Journal of the Geological Society, London, Vol. 147, 1990, pp. 1035-1050, 10 figs, Printed in Northern

Terrane assemblage of the Massif, SE Ireland, during the Lower Palaeozoic

M. D. MAX’.3,A. J. BARBER’ & J. MARTINEZ’ ’Geological Survey of Ireland, Beggars Bush, Haddington Road, 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- 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) Group rocks extend NE-SW across the nianrocks. Lower Palaeozoic sediments, plutonic and massif from 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 ‘ 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

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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 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 & DuNoyer (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 andtuffs 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 Theyhave a single steep cleavage and are affectedby 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 geology of Leinster shows more geochemistry of the lavas suggest extrusion in an extensional clearly than previous maps that the Leinster Massif is made environment,perhaps at a time of continentalbreak up. up of a series of structural and stratigraphical units trending There is no direct evidence for the age of the group but a NE-SW, bounded by faults oriented in the same direction. very latePrecambrian or earlyCambrian age has been Each of these units is a ‘terrane’ as conceived by Coney et suggestedfor the rifting event (Ryan & Max 1986). al. (1980;Coney 1989; cf. Barber 1985): mappablea Subsequently the group was compressed and deformed into structural entity with a distinct stratigraphic sequence and an its present attitude before it was intruded and hornfelsed by igneous,metamorphic and deformational history different the Carnsore Granite (430Ma, O’Connor et al. 1988). The fromthat of neighbouringterranes, from which isit boundingfault against theRosslare Terrane to the north separated by astructural break. In Leinster many of the cutsthe granite, but is cut by the statistically contem- boundingdiscontinuities are sinistral strike-slipfaults and poraneousSaltees Granite (c. 440 Ma, Max et al. 1979) someare shear zones (e.g. Slade Valley Shear Zone and (Fig. 1).

Fig. 1. Geological map of the Leinster Massif, generalized from new map compilationof the Geological Survey of Ireland and originaldata. Dashed lines offshore in SE show possible positions of Saltees and Carnsore granite margins. Location map: A, Anglesey; IS, Iapetus suture, MSL, Menai Straits Line. Inset map (top) shows sigmoidal dyke swarm in the Aghfarrell and Butter Mountain Formations to the NW of the Leinster granite (after Angus & Brindley 1970). Fault offsets after Briick. Arrows show shear sense, heavy dashed line shows trace of dyke trends. Recognized unconformable contacts shown bydots above contact. Maulin Fm. includes Ballybeg Pelite, Knockree Quartzite, etc. V in east and west Dublin terrane are larger volcanic horizons. AF, Athgarrett Fault; AG, Aghfarrell Fm.; BM, Butter Mountain Fm.; SVFZ, Slade Valley Fault Zone; B, Blackhall; BA, Bay; BB, Booley Bay; BC, Baltinglass; BR, Brownsford; CD, Carrigmore Diorite; CG, Carrickgollogan; CL, Carricklony; CM, Carrick Mountain; CO, Coolgarrow; CU, Cullenstown Strand. D, Duncannon; DC, Devil’s Glen; E, Enniskerry; FFZ, Ferns Fault Zone; G. Greystones; CL, Glencullen River; GR, Graiguenamanagh; IE, Ireland’s Eye; IN, Inistoige; K, ; KP, Kilmichael Point; MH, ; NR, ; PR,Pollaphuca Reservoir; R, Dargle River Bray Thrustcontact; RL, River Liffey; SH, St Helen’s; SMG, St Margaret’s Granodiorite; T, ; TR, Tramore; W, Wicklow Town; WH, Wicklow Head; WM, Whilkeen Member of Tuskar Terrane; WFZ, Wicklow Fault Zone.

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Murphy 1988). The southern belt shows the same sinistral sense of movement,and the age of the movementsis constrained by the cintrusive Saltees granite Ma, Fig. 1) which cuts the mylonites, DUBLIN < “WTH (440 butis itself deformed by latershear movements (Max & Ryan 1986). The amount of movement on the mylonite belts is unknown but no direct relationship can be demonstrated between the Tuskar and Rosslare terranes (Winchester et al. in press).

Tugout Group. Faultedoutliers ofOrdovician conglomerates, sandstones, flaggy siltstones and weakly cleaved shales are enclosed within thenorthern mylonite belt (Fig. 1). Claststhein conglomerates include quartzite, metaquartzite, chert and mylonite (Baker 1966). Thesesediments contain shellya fauna of brachiopods and trilobites which, together with graptolites in the shales,indicate a Late Arenig age (Brenchley et al. 1967). The shelly fauna, which shows a close affinity with the Arenig fauna of Anglesey, has been attributed to a Celtic Fauna1 Province distinct from the Anglo-Gallic Province of Wales and the Welsh Borders (Bates 1972) although this distinction has been disputed (Fortey & Cocks 1986; McKerrow & Cocks 1986).

(3) Cahore-Cullenstown terrane The relationship between the Rosslare terrane and the CullenstownGroup in the NW is obscured by the Carboniferous unconformity (Fig. 1).

Cahore Group. The CahoreGroup composedis of green feldspathic greywackes and quartzites with thin intervening shales. Sandstones showgrading and Bouma sequences, with bottom structures, flute and groove casts, indicating transport from the NE or SW,and slump foldsindicate a palaeoslope towards the NW (Crimes & Crossley 1968). These features are due to deposition by turbiditycurrents on asubmarine fan in acontinental slope environment (Mutti & Ricchi-Lucci 1978). Sandstones on the Bannow coast have a more distal aspect than thoseatCahore and are associated withpebbly mudstones containing clasts up to 5 m. Melanges occur in the CahoreGroup on the Bannow coast with blocks of various sizes and several different lithologiesin a fine grained cleaved matrix. Most blocks can be matched with adjacent sandstones, but some are exotic, including cross-beddedquartzites composed of well-roundedquartz grains, and one block of cataclastic quartzite (Dhonau 1972). Oldhumiu and other trace fossils recorded at Cahore and the Fig. 2. Tectonostratigraphic terranes of the Leinster Massif. Dotted Bannowcoast (Crimes & Crossley1968; Dhonau 1972) and line shows boundaries with Upper Palaeozoic rocks. Boundary acritarchs(Gardiner & Vanguestaine 1971)confirm aCambrian between Cahore-Cullenstown and Rosslare terranes is coveredby age. Upper Palaeozoic rocks. Granites unpatterned. Extension into Bedding dips steeply SE, younging predominantly to the north offshore area only in southeast. KP, Kilmichael Point. in the south part of the section and to the south in the north. Small scalefolds on NE-SWaxes plunge in either directionand are overturned to the NW. An axial plane cleavage in shales and rough spaced cleavage in sandstones and quartzites dips SE. The coastal (2) Rosslare terrane section is broken by many S dipping steep faults. The Rosslare Terrane (Fig. 1) is alargely amphibolite facies metamorphic complex of acid and basic gneisses and igneous rocks Cullenstown Group. The Cullenstown Group is also composed of with along intrusive, metamorphic and deformational history quartziticgreywackes and quartzites, but metamorphosed to (Winchester & Max1982; Max & Long1985). The complex greenschistfacies, with intervening shales altered to chlorite- originated at c. 2000Ma(Max 1975; Davies et al. 1985).A sericite schists (Max & Dhonau 1974). In less deformed sandstones mid-amphibotite facies event coincided with the intrusion of the St sedimentaryfeatures like graded bedding, parallel lamination, Helen’sgabbro in the Late Precambrian (626-618 Ma, Max & convolute lamination, ripple-drift bedding, and slump foldsmay still Roddick 1989).. Dykes were intruded during the Cambrian, and the be recognized. These features indicate deposition as turbidites on a wholecomplex was metamorphosed at highgreenschist-low proximal submarine fan in a continental slope environment. More amphibotite facies at 480 Ma (Max & Roddick 1989). distal features on the Bannow coast indicate transport was towards The Rosslare complex which is only 8 km wide, is bounded to the SW. Lithic grains of metamorphic origin and the abundance of NW and SE by mylonites. The NE belt is derived mainly from the quartz indicate a continental provenance. deformation of thegneisses (Tomhaggard Zone) butcontains Melanges occur in the Cullenstown Group of the Bannow coast importantsupracrustal elements (Ballycogly Group: Baker 1970; sectionwith large blocks of quartzsandstone and boulder

