules that 49 together constitute the Kinnairds Head Formation (Figure 32). 50 Calcareous rocks also occur in an isolated exposure south of the 51 harbour at NK 002 664. The contact with the underlying gneisses of 52 the Cairnbulg to St Combs GCR site is not seen. 53 The section from Broadsea to Rosehearty consists of tubiditic 54 rocks with graded units and typical elements of Bouma cycles. The 55 bedding is on the centimetre to metre scale. Various sedimentary 56 features can be seen including rip-up-clasts and convoluted 57 bedding; coarse pebbly psammite units might, in part, represent 58 channel fills. Good examples of sedimentary structures can be seen 59 throughout the section, for example at NJ 952 673, and partial and 60 complete Bouma cycles may be examined around NJ 986 673. One 61 62 63 64 65 1 2 3 4 rather spectacular rock type that dominates much of the section 5 consists of regular graded units, which are characteristically 20– 6 30 cm in thickness and consist of a finely layered psammite base 7 succeeded by pelite. The pelites, which comprise the greater part 8 of the graded unit are now recrystallized to coarse-grained 9 andalusite schist giving the appearance of reverse grading (Figures 10 33, 34). These rocks may be examined around Rosehearty and on the 11 12 foreshore west of Sandhaven. In general the rocks provide 13 excellent and abundant way-up evidence and allow the larger folds 14 to be traced out. This part of the section constitutes the 15 Rosehearty Formation. 16 The boundary between the Kinnaids Head and Rosehearty formations 17 is marked by the relatively sharp disappearance stratigraphically 18 upwards of calcareous material. However the background 19 sedimentation remains essentially unchanged with partial and 20 complete Bouma sequences identifiable throughout the section. 21 22 14.2.2 Structure 23 24 Minor folds ranging from centimetre scale up to amplitudes of tens 25 of metres are abundant throughout the section. There are two 26 generations of deformation present, defined as D1 and D3 on the 27 regional classification. The F1 folds are present throughout the 28 section. They trend approximately north to north-east, and are 29 o 30 subhorizontal or plunge gently to the north at up to 20 . The 31 characteristic structural profile shows a series of gentle vertical 32 or subvertical folds. In places along the section, these become 33 tightened to form asymmetric folds, which are steeply inclined or 34 overturned to the west with inverted limbs. In some cases the 35 axial planes become curved with fold noses ‘drooping’ to the west. 36 Way-up and cleavage-bedding relationships show that the folds face 37 consistently up to the west. The nature of D1 folding and cleavage 38 relationships is well seen on the foreshore around Sandhaven and 39 westwards to NJ 950 679, as well as in the area to the west of 40 Rosehearty harbour. Minor folds are spectacularly developed in the 41 mixed calcareous lithologies to the west of Kinnaird Head. S1 42 axial planar fabrics are developed throughout the section. 43 Cleavage planes that cross-cut the lithological banding in the 44 andalusite schists are spectacular between Rosehearty and Sandhaven 45 (Figures 33, 34). 46 The F3 folds are broadly coaxial with the F1 folds and are 47 48 overturned to the west, as seen for example at NJ 985 673 and NJ 49 998 677. They can be clearly shown to refold D1 structures and 50 excellent examples of refolded folds can be seen below the foghorn 51 at NJ 998 677 (Figure 32). Kneller (1987) also reported refolded 52 structures at NK 001 675. S3 axial planar fabrics are sporadically 53 developed and in places are associated with microfolds of S1. S3 54 characteristically forms a spaced clevage, for example at NK 001 55 675. 56 57 14.2.3 Metamorphism 58 59 Andalusite and cordierite are present throughout the section with 60 spectacular coarse-grained andalusite schists characterizing the 61 62 63 64 65 1 2 3 4 outcrops between Rosehearty and Sandhaven. Typical assemblages are 5 quartz-plagioclase-biotite-andalusite-cordierite. At Kinnairds 6 Head fibrolite appears, reflecting the general increase of grade to 7 the east. Kneller (1987) reported mixed assemblages in the 8 calcsilicate rocks, including calcic amphiboles, diopside, epidote 9 and zoisite. Fettes (1968) showed quite clearly that the peak of 10 metamorphism, marked by the major prophryroblast development, took 11 12 place in a static phase between D1 and D3, the porphyroblasts 13 overgrowing the S1 fabrics and being broken or rotated during D3 14 and mantled by the S3 fabrics. 15 16 14.2.4 Igneous and meta-igneous rocks 17 18 A coarse amphibolitic rock is shown on the BGS 1:10 000 sheets 19 immediately south of Fraserburgh harbour. In addition, Kneller 20 (1987) reported a number of igneous or meta-igneous rocks around 21 Kinnairds Head. He described a large crag in the middle of a small 22 bay at NK 000 675, which is composed of biotite-actinolite schist 23 and is apparently conformable with the bedding. Kneller also 24 described a similar, although less mafic ‘greenstone’, below the 25 harbour wall at NK 001 675. He noted that the mottled appearance 26 of the first occurrence is reminiscent of metamorphosed basic tuffs 27 found at similar stratigraphical horizons elsewhere in the 28 Tayvallich Subgroup. A number of aplitic and pegmatitic 29 30 leucogranite sheets (for example at NJ 998 677 and NJ 993 674) have 31 sharp contacts and post-date all the deformation structures; they 32 most probably relate to the late-Caledonian granites. 33 34 14.3 Intreptation 35 36 Sedimentological features clearly indicate that the rocks are 37 tubiditic and typical of the unstable conditions prevalent at the 38 time. This background sedimentation persists throughout the 39 section (see Kneller, 1987), although there is a relatively sharp 40 junction between the part of the succession that includes 41 calcareous rocks in the east and the part devoid of calcareous 42 lithologies to the west. The section comprises the Kinnairds Head 43 and Rosehearty formations (previously called ‘groups’ by Read and 44 Farquhar, 1956). The Kinnairds Head Formation is correlated with 45 the Tayvallich Subgroup (Argyll Group) and the Rosehearty Formation 46 is correlated with the lower part of the Southern Highland Group. 47 48 The Kinnairds Head Formation was previously regarded as equivalent 49 to the Whitehills ‘group’ in Banffshire. However the BGS 1:50 000 50 Sheet 96 (Banff, 2002) has placed the lower part of the Whitehills 51 ‘group’ in the Boyne Limestone Formation (Argyll Group) and the upper 52 part in the Macduff Formation of the Southern Highland Group. This 53 division is made on the first appearance, stratigraphically 54 upwards, of distinct, coarse-grained turbiditic units, albeit 55 within the part of the sequence that contains calcareous 56 lithologies. This boundary has no apparent equivalent in the 57 Rosehearty to Fraserburgh section, which obviously complicates 58 stratigraphical correlations across the Turriff Syncline and 59 questions the previously established criteria for delineating the 60 61 62 63 64 65 1 2 3 4 top of the Argyll Group at the top of the calcareous sequence 5 (Harris and Pitcher, 1975). 6 Read (1955) postulated that the rocks of the Buchan Block 7 constitute the upper limb of a recumbent Banff Nappe, an 8 allochthonous eastward-facing structure lying above a major 9 dislocation (the Boyne Line). The nappe structure was seen as 10 having been gently folded into the upright Turriff Syncline and 11 12 complementary Buchan Anticline. Read and Farquhar (1956) argued 13 that the Buchan Anticline was formed by an upsurging core of 14 gneissose rocks related to the translation of the Banff Nappe. 15 They postulated that the westward vergence and sense of movement of 16 the folds indicated that the metasedimentary pile had slumped off 17 the uprising anticline as a form of gravity slide. This view was 18 questioned by Loudon (1963) and it is now generally discounted. 19 The westward-verging F1 folds are consistent with those found 20 throughout the Aberdeenshire and Banffshire area and this is 21 evidence against a major D1 structure in the area. The westward 22 vergence of the D3 structures on the western limb of the Buchan 23 Anticline is also inconsistent with that structure being of D3 age, 24 suggesting, as noted elsewhere, that the Buchan Anticline might be 25 a relatively late structure, possibly of D4 age. 26 Read (1955) and Read and Farquhar (1956) believed that the 27 metamorphic pattern was imposed as a thermal aureole around the 28 upsurging gneisses in the core of the Buchan Anticline and was 29 30 essentially post deformation. Although the metamorphism is indeed 31 superimposed on the main structures, it is obvious that the 32 isograds are folded by the Buchan Anticline and the Turriff 33 Syncline. It is also clear from microstructural studies that the 34 metamorphic and thermal peak pre-dates the D3 and D4 structures 35 (Fettes 1968, 1970). 36 This coastal section, along with the adjacent Cairnbulg to St 37 Combs GCR site, exhibits typical Buchan metamorphism, characterized 38 by the zonal sequence cordierite → andalusite → sillimanite → 39 sillimanite+K-feldspar. Along with the Ythan gorge to the south, 40 these are the key exposures of the lowest P/T-style of metamorphism 41 (i.e. the lowest pressure/temperature ratios) present in the 42 Dalradian (the Buchan metamorphism as seen in Banffshire is 43 characterized by higher P/T values and the zonal sequence 44 cordierite → andalusite → staurolite → kyanite) (Harte and Hudson, 45 1979). The generally low P/T values of Buchan metamorphism 46 compared to the Barrovian areas might reflect a relatively 47 48 unthickened crust (Strachan et al., 2002): the role of the basic 49 intrusions of the North-east Grampian Basic Suite in this 50 situation, as either the cause or an effect of high heat-flow, is 51 uncertain. However, it is reasonable to note that the basic 52 intrusions, over much of their outcrop, are associated spatially 53 with the gneisses that represent the culmination of Buchan 54 metamorphism. Thus, Read and Farquhar‘s suggestion of an aureole 55 around a thermal gneiss dome might not be entirely wrong. 56 57 14.4 Conclusions 58 59 The superbly exposed coastal section between Rosehearty and 60 Fraserburgh is critical to understanding the geology of the Buchan 61 62 63 64 65 1 2 3 4 Block, which differs significantly in its structural and 5 metamorphic history from the Dalradian successions to the west and 6 south. It is of the highest national importance for three main 7 reasons. The sedimentological and stratigraphical evidence 8 demonstrates the nature of the Argyll Group–Southern Highland Group 9 transition, highlighting both similarities and contrasts with the 10 Perthshire and Banffshire successions. The structural history 11 12 provides a major piece of evidence to suggest that major D1 13 structures are absent from the Buchan Block, which is in marked 14 contrast to the major nappe-dominated structures of Perthshire. 15 The spectacular andalusite schists between Rosehearty and Sandhaven 16 and the transition to the sillimanite schists at Fraserburgh 17 constitute one of the type sections of Buchan metamorphism in the 18 Grampian Highlands, which is of international interest as one of 19 the first well-documented examples of low-pressure–high-temperature 20 regional metamorphism in the world. 21 22 15 CAIRNBULG TO ST COMBS 23 (NK 031 654–NK 062 626) 24 25 J.R. Mendum 26 27 28 29 15.1 Introduction 30 31 The coastal foreshore between Cairnbulg and St Combs, at the north- 32 eastern tip of the Grampian Highlands, provides an across-strike 33 section through migmatitic semipelites, pelites and psammites of 34 the Inzie Head Gneiss Formation (Crinan Subgroup) that are intruded 35 by numerous sheets, veins and irregular lenses of diorite and 36 granite. The metasedimentary rocks show evidence of amphibolite- 37 to granulite-facies metamorphism and anatectic melting. They have 38 been variously interpreted as gneissose basement forming the core 39 of the Banff Nappe (Read and Farquhar, 1956), allochthonous 40 Neoproterozoic basement gneisses (Ramsay and Sturt, 1979), and 41 migmatitic Argyll Group semipelites that have been subject to 42 partial melting (Kneller, 1987; Johnson et al., 2001a, 2001b). 43 These different designations have important consequences for models 44 of the structural and metamorphic history of the North-east 45 Grampian Highlands. 46 The gneisses lie in the core of the Buchan Anticline and their 47 48 outcrop extends south-west in a broad swathe to Mintlaw. Their 49 distribution, and western and eastern transition through the more- 50 calcareous Strichen and Kinnairds Head formations (Tayvallich 51 Subgroup) and into the overlying Southern Highland Group turbiditic 52 succession, suggests that they are Argyll Group rocks that belong 53 to the Crinan Subgroup. Thus, they are considered to be laterally 54 equivalent to the Cowhythe Psammite Formation, the Ellon Formation, 55 the Aberdeen Formation, the Queen‘s Hill Formation and the Ben Lui 56 Schist Formation (including the Duchray Hill Gneiss Member). All 57 of these lithological units generally show amphibolite-facies 58 metamorphic assemblages, ranging from garnet to sillimanite grade, 59 but in parts the mineral assemblages imply that conditions reached 60 granulite facies. In the Cairnbulg–St Combs section, peak 61 62 63 64 65 1 2 3 4 metamorphic temperatures of over 775° C and pressures of 3–4.5 kbar 5 have been inferred (Johnson et al., 2001a). Many of the rocks have 6 experienced significant retrogression, resulting in extensive 7 chlorite and sericite replacement of the high-grade mineralogies. 8 The area was first mapped as part of the primary geological survey 9 of one-inch Sheet 97 and a brief account of the gneissose and 10 intrusive rocks was given in the sheet memoir (Grant Wilson, 1882). 11 12 Somewhat later, H.H. Read and O.C. Farquhar visited the coastal 13 sections, as the Inzie Head gneisses were integral to their model 14 of the Banff Nappe (see Read, 1955; Read and Farquhar, 1956). 15 Sturt et al. (1977) obtained a Rb-Sr isochron from gneisses at 16 Cairnbulg Point that gave an age of c. 691 Ma, which was accepted 17 by Ramsay and Sturt (1979) as dating the migmatization. They 18 modified Read‘s hypothesis of the Banff Nappe and described a 19 complex and lengthy history for the gneisses, which they considered 20 to be allochthonous basement. Subsequently, Kneller (1987) 21 described the Cairnbulg–Inverallochy coast section and noted that 22 migmatization post-dated most of the deformation. He also 23 mentioned the presence of a gravity and magnetic anomaly beneath 24 the area implying the presence of a basic intrusion at shallow 25 depth. Later petrographical and geochemical work by Johnson (1999) 26 has documented the high-grade metamorphic assemblages, migmatitic 27 textures and detailed migmatite–granitic melt relationships 28 together with a link to mafic rocks of the North-east Grampian 29 30 Basic Suite (Johnson et al., 2001a, 2001b, 2003). 31 32 15.2 Description 33 34 The Cairnbulg–St Combs coastal section (Figure 35) is some 4.7 km 35 long and encompasses rocky foreshore, smooth glacially scoured rock 36 platforms, and sandy bays, all within the intertidal zone. Blown 37 sand backs much of the coast and there is no bedrock exposure 38 immediately inland. 39 The compositional banding and accompanying foliation in the Inzie 40 Head Gneiss Formation dip at between 40° and vertical towards the 41 north-west and west. The foliation is composite but basically 42 reflects the D2/3 deformation that accompanied the main period of 43 migmatization and partial melting. The section appears to lie on 44 the north-west limb of the open Buchan Anticline but it is not 45 possible to determine the detailed structural profile owing to the 46 degree of migmatization, partial melting, and number of intrusive 47 48 sheets present. In places early tight folds (F1) with axial planes 49 subparallel to the bedding are cross-cut by migmatitic segregations 50 and leucogranite veins (Johnson et al., 2001b). 51 The metasedimentary rocks consist of thin- to medium-bedded, 52 semipelite, micaceous psammite and feldspathic psammite with pelite 53 interbeds and subsidiary calcsilicate pods and lenses. Where 54 migmatized, the calcsilicate pods have remained relatively 55 refractory and obviously behaved in a more brittle manner. In 56 places a ‗ghost‘ bedding can be reconstructed from their incidence. 57 In the more-feldspathic psammites a spaced cleavage resembling that 58 seen in Southern Highland Group psammites of the Collieston– 59 Whinnyfold and Rosehearty areas is present. The psammite has been 60 recrystallized and the defining mineralogy is now high grade, but 61 62 63 64 65 1 2 3 4 the relict nature is still apparent. In places, pelitic rocks show 5 prominent cordierite and more-rarely andalusite porphyroblasts. 6 Sillimanite is common throughout the section. 7 Kneller (1987) was the first to mention the different types of 8 migmatite that can be seen in the section but Johnson (1999) has 9 mapped and studied their distribution and origin in detail. The 10 migmatites show variable development of segregation and partial 11 12 melting giving rise to abundant quartzofeldspathic material, termed 13 leucosome, which in most places resembles a pale-grey medium- 14 grained leucocratic granite. The material remaining after loss of 15 melt is termed melanosome and is richer in biotite and plagioclase 16 feldspar. Johnson et al. (2003) described the leucosome in thin 17 section as consisting of quartz, variably zoned plagioclase, 18 cordierite, biotite and potash feldspar with apatite, zircon and 19 opaque phases, mainly ilmenite as accessory minerals. Garnet, 20 orthopyroxene, tourmaline, muscovite and andalusite can occur as 21 additional phases. Potash feldspar is commonly altered to 22 myrmekitic blebs where it is in contact with plagioclase, and 23 larger muscovite and quartz symplectites, common in some 24 leucosomes, are interpreted as pseudomorphs after potash feldspar. 25 Many of the minerals are altered and pseudomorphed in these rocks, 26 with cordierite invariably altered to shimmer aggregate of chlorite 27 and white mica (rarely pinite) and no pristine orthopyroxene 28 preserved. The amount of leucosome present increases from north- 29 30 west to south-east with a concomitant decrease in the size and 31 abundance of the relict metasedimentary rocks (termed schollen). 