Journal of the Geological Society, London, Vol. 151, 1994, pp. 307-314, 5 figs. Printed in Northern Ireland

A late Caledonian melange in Ireland: implications for tectonic models

D.M.WILLIAMS,' J. HARKIN,'H.A. ARMSTRONG* & K.T.HIGGS3 I Geology Department, University College, Galway, Ireland 'Department of Geological Sciences, Durham University, South Road, Durham DH1 3LE, U.K 'Geology Department, University College, Cork, Ireland

Abstract: The Clew Bay area in western Ireland contains the remnants of a Caledonian terrane which separates the Dalradian of North Mayo from the Ordovician of South Mayo to the south. Rocks of thearea have been variouslyinterpreted as representing part of the Dalradiansuccession, as a Cambrian to Ordovician rifted margin to subduction related basin, and as a shear carpet derived from an accretionary wedge overridden by an ophiolite in the early Ordovician. The lithologies present include discontinuous outcrops of ultrabasic and basic rocks, together with quartzose and semipelitic schists on the south shore ofClew Bay. These are in contact to the north with a sedimentary and volcanic sequence exposed both on the mainland and on Clare Island for which a detailed stratigraphy had previously been established. We reinterpret this latter sequence as a melange containing blocks up to 500 m long of sandstone, conglomerate, chert and volcanics. Although microfossil evidence had previously shown thata chert block on Clare Island was of Late Llanvirn age, new fossil control shows that this date must be extended upwards. Microfossils extracted from a variety of lithologies within the melange showthat the cherts range from Middle Ordovicianto at least Caradoc. Further, spores extracted from the melange matrix show that the formation of the melange took place in the Silurian, probably in the Wenlock or later. These data indicate that the melange is unrelated to any early Ordovician obduction event and in fact represents the effects of a significant tectonic episode in the late Silurian of the British and Irish Caledonides.

The Clew Bay areain western Ireland lies betweenthe usage of the term Deer Park Complex by Phillips (1973) to South Mayo inlier of Ordovician rocks (Arenig-Llanvirn or denote the association of high grade schists and amphibol- later) to the south and the Dalradian of north Mayo to the ites. In view of evidence presented in this paper we suggest north. The pre-Devonian rocks of the area consist of several that the terminology be simplified and that the high grade elementswhose mutual relationships are in partunclear. rocks be termed informally the Deer Park Schists, as used Lower Devonian coarse clastics are restricted to the eastern by Max (1989), and that thewhole association be termed the end of thebay (Fig.1). Amongstthe pre-Devonian rocks Clew Bay Complex. aretwo Silurian successions, theLouisburgh Silurian and In the Clew Bayarea the northern boundary of the the Silurian of CroaghPatrick. These two units exhibit complex is defined by itsfaulted contact with Dalradian differences in stratigraphy, structure and metamorphic grade rocks on the small island of Achill Beg,south of Achill and may beallochthonous with respect toeach other Island. However,north of thiscontact a small sliver of (Williams & Harper 1991). Other rocks consist of a variety metamorphicrocks hasbeen equated with the Birchy of lithologies which may broadly be divided into two. The Complex of Newfoundland andthus possiblyof Lower first consists of a series of semipelitic and psammitic schists Cambrian age(Winchester et al. 1992). Thesouthern with amphibolites associated with serpentinites. These may boundary isa complex fault zone running E-W along the be coeval with the schists but show possible remobilization south shore of Clew Bay. This fault zone involves slivers of and intrusivecontacts with younger(lower-grade) rocks the CroaghPatrick Silurian. The eastern extent of the (Phillips 1973). Theserocks are in tectonic/stratigraphic complex is as yet unresolved. contact with the second group, sedimentary rocks of very low metamorphic grade (lowermost greenschist facies). This associationconsists of arenites,conglomerates, shales, Nature of the sedimentary rocks of the complex spilites, chertsand occasional (?chemical)limestones. A The Ballytoohy Formation on Clare Island was subdivided detailedstratigraphy for this sequence was established by stratigraphically by Phillips (1973) into a number of Phillips (1973), who recognized several formations, some of membersaffected byfive phases of tectonicdeformation. which weredivided into members, both on themainland The Killadangan Formation(mainland) has also been andmore especially on ClareIsland. These formations divided intofour members by Max(1989), with two consist of the Killadangan Formation (south shore of Clew recognizable phases of tectonicdeformation. However, Bay),the Ballytoohy Formation (Clare Island), the South none of these members can be shown to be in sedimentary Achill BegFormation (south of Achill Island)established contact with eachother. Soft-sediment deformation has by Max (1989) and possibly the Letter Formation in eastern been recognized asforming an important part of the Clew Bay (Graham & Smith 1981). TheBallytoohy and deformation of these two formations (N.Hoey, unpubl. BSc Killadangan Formations were thought to be equivalents by thesis, U.C.Galway 1987; Max 1989). In fact Max suggested Phillips (1973) and Max(1989). This whole association of thatpart of the Ballytoohy Formation was a mixtite. high and low graderocks was termed the ClewBay Re-examination of exposures on Clare Island has led us to Supercomplex by Harper et al. (1989) due to the previous the conclusion that the whole of this formation is a melange. 307 308 D. M. WILLIAMS ET AL.

