1 the Dalradian Rocks of Scotland: 6 7 an Introduction 8 9 10 D
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Revised manuscript 8/5/12 Click here to view linked References 1 2 3 4 5 1 The Dalradian rocks of Scotland: 6 7 an introduction 8 9 10 D. Stephenson, J. R. Mendum, D.J. Fettes and 11 A.G. Leslie 12 13 * David Stephenson British Geological Survey, Murchison House, 14 West Mains Road, Edinburgh EH9 3LA. 15 [email protected] 16 17 0131 650 0323 18 John R. Mendum British Geological Survey, Murchison House, West 19 Mains Road, Edinburgh EH9 3LA. 20 Douglas J. Fettes British Geological Survey, Murchison House, West 21 Mains Road, Edinburgh EH9 3LA. 22 A. Graham Leslie British Geological Survey, Murchison House, West 23 Mains Road, Edinburgh EH9 3LA. 24 25 * Corresponding author 26 27 Keywords: 28 Geological Conservation Review 29 Grampian Highlands 30 Dalradian Supergroup 31 History of Research 32 Metamorphism 33 34 Tectonic evolution 35 36 ABSTRACT 37 38 The Dalradian Supergroup and its basement rocks, together with 39 younger plutons, underpin most of the Grampian Highlands and the 40 islands of the Inner Hebrides between the Highland Boundary and 41 Great Glen faults. The Dalradian is a mid-Neoproterozoic to early- 42 Ordovician sequence of largely clastic metasedimentary rocks, with 43 some volcanic units, which were deformed and metamorphosed to 44 varying degrees during the Early Palaeozoic Caledonian Orogeny. 45 Sedimentation of the lower parts of the Dalradian Supergroup, 46 possibly commencing about 730 million years ago, took place 47 initially in fault-bounded rift basins, within the supercontinent 48 of Rodinia and adjacent to sectors of continental crust that were 49 later to become the foundations of North America, Greenland and 50 Scandinavia. Later sedimentation reflected increased instability, 51 culminating between 600 and 570 million years ago in continental 52 rupture, volcanicity and the development of the Iapetus Ocean. 53 This left the crustal foundations of Scotland, together with those 54 of North America and Greenland, on a laterally extensive passive 55 margin to the new continent of Laurentia, where turbiditic 56 sedimentation continued for about 85 million years. Later plate 57 movements led to closure of the Iapetus Ocean and the multi-event 58 Caledonian Orogeny. Most of the deformation and metamorphism of 59 the Dalradian strata peaked at about 470 million years ago, during 60 the mid-Ordovician Grampian Event, which has been attributed to the 61 62 63 64 65 1 2 3 4 collision of an oceanic arc with Laurentia. The later, mid- 5 Silurian Scandian Event, attributed to the collision of the 6 continent of Baltica with Laurentia and the final closure of the 7 Iapetus Ocean, apparently had little effect on the Dalradian rocks 8 but marked the start of late-orogenic uplift and extensive 9 magmatism in the Grampian Highlands that continued until Early 10 Devonian times. 11 The Dalradian rocks thus record a wide range of sedimentary 12 environments (alluvial, tidal, deltaic, shallow marine, turbiditic, 13 debris flow) and a complex structural and metamorphic history. In 14 areas of low strain, original sedimentary and volcanic structures 15 are well preserved, even at relatively high metamorphic grades. 16 There is convincing evidence for glacial episodes of worldwide 17 importance and economic deposits of stratiform barium minerals are 18 unique. The Grampian Highlands include two of the World's type- 19 areas for metamorphic zonation, Barrovian and Buchan, with 20 spectacular examples of the key metamorphic minerals, and various 21 stages of migmatite development. Polyphase folding is widespread 22 on all scales and gives rise to a range of associated cleavages and 23 lineations. Regional dislocations, both ductile and brittle, are 24 associated with a range of shear fabrics, breccias, clay gouges and 25 veining. 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 1 2 3 4 5 1.1 INTRODUCTION 6 7 D. Stephenson 8 9 10 11 1.1.1 The Dalradian Supergroup 12 13 The Dalradian Supergroup is a mid-Neoproterozoic to Early 14 Palaeozoic sequence of largely clastic sedimentary rocks, with some 15 notable carbonate and volcanic units that were all deformed and 16 metamorphosed to varying degrees during the mid-Ordovician Grampian 17 Event of the Caledonian Orogeny. The Dalradian rocks, together 18 with Caledonian intrusive igneous rocks, form the bedrock to most 19 of the Grampian Highlands of Scotland and the islands of the Inner 20 Hebrides between the Highland Boundary and Great Glen faults. Pre- 21 Dalradian basement crops out in parts of the Northern Grampian 22 Highlands and on the Isle of Islay (Figure 1.1). The Dalradian 23 sequence, its basement and the Caledonian intrusions comprise the 24 Grampian Terrane, one of several major crustal blocks that were 25 juxtaposed during the Caledonian Orogeny to form the northern part 26 of the British Isles (Figure 1.2). Dalradian rocks also occur in 27 the Shetland Islands, east of the Walls Boundary Fault, 28 conventionally as part of the Grampian Terrane but possibly part of 29 30 a separate terrane. Siluro–Devonian and Mesozoic cover rocks crop 31 out mainly around the margins of the Grampian Highlands. 