32 Johnson et al. (2001b) distinguished three main migmatite zones 33 based on the mineralogy of the leucosomes; Leucosome-cordierite (L- 34 crd), Leucosome-garnet (L-gt) and Leucosome-orthopyroxene (L-opx). 35 The indicator minerals, cordierite, garnet and orthopyroxene, which 36 formed at the time of melting of the metasedimentary host rock, 37 were in equilibrium with the melt and are termed peritectic phases. 38 Figure 36 shows the distribution of the various migmatite types and 39 diorite and major granite sheets across the section. 40 The limit of melting, marked by initial generation of leucosome, 41 is now concealed beneath Fraserburgh Bay. However, exposures at 42 West Haven and Cable Shore, south-west of Cairnbulg Point show 43 sections with less than 10% leucosome in the effectively unmodified 44 metasedimentary mesosome, the resulting migmatite being termed a 45 metatexite. The leucosome occurs as small blebs or discontinuous 46 streaky (stromatic) layers parallel to the bedding and foliation as 47 48 well as in dilatant zones such as shear-zones or boudin-necks. In 49 other instances the leucosome layers converge laterally or are 50 connected vertically by small extensional shears to give a veined 51 network, termed a diktyonitic structure (Johnson et al., 2001b). 52 Thicker sheets and veins of white to pale-grey leucogranite up to a 53 few metres across containing metasedimentary schollen and ragged 54 and diffuse schlieren are also present, notable on Cable Shore west 55 of Cairnbulg Harbour (Figure 37). Generally, the sheets trend 56 near-parallel to the foliation but some do show local discordance. 57 Typically, they have a consistent grain size (1–2 mm) and show 58 diffuse margins with the adjacent mesosome, commonly dark coloured 59 and leucosome deficient. The nearby 300 m-thick Cairnbulg 60 ‗granite‘ body shows a much higher degree of leucosome development 61 62 63 64 65 1 2 3 4 and is best described as a diatexite; only the more-refractory 5 elements of the original parent metasedimentary material remain, 6 forming abundant dispersed ‗ghost‘ schollen and resulting in a 7 nebulitic texture. 8 Immediately south-east of the Cairnbulg ‗granite‘ is a c. 125 m- 9 wide zone of veined (diktyonitic) metatexites with an enhanced 10 foliation, in turn bounded to the south-east by a c. 25 m-thick 11 12 diorite sheet. Psammite with relict spaced cleavage forms enclaves 13 in the migmatitic semipelite section beyond. The enclaves attain 14 some tens of metres thick, e.g. by Gowan Hole (NK 0448 6512) at the 15 east end of Inverallochy. 16 The boundary of the L-crd and L-gt zones is crossed between Gowan 17 Hole and the Point of Whitelinks at around Broad Hive. It is 18 marked by the incoming of dark purplish red almandine garnets up to 19 5 cm across in the white to pale-grey leucosomes; the garnets are 20 commonly retrograded wholly or in part to biotite, chlorite and 21 shimmer aggregate. Good examples are seen on the clean, striated 22 rock platform between Boat Hive and the Point of Whitelinks (Figure 23 38). Leucosome abundance reflects the original compositional 24 banding in the semipelitic rocks and is commonly focussed around 25 the calcsilicate-rock schollen and smaller pelitic inclusions. 26 Mafic selvidges occur at the margins of some of the plagioclase- 27 rich L-gt sheets and veins and they contain mafic schlieren (mainly 28 biotite) and ‗ghost‘ schollen that are manifested as rounded 29 30 concentrations of now-relict cordierite. In places flame-like L-gt 31 sheets are mingled with L-crd. 32 On the peninsula between Whitelinks Bay and Millburn Shore (by The 33 Gwights), diatexites with schollen are cut by a thin diorite sheet. 34 To the south-east, at the northern end of Millburn Shore, leucosome 35 of both granitic and granodioritic composition is dominant with 36 abundant dispersed biotite schlieren and schollen of calcsilicate 37 rock and psammite. Rare pelitic and semipelitic ‗ghost‘ enclaves 38 are also present. Sinuous contacts can be recognized between the 39 compositionally different phases of leucosome, but individual 40 garnet porphyroblasts lie across such boundaries showing that the 41 different leucosome melts were coeval. This zone must lie only a 42 short distance above the north-western boundary of the St Combs 43 diorite, which is concealed beneath the sands of Millburn Shore. 44 The section at St Combs is dominated by two thick diorite sheets, 45 some 100–200 m thick (Figure 35). Sandwiched between them is a 46 large metasedimentary raft consisting of coarse-grained garnet 47 48 and/or orthopyroxene, cordierite, biotite, quartz and feldspars and 49 containing irregular intergranular leucosome. Johnson et al. 50 (2001b) termed this unit a granoblastic restite. Larger 51 accumulations of leucosome form irregular pods and lenses that in 52 places form centimetre-scale stromatic layers parallel to the 53 regional foliation. Thicker discordant leucosome veins also occur, 54 which Johnson et al. (2001b) interpreted as channelways. 55 Underlying the diorite, to the south-east of Bailiff‘s Skelly, are 56 melanosome-rich migmatites (leucosome less than 10%) with a strong 57 foliation and abundant pseudomorphs after orthopyroxene. These 58 dark-green porphyroblasts are up to 3 cm long and are now composed 59 of green biotite and shimmer aggregate, but still retain their 60 prismatic form and perpendicular cleavages. Cordierite is also 61 62 63 64 65 1 2 3 4 present and biotite is variably abundant. Leucosome pods here are 5 discontinuous and locally form a fine millimetre-scale stromatic 6 layering. Only relics of the metasedimentary bedding are 7 preserved, notably where psammite beds are present. Leucosome 8 becomes more abundant towards Inzie Head as the migmatites grade 9 from metatexites up to diatexites with a moderate foliation defined 10 by aligned mafic schlieren and semipelitic schollen. Orthopyroxene 11 12 occurs in the thicker leucosome veins but the thinner ones 13 typically contain cordierite. 14 Microdiorite and diorite sheets, ranging from some 10 cm up to 300 15 m wide, are common throughout the migmatitic sequence and lie 16 subparallel to the regional banding/foliation. They show evidence 17 of only minor deformation and commonly cross-cut the 18 metasedimentary banding and early structures. The diorites consist 19 of essential hornblende, plagioclase and quartz with ilmenite and 20 titanite present. Johnson et al. (2001a) noted that they are 21 similar to uralitized gabbros near Ellon with no trace of 22 clinopyroxene now remaining. Good examples of the relationships 23 between diorite, granite, leucosome and mesosome are seen on Cable 24 Shore, some 200 m south-west of Cairnbulg pier (Figure 35). Back- 25 veining of the granitic leucosome material into the diorite is 26 common and granite-diorite contacts range from sharp to 27 gradational. Leucosome/granite veins penetrate the diorite, 28 commonly forming wispy tongues that invade lobate pillows of 29 30 diorite. Leucosome is developed preferentially marginal to the 31 diorite sheets, particularly adjacent to their upper margins. In 32 contrast, beneath the diorite sheets the foliation in the 33 migmatitic rocks is intensified and the amounts of leucosome are 34 generally low. The diorite commonly shows evidence of multiple 35 intrusion, with the darker, more-mafic parts of the diorite sheet 36 cross-cutting the earlier paler grey hybridized parts. Within the 37 Cairnbulg ‗granite‘, sheets of diorite show evidence of magma 38 mingling with the leucogranite, which in places is finely 39 porphyritic. 40 41 15.3 Interpretation 42 43 At various times the gneisses of the Cairnbulg–St Combs section have 44 been ascribed to an earlier succession of basement rocks. Read 45 (1955) interpreted them as part of an older Dalradian succession, 46 the ‗Keith division , overlain unconformably by younger Dalradian 47 ’ 48 rocks of the ‘Banff division’ in the Banff Nappe. However, Read and 49 Farquhar (1956) inferred that the migmatization post-dated the 50 metamorphism and that both were superimposed on the Banff Nappe. 51 Sturt et al. (1977) obtained an Rb-Sr whole-rock isochron from 52 gneisses at Cairnbulg Point that gave an age of 691 ± 39 Ma (674 Ma 53 with currently accepted decay constants). They interpreted this 54 age to date the formation of the migmatitic gneisses and the linked 55 diorite and granite bodies. Ramsay and Sturt (1979) subsequently 56 described a complex multistage tectonic, metamorphic and intrusive 57 history based on detailed field relationships in the gneisses. 58 Accepting the Rb-Sr isochron age, they viewed the lower gneissose 59 rocks as Cadomian basement rocks transported to the North-east 60 Grampian Highlands from elsewhere and inferred that the mid- 61 62 63 64 65 1 2 3 4 Ordovician Grampian Event was responsible only for the later 5 deformations. However, Rb-Sr isochron ages are dependent on the 6 Rb- and Sr-bearing minerals in the sample being in isotopic 7 equilibrium and having not been disturbed by subsequent igneous or 8 metamorphic processes or subjected to loss of Sr. The Inzie Head 9 gneisses fail most of these tests and the ages are now regarded as 10 spurious. A clear structural and stratigraphical pattern has now 11 12 emerged in which all of the dominantly semipelitic gneissose rocks 13 in the North-east Grampian Highlands can be allocated to the Crinan 14 Subgroup in the upper part of the Argyll Group (Kneller, 1987; 15 Stephenson and Gould, 1995; Strachan et al., 2002). 16 The Inzie Head gneisses lie on the western flank of an antiformal 17 structure that was originally recognized by Grant Wilson (1886) and 18 termed the Ellon Anticline. Read (1923, 1955) renamed this 19 structure the Buchan Anticline and envisaged it as an integral part 20 of the Banff Nappe, reflecting the presence of the migmatitic 21 geissose core of the nappe beneath a resistant ‗cap‘ formed by the 22 Mormond Hill Quartzite. However, as the anticline patently folds 23 both the foliation in the migmatitic rocks and the metamorphic 24 isograds, it is most probably a late-stage Caledonian structure, 25 complementary to the Turriff Syncline to the west (Stephenson and 26 Gould, 1995). Its precise age of formation is not known, but it 27 post-dates the early Ordovician and pre-dates the deposition of the 28 Early Devonian Crovie Group of the Turriff Outlier. 29 30 The migmatitic nature of the Inzie Head Gneiss Formation has long 31 been recognized and these rocks represent the acme of the classic 32 high-temperature–low-pressure assemblage characteristic of the 33 Buchan-type metamorphism (see Chinner, 1966; Harte and Hudson, 34 1979). Further work between 1980 and the present has shown that 35 the high-temperature metamorphism in these rocks was supplemented 36 by heat and an initial fluid input due to the intrusion of a 37 significantly large body of basic magma at shallow depth. 38 Johnson et al. (2001a, 2001b, 2003) documented the petrography and 39 whole-rock geochemistry of the semipelitic rocks together with the 40 leucosomes and leucogranite sheets from the Inzie Head Gneiss 41 Formation. They concluded that the granitic bodies were formed 42 effectively in situ by melting of the semipelitic host, initially 43 at c. 2.9 kbar and c. 650°C with abundant fluid present (aH20 =1). 44 Fluid input was ascribed to dehydration of the metasedimentary 45 rocks resulting from the emplacement of a basic magma accompanied 46 by the formation of shear-zones. The mineral assemblages and 47 48 nature of the peritectic phases suggest that P–T conditions 49 increased south-eastwards to c. 4.5 kbar and c. 775°C such that 50 melting occurred in the absence of a volatile fluid phase. When 51 the normative compositions of the granites are plotted on a quartz– 52 albite–orthoclase diagram they cluster around the minimum-melt 53 compositions at pressure and temperature conditions of c. 3 kbar 54 and over 750°C, with low aH20 (c. 0.3) as implied by the 55 mineralogies. The melting reactions largely consume biotite (in 56 addition to quartz and feldspar) in these circumstances. 57 The nature of the protolith semipelitic and pelitic rocks prior to 58 migmatization and partial melting is not fully known but the 59 mineralogies and geochemistry strongly suggest that they were 60 magnesium-rich aluminous pelitic and semipelitic rocks. Following 61 62 63 64 65 1 2 3 4 the method of Inger and Harris (1993) Rb vs Sr plots of the 5 pelites, migmatites, leucosomes and leucogranites imply that potash 6 feldspar differentiation (assuming c. 10% crystallization) has 7 controlled melt composition during its ascent to higher crustal 8 levels. The steeper Rb vs Sr trends for the related garnetiferous 9 aplitic veins that are found in the Fraserburgh area suggest that 10 plagioclase feldspar differentiation has controlled melt evolution 11 12 at a late stage (Johnson et al., 2003). 13 Johnson et al. (2001b; 2003) suggested that the anatectic melting 14 gave rise initially to melt generation in small patches and vein 15 networks from which the melt then passed upwards into channel ways. 16 The movement of melt material was accompanied by some potash 17 differentiation to give rise to the leucogranites. The rocks seen 18 in the Cairnbulg–St Combs section provide a ‗snapshot‘ of this 19 overall process. Johnson et al. (2003) argued that much of the 20 leucosome is still in situ but their geochemistry also suggests 21 that a variable proportion has moved out of the rocks. Melt has 22 moved to higher levels and some granitic material has certainly 23 moved up from structurally lower levels. The amount of melt 24 produced in the L-crd zone is only possible in the presence of 25 considerable quantities of fluid and Johnson et al. (2001a) argued 26 that fluid supply was enhanced by the formation of numerous shear- 27 zones. Quantities of melt seen in the L-gt and L-opx zones are 28 considerably less and are compatible with dry melting reactions. 29 30 The proportion of leucosome is greater directly above and below 31 the diorite intrusions, and the metasedimentary rocks beneath show 32 higher concentrations of more-mafic material suggesting that melt 33 has been generated there and migrated upwards. The presence of a 34 gravity and magnetic anomaly suggests that the diorite intrusions 35 are probably related to a mafic–ultramafic intrusion that underlies 36 the area at a very shallow depth and is a member of the mid- 37 Ordovician North-east Grampian Basic Suite (Johnson et al., 2001b). 38 Figure 36 shows the overall pattern envisaged by Johnson et al. 39 (2001b), whereby they postulated that this process could ultimately 40 have been responsible for the generation of the mid-Ordovician 41 granite plutons such as Strichen and Aberdeen. They hypothesized 42 that at the time of emplacement of the North-east Grampian Basic 43 Suite an overlying subhorizontal zone of migmatitic melts would 44 have been generated and that (D2) strain would have been 45 partitioned into such zones giving rise to a network of ductile 46 shear-zones. Field relationships and age dates from the Portsoy 47 48 and Huntly areas do indeed imply that major shearing, emplacement 49 of the mafic and ultramafic plutons, and formation of migmatitic 50 rocks all occurred in a short episode at around 470 Ma (Strachan et 51 al., 2002; Carty, 2001; Oliver, 2002). The older foliated 52 muscovite-biotite granite plutons were also intruded at about this 53 time. 54 55 15.4 Conclusions 56 57 The Cairnbulg to St Combs coastal section displays a spectacular 58 range of migmatites that developed in Dalradian metasedimentary 59 host rocks during mid-Ordovician (Grampian Event) low-pressure–high- 60 temperature metamorphism. The rocks belong to the Inzie Head 61 62 63 64 65 1 2 3 4 Gneiss Formation, which consists dominantly of gneissose 5 semipelites, with pelites, psammites and lenticles and bands of 6 calcsilicate rock. Some researchers have ascribed these gneisses 7 to various basement successions, which according to one theory were 8 pre-Dalradian and were transported to the North-east Grampian 9 Highlands from elsewhere. However, later work has rejected the 10 basement hypotheses and has re-affirmed the overall coherence of 11 12 the Dalradian succession across the area. The Inzie Head gneisses, 13 along with most other gneissose sequences in the North-east 14 Grampian Highlands, are now assigned to the Crinan Subgroup of the 15 Argyll Group. 16 The gneissose rocks are at the sillimanite grade of metamorphism 17 and show evidence of partial melting, at first in the presence of 18 excess fluid but later in a fluid-absent mode. The granitic 19 material generated (termed the leucosome) pervades the host rocks 20 as pale-grey veins, lenses, streaks, patches and larger irregular 21 bodies. Thicker discordant veins, commonly formed of white to 22 pale-grey leucogranite, are interpreted as channel ways, through 23 which granite that had been mobilized at slightly deeper levels 24 migrated upwards to be ‗frozen‘ in its present position. The 25 leucosome material has resulted from the melting of semipelite and 26 pelite at pressures of 3 to 4.5 kbar (equivalent to 10–14 km 27 crustal depth) and temperatures of 650–775°C. Minerals that 28 crystallized in the leucosome at the time of melt formation show 29 30 zonation of the migmatitic rocks from cordierite- up to garnet- and 31 ultimately orthopyroxene-bearing leucosome. The highest grades are 32 seen around St Combs, where the original bedding of the 33 metasedimentary rocks is preserved in places as a ‗ghost 34 stratigraphy‘. Only rarely has the accumulation of granitic 35 material been sufficient to result in larger granitic bodies, as 36 for example by Cairnbulg Point. This ‗migmatitic granite‘ contains 37 abundant metasedimentary relics that represent the more-refractory 38 components of the host rock. 39 Diorite sheets that have been intruded throughout the section show 40 textural and structural relationships with the granitic leucosome, 41 implying that their intrusion coincided with melt generation. The 42 diorite intrusions are probably related to a mafic–ultramafic 43 intrusion that underlies the area at a very shallow depth and 44 belongs to the mid-Ordovician North-east Grampian Basic Suite. If 45 this is the case, it is valuable evidence that the basic suite was 46 emplaced close to the peak of regional metamorphism in the Buchan 47 48 area. 49 The migmatization and associated partial melting post-date an 50 earlier penetrative deformation phase but were accompanied by a 51 secondary deformation that generated the overall foliation seen in 52 the migmatitic rocks. The foliation dips moderately to steeply to 53 the north-west, as the outcrops lie on the western flank of a major 54 late-stage antiform termed the Buchan Anticline, which controls the 55 distribution of Dalradian units and folds the metamorphic isograds 56 in the Ellon–Fraserburgh–Peterhead area. 57 This GCR site clearly has a wide significance in terms of the 58 overall history of the Grampian Event, including the peak of 59 metamorphism and the role of the mafic and ultramafic intrusions of 60 the North-east Grampian Basic Suite. It is also clear that studies 61 62 63 64 65 1 2 3 4 of the gneisses have been particularly significant to our 5 understanding of the small- and medium-scale processes of migmatite 6 formation and generation of granitic melts from semipelitic and 7 pelitic protoliths. In this they have potential international 8 importance. 