Journal of rhe Geological Society, London, Vol. 147, 1990, pp. 417-420, 1 fig. Printed in Northern Ireland

SHORT PAPER Colonsay asproducts of NW-directed simple shear,but noted that there are structures in Islay that do not readily Is the Colonsay-west Islay block of SW conform tothat interpretation.Rogers et al. (1989) sug- Scotland an allochthonous terrane? gested that the early structures were induced by the dock- Evidence from Dalradian tillite clasts ing of the Colonsay-west Islay terrane with the Grampian terrane. Bentley (1986), however, has re-interpreted the W. R.FITCHES, R. J.MUIR, A. J. early tectonicevents in Colonsay in terms of sub- MALTMAN & M.R. BENTLEY' horizontal transpression in which the simple shear was di- Institute of Earth Studies, University College of rected NNE. In Islay, the recumbentfolds, sub-horizontal Wales, Aberystwyth, Dyfed SY233DB, UK cleavage and shear-sense indicatorsrecorded in the Col- 'Present address: Shell UK Exploration and onsay Group and its basement by one of us (R.J.M.) point to SW-directed ductileshear. Defining the senses of dis- Production, 1 Altens Farm Road, Nigg, Aberdeen placement during the earlydeformation of the cover and AB92HY, UK basement of the Colonsay-west Islay terrane is asubject of our on-going research. On presentevidence, however, the displacements are not easily incorporated into a model In the Scottish Caledonides context, the Colonsay-west lshy ter- of NW-SE terrane collision, withoutresorting to convo- me hasseemed anomalous and thereforelikely to be dloch- luted explanations. thonous.New studies suggest, however, that its basement is not necessarily exotic but represents the digearly Proterozoic link Isotopic ages of basement and cover. The concept of a between Greenland and Scandinavia. Recent interpretations of the Colonsay-west Islay exotic terrane has been based on two Scottishmainland deformation history imply that the age of the main criteria.First, thereare fundamentalcontrasts be- Grampian events there resemblesthat of the late Proterozoic defor- tween the basement rocks of that terrane and those of the mation previously identified in the Colonsay Group cover succes- Lewisian Complex of NWScotland and the Outer Hebr- sion. Reconnaisance studies of Dalradian tillite provenance suggest ides (Muir et al. 1989). Secondly, the Colonsay Group that the Colonsay-west Islay basement was a source of Dalradian sediment and perhaps floored that basin. There is, therefore, less cover has been deformed by structures shown to be older reasonnow to regard theColonsay-west Islay terrane as doch- than c. 600 Ma by Bentley (1986), thereby seeming to rule thonous, and recent hypotheses of its tectonic emplacement being out correlation of that cover with the DalradianSuper- the cause of the Grampian orogeny should he viewed with caution. group which, until recently, was considered to havebeen deformed for the first time in the Phanerozoic. The basementcontrasts have been fully confirmed by The Dalradian Supergroup of Scotland and the Grampian recent field and isotope studies. Marcantonio et al. (1989), nappes that deform it are entirely Precambrian, according to from their reconnaissance isotope work, suggested that the the interpretations of new isotope data by Rogers et al. Islay basement comprises juvenilemantle-derived rocks (1989). Consequently,a major reappraisal of thelate emplaced c. 1800Ma ago, implying correlation with the Proterozoic history of theGrampian terrane is required. Ketilidian province of S. Greenland and the Svecofennian Amongtheir far-reaching interpretations, Rogers et al. of Scandinavia. Results of acomprehensive isotope study speculated that the c. 600 Ma Grampian deformation in the of the Islay basement by one of us (R.J.M.)supports SW Highlands region was caused largely by the docking of andelaborates on this suggestion. Moreover, his field the Colonsay-west Islay terrane against theGrampian andpetrographic studies have revealed that the basement terrane (Fig. 1).They suggested thattheterranes protolith is an alkalic igneous association of syenite, amalgamated by NW-SE compressional displacement, close granodiorite, gabbro, ultrabasicand other rocks. This as- to a tectonically active oceanmargin, ratherthan by the sociation has nocounterpart in the Lewisian terraneto strike-slip motion invoked by Bentley et al. (1988). the north. It is probable thatthe Colonsay-west Islay This new interpretation seems tobe an attractive basement partly completes the link between the Ketilidian explanation of the apparently anomalous lithological and and Svecofennian