Journal ofthe Geological Society, London, Vol. 153, 1996, pp. 109-120, 6 figs, 3 tables. Printed in Northern Ireland
U-Pb and Rb-Sr geochronology of magmatism and metamorphism in the Dalradian of Connemara, western Ireland
R. A. CLIFF, B. W. D.YARDLEY & F. R. BUSSY' Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK 'Present address: Institut de Mine'ralogie, Universite' de Lausanne, CH1015 Lausanne, Switzerland
Abstract: New geochronological data which clarify the timing of syn-orogenic magmatism and regionalmetamorphism in the Connemara Dalradian are presented. U-Pb zircon data onfour intermediate to acid foliated magmatic rocks show importantinherited components but the most concordant fractions demonstrate that major magmatism continued until 465 Ma whereas the earliest, basic magmatism has been dated previously at 490 Ma; a fine-grained, fabric-cutting granite contains discordant zircons which also appear to be 465Ma old. Arc magmatism in Connemaratherefore spanned a period of at least 25 Ma. Recent U-Pb data on titanite from central Connemara which gave a peak metamorphic age of 478Ma are supplemented by U-Pb data on titanite and monazite from metamorphic veins in the east of Connemara which indicate that low-P, high-T regional metamorph- ism continued there to 465 Ma, i.e. at least 10 Ma later than in the central region dated previously. New Rb-Sr data on muscovites from coarse-grained segregations in different structural settings range from 475 to 435 Ma: in part this range probably also reflects differences in age from west to east, with three ages close to 455Ma from theeastern area, which is also thesite of the lowest pressure metamorphism. Thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1-2 Ma. The new dates emphasize the complexity in the spatial and temporal distribution of high-level regional metamorphism caused by magmatic activity. The relatively simple overall distribution of mineral-appearance isograds revealed by regional mapping masks thecom- plexity of a prolonged but punctuated metamorphic history related to multiple intrusions, primarily in the southern part of Connemara. The later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism, and were localized in the eastern part of the region.
Keywords: Connemara, Dalradian, U-Pb, Rb-Sr, metamorphism, magmatism.
A major goal of metamorphic petrology in the 1980s was the the importance of extensional settings (Wickham & quantitativedetermination, not only of the pressures and Oxburgh 1987) whereas others propose an association with temperatures of specific metamorphicevents, but also the magmatism at convergent margins (Yardley et a/. 1987: De P-T path along which the rocks evolved before and after Yoreo et al. 1989). the metamorphicpeak. While uncertaintiesremain, especially at low and high grades, P-T paths have now been proposedfor many metamorphic belts (seefor example Geological setting and previous work Daly et al. 1989). Atthe same time,improvements in The Connemara region of western Ireland (Fig. 1) comprises geochronological techniques,and more sophisticatedsam- a fold belt of Dalradianmetasediments, closely correlated pling strategies, have made it possible to obtainmore with the Dalradian of Scotland (Harris et al. 1994), which precise datesformetamorphic rocks which can be hasbeen intruded by syn-orogenic calc-alkaline intrusions specifically related to growth of particularmetamorphic ranging in composition from gabbro with ultrabasic assemblages (Christensen et al. 1989: Mezger et a/. 1989: cumulates (Leake 1969) to granite (Senior 1973, Leake Burton & O'Nions 1991; Cliff et al. 1993). In this paper we 1989). re-examine the regional metamorphic history of the This cycle of igneous activity has been interpreted by Connemara region of the Irish Caledonides,and provide Yardley & Senior (1981) as having developed in the roots of new geochronological evidence for the timing of both a volcanic arc;a conclusion also reached subsequently by regional metamorphismand synorogenic magmatism. This Leake (1989). The peakmetamorphic grade of the allows us to re-assess the causative links between Dalradian rocks increases from north to southand ranges magmatism andmetamorphism, using simple thermal from the staurolitezone to upperamphibolite facies models to evaluate the magmatic history required to account migmatites (Yardley & Barber 1991), although higher grade forboth the metamorphic temperatures attainedand the hornfelses are locally present around some basic intrusions duration of high temperature events. The area ofis (Treloar 1985). Lower grade rocks have been reported particular interest because it sheds light on the problem of locally from northern Connemara, but may be thoroughly the causes of low-P, high-T regional metamorphism. The retrogradedamphibolite facies schists. The mineral as- high heat flow required to attainamphibolite facies semblages are indicative of low-P a metamorphism, temperatures at mid-crustal depths is commonly related to superimposed on pre-existing medium-P metamorphism synchronous magmatism, but some authors have emphasized (Yardley 1976; Yardley et al. 1987). 109 110 R. A. CLIFF ET AL.
0 U-PbSamples $ Diopside Rock Rb-SrSamples A Jagger et al. U-PbSample
Fig. 1. Geological Sketch Map of Connemara: sample localities referred to in the text are shown; metamorphic zones after Yardley et al. (1987) are indicated by grey shading.