9 10 11 16 COLLIESTON TO WHINNYFOLD 12 (NK 084 337–NK 042 287) 13 14 15 J.R. Mendum 16 17 18 16.1 Introduction 19 20 The c. 7 km-long coastal section between Collieston and Whinnyfold, 21 20 km south of Peterhead, exposes turbiditic psammites and pelites 22 assigned to the Collieston Formation of the Southern Highland 23 Group, which are disposed in a series of recumbent folds. The beds 24 show an abundance of grading from gritty bases to pelitic tops and 25 Bouma sequences can be recognized, albeit generally incomplete. 26 Basic sheets intrude the metasedimentary rocks, mainly as sills but 27 locally as dykes, and a possible tuffaceous unit occurs on the 28 north-east side of Collieston Bay. The grade of metamorphism 29 30 varies from greenschist facies in the north-east to lower 31 amphibolite facies in the south-west and the section encompasses 32 the andalusite and cordierite isograds. The relatively low 33 metamorphic grade and abundance of grading in the rocks enables the 34 geometry of the early-formed fold structures to be elucidated. The 35 beds form part of a large-scale recumbent syncline that faces 36 eastwards and most of the exposed succession lies on its upper, 37 inverted limb. Evidence for a second deformation episode can be 38 found towards the south-west end of the section. 39 The area was mapped during the primary geological survey by J.S. 40 Grant Wilson and the resulting one-inch Sheet 87 was published in 41 1885. A brief description of the metamorphic rock types and some 42 of the structures was given in the accompanying memoir (Grant 43 Wilson, 1886). Subsequently, the coastal section was mapped in 44 detail by O.C. Farquhar and a comprehensive summary of the 45 structure was given by Read and Farquhar (1956). The area was 46 revised by the Geological Survey in 1980-81 and an account of the 47 48 section was given in the excursion guide to the Geology of the 49 Aberdeen area by Mendum (1987). This account is based upon the 50 latter work. 51 Read (1955) and Read and Farquhar (1956) showed that the 52 Collieston–Whinnyfold section is important in terms of interpreting 53 the overall geology of the North-east Grampian Highlands. They 54 proposed that the rocks here form the hinge-zone of their Banff 55 Nappe, a regional east-facing recumbent anticline whose upper limb 56 crops out in the Banffshire coast section, east of the Boyne Line 57 (see the Cullen to Troup Head GCR site report). They envisaged 58 formation of the Collieston folds by gravitational processes linked 59 to the rise of the open Buchan Anticline farther west. However, it 60 is now clear that the Buchan Anticline, which exposes sillimanite- 61 62 63 64 65 1 2 3 4 grade gneissose Argyll Group rocks in its broad hinge-zone, is a 5 late-stage structure that refolds the earlier cleavages (see the 6 Cairnbulg to St Combs GCR site report). The Collieston–Whinnyfold 7 section remains anomalous as the only area of the North-east 8 Grampian Highlands that contains a regionally inverted succession 9 and a structural pattern similar to that seen in Southern Highland 10 Group rocks near the Highland Border. 11 12 13 16.2 Description 14 15 This GCR site consists of coastal cliffs, typically 25–30 m high but 16 in parts reaching 45–50 m, and intertidal foreshore. As the area 17 has been used for inshore fishing over many years the various 18 indentations of the coast have been given names; these are partly 19 topographical, partly historical and in places obviously personal 20 (Figure 39). 21 The section lies oblique to the strike of the bedding and to the 22 gentle NNE-plunging fold axes. Traversing south-west from 23 Whinnyfold, one moves structurally down but stratigraphically up 24 the section (Figures 39, 40). Recumbent folding on a scale of tens 25 of metres disrupts the simple progression but south-west of Old 26 Castle, fold amplitude and wavelength increase to several hundreds 27 of metres. This latter section appears to be part of the hinge- 28 zone to the large recumbent syncline. From Collieston south- 29 30 westwards one appears to be traversing out of the hinge-zone and 31 onto the lower limb of the main east-facing syncline; the overall 32 structure becomes more complex and the superimposed secondary 33 deformation and folding become more dominant. Impure 34 metalimestones and calcsilicate bands some 2.5 km south-west of 35 Collieston, between The Smithy (NK 0262 2658) and Rockend (NK 0215 36 2668), are possible representatives of the Tayvallich Subgroup. The 37 coast section is described below from north-east to south-west. 38 At Sandy Haven, 350 m north-east of Whinnyfold, locally brecciated 39 and hornfelsed psammites and semipelites are cut by the red 40 Peterhead Granite. The contact is sharp, steeply dipping and when 41 traced southwards it is offset in places by small faults. At the 42 south margin of the bay, large xenoliths of grey to olive-green 43 psammite, up to 30 m wide and 100 m long, show irregular contacts 44 with the granite. On the headland to the south-east (the Cruner) 45 thin- to medium-bedded psammites and semipelites contain gritty 46 bands that show flattened quartz and feldspar clasts. Some 47 48 boudinage is present and a discrete S1 spaced cleavage is developed 49 (spacing 5 mm to 13 mm). 50 South-west from Whinnyfold are further grey coarse- and medium- 51 grained psammites and semipelites with interbeds of fissile 52 cordierite-bearing pelite. In parts thicker pelitic units are 53 present. Calcsilicate lenses are common within the turbiditic 54 succession. Lenticular, fawn-weathering, cream, gritty quartzites 55 with notable quartz veining form discrete units several metres 56 thick within the turbiditic succession. The beds dip at 15° to 17° 57 to the north-north-east and excellent grading at NK 0809 3298 shows 58 that the succession is inverted. Large-scale tight recumbent folds 59 (F1) are present in Buck‘s Nose Bay and have a variably developed 60 axial-planar spaced cleavage. Well-defined grading and bottom 61 62 63 64 65 1 2 3 4 structures at the gritty psammite–pelite contacts show that the 5 folds face to the east. Some of the folds north-east of the bay 6 have disharmonic profiles and show discontinuities near parallel to 7 bedding. Similar features occur farther to the south-south-west at 8 Lady‘s Step (NK 0801 3274) where tight S-profile F1 folds with 9 attenuated middle limbs are seen. In these fold-pairs, the 10 syncline is much larger in amplitude than the corresponding 11 12 anticline, which when traced westwards diminishes to zero. The 13 fold axes are subhorizontal and trend 010°. 14 Cliffs on the south side of Green Craig Bay, at NK 0684 3154, 15 expose inverted graded beds with cuspate bottom structures trending 16 142°. In the pelitic units, which here form a greater proportion 17 of the succession, grey andalusite ‗slugs‘ and darker grey, small, 18 rounded cordierite porphyroblasts are abundant. The stack of Green 19 Craig itself shows recumbent F1 folds underlain by an apparent 20 discontinuity (Read and Farquhar, 1956, plate VI, figure 1). F1 21 fold hinges are exposed on the foreshore at the southern edge of 22 the bay. Fold axes here plunge very gently to the south-south- 23 west, with the S1 axial-planar cleavage dipping at 18° to the east- 24 south-east. However, the prominent rounded fold hinge in massive 25 gritty psammite units figured by Read and Farquhar (1956, plate IV, 26 figure 2) at this location folds the S1 spaced cleavage and is 27 attributed to the secondary deformation (termed D3). Although this 28 later fold is co-axial with F1 folds, the related axial plane dips 29 30 at 26° to the west-north-west. A further sign of this later, D3 31 deformation is shown by the abundant folded quartz veining in more- 32 pelitic units on the north side of the bay, which imply some 58% 33 shortening. 34 At the Devil‘s Study (NK 0606 3091), a spectacular example of a 35 recumbent westward-closing F1 syncline is exposed (Figure 41). It 36 occurs in thick-bedded, locally gritty psammites and subsidiary 37 interbanded cordierite-bearing schistose pelites and semipelites. 38 A prominent S1 axial-planar spaced cleavage (spacing c. 5 mm) is 39 uniformly developed across the fold profile. The lower limb is 40 strongly attenuated and the complementary antiform in the pelitic 41 beds below has a much smaller amplitude. The fold appears to be 42 underlain by a discontinuity. To the south-west, towards Radel 43 Haven, tight folds confined to individual beds are interpreted as 44 slump folds. In addition, the gritty bases of inverted psammite 45 units are exposed on the low foreshore. The dominant quartz and 46 feldspar clasts show little evidence of significant superimposed 47 48 strain in these thicker psammites, yet the more-elongate clasts 49 show a strong grain alignment that plunges gently to between 340° 50 and 020°. Grain alignment is also seen on the bases of inverted 51 graded psammite units just north of the Devil‘s Study, where the 52 resultant lineation plunges at 10° towards 007°. On the north edge 53 of Radel Haven (NK 0584 3083), more-complete Bouma sequences 54 contain both laminated and cross-laminated silty units. 55 The cliffs below Old Castle (NK 0525 3005) are composed mainly of 56 cordierite-rich schistose semipelite and pelite with coarse 57 andalusite present in parts. Abundant tight F1 folds plunge gently 58 to the north, and are commonly confined to particular 59 stratigraphical intervals. In parts a later S3 crenulation 60 cleavage is also developed. North of the Old Castle promontory, 61 62 63 64 65 1 2 3 4 psammites show a more closely spaced (c. 3 mm) S1 cleavage and in 5 the intertidal zone the S1 cleavage is folded by minor open to 6 close folds whose axes plunge about 25° to the north-north-east. A 7 fine-scale spaced S3 cleavage is developed locally. F1+F3 folding 8 is abundantly developed farther south at Portie Shore (around NK 9 0415 2991) involving the lenticular quartzite units. A weakly 10 developed S3 spaced cleavage is widely seen on weathered surfaces 11 12 in the psammites; it shows a spacing of 15–20 mm. A finely spaced 13 crenulation cleavage in the pelitic units attests to the oncoming 14 of penetrative D3 deformation. 15 Between Old Castle and Collieston, tight F1 and open to tight F3 16 folds are particularly abundant. An excellent example of a 17 recumbent fold train in interbedded psammitic and pelitic units is 18 displayed at Pottie Murlan (NK 0478 2916) (Read and Farquhar, 1956, 19 plate IV, figure 1). The fold axes plunge at 2° to 031° and axial 20 planes dip at 23° to the east-south-east. Although the fold 21 profile is coherent and individual hinges are complementary to one 22 another, closer inspection shows that there is a combination of F1 23 and F3 folds. Hence in some fold hinges the S1 cleavage is axial 24 planar but in others S1 is folded and a new S3 spaced cleavage is 25 developed. In thin section both cleavages are defined by biotite 26 concentrations but individual biotite laths are generally aligned 27 parallel to S3. 28 South-west of Pottie Murlan, thin- to medium-bedded psammites and 29 30 pelitic units show tight F1 and F3 folding but are right way up 31 overall. By Dowiestone Cave, a deformed metadolerite dyke has 32 cuspate margins and shows cleaved biotite amphibolite marginal 33 zones. Several other metadolerite intrusions are seen farther 34 south in the cliffs between Aver Hill and Collieston. 35 At Collieston, most of the psammite–pelite succession is inverted 36 but tight recumbent F1 folds are common giving rise to way-up 37 reversals. The quarry face behind the car park on the east side of 38 Tarness Haven displays a c. 10 m-high profile of a recumbent 39 syncline formed in gritty, locally pebbly psammites and subsidiary 40 semipelites and pelites. The fold faces east-south-east, has a 41 pervasive S1 spaced cleavage, and its axis plunges at 16° to 026°. 42 At the northern end of the quarry, similar pebbly psammites form 43 units up to 4 m thick and contain ripped-up mud clasts derived from 44 underlying pelitic lithologies. On the rocky slabs above St 45 Catherine‘s Dub, south-east of the car park, inverted psammites 46 with prominent gritty and pebbly bases show a strong grain- 47 48 alignment lineation. The white to pink quartz clasts are notably 49 elongate and the lineations are curved on some bedding planes and 50 of variable intensity. The lineations plunge gently to moderately 51 to the north-north-east. Their variability and association with a 52 linear fabric in the matrix suggests that here they are dominantly 53 of tectonic origin, and the overall strain patently reflects both 54 D1 and D3 deformation events. In the psammites, minor F3 folds 55 fold the pervasive S1 spaced cleavage and an accompanying S3 spaced 56 cleavage is developed locally. Near the low watermark by the arch 57 at the west side of St Catherine‘s Dub, tight F1 and F3 folds are 58 abundant in gritty psammites and cordierite-bearing pelites and 59 semipelites. F1 fold axes plunge gently to the north-north-east, 60 co-axial with an L1 lineation, and a discordant S3 cleavage is well 61 62 63 64 65 1 2 3 4 developed. Discordant quartz veins that post-date F1 folds and S1 5 cleavage are tightly folded here (F3). 6 A 4 m-thick discordant metadolerite sheeet with cuspate margins 7 and small boudinaged offshoots can be traced up the east side of 8 Tarness Haven and across the car park to the gully on the east side 9 of St Catherine‘s Dub. The marginal alteration to biotite-rich 10 foliated amphibolite is clearly seen. 11 12 Near low-water mark on the east side of Tarness Haven, weathered 13 fine-grained, green-brown, ?chlorite-rich units are interbanded 14 with dark-grey, thinly banded cordierite-rich pelites and 15 semipelites over a vertical interval of 1.5 m. These units might 16 represent tuffaceous horizons deposited in more-distal parts of the 17 turbidite fan. 18 South of Collieston Harbour, coastal outcrops show psammites with 19 excellent non-inverted grading, interbedded with pelites that 20 contain larger cordierite porphyroblasts and some andalusite. D3 21 deformation generally becomes more penetrative to the south, 22 although the earlier D1 structures are still generally apparent. 23 24 16.3 Interpretation 25 26 The Collieston to Whinnyfold section exposes beds that are assumed 27 to represent the lower part of the Southern Highland Group on the 28 basis of thin impure calcareous beds and quartzites, exposed at the 29 30 lowest stratigraphical level to the south-west of the GCR site, 31 which could be the sparse representatives of the Tayvallich 32 Subgroup in this area. Read and Farquhar (1956) interpreted these 33 calcareous and quartzose lithologies as part of the Mormond Hill 34 Quartzite, now the Mormond Hill Quartzite Member of the dominantly 35 semipelitic Strichen Formation. They believed that the Mormond 36 Hill Quartzite forms a thick lenticular unit in the core of the 37 Buchan Anticline but the distribution of units strongly implies 38 that there are significant lateral facies variations. Inland 39 bedrock exposures are few on the eastern limb of the anticline, and 40 it is difficult to correlate the stratigraphy with the better- 41 established succession on the western limb. 42 The psammites and pelitic rocks of the Southern Highland Group 43 succession are interpreted as having been deposited on a sub-marine 44 fan by density currents in the late Neoproterozoic. The lenticular 45 quartzose units appear to represent channel fills and might have 46 been produced by reworking of the turbiditic units by bottom 47 48 currents. Although quartz is the dominant clastic constituent of 49 the succession, locally with a bluish opalescent tint indicative of 50 inclusions, potash feldspar and more rarely plagioclase grains have 51 been recorded (Read and Farquhar, 1956). The matrix consists of 52 sericite, chlorite, altered feldspar and quartz in the low-grade 53 rocks but as metamorphic grade increases biotite, muscovite, 54 feldspar and quartz form a mosaic. Accessory iron-oxide minerals, 55 tourmaline, garnet, zircon and titanite have also been recorded. 56 Retrogression to chlorite and sericite is common locally. The 57 pelitic rocks show more-recrystallized mica fabrics south-west of 58 Bruce‘s Haven (NK 0680 3144) but cordierite porphyroblasts occur 59 throughout much of the section. These are rounded dark-grey spots, 60 except just south of Collieston Harbour where larger ‗black slugs‘ 61 62 63 64 65 1 2 3 4 are found. In thin section many are altered to pinite but as grade 5 increases some sector-twinned cordierite is present. Andalusite is 6 paler grey and forms elongate ‗slugs‘; it is first seen as 7 pseudomorphs around Berry‘s Loup (NK 0745 3217) but forms coherent 8 porphyroblasts farther south-west with laths up to 6 cm long 9 recorded south-west of Collieston (Munro, 1986). Both cordierite 10 and andalusite overgrow the S1 fabric and are wrapped and deformed 11 12 by the S3 fabric, thus placing the peak of metamorphism between the 13 two deformation events. 14 Structurally the Collieston–Whinnyfold section is important as it 15 shows the nature of the early D1 deformation and makes an 16 interesting comparison with that seen on the Banffshire Coast (see 17 the Cullen to Troup Head GCR site report) and near the Highland 18 Border (see the Little Glenshee GCR site report). In the 19 Collieston–Whinnyfold section the F1 folds are recumbent, face 20 eastwards and there are significant stretches of overturned strata. 