A Late Caledonian Melange in Ireland: Implications for Tectonic Models
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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 of Clew 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. l .. 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 yielded chitinozoa 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