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Is the Colonsay-West Islay Block of SW Scotland an Allochthonous Terrane

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Is the Colonsay-West Islay Block of SW Scotland an Allochthonous Terrane 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 recumbent folds, 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 and petrographic 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/3/417/4890342/gsjgs.147.3.0417.pdf by guest on 02 October 2021 118 W. R. FITCHES ET AL. 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
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