Journal of rhe Geological Society, London, Vol. 146, 1989, pp. 575-596, 19 figs, 1 table. Printed in Northern Ireland
The metagabbros, orthogneisses and paragneisses of the Connemara complex, western Ireland
BERNARD ELGEY LEAKE Department of Geology & Applied Geology, University of Glasgow, Glasgow G128QQ, UK
President's anniversary address 1987
Introduction...... 575 Dalradiancountry rocks ...... 577 Metagabbros ...... 578 Mineralogy ...... 581 Orthogneisses ...... 581 Petrogenesis ...... 582 Geochemical variations ...... 582 Hornblende ...... 586 Isotopicevidence ...... 587 Structure ...... 592 Tectonicsetting ...... 593
Abstract: Although now largely disappeared within the 400 Ma-old Galway Granite, the syntectonic Connemara metagabbro-gneiss complex, which crystallized at 490 Ma (Arenig), is the largest intru- sion into the Dalradian rocks. It is part of widespread Cambro-Ordovician volcanism and plutonic injection into continental crust in the Appalachian-Caledonide orogenic belt, being synchronous in crystallization with the widely obducted 490Ma ophiolites within the orogeny. The peridotites and hornblende gabbros were injected, broken up, and shouldered aside by quartz diorite, granodiorite, graniteand trondhjemite gneisses, andit is shownthat these orthogneisses are not the in situ differentiation products of the gabbro magma. The hornblende in the metagabbros is mainly igneous but is partlymetamorphic and replaces olivine(Fo,,-~J, orthopyroxene (En,,), clinopyroxene (Wo,,En,,,~s,,,) and plagioclasetypically An,, (range An,-). An aureole at least 10 km wide in the sillimanite zone was imposed, and cordierite-garnet-sillimanite Dalradian hornfelses were partially melted at temperatures up to 10o0 "C near to the gabbros at 4.5 f 1 kbar corresponding to c. 18 km depth. This suggests that the complex forms part of the root of a now eroded batholith of calc-alkaline affinities, the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths. Within the orthogneisses are sheets of paragneiss formed from Dalradian metasediments which have been folded up from the floor and down into sides of the magmachamber, whichprocess could be illustrative of themeans by whichbasic magma can intimately interact with crustal material. The '80/160,D/H, Nd and Sr isotope evidence, and the existence of inheritedzircons in the gneisses, indicate that the gabbro and gneiss magmas were significantly contaminated with crustal, possibly Dalradian, material both before and after intrusion, with the lowestinitial at490Ma being 0.708 evenin samples far removed from metasedi- ment. The metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc-alkalinegeochemistry. The chemistry, igneous hornblende and highlycalcic plagioclase match many arc plutons in the western USA. The syntectonic nature and the ubiquitousgneissic fabric in the quartz diorites, trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the Connemara massif, and which culminated in slicing off the largely inverted complex and carrying it southwards on the Mannin Thrust which underlies the whole of Connemara. The complex is thusa fragment of a now disruptedCambro-Ordovician continental magmatic arc or continent-island arc environment which can be traced along the SE edge of the Laurentian continent.
Introduction Connemara is southwardlya thrustedand eastwardly the largest igneous complex in the Dalradian rocks of strike-slip faulted fragment of Dalradian rocks (Leake et al. Scotland or Ireland.This E-W-striking metagabbro and 1983) in Co. Galway, western Ireland (Fig. 1). Intruded into quartz diorite-granodiorite-granite gneiss complex extends the Dalradianmetasediments and pre-dating the basal from Slyne Head in the west to east of Galway City, with no Mannin thrust which underlies the whole of Connemara, is visible termination at either end of the strike length of over
575 516 B. E. LEAKE
I DALRADIAN SCHISTS
10 Km
Fig. 1. Sketch map of the geology of Connemara, western Ireland showing the metagabbro-orthogneiss complex,fringing the paragneiss and the sillimanite isograd with sillimanite occurring to the south. A separate sillimanite isograd surrounds the northern intrusive complex,roughly corresponding to the paragneiss periphery. The Shannavara district shownin Fig. 2 is outlined showing the junction of the ortho- and paragneisses in particular. The true scale north-south cross-section, modified from Leakeet al. (1983), shows the thrust bottomof the complex although the metagabbro sequenceis inverted. B is the Wild Bellows Rock. Ornamentin section is as on map, except Ballyconneely Amphibolite which is shown stipplewith the ortho- and para- gneiss grouped together.
80 km (Fig. 1). Most of the complex has been lost in the the Batimoregabbroic complex in Maryland (Shaw & younger 400Ma Galway Granite (Leggo et al. 1966) but Wasserburg 1984) and the 496 * 14 Ma zircon U-Pb age from its northern intrusive edge with the Dalradian rocks, for the Elkahatchee quartz diorite, the largest gabbro-gneiss the complex now extendssouthwards 26km (andmore complex in the SouthernAppalachians (Drummond pers. originally) until it is truncated by the E-W Skird Rock fault comm.) TheConnemara metagabbro-gneiss complex is (Fig. 1). It includes, near its northern margin, paragneisses thereforepart of a major intrusive episode in the early formed from theDalradian country rocks. Similar Ordovician in the Appalachian-Caledonide orogenic belt. peridotite, metagabbro and gneiss intrusions occur in north Previous work on the complex (Fig. 1) includes studies Connemarawhere the disrupted Dawros-Currywongaun- of the Roundstone part of the intrusion by Wager (1932), Doughruagh intrusion (Rothstein 1957; Kanaris-Sotiriou & Morton (1964) and Bremner & Leake (1980), the area north Angus 1976; Bennett & Gibb 1983) is, like its southern and northwest of Roundstone by Benjamin (1968), Evans & counterpart, associated with a high grade metamorphic Leake (1970) and Downs-Rose (1985), thearea eastand aureole. northeast of Slyne Head by Ahmed & Leake (1978) and The metagabbro has been dated by U-Pb, and Pb-Pb Leake (1986), the Cashel district by Leake (1958, 1970a, studies of zircon at 490 f 1 Ma (Jagger et al. 1988), i.e. 1970b) Jagger (1985) and Jenkin (1988), the Glinsk district lower Arenig (Harland et al. 1982), which is closely similar by Harvey (1967), andthe Shannavara district by Senior tothe reported Rb-Sr age of theAberdeenshire gabbros (1973). Published work hasconcentrated largely on the (Pankhurst 1970), the zircon U-Pb age of 490 f 2 Ma for metagabbroswhereas the petrochemistries of thequartz THE CONNEMARACOMPLEX OF IRELAND 577 diorite to graniteorthogneiss series, andthe paragneisses Where the countryrocks come into contact with the produced fromthe Dalradian metasediments, are almost orthogneiss, as distinct from metagabbro, or form inclusions completely unpublished despitemajor thesis contributions within the orthogneiss (Fig. 2), there is extensive by Harvey (1967), Senior (1973), Keeling (1981) and Jagger development of paragneiss, typically quartz-porphyroblastic (1985). The presentaccount will therefore summarize the andesine (An,) biotite gneiss with or withoutvariable available information on the whole complex, especially the amounts of K-feldspar (<30%), magnetiteand ilmenite gneisses, and includes a detailed geological map of a critical (<10%), garnet (<10%), cordierite(<30%), andalusite area-the Shannavara district (Fig. 2)-which is crucial in ( The Dalradiancountry rocks were severely hornfelsed xx 055 and partially melted neartothe larger metagabbro X 1 xx X intrusions with the production of graniticpartial melts. 