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 stratigraphyand 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 kbarfor 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
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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).
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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) has where Mg is strongly partitioned into cordierite so that X,, provided arough calibration of this dependencein the of biotite is consistently lower than in the wholerock. Scottish Dalradian. K-S-B pelites inarea A are in Where analyses are available outside the aureole whole rock reasonableagreement with Baker'scalibration (Fig. 4) andbiotite X,, values in S-B pelites are similar (the although two anomalous samples are omitted from this plot, average relative error in using the former as an estimate of one with very high oxidation ratio (19058) and one with very the latter is only +1.3%). Accordingly we take whole rock high modal cordierite (19686). Inarea C, wherepeak X,, as the best estimate of biotite X,, ut the time of regional temperatures are some 50 "C lower, biotite X,, values are metamorphism; on this basis the meanvalue in K-S-B substantially too low to stabilize kyanite.Thus higher pelites would be 0.44 (standard deviation 0.03), which is temperaturesand XM, values in the west providea significantly greater than in S-B pelites. first-order explanation forthe restricteddistribution of
Table 1. Summarystatistics of rock and mineral Mg and Fe values from lower staurolite-sillimanite transition zone
K-S-B pelites' S-B pelites
Area A Area A Area C*
X snX snx S n t*
MgO whole rock 2.40 7 0.35 2.70 0.45 48 2.77 0.16 12 1.686 Fe0 whole rock 5.24 7 1.08 7.03 1.22 48 8.68 0.88 3.67212 X,, whole rock 0.45 7 0.04 0.040.41 48 0.33 -2.472 12 X,, biotite 0.404 0.0293 0.382 0.032 6 0.392 0.020 5 0.998 X,, garnet3 0.116 0.023 3 0.110 0.018 7 0.119 0.018 5 0.449 x,~stauro~ite~ 0.142 0.0143 0.144 0.012 5 0.139 0.016 0.216 4
X = mean, S = unbiased standard deviation, n = number of samples. 1, excludes sample 19058 which has very high oxidation ratio. 2, from Yardley et al. (1980). 3, rim compositions (or flanks where zoning is disturbed at very edge). 4, rim compositions (but all analyses used in K-S-B pelites; where rim and core analyses are available rims usually have lower X,, than cores). * Student's-t test between area A means.
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Formation all bearing theGroup 1 assemblage but with kyanite as an additional phase (triangles on Fig. 2). 0.60 1 Group 3. 11 pelites from the Lakes Marble Formation all bearing the regional assemblage bte + musc + qtz + plag + gt and free of calcic phases other than plagioclase, garnet and apatite (open circles on Fig. 2). A fourth groupof rocks will be discussed in the next section. Group 4. 3 pelites from the Ballynakill Formation which, like the K-S-B pelites, lie close to the LMF contact but which carry theGroup 1 assemblage (i.e. they are I kyanite-free). These rocks are shown as filled circles with a cross symbol on Fig. 2. 0 0 Representative analyses of Group 1 pelites,and all Groups 2 and 3 analyses, are given in Table 2 and some geochemical relations between groups are illustrated in Figs. l AREA C .... 5-8. Major element contents are given as Niggli parameters (Niggli 1954) by expressing the molecular proportion of each oxide as a percentage after SiO, is excluded. This helps to 0.35 minimize the constant sum effect in rocks with variable silica contents. 