21 Oncoming of the secondary deformation (D3) is gradual but coincides 22 approximately with the andalusite isograd. On the Banffshire coast 23 F1 folds are largely upright and verge and face to the west and the 24 secondary deformation comes on more sharply, again coincident with 25 the andalusite isograd. Fold axes plunge gently northwards in both 26 sections. Development of S1 spaced cleavage and metamorphic 27 assemblages are also similar. 28 The F1 fold geometry seen in the Collieston–Whinnyfold section 29 30 shows some unusual features. Fold profiles are variable along 31 their axial planes and in numerous instances the synclines and 32 anticlines that constitute fold-pairs have disparate amplitudes. 33 Dislocations are also numerous both in fold stacks and beneath 34 folded zones. This geometry suggests that either the turbiditic 35 Southern Highland Group sequence here might not have been fully 36 lithified at the time of deformation or that the mode of 37 deformation was shear dominated with abundant fluids present. 38 However, the related S1 cleavage is uniformly developed and fold 39 axis orientations are relatively consistent, showing that 40 deformation was tectonic and not syn-sedimentary. Although slump 41 or soft-sediment folds seem to be present locally, they are not 42 abundant. As this part of the sequence lies in the lower part of 43 the Southern Highland Group, which is little younger than 600 Ma 44 (Dempster et al., 2002), and the uppermost parts of the group were 45 deposited at about 515 Ma, it seems unlikely that the succession 46 would remain unlithified over some 80 Ma. 47 48 The lack of marker bands in the sequence makes it awkward to 49 construct an accurate cross-section, but Figure 40 shows a 50 projected, composite, down-plunge cross-section that represents an 51 overall structural profile of the exposed section. The orientation 52 data for the section between Whinnyfold and Old Castle, where D3 53 deformation is not well developed, are shown on the stereoplot 54 (inset to Figure 39). The fold geometry is consistent, whereas 55 farther south-west where D3 deformation is stronger, the F1 and F3 56 axes show a greater spread. 57 Read and Farquhar envisaged the Collieston folding to have been 58 initiated as a large recumbent fold, the Banff Nappe, which moved 59 eastwards generating the Buchan Anticline as a ‗tectonic drumlin‘ 60 due to the inclusion of the resistant Mormond Hill Quartzite in the 61 62 63 64 65 1 2 3 4 thrust succession. The folding of the Collieston turbiditic 5 succession was attributed to ‗gravity sliding‘ on the steep ‗brow 6 of the bulge‘ formed by the quartzite. 7 Ashcroft et al. (1984) and Harte et al. (1984) both suggested that 8 the Collieston structure could represent a lateral equivalent of 9 the hinge-zone below the Tay Nappe, albeit with a reduced 10 amplitude. Certainly, the styles of folding and metamorphic 11 12 conditions resemble those found in the eastern Highland Border 13 region, but the fold facing is to the east rather than south-east. 14 If the Collieston–Whinnyfold rocks do belong to the lower limb of 15 the Tay Nappe, it is interesting to speculate whether the upright 16 F1 folding of the Banffshire coast section was indeed coeval with 17 the recumbent F1 folding at Collieston. It seems clear that the 18 metamorphic peak in the Dalradian rocks in general relates to 19 orogenic events, termed D2, that are not represented by structures 20 in the Collieston–Whinnyfold section, or elsewhere in the Buchan 21 area. The D2 deformation occurred coeval with the intrusion of the 22 mid Ordovician-age North-east Grampian Basic Suite at c. 470 Ma 23 (Strachan et al., 2002). The D1 and D2 deformations were succeeded 24 by a further deformation event and metamorphism, D3, that is 25 represented here by the onset of reworking and enhanced metamorphic 26 grade. 27 28 29 16.4 Conclusions 30 31 The well-exposed coast section between Collieston and Whinnyfold 32 demonstrates the nature of early folding in the some of the 33 youngest Dalradian rocks seen in the North-east Grampian Highlands. 34 These Collieston Formation beds lie in the lower part of the 35 Southern Highland Group and comprise turbiditic, originally sandy 36 and muddy, psammites, semipelites and pelites. More-quartzose 37 gritty psammites form distinctive lenticular units up to c. 10m 38 thick in the turbiditic fan succession and probably represent 39 reworked sands within channels. Grading is common and the ‘Bouma 40 sequences’ that typify turbidite successions can easily be 41 recognized. Groove casts, pebble alignment and rare pebble 42 imbrication at the base of individual units imply that the density 43 currents flowed north, and hence the offshore depositional slope 44 appears to have been towards the north, at least in this area. 45 The rocks are folded into a large, composite, recumbent syncline. 46 Numerous smaller scale anticlines and synclines have axes that 47 48 plunge gently to the north and axial planes that dip gently to the 49 north-east. The beds become younger eastwards and hence the folds 50 face east. A penetrative spaced cleavage, defined by mica-rich 51 seams and intervening quartz-rich microlithons, is developed in the 52 psammites and a slaty to schistose cleavage is developed in the 53 pelitic lithologies. Much of the succession lies on the upper, 54 inverted limb of the main syncline, as is shown by the graded 55 bedding. The more-pelitic rocks contain abundant cordierite, and 56 andalusite is present in all but the lower grade north-eastern part 57 of the section. A secondary deformation becomes progressively more 58 important in the south-west. This has resulted in tightening of 59 the early structures and attenuation of the cleavages. However, in 60 61 62 63 64 65 1 2 3 4 many parts westward-verging minor folds fold the early cleavage and 5 a new spaced or crenulation cleavage is developed. 6 The Collieston–Whinnyfold folds could represent the most north- 7 easterly exposed extent of the Tay Nappe, although the structure 8 here seems to have reduced amplitude. It remains an area of 9 crucial importance in terms of regional interpretations of the 10 Dalradian structure and sequence and lends itself to further study 11 12 as well as providing an excellent teaching section. 13 14 15 ACKNOWLEDGEMENTS 16 17 This regional paper is the combined work of 7 authors, many of 18 whom, in addition to their own site descriptions, have made 19 valuable comments on other aspects of the work. 20 The paper has been compiled and edited by D. Stephenson. The GCR 21 editor was P.H. Banham and the referee was M.R.W. Johnson, who also 22 provided valuable editorial suggestions. The project was cofunded 23 by the Joint Nature Conservation Committee (JNCC) and the British 24 Geological Survey (BGS) and has been managed by N.V. Ellis for JNCC 25 and D.J. Fettes and M. Smith for BGS. 26 The initial site selection and site documentation for the 27 Dalradian block of the Geological Conservation Review was by S.J. 28 Moorhouse. Since then, much new mapping and refined interpretation 29 30 has taken place, and the site list has been revised, firstly by 31 J.E. Treagus and subsequently through a panel consisting of R. 32 Anderton, A.L. Harris, J.R. Mendum, J.L. Roberts, P.W.G. Tanner, R. 33 Threadgould and J.E. Treagus. The necessary amendments to the GCR 34 documentation were greatly facilitated by R. Wignall (for Scottish 35 Natural Heritage). 36 K.M. Goodenough and S.W. Horsburgh assisted with preliminary 37 drafts of key maps and all diagrams were drafted for publication by 38 S.C. White (JS Publications, Newmarket) and K.J. Henderson (BGS, 39 Edinburgh). Photographs were scanned and prepared by B.M. McIntyre 40 (BGS, Edinburgh). Photographs from the BGS collection are 41 reproduced by kind permission of the Director, BGS © Natural 42 Environment Research Council; all rights reserved (PR/23–27). 43 Several of the principal authors of the Dalradian GCR have been 44 involved in the writing of other reviews of the Dalradian of 45 Scotland and, inevitably, sections of introductory text have been 46 adapted and updated from their contributions to those earlier 47 48 works. In particular, large sections have been adapted from 49 British Regional Geology: the Grampian Highlands (Stephenson and 50 Gould, 1995) and some smaller sections have been adapted from a 51 chapter in The Geology of Scotland (Strachan et al., 2002) and from 52 a recent review of the evolution of the north-east margin of 53 Laurentia (Leslie et al., 2008). The original sources of many key 54 diagrams taken from these and other works are acknowledged in their 55 captions. 56 The first complete draft of the Dalradian GCR was submitted to the 57 JNCC in June 2009. In 2010, the JNCC terminated its involvement in 58 Earth Science conservation and abandoned its contractual agreements 59 to publish the remaining GCR volumes. So, the authors are greatly 60 indebted to Diarmad Campbell, Chief Geologist Scotland for the BGS, 61 62 63 64 65 1 2 3 4 for funding the drafting of remaining figures and to the 5 Geologists‘ Association and Elsevier, for ensuring that this volume 6 is published as a Special Issue of their Proceedings. We are 7 particularly grateful to Neil Ellis of the JNCC for his efforts to 8 secure a new publisher and to Professor James Rose, Editor in Chief 9 of the PGA, for making it all happen. 10 Finally, on behalf of all of the site authors, we would like to 11 12 record our thanks to the owners and managers of land and quarries 13 who have allowed access to the sites, either during previous work 14 or specifically for the GCR exercise. 15 16 REFERENCES 17 18 Agrell, S. O. (1942) 1. A petrological investigation of the 19 adinoles at Dinas Head, Cornwall. 2. A petrofabric study of the 20 Ben Vuroch granite and the adjacent schists in Perthshire. 21 Unpublished PhD thesis, University of Cambridge. 22 Ahmed-Said Y. and Tanner P. W. G. (2000) P-T conditions during 23 emplacement, and D2 regional metamorphism, of the Ben Vuirich 24 Granite, Perthshire, Scotland. Mineralogical Magazine, 64, 737–53. 25 Ague, J.J. & Baxter, E.F., 2007. 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Mineralogical Magazine, 68, 769-786. 47 48 49 Figure 1 Map of the North-east Grampian Highlands based upon BGS 50 1:50 000-scale maps and showing the location of Dalradian GCR 51 sites. 52 53 GCR sites: 1 Ben Vuirich, 2 Gilbert’s Bridge, Glen Tilt, 3 Glen 54 Ey gorge, 4 Cairn Leuchan, 5 Balnacraig, Dinnet, 6 Muckle Fergie 55 Burn, 7 Bridge of Brown, 8 Bridge of Avon, 9 Kymah Burn, 10 56 Black Water, 11 Auchindoun Castle, 12 Cullen to Troup Head, 13 57 Fraserburgh to Rosehearty, 14 Cairnbulg to St Combs, 15 58 Collieston to Whinnyfold. 59 BS Boundary Slide, KSZ Keith Shear-zone, PSZ Portsoy Shear-zone. 60 61 62 63 64 65 1 2 3 4 Figure 2 Principal stratigraphical units in the Dalradian of the 5 North-east Grampian Highlands, adapted from Stephenson and Gould 6 (1995, figure 10). The columns are not to scale. 7 8 Figure 3 9 (a) Generalized cross-section along the north coast of the North- 10 east Grampian Highlands from Cullen to Fraserburgh showing the main 11 structural features and dominant deformation/fold phases 12 (Stephenson and Gould, 1995, figure 21, partly after Loudon, 1963). 13 Stipple = the Old Red Sandstone outlier at Gardenstown. The entire 14 section is included in the Cullen to Troup Head and Fraserburgh to 15 Rosehearty GCR sites. 16 (b) Highly generalized cross-section across the Buchan Block to 17 illustrate the broad structure as envisaged by Read (1955) as 18 modified by Kneller (1987). The approximate locations of GCR sites 19 relative to the structure are shown. 20 21 Figure 4 Map of the Ben Vuirich Granite Intrusion and adjoining 22 country rocks, showing the locations of the two groups of exposures 23 (A and B) which comprise the GCR site. g exposure of pelitic 24 schist with garnet over 2 cm across. 25 26 Figure 5 Hornfels associated with the Ben Vuirich Granite, with 27 centimetre-scale, altered porphryoblasts of contact metamorphic 28 cordierite (pale grey), dotted with small garnets (white) that 29 formed during the subsequent D2 regional metamorphism. (Locality 30 A, Figure 4). (Photo: P.W.G. Tanner.) 31 32 Figure 6 Xenoliths of banded quartzose psammite and pelite in 33 the Ben Vuirich Granite at locality B, Figure 4. See text for 34 explanation. The scale is 5 cm long. (Photo: P.W.G. Tanner.) 35 36 Figure 7 Map of the area around the Gilbert’s Bridge GCR site, 37 based upon the BGS 1:50 000 Sheet 55E (Pitlochry, 1981). 38 39 Figure 8 Map of the area around lower Glen Ey adapted from the 40 BGS 1:50 000 Sheet 65W (Braemar, 1989). In this area most of the 41 information used for the BGS compilation was taken from Upton 42 (1983). 43 44 Figure 9 Schematic structural cross-section across the area 45 between Deeside and Glen Shee. Reproduced from Upton (1986, figure 46 3). The rectangle indicates the approximate position and exposure 47 level of the Glen Ey Gorge GCR site. 48 49 Figure 10 Map of the area around the Cairn Leuchan to Pannanich 50 Hill GCR site, adapted from the BGS 1:50 000 Sheet 65E (Ballater, 51 1995). The Coyles of Muick Intrusion is bounded on both sides by 52 ductile shears that define the Coyles of Muick Shear-zone. The 53 shear-zone also marks the position of the regional andalusite– 54 kyanite isograd; to the north-west metasedimentary rocks contain 55 andalusite ± staurolite (a Buchan-type assemblage), whereas to the 56 south-east they contain the assemblage sillimanite ± kyanite ± 57 staurolite, characteristic of Barrovian metamorphism. 58 59 60 61 62 63 64 65 1 2 3 4 Figure 11 Map of the area around Balnacraig, Dinnet. Modified 5 after Gould (2001, figure 11) 6 7 Figure 12 Xenolithic gneiss with folded remnants of psammite 8 dispersed in poorly foliated feldspar-porphyroblast gneiss. 9 Craigie, 1.5 km north-east of Dinnet (NJ 476 007). Coin is 26 mm 10 in diameter. (Photo: BGS No. P220491, reproduced with the 11 permission of the Director, British Geological Survey, © NERC.) 12 13 Figure 13 Amphibolite, with irregular patches of feldspathic 14 material, Balnacraig Cottage, north-east of Dinnet (NJ 4787 0056). 15 Coin is 25 mm in diameter. (Photo: BGS No. P 220371, reproduced 16 with the permission of the Director, British Geological Survey, © 17 NERC.) 18 19 Figure 14 Map of the Muckle Fergie Burn section, Glen Avon, based 20 upon British Geological Survey mapping, 1982–88. 21 22 Figure 15 Granitic cobbles, typically up to 10 cm across, in 23 metadiamictite in the lower part of the Muckle Fergie Burn (NJ 1657 24 1397). The smaller clasts include granite, quartz and ochreous 25 yellow-brown-weathering metadolostone (see example at bottom 26 right). (Photo: J R Mendum, BGS No. P 726597) 27 28 Figure 16 Map of the area around Bridge of Brown, based upon BGS 29 1:10 000 Sheet NJ12SW (1991). The line of part of the cross- 30 section in Figure 17 is indicated. 31 32 Figure 17 North-west–south-east cross-section across the area 33 surrounding the Bridge of Brown and Bridge of Avon GCR sites. The 34 line of section intersects Figure 16 and passes to the south-west 35 of Figure 18. 36 37 Figure 18 Map of the area around the Bridge of Avon, based upon 38 BGS 1:10 000 sheets NJ12SW (1991) and NJ12SE (1992) and on BGS 1:50 39 000 Sheet 75W (Glenlivet, 1996). The cross-section of Figure 17 40 passes the south-west corner of this map. 41 42 Figure 19 Asymmetrical minor F3 folds of thinly interbedded 43 metalimestone, calcsilicate rock and calcareous semipelite of the 44 Ailnack Phyllite and Limestone Formation at Bridge of Avon (NJ 150 45 201). The folds show attenuated limbs and fold axes plunge at 25o 46 to 152o. (Photo: BGS No. P 220186, reproduced with the permission 47 of the Director, British Geological Survey, © NERC.) 48 49 Figure 20 50 (a) Map of the Kymah Burn section, Glen Livet, based upon British 51 Geological Survey mapping, 1982–88. 52 (b) Cross-section along the line A–B showing the major fold 53 interference structure in the Kymah Burn section. 54 55 Figure 21 Map of the section through metavolcanic rocks in the 56 lower part of the Blackwater Formation, exposed in the Black Water. 57 Adapted from the BGS 1:10 000 Sheet NJ33SE (1993). 58 59 60 61 62 63 64 65 1 2 3 4 Figure 22 A typical fragmental ultrabasic volcanic rock from the 5 Kelman Hill Member of the Blackwater Formation. The dominant, 6 pale, subrounded to subangular clasts are derived from picritic 7 lavas and there are smaller, darker and more-rounded clasts that 8 are probably from metabasalts. Loose block near Shenval, Black 9 Water GCR site (NJ 363 308). Coin is 20 mm diameter. (from 10 Macdonald et al., 2005, figure 4.) (Photo: BGS No. P 582442, 11 reproduced with the permission of the Director, British Geological 12 Survey, © NERC.) 13 14 Figure 23 Map of the area around Auchindoun Castle, Glen Fiddich 15 from BGS 1:10 000 sheets NJ33NW (1993) and NJ33NE (1993). The 16 Portsoy Lineament lies some 4.5 km to the south-east of the castle. 17 Most of the exposures of pelites in this area show square cross- 18 sections of chiastolite in hand specimen and thin sections reveal 19 that this has been replaced by kyanite (Figure 24). 20 21 Figure 24 Chiastolite (=andalusite) porphyroblasts pseudomorphed 22 by fan-like sheaves of kyanite in a sample of pelite from the 23 Mortlach Graphitic Schist Formation close to Auchindoun Castle. 24 The original ‘crosses’ formed by graphite inclusions in the 25 chiastolite are still clearly visible. (from Beddoe-Stephens, 26 1990, figure 2a). (Photo: BGS No. P 254543, reproduced with the 27 permission of the Director, British Geological Survey, © NERC.) 28 29 Figure 25 Map of the coastal strip between Cullen and Troup Head, 30 based largely upon BGS 1:50 000 sheets 96W (Portsoy, 2002) and 96E 31 (Banff, 2002). Late-Caledonian minor intrusions are omitted for 32 clarity. 33 34 Figure 26 Cross-section of the coast between Cullen and Troup 35 Head, based largely upon sections accompanying BGS 1:50 000 sheets 36 96W (Portsoy, 2002) and 96E (Banff, 2002). Key as in Figure 25. 37 38 Figure 27 Ripples on bedding surface in the Findlater Castle 39 Quartzite at Findlater Castle. (Photo: BGS No. P 008614, 40 reproduced with the permission of the Director, British Geological 41 Survey, © NERC.) 42