050 X Some of the melts may havebeen incorporatedinto the basic magma, but at Cashel some of the melt was segregated into agranite sill in aslip zone along which the Cashel intrusion and its immediately adjoining mobilized hornfelses moved laterally relative to the more distant (c. 30-300 m) hornfelses. The concordant granite sill has been described and analysed (Leake 1970b; Ahmed-Said, Pers. Comm.); it is a K-rich granite tending to be syenitic; it closely agrees with the calculated composition of the material obtained by partially melting typical Dalradian Connemara schist and was clearly segregated ontothe movementzone. Jagger X (1985) has confirmed the partial melt origin by determining 0.20 X *X 030 1 the initial R7Sr/86Srratios at 490Ma as 0.720 which is well 0.10 n within the range of the hornfelsed Dalradian metasediments and much higher than any of the orthogneisses or metagabbros. The detailed description of the composition of the material removed from the hornfelses and the formation of 'S-type' granite is given in Ahmed-Said 8~ hake (unpublished). Metagabbros The metagabbros are the earliest intrusions in the complex and include peridotites and anorthosites but are atleast 90% composed of hornblendegabbros or hornblende gabbro- B a norites with hornblende and bytownite(often An,,,) or I ox 1 xx labradorite with relict diopside-salite and sometimes orthopyroxene. Both pyroxenes and plagioclase are considerably replaced by hornblende. Magnetite, apatite and pyrite are accessory minerals. Apart from Dawros (Fig. 1) which is mainly lherzolite, wehrlite and harzburgite with some chrome spinel-rich cumulates (Rothstein 1957), the peridotites form disruptedpods mainly atRoundstone. Everywhere the metagabbros are injected and broken up by thelater gneiss with extensive agmatiteformation, and hornblende is both late-statea igneous mineral and metamorphically-produced, as along the margins of metagabbroagainst gneiss. Igneous layering is generally poorly preserved,and rocks are massive or poorly metamorphically foliatedmetagabbros. The existence of xenoliths of earlier-crystallized basic rock in later phases and disruption of igneous layering by later gabbroicinjections reveals a complex igneous histroy only partly understood. Detailed descriptions of the various parts of the intrusion are given in the references already listed. Above the Mannin Thrust plane (Fig. 1) which underlies the whole complex, the metagabbros and orthogneisses were ground down, recrystallized into fine-grained, N-S lineated, 55 i 00 Ballyconneely amphibolites and siliceous tectonitesand thrustover the metarhyolites of the Delaney Dome Formation (Leake & Singh 1986) which underlie the Mannin Thrust (Leake 1986). As the Mannin Thrust is folded by F4 and F5 folds in the Connemara fold sequence, this dates the metagabbros aspre-F4. They are also folded by theF3 Cashel synform and antiform,and the writer previously Fig. 4. Plots of Niggli parameters for the metasediments (crosses), quartzofeldspathized metasediments i.e. paragneisses (crosses inside circles) and strongly hornfelsed pelites(circles) from the Shannavara district. Analyses from Senior(1973). No systematic I displacement of the paragneissesfrom the range of metasediment 50200 150 100 250 3W 350 400 variation is apparent. W is oxidation ratio. S1 0 5 10 15 20 25 0 5 10 15 20 25 '/o A1203 A 1203 Fig. 5. Plots of trace elements against wt 96 AI2O3for Shannavara metasediments (crosses) and quartzo-feldspathizedmetasediments i.e. paragneisses (crosses inside circles). Analyses from Senior (1973). 580 B. E. LEAKE Table 1. Chemical analysis of hornblendes from metagabbros andgneisses BL 1798 1443830 1951 931 1396 500 652 679 SiO, 48.80 44.16 44.05 43.53 42.85 50.31 41.90 40.33 42.19 TiO, 0.50 1.49 1.48 1.27 1.57 0.43 1.58 1.57 1.35 Al,03 7.30 11.02 10.05 12.23 12.59 5.89 11.10 11.02 10.54 Fe,O, 3.39 4.76 4.83 4.56 1.14 2.48 5.44 Fe0 7.88 9.30 9.43 10.28 13.36 9.25 13.59 20.40: 19.52: MnO 0.23 0.23 0.23 0.36 0.44 0.25 0.48 0.53 0.39 MgO 16.20 13.49 13.75 12.35 10.42 16.70 9.55 7.93 8.85 CaO 11.75 11.19 11.45 11.26 11.60 11.04 11.64 12.10 12.14 Na,O 1.14 1.12 1.85 1.21 1.88 0.91 1.OO 2.04 0.78 KZ0 0.75 0.71 0.90 0.75 1.33 0.50 1.48 1.92 1.61 P@, 0.02 0.00 0.22 0.05 0.03 0.01 0.02 0.00 0.00 H,O+ 2.12 2.082.49 2.27 2.31 2.16 2.34 Total 100.08 99.96 100.32100.1299.52 99.93 100.12 97.84 97.37 Ions to 24 (0,OH) Si 7.00 6.41 6.44 6.35 6.37 7.20 6.28 6.27 6.48 AI'" 1.00 1.59 1.56 1.65 1.63 0.80 1.72 1.73 1.52 AI"' 0.23 0.29 0.17 0.45 0.58 0.20 0.24 0.29 0.39 Ti 0.05 0.17 0.17 0.14 0.18 0.04 0.18 0.18 0.16 Fe3 0.36 0.52 0.53 0.50 0.13 0.27 0.61 Fe2 0.95 1.13 1.15 1.25 1.66 1.10 1.70 2.65 2.51 Mn 0.03 0.05 0.03 0.04 0.05 0.03 0.06 0.07 0.05 3.46 2.91 3.00 2.68 2.30 3.56 2.30 2.68Mg 3.00 2.91 3.46 2.13 1.84 2.03 Ca 1.80 1.74 1.79 1.76 1.85 1.70 1.86 2.01 2.00 Na 0.32 0.31 0.53 0.34 0.53 0.25 0.28 0.63 0.23 K 0.14 0.14 0.17 0.14 0.25 0.09 0.29 0.38 0.32 OH 2.02 2.41 2.02 2.21 2.29 2.06 2.34 [2.001 P.001 0 21.98 21.59 21.98 21.79 21.71 21.94 21.66 [22.00] [22.00] Cell dimensions aA 9.863 9.844 9.836 9.8319.853 bA 18.096 18.045 18.044 18.04318.111 CA 5.305 5.308 5.3195.309 5.306 B 104.95 104.925 104.873104.927 104.786 914.8 911.1 912.1 909.9VA912.1 911.1 914.8 915.5 * Total Fe as FeO. BL 1798. Magnesio-hornblende. Hornblende-saussuritized plagioclase gabbro. Peninsula SE of thenorthern island andNNE of thesecond most northern island in Loughaunemlagh, Cappaghoosh Townland. This, and all following samples are located on 6 inch (1 : 10560) sheet Co. Galway 51 and were (wet) analysed by B. E. Leake except BL652 & 679. BL 1443. Tschermakitic hornblende. Poikilitic 2 cm diameter hornblendes, partly replaced by prehnitized and chloritized biotite in a poikititic gabbro layer with some altered bytownite. Southern point of Lough Wheelaun; northern part of Lettershinna Townland. BL 1951. Magnesio-hastingsitic hornblende with saussuritized plagioclase, magnetite and traces of ortho- and clino-pyroxene in a gabbro. Chemical analysis and mode in Leake (1958, 1970a) 197 m NE of the summit of Cashel Hill. BL 830. Tschermakitichornblende with saussuritized An,,, magnetite,chlorite, apatite and sulphide in a gabbro sill, W of Cashel Hill Details in hake (1970a). BL 931. Potassian ferroan pargasitic hornblende with labradorite, quartz, chlorite, magnetite, epidote and apatite in gabbro injected by gneiss. 300 m SSW of the western point of Lough Nambrackkeagh, NW Lettershinna Townland. Rock analysis in Leake (1970a). BL13%. Magnesio-hornblende. Bright green metamorphic amphibole from hornblendite agmatite. Bunahown Townland. 