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421

428 LLANDOVERY F=438 ASHGILL 448 5 CARADOC 0 458 > LLANDEILO B 468 LLANVIRN 8 478 ARENIG 488 Z LATE 5- 525 [r m MIDDLE 5 540 0 EARLY 590

- CAHORE- WATERFORD-CAHORE- I TERRANES TUSUR I RoSSLARE I CULLENSTOWN I WICKLOW I BRAY.Howth&lrelands Eye I I Q. 3. Sedimentation, igneous, metamorphic andstructural history of tectonostratigraphic terranes in the Leinster massif, showing 'stitching' plutons and the Late Devonian-Carboniferous linking unconformity. Dashed boxes indicate nature of provenance for clastics in sedimentary units. Lithological symbols: bricks, limestones; circles, conglomerates; dots, sandstones; V, volcanic rocks; close spaced lines, pelagic sediments; alternating dots and lines, turbidite deposits. Time scale used is taken from Harlandet al. (1982).

conglomerates in which the boulders are also quartzites. As in the Inearlier accounts of thestructure of SE Irelandthe quartzite blocksin the Cahore Group, quartzgrains are Cullenstownand Cahore Groups have been regarded as well-rounded with frosted surfaces (Max & Dhonau 1974). separateunits. The higher grade of metamorphismand A dominantschistosity, sometimes intense, defined by the deformationin the Cullenstown Group has suggested that alignment of chlorite and sericite flakes, strikes NE-SW and dips to this is theolder unit. Because the Cahore Group is the SE. Minorfolds, often of sheathtype, plunge down the dip identified as of Cambrian age, the Cullenstown Group has towards the S, together witha lineation of elongatedchlorite beenregarded as latePrecambrian. The similarity in aggregates(Crimes & Dhonau 1967). Thesestructural features lithologyand sedimentary features in boththese units, contrast markedly with those of the Cahore Group. suggests that the Cullenstown and Cahore Groups may be parts of thesame sedimentary unit (cf.Tietzsch-Tyler & Bluckhnll Formation. Sandstones,siltstones, black slates, pebbly Phillips 1989), butsubsequently the two groups have mudstoneand melanges form afault-bounded sliver along the undergone different tectonothermal histories. contact between the Cahore and Cullenstown group on the Bannow Both Cahore and Cullenstown groups are composed of coast(Dhonau 1972) (Fig. 1). Pebblesin the mudstone are greywackes, and the melange,localized along the faulted east quartz-richturbidites, sometimes amounting toquartzites, boundary of the outlier, contains blocks of sandstone, quartzite and derived from a continental source. Well-rounded grains are chlorite schist, all of which could have been derived from adjacent indicative of longdistance transport with reworkingin a units.Brachiopods and echinoderms (Kinahan 1879) indicate beach or dune environment. Periodically these quartz-sands depositionin a shallow marine environment. Graptolites in the were mobilized as turbidity flows which were deposited on a slates indicate an early Caradoc age. However, a sample of shale submarinefan. The absence of volcaniclasticmaterial adjacent to the graptolitelocality yielded Tremadoc acritarchs, indicates that deposition took place on a passive continental scolecodontsand Chitinozoa of earlyArenig age (Gardiner & margin. Vanguestaine 1971). Thissample may have come from a faulted Quartziteblocks which occur in melanges within the slice of Ribband Group rocks. Cahore and Cullenstown Groups include reworked, lithified

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quartziteboulder conglomerates and cross-bedded quartz- NE,composed of schistose and mylonitizedrocks of itescomposed of roundedand frosted grains. These Ribband type. quartzites were evidently formed in a shallow water, tidal, beachand coastal dune, shelfenvironment on a passive RibbandGroup. Crimesand Crossley (1%8) and Brenchley & continental margin, the same environment from which the Treagus(1970) distinguish the Clones,Riverchapel, Breunoge, sand grains in the turbidites were derived, but the quartzites Glenbeg, Polkhone and Askingurrun formations by minor variation were lithified before they were incorporated in the melange. incolour and lithology in thetype area of the Ribband Group These melanges have been interpreted as olistostromes aroundCourtown (Fig. 4). Theseunits comprise the Courtown formedat a passive continental margin by contemporary Group in Fig. 1. Towardsthe SW the Bullyhoge, withdark grey submarine landslips (Max & Dhonau 1974), but lithification shalesand slates, Polldarrig, withslumped quartzites (Shannon of thequartzite blocks and particularly the presence of a 1978) BooleyBuy, withmelanges (Fig. 5) (Gardiner1967) and TramoreShale (Mitchell er al. 1972; Carlisle 1979) formations are cataclasticblock, suggests that themelanges were formed all of Ribband type. sometimeafter the deposition and deformation of the Trace fossilsindicative deep-waterofa environment of Cullenstown Group.Recently many melanges have been deposition have been described from the E coast section (Crimes & interpreted as the product of shalediaprism, formed long Crossley 1968). Graptolites and acritarchs indicate ages from Early after the depositionof the surrounding sediments in zonesof or Mid-Cambrian to Early Arenig (Gardiner & Vanguestaine 1971) activetectonic convergence. Where passive margins enter (Fig. 4). zones of convergence, distal deposits are thrust back over Inthe New RossUnit Ribband type. rocksare termed the thecontinental margin producing overpressuring in shale Kilmcreu Formation on the coast north of Wicklow, composed of units in the continental margin sequence which rise towards darkgrey slates andbanded siltstones, withthin quartzites, and the surface carrying blocks of lithified units to higher levels inland to the SW, the OuklundsFormution, incorporatingthe in the complex (Barber et al. 1986; Barber & Brown 1988). CunniunstownFormation ofTremlett (1959) and Reeves (1984), Teitzsch-Tyler & Phillips (1989) have suggested that the composed of laminated red-purple and green shales and siltstones relationshipbetween the Cullenstown Group and the RosslareComplex was originally an unconformity, but a

similar metamorphic grade and a similar structural sequence 0 1 2 3Km Kilmichael indicatesthat the Cullenstown Group forms part of the - \ Point mylonite belt along the NW margin of the Rosslare Terrane. KlLMlCHAEL These mylonites were formed by sinistral transcurrent fault movements (Murphy 1988) in Late Cambrian or very Early Arenigtimes, as blocks of myloniteoccur as clasts in conglomerates of theTagoat Formation (Baker 1966). Strand Crustalblocks in transcurrent fault zones are subject to subsidence in transtensile segments and uplift in transpres- sive segments. The Cullenstown Group was metamorphosed at depths of c. 15 km and during movement along the fault zone was then uplifted into juxtaposition with the equivalent CahoreGroup which had been deformed at highera structural level. Theamount of lateralmovement on the ffcarrick Rocks fault is unknown,but the total width of thezone of /’ mylonitized rocks, c. 15 km, suggests that it was substantial. ,” //‘ Later,post-Caradoc transcurrent fault movements are / indicated by horizontal fluting and grooving on the faulted / // contacts of the Blackhall Formation, which lies between the /’I two groups (Max 1975; Max & Long 1985). Thecontact between the Cahore-Cullenstown Terrane DendroidaraDtolites TREMADOCIAN and the Ribband Group of the Waterford-Wicklow Terrane tothe NW has been interpreted by Shannon(1978) as a sedimentary transition. In this study, wherever the contact hasbeen seen it is tectonicand on the map scale is cross-cutting to units recognized in the Ribband Group (Fig. RIBBAND .r\ ,X?PointRoney 1). This contact is regarded as a terrane boundary. GROUP

(4) Waterford- Wicklow terrane A complex series of units between the north margin of the Cahore Group and the Wicklow Fault Zone make up the Waterford-Wicklow Terrane (Fig. 1). TheTramore- CourtownUnit to the SE comprises both Ribband and DuncannonGroup rocks, separated by anunconformity. The New Ross Unit to the NWis made up of fault lenses of Fig. 4. East coast of Leinster between Cahore Point and Kilmichael Ribband and Duncannon Group rocks, separated by shear Point (location shown on Fig. 2), showing Formations and zones,frequently marked by basic andultrabasic lenses. palaeontological evidenceof age in the RibbandGroup, from Among these lenses is the Wicklow-Kilmichael Unit to the Crimes & Crossley (1968) and Brenchley& Treagus (1970).