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Fig. 1. Schematic geology of the Clew Bay area. Black, Clew Bay Complex (L, Letter Formation); dots, Dalradianof North Mayo; diagonal lines, pre- Dalradian basement; d, Devonian clastic c' Ireland rocks; S, Silurian of Clare Island and Louisburgh; sl, Silurian of Croagh Patrick; c, Carboniferous;A, Achill Ordovician of South Mayo Island.

Whilstinland exposures are generally insufficient to possibly fragmental, beds of grey sandstone are set obliquely demonstrate this unequivocally, the excellent exposures on to the cleavage, and protrude as glaciated bosses-above the thedangerous cliffs of the islandshow classic melange generalsurface of theground formed of thecontaining structures.Large blocks of areniteup to 10m long are slate'. enclosed in asqueezed, blackshaley matrix. Blocks of bedded chert/siltstone, still maintaining bedding coherence, are also enclosed. These blocks are of varying shape from Age of the melange irregularto rectangular torounded (Fig. 2a). Individual The rocks of the Clew Bay Complex were first ascribed to beds of arenite exhibit progressive disruption features with theDalradian (Phillips 1973).Rushton & Phillips (1973) coherent beds becoming separated by extensional mud-filled published an account of a fossil sponge (Protospongia detachments. The chert and siliceous siltstone blocks do not hicksi) from the Siorr Chert Member on Clare Island which showthese soft-sedimentary features. The blackshale they suggested implied a Cambrian age and thus confirmed matrix of the melange frequently exhibits an intense fabric to them the Dalradian association of the rocks. Cherts from which is wrappedaround enclosed clasts. The matrix, this member were subsequently examined by Harper et al. containingsmall clasts, intrudes large clasts of arenite (1989). Extracted microfossils suggested a late Llanvirn age showingthat melange formation was synchronous with for the cherts. The Letter Formation (of unknown affinities) sedimentarydisruption of somearenites (Fig. 2b). Also, at the eastern end of Clew Bay yieldedchitinozoa with a whilst much of the black shale matrix carries this fabric, the possiblemid- to lateOrdovician age (Graham & Smith areniteblocks commonly do not.Exposures of breccias 1981). occasionally demonstrate a black mudstone matrix without In an effort to constrainmore tightly the age of the this fabricand large mudstone blocks within arenites also melange,samples were taken from widera range of lack fabric. lithologies thanpreviously to analyseformicrofossil This evidence shows that much of the initial deformation content. These lithologies were: was accomplished while some of the sediment was still only (a) cherts from the Siorr Chert Member of Phillips (1973) partly consolidated. This proximity of high- and low-strain on ClareIsland, herein interpreted as part of a elements in melanges is confirmatoryevidence of a melange mega-clast; soft-sedimentorigin, asfor example in theDunnage (b)chert pebbles from conglomerates in the Ballytoohy Melange of Newfoundland (Jacobi 1984). Extrapolating the Formation on Clare Island, and chert blocks from the coastal exposures into the inland areas on the island implies Killadangan Formation on the south shore of Clew Bay that some enclosed blocks are up to 500 m long (Fig. 3) and also probably representing mega-clasts; still retain an internal coherence. This size is by no means (c) mudstone flakes (rip-up clasts) within arenites from the unusual in melanges. Clasts of chert, serpentinite or basalt island and the mainland. up to 2 km long occur in the Vara Complex of the northern (d) black shalesfrom the melangematrix on thesouth Appenines for example (Naylor 1981). shore of Clew Bay. We suggest therefore that these mega-blocks constitute The results indicate a radically new age range for these themembers of thestratigraphy of Phillips (1973). The rocks of the Clew Bay Complex. Cherts sampled from the KilladanganFormation on the mainlandcontains similar Siorr Chert Member (Fig. 4) yielded a possible mazuelloid lithological associations tothe Ballytoohy Formation,and together with the faunule mentioned in Harper et al. (1989). although of more limited exposure also shows evidence of Thechert pebble samples yielded no data. A chert block soft-sedimentdisruption without a coherent stratigraphy. from within the melange on the mainland yielded a probable Here massiveconglomerate beds exhibit progressive S-element of the conodont Periodon (too fragmentary to be disruptionwith resulting blocks up to 2m across(and figured), having a large proclined cusp and short posterior possiblymuch larger) enclosed in the shalymatrix. In an process. Siliceous mudstone flakes from within conglomer- early description of these rocks on Clare Island (Cole et al. ates and sandstones on Clare Island showed the presence of 1914) the implication of their melange nature was realized five specimens of mazuelloids with a distinctive double wall when theauthors wrote 'A fewthick non-continuous, structureand smooth morphology. Also containedwere LATE CALEDONIAN MELANGE IN IRELAND 309

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Fig. 2. (a) Detached block of arenite (dotted) with bedding preserved, enclosed in shaly matrix (dashed lines).(b) Necking of arenite block with soft-sedimentary melange intrusion into extensional areas (hammer is 33 cm long). (c) Mega-clast of conglomerate within the black shaly matrix of the melange (pencil for scale). (d) Arenite block with irregular outlines enclosed in melange matrix (block is approximately 1 m in long dimension). (e) Typical field appearance of the melange with large arenite blocks separated by shaly matrix with fabric. enclosing smaller clasts.

chitinozoa in a simple chain of three. The best preserved in assemblage of triletemiospores (Fig. 5). The sporesare the chain is ascribed to Conochitina primitiua Eisenack (Fig. dark,brittle, and commonlypitted with pyrite which was 4). Black shales from the melange matrix on the mainland removed in the analysis. One samplecontains simple were processed for palynological study. All samples yielded one-layered, unornamented forms with a clear trilete mark relatively small amounts of organic residue including plant which closely resemble the genus Arnbitisporites. However, material.Two samples revealed poorly a preserved the majority of themiospores are equatorially-thickened 310 D. M. WILLIAMS ET AL

Fig. 3. Geological sketch map of the Ballytoohy Formation on Clare Island. Inland exposures are separated by unexposed ground (blank) but coastal exposures consist entirely of melange which is occasionally exposed inland. one-layeredspores with a distal verrucate or muronate (Aldridge & Armstrong 1981). However the Llandeilo age ornament.Due to poor preservation these forms are not previouslyobtained from this samechert (Harper et al. positively identified here; however they are comparable to 1989) suggests that the mazuelloid may be older than any so the genera Synorisporites and Actinosporites. There are also fardescribed. Typically Ordovician mazuelloids appear to severalpoorly preserved reticulate forms which appear be most abundant in deep slope facies often associated with similar to Chelinospora or Brochotriletes. Finally,several radiolariancherts anddeep water conodonts. The specimens of cryptosporesare present and the forms are chitinozoan Conochitinaprimitiva Eisenack was recovered tentativelyassigned thetogenera Dyadospora and from siliceous shale fragments in arenites from Clare Island. Segestrespora. Alsopresent in theorganic residue are In the Balto-Scandinavianregion C. primitiva ranges microscopic plant fragments (Phytoclasts) such as structured through the Volkhov(Arenig) to Johvi(Caradoc) stages tissue and banded tubes (Fig. 5). These tubes show annular (Grahn 1984). However the presenceof mazuelloids in these and spiral internal thickenings and may represent primitive samples suggests that they are Caradoc or later. Since these vascular tissue. arenites and associated