components of the earlyProterozoic tectonic aspects of the Colonsay-west Islay terrane mobile belt that fringes the southern margin of the Laur- highlighted by Fitches & Maltman (1984), Bentley (1988) entian craton; there is no longer reason to explain it as an and Bentley et al. (1988), and of theGrampian events. exotic terrane. However, we see three reasonsfor urging caution in The Colonsay Group of sedimentary rocks was deposited accepting this model uncritically: early structures and fabrics unconformably on the plutonic basement of the Colonsay- in the Colonsay-west Islay terraneare notentirely west Islay terrane, deformed by flat-lying structures, consistent with the NW-SE terrane closure model;the then intruded by alkaline-subalkaline igneous plugs at concept of a Colonsay-west Islay terranethat evolved 600-635 Ma (Bentley 1986; Bentley et al. in prep.), independently of SW Scotland is questioned in the light of anddeformed again. This evidence of late Proterozoic recently acquiredisotope data and field-based studies in deformationseemed to distinguish the Colonsay Group Islay; and thereare indicationsfrom clasts in Dalradian from the DalradianSupergroup. The recognition of a sedimentary rocks thatthe basement tothe terrane was pre-595 Ma age for the Grampian deformation by Rogers et already nearthe Dalradian basin duringits subsidence al. (1989), however, begins to remove this distinction. There history. These three points are treated in turn. remain difficulties in structural and stratigraphic correlations between the Colonsay Group and the DalradianSuper- Tectonicdisplacement directions in the Colonsay group, but the need to view terrane as allochthonous is now Group. Fitches & Maltman (1984) explained theearly, greatly diminshed. Any modelbased on docking of the flat-lying structures andfabrics in the Colonsay Group of terrane should therefore be regarded with circumspection. 417

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Fig. 1. Geological map of Scotland and NW Ireland (after Dunning 1985) to show the setting of the Colonsay-west Islay terrane and the distribution of late Proterozoic Dalradian tillites. Tillite localities after Harris & Pitcher (1975).

Provenance of Dalradian sediment. Perhaps the main im- Kinlochlaggan Boulder Bed); and vein quartz, gneiss, and pediment to determining precisely the geological relation- white, pink and redgranite (Howarth et al. 1966 and ships between the Colonsay-west Islay and Grampian ter- Howarth 1971, Glencolumkille Boulder Bed). Sand-size ranes is that they are separated by faults or the boundary particles from the tillites, listed by Spencer (1971), com- is submerged. It hasbecome apparentfrom our recent prise a wide range of minerals: quartz, chess-board albite, studies,however, thatthe Colonsay-west Islay basement albite-oligoclase, untwinned plagioclase, microcline, mus- could beasource of clasts in parts of the Dalradian covite, biotite, dolomite, magnetite and pyrite, with acces- Supergroup succession. If so, that basement lay exposed sory apatite, greentourmaline, zircon andrutile. Quartz close to the site of Dalradian sedimentation and was not with acicular rutileneedles (in our opinion,probably the somedistant microcontinent thatlater impinged onthe blue quartz observed in hand specimens), chess-board alb- basin asit collided with theGrampian terrane. On this ite, albite-oligoclase, apatite, zircon andrutile are also basis, the wedge-shaped mass imaged under the SW High- components of the crystalline stones. lands onthe WINCH seismic profile (Hall et al. 1984), The crystalline stones and mineral fragments have been can be envisaged as Colonsay-west Islay basementcon- described as ‘extra-basinal’ and ‘exotic’ by various authors, tinuing beneath part of the Dalradian basin as in situ base- and their source has been the subject of speculation. The ment,rather than aslab introduced tectonically beneath upward change fromdolomite-dominated to crystalline- the basin as suggested by Rogers et al. dominated stones has been explained by either a progres- Our provenance arguments, still at the tentative stage, sive blanketing of the source rocks by the glacial beds are based on observations madeon clasts in thePort (Kilburn et al. 1965) or by stripping of adolomite cover Askaig Tillite (Fig. l), lying atthe base of the Argyll from the source region to exposea granite-metamorphic Group (‘middle’ Dalradian),and to someextent on pub- basement (Spencer 1971). One of the problems in locating lished information on other Dalradian siliciclastic units. the source is that the