43 Figure 28 View westwards from Portsoy Old Harbour towards 44 Redhythe Point. The rocks of the near and middle distance are 45 mostly highly deformed semipelites, quartzites, metalimestones and 46 graphitic pelites within the Portsoy Shear-zone; note the very 47 strong, steeply plunging lineation in the quartzite on the left. 48 The second promontory away from the camera consists of 49 serpentinized peridotite, worked and sold locally as Portsoy 50 ‘ 51 Marble’. (Photo: J.R. Mendum, BGS No. P 001134.) 52 53 Figure 29 Andalusite porphyroblasts in the semipelitic part of 54 the Knock Head Grit Member of the Macduff Slate Formation at 55 Boyndie Bay. The hammer shaft is 35 cm long. (Photo: BGS No. P 56 221160, reproduced with the permission of the Director, British 57 Geological Survey, © NERC.) 58 59 Figure 30 Quartzofeldspathic boulder, interpreted as a dropstone 60 from a floating iceberg in the Macduff ‘Boulder Bed’, Macduff Slate 61 62 63 64 65 1 2 3 4 Formation, The Sclates, Macduff. Professor Janet Watson provides a 5 scale. (Photo: J.R. Mendum, BGS No. P 726598.) 6 7 Figure 31 Map of the coast section between Fraserburgh and 8 Rosehearty, based upon the BGS 1:50 000 Sheet 97 (Fraserburgh, 9 1987). The diagrammatic cross-section is adapted from Loudon 10 (1963) and is not to scale. 11 12 Figure 32 Refolded folds (F1 + F3) in a mixed sequence of thinly 13 bedded calcareous semipelites, calcsilicate rocks and impure 14 metalimestones of the Kinnaird’s Head Formation at Kinnaird’s Head, 15 Fraserburgh. 16 (a) Tight F1 folds refolded by close to tight F3 folds, 17 immediately east of the foghorn at NJ 998 677. A prominent S3 18 axial-planar cleavage and an L3 lineation are well developed in 19 places. Hammer head is 16.5 cm long. (Photo: J.R. Mendum, BGS No. 20 P 726599.) 21 (b) Refolded folds (F1 + F3), viewed to the east, at NJ 998 676. 22 Key fob is 4 cm in diameter. (Photo: D.J. Fettes, BGS No. P 23 726600.) 24 25 Figure 33 Cyclic bedding with graded units in the Rosehearty 26 Formation. Andalusite is abundant in the finer-grained units. 27 Note the cross-cutting cleavage. West of Sandhaven, NJ 952 679. 28 Key fob is 4 cm in diameter. (Photo: D.J. Fettes, BGS No. P 29 726601.) 30 31 Figure 34 Graded units with thin sandstone layers in a dominantly 32 pelitic facies of the Rosehearty Formation. Abundant andalusite is 33 clearly visible. Location as Figure 33. Pen is 15 cm long. 34 (Photo: D.J. Fettes, BGS No. P 726602.) 35 36 Figure 35 37 (a) Map of the Cairnbulg to St Combs coast section showing the 38 textural varieties of migmatitic rocks within the Inzie Head Gneiss 39 Formation. Modified after Johnson et al. (2001b). See text for 40 explanations of the terminology. 41 (b) Map summarizing mineralogical characteristics of the leucosome 42 element of the gneisses. 43 44 Figure 36 Schematic crustal section prior to D4 folding and 45 uplift, showing the nature of migmatitic rocks in the Inzie Head 46 Gneiss Formation and their relationships with the mid-Ordovician 47 mafic and felsic intrusions. From Johnson et al. (2001b). 48 49 Figure 37 Cairnbulg Granite intruded into diktyonitic metatexite 50 and nebulitic diatexite with relict metasedimentary schollen of the 51 Inzie Head Gneiss Formation. Top of beach, south of Cairnbulg 52 Harbour. The hammer shaft is 35 cm long. (Photo: J.R. Mendum, BGS 53 No. P 726603.) 54 55 Figure 38 Schollen diatexites (L-gt zone), with large garnets now 56 mainly retrogressed to chlorite in the Inzie Head Gneiss Formation. 57 The calcsilicate lenses and psammites form metasedimentary 58 schollen. The rocks were derived by anatectic partial melting of 59 the dominantly semipelitic rocks. Point of Whitelinks, east of 60 61 62 63 64 65 1 2 3 4 Inverallochy, Cairnbulg to St Combs GCR site. The hammer shaft is 5 35 cm long. (Photo: J.R. Mendum, BGS No. P 726604.) 6 7 Figure 39 Map of the Collieston to Whinnyfold coast section 8 showing D1 and D3 structural elements, adapted from Mendum (1987). 9 The inset equal-area stereographic projection shows structural 10 elements of the section between Whinnyfold and Old Castle. 11 12 Figure 40 Composite cross-section of the coast section between 13 Collieston and Whinnyfold, showing the overall fold pattern in a 14 plane normal to the fold axes. From Mendum (1987). 15 16 Figure 41 An excellent example of a recumbent, east-facing F1 17 fold in dominantly inverted gritty psammites of the Collieston 18 Formation, viewed towards the north-north-east. Devil’s Study, 19 Collieston to Whinnyfold coast section. J.R. Mendum provides a 20 scale. (Photo: BGS No. P 002878, reproduced with the permission of 21 the Director, British Geological Survey, © NERC.) 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure370 6.1 910 390 880 14 410 860 420 Fraserburgh N 13 840 Peterhead