32 The Grampian Terrane extends south-westwards into the northern and 33 north-western parts of Ireland, where Dalradian rocks crop out over 34 wide areas (Figure 1.2). There, the south-eastern terrane boundary 35 is largely buried beneath younger rocks and is difficult to define. 36 An extension or major splay of the Highland Boundary Fault probably 37 does extend from Cushendun in the east to Clew Bay in the west but, 38 unlike in Scotland, it is not defined by any strong geophysical 39 feature. A Dalradian sequence with remarkable similarities to that 40 of the Grampian Highlands also crops out in Connemara, well to the 41 south of Clew Bay, and hence it seems likely that the boundary of 42 the Grampian Terrane does not coincide with an extension of the 43 Highland Boundary Fault in the west of Ireland and possibly extends 44 south-eastwards as far as a line between south Antrim and Galway 45 (Ryan et al., 1995). 46 On a broader scale, the terranes of the northern British Isles are 47 48 inherently linked geologically to eastern North America and 49 Greenland, which were in close proximity prior to the opening of 50 the North Atlantic Ocean in Palaeogene times (c. 55 Ma ago) (Figure 51 1.3). The Dalradian Supergroup is similar in age to the Fleur de 52 Lys Supergroup in Newfoundland (Kennedy, 1975) and its lower parts 53 are equivalent to the Eleonore Bay Supergroup in East Greenland 54 (Soper, 1994b; Leslie et al., 2008); the three sequences might have 55 been deposited in adjacent basins. The Geological Conservation 56 Review, and hence this volume, considers only the Dalradian rocks 57 of Scotland and Shetland; for reviews of the Irish Dalradian see 58 Alsop and Hutton (1990), Leake and Tanner (1994), Harris et al. 59 (1994), Cooper and Johnston (in Mitchell, 2004) and chapters by 60 J.S. Daly and D.M. Chew in Holland and Sanders (2009). 61 62 63 64 65 1 2 3 4 The current best estimate for the age of deposition of the oldest 5 Dalradian rocks, adopted in this volume, is about 730 Ma. The 6 youngest strata that can be assigned to the Dalradian has been a 7 matter of recent debate, but it is now generally accepted that 8 there is stratigraphical and structural continuity through from 9 undisputed Dalradian strata into fossiliferous strata of Early 10 Cambrian age (c. 515 Ma) and possibly continuing up into mid-Arenig 11 12 age strata (Tanner and Pringle, 1999; Tanner and Sutherland, 2007). 13 Those Early Palaeozoic strata were formerly thought to have been 14 juxtaposed tectonically against the Dalradian sequence and 15 consequently they have been described separately in the British 16 Cambrian to Ordovician Stratigraphy GCR volume (Rushton et al., 17 1999). They were assigned originally to the Highland Border 18 Complex, which also includes elements of an ophiolite obducted 19 during the Caledonian Orogeny, but Tanner and Sutherland (2007) 20 have suggested that they should be designated as a separate 21 ‗Trossachs Group‘ and included in the Dalradian Supergroup (see 22 Introduction to Chapter 4). 23 The name ‗Dalradian‘ is derived from that of the ancient Scots‘ 24 kingdom of Dalriada, which united the coastal areas of Argyll, 25 Arran and Antrim between the 5th and 9th centuries AD. It was 26 first applied to all of the metamorphic rocks that crop out between 27 the Moine Thrust and Highland Boundary Fault by Sir Archibald 28 Geikie in 1891. However, in his explanatory notes to the 1892 29 30 edition of Bartholomew‘s 10-miles-to-one-inch Geological Map of 31 Scotland, he did make it clear that the ‗Moine Schists‘ of the 32 Northern Highlands are different in character to the ‗Dalradian‘ 33 rocks south-east of the Great Glen. As survey work progressed, 34 quartzofeldspathic rocks of apparent ‗Moine Schist‘ facies were 35 also identified in the northern part of the Grampian Highlands and 36 the term ‗Dalradian‘ sensu stricto became restricted to the 37 lithologically more-diverse strata now assigned to the Appin, 38 Argyll and Southern Highland groups of the Dalradian Supergroup. 39 The so-called ‗Moine‘ rocks of the Grampian Highlands have been 40 the subject of much debate and revision of assignment and 41 terminology over the past 100 years.