The Dalradian Rocks of the North-East Grampian Highlands of Scotland
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Revised Manuscript 8/7/12 Click here to view linked References 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.