204111 east of BM63.1. BL 500. Potassian tschermakite in hornblende quartz diorite gneiss. N of Lough Alurgan and E of BM99.0, SE of theCashel road junction, Lettershinna Townland. Rock analysis in Leake (1970~). BL 652. Potassian ferroan pargasitic hornblende in a K-feldspar-rich hornblendic quartz diorite gneiss. Lettershinna Hill triangulation point, Lettershinna Townland. Electron microprobe analysis. BL679. Potassian ferroan pargasitic hornblende from a gabbro injected by K feldspar granite gneiss 325 m SE of the Zetland Arms Hotel, Bunahown Townland. Electron microprobe analysis. THE CONNEMARACOMPLEX OF IRELAND 581 concluded (1970~) that the syn-magmatic deformation was The extremely calcic compositions suggest crystallization therefore. syn-F2 in theConnemara fold sequence. under high pH,0 which promotes enhanced An percentages Tanner (pers. comm.)however has pointed out that thereis no in typical basaltic magma (Thy 1987). evidence of F2 involvement, and that intrusion pre-F3 or The amphiboles (Table 1; BL1798, 1443, 1951, 830, 931) contemporaneous with the earlyphases of F3 fits the are original igneoushornblendes ranging from tscher- available evidence best. This seems most likely to be correct makitic, pargasitic, and magnesio-hastingsitic, hornblende inview of the syn- topost-F3 nature of the sillimanite to magnesio-hornblende using the nomenclature of Leake isograd in Connemara, which appearsto be the result of (1978). Often significant is the enrichment in K and OH, regional heating by the igneous complexes of both south and with the latter frequently exceeding the commonly assumed north Connemara. 2.0 OH per 24 (0,OH) and resulting in cation totals of c. Everywherehydrous alterations of hornblendeto 15.3 instead of themore usually supposed 15.5. These chlorite andepidote are usual, also of plagioclase to features suggest a basic magma unusually rich inK and saussurite andsericite, of pyroxene to chlorite and water. Fe3 is often 25-33% of the total iron which indicates serpentine, of olivine to serpentine and magnetite, and of a rather oxidized magma,a conclusion supported by the biotite to chlorite and sometimesprehnite. Jenkin (1988) common presence of significant amounts of magnetite. has shown that this low temperature episode was distinct Metamorphic replacements of orthopyroxene by anthophyll- from the magmatism of the rocksand was due a later ite, of clinopyroxene by tremolite-actinolite and of hydration unconnected with the water-rich gabbroic magma. pyroxene plus plagioclase by actinolitic of magnesio- hornblende are usual, especially in amphibole vein networks and in the peripheries of basic agmatitic masses broken off Mineralogy by intruding gneiss (Table 1, BL 1396). Gabbro injected by Olivine is invariably much serpentinized but relict grains K-feldspar granitic gneiss can develop K-rich hornblende from Dawros yield F09242 (Bennett & Gibb 1983) although demonstrating late, postmagmatic (with respect tothe the most commonrange in theRoundstone, Cashel and gabbro) crystallization (Table 1,BL679). In short, the other bodies in the main intrusive complex is F07~90with a amphibolesrange from igneous megacrysts through very strongconcentration atFomf2. The most iron-rich late-stage igneous to metamorphic types. grains determined are F075 and thus there is comparatively Biotite,sometimes grown along the amphibole cleav- limited iron-enrichment, probably because of the common ages, together with sulphides, apatite and small amounts of crystallization of magnetite. late replacive quartz are widespread. Some samples contain The clinopyroxenes are extensively replaced by horn- up to 10% of ilmenite plus magnetite. blende but relict cores survive especially in plagioclase-poor rocks. The compositions determined rangefrom Orthogneisses WO~E~,,.,FS~.,at Roundstone, WO~~.,E~,,.,FS, on Inish- In terms of internationallyagreed nomenclature (Strecke- dawros and Wo4,En5,Fs5 at Dawros, to atleast WO,,E~~,FS~, isen 1976) these gneisses range from quartz diorite through atRoundstone (Rothstein 1958; Bremner & Leake 1980). tonalite, granodiorite and granite to alkali granite but also These pyroxenes are slightly but significantly richer inCa include some trondhjemite and are unusually quartz-rich, a thanthose crystallized from typical tholeiitic magmas (c. fact first emphasized by Benjamin (1968). There is complete Ca,,, at En4-), andapproach the high Ca values long gradation between all these varieties and it is convenient to known to occur in some distinctly alkalinesuites (Murray consider the spectrum of orthogneisses under three headings 1954; Wilkinson 1956). Nevertheless the low AI2O3, which in order of intrusion are hornblendic quartz diorite generally less than 2.5%, low TiO, (<0.50%) and high SiO, gneiss, hornblende-free quartz diorite gneiss and K-feldspar (>51%) contents of the clinopyroxenes, together with the gneiss. K-feldspar can occur in any of thequartz diorite common coexistence of orthopyroxene makes it quite clear gneisses but an arbitrary minimum of 20% is taken as the that the magma did not have alkaline affinities (Le Bas 1962; lower limit of the K-feldspar gneiss, which therefore is Deer et al. 1978). The high Ca of theConnemara usually granitic in composition. clinopyroxenes almost certainly results from the high water Hornblendic quartz diorite gneiss is formed of roughly content of the magma leading to depression of the liquidus equal proportions of green hornblende, plagioclase An,, andsolidus, a conclusion supported by the scarcity of and quartz with lesser biotite and a grain size of 1-5 mm. It exsolved orthopyroxene lamellae(Elsdon 1971) together is both intimately associated with the metagabbros and quite with the abundant amphibolitization of the pyroxenes. separate. Some varieties containaugite relicts inside the The orthopyroxenes show a wide range of composition, hornblende, others havevariable contents of K-feldspar, from En,, to En,, at Dawros (Bennett & Gibb 1983) and and in thelatter rocks amphibole may form upto 1cm En8=, at Currywongaun-Doughruagh (Kanaris-Sotiriou & poikilitic crystals of K-rich hornblende (Table 1, Angus 1976), but compositions more magnesian than En,, BL 500,652). Apatite, titanite, zircon, orthite and iron ore are generally unusual; most range between En, and En,,; are accessory minerals. Quartzand K-feldspar commonly with the lower limit (below En,) remaining to be corrode the plagioclase which is heavily saussuritized and determined. Nevertheless the absence of inverted pigeonite sericitized. The hornblende-free varieties are similar but even in compositions near En, implies that the temperature have a more sodic plagioclase, typically An,,. They grade of crystallization of the magma was unusually depressed into trondhjemites or K-feldspar gneisses andare not relative to the orthorhombic-monoclinic pyoxene inversion intimately associated with the metagabbros although, like all curve and the most likely cause of this was the high water the gneisses, they disrupt the metagabbros. The K-feldspar content of the magma. genisses contain microcline (20-60%), quartz (20-40%), The plagioclases, which are invariably partly saussur- andesine (10-30%) and up to 10% biotite 2-6 mm in grain itized and sericitized, rangefrom An, (Leake 1964) to diameter with accessory apatite, zircon, magnetite,pyrite about An50 and compositions An,,, arequite common. and zoned orthite. Finegrained K-feldspar-rich aplites, 582 B. E. LEAKE sometimes extending