palaeocurrentindicators in the gla- The PortAskaig Tillite is correlated with the cial beds offer no clues on regional sediment transport Schiehallion Boulder Beds of thecentral Highlands of directions. Spencer (1971, p. 29) concluded that ‘a single Scotland and the Glencolumkille Boulder Beds of Ireland palaeoslope of regional extent was absentthroughout the (Spencer 1969, 1971, 1975), and probably equates ap- deposition of the whole formation’,and Kilburn et al. proximately with the Varanger Tillites of Norway (Pringle (1965, p. 358) considered that evidence of transport direc- 1972). It is widely exposed on eastern Islay, within 10 km tion is ‘too slight to be seriously discussed’. of the Colonsay-west Islay terrane but separatedfrom it Spencer (1969, 1975) favouredderivation from regions by the Loch Gruinart fault.Most stones (using theter- southeast of the Dalradian outcrop, partly on the evidence minology of thoseauthors concerned with the glacial as- of rare NW-verging glacial push folds in the tillites and, as pects of the deposits) in the lower part of the succession Kilburn et al. (1965) also noted, because of the con- are dolomite,probably derived fromolder Dalradian spicuous absence of Lewisian-type stones.Spencer (1969, strata. Higherin the succession thestones are pre- 1975) further pointed outthat somestones resemble the dominantly of crystalline rocks. Their compositionshave Rapikivi granites of Sweden andFinland, and suggested notbeen thoroughly described in theliterature but are that theirsource therefore might lie within a southern said to include: unfoliated granite and minor granitic gneiss extension of the 1000 Ma ‘Gothides’ (Sveconorweigan Pro- (Spencer 1971); granite with Rapikivi-type textureand vince) now hiddenbeneath thenorthern European plain. albitized granite (Spencer 1969, 1975); coarse, pink biotite He cited as supportfor this interpretation a c. 1000 Ma alkali granite,granophyre, granite porphyry, gneissose Rb/Sr whole rockage obtained from albitized granite granite,granitic gneiss andrare, altered basic igneous stones, which he considered might date ametamorphic rocks (Kilburn et al. 1965); alkali granite (Treagus 1969, event (data of P. Leggo, reported by Spencer 1975).

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Spencer (1975) also recorded K/Ar ages of 1807 f 50 Ma c. 1900 Ma crust was a major contributor to those and 1816 f 40 Ma obtained from muscovite in a ‘quartz- sedimentary rocks. Moreover, recent studies by one of us muscovite-granulite’ boulder fromthe tillite by S. Moor- (R.J.M.) have shown that the islands of Inishtrahull and Tor bath. Here, we cannot comment constructively on the in- Rocks, off Malin Head in Ireland, comprise deformed terpretation of these isotopeages, and simply point out syenites and gabbros thatare petrographically and that theK/Ar ages resemblethose fromthe Colonsay- geochemically indistinguishable from those of the Colonsay- west Islay basement using other methods. west Islay basement. The 39Ar-40Arisotope age of 1710 Ma It was recognized by Spencer that derivation of stones obtainedfrom amphiboles of theTor Rocksgabbros by from southeasterlya direction seems torequire some Roddick & Max (1983) augments the similarities with west method of transporting immense numbers of stones across Islay, and supports the hypothesis that Colonsay-west Islay the Iapetus Ocean. That problem is alleviated if the Iapetus type basement extended from the Inner Hebrides to Ireland Ocean opened widely after the glacial event or the source and provided a substantial component of the NW margin of was continental crust lying between the Dalradian basin and the Dalradian basin. that ocean (Anderton 1980). Eyles & Clark (1985) suggested Our observations on the tillite stones are ata preliminary the ocean was already open and was receiving large volumes stage andtherefore we makeno categoricalcorrelations of glacial debris from a mainland source to the southeast. A with the west Islay basement. On-going studies of the close source might best explain the uniformity of stone geochemistry, petrography and isotopic aspects of the stones types, whereas distant sources would supply a much greater should enable us to test this hypothesis, and thereby diversity (A.Stewart in discussion of Spencer 1969). demonstratewhether or not the Colonsay-west Islay In the absence of Lewisian-type stones, a westerly source igneous association lay close to,and perhaps continued of stones has been largely discounted, although Llewelyn (in beneath, part of the Dalradianbasin. Inthe meantime, discussion of Spencer 1969) favoured