0 kilometres 15

12 Banff 350 Portsoy 15 820

Ellon P P Ellon S Z Formation Buckie

KSZ Z

S

P P Aberdeen Keith Formation Aberdeen Huntly Inverurie 800 330

Dufftown 11 Alford 10 Craigievar Stonehaven Formation 780 Banchory PSZ 9 Aboyne

310 5

7 8 Tomintoul Silurian, Devonian and younger sedimentary and volcanic rocks 6 Ballater 4 Southern Highland Group Crinan and Tayvallich subgroups (undivided) Islay and Easdale subgroups (undivided) Braemar Blair Atholl Subgroup 3 t 2 l u 90 a Ballachullish Subgroup F

y r a BS d n Lochaber Subgroup u o B d n Grampian Group la h ig H felsic Caledonian intrusions mafic and ultramafic 1 Caledonian intrusions felsic pre-Caledonian intrusions 2

BS Blair Atholl mafic Neoproterozoic intrusions Pitlochry major faults, slides and shear-zones 3 GCR site in Chapter 6 Figure 6.2a Group Subgroup Glen Tilt to Glen Shee Deeside

SOUTHERN for details of Southern Highland for details of Southern Highland HIGHLAND Group in this area see Chapter 4 M Group in this area see Chapter 4 M M M M M M M M M C C C C C C C C C C C C C C C C C C C C C C C C C C C C Tarfside Psammite Fm Tayvallich Loch Tay Limestone Fm

C C C C C C Water of Tanar Limestone/ C C C C C C C C C C C Deeside Limestone Fm

Ben Lui Schist Formation (mostly Duchray Hill Queens Hill Fm Crinan Gneiss Member) Balnacraig Metabasite Member ? ? Coyles of Muick Shear-zone ARGYLL Farragon Volcanic Fm Meall Dubh Metabasite Fm C C C C C C C C C C C C C C C C C C C C C C Glen Girnock Calcareous Fm C C C C Ben Lawers Schist Fm C C C C Easdale C C C C Creag nam Ban Psammite Fm/ C C C C C C Glas Maol Graphitic Schist Fm Ben Eagach Schist Fm Cairn of Claise Transition Fm

Schiehallion/Creag Leacach Creag Leacach Quartzite Fm Islay Quartzite Fm

Sron na Gaoithe Boulder Bed Rimarsin Limestone Fm C C C C C C Gleann Beag Schist Fm C C C C Lawsie Psammite Fm C C C C Blair C C C C Atholl Tulaichean Schist Fm Crathie Schist and Sron nan Dias Pelite Limestone Fm and Limestone Fm

C C C C Glen Loch Phyllite and Birchwood Semipelite Fm Limestone Fm Ballachulish C C C APPIN An Socach Quartzite Fm An Socach Quartzite Fm (Transition) Beinn a Ghlo Transition Fm Glen Clunie Graphitic Schist Fm Glen Clunie Graphitic Schist Fm Baddoch Burn Dolomite Member position of Loch Tay Fault C C C C C C C C C C C C C C C C C C Glen Banvie Fm Tom Anthon Mica-schist Fm C C C C Lochaber

Boundary Slide-zone ?

