The Dalradian Rocks of the Shetland Islands, 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 Shetland Islands, 6 7 Scotland 8 9 10 D. Flinn, P. Stone and D. Stephenson 11 12 Derek Flinn (deceased) formerly The Jane Herdman Laboratories of 13 Geology, University of Liverpool. 14 Phillip Stone British Geological Survey, Murchison House, West 15 Mains Road, Edinburgh EH9 3LA. 16 * David Stephenson British Geological Survey, Murchison House, 17 West Mains Road, Edinburgh EH9 3LA. 18 [email protected] 19 0131 650 0323 20 21 * Corresponding author 22 23 24 Keywords: 25 Geological Conservation Review 26 Shetland Islands 27 Dalradian Supergroup 28 Lithostratigraphy 29 Structural geology 30 Metamorphism 31 32 33 34 35 ABSTRACT 36 37 Metasedimentary and metavolcanic rocks to the east of the Walls 38 Boundary Fault on Shetland have lithological similarities to those 39 of the Dalradian Supergroup of the Scottish mainland. In 40 particular, the middle part of the succession, termed the Whiteness 41 Group, includes numerous metalimestones and associated pelites in a 42 shallow-marine succession that recalls the upper parts of the Appin 43 Group and the Argyll Group of mainland Scotland. Metavolcanic 44 rocks within the deeper water turbiditic sequence of the succeeding 45 Clift Hills Group might be broadly coeval with those of the 46 Southern Highland Group of Scotland. Beyond that, correlations 47 with the established Dalradian succession are tenuous and are not 48 possible at formation level. A local succession immediately west 49 of the Walls Boundary Fault is of even more-dubious Dalradian 50 affinity. 51 The dominant structure is the regional-scale, downward- and east- 52 facing East Mainland Mega-monocline. This has a vertical western 53 limb, which youngs to the east, and an eastern top limb that dips 54 to the north-west at 20–30o. Strata on the eastern limb are 55 inverted on Mainland, Whalsay and Out Skerries but are right way up 56 on the west side of Unst, having been folded around the tight Valla 57 Field Anticline. The Shetland Ophiolite-complex has been thrust 58 over the inverted limb of the Valla Field Anticline on the east 59 side of Unst. The regional monocline folds earlier small- to 60 61 62 63 64 65 1 2 3 4 medium-scale, tight to isoclinal folds with associated planar and 5 linear structures, which are all assigned to a single ‗Main 6 Deformation‘. It also post-dates the regional metamorphism, which 7 ranges from chlorite to garnet grade, with localized development of 8 staurolite-kyanite, gneissose fabrics, and the emplacement of 9 schistose granitic sheets in the Colla Firth Permeation Belt. 10 The GCR sites have been selected mainly to be representative of 11 the East Mainland Succession with its associated structures and 12 metamorphism. Highlights include well-preserved sedimentary 13 structures, high-grade gneisses permeated by granitic material, 14 basaltic pillow lavas and serpentinized ultramafic rocks. Some of 15 the latter contain enigmatic skeletal pseudomorphs after olivine 16 and have been interpreted as former high-magnesium lavas. 17 18 19 20 21 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 1 2 3 4 1 INTRODUCTION 5 6 D. Flinn, P. Stone and D. Stephenson 7 8 The Shetland Islands lie about 165 km north-east of the Scottish 9 mainland, and are almost half way between Scotland and Norway. The 10 islands comprise an inlier of Caledonian and pre-Caledonian 11 12 metamorphic rocks, which is completely surrounded by Devonian (Old 13 Red Sandstone) and younger rocks (Figure 1). Correlation of the 14 metamorphic rocks with those of the Scottish mainland has been 15 based on lithological similarities, aided by some radiometric 16 dating. For general summaries of the geology see Mykura (1976) and 17 Flinn (1985). 18 The Walls Boundary Fault, a likely northward continuation from the 19 Scottish mainland of the Great Glen Fault (Flinn, 1961) (Figure 1) 20 divides the metamorphic rocks of Shetland into two mutually 21 uncorrelatable successions associated with two distinct sets of 22 post-metamorphic granites. 