Journal of the Geological Society, London, Vol. 148, 1991, pp. 165-171, 4 figs Printed in Northern

End-Silurian modifications of Ordovician terranes in western Ireland

D. M.WILLIAMS & D. A. T. HARPER Geology Department, University College, Galway, Ireland

Abstract: The Ordovician elements in the Caledonides of western Ireland may be divided into the Clew Bay, South Mayo, North Connemara and South Connemara sub-zones. Evidence suggests the presence of a number of proximal suspect terranes, and the remnants of one, or possibly two, arc complexes.Palaeontological and sedimentaryevidence suggests an allochthonousrelationship be- tween the Silurian rocks of Croagh Patrick and North Galway. These successions may therefore not represent a single sedimentary overlap to the Ordovician suspect terranes. End-Silurian deformation was the result of transpression, which was responsible in part for the present configuration of the area’s geology. Deep seated shear zones, with fuchsite and gold mineralization were foci for lateral translationconcomitant with southwarddirected thrusting, northward directed back thrusts and variable degrees of metamorphism being of higher grade in thenorth. The end-Silurianevent is intermediate betweenthe high grade metamorphism in parts of the Appalachians andthe non- metamorphic Silurian sequences of Scotland.

The Caledonides of westernIreland occupy a position overlain by the Silurian succession of North Galway as an spatially betweenthose of northeast Canada and Scotland overlap sequence (Fig. 1). (Fig. 1).The area exhibitsgood exposure of Dalradian, The South Connemara sub-zone consists of some 3.2 km Ordovician and Silurian sequences and is crucial tothe of Ordovician rocks (Ffrench & Williams 1984; Williams et understanding of the development of theorogen. Within al. 1988). These amphibolites, basic lavas,cherts, this part the Clew Bay, South Mayo, North Connemara and conglomerates and sandstones are intruded by the Devonian South Connemara sub-zones may berecognized. The Galway Granite which represents a stitching pluton cutting contactsbetween these sub-zones coincide with the both this sub-zone and that of North Connemara (Leggo et proposed terrane boundaries of some authors (Fig. 1). al. 1966). The Clew Bay sub-zone is bounded to the north by the Dalradian of North Mayo and to the south by the Silurian sequences of Louisburgh and CroaghPatrick. The Geological evidence for Ordovician terranes structurally lowest element comprises an association of Caledonianelements in westernIreland have figured amphibolites, serpentinites, gabbros and schists considered prominently in terrane accretion models. Dewey & to representanorthward dipping slab of dismembered Shackleton (1984) andHutton & Dewey (1986) cited the ophiolite (Anderson 1960; Ryan et al. 1983). These area as a locus of strike-slip terrane migration,intermit- meta-igneous rocks are adjacent to a melange unit cropping tently, from the Llanvirn tothe earlyDevonian. Hutton out along the southern shore of Clew Bay, the Killadangan (1987) envisaged the Ordoviciansequences of western Formation. Overlying thismelange-ophiolite sequence are Ireland as laterally accreted remnants of a single fore-arc to the basic volcanic rocks,shales, cherts and sandstones back-arc complex originally marginal tothe Laurentian exposed onClare Island (Phillips 1973). Microfossil continent. Excluding the rocks of the Delaney Dome in the evidence indicates an Ordovician age for this part of the North Connemara sub-zone,these Ordovician sequences Clew Bay sub-zone and the term Clew Bay Supercomplex exhibit certain similarities (Fig. 2). All contain thick basic hasbeen employed to encompassboth the ophiolite- igneous associations at their bases, probably of similar ages, melange and its cover (Harper et al. 1989). The Silurian of and overlain by sedimentary sequences ranging in age from rests unconformably on part of the supposed Arenig to ?Caradoc. These similarities could suggest a ophiolite (Phillips 1974). common origin as a single arc-related basin whose parts are TheSouth Mayosub-zone comprisesOrdovician rocks now seen in their original relative configuration. Aspects of with a basal 2.6 km of basic volcanic rocks and sedimentary the geology however imply that this present configuration is intercalations. These are overlain by approximately 7 km of a post-Ordovician feature. sedimentary rocks representingdeposition in progressively shallower environments,from thesubmarine fans of the Rosroe Formation and equivalents (Archer 1977) tothe The Clew Bay sub-zone alluvial fans and fan deltas of the Group (Williams The Clew Bay Supercomplexhas been equated with the 1984; Pudsey 1984). The age of this succession is Arenig to Highland Border Complex of Scotland (Hutton 1987; at least Llanvirn. Harper et al. 1989; Max 1989). Thus itscontact with the The North Connemara sub-zone comprisesDalradian Dalradian of North Mayo tothe north of Clew Bay is metamorphics in thrust contact with underlying remnants of probably the equivalent of the Highland Boundary Fault of Ordovician silicic volcanic rocks (Leake et al. 1983). The Scotland.However, the surface expression of this faultin Dalradianstratigraphy is comparableto that of the main Scotland and in Tyrone is anorthward dipping thrust. Scoto-Irish Dalradian outcrops to the north. Both the North Geophysical evidence suggests that this thrust obscures the Connemara and South Mayo sub-zones are unconformably original line of crustal discontinuity which lies to the north