560 580 600 620 640 660 680 On a triangular Fe-Na-Ca plot the S-B and LMF pelites estimatedtemperature /"C show almost no overlap, the latter being distinctly Ca-rich Fig. 4. Plot of estimated temperature vs. X,, in biotite or whole and Fe-poor (Fig. 5a). The K-S-B pelites fall in the S-B rock. Open circles-biotite X,, in assemblage kyanite-staurolite- pelite field (except forone conspicuous outlier)but are biotite, E Dalradian of Scotland (from Baker 1985); restricted to the Fe-poor part. Trace elements show similar rectangles-whole rock X,, in K-S-B pelites, area A,with f5% characteristics (e.g. Fig. 5b), the K-S-B pelites plotting in a relative error (temperatures interpolated from samples 19701, 19740 field transitionalbetween S-B and LMF pelites. The and 19072, Fig. 12, with f10 "C error); dots-whole rock X,, in transitionalcharacter ofK-S-B pelites in terms of Ca S-B pelites, area A (temperature interpolated as before). Area C content is also clearly reflected in garnetand plagioclase rectangle shows estimates for lower staurolite-sillimanite transition analyses (Fig. 6). A systematic illustration of differences zone (from Yardley et al. 1980). between groups l and 2 is shown in Fig. 7 where the ratio of groupmeans provides a simple measure of relative enrichment or depletion in K-S-B pelites relative to S-B kyanite in Connemara.The overlapbetween K-S-B and pelites. Note in particular that AI, Ca and Na are relatively S-B pelites in area A remains a problem because, by the enriched inK-S-B pelites. The weak potashenrichment, same argument, some 10 or 15 samples from the latter group which is not reflected in Rb, is not significant at the LY = 0.05 should have developed kyanite. We return to this problem level (twosample t-test) and is probably best discounted. later. MnO is not included in Fig. 7 because of very high standard deviation in the K-S-B pelites. Nevertheless, it is clear that this group is appreciably enriched in Mn relative to S-B Transitional chemistry of kyanite pelites in area A pelites, a conclusion supported by MnO contents of garnet The most striking feature of the distribution of kyanite- andilmenite (Table 3).The most markeddepletions in bearing pelites in area A is that all except one are found K-S-B pelites areFe and Mg together with associated close to the base of the Ballynakill Formation (BF) within minor and trace elements (Ti, V, Cr, Ni, Zn). Cl, Cu and about100m of the contact with the underlying Lakes Ba are also depleted although the significance of this is not Marble Formation (LMF). The main field criterion used in clear.It is interesting tonote, however, that Cu is often mapping this contact was the occurrence of calc-silicate considered toshare the mobility of volatiles during assemblages or distinct 'calc' weathering in the LMF, even diagenesis andmetamorphism (Holland & Winchester though this often resulted in including garnet pelites which 1983), while the mobility of Ba in aqueous solution during wereindistinguishable in the field from many in the BF. metasomatism was demonstrated by Senior & Leake (1978). However, subsequent examination of several hundred thin Relative to LMF pelites the K-S-B pelites show fewer sections demonstratedthat the mappedcontact exactly systematic differences (Fig. 8). Predictably Ca and Sr are separates staurolite-bearingpelites (in the BF) from relatively depleted. AI is somewhat enriched but the alkalis staurolite-free pelites (in the LMF), and that kyanite never are broadly similar. Fe is slightly enriched in K-S-B pelites occurs instaurolite-free rocks. Nevertheless, geochemical and Mgis slightly depleted, while the associated trace analyses of the kyanite-bearing pelites reveal an interesting elements show no appreciable differences. Curiously Cl, Cu transitional character between typical (i.e. kyanite-free) BF and Ba are depleted relative to the LMF also, although the pelites andLMF pelites. These geochemical relations are dispersions are fairly large.Overall, similarities between examined using three groups of rocks. groups 2 and 3 are more striking than differences. Indeed, Group 1. 45 S-B pelites from the Ballynakill Formation with the sole exception of Nb, all elements enriched bearing the regional assemblage bte + musc + qtz + plag + (depleted) in LMF pelites relative to S-B pelites are also gt + staur f fibrolite. These are shown as filled circles in Fig. enriched (depleted) in the K-S-B pelites, although some of 2. the enrichments or depletions (including Nb) are not Group 2. 8 K-S-B pelitesfrom the Ballynakill statistically significant.