conglomerates often containclasts of In terms of spore colouration, a thermal alteration index siliceous shale,siltstone and chert it appearsthey must of 4-5 on the Staplin scale (1969) is indicated. Correlation post-datedeposition of thechert-shale association.Main- of this value with publishedthermal alteration indices of land shale samples contain spores of one-layered trilete type conodontsand palaeotemperatures suggeststhese rocks with a distal ornament of verrucate and muronate elements, have undergone thermal alteration in the region 250-300 "C. indicating thatthe assemblage is noolder than Wenlock This may be compared with a thermal alteration index value (Edwards & Burgess1990). The Ambitisporitestype fromthe fossils in thecherts of Clare Islandindicating ranges down to the Llandovery. The cryptospores from the heating to a value of at least 400 "C. same samples are less well constrained in age but in broad Theconodont Periodon fromcherta block in the termsdyads and pseudodyads appear in theCaradoc and melangeon the mainlandsuggests an age of early or become very rare at the top of the Gedinnian. mid-Ordovician. The state of preservation is too poor to be figuredand does notallow species identification. The mazuelloid from the chert block on Clare Island suggests an Origin of the melange age of Caradoc or later. Mazuelloids are known to range Melanges may form in anumber of geologicalenviron- from the Caradoc (Burrett 1985) to possibly the Devonian ments.Jacobi (1984) listed 16different sites at which

Fig. 4. Some microfossils from the Clew Bay Complex. Figures in brackets are magnification values. (1-10) Panderodus sp. elements (63); (11-12) ?Profopanderodussp. (84); (13-14) Indeterminate oistodiform elements (63); (15-16) Drepanoisfodus sp., r-element (42); (17-18) Panderodus sp. asymmetrical q-element (63); (19-20) Paltodiform element (63); (21-22) Conical elements (84); (23-24) 'Dapsilodus' sp. element (63); (25) slender tube (42); (26) Conochifinaprimitiua (87); (27) Echinoid spine (86); (28) External of mazuelloid (39); (29) Internal of mazuelloid (44);(30) High magnification of mazuelloid showing double wall structure (217).

Fie. 5. Some miospores from the Clew Bay Complex. All magnifications X 1000 unless otherwise stated.(1) Arnbitisporifes sp; (2) Arnbirisporires ?sp; (3) Cingulate miospore; (4) muronate miospore with gaping trilete mark; (5) verrucate miospore with gaping trilete mark; (6) unknown palynomorph, possibly a cryptospore polyad; (7) reticulate miospore? (8) apiculate miospore with labrate trilete mark;(9) banded tubes, possibly vascular tissue; (10) structured plant tissueX750; (11-12) Segesfresporu sp., a pseudodyad showing central encircling dark band and thin outer envelope. LAT E CALEDONIAN MELANGE IN IRELANDCALEDONIAN INMELANGE LATE 313 melanges involving oceanic elements may occur. Those not and in common with many other authors equated the Clew involvingsuch elements mayalso occur in passivemargin Bay Complex with the Highland Border Complex in environments.From the evidence cited above we believe Scotland. The importance of the complex to tectonic models that the Clew Bay melange was originally generated as an was emphasized by Dewey & Shackleton (1984). They olistostrome,i.e. a chaotically deformed body with some proposed an early Ordovician northward emplacement of a stratigraphic order preserved and producedwhile in a partly large ophiolite sheet onto the Dalradian of the Laurentian consolidated state by gravitysliding (Robertson 1977). margin in this area, generating D2-D3 Dalradian structures. Certainly it hasbeen subjected to subsequenttectonic The Clew Bay Complex in this context was envisaged as a deformationbutthe original dismemberment of the shear carpet over which the ophiolite advanced northwardin stratigraphy was caused by gravitationalcollapse. The Tremadoc to Arenigtimes. Dewey & Ryan (1990) serpentinites and associated basic rocks poorly exposed on perpetuated this view and saw the Clew Bay Complex as an the south shore of