Connemara is flanked to north and south by low grade lower plate (Leake & Singh1986) but their thickness is lowerPalaeozoic sedimentary and volcanic rocks. The unknown. contactbetween Ordovician rocks and the Dalradian metasediments is invariably faulted(Dewey 1963; Ryan et al. 1980),but Silurian strata rest unconformably on both Previous isotopic dating Dalradianand Ordovician rocks (McKerrow & Campbell Geochronological data relevant to thethermal history of 1965; Bluck & Leake 1986). The lower part of the exposed Connemara has accumulated since thepioneering work of Ordovician succession of the South Mayo trough, immedi- Giletti et al. (1961), but most of it consists of K-Ar and to a ately to the north of Connemara, is Tremadoc in age, and is lesser extent Rb-Sr mineral ages which are often scattered succeeded by predominantlya volcanogenic succession even on the scale of a single outcrop. All of the ages extending to the mid-Llanvirn. Sedimentation continued to discussed in this paper have been calculated or re-calculated the end of the Llanvirn but is dominated by sandstones and using the constants recommended in Steiger & Jager (1977). conglomerates, including clasts of rhyolites, granitesand The problems of interpreting many of the mineral ages are metamorphic material(Dewey & Ryan 1990). One of the well illustrated by the recent work of Miller et al. (1991), points whichwill be addressed here is the extent to which who found a scatter of hornblende and mica ages ranging this range of ages for volcanism in south Mayo overlaps the from 397 to 481Ma in a single 30 m quarry, unrelated to age of plutonism in Connemara,and hence theextent to mineral composition orother obvious geological features. which Ordovician magmatism in Connemaraand south The number of well-constrained ages is severely limited but Mayo may be connected and may document the docking of three points are well established. Firstly, Jagger et al. (1988) these terranes. presented U-Pb data on zircons from the Cashel-Lough The youngest major structure in Connemara is the Wheelan gabbro, a sub-set of which have tightly clustered Mannin Thrust (Leake et al. 1983; Leake 1986), exposed in 2'17Pb/20hPbages at 490 f 1 Ma, and this is taken to be the the western part of the region. This thrust is marked by the age of the gabbro. Secondly, Cliff et al. (1993) have recently development of extensive mylonites in both the lower and presented a precise age for fluid infiltration associated with upper plates. The main Dalradian succession and its production of metasomatica diopside rock within the associatedintrusions comprise theupper plate of the Connemara Marble Formation, described and discussed by Mannin Thrust,and are retrograded in its vicinity. Acid Yardley et al. (1991a, b). This age of 478 f 3 Ma was based mylonites of poorly understood affinities are present in the on analysis of uranium-richtitanite from three Rb -Sr AGES FROM CONNEMARAFROMU-Pb AGES & Rb-Sr 111 outcrops of the diopside rock in the sillimanite-muscovite clear that this linear fabric is in fact a younger feature, and zone. Thirdly the end of the regional metamorphic history is Tanner (1990) has demonstratedthat thesegabbros are defined by thrusting of the high-grade rocks over post-D2, pre-D3, a conclusion which fits better with other lower-grade rocks onthe Mannin Thrust; acid mylonites lines of evidence. It has been established from palaeomag- from the thrustzone have yielded a well-defined Rb-Sr netic studies (Morris & Tanner 1977; Robertson 1988) that whole rock isochron at 447 f 4 Ma (Tanner et al. 1989). thegabbro bodies of Currywongaun-Doughraugh inNW Subsequently, post-orogenic granites were emplaced in late Connemara and Cashel in the south cooled through the Silurian to Devonian times (Max et al. 1978; Pidgeon 1969) Curie point after D3 but before D4 folding, but apart from this there is only circumstantial evidence to link these widely spacedbodies (Leake 1970). The hornfelses around the Geological and textural evidence on the timing of gabbros of north Connemara exhibit intense and disrupted magmatism, metamorphism and folding D3 folds indicative of deformationduring anatexis. This could imply that the gabbros in the north are younger than those tothe south (i.e. syn-D3) but may just reflect the imprecision withwhich the structural ages are known. The deformation chronology Similar disrupted D3 structures are widespread in the The Dalradian rocks appear to have been affected by three migmatite belt to the south but could reflect later, more acid major periods of folding, corresponding to structures known magmatism. Until more gabbros have been reliably dated it as D2, D3 andD4 in the extantliterature (Yardley 1976; remains uncertain whether they are all the same age. Leake & Tanner 1994). No D1 folds are now recognised The relationship of younger, more acid members of the (Tanner & Shackleton 1979). The earliest, i.e. D2, phase synorogenic magma series to the D3 folding, is less obvious produced a pervasive schistosity in all Dalradian lithologies from the map, but they appear to overlap with it. The major andresulted in the development of at least one major ‘quartz-diorite gneiss’ bodies are coarse, plutonic rocks isoclinal fold structure which extends east-west through which have been affected, at least in part, by D3 centralConnemara. The oldest(Appin Group) lithologies deformation and are generally foliated. Leake (1989) reports occur in the hinge region of this fold, and the succession that thefabric is leastintense in parts of the K-feldspar youngs away from the central belt in which this fold hinge gneiss, the youngest member of the series. Thereare lies, to both northand south. The most abundant however many small granite bodies in the associated mesoscopic folds are tight D3 structures which form tight migmatitic metasediments, sometimes barely a metre across, northward facing nappes in centraland northern Con- which are unfoliated and have a fine equigranular texture. nemara, possibly rooted in a steep belt to the south (Yardley These bodies are very similar in appearance to parts of the 1976). These folds formedat the metamorphicpeak, and Oughterard Granite, a complex of poorly exposed multiple have axial-planar fibrolite fabrics in appropriate lithologies intrusions in the easternmost part of the Connemara inlier and grades (Yardley 1976). In the steep belt, there is intense (Bradshaw et al. 1969) whose age has