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Table 2. Representative analyses of Group 1 pelites, and all Group 2 and Group 3 analyses S-B pelites (Group 1)
702 703 707 710 713 690 713699 710 707701 703 702 724 722 721, 716
54.80 54.09 59.15 56.74 56.37 58.39 56.44 54.85 56.08 57.81 50.02 55.56 1.25 1.41 1.28 1.32 1.50 1.27 1.25 1.24 1.38 1.20 1.16 1.13 20.78 20.60 18.06 18.68 18.85 20.14 20.06 20.79 19.50 18.38 25.05 20.50 2.52 4.23 1.19 1.07 1.69 1.66 2.01 2.28 1.46 4.20 3.06 2.05 6.84 7.63 6.92 6.95 7.10 7.27 6.64 6.88 7.79 4.88 6.92 6.64 0.11 0.14 0.13 0.12 0.18 0.15 0.16 0.31 0.12 0.27 0.18 0.28 2.86 2.72 2.99 2.96 2.83 2.49 2.48 2.90 2.86 2.50 2.76 2.50 1.03 1.11 1.49 2.24 2.41 1.41 2.19 1.53 1.51 1.59 1.27 2.02 1.35 1.35 2.40 3.56 2.91 1.79 2.86 2.49 2.01 2.37 2.65 2.87 4.41 3.71 3.47 3.01 2.57 3.04 2.66 2.83 3.43 3.74 3.03 2.36 0.31 0.22 0.24 0.26 0.42 0.21 0.28 0.19 0.23 0.21 0.26 0.19 2.76 1.79 2.19 2.33 2.79 2.84 1.84 2.22 1.97 1.87 2.43 2.52
Total 99.02 99.00 99.51 99.24 99.62 100.66 98.87 98.51 98.34 99.02 98.79 98.62 OR 24.90 33.28 13.40 12.17 17.64 17.04 21.41 22.97 14.43 43.64 28.46 21.74 M/FM 0.43 0.39 0.44 0.43 0.42 0.38 0.40 0.43 0.40 0.48 0.42 0.40
Ba 1056 841 683 601 656 907 732 800 1019 486 733 549 Ce 103 77 117 118 96 90 128 112 76 116 123 88 Cr 122 128 119 132 122 136 151 90 126 84 121 102 cu 25 48 24 19 62 58 42 56 39 227 48 37 La 55 42 54 55 43 40 60 56 37 57 59 51 Nb 29 21 38 38 22 26 27 23 23 37 22 22 Ni 49 62 59 54 52 47 57 80 51 52 70 50 Pb 23 14 20 22 21 20 25 26 22 19 26 27 Rb 148 143 126 127 102 130 121 126 151 134 124 96 S 110 32 67 30 1216 1082 126 31 211 82 38 33 Sr 140 103 165 222 248 185 265 296 184 214 353 255 V 149 176 148 146 175 148 148 165 166 128 167 144 Y 38 34 40 39 43 30 42 34 36 65 42 37 Zn 129 155 130 121 116 117 123 142 138 127 167 122 Zr 276 207 313 282 324 285 316 186 298 312 187 191
Qtz + Fsp 37.0 36.6 48.9 51.3 62.1 42.6 54.7 48.9 49.0 50.6 37.6 58.0 Biotite 23.1 27.1 21.0 24.3 18.0 5.7 23.5 22.3 27.0 14.8 23.3 13.9 Chlorite* 2.9 0.3 0.3 2.2 3.4 13.3 0.3 1.7 1.1 0.3 3.9 9.7 Muscovite 24.5 15.5 16.9 14.8 5.0 16.4 10.4 1.7 3.1 23.0 9.2 3.8 Garnet 0.9 5.1 1.5 0.3 3.9 5.8 4.0 7.2 4.4 2.8 1.5 2.1 Staurolite 4.6 10.8 2.1 0.4 0.5 0.03 3.0 9.7 0.6 1.0 15.1 2.3 Shimmert 4.0 0.4 6.3 4.7 1.9 11.8 0.5 2.5 - 2.1 1.1 Cordierite Kyanite Andalusite - Sillimanite - tr - - - - - 2.4 12.7 1.7 4.2 6.6 Opaques 3.6 2.6 2.6 1.9 4.3 3.6 2.6 2.4 1.8 4.6 2.4 1.9 Others$ 0.7 0.4 0.4 0.1 0.9 0.9 1.0 1.3 0.3 0.5 0.7 0.6 ______~ * Secondary (mostly after biotite) t fine grained alteration product mainly from staurolite $ Mainly tourmaline and apatite. Also graphite (588 and 756 only), zircon, calcite, zoisite, sphene Sedimentary geochemistry and a depositional model Therefore one might expect a positive correlation between for area A pelites al-alk andthose elements introduced intothe original sediment via phyllosilicates, and a negative correlation for In this section we attempt to relate the variations in pelite elementsintroduced in other minerals. In fact no clear geochemistry outlinedabove to differences in