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d

Fig. 5. Melanges in Booley Bay. (a) North side of Booley Bay. Pre-cleavage melange cross-cutting overturned distal greywacke siltstones. Greywackes were drawnout into the scaly clay matrixof the melange ina ductile-appearing manner suggesting that both the sediments and the melange were water saturated and capableof local fluidization. These featuresare diagnostic of diapiric melange (cf. Barberet al. 1986). Medium-sized rabbit for scale. (b) Melange in central Booley Bay section. Greywacke (patterned) in disrupted blocks retain some internal bedded character but finer grained sediments vary from scaly clay to complexly disposed, streakedout fragments in scaly clay matrix. Heavy lines are later shears intowhich earlier melange fabric isrotated, showing polycyclic nature of the melange formation. Scale baris 1 metre. (c) Blackhall Formation west of Cullenstown. Melange in banded, Caradoc siltstones. Pocket notebook (black)for scale. Complex movements indicated by folding of earlier shear bands. (a) Cliff exposure above beach, north sideof St Petit's Bay, immediatelywest of Baginbun Head. Notebook for scale. Tectonic relictof bedded black shale in fragmental cleavage/bedding parallel zone.Dotted areas are scaly clay. Heavy lines are later shears. Dashed linesin the boudin indicate cleavage (S2?) that elsewhereis about parallel to foliation. Steep NW-plunging, sheared, 'S' shaped folds indicate sinistral shearin this zone and northof Baginbun Head. On a larger scale, larger exposure areasof preserved bedding may all be large boudinsin an anastomosing shear zone comprising the wholeof the Booley Bay Formation.

withthin greywackes andbasic to intermediate tuffs. Acritarchs The WicklowHead-Kilmichael unit consists of silverygrey indicate a Late Arenig (c. 480 Ma) age (Van Lunsen 1976). To the mica-chlorite-garnetschists of the WicklowHead Formation SW the BallylaneFormation near New Ross, incorporatingthe (Reeves1984), whichform Wicklow Head, andthe Kilmichael Stump-of -the-Castle, Carrickloney and Glenmore formations in E, Formation (Brenchley & Treagus 1970) which occupies the coastal Kilkenny(Briick et al. 1979)in Fig. 1, is composed of similar sectionbetween Mizen Head and Kilmichael Point and extends lithologies,but also includes manganese nodules (20-@cm) and 20 km SW along the Ferns Fault Zone (Fig. 1). Banded lithologies manganiferous cherts, altered to coticules in the aureoles of dolerite and deformed pillow lavas in this unit suggest that the schists were intrusions. The PalaceGreywacke Formation outcropping in the formed from a Ribband type protolith (Brenchley & Treagus 1970). central part of the SE margin of the New Ross Unit (Fig. 1) may representdetached lenses of theBray Group of Ordovician The Ribband Group rocks of the Waterford-Wicklow greywackes caught up in the Wicklow Shear Zone. terraneare distal turbidites and pelagic and hemipelagic

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sediments,locally associated with pillow lavas and pillow A 10 CM B IM ssw ; NNE ssw ' NNE breccias, indicative of deposition in an abyssal ocean floor .. environment. The presence of manganiferous cherts among the pelagicsediments suggests that at times the area of deposition was beyond the reach of temgenous sediments, in an open ocean environment, rather than in an enclosed ensiaiicbasin. Greywacke sandstone beds and thin quartzites,interbedded with the dominant argillaceous pelagicdeposits, represent thedistal extensions of Fig. 6. Details of the Duncannon-Ribband Group unconformity submarine fans derived from island arcs or from a passive along the west side of Tramore Strand drawn from field sketches. continentalmargin spread across the ocean floor from a Overlying DuncannonGroup youngs to the west at 65"-80" dip continentalslope. Slumped quartzites of thePolldarrig through section. (A) Southern exposure, low in beach. Dark Formationmay indicate closer proximity to the slope. A psammite ribs (dotted) in black slaty RibbandGroup below more distal assemblage of rock types in the New Ross unit unconformity. Bedding is overturned and youngs to the south. than further SE suggests that the open ocean layto the NW. Weak bedding plane cleavage does not pass across unconformity.S2 Acritarchsand graptolites indicate that deposition on the in Ribband Group is S1 in overlying DuncannonGroup. Minor fold ocean floor persisted from Early Cambrian to Late Arenig may be F1 or sedimentary deformation. (B) Northern exposure, times, a period of the order of 100 Ma. about 10 mnorth, in more irregular rock surface thatwas Ribband Group rocks in the Waterford-Wicklow terrane topographically slightly elevated above southern exposure during havea general SEdip, interrupted by frequentreversals deposition of first DuncannonGroup sediments. Early fold plunges relatedto major and minor asymmetrical and overturned 51" to SW in weakly banded black slates with thin, impersistent folds (Fl) with a NW vergence. An intense (Sl) axial plane bands of dark, ripple drifted psammite. Early bedding-parallel cleavageis commonly folded (F2) witha (S2) crenulation cleavage does not appearto be axial planar to the fold.S2 passes cleavage.Way-up evidence in thin turbidite sandstones across unconformity. Bedding is overturned and youngs towards the youngs tothe SE, although localreversals on steep or south. overturned fold limbs are common, while fossils indicate an overall younging from Early Cambrian in the SE to Early Ordovician in the NW. (Fig. 4). The rocks are imbricatedby proportion of volcanic rocks to interbedded sediments, indicating steep NW directed thrusts (Crimes & Crossley 1968, fig. 6; eruptionfrom separate volcanic centres with different eruptive Brenchley & Treagus 1970). These features are characteris- histories. tic of accretionary complexes formed at the hangingwall of A limestone, the Courtown LimestoneFormation also overlies theunconformity Courtown,at with basal a conglomerate subductionzones (Dickenson & Seely1979; Karig et al. containingclasts which could have been derived from the 1979; Leggett et al. 1979) Diapiric melanges in the Booley underlying Riverchapel Formation (Brenchley & Treagus 1970, p. BayFormation (Fig. 5a) are also compatible with a 90, plates VI and VU). Blackshales and volcanic rocks, with subduction environment (Barber et al. 1986).Reversals in intermediate and acid tuffs and laharic breccias, follow. The lahars vergence as seen in the Booley Bay Formation (Tietzsch- contain fragments of limestone and black shale indicating uplift and Tyler pen. comm.1988) can beaccommodated as back erosion of the volcanic basement. thrustsin the accretionary model. Evidently the Ribband The Duncannon Group rocks are folded by large scale upright Group was accreted from an ocean floor which was being folds (Fl) on axestrending NE-SWwith a variableplunge subductedatconvergenta continental margin, with a (Brenchley & Treagus 1970,fig. 2, plate V). Anaxial planar deformation front orientated in a NE-SW direction, along pressure solution cleavage is developed in the limestone, and slaty the margins of a continent which lay to the southeast. cleavagein the black shales. F1 inthe Duncannon Group is equivalentto F2 in theunderlying Ribband Group (Crimes & Duncannon group. Duncannon Group rocks form the complex and Crossley 1968). highly faulted Campile Syncline (Gardiner 1970) in the central part of the Tramore-Courtown unit. They rest unconformably on the Early to Middle Cambrian acritarchs have been obtained Ribband Group at both Tramore and Courtown. from the Booley Bay Formation adjacent to the Duncannon At Tramore Strand the unconformity, exposed at low-tide, dips Groupat Duncannon (Gardiner & Vanguestaine 1971) steeply to the west and is irregular on a small scale (Fig.6). Beneath while the Riverchapel Formation below the unconformity at the unconformity shales and thin sandstones of the Tramore Shale Courtown has yielded Early Arenig (c. 483 Ma) graptolites Formation are isoclinallyfolded with bedding-parallel cleavage (Crimes & Crossley1968). Above the unconformity, (SI). Thesestructures are truncated by the TramoreLimestone limestones of the Duncannon Group are richly fossiliferous Formarion with abasal conglomerate containing pebbles of vein with a prolific shelly fauna, and the shale units frequently quartz, sandstone and cleaved slate. Cleavage (Sl) in the limestone contain graptolites. Mitchell et al. (1972) and Carlisle (1979) is parallel to the unconformity, and in the slates beneath, the same show that the oldest members of the Duncannon group are S2) cleavage (here is parallel to the axial planes of small scale folds of MiddleLlandeilo age (c. 462 Ma)and that volcanic which affect both bedding and the earlier cleavage. (E) activity continued until at least the MiddleCaradoc (c. Thelimestones are followedby black shales withtuffaceous 452 Ma). Conformable relationships between the Ribband horizons and then c. 2000 m of andesitic lavas, agglomerates, lapilli and Duncannon groups, claimed locally by Shannon (1978), andcrystal tuffs and ignimbrites (Carlisle 1979). Some lavas are are unlikely in view of the c. 20 Ma time gap represented by pillowed,indicating submarine eruption, while others, more massive, associated with lahars and welded tuffs, indicate subaerial the unconformity and the presence of cleaved slate clasts in activity.Rhyolite sheets intrude the volcanics. Similar volcanic thebasal conglomerate of theDuncannon Group at assemblages at Duncannon, Annagh Hill, Avoca and Arklow Head Tramore,Duncannon and Courtown, which indicate that (Williams et al. 1987; Gardiner 1974) show marked variation in the the Ribband Group rocks were already deformed, with the