Struan Flags

GRAMPIAN Figure 6.2b Strathdon, Tomintoul, North coast and North-east coast and Dufftown, Huntly Turriff Syncline Buchan Anticline

Macduff Fm Macduff Fm Collieston Fm/ Rosehearty Fm/ Clashindarroch Fm Whitehills Grit Fm Methlick Fm MMMMM Garnel Burn Pelite Member

C C C

C C C C C C C C C Kinnairds Head Fm/

C C C C Boyne Limestone Fm C C C C Strichen Fm/ Blackwater Fm (upper)

C C C

Mormond Hill Quartzite Member/Strichen Fm (lower) ?? Cowhythe Psammite Fm Inzie Head Gneiss Fm/ Ellon Fm Scors Burn Schist Fm/ Beldorney Pelite Fm Portsoy Shear-zone Reppachie Semipelitic Schist Fm/ Portsoy Limestone Fm C C Badenyon Schist and Limestone Fm Glenbuchat Graphitic Schist Fm Culchavie Striped Fm Castle Point Pelite Fm quartzite

flaggy and micaceous Kymah Quartzite Fm psammite Durn Hill Quartzite Fm psammitic metagreywacke and gritty psammite Ladder Hills Fm/Nochty Arnbath Psammite Fm Semipelite and Limestone Fm semipelitic metagreywacke Glenfiddich slaty and phyllitic Pelite Fm semipelite, mica-schist Tornahaish Fordyce Limestone Fm Inchrory Dark Schist and black slaty and Limestone Fm Limestone Fm schistose pelite Clashnoir metalimestone, Semipelite Fm metadolostone Cock Burn Calcareous Schist/ Tarnash Phyllite and Ailnack Phyllite and Limestone Fm Limestone Fm lava Corryhabbie Quartzite Fm volcaniclastic rocks, Mortlach Graphitic (Transition) green beds Schist Fm Mortlach Graphitic Schist Fm Sandend Harbour tillite and boulder beds Dufftown Limestone Member Limestone Member C C C C C C C C C C C C C C Garron Point C C C C C C C C C calcsilicate beds K K Fodderletter Calcareous Flag Fm/ K K Member Cairnfield C C C C C C C C C C C C Calcareous C C C C C C C C Pitlurg Calcareous Flag Fm C C C C C C C C Crathie Point C C C C C C C C Flag Fm K K K K K kyanite gneiss in upper Member Lochaber Subgroup Dalvrecht Slate/ Findlater Flag Fm M M M M M Findlater Flag Fm magnetic beds

Cullen Quartzite Fm Figure 6.3 (a)

W Whitehills Portsoy–Duchray Hill Boyndie Lineament Syncline Findlater Stocked 2 Cullen Castle Portsoy ‘Boyne Line’ Banff Macduff Head km 0

Portsoy Shear-zone

D1, D2 D1 and D3 D1 and D3 structures and D3 structures D1 structures predominant structures

Stocked E Head Troup Gardenstown Head Pennan Rosehearty Fraserburgh 2 km Turriff Syncline

0

0 kilometres 4

D1 structures D1 and D3 structures

(b) WNW ESE position of Boyndie Turriff Fraserburgh to Buchan ‘Boyne Line’ Syncline Syncline Rosehearty Anticline of Read (1955) Cullen to Troup Head

Cairnbulg to St Combs Collieston to Whinnyfold Southern Highland Group 0 kilometres 10

Tayvallach Subgroup

Crinan and Easdale subgroups Argyll Group Islay Subgroup

Appin Group Figure 6.4

99 00 44 0101 fault 60 GS 60 50 Allt Glen Loch tectonic slide 62 50 (ductile fault) 54 42 70 direction of 42 younging 46 64 58 71 inclined bedding, 60 76 dip in degrees 58 inclined S2 cleavage, 68 dip in degrees ? vertical S2 cleavage 70 ? 42 A 98 ? 58 70 ? N 0 metres 500 50 52 84 64 62 70 Ben Vuirich 72 84 74 52 52 62 74 26 84

? 72 86 86 Loch 47 76 B Valigan Carn NF Dubh 80 68 NF 84 60 85 66 70 74 69 NF 82 77 g NF 66 74 NF 76 GS 74 Creag nan 74 NF Gobhar NF 42 67 CS NF NF NF 75 NF 55 Ben Vuirich Granite (mostly foliated) 50 granoblastic to weakly 68 NF foliated granite

grey and white metacarbonate rocks strongly foliated granite 67 hornfels Crochton 75 82 Ben Lawers Schist Formation 65 Creag Chlacharnach Easdale 82 pelite Subgroup 78 CS ? Creag Uisge Slide Carn 67 Tulaichean Schist Formation 75 Liath 80 Allt na Leacainn Moire 77 NF ? garnet-mica schist 57 g NF 88 NF ? quartzose psammite/ ? CS quartzite 58 NF Blair Atholl 69 NF 65 GS g Killiecrankie– Subgroup Cnoc Dubh Glen Loch Slide Sron nan Dias Pelite and GS g Limestone Formation ? Clunskea Burn 48 77 pelite 72 66 mainly metacarbonate rock Figure 6.7

88 89 90 91 Clachghlas Allt Mhairc Argyll Group 30 54 Coille Sron an Duine Blair Atholl Subgroup 72 25 26 Ballachulish Subgroup Appin River Tilt Group Marble 35 Lodge marble Glen Banvie Formation with 38 lt quarry quartzite (q) ?Lochaber Subgroup au F 26 ay h T Grampian Group oc 71 L 33 Sron a’Chro Granite (Caledonian) Glen Tilt 24 Clachghlas Granite (pre-Caledonian) Allt Fas-charaidh 24 Gilbert’s amphibolite Bridge 28 70 32 fault 14 16 slide r Clachglas a u r inclined main foliation (S2), 16 B Marble

n dip in degrees

e Lodge

l Glen Tilt 69 G Gilbert’s 32 Bridge Glen Banviearea of Glen Fender N figure 36

Blair Loch Castle 68 Blair Moraig 0 kilometres 1 Struan River Garry Atholl Figure 6.8 e Mar Lodge River De

N Inverey BS

0 metres 500 16 4 89 19

5 Craig a’ 29 Chait 11

16 14 Tom Anthon 88 8 23 72 17 35

27 Allt Connie The Colonel’s 50 Bed 5 87 20 16 23 82 Carn Deag BS Tom AnthonAuchelie Fault Ey Burn

86 08 09 10 felsic intrusions (late Caledonian); minor intrusions omitted amphibolite (Neoproterozoic), probably metamorphosed basic intrusions Dalradian

Birchwood Semipelite Formation

An Socach Quartzite Formation and Beinn a’Ghlo Transition Formation Ballachulish Glen Clunie Graphitic Schist Subgroup Formation Baddoch Burn Dolomite Member Lochaber Tom Anthon Mica-schist Formation Subgroup ? ? ? Boundary Slide-zone Grampian Group

BS Boundary Slide

fault

inclined bedding, dip in degrees

inclined dominant schistosity, dip in degrees direction of younging determined from sedimentary structures Figure 6.9

NW SE

C BBS C

BS

Glen Ey Gorge

BS AL M AE BBS B AS DE

Easdale Subgroup slide-zone

Islay Subgroup F1 axial trace

Blair Atholl Subgroup F2 axial trace

Ballachulish Subgroup F4 axial trace Birchwood Semipelite Formation F1 facing An Socach Quartzite and Beinn a’Ghlo Transition formations AE Allt Elrig–Carn Aosda fold trace (F2) Glen Clunie Graphitic Schist Formation AL Altanour Lodge fold trace (F2) AS An Socach fold trace (F2) Lochaber Subgroup B Baddoch Burn fold trace (F2) BBS Baddoch Burn–Morrone Slide (D2) Tom Anthon Mica Schist Formation BS Boundary Slide (D2) C Cairnwell–Gleann Beag fold trace (F1) Grampian Group DE Devil’s Elbow fold trace (F4) Struan Flags M Morrone fold trace (F2) Figure 6.10 Ballater e e D n er 7 Riv 95 i o s u r t Pannanich Hill n I

k ck 7 c i 94 i u M u er 82 iv M R

f o

7 s 18 93 e l y o 82

C

792 36 30 45

Cairn 791 Leuchan

Am Mullach 60

790 74 48 75 ater of Tanar 52 W

789 56 N 28

7 88 56 Drum Cholzie Creag Dearg 52 0 kilometres 1 332 333 334 335 336 337 338 339 340 plutons, largely silicic (late Silurian; late-Silurian minor intrusions omitted) intrusive basic and ultrabasic rocks (Ordovician) inclined metavolcanic rocks, dip in degrees inclined foliation or gneissose fabric, intrusive basic meta-igneous rocks (Neoproterozoic) dip in degrees

Dalradian Supergroup fault Tayvallich Subgroup ductile shear-zone Crinan Subgroup (Queen’s Hill Formation); areas of intense migmatization indicated by overlay Basic metavolcanic rocks within the Easdale and Crinan subgroups Easdale Subgroup Figure 6.11 346 347 348 349 350 N Tarland B9094 B9119

802 A97 84 72

A b Scar Hill o y n Craig Dhu e 78 42 801 Craigie Balnagowan 66 84 Hill 40 Mulloch Balnacraig 20 42 Braeroddach Loch 20 800 86 Tomachallich 40 20 Craig Ferrar

799 Aboyne Dinnet A93 A93 Ballater 0 kilometres 1 ee D B976 er Riv

Silurian-Devonian Caledonian IgneousCaledonian Tomnaverie Granodiorite geological boundary Supersuite Logie Coldstone Tonalite fault, with sense of displacement shown by tick Ordovician inclined gneissose foliation, Tarland Intrusion: dip in degrees Norite (stipple shows area of shearing) NB in this area, all of the Neoproterozoic Neoproterozoic rocks amphibolite: metamorphosed tholeiitic sheets are migmatitic Tayvallich Subgroup road Deeside Limestone Formation: calcsilicate rock and metalimestone with beds of psammite and minor semipelite Dalradian SupergroupDalradian Queen’s Hill Formation Argyll GroupArgyll

psammite with minor semipelite layers SubgroupCrinan

pelite and semipelite, coarse grained, with large feldspar porphyroblasts mixed psammite, semipelite and pelite, becoming poorly foliated and xenolithic in patches heterogeneous xenolithic gneiss of metasedimentary origin (stipple shows area of shearing and retrogression) Figure 6.14

50 0 metres 200 8 60 143 N

Auchnahyle 8142

8141 35

n

o v 41 A

r e 8 v 140 i

R 69 60 Sìdh Mòr Sidh Beag 65 52

40 69 rn 8 Bu 138 ergie Muckle F

8137

8136

Torbain 3161 3162 3163 3164 3165 3166 3167 3168 3169 3170 3171

Argyll Group Kymah Quartzite Formation: inclined bedding, way up not known, quartzite, feldspathic psammite dip in degrees where known Auchnahyle Formation: vertical bedding Psammite and subsidiary semipelite and pelite metalimestone inclined schistosity, dip in degrees Islay metadolostone Subgroup direction of younging given by cross bedding structures metadiamictite fault amphibolitic mafic bodies road, track (lavas and intrusive sheets) Appin Group Glenfiddich Pelite Formation: Blair Atholl graphitic pelite and semipelite, partly calcareous Subgroup Later intrusions (?Caledonian) metadiorite Figure 6.16

N Appin Group Ballachullish Subgroup Creag an Iaruinn Ailnack Phyllite and Cross-section (Fig 6.17) 20 Limestone Formation 0 metres 200 Mortlach Graphitic Schist Formation 44 Blar an Lochain (not exposed in burn) 30 Lochaber Subgroup 20 29 821 Fodderletter Calcareous 24 Flag Formation. Fireach 37 35 Beag Kyanite Gneiss 37 Bur Member in middle part n o f B Dalvrecht Slate ro wn Formation 36 41 Grampian Group Tomintoul Tormore Psammite Formation 35 Strathavon Psammite Member 43 A939 feldspathic psammite A939 Bridge of Brown Grantown 40 quartzite Wade Bridge 38 feldspathic psammite h mixed psammite aid 40 ad 40 and semipelite m 41 o lt I G 44 inclined bedding, way up Al l 51 e 42 not known, dip in degrees n 48 43 B fault r 43 o w 44

n slide

farm or house

820 312 313 Figure 6.17

NJ 1218 2183 NJ 1488 1812 NW SE Burn of Creag an Tom Allt na Cnocan Lochy Iaruinn Beag Cluaine Eirich 1000

500

0

-500

-1000 0 kilometres 1

Old Red Sandstone Supergroup Tomintoul Group (late Silurian or Early Devonian) Grampian Group Delnabo Conglomerate Formation Tormore Psammite Formation Strathavon Psammite Member Dalradian Supergroup (Neoproterozoic) feldspathic psammite Appin Group quartzite Blair Atholl Subgroup feldspathic psammite Glenfiddich Pelite Formation mixed psammite and semipelite psammite Inchrory Limestone Formation

Ballachulish Subgroup Cromdale Hills Quartzite Member Ailnack Phyllite and Limestone Formation fault

Corriehabbie Quartzite Formation slide

Mortlach Graphitic Schist Formation

Lochaber Subgroup Fodderletter Calcareous Flag Formation Fireach Beag Kyanite Gneiss Member in middle part

Dalvrecht Slate Formation Figure 6.18 A939

B9136

30 N 57

25 0 metres 200 81

40

65 Old Bridge of Avon

38 41 26 820 Urlarmore

Kylnadrochit Lodge 39 Campdalmore

33 23 57

9 3 9 A Torrans 26 l Creag ou 21 42 int 35 om R Chalcaidh T i ve r Av e on ain 36 27 Clu 33 a 34 n

t l l A 34 40 48 35 27 57 3 15 819 Appin Group Blair Atholl Subgroup inclined bedding, dip in degrees Inchrory Limestone Formation inclined bedding, way up Ballachulish Subgroup not known, dip in degrees Ailnack Phyllite and Limestone Formation Contains several distinctive members fault