original pattern emerges in the area A pelites, there being a notable sediment sources. Trace element concentrations in metased- lack of correlation between al-alk and almost all elements iments are mainly controlled by clay mineraland mica analyzed.This appearsto conflict with Senior & Leake content of the original sediment. The Niggli parameter (1978) who demonstrated positive correlationsbetween al-alk (mol. % (A120,-Na,O + K,O)) is a measure ofAI al-alk and many trace elements.However, their correla- introduced in clay minerals and micas because al-alk in tions areapparent only when a wide range of rock albiteand K-feldspar is zero (Senior & Leake 1978). types-marble, calcsilicate, psammiteand pelite-are in-
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Table 2. (continued) K-S-B pelites (Group 2)
058 072 633 672 673 676 686 757 686 676 673 672 633 072 058
49.71 54.92 58.77 45.59 54.56 58.39 57.79 56.00 0.57 1.27 0.75 0.98 0.81 0.70 0.68 0.66 24.59 22.14 21.02 32.98 22.99 19.91 21.27 19.38 8.47 1.44 1.05 0.93 1.90 1.17 0.87 1.28 3.80 4.71 5.00 3.88 5.40 4.47 6.15 7.06 1.08 0.06 0.20 0.10 0.25 0.43 0.22 0.16 1.20 2.65 2.02 1.92 2.47 2.47 2.33 2.95 1.83 1.91 0.77 1.68 1.65 2.82 1.27 2.95 4.00 1.73 1.93 1.92 2.69 5.45 2.14 2.79 2.01 5.01 3.98 5.34 3.39 1.62 3.26 2.23 0.06 0.51 0.11 0.12 0.20 0.14 0.11 0.22 2.19 3.03 3.11 3.65 3.27 1.73 2.54 3.27
99.51 99.38 98.71 99.09 99.58 99.30 98.99 98.90 66.73 21.57 15.89 17.74 24.05 19.06 11.29 14.03 0.36 0.50 0.42 0.47 0.45 0.50 0.40 0.43
523 687 1112541 687 523 700 365 635 392 131 112 123 112 131 191 110 102 108 145 85 98 85 123 91 93 86 86 90 67 22 67 9 14 12 7 29 42 56 5762 56 99 59 51 60 63 30 22 25 2252 30 30 24 26 27 30 34 30 51 44 90 46 46 49 35 19 27 19 35 43 38 57 29 19 151 146 16374 146 151 141 99 143 95 3975 42528 244 0 32 476 1090 286 114 261 443 261 114 286 304 402 259 316 81 177 104 158 144 92 95 79 42 30 30 3059 42 34 38 28 69 100 103 100 96 116 133 53 131 210 325 281231 219 189 162 185 176
68.4 23.9 45.6 36.6 48.9 72.2 39.4 53.6 2.1 18.8 7.8 7.6 16.3 19.8 19.7 tr 0.3 0.3 2.1 - 0.3 0.3 0.5 3.4 - 26.9 10.7 31.7 24.8 9.141.7 0.1 18.2 4.1 14.5 0.1 5.7 2.6 7.0 2.3 10.4 4.6 0.1 1.5 2.3 2.8 2.3 0.4 1.5 0.1 1.2 0.2 tr 2.1 - - 2.1 - - - - 14.4 1.2 - - 22.6 - 0.8 1.5 0.3 0.1 0.6 0.3 0.6 0.10.3 0.3 0.05 1.5 0.8 0.9 - 1.0 2.1 - - - - - 20.7 0.1 - 0.1 - - - 1.2 1.8 0.7 0.9 0.2 0.4 1.6 2.1 1.5 0.4 0.3 1.5 2.7 0.6 1.2 2.4
cluded in the analysis (Senior & Leake 1978, fig. 5); little or sediment. Fig. 9a suggests that the Ca contentin K-S-B no correlation exists within each rock type (Senior & Leake pelites is similarly derived. 1978, fig. 3). This probably reflects mobilization of elements Localized carbonate influxes into Ballynakill sediments duringdiagenesis and metamorphism. Theone clear are entirelyconsistent with probable facies relationships exception is the marked negative correlation between al-alk during Lakes Marble and Ballynakill times. In east-central andCaO in LMFand K-S-B pelites (Fig. sa), a trend Connemara (area B, Fig. 1) the Lakes Marble Formation is reflected only weakly if at all in S-B pelites (Fig. 9b). There subdivided into four members (Badley 1976), the youngest seems little doubt that, given the association with marbles of which (Finnisglin member) is dominated by banded and calcsilicates, the high Ca content in LMF pelites is due amphibolites (Fig. 10). Yardley (1976) recognized the same to a relatively high carbonatecomponent in the original four members in area C. The Bennabeola area of central
