Terrane Assemblage of the Leinster Massif, SE Ireland, During the Lower Palaeozoic
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Journal of the Geological Society, London, Vol. 147, 1990, pp. 1035-1050, 10 figs, Printed in Northern Ireland Terrane assemblage of the Leinster Massif, SE Ireland, during the Lower Palaeozoic M. D. MAX’.3,A. J. BARBER’ & J. MARTINEZ’ ’Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland 2Department of Geology, Royal Holloway and Bedford New College, Egham, Surrey TW20 OEX, UK 3Present address: Naval Research Laboratory, Code 5110, Washington DC 203/5-5000, USA Abstract: Compilation of Irish Geological Survey mapping is used to construct a synthesis of the tectonic evolution of the Leinster Massif during the Lower Palaeozoic. The massif is interpreted as a series ofNE-SW trending tectonostratigraphic terranes of diverse origin and provenance brought together along major sinistral transcurrent faults. With minor discrepancies the age of these terranes decreases from SE to NW. The Precambrian basement Rosslare terrane is succeeded by the Early Carnbrian continental margin Cullenstown-Cahore terrane. In the Waterford-Wicklow terrane Late Cambrian-Early Ordovician accreted oceanfloor materials are overlain by a Late Ordovician volcanic arc. The Early to Mid-Ordovician accreted ocean floor Dublin terrane to the NW is overthrust by the Early Carnbrian continental margin Bray terrane and is followed to the NW by the mainly Silurian Midlands terrane. These terranes were juxtaposed after substantial sinistral movement along transcur- rent faults during the Silurian. Consolidation of the massif also took place from SE to NW. Stages in the consolidation are marked by the intrusion of the Saltees (437 Ma), Carrigmore (415 Ma) and Leinster (404 Ma) stitching plutons. Fault movement ceased before the deposition of Late Devonian and Carboniferous sediments on a linking unconformity across the massif. The Leinster Massif in SE Ireland (Fig. 1) forms an inlier of Bray Group rocksform the Brayand Cahore- LowerPalaeozoic rocks overlain unconformably by Cullenstown terranes on Fig. 1. Permo-Triassic,Carboniferous, and possibly UpperDevo- (3) Duncannon Group rocks extend NE-SW across the nianrocks. Lower Palaeozoic sediments, plutonic and massif from Courtown to Tramore and form fault-bounded volcanicrocks forming the massif weredeformed and lenses to the SW of Wicklow Town (Fig. 1). At Tramore intruded by theLeinster Granite batholith during the and Courtown these rocks rest unconformably on rocks of Caledonianorogeny. Four major lithological assemblages theRibband Group. The sequence commences with a have been distinguished in the Lower Palaeozoic rocks: (1) limestone, followed by black shales and then a thick series RibbandGroup; (2) Bray Group; (3) DuncannonGroup; of basaltic, andesitic and rhyolitic lava flows, agglomerates (4) Kilcullen Group (e.g. Bruck et al. 1979). and tuffs. Abundant fossils indicatethat the Duncannon (1) Ribband Group rocks in the central partof the massif Group is of Mid- toUpper Ordovician age (Crimes & arepredominantly fine-grained sediments, shales, mud- Crossley1968; Brenchley & Treagus 1970; Mitchell et al. stonesand siltstones, often graded and altered to slates, 1972; Gardiner 1974; Brenchley et al. 1977). with a fine-banded structure on the scale of a few to tens of (4) Kilcullen Group rocksoccur tothe NW of the centimetres.These sediments are varicoloured from grey, Leinster Granite and are coarse to fine-grained turbidites of black,red-purple to green, where colour striping often Ordovician to Late Silurianage (Bruck & Downie 1974). relates to the banding (hence ‘Ribband’, Jukes & Du Noyer They are shown forming the Midlands terrane on Fig. 1. 1869). Interpretations of the evolution of the Leinster Massif Subordinatelithologies include basic pillowed and (e.g. Briick et al. 1979) have proceeded from the basis thatit vesicularlavas, basic tuffs, andmanganiferous cherts. In formed a relatively straightforward sedimentary basin in the someareas finer grained sediments are interbedded with Lower Palaeozoic with the same NE-SW orientation as the graded volcaniclastic greywacke sandstones or graded quartz presenttrend of themajor lithological units within the sandstones.Graptolites and microfossils indicate an Early massif. Thisbasin was situated between an ‘Irish Sea Cambrianto Mid-Ordovician age (Brenchley et al. 1967; Landmass’,now represented by the Precambrian Rosslare Crimes & Crossley1968; Brenchley & Treagus1970; Complex, which was then attached to Cadomia (‘Avalonia’, Brenchley et al. 1977; Bruck et al. 1974). Kelling et al. 1985). During the Cambrian, coarse proximal RibbandGroup rocks are incorporated within the turbidites of the Bray Group were deposited on the NW and Dublin and Waterford-Wicklow