165 166 D. M. WILLIAMS & D. A. T. HARPER

B::...!\‘”’:, , ....:...,.,p ,;: . .‘.‘..i......

_I sub-zone

South Mayo sub-zone

Fig. 1. (A) Location of western Irish Caledonides with respect to zonation of the Appalachians and Scotland. (B) Sub-zone divisions in western Ireland: CP, Croagh Patrick Silurian; FF, Far- nacht Formation; L, Louisburgh/Clare ___--- Island Silurian; NG, North Galway SC wth Connemara sub-zone Silurian.

Clew Bay South noye South Connemara underoverthrust Dalradian (Max & Rddihough 1975; Dentith et al. 1989). Bluck (1984) suggested that the age of this thrusting was Devonian. In western Ireland, at (Fig. l), a northerly dipping slide separates theLower and Middle Dalradian from a sequence correlated with the Southern Highland group(Kennedy 1980). On Achill this sequence consists of melange units and serpentinites interpreted as ophiolite fragments by Kennedy (1980). Gray & Yardley (1979) documented crossite in this sequence and recognized a blueschist assemblage although the regional grade is lower greenschist on south Achill Island, similar to the Ordovician rocks of Clare Island. Moreover the Achill sequence bears lithological similarities tothe melange- ophiolite sequence in southern Clew Bay. All were originally regardedas part of theUpper Dalradian by Kennedy (1980). Other fragmentaryoutcrops of anomalo- shalt!S usly low-grade rocks occur further east adjacent to the Ox Mountains (Long & Yardley 1979). Whilst these rocks are cherls as yet undated, some may be of Ordovician age. Geochemically someelements of volcanics in theOx 0 Mountains are more similar to those of Clare Island than the Z W Dalradian of North Mayo (Winchester et al. 1987). The K Q Clew Bay sub-zone may thus beregarded as a complex arrangement of Dalradian, ?Cambrian and Ordovician units now in mutual tectonic contact. Whilst the present contactbetween the Dalradian and Highland Border complex is in the form of a thrust or higher angle fault, the original relationshipbetween them is debatable. In Scotland despitesubstantial uplift of the Dalradianduring the Ordovician (Dempster 1985), no unequivocal Dalradian detritus hasbeen found in the sediments of the Highland Border Complex. Bluck (1984) therefore suggested that this Cambro-Ordovician basin was atsome distance from the Scottish Dalradianduring its evolution.Definite Dalradian detritus appears first in the Upper Devonian of the Scottish Midland Valley according to Bluck (1984), or Lower Devonian according to Haughton Fig. 2. Generalized lithostratigraphy of the main Ordovician & Farrow (1989). However, the revised dating of the Ben successions in western Ireland. Vuirich Granite in the Scottish Dalradian at 590 Ma has led O RD OV ICIA N TERRANES, WESTERN IRELANDORDOVICIANWESTERNTERRANES, 167