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development of slatycleavage, before the Duncannon KilmichaelSchists with high greenschist facies metamor- Group rocks were deposited unconformably upon them. phism were formed by ductile deformation at depth within The Duncannon Group represents a shallow water shelf theshear zone. The later development of discretethrust sequence of limestones and shales, deposited unconformably planes,related fold structures and crenulation cleavages onthe previouslyaccreted, uplifted and eroded Ribband indicate that the schists were uplifted into regions of lower Group.A volcanicisland arc, presumably related to temperature and pressure, but still with a sinistral sense of continued subduction some 100-200 km further to the NW transcurrent movement. Finally the schists were extruded by (Stillman & Williams1979), was built up inthis shelf transpressionfrom the Ferns Shear Zoneand overthrust environment on top of an older accretionary complex, and a north across the Kilmacrea Formation (Fig. 8). series of individual volcanic edifices were constructed which Evidence of theage of deformationin the New Ross rose above sea level as volcanic islands, surroundedby areas Shear Zone is seen in the area to the SWof Wicklow, where in which shallow water sedimentation continued, with a high slates of the KilmacreaFormation areintruded by volcaniclastic input. coarse-grained dioritic rocks. The largest of these intrusions Accretion occurred after the deposition of the youngest is the Camgmore Diorite which has yielded a Rb-Sr whole members of theRibband Group, in the Lower Arenig rock age of 415 f 21 Ma (O’Connor & Reeves 1979). The (483 Ma)and was completed by Middle Llandeilo times aureolerocks surrounding this intrusioncontain chloritic (462 Ma), when the oldest part of the Tramore Limestone spotswith aligned inclusions, indicating that the country Formation was deposited unconformably on the accretion- rockwas already cleaved prior tothe intrusion of the arycomplex. By thisstage the accretionary complex had diorite.However, thechloritic spots themselves are thickened, been uplifted and eroded, to stabilize as newly flattenedand the diorite is locallysheared and cleaved formed continental crust upon which shelf carbonates could parallel to the cleavage of thecountry rocks, indicating be deposited. Accretion, uplift and erosion of the Ribband continued deformation (Reeves 1976). Group occurred within a period of 20 Ma. Fossil evidence In the Waterford-Wicklowterrane as awhole, the from intercalated sediments among the volcanic rocks of the RibbandGroup shows evidence of severalphases of arc indicates that volcanic activity lasted for at least 10 Ma deformation,the earlier of whichmay be attributed to (462Ma-452 ma),a life spancomparable to Tertiary accretion, while the overlying Duncannon Group shows only volcanic arcs. onephase of uprightfolding with vertical axial plane cleavage in finer grained units. If it is not related to the final closure of Iapetusit ispossible that thislast phase of Wicklow Head-KilmichaelSchbts. The northern part of the Waterford-Wicklow terrane, between the Ferns and Wicklow fault deformation is dueto transpression affecting the whole zones(Fig. l), ismade up of lenses of shearedand disrupted terrane during transcurrent fault movements. Ribband and Duncannon group rocks. Among these lenses are the The Wicklow-Waterford terrane is separated from the Wicklow Head and Kilmichael Schists derived from a Ribband type Dublin and Bray terranes to the NW by the Wicklow Fault protolith, butmore highly metamorphosed and more complexly Zone. Lenses of gabbro and serpentinite, together with a deformed (Fig. 7A-F). prominentmagnetic lineament (Max el al. 1983; Max & Mineral banding and earlier schistosities are folded into isoclinal Inamdar 1985) suggestthat this faultmarks azone of intrafolial folds to which the dominant schistosity is axial planar. In substantial movement and may correspond with the major thin section mica and chlorite foliae diverge around clastic grains zones marked by serpentinite belts in Newfoundland which andquartz segregations to formaugen (Fig. 7C). On an outcrop areregarded as ophiolites representing vestiges of the scale, earlier schistosities are folded and enclosed in the dominant subducted Iapetus ocean floor (Dunning & Chorlton 1985). schistosity(Fig. 7B), showing that thisis a composite structure formedbythe shear-fold-shear mechanism characteristic of mylonites formed in majorshear or thrust zones(Johnson 1959, (5) Bray terrane 1961). The Bray terrane in the NE of the LeinsterMassif lies on a In well-exposedcoastal sections thrusts are seen to have flat-lying thrust above the Dublin terrane (Bruck er al. 1974) (Figs 1 developedalong decollement surfaces within the schistosity(Fig. and 8). A klippe occurs at Carrickgollogan, and thrust and faulted 7E), and the adjacentrocks are intenselyveined by quartz, lenses at Carrick Mountain and Coolgarrow lie along the Wicklow indicating high fluid pressures during thrusting. Folded and sheared Fault Zone (Fig. 1). quartz veins along some thrust surfaces show multiple movement. Bruck & Reeves (1976) distinguish the Devil’s Glen and Bruy F2 folds of the schistosityform above hanging wall ramps and Head formations within the Bray Group. The older, Devil’s Glen develop a south-dipping axial plane crenulation cleavage (Fig. 7F). Formationis composed of massiveunlaminated greywackes and The plunge of the fold hinges is to the E or SE down the fold axial slates.Rare quartzites and pillow lavas recorded near the basal planes.All movement indicators show sinistral transcurrent shear thrust (Brindley & Millan1973; Briick & Reeves1976) in poorly towards the SW (e.g. Fig. 7E). exposed ground are probably tectonic intercalations of Bray Head These earlier structures are cut at intervals of a few metres by and Dublin Terrane lithologies respectively. brittleSE-dipping thrusts, whichby displacement of marker The BrayHead Formation composedis of laminated horizons,indicate late stage overthrustingtowards the NW. greywackes, with well-developed Bouma sequences, slates and thick Large-scale thrust planes are seen from the sea cutting the schists in interbedded quartzites. The quartzites are composed of mono- and Wicklow Head. It is inferred from the contrast inmetamorphic polycrystallinequartz and vein quartz. Greywackes,while still grade that theschists rest on basala thrust, with the same quartz rich, are of lithic and feldspathic type. Constituents include orientation and sense of NW movement as seen in the cliff section, K-feldspar,plagioclase epidote, biotite, muscovite and above the Kilmacrea Formation (Fig. 8). clasts of igneous and metamorphic rocks, rare cherts, sandstones, siltstone and shale, indicating provenance in an amphibolite facies The New Ross unit is interpretedas a major sinistral continental basement terrane (Bruck & Reeves 1976). shearzone. The intensely deformed Wicklow Head and Bothformations were deposited by turbidity currents on a