from aplopegmatite patchesare common compared with those from Glinsk and Cashel, it is clear that in and near K-feldspar gneiss. they are chemically very similar (Fig. 6) except for Niggli k The orthogneisses invariably inject, break up or enclose (i.e. mol K,O/mol (K,O + Na20)).The Shannavara fragments of metagabbroand are clearly laterthan the metagabbros are systematically higher in K throughout the metagabbros. The gneisses were emplaced during strong F3 whole mg range which indicates that the enrichment in K is tectonicmovements which resultedsyntectonic ain not due to magmatic differentiation but probably correlates foliation, drawing out of Dalradian countryrockxenoliths with K metasomatism linked to the abundance of K feldspar and intense disruption of the earlier metagabbros, as clearly granite gneiss in the Shannavara arearather than brought out both in individual outcrops and in the overall trondhjemite. This is manifest in the partial replacement of mappeddistribution of metagabbro (Figs 1 and2). The hornblende by biotite growing along the amphibole foliation in the gneisses is usually free of small folds and is cleavage. Accompanying the increase in K were Ba and Rb weakest in massive K feldspar-rich granite patches which are while Caand sometimes Na may have been lost (Fig. 8). especially well developed in the Cashel and Shannavara Similarly, throughout the whole complex, silicification of districts e.g. Lettershinna Hill (Leake 1970a; Jagger et al. patches of metagabbro, especially in agmatites and when 1988). This suggests the crystallization of the K-feldspar injected by quartzdiorite gneiss, gives rise to quartz gneiss was last in the gneiss sequence while the injection of replacing hornblende or plagioclase. Most silicified samples K-feldspar andhornblende-free quartz diorite intothe have been excluded from the summary plots, but the process hornblendic variety identifies the latter as the earliest gneiss. was studied in detail at Roundstone by Downs-Rose (1985) There is no regular distribution of gneiss types within the who showed that other elements such as P, Zr, Sr, Rb, Ba intrusive complex in the manner commonly found in zoned and light to middle REE (La-Gd) were added with the Si. diorite,granodiorite orgranite plutons,but K-feldspar Overall the metagabbros clearly belong to a tholeiitic gneiss is rare west of Cashel whereas it is extremely common magma based onan AFM plot (Fig. 9), the common within and to the east of the Cashel district. Typically layers occurrence of two pyroxenes, andthe presence of small of different orthogneisses lie next to each other enclosing amounts of q, not ne, in the norm although a number of fragments of metagabbros andsheets of country rock analysed metagabbros just spreadfrom the tholeiite fold xenoliths,often over 1 km long,forming lensoid shapes. into the edge of the alkaline fold on the K,O + Na,O versus Where K-feldspar gneiss adjoins quartz diorite gneiss there SiO, plot of Irvine & Baragar (1971). There is astrong may bea gradation to K-feldspar-bearing quartz diorite tendency to high-alumina basalt magma with about 40% of gneiss, and K-feldspar porphyroblasts suggest K- the available analyses possessing over 17% AI,03 which is a feldspathization has sometimesoccurred. Clearly the feature recognized as common in arc basalts. It is important distribution of the different gneisses andmetagabbros is to notethat although superficially the gneisses appear to inconsistent with fractionation in situ, but is consistent with follow on from the metagabbro fractionation trends this is injection of different magma pulses into the Dalradian rocks not the correct interpretation if thedata are examined and into earlier injections, with some marginal mixing and carefully. Thus the Na,O plot shows that the metagabbros metasomatic activity. The last two processes are well fractionate with increasing SiO, tothe Ballyconneely demonstrated by the development of K-feldspar-rich quartz amphibolites with 3-6% Na,O, whereas the gneisses form a diorite containing potassian hornblende (e.g. BL 652, Table completely different trend, anda similar contrast occurs 1) where hornblendic quartz diorite adjoins K-feldspar-rich with Ti0,. The metagabbro CaO trendat SiO, values above granitic gneiss. 50% parallels at a lower level the gneiss trend rather than abutting it. Likewise, in the Niggli si versus mg plot (Fig. Petrogenesis lO), the gneisses clearly do notfractionate from theend differentiate of the metagabbro which is the Ballyconneely Geochemical variations amphibolite. Cr and Ni plots confirm this interpretation as The metagabbros display trends of chemical variation there is substantialoverlap of thelate, lowNi and low consistent with overallcontrol by igneousfractionation, Cr-bearing metagabbros fractions with the gneisses whereby probablyfrom oneparent magma. Thus Niggli mg the Ballyconneely amphibolitesreach much lower values (= Mg/(Mg + Fe2+ Fe3+ Mn)) falls with declining Cr, Ni than a great many of the gneisses (Figs 10 and 11). There and fm and increasing ti, p, Rare Earth elements and alk are distinctly different trends of variation forthe (Fig. 6). The peridotites are among the earliest crystallized metagabbros and gneisses which are brought out by Figs 7,9 metagabbros, andthe Ballyconneely amphibolites (Fig. 1) and 11 and the si-mg plot of Fig. 10. are the most differentiated and latest rocks. Figure 7 shows The gneisses asa whole show calc-alkaline (not rather poorcorrelations of TiO,, FeO,MgO, A1,03 and tholeiitic) differentiation trends similar to those determined K,O with SiO, for the metagabbros but a significant decline for the classical ‘Caledonian’ igneous suite of Scotland by of CaO and Niggli mg and a rise of Na,O with increase in Nockolds & Mitchell (1948) and Nockolds & Alien (1953). SiO,. The scatter on the metagabbro plots is indicative of Figures 7 and 9 show that there is a clear fractionation trend accumulative assemblages e.g. olivine-rich (high MgO low from the hornblendic quartz diorite gneisses through the CaO, A1203,TiO, and Na,O), plagioclase-rich (high CaO hornblende-free varieties to the K-feldspar and trondhjemi- and Al,03, low MgO and FeO) or magnetite (high Fe0 and tic gneiss with only the K,O showing great scatter at high TiO,, low SiO,). The highest Na20 values of the SiO, contents. This reflects the K-rich and K-poor nature of Ballyconneely amphibolite might in part be influenced by the K-feldspar andtrondhjemite gneisses, with many shearing above the Mannin Thrust. intermediatesamples having low to moderate contents of However thereareother influences than simple K-feldspar. The existence of trondhjemiteand K-feldspar magmatic differentiation. Thus if the metagabbros from the gneisses does not favour in situ fractionation, which would Shannavaradistrict, asstudied by Senior (1973), are be expected to give a more uniform end-stage magma, but THE CONNEMARACOMPLEX THE OF IRELAND 583 4c 3c 2c 0.6 1 30 0.6 0.4 0.2 m'O 0.5 0.4 m p 0.3 0 0.2 m m m -30 La 0.1 2.0 ti i.o 0 70 60 fm 240 50 40 160 Cr 80 25 20 15 alk 'O 5 0 160 Ni 140 si 120 80 100 80 0 0.50 0.55 0.60 0.65 0.70 045 0.50 0.55 0.60 0.65 0.70 mg mg Fig. 6. Plots of Niggli parameters and trace elements(in ppm) against Niggli mg for 44 metagabbros from Cashel (Leake1958) and Glinsk (Harvey 1967) which plot within the field outlined. 