Rogers et al. (1989) to suggest an alternative view. The sub-zones (Hutton & Dewey 1986), compatible with a possible pre-Arenig emplacement of an ophiolite above the mutually allochthonousrelationship. Additionally, a vol- Dalradian may haveprovided ophiolitic ratherthan canic sequence in thenortheast part of the South Mayo Dalradian detritusto the Highland Border basin. Thus sub-zone, called the FarnachtFormation (Fig. 1) was Rogers et al. (1989) suggested the possibility of some considered a suspect terrane by Max et al. (1988). interaction between the Dalradian and this basin during the Ordovician. Irrespective of thedetrital lithologies infilling the basin, theamounts of this detritusare of equal The North Connemara sub-zone importance. Significant uplift of the Dalradian and possible The North Connemara sub-zone is thought by many authors ophiolite cover has left little trace of massive detrital input to be a terranedetached from the main Scoto-Irish in the Ordovician sediments of Clew Bay or of the Highland Dalradian block (Leake et al. 1984; Leake 1989; Hutton & Border complex (Curry et al. 1984; Williams 1990). Dewey 1986; Hutton 1987; Williams & Rice 1989). Certainly Geophysical and other evidence suggests that the Midland the substantial uplift experienced by the Dalradianduring Valley of Scotland was notunderlain by the Dalradian much of the Ordovician makes it unlikely that South Mayo during the Ordovician (Aftalion et al. 1984; Bluck 1985). At is underlain by Dalradian rocks. The apparent absence of least for much of the Ordovician, clearly the preserved parts typical Dalradian lithologies in the Arenig-Llanvirn detritus of the Highland Border basin and itsIrish equivalents of the South Mayo sub-zone led Bluck & Leake (1986) to evolved at somedistance from any major uplifting block. conclude that the two sub-zones were separated at least until Theirpresent proximity could be the result of orthogonal the Llanvirn-Llandeilo. Sediment derived from the general shorteningbut in view of the evidence of transpressional direction of Connemara into South Mayo is dominated by movements, discussed below, it seems more likely to be at granitoid detritus in the Arenig to early Llanvirn (Archer least partly the result of oblique slip. Thusthe Highland 1977). Granites of this age are not preserved in Connemara Border complex and itsIrish equivalents are suspect with with the exception of small granite sills (Leake 1989). These respect to the Scottish-Irish Dalradian. early Ordovician sediments (RosroeFormation) are of deep-water facies (Archer 1977), and are now immediately The South Mayo sub-zone adjacent to the Connemara Dalradian although the contact is overlain by the Silurian of North Galway (Fig. 1). A rapid The contactbetween the Clew Bay sub-zoneand that of phase of uplift at 480-490Ma affected Connemara (Barber South Mayo is obscured by Silurian rocks (Fig. 1).The & Yardley 1985; Leake 1989) thus suggesting that the two Silurian of Croagh Patrick is in fault contact with the Clew sub-zoneswere separated by atleast a fault in the Bay rocks and is apparently unconformable onthe Arenig-Llanvirn. Differences in REE patterns between the Ordovician of SouthMayo (Kelly 1976; Hutton & Dewey basic volcanics of the South Mayo sub-zone and the injected 1986). The Louisburgh-ClareIsland Silurian is unconfor- metagabbro-gneiss complex of Connemara, approximately mable on the Clew Bay supercomplex and in fault contact synchronous in age, suggested to Leake (1989) that the two with other rocks. Ageneralized lithostratigraphy of the were geographically separated in the earlyArenig. Later South Mayo Ordovician is shown in Fig. 2. The rocks are sediment of Llanvirn-?Llandeilo age contains abundant disposed in largea syncline and a satisfactory mutual metamorphicand silicic volcanic detritus (McManus 1967; correlation of the two limbs has not been fully established Pudsey 1984; Williams 1984), although this is not (e.g. Dewey 1963; Archer 1977; Pudsey 1984). The Arenig conclusively proven to have aprovenance in thenorth volcanic rocks arethought to be of island-arc affinities Connemara sub-zone. The simplest model explaining the (Ryan et al. 