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Geochemistry of Dalradian Pelites from Connemara, Ireland: New Constraints on Kyanite Genesis and Conditions of Metamorphism

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Geochemistry of Dalradian Pelites from Connemara, Ireland: New Constraints on Kyanite Genesis and Conditions of Metamorphism Journal of the Geological Society, London, Vol. 143, 1986, pp. 237-252, 14 figs, 4 tables. Printed in Northern Ireland Geochemistry of Dalradian pelites from Connemara, Ireland: new constraints on kyanite genesis and conditions of metamorphism C. C. FERGUSON' & S. I. AL-AMEEN2 Department of Geology, Birkbeck College (University of London), London WlP lPA, England Department of Geology, University of Nottingham, Nottingham, NG7 2RD, England Abstract: Whole-rockand mineral analyses of Dalradianpelites from W Connemarashow that kyanitepelites are not unusually Mg-rich as previouslysupposed. Their chemistry is transitional, especially in Ca content, between overlying Fe-rich staurolite pelites and underlying Ca-rich rocks. The flysch basin in which they were deposited was established relatively earlyin the west, its sediment fill including carbonate-rich influxes derived from an adjacent swell to the east which later subsided to form an enlarged basin. Kyanite has a very restricted spatial distribution because its development was controlled by Fe deficiency associated with the carbonate-rich influxes, which in turn led to relatively high XMg during metamorphism.P-T estimates for themetamorphic peak are 650" f 25 "C and -8 kbar, about 50°C and 3 kbar greater than for similar rocks in the east. The difference can be related to structural level with respect to regional-scale folds, which may have developed as a huge backfolded complex during the emplacement of the Connemara allochthon. The Connemara Schists (Dalradian Supergroup) comprise a greaterthan 200 km'. Area A is clearly of central varied sequence of pelites,psammites, quartzites, marble- importance in understanding the genesis and significance of calcsilicate units, and a distinctive diamictitehorizon. The kyanite in Connemara.It occurs only in the Ballynakill stratigraphy and structure are now reasonably well known Formation,the uppermostunit of the Argyll Group (see (Badley 1976; Tanner & Shackleton 1979; Leake et al. 1981) Ferguson & Harvey 1979 for a brief review of stratigraphy and correlations with Donegal and Scotland are well- and structure in area A; also Cobbing 1969 and Leake et al. establishedin the lower and middle (Appin and Argyll) 1981). The typical regional assemblage is quartz + parts of theSupergroup (Kilburn et al. 1965; Harris & plagioclase + biotite + muscovite + garnet + staurolite, with Pitcher 1975). Detailed studies of pelitic rocks (Yardley fibrolite forming an additional phase in central and southern 1976; Yardley et al. 1980) have greatly improved our parts of the area (Fig. 2). In the extreme SE the rocks were understanding of metamorphic evolution in the Connemara close tothe upper stability limit of the assemblage Dalradian. The amphibolite facies regional metamorphism is staurolite + muscovite + quartz and locally staurolitehas characterized by the almostcomplete lack of kyanite reacted out. Of the 20 or so kyanite localities known to us in throughout most of the region; in contrast, andalusite (and area A, all but one (sample 19072) lie close to the contact to a lesser extent cordierite) are sporadically developed in with the underlying LakesMarble Formation. This eastern and central partsbut are absentin the west. The formation comprises discontinuousmarble and calcsilicate currentlyaccepted view (Yardley et al. 1980) is that horizons, and a variety of psammitic, semipelitic and pelitic Barrovian(staurolite-kyanite) metamorphism at pressures schists many of which are not obviously calcareous. The around 5-6 kbar was followed by Buchan-type metamorph- pelites are often similar in matrix mineralogy to those in the ism, reflecting asteepening of thethermal gradientand Ballynakill butstaurolite never occurs andgarnet is less uplift to pressures of 3-4 kbar. Usinga revised geobaro- abundant. Most kynaite localities are within the aureole of metric calculation for kyanite-bearingrocks, Barber & theLate CaledonianOmey granite (theouter limit of Yardley (1985) haverecently proposed 7-10 kbar for the contact andalusite is shown in Fig. 2). Contact and regional Barrovian metamorphism, a pressure range similar to that in effects are mostly not difficult to disentangle in the the classic Barrovian terrain of Scotland. These studies all Ballynakill, but in the Lakes Marble Formation the aureole relate to E Connemara (area C, Fig. 1). However, in spite overprint has produced thorough textural and mineralogical of very detailed investigations, kyanite is known from only a reconstitution, especially in calcsilicate rocks. single thin section in this area (B. E. Leake