19 metagabbros from Shannavara (squares)confirm the same trends except forNiggli k(= mol K,O/mol (K,O + Na,O)) which is systematically higher in the Shannavara samples. La,Ce, Ga and Zn values for the Shannavara samples only (Senior 1973). favours injection of different magma batches which variably the arrangement of the different magmatic members, which mixed in the complex. is not systematic from metagabbro through quartz diorite Moreover, the distribution of the metagabbros and gneiss to K-feldspar granitic gneiss, does not agree with gneisses shows (Figs 1 and 2) that the latter cross-cut and significant igneous fractionation at the level now exposed. inject the former in a manner most compatible with separate The distribution of the Rare Earth elements (REE) in injections intothe Connemara complex. The existence of the complex is summarized in Fig. 12 based on Downs-Rose numerous metasedimentary layers within the complex, and (1985). There is small, but significant, increase in REE with 584 B. E. LEAKE . . .a METAGABBROS 16 t 0. A BALLYCONNEELY AMPHIBOLITES SlLlClFlEDMETAGABBROS + ANDQUARTZOSE BASICROCKS 0 GNEISSES 0 A Fig. 7. Plots of SiO, wt % against 16 Af various oxides (wt%) and Niggli mg for the whole Connemara metagabbros- orthogneiss suite. The most fractionated HORNBLENDEQUARTZ DIORITES> metagabbro samples are the Bally- HORNBLENDE-FREEQUARTZ DIORITES> conneely amphiboliteslying above the K FELDSPARGRANITE a Mannin Thrust (Fig, 1). The single line Connemara complex generally matches 0 except for Na,O. Si02 wt % fractionation of themetagabbros, the latestdifferentiates, generally insignificant, positive anomaly which suggests that the Ballyconneely amphibolites, having a narrow range of the magma was sufficiently oxidized that EuZ+ substitution variation with generally the highest REE contents. There is into plagioclase was not important. The overall pattern is a slight light REE (LREE) enrichment with the normalized similar tothat of many tholeiitic to high-alumina or calc La/Lu ratio (i.e. *La/*Lu) being generally in the range 1.2 alkaline arc basalts (e.g. Kay et al. 1982; Gromet & Silver to 4. TheEu anomalies vary from slightly negative to 1987). strongly positive but most metagabbros have a very small, The quartz diorite gneisses have a different REE pattern THE CONNEMARACOMPLEX OF IRELAND 585 Ot 0 ot 00 mg 0 00 or 0000 0: 5 I 00 0 0 7 0 < 00 O0 8O K2C X e Ar A A A 0 0 4 W!o2 % 2 1 28 26 24 22 + 0 20 *** 18 16 A' 2'3 % 12 10 0 8 .* :m** 6 '** 4 0. .* 2 1IIlI IIIIIIIII I1 1111 38 40 42 44 46 48 50 52 54 56 58 6062 64 66 68 70 7274 76 7880 Si02 wt % Fig. 7 cont. 586 B. E. LEAKE from the metagabbros, being strongly LREE enriched with typically *La/*Lu values of 5-15 (Fig. 12). They grade into the K-feldspar granitic gneiss which has an even more pronounced LREE enrichment with *La/*Lu in the range 15-60. All gneiss varieties have arange of Eu anomalies from negative to positive. Although there are differences between the metagabbro and gneiss patterns, the lowest silica gneisses (54-56% SO2) overlap in their REE patternswith the most LREE-enriched metagabbros, andthe overallpicture of variable Eu anomalies and variable LREE enrichment is similar to the patterns of agreat many calc-alkaline magmatic arc and batholithic rocks (e.g. Arth et al. 1988; Henderson 1983; Snoke et al. 1981). The patterns are consistent with either a mixing or fractionationrelationship between the gabbros and the gneisses. As typical post-Archaean metasediments, including Dalradian rocks, exhibit a similar LREE-enriched pattern, assimilation of such metasediments would promote the strong LREE enrichment but it is entirely feasible that the REE patterns areessentially those of a primary tholeiitic magma initially enriched in LREE. Gromet & Silver (1987) modelled the production of LREE-enriched tonalite magma by 30% partial melting of tholeiitic to high alumina basalt at high pressure, while the formation of arc basalt is generally acceptedas partial melting of the mantle wedge under the influence of water released from subducted oceanic crust (Wyllie 1984; Plank & Langmuir 1988). Hornblende A majorquestion is the origin of the metagabbro hornblende which partially replaced both pyroxene and plagioclase. The evidence is clear that it is late magmatic to metamorphic in crystallization and was nota primary fractionatingigneous mineral. Thusan AFM plot of the metagabbros (Fig. 9) shows a clear trend away from the position of olivine and pyroxenes, using the range of compositions determined in theConnemara metagabbros, but not away from the range of analysed hornblendes until the quartz diorite gneisses formed. Hornblende occurs on the end of the metagabbro tholeiite trend. Likewise log-log plots of Ti02 and Zr (Fig. 13) show that olivine and,or pyroxenes, dominated the mafic fractionation of these elements with somemetagabbro compositions being influenced by the common modal magnetite but hornblende apparently having little influence. Similarly Keeling (1981) showed by means of the CMAS plot (O'Hara 1976), using a range of analysed metagabbro minerals, that hornblende could not have been a significant fractionating phase. This was further confirmed by the flat pattern of chondrite- normalized heavy to middle Rare Earth element (*HREE to *MREE) plots which is inconsistent with substantial hornblende fractionation from the magma (Fig. 12). Finally Jenkin (1988) showed thatthe 6D and 6"O values of definite subsolidus-grown hornblendesin the metagabbros arethe same as in the 'magmatic' hornblende, which suggests that all the amphiboles crystallized from similar late-stage magmatic fluids and range from late magmatic to Fig. 8. Plots of Connemara metagabbros demonstrating theK, Ba and Rb-richer natureof the Shannavara metagabbros comparedto metagabbros from elsewhere in Connemara which are notso intimately associated with K-feldspar granitic gneiss. THECONNEMARA COMPLEX OF IRELAND 587 m METAGABBROS + HORNBLENDICGNEISS 0 HORNBLENDE-FREE GNEISS 8 HORNBLENDE- KF GNEISS 0 KF GNEISS Analysed @ KF APLITES / HBDES c PX Fig. 9. A(Na,O + K,O) F(tota1 Fe as FeO) M(Mg0) of a representative sample of 180 Connemara metagabbros and orthogneisses. The broken line separates the tholeiite field (above) from the calc-alkaline field (below) according \ to Irvine & Baragar-. (1971). . HBDES, hornblendes. A M auto-metamorphic, the fractionation of themetagabbro Isotopic evidence magma being accomplished by other minerals. The crystallization of hornblende is therefore ascribed tothe Jagger (1985) showed that the initial 87Sr/ssSr ratios of the high water content of the magma. The fact that the K-Ar metagabbros at490Ma are never below 0.708, andthis age of latemagmatic hornblende from the Cashel-Lough value is obtained uniformly from the Dawros peridotite, the Wheelaun intrusion is 486 f 9 Ma (Elias et al. 1988) while Roundstone intrusion (which is several kilometres from any the same handspecimen yields zircon with a Pb-Pb age of metasediment) and the centre of the Cashel intrusion. The 490 f 1Ma (Jagger et al. 1988) suggests that this hornblende metagabbro magma had thereforean