1980), possibly founded onan obducted presence of this anomalousDalradian block is that of an Tremadoc ophiolite (Dewey & Shackleton 1984). Some infill extensionally detached terrane, combined with sinistral of this basin was derived from the north in the Arenig and translation approaching the site of the South Mayo sub-zone containedophiolitic detritus. This led Wrafter & Graham in the Arenigand finally docking perhaps in the early (1989) to suggest that derivation of this material was from Llanvirn (Dewey & Shackleton 1984; Williams & Rice the Clew Bay ophiolite thus forming a sedimentary linkage 1989). The North Connemara sub-zone itself is envisaged by between the two in the Arenig. Williams (1990) made the Leake (1989) and Williams & Rice (1989) as that of an point that the Ordovician sedimentary histories of Clew Bay Ordovician arc founded on continentala (Dalradian) and South Mayo are so different as to suggest an original basement. separation. Thus if the Clew Bay ophiolite was the source of this Arenig detritus then the ClareIsland succession must be separated from the ophiolite sequence by a major fault or The South Connemara sub-zone terrane boundary. Most of the sedimentary input to South The South Connemara sub-zone is in fault contact with the Mayo was from the south in the form of submarine fans, Connemara Dalradian along the Skird Rocks Fault (Fig. 1). alluvial fans and fan-deltas (Archer 1977; Williams 1984; Elsewhere the contact is obscured by the Devonian Galway Pudsey 1984). TheSouth MayoOrdovician has been Granite. Geochemical analysis of the basalts suggested an modelled as representing theremnants of apre-Llanvirn ocean floor origin, perhaps during the early stages of an fore-arc basin evolving into a closing, middle-upper arc-marginal basin (Ryan et al. 1983). Sandstone and Ordovician back-arc basin (Williams 1990). Thisdevelop- conglomerate petrology indicated derivation from a recycled ment was the result of a change in subduction polarity. orogenprovenance (Ffrench & Williams 1984). The The complicated structural framework of the Clew Bay sequencehas been equated with the northern belt of the sub-zone makes it difficult to ascertainits original SouthernUplands of Scotland(Ffrench & Williams 1984; relationship with that of South Mayo. There is however Williams et al. 1988). Palaeocurrentswere principally evidence of sinistral shear along the boundary between the derived from the west with aminor component from the 168 D. M. WILLIAMS & D. A. T. HARPER south, similar butmore variedpalaeocurrents have been (1987). Metamorphism was also largely late Silurian since recordedfrom the meta-clast petrofacies of theNorthern Silurian rocks are affected by it. The FarnachtFormation Belt in Scotland by Morris (1987). Evidence of significant underwent a metamorphic episode with growth of biotite at strike-slip has been recorded from faults within the Southern 413 Ma (Max et al. 1988). Uplands. In fact Anderson & Oliver (1987) andElders Any attempt to synthesize the final stages of structural (1987) suggested thatthe Southern Upland Fault was a development of this sector must takeinto account the majorterrane boundary. Miller & O’Nions (1984) evidence of transpression, the presence of metamorphic demonstrated through Sm-Nd analysis thatthe Scottish shearzones andthe northwardincrease in metamorphic Dalradian was not a source of detritusfor the lower grade in the South Mayo sub-zone both in the Ordovician Palaeozoic rocks of the Southern Uplands. By analogy, the and Silurian, together with the relationshipbetween the Connemara Dalradian may not be the source of continental Silurian of Croagh Patrick and that of North Galway. The detritus in the South Connemara sub-zone. This sub-zone in supposedlateral equivalence of these two sequenceshas common with the Northern Belt in Scotland must therefore been used demonstrateto their contiguity andthe be regarded as a suspect terrane. continuous overstep of the Ordovician and Dalradian of the South Mayo and North Connemara sub-zones respectively (Hutton & Dewey 1986; Phillips 1974; Laird & McKerrow Late Silurian events 1970). Whilst it seems likely that some elements in western Ireland The poor