pers. comm.). This paper addresses three main problems. First we show Supposed differences inmineral compositions between that kyanite is not'restricted to anomalously Mg-rich kyanite-free and kyanite-bearing rocks (Yardley et al. 1980) horizons' assupposed by Yardley et al. (1980), although are based on three samples supplied by one of us (C. C. F.) kyanite pelites are geochemically distinct from other from area A some 40 km further west (Fig. 1). In this paper staurolitepelites, especially intheir Caand Fe contents. we present the results of the first geochemical study in area Second, we proposea model for the evolution of the A, indeed the first detailed study of pelitic rocks in western 'Ballynakill basin' which satisfies all available geochemical Connemara. andstratigraphical constraints. The basin was established Kyanite is extremely rare in Connemara.Apart from relatively early in W Connemara; its sediment fill included area A, it has been recorded in only three widely separated localized carbonate influxes derived from an adjacent swell areas (and is known from only a single thin section in each), tothe east which later subsided to form an enlarged eventhough rocks of suitable grade occur overan area Ballynakill basin. The localized carbonate component in the 237 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/237/4893018/gsjgs.143.2.0237.pdf by guest on 25 September 2021 238 C.C. FERGUSON & S. I. AL-AMEEN Fig. 1. Outline map of Connemara showing location of Streamstown-Cleggan area (area A), Maamturk Mountains (area B) and Maam-Cornamona area (area C). BallynakillFormation (amphibolite horizon marked l Lakes Marbleand older formations 0 staurolitepelites A staurolite-kyanitepelites \ o garnetpelites (LMF 1 \ \ \ ---A---A outerlimit of \ \ andalusitethermal \\ ---S--S sillimanite \ \ ‘isograd’ STREAMSTOWN BAY -1 km Fig. 2. Detailed map of Streamstown-Cleggan area A showing locations of analyzed samples. The major fold structures in the area are the Connemara Antiform (an F4 structure in the regional sequence of folding events) which refolds the F3 Cleggan Syncline (shown as a synform south of the Connemara Antiform andas a downward facing antiform to the north). Localities with sample numbers are those from which P-T estimates have been derived (see Table 4 and Fig. 12) but prefix ‘19’ is dropped from 5-figure sample numbers (i.e. 740 refers to sample 19740 etc). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/237/4893018/gsjgs.143.2.0237.pdf by guest on 25 September 2021 GEOCHEMISTRY OF DALRADIAN PELITES FROM CONNEMARA, IRELAND 239 Ballynakill pelites influenced kyanitegrowth because the associated Fe deficiency led to relatively high X,,(= mol. MgO/(FeO + MgO)). Third, we deriveP-T estimates for A kyanite f fibrolite area A using the garnet-biotite geothermometer of Ferry & fibrolite Spear (1978), and two garnet-plagioclase geobarometers. o no AIzSiOg phase Our results suggest that the metamorphic peak occurred at pressures -3 kbar greater than in the east. We explain this difference in terms of structural level controlled by major D3 folds, which may have evolved during southerly transport of the Connemara allochthon (Leake et al. 1983). Compositional control on kyanite formation Inan extensivestudy of Dalradian pelites within the aureoles of the Donegal granites, Naggar & Atherton (1970) showed that kyanite was confined to rocks with X,, greater than 0.54. Inthe Scottish Dalradian pelites studied by Atherton & Brotherton (1972) only six rocks of suitable gradehad X,,,>O.54 and all werekyanite-bearing. A further 14 kyanlte-pelites with X,,< 0.54 were explained F M using amodel of 'available' bulk composition.Guided by 00'l4 l this approach Yardley et al. (1980) assumed that kyanite in Connemara is restricted to anomalously Mg-rich horizons. In fact the kyanite-staurolite-biotite (K-S-B) pelites biotite from area A are less magnesian thanstaurolite-biotite v Fig. 3. AFM projection (muscovite + plagioclase + quartz + H,O in (S-B) pelites,although X,, is greaterdue to amarked excess) of the 8 K-S-B pelites (black triangles) and the 48 S-B deficiency in Fe0 (Table 1). Even so, overlap between the pelites (circles) from area A. two groups is substantial;in Donegal the kyanite-bearing and kyanite-free fields separate clearly on an AFM diagram Chinner (1965) first showed that first appearance of (Naggar & Atherton 1970) but no such separation emerges kyanite in staurolite-biotite schists (muscovite + quartz + in area A (Fig. 3). X,, of biotite is somewhat greater in plagioclase in excess) is an isograd only for fixed X,, in K-S-B pelites (Table 1) although the difference is not biotite. With increasing temperature biotite in the significant at the a = 0.05 level. However, almost all kyanite assemblage K-S-B becomes moreferroan (Harte & in area A is found within the Omey Granite aureole (Fig. 2) Hudson 1979; Chinner 1980), and Baker (1985)
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