unusually high must have grown as part of the magmatic episode and is not 87Sr/ssSrratio for a basaltic magma which suggests either an an entirely later superimposed effect. enriched mantle source or significant crustal contamination It is clear however thatthehornblende grown before emplacement. Samples from the edge of the Cashel peripherally inwards into metagabbro fragments in agmatite, metagabbro, which contain visible metasedimentary xeno- the hornblende grown around veins of quartz diorite or liths in all stages of solution, invariably yield initial 87Sr/86Sr K-feldspar gneiss injections intometagabbro, and the ratios higher than 0.708 at 490 Ma, ranging up to 0.714 (Fig. actinolite-tremolite replacements of pyroxenite layers, are 14). This is consistent with the assimilation of Connemara metamorphic even if immediately post-magmatic amphibole Dalradianmetasediment which Jagger (1985) has shown growth passed into metamorphic growth. typically had 87Sr/86Srat 490 ma of 0.716 to 0.727. The hornblende in the orthogneisses is clearly entirely of Jagger (1985) also showed that the Cashel metagabbros igneous origin, and the high K,O (1.40-2.00 wt %) content without xenoliths had '43Nd/'44Nd ratios of 0.5116 to 0.5118 of the amphibole is directly relatedto the amount of at 490 Ma,the metasedimentcontaminated metagabbros K-feldspar in the gneiss. The AFM plot (Fig. 8) shows the have very slightly lower ratios (0.5115 to 0.5116) whereas gneisses fractionating away from the hornblende field. The the metasedimentsrange from 0.5111 to 0.5114. An distribution of the gneisses on the TiO, & Y-Zr plot (Fig. ESr-eNd plot (Fig. 15) suggests thatthe non-xenolithic 13) is ambiguous as regards hornblende fractionation metagabbroshad suffered somecrustal contamination presumably because other minerals than hornblende such as beforeemplacement intothe presentmetasedimentary zircon, biotite and very largemodal contents of feldspar, envelope. It is unlikely to be metasediment assimilation at influence the plot andthe affect of the relatively small the exposed level of erosion because of the distance from proportion of hornblende is hidden. The REE patterns of any metasediment shown by some of the non-xenolithic the gneisses are overall convex downwards which is metagabbros.Figure 15 also shows that it is unlikely that indicative of someMREE depletion which would be Cambrian-Ordovician seawater was a significant circulating consistent with hornblende fractionation (Hanson 1980). fluid responsible for the Sr-Nd contamination, as most of 588 B. E. LEAKE L 0 0 0 0 0 0 0 0 0 0 v) 0 80 0 0 v) N N v, oR 5 8vP @48 0 (0 0 O::9 r I Ullllllllll1 fa.. m . Y 0 F .OtX@ . e1 f me €6 X X XX X xx 0 Y X0 X X X 0 0 4000- . 3000 - 2000 - 1000~- 500 *P 450 - 400 - . . 0 350 - METAGABBROS 300 - A RALLYCONNEELY A AMPHlBOLlTE .A 250 - o GNEISSES K-FELDSPAR \. GRANITE GNEISS 200- O 180 - Cr ppm 160- l40 - 0 120 - 8 100- V 80 - 60 - 40- 20 - 0 0 . 0 0 00 lot-- 36 38 40 Si02 wt % Fig. 11. Plot of SiO, wt % against Ni and Cr ppm for the Connemara metagabbros and gneisses. 590 B. E, LEAKE the samples of both metagabbros and gneisses have too high - LATESTSTAGE ESr and too low ENd. This is confirmed by the 6l80 and SD I- studies of Jenkin (1988). I LATESTAGE Both Jagger (1985) and Jenkin (1988) have concluded 0 from 6l80 studies of the Cashel metagabbro (typically 6'*0 7 to 10%) that the magma (calculated 6I8O c. 7.0%) was enrichedin "0 before intrusion and this enrichment was 50 - most probably the result of assimilating metasediment- possibly Dalradian metasediment-at depth and was quite distinct from the evident results of assimilating metasedi- ment at the exposed level of erosion. In particular Jagger (1985) showed that there was no correlation of Sl8O and (87Sr/86Sr)4w. Jenkin (1988) has also shown thatthe 10: magmatic hornblendes in the metagabbro were crystallized from a magma with SD - 70 f 5% and that much of the water in the magma was probably crustally derived, 5- putatively from the assimilation of Dalradian metasediment or possibly an amphibolitic Lewisian-like basement. It is not possible at present to identify or exclude specifically a contribution of waterderived from a subducting slab of 1 METAGABBROS oceanic crust. Althoughthe gneisses do not appearto be the differentiation productsfrom the crystallization of the metagabbros, there is nevertheless a close genetic relationship between the magmas which produced the metagabbro and the gneisses. This is well brought out by the constant mutual association of the two suites and the similar 50- range of initial 87Sr/86Srratios at 490 Ma extending from 0.708 to 0.715 within both suites (Figs 14 and 15). Over 20 years ago Leggo et al. (1966) demonstratedthat closely Lu != geographically associated K-feldspar gneisses did not yield a a- n Rb-Sr isochron, implying that either different samples had Z variable initial 87Sr/86Srratios or that the rocks had been 0 variably 'reset' by later events or that both possibilities were I 10: v- true. It is still unclear what is the cause of this variation. \- Jagger (1985) identified two parallel isochrons, a lower one X- v 5- comprising nine samples of gneisses and nine samples of 2- metagabbros giving an intercept of 0.709874 f 0.00008 (MSWD 31) and an age of 517 f 8 Ma or 462 f 28 Ma if the gneiss samples alone are included, and an upper isochron, QUARTZDlORlTE GNEISS composed of 24 gneiss samples which gave an intercept of i 0.71240 f 0.0035 (MSWD 137) and an age of 508f 30Ma (Fig. 16). Ages are calculated with Ag7Rb 1.42 X 10-"a-'. All these ages overlap or are close to the Pb-Pb zircon age of the metagabbro of 490 Ma (Jagger et al. 1988) and might result from mixing of slightly inhomogeneous magmas. Geographically however, closely associated samples (tens of metres apart) fall on different isochrons with no intermediatesamples being identified. The fact thatthe chlorite and epidote in the metagabbros and gneisses are of post-magmatic generation, possibly much later according to 60l8 and 6D values (Jenkin 1988), raises the prospect of late fluid disturbance of the Rb-Sr systems, but leaves unresolved the time when this may have taken place or the reason for the dual distribution of the gneiss samples. Jagger et al. (1988) have shown thatthe K-feldspar gneiss Cashelat contains zircons of quite different Fig. 12. Representative normalized-Rare Earth Elementranges of the metagabbros, quartz diorite gneissesand K-feldspar granitic K-FELDSPARGRANITE GNEISS gneisses from the Roundstone area. The metagabbros are divided into three broad fractionation stages withthe latest being the 1 Ballyconneely amphibolites. Data from Downs-Rose (1985) based Lace Pr Nd Sm EuGdDy Ho Er Yb Lu on 40 rock samples, normalized to Nakumura (1976) values. TH E CONNEMARACOMPLEX THE OF IRELAND 591 Metagabbrowith no xenoliths Metagabbrowith xenoliths I lssl METAGABBROS 5 0 0 *+ 00 0 e* -a + e. bat ++eo 0 *+ L + ++ + ++ 0 0 1- I+ I.,. v , +*C+-" , 5 I0 50 100 500 Zr PPm Fig. 13. Log-log plots of TiO, wt % and Y ppm against Zr ppm for representative Connemara metagabbros andorthogneisses. morphology from theprimary igneous ones in the metagabbro. The gneiss zircons yield a lower intercept age 0.7080.710 0.712 0.714 0.716 of 454 