proved controversial rotation of fabrics towards parallelism, so that separate fold because combination a of alterationand extensive phases can be difficult to unravel, andthere is also contamination of the original magma has made reliable considerabledisruption along later faults. The final major geochronology difficult (Kennan et al. 1987; Leake 1988). folds are broadopen upright D4structures, trending Indeeda few larger examples are shown as Oughterard east-west; most notably the Connemara Antiform of central Granite on the detailed map in Leake (1989). Senior (1973) Connemara. The relationship of D4 folds to the Mannin considered that these unfoliated bodies may belong to the Thrust is not directly apparent, although Leake et al. (1983) same magma series as the foliated gneisses, which suggests have suggested a genetic relationship, with D4 folds forming that the magmatism outlived D3 deformation. in the upper plate as it rose on the Mannin Thrust prior to Melting in the migmatitic metasediments also spanned emplacement at its present position; geochronological data D3. Minor D3 folds of competentand resistant beds are on this point will be presented below. sometimes distorted and disrupted because of melting of the host schist, and migmatite leucosomes sometimes parallel D3 fold axial surfaces. However minor leucosome veins are sometimes clearly later and crosscut the D3fabric and Synorogenic magmatism structures. Thus the clear evidence from the field for a close Early work on the synorogenic intrusions of Connemara relationship in space and time between magmatism and concentrated on the basic bodies (Wager 1932; Leake 1958). low-Pmetamorphism (Yardley et af. 1987)is now more A major obstacle to understanding the intermediate to acid strongly established by the recent work of Tanner (1990). members of the series, was discriminating betweenthe intrusive rocks andthe migmatized metasediments with which they are intimately interspersed; a problem which was Distribution of the intrusive rocks resolved by Senior (1973, see also Leake 1989). Nevertheless The early basic members of the synorogenic magma series it is clear that there is a progression with time from more are in fact the most widely distributed, occurring in both the basic to more acid magma types (Leake 1958). The northern and southern parts of Connemara; indeed Leake correlation of the magmatism with the structural sequence in (1970) suggested that all the gabbro bodies might belong to the metasediments has beenmore problematical. Leake a single disruptedsheet. The more acid members of the (1958, 1970) was able to show that the emplacement of the calc-alkaline magma series are restricted tothe southern basic body at Cashel (Fig. 1) predated D3 folding, but also migmatite belt, and within this region there is a progressive claimed that it, and adjacent bodies were emplaced prior to changefrom west to east (Leake 1986). The younger, D2 because they were affected by a D2 lineation. It is now K-feldspar gneisses are progressively more abundant 112 R. A. CLIFF ET AL. towards the east, while the unfoliated granite pods are also can have two possible explanations: either subsequent tilting most abundant in the east and become scarce to the west of has caused higher structural levels of a single, contem- Recess. If the Oughterard Granite, at the eastern end of the poraneous zonal system to be exposed in the east; or the inlier, is indeedpart of this magma series,then this time of metamorphismvaried, with higher temperatures concentration of the later stages of intrusive activity in the persisting later in the east, while regional uplift and erosion eastern part of the area is even more marked. affected the entire inlier. Textural studies demonstrate that the low-pressure andalusite-sillimanite-cordierite meta- morphism was syn-D3and thus distinctly later than the Distribution of metamorphic zones staurolite (kyanite) metamorphism with its MP2 peak grade. The overalldistribution of metamorphicgrades in Staurolite zone rocks in the west and centralparts of Connemara broadly parallels the distribution of the main Connemarareached their metamorphic peak prior to D3 belt of synorogenicintrusives in the south.Dalradian (Badley 1976), whereas in the east there is evidence for the metasedimentsassociated with the intrusives across the approximately syn-D3 low-P overprint (Yardley1976). Thus southernpart of Connemaraare assigned toa series of the latter of the two models for the distribution of zones is migmatitezones (Barber & Yardley 1985) andthe grade favoured. Yardley et al. (1987) demonstrated from the decreases northwards to at least the staurolite zone. In both distribution of andalusiteand of sillimanite pseudomorphs NW and southern Connemara, basic intrusions are enclosed afterandalusite, that isotherms in easternConnemara by envelopes of hornfels which haveattained extreme moved northwardsatthe same time that progressive metamorphicconditions (Leake & Skirrow 1960; Treloar unroofing moved isobars down throughthe metamorphic 1985). In detail however, there are important differences in pile. We conclude that this scenario is only applicable in the the metamorphism from west to east which suggest higher eastern part of Connemara, because intrusive activity had metamorphicpressures in the west. Theprograde zonal already ceased in the west. sequence in the western part of thearea includessparse The youngest prograde metamorphism may be as late as occurrences of kyanite with staurolite,where bulk syn-D4, although the evidence for it is indirect. On a ridge compositions are suitable (Ferguson & AI Ameen 1986). No at Knockaunbaun, to the north of the Maam Valley (Fig. 2), distinctzone of unmigmatised rocks with sillimanite + D3 fold axial surfaces,and the dominant schistosity, have K-feldspar has been recognized,although such azone is relatively shallow (c. 30") dips to the SW, and near vertical present furthereast, and cordieriteand andalusite are D4 crenulationsare common. On this ridge, staurolite- restricted to the immediate aureole of the post-metamorphic biotite-chlorite schists pass rapidly northwardsinto Omey Granite. Further east (Yardley et al. 1980), kyanite is apparently low-grade muscovite-chlorite-biotite-albite sch- very rare, occurring as relict inclusions in one sample from ists. Thepresence of occasional relict oligoclase coresto Cornamona, but cordieritebecomes important. Cordierite albite and theshort space of the transition (~.200m), and andalusite are rare components of staurolite schists in suggests that the staurolite-out isograd in this region may in the Maam Valley, but andalusite increases in abundance fact be a retrograde one. This conclusion is reinforced by the towards Cornamona. In the easternmost part of the inlier, presence at Knockaunbaun of a distinct band of schist with cordierite is abundant (Bradshaw et al. 1969; Barber 1985). coarse (1-2 cm)poikiloblasts of alteredcordierite in a Thisdistribution of mineral assemblagesappears to muscovite-rich matrix. Thesame, distinctive band with imply a decrease in metamorphic pressures to the east that centimetre sized cordieritesalso occurs at Claggan bends,