Clew Bay have been interpreted as the accretionaryprism related to northwardobduction of an remnants of a dismembered ophiolite by Ryan et al. (1983). ophiolite upon which the SouthMayo volcanic arc was Theserocks arejuxtaposed against, and tectonically originally founded. interleavedwith, both the Deer Park Schists and the The new data presented here place constraints on such KilladanganFormation (melange). However the melange models. Many models relating the Clew Bay area to a site of comprising the Ballytoohy and Killadangan Formations does Ordoviciansubduction/obduction employ the geology of not contain mega-clasts of an ophiolitic nature. Mega-clasts AchillIsland as supporting evidence. This includes the aredominated by quartz-richsandstones and quartz presence of aserpentinite melange described by Kennedy conglomerates.These are mixed with minor amounts of (1980) and the presence of crossite in blueschist facies rocks cherts, spilites andrare limestones. No clastsindicating described by Gray & Yardley (1979). It must be noted that high-Pllow-T conditions such as blueschist fragments have boththese occurrences are withinrocks which are been found.The content of the melangeindicates the considered to be Dalradian. The serpentinite melange is of presence of deep-waterassociations (chert, black shale, very localizedand narrow extent and was considered of spilite) mixedwith latersandstones and conglomerates soft-sediment origin by Kennedy (1980). Thus in view of the whose petrology, nature and thickness suggest proximity to probablepre-Cambrian age of theDalradian (Rogers et a continental source area. The new age dates obtained for al. 1989) this small melange must be considered much older the melange indicate that whilst it containsmega-clasts of thanOrdovician events. Similarly the blueschistmeta- Llanvirn toCaradoc age its generation occurred in the morphism present was stated by Gray & Yardley (1979) to Silurian. Theapparently undisturbed stratigraphy exposed haveoccurred early in themetamorphic history of the south of AchillIsland (South Achill Beg Formation) Dalradian and must therefore also be pre-Cambrian in age, suggests that these rocks may post-date melange generation i.e.pre-D3. The earlyCambrian age proposed for these or may be lateral equivalents. This may also be true of the rocks by Winchester et al. (1992) also precludes an origin in sediments of theLetter Formation to theeast although the Arenig. exposures do notallow for a definite correlation with the Models involving the northward obduction of ophiolite Clew BayComplex. The Silurian of CroaghPatrick has sheets in the British and Irish Caledonides face difficulties longbeen assumed to beunconformable on what is now with the timing of such events. There is no evidence of the termedthe Clew Bay Complex (e.g. Stanton 1960). Clew Bay melange having been overridden by a 10-15 km However,the so-calledbasal conglomerate of the thickophiolite. High-strain zones are localized and in CregganbaunFormation is nowhereseen in sucha general thearenaceous lithologies show little evidence of relationship with other rocks. In fact this conglomerate (of significant recrystallization or shearing. The vergence of Llandovery age) outcrops in a number of northward-dipping structureswithin the melange (thrusts, cleavage and some thrust-bound slices within rocks of the Clew Bay Complex folds) is to the south and not to the north as implied by such on the south side of Clew Bay. Thus the age of the melange models. A significant fraction of thesedimentary clast is constrainednot by this supposedLlandovery lithologies in themelange is of oceanicaspect, and this unconformity. means that such anophiolite must have overridden the melange in anorthward direction prior to obduction onto the continental margin. Thus the proposed ophiolite cannot Implications for Caledonian tectonics have been finally obducted onto such a margin until at least The rocks of Clew Bay have been interpreted in a variety of the late Silurian. ways, Phillips (1973) consideredthe