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Fig. 7. Structural details fromthe Wicklow Head Schists and the Kilmichael Formation, drawn from photographs. (A) Folded sandstone band (dotted) in striped Wicklow Head pelitic schists, eastof east Harbour wall. View to west. Edge of 11 cm long notebook for scale.S1 striped pressure solution cleavagein pelites intersects the beddingof the sandstone bandat a low angle. If the bedding were unfolded to the horizontal, the cleavage would dip more steeply than beddingto the SE. Open F2 fold hinge. (B) SW-NE decollement surfacein striped light green schist,SW end of Kilmichael section. View to west. Knifefor scale is 12 cmlong. The thrust surface is parallel to the composite bedding/Sl fabric, but diverges arounda contorted lens in which S1 is highly deformed. Quartz veins shown in black. Dashed lines indicate orientation of S2 crenulation cleavage. (C) SW-NE Quartz veins and augen in Wicklow Head-Kilmichael schists, Geological Surveyof Ireland specimen 80-1268 (6 inch sheet Wicklow31/4, specimen 23) from southernmost Wicklow Head schists. Scale 1.5 mm. Quartz vein-filled D2 shears andF2 folds common in section; section cut normal to steep plunging axesof early folds. Note sub-parallelismof slip surfaces (heavy lines) with S1.(D) Specimen 80-1268, scale 1.5mm. Thin silty layer of composite bedding/Sl folded into an intrafolial fold. Slip surfaces (heavy lines)are sub-concordant to S1. S2 crenulations intersect S1 at a high angle. (E) Far SW end of Kilmichael Point section, in striped green and white slates.View to west, dip of foliation 35" to 65" to NW, knife for scale. Thrust hanging wall ramp, sinistral shear with thrusting to SW. S2 (main cleavage) along dashed lines.Quartz veins shown in black, dashed lines indicate orientationof S2. Inset extension to SW.(F) F2 folds of composite bedding/Sl, mierolithon strain bands and cleavage(S2) merge with microlithons. Specimen 80-1268. Scale, 1.5 mm.

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Fig. 8. Diagrammatic cross-section through the Bray Head-Kilmichael Area. Double headed arrows indicate WICKLOW HEAD DUNCANNON GP IN direction of younging, single headed arrows indicate thrust movement sense. C, Carrickgollagan outlier; G, Deformed gabbro at southern end of Wicklow Fault Zone; GWSZ, Graiguenamanagh-Wicklow shear zone (matrix is Ribband Group);S; Serpentinite lenses along north-central part of Wicklow Fault; SDSZ, South Dublin shear zone (unnamed shear zone of Cooper & Briick 1983); WF, Wicklow Fault (southern sideof shear zone). Shear zonesare depicted by dashed lines.

continental slope, but the more massive greywackes of the Devil’s environment;beach, river, delta and coastal dune environments GlenFormation suggest deposition in amore proximalenviron- have been suggested (Gardiner & Robinson 1971). ment.Bottom structures indicatesediment transport from the north. Slump structures indicateapalaeoslope towards the W It is probablethat the Howth and Ireland’s Eye and NW. occurrences, like the main outcrop of the Bray Group, rest A Cambrian age, long attributed to the Bray Group from the on a thrust plane, as their structural complexity (Gardiner& occurrence of the trace fossil Oldhamiu, has been confirmed by the Robinson 1970; van Lunsen & Max 1975) makes it unlikely discovery of late EarlyCambrian to earlyMiddle Cambrian that they are still attached to their original basement. acritarchs in the Bray Head Formation (Briick et al. 1974). Althoughthe Bray terrane, with Howth and Ireland’s Coastaloutcrops in the Bray Group at Brayand Greystones Eye,bounded by tectonicdisordances, complies with the shownorth-directed overturned folds with a south-dipping axial definition of a distinct tectonostratigraphic terrane, there are plane cleavage (Sl); thrusts along the cleavage appear to be related strongresemblances to the Cahore-Cullenstown terrane. to its formation. Briick & Reeves (1976) interpret the Bray Nappe as a largeasa scale recumbent anticline/syncline fold pair with Thereare, however, differences thein direction of subhorizontal fold axes and a fold trace, marked by the outcrop of palaeoslope,indicated by the slumpedbeds, and of the Devil’sGlen Formation, trending NNE-SSW. ESE- or sediment transport directions, indicated by turbidite bottom SE-dipping beds in both fold limbs indicate overturning towardsthe structures. These differences may be accounted for either by NNW (Fig. 8). Briick & Reeves (1976) relate the NNW-facing folds irregularities in the original passive margin, or by rotation of to the formation of the Bray Nappe and its movement on the basal theBray terrane during transcurrent fault movement thrust. In the coastal outcrops the cleavage (Sl) is cut by quartz (Crowell 1985). veins and both cleavage and veins are crenulated and develop an The present tectonic situation of the Bray terrane is the upright crenulation cleavage (S2). This may be the same cleavage result of movement along the Wicklow Fault Zone (Fig. l), seen to cut the basal thrust in the DargleRiver near Inniskerry with extrusion during transpressive movements, to be thrust (Briick & Reeves 1976) and may be related to folds seen to affect over the Dublin terrane to the north (Fig. 8), followed by the outcrop pattern of the basal thrust (Fig.1). At Greystones folding in the thrust and the overlying nappe. earlier structures are cut every few metres by steep south dipping reverse faults. (6) Dublin Terrane The Dublin terrane, composed entirely of rocks of Ribband type, is Howth and Ireland’s Eye. Quartz sandstones, shales and melanges separated into two outcrop areas by the Leinster Granite (Fig. 1); inHowth and Ireland’s Eye, with Oldhamiu, althoughdetached, The Maulin Formation (Briick er al. 1974) and its SW extension have traditionally been regarded as part of the Bray Group (Fig. 1). in the BallybegPelite Formation (McArdle 1981) consists of dark Recently acritarchs have confirmed a similar early Middle Cambrian grey or dark blue-black, often graphitic phyllites and slates with age (Gardiner & Vanguestaine 1971; Smith 1977). thin quartzites, bentonitic bands and manganiferous cherts, altered The older CensureGroup of shallow-watersandstones and to ‘coticules’ in the aureole of the granite. Globular garnet crystals shales, is overlain by the younger Nose of Howth Group of deeper in these coticules (Kennan 1972) may have nucleated on deformed waterturbidites and melanges (Gardiner & Robinson1970; van Radiolaria.Thicker quartzites, the KnockreeQuartzite Member, Lunsen & Max 1975). Clasts in sandstones show derivation from an 7-70111 thick, occur at the north end of the outcrop (Briick et al. uplifted amphibolite facies continental basement terrane. Slumps in 1974), the Censure Group showpalaeoslopes from the W and NW and The SleumaineFormation (Briick et al. 1974)consists of grey transport indicatorsin the turbiditesindicate derivation from the laminated shales and siltstones with cross lamination. Correlatives SE. Blocks in the melanges include sandstones of local derivation, probably occur within the Ballybeg Pelite Formation along strike to with soft-sediment deformation features, and lithified joint-bounded the SW(McArdle 1981). Here thepelites contain thin lenses of quartzite blocks,over a hundred metres across. An olistostrome greywacke and quartzite. McArdle (1981) interprets the quartzites origin has been proposed (van Lunsen & Max 1975). The quartzites asmass flow deposits and correlates them with the Bray Group. contain well-rounded pebbles, a high degree of sorting and coarse Thin section study shows, however, that the fragments are mainly cross-beds indicating deposition in a shallow water continental shelf volcaniclastic and cannot be matched in the Bray Group. Coticules

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sandstone units (Angus& Brindley 1970). Refolding of earlier folds and folding of the cleavage are common.