preservation and tectonic deformation of the wereamalgamated during the Ordovician, the final faunas from the Silurian of Croagh Patrick have tended to configuration now seen is duetolate Silurian-early preclude a very precise and unequivocal assessment of their Devonianmovements. These movements are expressedas age. Kilroe (1907) recorded the presence of both thrust and oblique-slip contacts between many of the Pentamerus knighti and Pentamerus oblongus from what is tectonic elements involved. The contact between the Mayo now termed the Bouris Group. He correlated this with the Dalradian andthe Clew Baysub-zone has alreadybeen Wenlock and considered the Kilbride Formation of North discussed. Furthersouth the ophiolite-melangerocks of Galway (Fig. 3), now correlated with the uppermost Clew Bay are in tectonic contact with the Silurian of Croagh Llandovery (upper Telychian), to be of thesame age. Patrick. Within the Silurian there is evidence of sinistral Williams (in Anderson 1960) considered the shelly shear during D1 (Hutton & Dewey 1986), and the Croagh assemblages from the Bouris Groupto be of eitherlate Patrick area is seen by theseauthors aspart of a flower Llandovery or early Wenlock age.Anderson (1960) structure produced by sinistral transpression.Within the supporteda lower Wenlock correlation based onan SouthMayo sub-zone approximately E-W trending equivalence of the Cregganbaunconglomerate with the fuchsite-bearing shear zones exist in the northern half of the Gowlaun conglomerate of the North Galway succession. belt. Such shear zones are also present in the Croagh Patrick Reassessment of all existing collections from localities at Silurian (Young et al. 1989; Aherne et al. 1989). An both and Bourisschool, together with increased metamorphic grade in the northern parts of the investigation of new collections from bothareas, has SouthMayo sub-zone is matched by a similar contrast in confirmed the presence of Pentarnerus oblongus within a set Silurian rocks. Those at Croagh Patrick are at greenschist of Llandovery assemblages (Harper & Williams in prep.). facies (Kelly 1976) whereas those of North Galway are virtually unmetamorphosed.However these rocks also Galway-HayoBorder Croagh Patrlck Range exhibit evidence of oblique-slipin their deformation. The Salrock Fault which juxtaposesOrdovician rocks of the Fossil control (qeneralized) South Mayo sub-zone against the Silurian has a component + SalrockFm of transpression. Subordinateshear zones within this sequence confirm aductile phase of transpressionalong Lough Iluck Fm approximately E-W trends.Late thrusting is manifest as Glencraff Fm southward-dippingfaults emplacing theDalradian of the North Connemara sub-zone over the Silurian in a series of Le!tergesh Fm breached anticlines (Williams & Rice 1989). Thus Lough Nacorra Fm deformation of the North Galway Silurian can be related to Tonalee Fm sinistral transpression promoting northward and southward Kllbride Fm ’ BGUrlS Group Fm directedthrusts, approximately E-W extension with both Cregganbaun Frn major and minor riedel shear patterns. Due to the fact that Cregganbaun Conglomel*at e this Silurian sequenceunconformably overlies boththe Derryheagh Fm Ordovician of SouthMayo andthe Dalradian of North Connemara, these subzones would also have been affected Schema!ir lithologlcal key by thistranspressional episode. The fuchsite/gold-bearing shear zones are present in both Ordovician and Silurian Fine clastics Calcareous medium cIastIcs rocks implying that they are of late Silurian-early Devonian age. The association of intrusive quartz porphyries with one Fine-medlum clastlcs Red and green clastics of the shear zones in the Ordovician again suggests a late Medlum clastics Alternating flne and medium clastrcs stage origin since early Ordovician igneous activity consisted of basic volcanism (Ryan et aI. 1980; Young et al. 1989). The Coarse clastics Dominantly fine clastics final intrusion of the Corvock Granite post-dates much of Fig. 3. Proposed temporal correlation between the North Galway this deformation and is dated at 387 Ma by O’Connor et al. and Croagh Patrick Siluriansuccessions. ORDOVICIANTERRANES, WESTERN IRELAND 169