E Fig. 2. Outcrop photograph of veining on Knockanbaun; sample 51089 was taken from this vein. The steeply dipping I i vein of coarse quartz. chlorite, muscovite and sericitized albite cross-cuts retrog- ressed pelite with a strong S3 foliation; note concordant apophyses. D4 crenula- tions are visible in the schist and the vein overall is parallel to their axial planes; it is also parallel to other veins with staurolite and/or andalusite enriched margins in the same immediate area (Yardley 1986). Rb-Sr AGES FROM CONNEMARAFROMU-Pb AGES & Rb-Sr 113
between Maam and Cornamona (Fig. l), but at Claggan the pegmatitessampled formedat a late stage in thestructural associated rocks are staurolite-biotite schists, andthe sequence, i.e. late- to post-D3. A total of seven coarse muscovite cordierite poikiloblast layercontains biotite as well as samples were analysed. Sample localities are shown on Fig. 1 and muscovite in the matrix. Within the retrograded schists at details are given in the Appendix. Knockaunbaun are series a of quartz veins in an east-west-trendingen-echelon array. These veins are U-Pb isotopic data. The results of the U-Pb analyses are generallysteep, and approximately parallel to theD4 givenin Table 1. Neither of the titanite samples is as crenulations,although their form is somewhat irregular. uranium-rich asthose from the diopside rock (Cliff ef al. They cross-cut the D3 fabric, although a few apophyses are 1993) and in the case of the thin calc-silicate band only the parallel to the foliation. We interpret the veins as syn-D4. 2MPb/23XUage of 469 f 15 Ma is sufficiently precise to war- The remarkable feature of these veins is themetasomatic rant further discussion. The data for the pegmatite titanite alteration of the schists at their margins, described briefly by are plotted with other data on a concordia diagram in Fig. 3, Yardley (1986). The immediate wall rocks for 1-5 cm are where the titanite data have been corrected for initial lead discoloured yellow-brown by abundant staurolite.Some- using the K-feldspar analysis. This sample appearsto be times prismatic andalusite may bepresent as clusters of concordant with a 2'"Pb/2'xU age of466 f 3 Ma; the centimetre long elongate prisms within the staurolite-rich 207Pb/2'JhPbage is less well defined at 470 f 12 Ma. zone. There is a progressive decrease in apparent Data for three optically distinct fractions of monazite metamophicgrade away from the veins, througha garnet from sample 49495 are also presented in Table 1. The zone to a biotite-chlorite zone, over a distance of <20cm. fraction comprising 11 dark grains yielded a perfectly The gradient is in large part of metasomatic origin and the concordant result at 465 f 2 Ma. Twenty-two pale grains are innermost zonehas been strongly acid-leached of cations. also concordant within error at 461 f 2 Ma. Theother The key feature of the metasomaticalteration around the fraction of 40 cleargrains lies slightly above concordia. veins is that it producesa mineral assemblage typical of Similar results have frequentlybeen reported for younger staurolite-andalusitezone metapelites, and this can only monazites and they have generally been attributed to excess realistically result if the fluid that infiltrated along the veins "'Pb, produced by the decay of 230Th incorporated in excess wasitself derivedfrom a location wherethe staurolite- during crystallization as a result of the strong partitioning of andalusitezone assemblage was stable.At Knockaunbaun thoriumrelative to uranium in the monazite lattice. It is this D4event was apparentlysynchronous with the noteworthy that this fraction has the highest 20XPb/ZMPbof retrogression of the original staurolite-chloritezone thethree fractions, reflecting a higher Th/U ratio. This assemblages away from the veins walls, but we conclude that fraction has a 21J7Pb/23sUage of 462 f 2 Ma similar to the in the general vicinity, perhaps a few kilometres to the east other two fractions. As this age is unaffected by initial '"Yh along strike, peak metamorphicconditions were locally excess the mean of the three Z'17Pb/23sUages, 463 f 4 Ma is being attainedduring D4, ata time when the rest of taken as the best estimate of the age of monazite in this Connemara had begun to cool. A few veins at rock. Knockaunbaun, occupying structurally identical situations to those with staurolite-enriched margins, containa typical Rb-Sr isotopic data. The Rb-Sr data for the coarse musco- greenschist facies assemblage with quartz, coarse muscovite vites are presented in Table 2. In each case individual mica (dated below) sericitized albiteand fine aggregates of books, 1-2cm in diameter were extracted from pegmatitic chlorite (Fig. 2). Theseare inferred to have formed from segregations. Initial ratios were estimated from analyses of fluid that had moved more slowly and cooled and feldspar grains located asclose as possible to each muscovite re-equilibrated en route. On the basis of these observations, book. The ages range from 435 to 475Ma with analytical we concludethat the final stages of local magmatism and uncertainties between 3 and 7 Ma. The data set is small but metamorphism persisted into D4, although most of there is no indication of pre-500 Ma metamorphismpre- Connemara had begun to cool at that time. This conclusion served in any of the coarse muscovites. The two samples is strongly reinforced by the geochronological results from Renvyle (51087, 51088) are the most remote from the presented in the next part of this paper. effects of synmetamorphic magmatic activity and are located in the garnet zone, as shown on the Yardley et al. (1987) metamorphic map. These arethe most likely to preserve Geochronological investigation of