Ribband Group rocks of the Dublin terrane, like those of theWaterford-Wicklow terrane, consistpredominantly of bandedshales and siltstones with thin quartzites, greywackes and tuff bands, and represent distal sedimenta- tion. The association of these rocks with pillow lavas, pillow brecciasand manganiferous cherts suggests abyssal sedim- entationon an ocean floor.Palaeontological evidence suggeststhat this ocean floor was of LateCambrian to Mid-Ordovician(Llandeilo) age.Again, in as the Waterford-Wicklow terrane, the dominance of ocean floor Fig. 9. Palaeontological evidenceof age of the formations in the assemblages,consistent dip of beddingand younging Bray, north Dublin and Midlands Terranes. Simplified geological towards the SE, with vergence in folds and thrusts towards map; comparewith Fig. 1. Fossil localities indicatedby filled circles the NW while fossils indicate that the overall direction of with ages assigned by Briick & Downie (1974) and Briick et al. younging is towards the NW, and the width of the outcrop, (1974). Heavy lines mark terrane boundaries. AG, Aghfarrell Fm., 20 km, all suggest that Ribband Group sediments have been BM, Butter Mountain Fm.;CH, Carrighill Fm.; GD,Glen Ding imbricatedinto an accretionary complex formed ata Fm.; GR, Glencullen RiverFM.; PO, Pollaphuca Fm.; SL, convergent plate margin. Sleamaine Fm.; SQ, Slate Quarries Fm.; SVSZ, Slade Valley Shear Zone; TK, Tipperkevin Fm. Nodetailed geochemical studies have yet been carried out on the basaltic and andesitic lavas andtuffs of the Butter occur within the aureole of the Leinster Granite. Rare tuff bands Mountain,Glencullen River and Kilcarryformations, but are interpreted as air-fall tuffs (McArdle 1981), and vesicular pillow these may represent fragments of volcanic arcs. It has been lavasand pillow breccias occur at Roundwoodand Devil’s Glen suggestedthat these volcanic rocks are related tothe (Brindley & Millan 1973). DuncannonGroup (Brindley et al. 1973), butthere is no The GlencullenRiver Formation (Briick etal. 1974) consists evidence of unconformity with Ribband Group rocks, nor predominantly of volcanic rocks, including buff and grey acid tuffs, any sign of the sediments which are usually associated with with subordinate lavas. Similar volcanic rocks, but with porphyritic DuncannonGroup volcanic rocks. Indeed the association andesites and locally deformed into schists, have been described by with ocean floor sediments, and the absence of continental McArdle (1981) alongthe SE margin of Leinster Granite and influence,suggests that these volcanic rocks represent named the Kilcarry Volcanic Formation. fragments of oceanicisland arcs, rather than part of an The AghfarreN Formation consists of dark blue or grey phyllites Andean volcanic arc. and slates with thin siltstone bands and occasional green or grey If the fossil evidence is accepted, the Ribband Group of greywackesand rare quartzites (Angus & Brindley 1970). Basic volcanic rocks and intrusives are common. the Dublin terrane represents a younger segment of ocean The ButterMountain Formation alsoconsists ofblue or grey floorthan that represented by theWaterford-Wicklow phyllites,slates and rare sandstones with extensive basic and terrane, with deposition continuing into the Llandeilo. Nor andesitic rocks forming a major outcrop near Baltinglass (Fig. 1). canit represent part of thesame accretionary complex, They are lavas, sometimes pillowed or brecciated, and tuffs, cut by slightlydisplaced along the WickowFault Zone,as the intrusions (Briick 1976). youngest sediments of the Dublin terrane were stillbeing Ribband Group rocks from the Dublin terrane have been dated depositedafter the Ribband Group at the Waterford- palaeontologically by Briick et al. (1974). Palynomorphs from the Wicklow terrane had been accreted uplifted and eroded and type locality of the Glencullen River Formation in the east indicate the shelf limestones of the Duncannon Group were being a late Cambrian to Arenig age and acritarchs, more doubtfully, an deposited unconformably on the eroded surface. Nor can, in Early Ordovician age. Chitinozoa from the Butter Mountain and its present position, the Dublin terrane represent part of the Aghfarrell formations to the west indicate a Llanvirn to Llandeilo accretionarycomplex related tothe formation of the age.These results suggest that the rock units become younger DuncannonGroup volcanic arc. By analogy with modern towards the NW (Fig. 9). examples, deposition on undisturbed ocean floor in front of Beddingboth east and west of theLeinster Granite, dips an accretionary complex must lie some 100-200 km in front generally to the E or SE. Way-up evidence from graded beds in of the volcanic arc. At the present time Llandeilo rocks of greywackes and cross-laminationin siltstones indicates that the beds the Dublin terrane lie only 20 kmaway from the nearest drepredominantly the right wayup and young towardsthe SE. Duncannon Group volcanic rocks. Reversals of dip are common,however, as the rocks are often intensely folded on a small scale into asymmetrical folds on NE or SW plunging axes, overturned towards the NW. Angus & Brindley (7) Midlands terrane (1970, fig. 1) haveinterpreted the Aghfarrell Formation, where In the NW part of the Leinster Massif between the Slade Valley reversals of dip are particularly common and the beds frequently ShearZone and the Carboniferous unconformity Briick (1970, young towards the NW, as the inverted limb of a large scale fold, 1973,1975) andBruck et al. (1979) defined the Kilcullen Group overturned towards the NW. A similar large scale fold has been composed of the Carrighill, Tipperkevin, Glen Ding, Slate Quarries mapped by Briick (1975) in the Butter Mountain Formation NE of and Pollaphuca formations (Fig. 1). Apart from the Slate Quarries Baltinglass.Cleavage is pervasive throughout the finergrained Formation,these are all greywacke units composed of thick sediments of the Dublin terrane, frequently axial planar to minor sandstonesand alternating shales with well developed Bouma folds and dips to the SE more steeply than the general dip of the sequences (Briick 1970). bedding.Spaced cleavage may be developed in siltstone and The Pollaphuca Formation has yielded Chitinozoa of Early to