thatoption. This preliminary findings supportthe hypothesis,prompting alternative becomes more credible in the Islay region in the caution about invoking collision of formerlya remote light of Anderton’s (1985, 1988) argumentsfor a linked Colonsay-west Islay terrane with the Grampian terrane as extensional fault system affecting Dalradiansedimentation an explanation of theGrampian deformation in SW patterns in that district. Footwall uplift on half-graben faults Scotland. and elevation of blocks between transfer faults could have exposed the basement locally, whilst west Islay and other Concluding remarks. The hypothesis thatthe Colonsay- more westerly parts of thatterrane perhaps remained west Islay terrane is exotic was prompted largely by evid- emergent. ence of deformation in the Colonsay Group being older In our view, the Colonsay-west Islay terrane basement than any tectonic eventthen dated in the Dalradian or itsextension beneaththe SWHighlands Dalradian Supergroup, the lack of any convincing means of correlat- provides a ready source of the tillite crystalline stones. The ing the Colonsay Group with late Proterozoic successions basement on Islay hasbeen strongly deformed butthe on neighbouring islands andthe mainland, andthe ano- protolith is still recognizable asa suite of mainly syenites malous petrography of the basement. Since the formula- andgabbros, with localized granodiorite, together with tion of that hypothesis,however, the age of theearlier, a wide range of minorintrusions ranging in composition Grampiandeformation of the Dalradianhas been revised from acidic to ultrabasic. Thereare close petrographical and shown to be no longer conspicuously different from similarities betweenseveral of these rock types andthe the earliertectonic events in the Colonsay Group.The tillite stones from the Port Askaig area, the deformed and basement of the Colonsay-west Islay terrane hasbeen undeformedgranitic and syenitic rocks,inparticular, confirmed asprofoundly different from the Lewisian appearing beto common both.to Moreover, the Complex of NW Scotland andthe Outer Isles. It readily nordmarkitic,granitic and porphyritic acidic stones in the provides the ‘missing link’ between the Ketilidian province tillite point to derivation from a region with alkalic to of S Greenlandand the Svecofennian province of Scand- sub-alkaline affinities; the Islay basementhas just these inavia, and is therefore not necessarily allochthonous or affinities. Virtually all thedetrital minerals in the tillites exotic in its NAtlantic context. Moreover,there are str- listed by Spencer (1971) can be identified in these basement ong indications that rocks of Colonsay-west Islay base- rocks, which provide in particular an abundance of various ment type supplieddebris tothe late ProterozoicDal- feldspars. radian tillites andother siliciclastic deposits. If so, that It is also relevant to note that Anderton (1980) showed basement was exposed close tothe subsiding Dalradian that clasts in theJura Quartzites andother Dalradian basin andperhaps lay beneathit. In view of these new psammites appearto bederived from the northwest. He data and interpretations, any model of Grampian orogeny suggested that the ultimate source may have been an early relying onthe tectonicinsertion of an exotic Colonsay- Proterozoic terrane of the type found in N America but that west Islay terrane is best regarded cautiously. the more immediate source could have been the Torridonian The configuration of the Colonsay-west Islay terrane Supergroup of NW Scotland. Thechert, jasper and and its relationships with neighbouring Archaean and early ferruginous clasts described by Anderton have no known Proterozoic regimes have yet to be clarified. Our studies counterparts in the Colonsay-west Islay terrane, but that show that the high grade gneisses of Tiree, Col1 and Iona, basement could havesupplied theother components, islands tothe north of Colonsay, have much in common especially the abundant microcline. with the Lewisian of the Outer Hebrides. The boundary, or Recent isotope data from Ireland are also consistent with transition zone, between the Colonsay-west Islay terrane a Colonsay-west Islay-type terrane supplying detritus to the and the Lewisian therefore probably lies between Colonsay southwesternparts of the Dalradian Supergroup. Daly & and Iona. These islands are separated by the Great Glen Menuge (1989) haveshown from Sm-Nd datathat Fault, however, so the original junction has probably been