terranes on Fig. 1. SE margins of thisbasin, while distal turbidites and (2) Bray Group rocksare coarse greywackes and extrusivevolcanic rocks of theRibband Group occupied quartzitesinterbedded with thin siltstones and shales. the deeper water at its centre. During the Early Ordovician Sandstonesand quartzites show bottom structures, slump the basinbecame more extensive, so thatRibband foldingand Bouma sequences (Briick & Reeves1976) deposition became more widespread. After minor tectonic indicative of deposition by turbidity currents in a proximal activity the basin shallowed in the Upper Ordovician so that environment. Micro- and trace fossils indicate an Early to shelf carbonates of the Duncannon Group were deposited, Middle Cambrian age (Bruck et al. 1974). with localunconformity, on various members of the 1035 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/6/1035/4890737/gsjgs.147.6.1035.pdf by guest on 28 September 2021 M . D. MAX1036 D. M. ET AL. Ribband Group, to be followed by a great thickness of basic Wicklow Shear Zone in Fig. 1) enclosing lensesof mafic and intermediate and acid volcanic rocks to build an island arc. ultrabasic rock, suggesting that they are transcurrent faults Deposition in the Silurian was restricted to the NW part of of majortectonic significance; others are thrusts, such as the Massif (Briick et al. 1979). that bounding the NW margin of the Bray terrane (Fig. 1). Structurallythe massif hasalso been regarded as Designation of a structural unit as a ‘terrane’ does not relatively simple (Briick et al. 1979, fig. 1). The Duncannon carryany genetic implications. A terrane is purelya Group is considered to occupy a broadly synclinal area, the mapping unit with no implications concerning the amount of Campile syncline, while the Ribband Group forms anticlinal movement on the bounding faults or thrusts, nor that the areas,although the structure is farfrom clear (Gardiner terranesare ‘exotic’,‘allochthonous’ or even‘suspect’ in 1970), with a major culmination in the area now occupiedby theirpresent tectonic setting (Coney 1989). does,It the Leinster batholith (Briick et al. 1979, fig. 1). Additional however,allow each unit tobe looked at objectively, structural complexity is seen in the area to the southof Bray without any assumptions concerning its original relationship (Fig. 1) wherethe Bray Group forms a nappe structure toadjacent units, so thatthe structural, stratigraphical, above a thrust plane along which it has been thrust to the sedimentologicaland palaeontological evidence for their north across the Ribband Group (Briick & Reeves 1976). original relationship may be critically assessed. Onthis basis seven terranes are recognized in the A new geological map of Leinster LeinsterMassif (Fig. 2). FromSE to NW these are: (1) Tuskarterrane; (2) Rosslareterrane; (3) Cahore- The earliest geological maps of Leinster were prepared by Cullenstownterrane; Waterford-Wicklowterrane; (5) Jukes & Haughton (1859) and Jukes & Du Noyer (1869). (4) Bray terrane; (6) Dublin terrane; (7) Midlands terrane. The Since that time almost the whole of the Leinster massif has stratigraphic sequences and the intrusive and deformational beenmapped at the scale of six inchesto one mile (1: events in each of these terranes is shown in Fig. 3. 10 560). Many of the Geological Survey of Ireland sheets showexposures which have since been obliterated, and (1) Tuskar ierrane temporary exposures, opened up during pipe-laying or road and harbour works. These maps, with six-inch mapping by Rocks of the Tuskar terrane at the SE extremity of the Leinster Geological Survey geologists, principally by P. M. Briick, T. Massif and around Tuskar Rock (Fig. 1) are vesicular pillow basalts and tuffs with associated greywackes, siltstones and black shales. J. Reeves and H. A. van Lunsen, together with published The rocks dip steeply to the north and young in the same direction. maps and accounts the geology and all the unpublished of They have a single steep cleavage and are affected by low grade University and company data which could be obtained, have metamorphism.Geochemical discriminant plots show that the beencomplied by J. Martinezand M. D. Max(Fig. 1). basalts and tuffs are sub-alkaline and of within-plate origin, while Extensive additional fieldwork has also been carried out by theassociated sediments show both volcanic and continental the present authors. Original data and compilations are on provenance (Mm & Ryan 1986). 0.5 inches to 1 mile and 1: 25 OOO scale held by the Mapping Division of the Geological Survey of Ireland. The pillow basalts and the sediments indicate a marine origin for the Tuskar Group; vesicles (6 mm) in the basalts Terrane analysis of the Leinster Massif indicate that extrusion was at less than abyssal depths. The The new compilation of the