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Mid-Ordovicianage (Briick et al. 1974),the Slate Quarries NWmargin of Cadomia(Gibbons 1987). All the Formation yielded acritarchs of Mid-Ordovician to Early Silurian componentswhich make up the Leinster Massifmust be age, and the greywacke units to NW contain crinoids, bryozoa and allochthonous to the Cadomian margin of Wales, as they lie brachiopodfragments of probableCaradoc to Silurianage and entirely to the NW of the SW extension of the Menai Strait Chitinozoa of Llandovery to Wenlock age (Bruck & Downie 1974) line (cf. Barber & Max 1979, fig. 8). (Fig. 9). The oldestcomponent of theLeinster Massifis the Continuous greywacke deposition from early Ordovician to Late Rosslareterrane, a narrow sheared sliver of c. 2000 Ma Silurian is most unlikely; either the Chitinozoa in the Pollaphuca gneisseswhich must once have formed part of amore Formation are derivedfrom older deposits, assuggested for the extensive continental basement terrane. No basement of this Slievenamoninlier to the SE (Briick et al. 1979), or there are ageis known beneath England and Wales. The closest importantstratigraphical or structural breaks in theKilcullen Group. analogues arethe kart Gneisses of Guernseyand the The greywackeunits in the Midland terrane are allhighly Pentevrian of Brittany (Adams 1976). Metamorphism of the quartzosebut include plagioclase and K-feldspar whichbecomes Rosslare terrane at c. 620 Ma does correspond with the age less common in theCarrighill Formation. Quartzite clasts are of the Late Proterozoic arcof England and Wales, as well as abundant in the Pollaphuca Formation, but become less common to with the intrusion of the Coedana Granite of Anglesey and the NW, while chert clasts occur throughout the sequence. Other theCadomian of Brittany,suggesting that the Rosslare lithic clasts include trachyte and granodiorite (Briick 1970, 1972). terrane formed part of the Cadomian margin at that time The composition of the clasts shows that the sediments were derived (Winchester et al. in press). froman earlier accretionary,island arc and basement terrane. The age and origin of the intraplate basalts of the Tuskar Current direction indicators show that the sediments were sourced terrane to the SE are unknown, but may relate to the phase from the SE, the position now occupied by the rest of the Leinster of extension of theCadomian margin represented by the Massif. latebasic dykes in the Rosslare terrane. Whatever their The rocksof the Midlands terrane have a relativelysimple original relationship, the Rosslare and Tuskar terranes are structure, with one major set of large scale asymmetrical folds on now separated by mylonitea belt indicating that a horizontal or gently NW plunging axes, and associated minor folds substantialamount of sinistral transcurrentmovement has with a steeply SE dipping axial plane cleavage in slaty horizons and takenplace between them. The age of the last major spaced cleavage in the sandstones (Angus & Brindley 1970, Briick movementsin the mylonite belt are tightly constrained as 1970). lateOrdovician toearly Silurian (440-430 Ma)as the The Slade Valley Shear Zone, marked by sheared phyllites and Carnsoregranite is cutby the bounding fault while the crushed greywackes (Angus & Brindley 1970) is regarded as the SE broadlycontemporaneous Saltees granite intrudes the boundary of the Midlands terrane, separating the greywackesof the Kilcullen Groupfrom the Ribband Group rocks of the Dublin mylonites(Fig. 1) and is affected by onlyminor later terrane. Towards the SW the Aghfarrell Formation is cut out along shearing. this zone, bringing the Pollaphuca formation into direct contact with The northern mylonite belt, between the Rosslare and the Butter Mountain Formation (Fig. 1). At this point a series of Cahore-Cullenstown terranes, incorporates lenses of quart- ultramaficlenses are localizedalong, or closely adjacent, tothe zite which could have been derived from the Cullenstown shearzone (Bruck 1976,figs 1 & 2) suggesting that thiszone Group, and much of the latter is mylonitized. The width of represents a major structural boundary. thismylonite belt, up to three miles, suggests that any originalunconformable relationship between the Cullens- town Groupand the Rosslare complex (Teitzsch-Tyler & Evolution and assembly of the Leinster Massif Phillips 1989) hasbeen drastically modified by substantial Current concepts for the tectonic evolution of the Irish and sinistral transcurrentmovements. Age of movement is British Caledonides during the Lower Palaeozoic visualise a constrained by mylonitization of theEarly Cambrian three-plate model in which three major continental blocks, Cullenstown Group and the occurrence of mylonite clasts in Laurentia,Baltica and Cadomia (Gondwanaland) were the Late Arenig Tagoat Group. Late movements sliced the separated by the Iapetus Ocean, and by Tornquist’s Ocean Tagoat sediments into a lozenge enclosed by the mylonites whichis now represented by remnantsextending through and imposed a weak cleavage. easternEurope (Cocks & Fortey1982; Soper & Hutton The origin of the Cahore-Cullenstown terrane may be 1984; Hutton 1987; Oliver & Murphy, pers. comm. 1987). It considered together with that of the Bray terrane (including is consideredthat the Iapetus Ocean was subducted Howth and Ireland’s Eye) as the sediments of these terranes obliquelybeneath Laurentia forming anaccretionary arethe same age and show the same lithological and complex and inducing transcurrent fault movements within sedimentary features. Greywackes and quartz sandstones in the accreted material. Iapetus was also subducted to the SE theseterranes are derived from an amphibolite facies beneath Cadomia and Leinster is considered to have formed basement terrane and the clasts show evidence of extensive part of this SE margin (Fitton & Hughes 1970; Phillips et al. workingon a continental shelf before being mobilized as 1976). mass flows to be deposited as submarine fans along a passive Scatteredinliers of volcanicandplutonic rocks continental margin. distributedamong Palaeozoic and younger rocks between Closer relationships to the shelf environment are shown Pembroke and the Welsh Borders are regarded as fragments by clasts of shelf quartzite within melanges of the Cahore of aLate Proterozoic volcanic arc formed along the NW and Cullenstown Groups and the occurrenceof shallow shelf margin of Cadomia(Thorpe et al. 1984). To the NW the sedimentsin Howth and Ireland’s Eye. Thesesediments marginis abruptly truncated by theMenai Straits Line, couldnot have been derived directly from theLate interpreted as a major transcurrent fault which juxtaposed Proterozoic magmatic arc of the adjacent Cadomian margin thedisparate terranes of Angleseyincluding theEarly in England,from which they are separated by theWelsh Cambrian,subduction-related blueschist belt, against the Basin, (with continuous sedimentation throughout the Early

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Cambrian), the Menai Strait Line and the Early Cambrian Late Ordovician, conversion from a convergentto strike-slip blueschists of Anglesey. The Cahore-Cullenstown and Bray margin being marked by the cessation of volcanic activity in terranes are entirely allochthonous in their present position. the Lake District, North Wales and Leinster at that time. RibbandGroup rocks of the Waterford-Wicklowand Hutton & Murphy (1987) arguethat collision between Dublinterranes have been interpreted in this account as Cadomia and Laurentia took place in the Late Ordovician Ordovician ocean floor materials imbricated into accretion- and that the collision zone was marked by the development ary complexes by southeasterly directed subduction. Fossil of a series of successor basins, filled by turbidite deposits, evidencetaken at its face value suggests that the Dublin now represented by the greywacke terrains of the Southern terrane is youngerthan the Waterford-Wicklow terrane Uplands, the Lake District and the Kilcullen Group of the with the latter having consolidated, with the construction of Midlands terrane in Leinster. The change in sedimentation the Duncannon Group volcanic arc, before the former was patternsmay, however, be due tothe change from a accreted.It is alsosuggested that these two terranesare subduction to a strike-slip environment which took place at separated by a major structural discontinuity in the Wicklow that time, rather than to collision. Fault Zone. Thedolerite dyke swarm which cuts Ribband Group Accretion of the Waterford-Wicklow terrane from Late rocks in the area NW of the Leinster granite indicates that Arenigto Llanvirn coincides with the Arenig to Llanvirn crustal extension followed accretion of the Dublin terrane volcanism of North Wales and the extrusion of the Eycott (Angus & Brindley1970; Murphy 1987). The sigmoidal Lavas of the Lake District. Accretion of the Dublin Terrane pattern of the dyke swarm (Fig. 1 inset) results from sinistral in the Caradoc coincides with the formationof the Snowdon movementsalong ashear zone localized at thegranite volcanics of NorthWales and the Borrowdale Volcanic contact and along the Slade Valley Shear Zone. Since the rocks of the Lake District, as well as the Duncannon Group dyke swarm does not continue across the shear zone into the rocks of Leinster. Pollaphuca Formation to the NW, the Pollaphuca and the There are no equivalents to the Waterford-Wicklow or Dublinaccretionary complexes, along strike to the NE. Indeed Leggett et al. (1983), in their interpretation of the relationships between the Southern Uplands and the Lake District, drew an analogy between the situation of the Lake District in Silurian times and the Queen Charlotte Islandsof BritishColumbia at the present time. Age relationships suggestthat the Dublin terrane may have been removed from the Lake District margin by strike-slip faulting in the

Fig. 10. Map showing positionsof terrane boundaries, shear belts, major faults and shears. Segmented terranes and sub-terranesare patterned; otherwise named fault and shear zonesseparate tectonostratigraphic units. Dotson line show limitsof Upper Palaeozoic rocks. Black, deformed gabbros and serpentinites; toothed lines are thrusts. AF, Athgarrett fault; AHC, Annagh Hill complex; BCFZ, Bannow-Cahore fault zone; DSZ, Dublin Shear Zone (the unnamed shear zoneof Cooper & Briick 1983); FFZ, Ferns fault zone; GWL, western endof Graiguenamanagh- Wicklow Line, the magnetic linear (Maxet al. 1983) that is coincident with the GWSZ elsewhere. Very poor exposuresto the SW of the Leinster Granite has not allowed for good delineationof the shear zone boundaries; GWSZ, Graiguenamanagh-Wicklow Shear Zone, in part coincident with the East Carlow Deformation Zone of McArdle & Kennedy (1985) and merging with the Dublin Shear Zone, shown close dotted inclusiveof relict sheared textures in Leinster granite; RFZ, Rosslare Fault Zone; SFZ, Saltees Fault Zone; WFZ, Wicklow FaultZone; WKS, Wicklow Head- Kilmichael Schists. Imbricated zones (open dots) appearto have been stacked from NE to SW. The most prominent sinistral stacking occurs along the NE end of the WFZ. The accumulation of faulted lenses intoa strike-slip duplexmay have diverted the WFZ intoa new course along the GWSZ. Inset showsthe diagrammatic relationshipof the units between FFZ and the BCFZ. BA, Ballaghloughkeen granite; BF, Ballyhogue Formation; BB, Booley Bay Formation; CC, Cahore Group; PF, Polldarrig Fm. Dashed lines show interpreted extension of Ribband Group boundaries beneath the DuncannonGroup (V pattern).