c, 170 WILLIAMSD. M. & D. A. T. HARPER

Although parts of this sequence exhibit mid-shelf associa- now located in the northern Ordovician of South Mayo and tions within a transgressive sequence, similar to horizons sympathetic structures within the Silurian of Croagh Patrick within the Kilbride Formation of North Galway, the latter is itself. Examples of such zones, which need not be characterized by Pentameroides in adiverse shelly fauna exclusively sinistral, include the fuchsite-bearing schistose (Harper & Doyle in prep.), and is thus younger. Current shearzones of Young et al. (1989). Additionally, the biostratigraphical dataforthe Llandovery suggest a anomalously high-grade schistose volcanic rocks of the correlation with part of theupper Aeronian to lower FarnachtFormation (Max et al. 1988) are bounded by Telychian (Cocks et al. 1984). The Croagh Patrick Silurian is mylonitic beltscontaining syn-metamorphic indicators of thusanomalous in having marine strata significantly older both sinistral and dextral shear. than those of the Louisburgh-Clare Island Silurian (Palmer It is suggested therefore, that the Silurian succession of et al. 1989) and of the North Galway Silurian (Williams & Croagh Patrick achieved itspresent position in the early Harper 1988). The resultingcorrelation (excluding the Devonian (Fig. 4A). Movement was dueto sinistral Louisburgh succession) is shown in Fig. 3. This correlation is transpression whose locus was in the Clew Bay area but strengthened by theappearance in the immediately whose effects extendedsouthwards intothe South Mayo overlying formationsin both CroaghPatrick andNorth sub-zone. These deep-seatedshear zones may have been Galway of large amounts of volcanic detritus together with responsible for exploiting an ophiolitic foundation to South tuff horizons (Williams & Harper 1988). The Bouris Group Mayo liberating chromium-rich fluids for the generation of however is underlain by a thick sequence of marine fuchsite (Young et al. 1989). Theend results of quartzites and conglomerates(Cregganbaun Formation). transpression were the generation of southwarddirected This, in turn, overliesa shallow marine sequence,the thrusting, together with northward-directed back thrusts. DerryheaghFormation (Fig. 4). Thus, braided river This thrusting was responsible in part for tectonic loading sedimentation inthe North Galway sector inthe Upper and increased metamorphic grades in the northern parts of Llandovery, was coeval with marine deposition in the South Mayo (Fig. 4B). The loading may also have been Croagh Patrick sequence. It is therefore unlikely that these responsible forthe protection of these two Silurian two sequences are now in their original configuration. If this sequences from subsequent erosion. were the case the northern provenance for the braided rivers Thus end-Silurian events in western Ireland represent an of the Lough Mask Formation would itself havebeen an intermediate milieu between those of Canada and Scotland. element of a subsiding basin receiving significant clastic End-Silurian deformation in Canada is manifest as input. allochthon emplacementand up-faulting of ophiolite The original site of the CroaghPatrick Silurian is a fragments (Colman-Sadd & Swinden 1984). This event also matter of someconjecture. Williams & Harper (1988) included major metamorphicepisodes (Plint & Jamieson suggested thatthe establishment of relatively stable shelf 1989) andterrane accretion(Goldsten 1989). Incontrast, conditions during a marine transgression was a diachronous Silurian deformation in the Midland Valley of Scotland event affecting the Silurian of the northern part of Ireland resulted in little or no metamorphism and aparautoch- andthe Midland Valley of Scotland.According to this thonous relationship between the Silurian inliers. scenario the Silurian sediments of CroaghPatrick would have been deposited some distance east or southeast of their References present position with respect to North Galway. This is not to suggest that this element constitutesa separate Silurian AWALION, M,,VAN BREEMEN,0. & BOWES,D. R. 1984. Age constraints on basement of the Midland Valley of Scotland. Transactions of the Royal terrane.Rather it may bea sequence that was sinistrally Society of Edinburgh: Earth Sciences, 15, 53-64. transposed to its present position over a distance of some AHERNE,S., REYNOLDS,N. A., BURKE,D. J., HARRINGTON, K.,O’KEEFFE, 40-50 km (Williams & Harper 1988). 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These two Transactions of the Royal Society of Edinburgh: Earth Sciences, 78, Silurian successions have an allochthonousrelationship. 223-225. That of Clare Island-Louisburgh is unmetamorphosed, it is ARCHER,J. B. 1977. Llanvirn stratigraphy of the Galway-Mayo border area, generally younger than that of Croagh Patrick (Palmer et al. Western Ireland. Geological Journal, U,71-98. BARBER,J. P. & YARDLEY,B. W. D. 1985. Conditions of high grade 1989), and the two are not lithostratigraphically similar. metamorphism in the Dalradian of Connemara, Ireland. Journal of the Currently there is no obvious single zone along which the Geological Society, London, 142, 87-96, movement of the CroaghPatrick Silurian may have BLUCK,B. J. 1984. Pre-Carboniferous history of the Midland Valley of occurred.However thenorthern Ordovicianunits in the Scotland. Transactions of the Royal Society of Edinburgh: Earth South Mayo sub-zone (Letterbrock,Derrymore and Sciences, IS, 275-295. -1985. 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Received 20 December 1989; revised typescript accepted 2 July 1990.