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dismembered andperhaps rendered indecipherable.It is FITCHES,W. R. & MALTMAN,A. J. 1984. Tectonic development and considered inadvisable, on present evidence, to speculate on stratigraphy at the westernmargin of the Caledonides:Islay and the apparent coincidence of the Great Glen Fault with the Colonsay, Scotland. Tramactions of the Royal Society of Edinburgh: Earth Sciences, 75, 365-382. junction. HALL,J., BREWER,J. A., MATTHEW,D. H. & WARNER,M. R. 1984. Crustal One potential method of assessing the broad configura- structure across the Caledonides from “WINCH’ seismic reflection tions of theArchaean and early-mid-Proterozoiccrustal profile: influences on the evolution of the Midland valley of Scotland. components of Scotland and Ireland, now largely hidden by Tramactions of the Royal Society of Edinburgh: Earth Sciences, 15, 97-109. the Upper Proterozoic cover, is to use information offered HARRIS,A. L. & PITCHER,W. S. 1975. The Dalradian Supergroup In: by the Dalradian tillite formation. Its clasts probably include HARRIS,A. L. et al. (eds) A correlation of Precambrian rocks inthe samples of those older rocks. By this method,under British Isles. Special Report of the Geological Society, London, 6, 52-75. evaluation,a clearer picture of the floor to thenorthern HOWARTH,R. J. 1971. The Portaskaig Tillite succession (Dalradian) of Co. Caledonides, and particularly the Dalradianbasin, may Donegal. Proceedings of the Royal Irish Academy, 71B, 1-35. -, KILBURN,C. & LEAKE,B. E. 1966. The Boulder Bedsuccession at emerge. Glencolumkille,County Donegal. Proceedings of the Royal Irish Academy, 65, Section B, 117-156. M. Bentleyand R. J. Muiracknowledge NERCstudentships. KILBURN,C., PITCHER,W. S. & SHACKLETON,R. M. 1965. The stratigraphy and origin of the Portaskaig Boulder Bed Series (Dalradian). Geological Institute Earth Studies Publication No. 73. of Journal, 4, 343-360. MARCANTONIO,F., DICKIN, A. P,, McNurr, R. H.& HEAMAN,L. M.1988. A 1,800-million-year-old Proterozoic gneiss terrane inIslay with implica- References tionsfor the crustal structure andevolution of Britain. Nature, 335, 62-64. ANDERTON,R.1980. Distinctive pebbles as indicators of Dalradian MUIR,R. J., FITCHES, W.R. & MALTMAN,A. J. 1989. An Early Proterozoic provenance. Scottish Journal of Geology, 16, 143-152. linkbetween Greenland andScandinavia in the Inner Hebrides of - 1985. Sedimentation and tectonics in the Scottish Dalradian. Scottish Scotland. Terra Abstracts, 1, 5. Journal of Geology, 21, 407-436. PRINGLE,I. R. 1972. Rb/Sr age determinations on shales associated with the -1988. Dalradian slides and basin development: a radical interpretation of Varanger Ice Age. Geological Magazine, 109, 46-472. stratigraphy and structure in the SW and central Highlands of Scotland. RODDICK,J. C. & MAX,M. D. 1983. A Laxfordian age from the Inishtrahull Journal of the Geological Society, London, 145, 669-678. Platform, County Donegal, Ireland. Scottish Journal of Geology, 19, BENTLEY,M. R. 1986. The Tectonics of Colonsay,Scotland, PhD Thesis, 97-102. University College of Wales, Aberystwyth. ROGERS,G., DEMPSTER,T. J., BLUCK,B. J. & TANNER,P.W. G. 1989. A - 1988. The Colonsay Group In: WINCHESTER,J. A. (ed.) Later high precision U-Pb age for the Ben Vuirich granite: implications for the Proterozoic Stratigraphy of the Northern AtlanticRegions. Blackie, evolution of the ScottishDalradian Supergroup. Journal of the 119-130. Geological Society, London, 146, 789-798. -, MALMAN,A. J. & FITCHES,W. R. 1988. Colonsay and Islay: a suspect SPENCER,A. M. 1969. Late Pre-Cambrian glaciation in Scotland. Proceedings terrane within the Scottish Caledonides. Geology, 16, 26-28. of the Geological Society, London, 1657, 177-198. DALY,J. S. & MENUGE,J. F. 1989. Nd isotopic evidence for the Provenance - 1971. Late Pre-Cambrian Glaciation in Scotland. GeologicalSociety, of Dalradian Supergroup sediments in Ireland. Terra Abstracts, 1, 12. London, Memoir 6. DUNNING,F. W. (ed.) 1985. Geological Structure of Great Britain, Ireland and - 1975. Late Precambrianglaciation in the North Atlantic region In: Surrounding Seas. Geological Society, London, Mapchart. WRIGHT,A. E. & MOSELEY,F. (eds) Ice Ages: Ancient and Modern. See1 EYLES,N. & CLARK,B. M. 1985. Gravity-induced soft-sediment deformation House Press, 217-240. in glaciomarine sequences of the Upper Proterozoic Port Askaig TREAGUS,J. E. 1969. The KinlochlagganBoulder Bed. Proceedings of the Formation, Scotland. Sedimentology, 32, 789-814. Geological Society, London, 1654, 55-60.

Received 25 October 1989; revised typescript accepted 5 January 1990

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