23 24 1.1 West of the Walls Boundary Fault 25 26 To the west of the Walls Boundary Fault, the overall tectonic 27 arrangement is a series of structural slices, separated by thrusts 28 and shear-zones that interleave pre-Caledonian basement gneisses 29 30 with metasedimentary cover sequences (Flinn et al., 1979; Flinn, 31 1985). Many of the component slices exhibit similarities to parts 32 of the Lewisian, Moine and Dalradian sequences of the Scottish 33 mainland and Western Isles. Quartzofeldspathic orthogneisses 34 contain hornblende gneisses that have yielded radiometric ages up 35 to c. 2900 Ma and have been correlated with the Lewisian Gneiss 36 Complex. The orthogneisses are in contact to the east with a belt 37 of predominantly schistose psammites containing zones of coarse 38 hornblende gneisses, which are locally blastomylonitized. The 39 psammites have been tentatively correlated with the Morar Group of 40 the Moine Supergroup, whereas the hornblende gneisses might 41 correspond to the inliers of Lewisianiod rocks that are common in 42 the Morar Group. A blastomylonite shear-zone, which separates the 43 Moine-like rocks from the orthogneisses has been correlated with 44 the Moine Thrust. GCR sites to represent these units are described 45 in the Lewisian, Moine and Torridonian rocks of Scotland GCR volume 46 (Mendum et al., 2009). The Moine-like rocks, with their 47 48 Lewisianoid inliers, are limited to the east by the Virdibreck 49 Shear-zone, along which low-grade phyllitic to schistose 50 metasedimentary and metavolcanic rocks of possible Dalradian 51 affinity (the Queyfirth Group) have been thrust westwards. 52 53 1.2 East of the Walls Boundary Fault 54 55 On the mainland of Shetland, to the east of the Walls Boundary 56 Fault, a dominantly metasedimentary sequence has been correlated 57 with the Moine and Dalradian successions of the Scottish mainland 58 and is referred to as the East Mainland Succession. This 59 succession has been split into four major, lithostratigraphically 60 61 62 63 64 65 1 2 3 4 distinct ‘divisions’, now formally defined as groups (Figures 1 and 5 2; Flinn et al., 1972). 6 The oldest part of the East Mainland Succession, the Yell Sound 7 Group, crops out in the west, where it has been truncated obliquely 8 by the Walls Boundary Fault. It has a maximum exposed width of 10 9 km and possibly half as much again allowing for sea cover. It is 10 composed of variably gneissose quartzofeldspathic psammites, 11 12 alternating with major lenses of mica schist and quartzite. It 13 also contains layers of garnet-studded hornblende schist together 14 with half a dozen Lewisianoid inliers. This lithological 15 assemblage distinguishes the Yell Sound Group from the rest of the 16 East Mainland Succession and has allowed it to be correlated with 17 the Loch Eil and Glenfinnan groups of the Moine Supergroup in 18 Scotland (Flinn, 1967, 1992). 19 The Yell Sound Group is separated from rocks to the east by the 70 20 km-long and c. 1 km-wide Boundary Zone that extends across the 21 islands of Mainland, Yell and Unst. The western margin of the 22 Boundary Zone is marked by occurrences of a microcline-megacryst 23 augen gneiss, the Valayre Gneiss (Flinn 1992, Flinn in Mendum et 24 al., 2009), and its eastern margin by the Skella Dale Burn Gneiss. 25 Between these two augen gneisses the Boundary Zone contains lenses 26 of locally blastomylonitized Lewisianoid hornblende gneisses, basic 27 metavolcanic rocks and a variety of other gneissose psammites and 28 semipelites together with a metalimestone. 29 30 To the east of the Boundary Zone, the rocks of the East Mainland 31 Succession have very general lithological similarities with the 32 Dalradian succession of mainland Scotland and are the subject of 33 this paper. They extend the length of Shetland from north to south 34 and have been divided into three groups. From west to east and 35 older to younger these are the Scatsta, Whiteness and Clift Hills 36 groups. 37 Along part of the south-east coast of the Mainland, the Dalradian 38 rocks have been overthrust, from the east, by a tectonic nappe 39 containing gneisses and various metasedimentary lithologies. The 40 nappe overlies an imbricate zone, containing some serpentinite that 41 was termed a tectonic mélange by Flinn (1967). Some of the 42 constituent rock types are similar to Dalradian lithologies seen 43 farther west within the East Mainland Succession (e.g. in the 44 Scalloway GCR site) but, despite these similarities, the tectonic 45 style is distinct and this south-eastern fringe is recognized as 46 the separate Quarff Nappe Succession. The emplacement of the 47 48 ‗Quarff Nappe‘ probably took place late in the Caledonian Orogeny, 49 during the Scandian Event. Farther north, part of an Early 50 Palaeozoic ophiolite crops out on the islands of Unst and Fetlar 51 (Figure 1). This, the Shetland Ophiolite-complex, was tectonically 52 emplaced at about 500 Ma above rocks of likely Dalradian affinity; 53 its geology has been summarized by Flinn (2001, in press) and its 54 GCR sites are described in the Caledonian Igneous Rocks of Great 55 Britain GCR volume (Stephenson et al., 1999). Elsewhere, across 56 much of the east and south of Shetland‘s Mainland, the Dalradian 57 rocks are unconformably overlain by sedimentary and volcanic rocks 58 of the Old Red Sandstone Supergroup and are intruded by late- 59 Caledonian granites.