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younger Silurian Formations of the Midlands terrane must References have been moved into their present position relative to the ADAMS,C. J. D. 1976. Geochronology of the Channel Islands and adjacent rest of theLeinster Massifby sinistraltranscurrent FrenchMainland. Journal of the Geological Society;London 132, movementalong the Slade Valley Shear Zone at a later 233-250. stage. The contrast in lithologies, the degree of shearing of ANGUS,N. S. & BRINDLEY,J. C. 1970. A swarm of dolerite intrusions in the Tallaght Hills, Co. Dublin. Proceedings of the RoyalIrish Academy, the rocks and the absence of the dyke swarm or of volcanics 69B, 165-178. to the NW of the shear zone all suggest that the amount of BAKER,J. W. 1966. The Ordovician and other post-Rosslare Series rocks in movement was substantial. Southeast Co. . Geological Journal, 5, 1-6. The Dublin and Midland terranes had reached more or - 1970.Petrology of the metamorphosed pre-Cambrian rocks of less their present relationship by the time of intrusion of the south-easternmost Co. Wexford. Proceedings of the RoyalIrish Academy, 69, 1-20. Leinster Granite at 404 Ma (early Devonian), as the granite BARBER,A. J. 1985. A new concept of mountain building. Geology Today, 1, cuts across the Slade Valley shear zone in the neighbour- 116121. hood of Baltinglass(Figs 1 & 2). TheLeinster granite - & BROWN,K. 1988.Mud diapirisrn: the origin of melangesin constitutes a ‘stitching pluton’ in the terminology of Jones et accretionary complexes. Geology Today, 4, 89-94. - & Mm,M. D. 1979. A newlook at the MonaComplex (Anglesey, al. (1983). North Wales). Journal of the Geological Society, London, 136, 407-424. Recently Cooper & Briick (1983) have suggested that the - TJOKROSAPOETRO,S. & CHARLTON,T. R. 1986.Mud volcanoes, shale presence of cleavagein the country rocks influenced the diapirs, wrench faults and melanges in accretionary complexes, Eastern emplacement of thegranite, so thatthe plutons are Indonesia. American Association of PetroleumGeologists Bulletin 70, 1729-1741. elongatedNE-SW parallel to the general trend of the BATES,D. E. B. 1972. The stratigraphy of the Ordovician rocks of Anglesey. cleavage.They have also suggested that later sinistral Geological Journal, 8, 29-58. movement along shear zones on either side of the granite BRENCHLEY,P. J. & TREAGUS,J. E. 1970. The stratigraphy and structure of hasmodified thetrend of the cleavage andhas locally the Ordovicianrocks between Courtown andKilmichael Point, Co. Wexford. Proceedings of the Royal Irish Academy, @B, 83-102. developed a foliation within the granite. One of these shear - HARPER,J. C. & SKEVINGTON,D. 1967.Lower Ordovician shelly and zones corresponds with the Slade Valley shear zone and the graptolitic faunas from south-eastern Ireland. Proceedings of the Royal other broadlyfollows thesoutheastern margin of the Irish Academy, 65B, 385-390. granite,where Brindley (1974) andMcArdle & Kennedy --, , MITCHELL,W. I. & ROMAN~,M. 1977. A reappraisal of some (1985)recognized theEast Carlow Deformation Zone in Ordovician successions in Eastern Ireland. Proceedings of the Royal irish Academy, 77B, 65-68. which shearing affects both theSE margin of the granite and BRINDLEY,J. C. 1974. The Graiguenamanagh Belt in the Leinster Granite. the rocks of the aureole (McArdle 1981). Further SE this Scientific Proceedings of the Royal Dublin Society, 5A, 137-143. shearzone passes between the Tullow-Lowlands and the - & MILLAN,S. 1973.Variolitic lavas from south CountyWick- BlackstairsUnits of theLeinster Granite as the Craig- low, Ireland. ScientificProceedings of the RoyalDublin Society, 4.4, 461-469. nuenamagh Shear Zone (Fig. 10). NE the shear zone passes -, MCARDLE,P. & SCHIENER,E. J. 1973. The Lower Palaeozoic volcanic into the Wicklow Shear Zone and coincides with boundary rocks of south-east Leinster, Ireland. Scient& Proceedings of the Royal betweenthe Dublin Terrane and the New Ross Unit. Dublin Society, 4A, 431-438. Movementalong these zones after the intrusion of the BRUCK,P. M. 1970. Stratigraphy, petrology and structure of the greywacke Leinstergranite cannot have been more than fewa formations of the Blessington area. Bulletin of the Geological Survey of Ireland, 1, 31-45. kilometres, as the margins of the Leinster granite are not - 1972. Stratigraphy andsedimentology of the LowerPalaeozoic substantially displaced. Many of the minor granites (Fig. l), greywacke formations in Counties KildareandWest Wicklow. however,appear to have been intruded earlier than the Proceedings of the Royal Irish Academy, 71B, 25-53. - 1973. Structure of the Lower Palaeozoic greywacke formations west of main plutons during intensive shearing. the Leinster Granite in Counties Kildare and West Wicklow. Scientific Transcurrentmovement along all terraneboundaries Proceedings of the Royal Dublin Society, 4A, 391-409. within the Leinster Massif had ceased by the Late Devonian -1975. A map and outline description of the Lower Palaeozoicrocks of S. as Upper Devonian to Lower Carboniferous rocks overstep W. Wicklow and S. Kildare (One-inch sheets 128 and 129). Report of the all theterrane boundaries unconformably. Minor normal Geological Survey of Ireland. 75/2. - 1976. The andesitic and doleritic igneous rocks of westWicklow and faultingand small scale NW-SE wrench faults, related to south Dublin. Bulletin of the Geological Survey of Ireland, 2, 37-51. Hercynian or Mesozoic-Tertiarymovements, have locally - & DOWNIE,C. 1974.Silurian microfossils from west of the Leinster modifiedthe outcrop pattern toproduce the present Granite. Journal of the Geological Society, London, 130, 383-386. Leinster Massif. -& REEVES,T. I. 1976. Stratigraphy. sedimentology and structure of the Bray Group in and south . Proceedings the Royal Irish Academy, 76B, 53-77. The authorsare indebted to the geologists who contributed the of -, COLHURST,J. R. J., FEELY,M. GARDINER,P. R. R., PENNEY,S. R., primarydata on which this synthesis is based.Most are REEVES,T. J., SHANNON,P. M,, SMITH, D. G. & VANGUESTAINE, M. acknowledged in the reference list but we are particularly grateful 1979. South-east Ireland: Lower Palaeozoic stratigraphy and depositional to P. M. Briick, T. J. Reeves, H. A. van Lunsen, J. H. Morris, P. history. In: HARRIS,A. L., HOLLAND,C. H. & LEAKE,R. E. (eds) The Shannonand P. R. R. Gardiner who madetheir maps and field Caledonides of the British Isles-reviewed. Geological Society, London, notesavailable and took part in extensive discussions concerning Special Publication, 8, 533-544. -, POTIER,T. L. & DOWNIE,C. 1974. The Lower Palaeozoic stratigraphy the geology of Leinster over many years. We are also grateful toP. of the northern part of the Leinster Massif. Proceedings of theRoyal Brenchleyand M. Bennett for discussionand to the Geological Irish Academy, 74B, 74-84. Society’sreferees for useful comments on the first draft. A. J. CARLISLE,H. 1979. Ordovician stratigraphy of the Tramore area, County Barber’s written contribution to this paper was prepared while he Waterford, with a revised Ordovician correlation for southeast Ireland. was Visiting Research Fellow at the Australian National University, In: HARRIS,A. L., HOLLAND,C. H. & LEAKE,B. E. (eds) The Canberra, and he is grateful to K. S. W. 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Received 18 October 1988; revised typescript accepted 16 February 1990.

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