metamorphism evidence of any early metamorphic history; in fact they span The recent determination of the age of metasomatism of the the complete range of muscovite ages. diopside rock (Cliff et al. 1993) is supplementedhere by Of the remaining five samples, 54393 and 51107 are peak furtherU-Pb analyses on other lithologies. In addition a metamorphic segregations and yield similar ages of 463 and range of samples containing coarse white mica segregations, 467 Ma. spanning various metamorphic grades and structural settings Sample 51089 is from latea cross-cutting vein at has been dated by Rb-Sr measurements. Knockanbaun (see aboveand Yardley 1986). This vein is apparentlysyn-D4 and is associated with retrogression. Consistent with this its age, of456 * 6 Ma, appears to be Sample description and isotopic results slightly younger than those of most other muscovites. Titanite ages have been determined on a sample of two-feldspar- Samples 54390 and 54391, from pegmatitic segregations clinopyroxenepegmatite (sample 51580) from the migmatite zone associated with andalusite at Claggan give ages 450 f 3 and and a thin discontinuous calc-silicate layer (51069) in quartzite from 459 f 4 Ma, very similar to the Knockanbaun sample which thestaurolite-biotite zone, north of the Connemara Antiform at is from thesame stratigraphic unit. The locality is further Kylemore Lough (Fig. 1). Monazite has been dated from a east than the other samples dated and the relative timing of quartz-cordierite pegmatite (4Y495) in the migmatite zone. The two andalusite growth here is not well constrained. It grew 114 CLIFF R. A. ET AL
Table 1. U-Pb isotopic data on metamorphic minerals
49495 Cordierite pegmarite FSP nd 126 0.052962.037 0.8298 Monazite 11 Dark 7027 529.4 0.000227 3.4440.05970 0.05638 0.5812 0.07477 (467 f 9) (465 f 2) (465 f 2) 22 Pale 1790 132.5 0.000070 0.36190.05741 0.05638 0.5748 0.03940 (468 f 9) (461 f 2) (460 f 2) 40 Clear 1344 100.5 0.000144 0.05814 5.873 0.05603 0.5762 0.07459 (454 f 4) (462 f 2) (464 f 2)
51580 calc-silicate pegmatite Kfsp 0.007 14.1 0.0555 0.8616 2.085 Plag 0.18 2.56 0.05520* 0.8585 2.086 Titanite 52.3 5.88 0.0038 0.11156 0.52 0.05646 0.5830 0.07489 (470 f 12) (466 f 4) (466 f 3)
51069 calcsilicate Titanite 3.58 2.29 0.04128 0.6474 1.519 0.07541 (469 f 15) Kfsp 0.01 0.431 0.06088 0.926 2.225
* Plagioclase isotopic composition corrected for in situ radiogenic growth over 480 Ma using the measured Z3XU/Z'UPb value of 5.06. U-Pb analyses were performed using standard techniques, using the same 21r2Pb-2"U-z36U spike as Cliff et al. (1993). It should be noted that this spike has been recalibrated and compared with a variety of interlaboratory standards; this resulted in a significant adjustment to the uranium concentration which reduces the U/Pb ratios in Cliff et al. (1993) by 2.7%. The concordia age and conclusions of that paper are not affected. duringthe syn- to post-D3low-P metamorphic overprint, Discussion b ut unlikebut Knockanbaun there is no associated retrogression. Early metamorphichistory. To date no evidence has been found for apreserved record of any Precambrianmeta- morphism in Connemara. The samples from Renvyle, from 0.080 I I I the area of lowest metamorphic grade, which a priori were most likely to preserve such a record, yield Palaeozoic ages. We note that this is consistent with recent revision of the geological interpretation of the metamorphic history of the 206ph0'07a Scottish Dalradian (Tanner & Leslie 1994). Samples 51087 and 51088 were collected within a few metres of each other 230 U but differ in measured age by 40 Ma. No explanation for this large difference is apparent. Later alteration is a possibility 0.076 but the feldspars which are most susceptible to alteration gave closely similar results for both samples. It is uncertainwhether more suitable samples can be found; a search for sphene-bearing calcareous rocks in the 0.074 lowest grade zones was not fruitful.
Peak metamorphism in the high-grade areas. Age measure- 7 CONNEMARA METAMORPHISN 0.072 I I I ments which approximate to the time of peak metamorph- 0.55 0.57 0.59 0.57 0.55 0.61 ism are now available for three areas. (i) The diopside rock localities in centralConnemara, 235 where Cliff et al. (1993) reported titanite ages, the most U precise of which is 478 * 3 Ma. Fig. 3. Concordia diagram for metamorphic samples: ellipses (ii) U-poor titanite from a calc-silicate band in quartzite represent 2a errors; monazites in black, titanite from pegmatite from Kylemore Lough analysed here has a Z'mPb-23xUage of stippled, other symbols refer to fractions of titanite from the 469 f 15 Ma. diopside rock, recalculated from Cliff et al. (1993). Note that (iii) In the vicinity of Maam Cross, concordant titanite in recalibration of the spike used here and in that paper brings the a calc-silicate pegmatite at Lough Shindilla has a 2'aPb-z3xU most precise titanite analysis on to concordia. age of 466 f 3 Ma while monazite from the cordierite U-Pb & Rb-SrAGES FROM CONNEMARA 115
Table 2. Rb-Sr isotopic data
Sam ple MineralSample Rb Sr ”Rb/%r X7Sr/R6Sr Age (IJpm) (PPm) (IJpm) (M4
54393 Mu 156 26.9463 0.83543 17.02 f 4 68.4 15.63 0.82801 36515.63 68.4 5.8 42.4 0.397 0.72672 0.397 FSP 42.4 5.8 51107 Mu 334 35.9 27.44 0.90250 467 f 6 FSP106 138 3.78 0.745 54391 Mu 285 73.4 11.32 0.79581 459 f 4 179 1.97 0.73467FSP 1.97122 179 54390 Mu 261 18.3 42.4 0.99371 450 f 3 122 179 1.97 0.73467 1.97 FSP179 122 51089 Mu 382 97.8 11.4 0.79783 456 f 6 57.5 344 0.485 0.72934 0.485 FSP 344 57.5 36.8 130 0.822 0.72900 0.822 130 36.8 5 l087 Mu 192 51.3475 0.80360 10.94 f 7 FSP 0.353247.2 387 0.73183 51088 Mu 0.77627183 7.808 68.2 435 *S FSP 0.371734.6 270 0.73019