sedimentary elements The proposal by Dewey & Ryan (1990) thatthe Clew of the Clew Bay Complex to represent the results of latest Bay Complexrepresents an accretionary wedge related to Dalradian sedimentation, whilst the Deer Park Schists were southward-directed subduction must also be reviewed. This seen by him as the remnants of a pre-Dalradian horst within model implies the generation of the Arenig volcanic arc of theDalradian basin of deposition.Ryan er al. (1983) South Mayo by southward subduction followed by a reversal interpreted the basic igneous rocks on the south shores of of subduction direction so that the South Mayo trough was a Clew Bay representingas dismembered a ophiolite fore-arc basin in the early Ordovician and a back-arc basin separatingthe ortho- and para-tectonic Caledonides. Both in the mid-Ordovician.This model was suggested by these interpretations were based ona supposed Dalradian Williams (1990),who did not interpretthe Clew Bay age for the Clew Bay Complex. Max (1989) suggested that Complex to be an integral part but rather suggested that it the Clew BayComplex originated at a Cambrian passive represented a separate terrane to South Mayo for at least rifted margin which later developed into a subduction zone some of theOrdovician. Since the melange formation has along the FairHead-Clew Bay line. Harper et al. (1989) beenshown to be Silurian it is unlikely to berelated to obtained an Ordovician age for a chert from Clare Island Arenig or pre-Arenigsubduction and arc generation. The 314 D. M. WILLIAMS ET AL. actualage of themelange and some of itssedimentary EDWARDS. D.& BURGESS, N.D.1990. Tcrrestialization-Plants. In: BRIGGS, components does not, of course, determine the age of other E.G. & CROWTIIER,P.G. (eds) Palaeobiology:a synthesb. Blackwell components such as the amphibolites andschists of the Deer Scientific Publications, 60-64. EISENACK,A. 1939. Chitinozocn und Hystrichosphaerideen im Ordovicium Park Schists. des Rheinischen Schiefergcbirges. Senckenbergiana. 21, 135-152. Whilst it is not thepurpose of thispaper to suggest GRARAM. J.R.& SMI.I.H, D.G.1981. The age and significance of a small lower alternativemodels for this part of theCaledonides, the Palacozoic inlier in County Mayo. Journal of Earth Sciences of the Royal tectonic setting of the melange can broadly be summarized Dublin Society, 4, 1-5. asfollows. Initial sedimentationtook place in deepwater GRAIIN,Y. 1984. Ordovicianchitinozoa from Tallinn, northcrn Estonia. Reviews of Palaeontology and Palynology, 43, 5-31. with associatedoceanic material. Deepwater associations GRAY, J.R.& YARDLEY,B.W.D. 1979. ACaledonian blueschist fromthe persistedfrom at least the Llanvirn tothe Caradoc. The Irish Dalradian. Nature, 278, 736-737. basin of depositioncontracted subsequently to allow the HARPER, D.A.T., WILLIAMS, D.M.& ARMSTRONG, H.A. 1989. Stratigraphical approach of a continental margin from one or both sides. correlationsadjacent to the Highland Boundary Fault in the west of Ireland. Journal of the Geological Society, London, 146, 381-384. The basin was not proximal to a significant arc structure due JACOBI, R.D.1984. Modern submarine sediment slides and their implications tothe absence of large amounts of volcaniclasticdebris. for melange and the Dunnage Formation in north-central Newfoundland. Initial collapse of thestratigraphy was by soft-sediment Geological Society of America, Special Papers, 198. failurepossibly in thelate Ordovician to mid-Silurianand KENNEDY,M.J. 1980. Serpentinitc-bearing melange in the Dalradian of Co. Mayo and its significance in thedevelopment of theDalradian basin, the whole was tectonically deformed by strike-slip faulting Journal of Earth Sciences, Royal Dublin Society, 3, 117-126. and/or thrusting no earlier than the Wenlock. MAX,M.D. 1989. The Clcw Bay Group:a displaccd terranc of Highland Detailed tectonic correlations of the Clew Bay Complex BorderGroup rocks (Cambro-Ordovician) in northwestIreland. will be presented in a later paper. However it is apparent Geological Journal, 24, 1-17. NAYLOR,M.A. 1981. Debris flow (olistostromes)and slumping on a distal that significant tectonic and sedimentary Caledonian events passive continental margin: the Palombini limestone-shalesequence of related to basin closure and collisions of this age can be seen the northern Apennines. Sedimentology. 