f ;4 Ma and project back to a Proterozoic pre-history (87Sr /86Sr) 490 of 1807 f EMa. This demonstrates that thegneisses contain I MORB+IA I I +l0 CAMBRO - ORDOVICIAN SEAWATER 00 (X * BULK - EARTH D z orroy " -10 - 20 - 30 1 I I l I I I I .l00 -50 0 + 50 +l00 +l50 +200 +250 +300 +350 Fig. Plot of cNdand eNdat 490 Ma 0 METAGABBRO 0 DlORlTEQUARTZ 0 CONNEMARA 15. GNEISS SCHIST for the Connemara metagabbros,or- METAGABBRO WITH CONNEMARA A KF GRANlllC HORNFELSED thogneisses and Dalradianmetasedi- SCHIST XENOLITHS GNEISS CONNEMARA ments. Data from Jagger (1985). SCHIST 592 B. E. LEAKE A A A o QUARTZ D!ORITE 0.7121+ A , A , K-FELDSPAR GRANITE , 1 0,708 1 2 3 I I I 0 50 % 100 % 150% 200% DISTANCEABOVE THE MANNIN THRUST 16. Plot of 87Rb/MSragainst *'Sr/% for the Cashel Fig. Fig. 17. Plot of Niggli mg against distance above the Mannin Thrust metagabbros and gneisses showinghow the gneisses plot on two expressed as a percentage of the thickness of the Ballyconneely nearly parallel isochrons (Jagger1985). amphibolite at the sample locality.This scale is necessary to overcome the varying thicknessesof the amphibolite in different Pidgeon (1969) showed thatthe Dalradian rockscontain localities around the domal structure. detrital zircons with an originalage of crystallization of 1300-1700Ma, so it is possible that the dissolved crust was either Dalradian or older Proterozoic basement. ubiquitous strong foliation and causing the disruption of the In conclusion the isotopicevidence does not ruleout metagabbro by synmagmatic movements.Tectonic move- derivation of amantle-derived magma from an enriched ments must however have taken place during the mantlesource but that alone is insufficient to explain the metagabbro crystallization, as the complex relationships mineralogical, chemical and isotopicpicture. Substantial entirely within the metagabbros, seenfor instance at crustal involvement is essential to explain the relict zircons, Roundstone (Bremner & Leake 1980), must in part precede and the 0, D, Sr and Ndevidence. The complex has an the gneiss injections. These gneiss injectionsforced apart early tholeiitic to high alumina light REE-enriched gabbroic thenorthern edge of theRoundstone body from the component and a slightly later calc-alkaline component, and metasediments to the north. the similar isotopic Sr and Ndresults suggest thatthe Judging from the flat nature of the metagabbro from east gneisses were derived by crystal fractionation from a deeper, of Slyne Head to east of Glinsk and down to the south at unexposed magma chamber in which gabbroic magma least as far as Mile Rock (Fig. l), a major part of the body differentiated and assimilated country rock. The fractiona- was flat-lying before the intrusion of the Galway Granite tion was not in the chamber exposed at present. Whether destroyed most of the complex. Chemical traverses show the trondhjemites represent mobilized restite out of which that in addition tothe Ballyconneely amphibolites, the K-feldspar has been melted, or whether they represent the Glinsk metagabbros(Harvey 1967) andthe metagabbros fractionation products of a separate magma chamber from west of Roundstone(Bremner & Leake 1980) are also tbe K-feldspar granitic material, remains to be determined inverted fractionation sequences so that a major part of the by detailed isotopic studies of other systems such as Pb. metagabbro is upside-down. Using the way-up evidence of rare gravity-graded layers, the cryptic variation of the pyroxenes and the fractionation trends of the rock series, Structure of the complex the overall structure of the complex is shown schematically The occurrence of talc serpentiniteand rare peridotites in Fig. 18. nodules along the tectonized northern edge of the complex As an example of a mid-crustal contaminatedmantle together with the fractionation sequence deduced at Cashel, intrusion, it is significant that Fig. 18 shows complex provideevidence thatthe northern contact was originally infolding of metasedimentand intrusive rock with long close to the bottom of the metagabbro intrusion and the prongs of Dalradian rock extending both up from the floor Ballyconneely amphibolite, now inverted, is clearly the of the intrusion and down from the sides of the body. latestfraction. This is demonstrated notonly by the Such geometries, when combined with the syntectonic Ballyconneely amphiboliteanalyses plotting at the end of nature of the complex,enable the assimilation and the metagabbro geochemical trends (Figs 7 and 10) but also interaction of magma and country rock tobe far more by the changes in chemical composition across the thickness extensive than is possible with either a simple sheeted model of the amphibolite. There is a general decline in Ni, Cr, or globulara intrusion. This reduces the difficulty of normative An percentage and Niggli mg (Fig. 17) from the explaining the widely internally distributed inherited zircons metagabbro, through the variably sheared transition zone to and the crustally contaminated isotopic contents of many the Ballyconneely amphibolite and through the amphibolite granitoid plutons, especially if the crust-magma interaction to the basal Mannin Thrust (Leake 19706; Singh 1984). The can be intimate and pervasive through the intrusion and not inversion may havebeen connected with the early confined tothe periphery of an intrusion of simple movements onthe Mannin thrust,and theseprobably geometry. accompanied the injections of the orthogneiss imposing the Accepting the pressure estimates of Treloar (1981, 1985) TH E CONNEMARACOMPLEX THE OF IRELAND 593 and Ahmed-Said & Leake (unpub.) of 4.5 f 1kbar for the hornfels indicates that about 18 km of cover has been stripped off so thatthe present level of exposure of the batholithic complex is likely to be towards the lower part of the original body rather than towards the top. This suggests that the flat metagabbro ‘bottom’ to the complex, with its underlying thrust and overlying calc-alkaline granitoid gneiss, might correspond tothe interpreted thrust and reflective lower crust identified in deep seismic profiles as seen across the Cornubianbatholith (Brooks et al. 1984; IN BIRPS & ECORS 1986). Tectonic setting of the complex The platetectonic setting which matchesbest the Connemara intrusive complex is that of a magmatic arc at a continental margin above a subductionzone. The assemblage has great similarities with numerous hornblendic and very calcic plagioclase-bearing plutonic complexes occurring along the western margin of the Americas such as the amphibole- and anorthite-richgabbros and olivine- pyroxene gabbro-norites of the Peninsular Ranges gabbros, California (Smith et al. 1983); the Bear Mountain igneous N S complex of the Klamath Mountains, California (Snoke et al. 1981)which has a sequence with early ultramafic and hornblende-plagioclase rocks, later diorites, hornblende-rich gabbros to diorite and then leucocratic rocks, chiefly tonalite andgranodiorite; the Smartville intrusive complex of the northern SierraNevada with its hornblendic gabbro pegmatites rich in calcic bytownite or anorthite (Beard & Day 1986, 1988); the quartz diorite, tonalite,granodiorite and granite of the Coast Range batholith, British Columbia (Arth et al. 1988) and