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Table 2. (continued) Lakes Marble Formation pelites (Group 3)
062 587 588043 587 062 756 657 654 645596 641 640
49.74 59.74 42.04 61.56 56.72 55.66 60.31 59.27 57.42 55.50 60.72 SiO, 0.97 0.66 1.73 0.62 0.70 0.70 0.62 0.95 0.63 0.73 0.59 TiO, 25.07 20.26 26.03 17.77 21.21 20.80 20.09 18.40 20.22 20.05 18.53 AI,O, 0.90 1.28 2.11 0.49 1.11 0.90 0.94 1.64 0.67 2.04 0.88 Fe,O, 6.84 4.04 9.58 4.81 5.24 5.58 4.12 4.30 5.42 4.52 3.62 Fe0 0.11 0.13 0.22 0.14 0.19 0.23 0.20 0.14 0.18 0.19 0.23 MnO 2.20 2.46 2.73 2.48 2.74 2.77 2.12 2.94 2.42 2.76 2.11 MgO 2.56 1.75 4.64 4.26 2.66 4.02 2.28 1.69 3.91 4.70 4.70 CaO 2.81 1.99 1.99 3.00 2.40 3.01 2.41 2.18 3.23 2.69 2.49 Na,O 4.80 3.85 3.98 2.32 3.82 3.86 3.12 4.71 3.09 3.78 3.67 K,O 0.27 0.06 0.02 0.12 0.10 0.12 0.11 0.21 0.10 0.14 0.09 P,O, 2.39 3.26 2.76 1.73 2.27 1.78 3.11 2.05 1.80 3.54 3.68 H,O+
98.66 99.48 97.83 99.30 99.16 99.43 99.43 98.48 99.09 100.64 101.31 Total 10.59 22.18 16.54 8.40 16.01 12.67 17.03 25.55 10.01 28.88 17.95 OR 0.36 0.52 0.34 0.48 0.48 0.47 0.48 0.55 0.44 0.52 0.510.52 0.44 0.55 0.48 0.47 0.48 0.48 0.34 0.52 0.36 M/FM
843 658436 6561120 698 552 1121 593 702 719 Ba 162 117143 142 135 150122 115 161 228 139 Ce 144 73 83 83 10483 7773 144 82 80 76 120 83 Cr 1637 3026 86 29 35 80 40 91 17 cu 77 61 67 78 71 6581 66 78 115 76 La 2717 2718 33 26 20 20 18 35 18 Nb 44 6644 4980 36 38 48 50 90 27 Ni 36 24 2831 2433 36 30 30 37 31 41 28 Pb 172 155 216 138 101 155 158 208 112 252 136 Rb 3597 295 1574 1376 234274 1376 33121574 295 3597 18 1512 3980 718 S 478 156 516244 317 155 348 474 385 916 386 Sr 266 82 96 102 75 144 75 102 12996 8482 266 78 163 82 V 53 22 81 34 30 38 60 46 30 52 32 Y 114 131 192 88 123 125 88 144 107 190 87 Zn 249 151 240 156 158 173 130 242 154 307 171 Zr
30.0 40.2 51.1 68.8 54.2 62.7 64.0 50.8 64.2 67.3 66.7 Qtz + Fsp 23.2 20.5 30.4 16.3 22.6 29.1 15.5 20.4 20.1 18.5 24.4 Biotite 1.110.4 0.4 0.4 0.70.1 8.6 - 1.2 0.5 Chlorite* 33.3 35.133.3 8.2 1.6 13.0 1.6 12.2 27.3 10.8 1.4 2.7 Muscovite 1.1 0.03 1.0 0.7 0.6 3.2 4.6 tr 0.5 0.1 0.9 Garnet ------Staurolite Shimmer? Cordierite ------Kyanite 10.32.2 0.3 - 0.6 ------Andaluste ------Sillimanite 2.1 1.6 4.2 0.4 2.7 2.2 3.1 0.03 2.1 2.8 2.0 Opaques 0.2 1.1 2.6 1.7 0.6 1.0 0.6 0.8 1.0 1.3 2.7 Others$