Rh-Sr analyseswere performed usingstandard techniques following the procedures described in Cliff et-al. (1985). pegmatite locality at Oorid Lough has mean z07Pb-z”U ages on muscovite and biotite (Elias et al. 1988; this paper) and of463 f 4 Ma. The precise relative timing of thesetwo K-Ar ages on hornblende, muscovite and biotite (Elias et pegmatites is not clearbut their ages are not significantly al. 1988; Miller et al. 1991). For the larger argon data set itis different. These pegmatites are irregular and discontinuous, evident that ages are highly variable on an outcrop scale, as so that their field relations are somewhat ambiguous. They demonstrated by Miller et al. (1991). That work, together arelater than much of thefabric development, but with the stable isotope study of Jenkin et al. (1992), strongly reactivationhas wrapped fabrics around them locally. The suggests that many of the mineral ages have been influenced mean age of 465 Ma probably reflects the continuing minor by post-metamorphic fluid-rock interaction.Widespread post-D3magmatism, maintaining nearpeak metamorphic petrographic evidence of such interaction is found: plagioc- temperatures to a late stage in this part of Connemara. lase is generally at least partially sericitized, and biotite free These results indicate asignificant difference inage of alteration to chlorite is rare. between the samplesfrom central Connemara and the Maam Cross area to the east. The difference of 13 f 4 Ma Geochronological investigation of magmatic activity may reflect genuine geographical variations in the timing of metamorphism; this possibility is discussed later. A second Sample description and isotopic results possibility is that the metasomatic fluid influx dated by the diopside rock sphenes is not directly correlated with peak Zirconswere separated from five samples. Four were collected temperature conditions. Strongarguments linking the along a traverse in the eastern part of the magmatic belt, south of metasomatism to peakmetamorphic reactions in marbles Maam Cross. They comprise two quartz-dioritegneisses (51104, and fluid loss fromthe surrounding pelites havebeen 51105) and two samplesfrom a singleoutcrop of granodioritic ‘K-feldspar gneiss’ (51102, 51103). The fifth sample (51106) comes presented by Yardley et al. (1991b). It is possible that peak from a smallbody on the northshore of OoridLough; it is a fluidflow precededthe temperature maximum although fine-grained, unfoliated granite which post-dates the main fabric in whether by as much as 13 Ma is doubtful. The fluidflow the host gneisses. In addition, in order to supplement existing data occurred at temperaturesthat were certainly close tothe on potential inherited components, zircons from the metasedimen- maximum attained,and it is shown below that a simple tary calc-silicate sample (51069) were also analysed. magmaticheating model for metamorphism is not able to In each case small fractions of the better quality grains, with a sustain peak temperatures for such prolonged periods. minimumof cracks and inclusions, were hand-picked. Before Both the evidence for continued prograde metamorph- analysismost fractions were air-abraded using the technique ism during D4 times, outlined above,and the 456 Ma developed by Krogh (1982). Theresults of theseanalyses are muscovite age at Knockanbaun suggest that the D4 folding summarized in Table 3 and plotted on a concordia diagram in Fig. 4. predatedthe development of the ManninThrust with its Allexcept one fractionfrom 51105 are significantly associated retrogression, and is not directly related to it as discordant and this complicates interpretation of the isotopic proposed by Leake et al. (1983). data. However for samples 51102,51103, 51104 and 51105. there is a systematic pattern in the results which is consistent Post metamorphic cooling. Comparison of our new data on witha similar cause of discordance in all four samples. In the age of peak metamorphism with other mineral ages can 51105 the error ellipse for the 43pm fraction just touches potentially yield information on thepatterns of post- concordia withzO’Pb/z‘EPb age of 475 Ma, whereas the metamorphic cooling, provided that mineral behaviour dur- 75-125 pm fraction is discordant with a z‘l’Pb/zOhPbage of ing cooling was essentially thermally controlled. The avail- 478 Ma. The data for the other threesamples define an array able mineral age data comprises a limited set of Rb-Sr ages extending away from the most concordant analysis towards a 116 CLIFF R. A. ET AL
51105 75-125 pm 187.9 14.1 0.000568 0.06493 0.271 0.05664 0.5807 0.07436 (478 f 9) (465 f4) (462 f 3)
Precambrian upper intercept. Zircons from the metasedim- entarysample 51069 provide an indication of the 0.081 composition of potential contaminants at the present erosion level; this composition is consistent with the pattern of 0.079 discordance defined by the gneiss zircons alone as shown in Fig.4. Fitting a discordia chord to the data yields a lower 0.077 intercept at 461 f 4 Ma; if the metasediment zircon analysis is taken as a direct indication of the inherited component 206PL2 the regression gives a lower intercept of 463 f 4 Ma. 238 0.075 U The two fractions for 51106 have similar 207Pb/Z(KPb Granite Gneisses ratios but both plot below concordia with 2'fiPb/238Pb ages (Syn-tectonic) of 448 and 433 Ma. Both points plot within erroron a discordia chordforced through the origin with an upper intercept at 466 f 5 Ma, which is statistically indistinguish- LateGranite 51106 able from thecalculated age for the granite gneisses. (post-tectonic)
CONNEMARA MAGMATISM Discussion 0.067 0.50 0.54 0.58 0.62 0.66 As in the earlier study by Jagger et al. (1988) the zircons in 207 Pb the gneisses analysed hereare seriously affected by an 235 inherited component. Despite careful picking to select grains U which containas little as possible of such material, the Fig. 4. Concordia diagram for magmatic zircons: ellipses show 20 proportion of the analyses which have yielded chronological errors. Data for the dioritic-granitic gneisses are shown in grey data with the precision needed is small. Only one of the shading; the arrow represents a best-fit discordia tied to the detrital analysed fractions is concordant within error; that is from zircon analysis from the metasediment 51069. Two fractions of the thequartz-diorite sample 51105. Thisfraction strongly fine-grained post-tectonic granite are shown in black with a controls the 463 f 4 Ma age calculated for the gneisses; it is discordia based on the assumption of recent lead-loss. The inset useful, in conjunction with the 490 Ma age for the Cashel shows the relationship between the data from 51104/51105 and Lough Wheelaun gabbro, in suggesting a minimum duration concordia at a larger scale. of approximately 25 Ma for syn-orogenic magmatism. U-PbAGES & Rb-Sr FROM CONNEMARA 117