28, 837-852. from Newfoundland to Scandinavia. In the Dunnage Zone F'HILLIPS, W.E.A. 1973. The pre-Silurianrocks of ClareIsland, Co. Mayo, in Newfoundland, late Ordovician to Silurian melanges are Ireland, and the age of the metamorphism of the Dalradian in Ireland. Journal of the Geological Society, London, 129, 585-606. common (e.g.Arnott 1983). In Scandinavia there is ROBERISON, A.H.F. 1977. The Moni Melange,Cyprus: an olistostrome evidence for the accretion of Ordovician oceanic crust prior formed at a destructivc plate margin. Journal of the Geological Society, tothe Ashgillfollowed by thecreation of second-phase London, l33,447-&. basins founded on new oceanic crust in the late Ordovician ROGERS,G., DEMPSTER,T.J., BLUCK, B.J.& TANNER,P.W.G. 1989. A high and Silurian. It has become apparent that the Silurian was a precision U-Pbage for the Ben Vuirich granite: implications forthe evolution of the Scottish DalradianSupcrgroup. Journal of the timewhen the last vestiges of Iapetusoceanic crust were Geological Society, London, 146,789-798. beingdestroyed, coeval with volcanism and collision of RUSHTON,A. & PIIILLIPS.W.E.A. 1973. Aspcciman of Protspongia hicksi smallermarginal basins (Soper et ~1.1992;Williams et al. from the Dalradian of Clare Island, Co. Mayo, Ireland. Palaeontology, 1992). 16, 223-230. RYAN,P.D., SAWAL, V.K. & ROWLANDS,A.S. 1983. Ophioliticmelange separatesortho- and para-tectonic Calcdonidcs in westcrnIreland. J.H. thanksEolas and Donegal Co. Council for financial support Nature, 301, 50-52. and D. McLaughlin, Westport for assistance. We thank the referees SOPER, N.J., STRACHAN, R.A.,HOLDSWORIH, R.E., GAYER,R.A. & GREILING, R.O.1992. Sinistral transpression and the Silurian closure of for their helpful comments. lapetus. Journal of the Geological Society, London, 149, 871-880. SrANTON, W.I. 1960. Lower Palaeozoicrocks of south west Murrisk.Eire. References Quarterly Journal of the Geological Society of London, 116, 269-296. SrAPLIN, F.L. 1969. Sedimentary organic matter, organic metamorphism and ALDRIDGE, R.J. & ARMSIRONG. H.A.1981. Sphcrical phosphatic microfosils oil and gas occurrcncc. Canadian Petroleum GeologistsBulletin, 17, from thc Silurian of North Grccnland. Nature, 292, 531-533. 47-66. ARNOIT, R.J. 1983. Scdimcntology of Uppcr Ordovician-Silurian scqucnccs WILLIAMS, D.M.IY90. Evolution of Ordovicianterranes in westernIreland on New World Island,Ncwfoundland: scparatc fault-controlled basins? and their possible Scottish equivalents. Transactions of the Royal Society Canadian Journal of Earth Sciences. U), 345-354. of Edinburgh, Earth Sciences. 81, 23-29. BURREIT.C. 1985. Problcmatic phosphatic microsphcrcs (mazuclloids) from - & HARPER,D.A.T. 1991. End-Silurian modifications of Ordovician the Ordovician of Tasmania, Australia. Alcheringa, 9, 158. terranes in western Ireland. Journal of the Geological Society, London, COLE,G.A.J., KILROE. J.R., HaLLlssY. T. & ARBER,E.A.N. 1914. The 148, 165-171. geology of Clare island.County Mayo. Memoirs of theGeological -, O'CONNOR, P.D.& MENUGE,J. 1992. Silurian turbidite provenance and Survcy of Ireland. the closure of Iapetus. Journal of the Geological Society, London, 149, DEWEY,J.F. & RYAN,P.D. IW). The Ordovicianevolution of theSouth 349-357. Mayo trough, western Ireland. Tcctonics, 9, 887-903. WINCHESTER,J.A., WILLIAMS,H., MAX, M.D. & VAN STAAL, C.R. 1992. -& SIIACKLEI'ON, R.M. 1984. A model for thc cvolution of the Grampian Does the Birchy Complex of Newfoundland extend into Ireland? Journal tract in the carly Calcdonidcs and Appalachians. Nature, 3U, 115-121. of the Geological Society, London, 149, 159-162.

Rcccivcd 11 March 1993; rcviscd typescript accepted 15 June 1993.