Kylnadrochit Semipelite Member farm, lodge

Torulian Limestone Member road

Corriehabbie Quartzite Formation former limestone quarry

Mortlach Graphitic Schist Formation Figure 6.20a

N h

450 c

t 73 i D

k c a 58 l 0metres 200 Bachd B Buidhe A 80 85 85 The Eachrach 85 Ky mah 650 Bur n 600 500 87 81 85 86 81 550 79 80 823 550 82

600 81

650 73 e 78 ip tr 44 S 80 ry 74 700 D B 65 68 77 70 65 65 73 69 67 62 h 550 75 ic a 70 Carn na Glascoill n a 68 733m R n 53 CCC a 88 h c 63 o a 78 C 600 67 77 78 84 822 70 86 74 in a 69 h m 80 o 650 D n a h 62 c ao C

329 330 Late-Caledonian Igneous Rocks microgranite (part of Glenlivet Granite Pluton) inclined bedding, dip in degrees Argyll Group inclined bedding, known to be overturned, Islay Subgroup dip in degrees Kymah Quartzite Formation inclined bedding, way up not known, dip in degrees Nochty Semipelite and Limestone Formation vertical bedding

inclined cleavage, dip in degrees Ladder Hills Formation vertical cleavage metabasalt (possibly lava) metavolcaniclastic rocks direction of younging metadolostone fault meta-igneous rocks (possibly early Caledonian) metadolerite sheet Figure 6.20b

Caochan Caochan Ranaich Domhain A B C

metres 800

700

600

500

400

300

200

axial plane trace of early syncline Figure 6.21

Dufftown 3 3 3 36 37 38 Huntly L Tom na A 0 metres 500 K Vowin A 65 N A 70 831 Torr of 86 85 Shenwell A 72 P Shenval 68 P 32 L Blackwater (Ardwell)

65 R Bridge The iv 46 Grouse Inn er D eve Upper ro 74 n Ardwell 64 72 68 Bl er ack Wat K c h ra L ab L A A C A941 830 Old Red Sandstone Dalradian Supergroup P pillow lavas breccia of local clasts Argyll Group inclined composite fabric Blackwater Formation Blackwater Intrusion (bedding and foliation), dip in degrees Corinacy Pelite Member vertical composite fabric metagabbro graphitic pelite and andalusite schist (bedding and foliation) mainly gritty psammite with some direction of younging of serpentinite schistose semipelite and pelite graded beds mainly metabasalt lavas; L = Lynebain Member fault A= Ardwell Bridge Member mainly metapicrite lavas; road K = Kelman Hill Member track Figure 6.23

S 3 3 m 35 22 36 a ll B u r n

75 35 N 38

h 45 ic Rothes Fault d

d

i

F

r

e

v metres i 0 500 30

838 R 22 Gallow Hill 13 45 45 60 72 65 20 30 60 Red 80 Burn 65 55 30 70 Dufftown Auchindoun 80 43 Castle 60

A941 52 74 A l 60 l t

a ’ 30

45 25 C h o 43

i l e 53 a old slate c h quarries 8 a 48 37 i 43 n 30 35 45 38 45 45 32 75 60 45 38 50

Appin Group inclined bedding, Ballachulish Corriehabbie Quartzite Formation Subgroup dip in degrees inclined dominant cleavage (?S2), Mortlach Graphitic Schist Formation dip in degrees Dufftown Limestone Member at base inclined coplanar bedding and Subgroup Lochaber slaty cleavage, dip in degrees 55 Pitlurg Calcareous Flag Formation axis of minor fold, 836 plunge in degrees fault

A941 25 road

Cabrach track Figure 6.25

351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 Logie Head 82 N 868 Cullen 66 78 Crathie Bay 82 76 Point Redhythe Garron Point East Cullen Point 68 Head 78 65 Links Cowhythe 0 kilometres 2 867 Sandend Bay Head 85 Bay 75 80 50 Old Hythe 76 35 Whyntie Knock 36 77 Stake Ness Head A98 Boyne Bay Head 56 Sandend 55 55 65 866 70 70 Portsoy 43 B 30 Boyndie ur 70 n Whitehills Bay of D 8 e 65 s 62 k

f B

o 72 u

r 80 d d r n B9121

o A98 f 8 52 B 64

53 o

y y

n n e B9018 e

367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383

868 N Troup 30 Head

867 40 27 0 kilometres 2

866 More 32 Head River Howe of Gamrie Tarlair Stocked 88 Boyndie Deveron 60 80 Bay Crovie Head 53 8 Bay 80 45 76 38 B9031 65 52 44 68 22 Gardenstown 55 A98 Macduff 67 30 15 44 864 Banff B9031 B9123 A947 A97 72 A98

76

74 Figure 6.25 + 6.26 key

Old Red Sandstone Supergroup Appin Group Intrusive Igneous Rocks Blair Atholl Subgroup Fordyce Limestone Formation. Longmanhill Granite (Ordovician) Dalradian Supergroup (Neoproterozoic) includes prominent metalimestone unit, Southern Highland Group shown in the cross-section only pegmatitic granite veins mainly in Macduff Formation, Ballachulish Subgroup Portsoy Complex showing prominent psammite units Tarnash Phyllite and Limestone Formation. ultramafic and mafic bodies (early Ordovician) includes the Linkbrae Limestone Member amphibolitic mafic sheets and dykes Knock Head Grit Member (shown in the cross-section only) (mainly early Ordovician) Mortlach Graphitic Schist Formation Whitehills Grit Formation, Portsoy Granite (Neoproterozoic) showing calcareous gritty psammite and lower Sandend Harbour Limestone Member gritty psammite units (in cross-section only) inclined bedding, Lochaber Subgroup dip in degrees Argyll Group Garron Point Tremolitic inclined bedding known to be overturned, Tayvallich Subgroup Flag Member Cairnfield Calcareous dip in degrees Whyntie Brae Calcsilicate Member Boyne Crathie Point Flag Formation inclined bedding, way up not known, dip in degrees Boyne Castle Limestone Member Limestone Calcsilicate Member Formation Old Hythe Semipelite Member Findlater Flag Formation. vertical bedding includes Findlater Castle Crinan Subgroup Quartzite Member geological boundary West Sands Mica Schist Member Cowythe Psammite Formation, fault including prominent metalimestone unit Grampian Group

Easdale Subgroup Cullen Quartzite Formation (undivided) thrust Sunnyside Psammite Member Portsoy Limestone Formation road Dicky Hare Semipelite Member Castle Point Pelite Formation, including related semipelitic and pelitic rocks Logie Head Quartzite Member Islay Subgroup Durn Hill Quartzite Formation

Arnbath Psammite Formation Figure 6.26

Logie Findlater Head Castle Sandend Portsoy Boyne Bay [5262 6793] [5612 6625] metres 500 250 OD -250 -500 -750

Knock Head Whitehills Banff Macduff [6361 6589] [6925 6445] [7186 6512]

Troup More Head Crovie Head [7935 6469] [8301 6719] metres 500 250 OD -250 -500 -750 Figure 6.31

392 393 394 395 396 397 398 399 400 N

Typical fold profiles 8 68 refolded folds 70 Typical fold profiles 50 28 23 30 60 28 23 23 Kinnairds Head Excellent sedimentary 23 structures in Rosehearty Sandhaven 40 Broadsea this area 20 867 42 Harbour 23 Fraserburgh

Fraserburgh 8 coarse-grained basic 66 Bay meta-igneous body and mixed calcareous rocks

Kinnairds Head Formation inclined foliation, dip in degrees boundaries of built-up areas 0 kilometres 1 Rosehearty Formation direction of younging, fault indicated by graded bedding Prominent psammite beds Figure 6.35

Cairnbulg 404 405 inclined foliation or lithological banding, (a) Point dip in degrees vertical foliation or lithological banding, 865 geological boundary 8 Gt 66 Gowan in Broad 50 58 Hole Hive Boat fault 85 60 74 60 45 Hive Point of 41 metamorphic boundary (peritectic phase boundary. 52 Whitelinks 40 64 Crd – cordierite, Gt – garnet, Opx – orthopyroxene) Cable 65 54 49 36 61 Shore 54 road Cairnbulg 60 West Inverallochy track Haven 60 LWM 4 71 HWM 03 boundaries of built-up areas

40 Whitelinks 864 406

h The t

r Bay N

o

l

i 4 Gwights Kittyloch Shore

h 04

P 43

f

o Types of migmatitic rocks in the

r te Inzie Head Gneiss Formation a Intrusive rocks (early Ordovician) 41 W diktyonitic (network-veined) Millburn 0 metres 300 leucogranite and granite Shore Opx 865 403 metatexites 864 in 863 diorite schlieren metatexites Bailiff ’s Skelly schollen diatexites Gt Inzie out 56 Head schlieren diatexites 48 St Combs 29 hybrid diatexites 41 59 granoblastic restite 44 405 406

4 4 4 4 04 05 06 07 Intrusive rocks (early Ordovician) (b) Cairnbulg Point N Gt in 0 kilometres 1 leucogranite and granite geological boundary

diorite fault Fraserburgh Whitelinks Opx Bay Bay in Migmatitic rocks in the Inzie Head Gneiss Formation 866 Gt metamorphic boundary 7 Inzie 0 out leucosome characterised by cordierite (L-crd) 1 Inverallochy Head M 9 402 W B road L leucosome characterised by garnet (L-gt) Cairnbulg WM H B9033 St Combs leucosome characterised by orthopyroxene (L-opx) 865 864 863 Figure 6.36 granite/leucosome L-opx zone L-gt zone L-crd zone unmigmatized metasedimentary rock deformed unmigmatized deformed metasedimentary rock metasedimentary L-opx L-gt L-crd L-gtdiatexites diktyonitic L-crd diktyonitic metatexites L-opxdiatexites hybrid diatexites hybrid gabbro body gabbro granoblastic hybrids restites restites diorite and gabbro and diorite restite leucogranite peraluminous garnetiferous aplitic veins aplitic diorite leucosome channel leucosome granite Figure835 6.39 834 17 Intrusive rocks 833 Peterhead Granite (late Caledonian) Sandy Haven metadolerite sheets

4 N 09 Dalradian Supergroup The Cruner 16 Southern Highland Group Whinnyfold Slagduff Collieston Formation Buck’s Nose 0 metres 500 15 geological boundary Lady’s Step inclined bedding, right way up, 832 dip in degrees inclined bedding, inverted, dip in degrees oil pipeline 10 Berry’s Loup 5 axis of F1 minor fold, plunge in degrees booster station N 408 road

9 buildings (mainly farm buildings)

Green Craig 831 3 Stereogram Bruce’s Haven F1 fold axis

10 F3 fold axis

A975 407 pole to S1 cleavage and/or F1 axial plane Devil’s Study 3 pole to S3 cleavage and/or F3 axial plane 2 8 Radel Haven 30

5

Broad Haven 5 Old Castle Portie Shore 406 20 829

18 Pottie Murlan Meikle Loch Dowiestone Cave Aver Hill

A975 30 18 405 St Catherine’s Dub B9003 828 8 Tarness Haven Collieston Collieston Harbour Cotehill Loch 20 Sand Loch B9003 403 404 Figure 6.40

2 Old Whinnyfold The Pier Castle Collieston

D1 deformation dominant

1 D3 deformation

penetrative kilometres The Smithy

0 2 1 kilometres

direction of younging of strata Figure 6.5 colour Click here to download high resolution image Figure 6.6 colour Click here to download high resolution image Figure 6.12 colour Click here to download high resolution image Figure 6.13 colour Click here to download high resolution image Figure 6.15 colour Click here to download high resolution image Figure 6.19 colour Click here to download high resolution image Figure 6.22 colour Click here to download high resolution image Figure 6.27 colour Click here to download high resolution image Figure 6.28 colour Click here to download high resolution image Figure 6.29 colour Click here to download high resolution image Figure 6.30 colour Click here to download high resolution image Figure 6.32a colour Click here to download high resolution image Figure 6.32b colour Click here to download high resolution image Figure 6.33 colour Click here to download high resolution image Figure 6.34 colour Click here to download high resolution image Figure 6.37 colour Click here to download high resolution image Figure 6.38 colour Click here to download high resolution image Figure 6.41 colour Click here to download high resolution image Figure 6.24 Click here to download high resolution image Figure 6.5 B&W Click here to download high resolution image Figure 6.6 B&W Click here to download high resolution image Figure 6.12 B&W Click here to download high resolution image Figure 6.13 B&W Click here to download high resolution image Figure 6.15 B&W Click here to download high resolution image Figure 6.19 B&W Click here to download high resolution image Figure 6.22 B&W Click here to download high resolution image Figure 6.27 B&W Click here to download high resolution image Figure 6.28 B&W Click here to download high resolution image Figure 6.29 B&W Click here to download high resolution image Figure 6.30 B&W Click here to download high resolution image Figure 6.32a B&W Click here to download high resolution image Figure 6.32b B&W Click here to download high resolution image Figure 6.33 B&W Click here to download high resolution image Figure 6.34 B&W Click here to download high resolution image Figure 6.37 B&W Click here to download high resolution image Figure 6.38 B&W Click here to download high resolution image Figure 6.41 B&W Click here to download high resolution image