The inherited component is greatest in 51103, while in of the calc-alkaline magmatic suite in southern Connemara 51102 the selected fractions have variable proportions of this hasbeen examined using a highly simplified numerical component. In both samples the data indicate an inherited model. The advective introduction of heat was simulated by component with Proterozoica mean age. That such a a 2 km thick sheet of basic magma. The initial country rock component was available for anatexis/assimilation at least at temperature was setat 500°C and this was maintained the presentexposure level is confirmed by the analysis of constant at adistance of25 km from thecentre of the zircons from the sediment sample 51069. Figure 4 shows a intrusion. The temperature evolution of the region between discordia line drawn from these sedimentary zircons through and0 25 kmwas calculated using the explicit finite the concordantfraction of 51105. It is apparent thatthe difference expression given in Peacock (1989). The effects of zircon data from the gneisses are compatible with formation latent heat on cooling of the basic intrusion as well as on of the syn-orogenic acid-intermediate magmas at 460- melting and crystallization in the country rock were taken 470Ma with variableproportions of thesame inherited into account by modifying the thermal diffusivity (K) in the component. Field relationships, however, indicate thatthe manner described by De Yoreo et al. (1989). Yardley & more acid, K-feldspar gneisses were emplaced later than the Barber (1991) have argued that melting in the Connemara quartz-diorite gneisses. It is thus important to evaluate how migmatites took place by hydrous melting reactions and the much younger than the quartz-diorites they could be. Taking latent heat contribution was accordingly distributed through the analysed sedimentary zircon as a fixed upperpoint, a the crystallisation interval (DeYoreo et al., 1989). The discordia drawn through the lower error limit for the least details of the thermal model are summarized in Fig. 5a. discordantfraction of51102 suggests a minimum age of It is quite clear that an intrusion of a single magma pulse 457 Ma. Together the data suggest a maximum time span for of the given dimensions could not generate the observed the evolution from dioritic to granitic magma compositions metamorphic conditions. Given the probability that the from 463 f 4 to 457 Ma, i.e. 6-10 Ma. exposed intrusions were linked in some way to volcanic The fine-grained granitesample 51106 provides a activity over a significant period, the model was modified so potentialconstraint onboth the end of K-feldspar gneiss that the 2 km sheet representeda conduit continuously magmatism and the end of fabric forming deformation since flushed by fresh basic magma for varying periods of time it cross-cuts the main foliation in the host gneisses but is after which the sheet was allowed to cool and solidify. The itself unfoliated. Neither of the analysed fractions is results of these calculations are presented in Fig. 5b where concordant; however they have identical 2"7Pb/Z'KPbages they can becompared with the peakmetamorphic within error, lying on a discordia passing through the origin, conditions estimated by Yardley et al. (1987). For distances with upper intercept at 466 f 6 Ma. Interpreting the data in greater than 4 km from the contact with the basic sheet a this way assumes thesefractions contain no inherited close match to the estimated conditions is achieved if the component; if the disposition of the data in the concordia magma conduit was active for a period of 0.1-0.5 Ma. Close diagram results from a combination of inheritance and lead to thecontact however therecorded conditions are loss then a minimum age as low as 445 Ma is possible. anomalously cool for all periodsabove 0.05Ma. However The overall duration of magmatism in southern this region is occupied by intermediate and acid magmatic Connemara was at least 27 Ma, beginning with the rocks. These rocks probably represent some hybrid mixture emplacement of the Cashel gabbro at 490Ma and lasting betweenintruded magma and local anatectic melts, as through quartz-diorite formation at 463 Ma. The termination suggested by the abundance of inherited zircons similar to of syntectonic magmatic activity is not unambiguously those in the country rocks. The resultant melts may have defined by the data but the coincidence of the quartz-diorite convected andprobably migrated so thatmetamorphic age with data for the K-feldspar gneisses, consistent with the conditions in locally preserved, restitic country rocks are same age, suggests that the more acid magmatism followed unlikely to reflect the conditions predicted by the very shortly afterthe quartz-diorites. The two metamorphic simple numerical model considered. pegmatite ages at close to 465 Ma indicate that associated peak metamorphism coincident with this magmatism. The latest magmatism may be represented by the Concluding discussion OughterardGranite (Senior 1973). Its age remains The results of this study, with selected published data are controversial (see above), but the best estimate is based on a summarized together in relation to the geological timescale single Rb-Sr muscovite mineral isochron (Leggo et a/. 1966) in Fig. 6. Magmatic ages range from 490 to c. 460 Ma; they which recalculates to 460 f 5 Ma. This is similar to the age may represent discrete events rather thana contiunuum. of veining at Knockanbaun, discussed above. U-Pb ages on metamorphic minerals which are most likely to reflect the time of crystallization are between 479 and 465Ma. Muscovite Rb-Sr ages of coarse vein material A simple thermal model strongly suggest at least local continuation of metamorphic In order to examine the feasibility of using geochronological veining to at least 460 Ma, and probably 455 Ma. One major data to evaluatealternative models for the low-pressure result of this study is our finding that the progression of thermalpeak metamorphism some simple thermal model synorogenic magmatism in Connemara, from early basic (arc calculations have been performed. Individual bodies of basic tholeiite) intrusions to late stage granites, spanned a period and intermediate plutonic rock appear to have diameters in of at least 20Ma. This has important ramifications for the range 2-10 km; together they constitute a belt with a understanding the regional metamorphism of the Dalradian preserved width of 5-10 km. Such length scales imply rocks, which is clearly closely linked tothe magmatism. thermal time constants on the order of 1 Ma. Despitetheir higher temperatures and the presenceof The thermal evolution resulting fromthe emplacement distinct aureoles of high-T hornfelses around them, the basic 118 R. A, CLIFF ET AL.