Connemara (between areas A and B) has not been mapped marble is uncommon and the member comprises a variety of in the same detail, and the Lakes MarbleFormation has not pelites, psammites and calcsilicates. These are succeeded by been subdivided.Nevertheless the characteristic area B staurolite pelites assigned to the Ballynakill Formation. The lithologies are all found, psammitic rocks show graded Finnisglin bandedamphibolites are thought to have bedding and pebblybands typical of the Luggartarriff originated as ash bands and are treated as an important time member, and banded amphibolites occur at or near the top marker in Connemara (Evans & Leake 1960; Badley 1976). (Tanner & Shackleton 1979; Evans & Leake 1970). In area The only bandedamphibolite in area A that could be A the Sruffaunbaun and Luggartarriff members are clearly correlated with them is within the Ballynakill Formation recognizable although the latter is finer grained than further (see Figs 2 and 10). Although thin, it can betraced for east and pebbly bands are rare. The Muingboy member is a considerable distances along strike.This correlation is rather pure calcite marble in areas B and C but in area A supported by other workers (e.g. Tanner & Shackleton
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Fe
I ll I l I l 5 10 15 20 25
Oh grossular in garnet
Na Ca Fig. 6. Plot of Ca content in garnet and plagioclase expressed as % ,,-Cr:O ,,-Cr:O 6 grossular vs. % An for area A pelites.Symbols show mean values of y microprobe spot analyses distributed overX grains (X,y shown on plot). Bars represent2x standard error intervals.
1979) and, if correct, implies that Ballynakill staurolite pelitesbelow the amphibolitehorizon in area A arethe chronostratigraphicequivalents of theupper part of the Muingboy member in areas B and C. These stratigraphic relations are consistent with a basin and swell depositionalmodel for Muingboy times in \ Rb Sr Connemara, the swells being sites of limestone deposition and, locally, of erosion. In adjacent basins shale sequences Fig. 5. (a) Triangular Fe-Na-Ca plot (Niggli parameters). predominated, with periodic influxes of carbonate-rich (b) Cr-Rb-Sr plot of analyzed pelites from areaA. K-S-B pelites sediment introduced via turbidity currents shedfrom the shown by black triangles or marked field, S-B pelitesby black dots unstable swell margins.Strips of staurolite pelite found or marked field, LMF pelites by open circleor marked field. within theLMF inthe Bennabeola area (Tanner &
Fig. 7. Enrichment or depletion factors for 24 elements in K-S-B pelites relativeto S-B pelites in area A. The datum lineis set by the mean values of 45 S-B pelites (group 1 in text), the shaded zone indicating2 X standard error intervals. Black dots indicate the mean value of 8 K-S-B pelites (group2 in text) dividedby the group 1 mean for each element, the reciprocalof this ratio being plotted for relative depletions. Vertical bars indicate 2X standard error intervals. Major element ratios are basedmean on Niggli values.
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Table 3. MnO contents in whole rock, garnet and ilmenite
Whole rock Garnet Ilmenite
X snx S mn X S rnn
Groups 1 +4 0.17 0.07 48 3.23 1.54 6 64 1.02 0.75 5 11 Group 2 0.31 0.33 4.14 8 2.10 3 30 2.04 0.294 2 Group 3 0.18 0.04 11 4.62 1.65 13 2 - - --
,t = mean, S = unbiased standard deviation, n = number of samples, m = number of point analyses.