mahy other parts of the magmatic arc of the western Americas. Some of these closely match the relations seen in the Connemara complex, particularly those of the gabbros of the Peninsula Ranges(Walawender & Smith 1980) where the plutons are multiple intrusions with heterogeneouslydistributed peridotite, anorthosite, trocto- lite, gabbronorite,norite, hornblende gabbro, sometimes with diorite and quartz diorite. This implies generation of the magmas above a AGAINSTDELANEY subductionzone underthe SE continental edge of Laurentia.Subsequent major strike slip faulting which is FORMATION generally agreed to have taken place (Dewey & Shackleton 1984; Hutton & Dewey 1986; Soper & Hutton 1984; Soper D Quartz - diorite gneiss 1986; Hutton 1987), would have laterally displaced portions of the plutonic complexes into widely separated regions in a Graniticgneiss manner similar to that of the displaced terranes of the west m coast of the United States and Canada. It is significant that the southern edge of the Dalradian Metagabbro & Peridotite block of Connemara should bemade of this igneous complex which is not present in the southwestern part of the Metasediment Dalradian outcrop of western Scotland or northern Ireland. This suggests thatthere was a relationshipbetween the Direction of younging in emplacement of the intrusion andthe displacement of basicrocks Connemara relative to the main Dalradian outcrop. It may f berelevant that recentstudies have revealed the tectonic emplacement of theAberdeenshire gabbrosshortly after Fig. 18. Schematic profile through the western and central part intrusion along major steeply dipping shear zones, probably of the Connemara metagabbro-gneiss complex. C, Cashel;R, during D3 of the regional deformation, atan estimated Roundstone; S, Shannavara; M, Clifden;G, Glinsk. Localities on 481 f 15 Ma (Ashcroft et al. 1984). The Connemara block is Fig. 1. thought to have been displaced by a combination of major southward-thrusting with east to northeast strike-slip motion 594 B. E. LEAKE (hake et al. 1983) froman original west orsouthwest remnants of a magmatic arc stretching from Connecticut to extension of the main Dalradian outcrop across Ireland. Maine (Tracy et al. 1984; Wones & Sinha 1988). In The REE patterns clearly distinguish the complex from Newfoundland, especially in theDunnage zone central the LREE-depleted Tremadoc-Arenig volcanic rocks of the volcanic belt,there are both plutons and calc-alkaline LoughNafooey area (Fig. 1) which are virtually non-ophiolitic volcanic rocks of Lower-Middle Ordovician synchronous in age (Ryan et al. 1980). This makes it most age (Mattinson 1977; Kusky & Kidd 1984). Althoughthe unlikely thatthe Lough Nafooey volcanic rockswere possible collision of island arcs with the SE edge of eruptedanywhere geographically near the Connemara Laurentia must complicatemodelling, the position of the massif,although they were deposited in association with Connemarametagabbro-gneiss complex on the southern deep water cherts and probably in a back-arc basin (Ryan et side of theDalradian exposure suggests a magmaticarc al. 1980). Because many plutonic rocks in magmatic arcs are abovenorthwestward-dippinga subduction zone. The accompanied by volcanic effusions of calc-alkaline andesites metarhyolites of the Delaney Dome Formation under the and basalts (Snoke et al. 1981), it must be a possibility that Mannin Thrust must be older than 454 f 11 Ma (hake et al. volcanic rocks wereformed above Connemara c. 490Ma 1984) and younger thanthe Dalradian rocks,and might ago but have been completely stripped off by erosion. Plots therefore be part of the volcanism associated with the of the gneisses on trace element discrimination diagrams for magmaticarc. Likewise thesouth Connemara putative the tectonic setting of granitic rocks as formulated by Pearce Ordovician (Fig. 1) pillow lavas and breccias are probably a et al. (1984), fallclearly into the field of volcanic arc fragment of the fore-arc formed above the subduction zone. granites (Fig. 19). Also in a general position near the southeast side of the Fragments of this Cambro-Ordovician continental arc or Dalardianoutcrop is thesynkinematic Slieve Gamph continent-island arc can be identified along the Appalachi- tonalite-granodiorite-granite complex of the OxMoun- ans and in Newfoundland. Thus in the central and southern tains, Mayo, Ireland dated at 477 f 6 Ma by Rb-Sr whole Appalachians,early Ordovician tonalitic to granodioritic rock studies (Pankhurst et al. 1976) and the Aberdeenshire bodies constitute the intrusive complexes of Leatherwood, gabbros, dated by thesame method at 489 f 17Ma Melrose, Columbia, Occoquan, Georgetown, Port Deposit, (Pankhurst 1970). TheConnemara complex is among the Arden and mostextensive of all, the Elkahatchee Quartz best preserved and best studied plutonic remnantsof a once Diorite (Wones & Sinha 1988; Drummondpers. comm.). major Cambro-Ordovician magmatic arc. with these plutonic rocks are thevolcanic suites of the James RunFormation (and correlated suites), well preserved in References central Virginia (Pavlides 1981). Thereare many mafic plutons in the central and southern Appalachians but only AHMED,A. A. & LEAKE,B. E. 1978.The Inishdawros meta-peridotite, Callow, Ballyconneely, Connemara,western Ireland. Mineralogical theBaltimore complex has been studied in detail,dated Magazine, 42.69-74. precisely (U-Pb 490 f 2 Ma) and demonstrated to involve Am, 0.& EYAL, Y. 1976. The genesis of WadiMagrish migmatites (NE extensive crustal contamination (Shaw & Wasserburg 1984) Sinai). Contributions to Mineralogy and Petrology, 59, 95-110. as part of theCambro-Ordovician continental margin arc ARTH,J. G.,BARKER, F.& STERN,T. W. 1988. Coast Batholith and Taku plutons near Ketchikan, Alaska: Petrography, geochronology, geochem- complex(Sinha & Hanan 1987). In New England,Lower istryand isotopiccharacter. AmericanJournal of Science, t88A, and Middle Ordovician volcanic sequences (e.g. Ammono- 461-489. osuc volcanics) andplutons of tonaliteand granodiorite ASHCRO~,W. A., KNELLER, B.C., LESLIE,A. G. & MUNROE,M. 1984. associated with theBronson Hillanticlinorium form Majorshear zones and autochthonousDalradian in the NE Scottish Caledonides. Nafure, 310, 760-762. BARBER,J. P. 1985. 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LEGEND METASEDIMENTS m PELITE QUARTZITE MARBLE SEMIPELITE.IMPURE PSAMMITE B CALCSILICATE MOBILIZEDHORNFELSED METASEDIMENT METAMORPHOSED BASIC ROCKS STRIPEDAMPHllOLlTE METAGAUBRO ORTHO-LPARA-GNEISSES mast contacts grodotionol B QUARTZO-FELDSPATHLZED METASEOIMENT 0 QUARTZ-PlAGlOClASE-HORNELENDE k BlOTlTE GNEISS QUARTZ-PLACIOCLASE-HORNULFNDE BIOTITE- K-FELDSPAR GNEISS m QUARTZ-PLAGIOCLASE-BIOTITEMEW K-FHDSPARRICH GNEISS this ornament is combined withother gneiss ornaments to indicate moderate K-feldspothiurtion LATE INTRUSIVEROCKS B OUGHTERARDGRANITE TJ' GALWAYGRANITE H FOLIATED GALWAY GRANITE / PORPHYRYDYKE FELSlfEDYKE DOLERITEOYKE SYM80LS .--4 ;g;EGOJCA;O;;;P;I;;RY 4 FAULT I INFERREDFAULT 55 DIP55 L STRIKE OF MAIN J F01IATION SCHISTOSITY H STRIKE OF VERTICALFOLIATION ,--, /I :ne,/ NO EXPOSURE kd e complex. The relationships of the kgabbro bodies are displayed with mobilized strip which is by Leake. This map exactly