Ma Magmafrom Depth Melted SedimentsCountry Rocks MANNIN THRUST 450 perrane Docking
Coarse Musc.(eastf D4 T=500" C 460 PegMonazitd -Granitic gneiss4 Musc. (west)' T<700: K = K-. METAMORPHIC
t 'tactive T>1000: K = f(Kb, Lb) Diopslde Rock' T<1000: K = K,
a) Characteristics of the model 490 'Cashel-Lou h Wheelaun 8abbd
1200 MaximumTemperatures Fig. 6. Timing of magmatism and metamorphism in Connemara proposed in this paper. Geochronological data are from Jagger et al. ' E 0.5 Ma Magma at 1100 C (1988)'. Tanner et al. (1989)'. Cliff et al. (1993)3 and this paper4; ,B 0-25 Ma Host at 500 C at25 km U-Pb data shown by solid symbols, Rh-Sr data by open symbols. 1000 A .2 v 0-125 Ma Conduit2.5 km wide - 3 5. 0.063 Ma The time scale is from Harland ef al. (1990). , E A Instantaneous "C v* I. 0.125 Ma -since conduit was active and reflects the imprecision withwhich they have been mapped. Within the staurolitezone, staurolite-kyanite cp 0.5 Ma? assemblages are most common in the west, where metamorphism was earlyand relatively deep, while staurolite-andalusite or cordierite-bearing assemblages occur in the east and developed later. Differences in P and T betweeneastern and western Connemara probably reflect . * Metamorphic Temperature the progressive shift of the magmatic activity towards the from Yardley et a1 1987 east with time, while the region was simultaneously 400 l,l,l,l,l,I undergoing uplift and erosion. The final prograde 0 4 8 12 16 24 20 metamorphism in the east was possibly as late as syn-D4 in Distance (km) the vicinity of the Oughterard Granite, which we interpret b) Results of modelling calculations as the final phase of the syn-orogenic magmatic cycle. At any one locality, however, the observed assemblages Fig. 5. Thermal model: (a) the diagram shows the thermal probably developed and partially cooled over a time span of parameters used, as functions of position, time and temperature. 1-2 Ma. b = gabbro; g = granitic melt; cr = country rock; K = thermal One of the main reasons why the intermediate to acid diffusivity (K* = value for basalt magma), L = latent heat of fusion, magmatism hasbeen considered hitherto as essentially t,,,,,, = time at which magma conduit ceased to be active. synchronous with the regional peak of metamorphism, and (b) Thermal model: results of calculations, showing maximum of relatively brief duration, has been the relationship of temperature reached as a function of distance from the magma magmatic rocks to tectonic fabrics in the 'steep belt' of near conduit; data are shown for durations of conduit activity from 0 to 0.5 Ma; the black contours show time taken to reach maximum vertical structures which extends east-west across Con- temperature after conduit activity ceased; the stars show estimated nemara in the migmatite zone. Intensely disrupted D3 folds peak metamorphic temperatures at different distances form the belt in migmatised metasediments are accompanied by a of basic intrusions, from Yardley et al. (1987). well-developed fabric in the intermediate to acid intrusives, some of which can be considered orthogneisses. Only a few minor bodies lack such a fabric. The results presented here intrusions appearto predate the peakstage of syn-D3 demonstratethat some of the foliatedintrusions are metamorphism by 11-25 Ma. Ratherthe main stage of 10-15 Ma younger than syn-D3 peak metamorphism in the metamorphism in much of Connemara is probably fold belt a few kilometres to the north of the steep belt in coincident with the emplacement of more voluminous central Connemara. It is inconsistent with what is known of magmas of intermediate composition in the southern part of strain rates today to suppose that a single co-axial the region. Thermal modelling has emphasized that the P-T deformation spanned such a long period, and we conclude conditions attained during regional metamorphism of instead that the fabric in the synorogenic intrusive rocks of Connemaraare inconsistent with prolonged magmatic thesteep belt is probablya complex one, developed by activity throughout the length of the belt. Instead we suggest reactivation accompanying continued magmatism, and that the locus of magmatism changed with time and that the intensestrain. It developedover a much longer period of regional metamorphism was diachronous. The continuity of time than D3 folding in the metasediments,and is nota isograds from west to east across the region is only apparent simple equivalent of a D3 fabric. R h- Sr AGES FROM CONNEMARAU-PbFROM AGES & Rh-Sr 119
The magmatic episode in Connemara appears to extend zircon fraction comprising selected clearest grains was into the period during which the lower Palaeozoic rocks of abradedand analysed. LoughNahaslean section, South Mayo docked with the Connemara Terrane. Theearly along roadrunning south from Maam Cross intrusive stages are broadly contemporary with volcanism in [L974430]. South Mayo (Dewey & Ryan 1990), but the intermediate to S1102 ‘K-feldspar gneiss’, a weakly-foliated granodiorite acid phases dated here appearto post-datethe preserved with extensive retrogression to epidote and chlorite: volcanic record in South Mayo, which does not continue three zircon samples were analysed - 36 clearest beyond the Llanvirn, andto be coeval with folding and prismatic grains fromthe >l25 pm fraction, 26 erosion in South Mayo. Late evolution of the syn-orogenic relatively clear prismatic grains from the 100-12s pm magmatism and metamorphism in Connemara, development fraction and 40 glass-clear oval grainsfrom the of the Mannin Thrust and docking of Connemara with the 100-12s pm fraction. All three samples were abraded before dissolution. Lough Nahasleam section,along South Mayo Trough were accomplished in aperiod of road running south from Maam Cross [L974430]. 10-20Ma during the Caradoc, with thelater magmatic S1 106 Fine-grainedunfoliated granite, 10 m scale, cross- stages of only local influence. cutting intrusion in migmatite: two fractions compris- ing very delicate,elongate prisms were picked for This work was supported by NERC grant GR3/S932. We are analysis. Road-side quarry at north of Oorid Lough. grateful for constructive comments from the referees, in particular a [L924462]. very thorough review by P. W. G. Tanner. Correspondence to R. A. Cliff (c-mail: [email protected]). References BADLEY,M.E. 1976. Stratigraphy, structure and metamorphism of Dalradian Appendix: sample details and grid references rocks of the Maamturk Mountains, Connemara, Ireland. Journal of the S1069 Bedding-parallel calc-silicate lens in quartzite, road- Geological Society, London, 132, 509-520. BARBER,J.P. 1985. High-grade metamorphism and melting in the Dalradian of sidequarry nearnorth end of Kylemore Lough Eastern Connemara,Ireland. PhD thesis, University of EastAnglia, [L782586]. Norwich, UK, S1580 Coarse (
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Received 12 September 1994; revised typescript accepted 10 August 1995. Scientific editing by Simon Kelley.