Shackleton 1979) probably represent atransitional facies SW Highlands, where facies patterns were controlled by a between basin and swell, althoughtectonic intercalation series of fault-bounded blocks and basins. Ballynakill cannot be ruled out. The Finnisglin volcanic episode then sedimentation was probablycomparable tothat of the providestimea marker across the basin and swell Ardrishaig Phyllites; distal turbidites predominate (probably palaeogeography.Psammites in the lower part of the introduced along the axis of the trough) with local succeeding Ballynakill sequence in the Bennabeola area are introductions of carbonate detritus from adjacent uplifted locally calcareous and hornblende-bearing while, in area A, (and locally emergent) areas. zoisite and epidote are occasionally recorded in Ballynakill We suggest then that two distinct componentscontrib- pelites above the amphibolitehorizon although neither uted to the basin fill in area Aduring Lakes Marble and mineral has been recorded below (nor in the LMF pelites). Ballynakill times. (1) Turbidites introduced along the axis of This suggests that some Finnisglin volcanic detritus was the basin contributeda pelitic fraction consisting of clay reworked intothe adjacent basin tothe west before the minerals (probably illitic), finely divided alkali feldspar and swell finally subsided to produce an enlarged Ballynakill quartz,and a significant dissolved Fe component. basin receiving sediment mostly of distal flysch type. Feldspathic quartzites and graded pebbly units occur in both The facies relations outlined above are compatible with formations. (2) Carbonate-richdetritus reworked from an Dalradiansedimentation models worked out in the SW adjacent,and probablyfault controlled, basin margin Highlands of Scotland (Anderton 1979, 1982). Although probablyincluded an Na-rich,Fe-poor illite fraction.This exactageequivalence cannotbedemonstrated, the source waned in importanceas the margin subsided Ballynakill andLakes Marble Formations can be broadly following the Finnisglin volcanic episode, and changes in correlated with theEasdale and Crinan Subgroups in the basin and swell geometry may have been related to volcanic
Nb
T T
Fig. 8. Enrichment or depletion factors for 24 elements in K-S-B pelites relative to Lakes Marble Formation pelites. The datum line is set by the mean values of 11 LMF pelites (group 3 in text); black dots indicate the mean value of 8 K-S-B pelites (group 2 in text) divided by the group 3 mean, or the reciprocal of this ratio. Shaded area and vertical bars are explained in Fig. 7.
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00
0 a 0
4 0 0
3 A
0 0 0 0 0 S 0 2 A A 0 0 A A
A
1 A
I I I I I 20 22 20 24 26 28 3b al-alk 'tti-
- 3 e e b
e e bl e
0 c e e :2 - e I
e e
1
202L 22 26 28 30 al-alk
Fig. 9. Plots of Niggli al-alk vs. % CaO. (a) K-S-B (black triangles) and LMF (open circles) pelites; (b) S-B pelites.
activity at this time.Kyanite subsequently developed near although close to the LMF contact, do not contain kyanite. the base of the Ballynakill Formationwhere the Relative to K-S-B pelites these rocks are rich in K, Al, Si carbonate-rich component was significant, although the and Fe and poor in Ca and Sr (Fig. ll),suggesting that they geochemical control was the associated Fe deficiency. This is represent lenses in which thecarbonate component was supported by the three staurolite pelites (group 4) which, absent.
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zoisite,epidote *B F . . ..J recorded ...... I” ----___
- KYANITE
LMF R---...... --__ . ._I...... :, ..:. .
? ,r T Bennabeola Area B Area A Area I \L
Muingboy Mb. tlmes
Fig. 10. Lithostratigraphical logs for Lakes Marble Formation (LMF) and lower part of Ballynakill Formation (BF) in areas A and B, andin the intervening (Bennabeola) area. Brokenlines indicate probable chronostatigraphic correlations with the four members of the LMF established in area B. Themain volcanic horizon (Finnisglin Member of area B) is now represented by banded amphibolites. Logs, and facies relationships for Muingboy Member times, are schematic and not to scale. Black ornament indicates shale; cc ornament indicates carbonate rich shale or silt; other rock types are shown conventionally.
Thermobarometry and a structural model samples also contain fibrolite and/or kyanite, except sample 19701 which is included because it lies almost exactly on the In order to estimate conditions of metamorphism in area A sillimanite-in ‘isograd’. We have also included two Lakes we have applied one cation exchange geothermometer and Marble Formation pelites with the same mineralogy except two geobarometersto ninesamples from the Ballynakill that staurolite is lacking. Eight of the 11 samples are from Formation, all bearing the assemblage quartz + within the Omeygranite aureoleand these include three plagioclase + muscovite + biotite + garnet + staurolite. All (4278, 4296, 3163/2) used in the earlier work of Yardley et
K cu
T SI c1 T l: T
C .-0 c -al a W U l
Fig. 11. Enrichment or depletion factors (24 elements) for group 4 rocks (see text) relative to the K-S-B pelites (group 2 in text). Shaded area andvertical bars indicate one standard errorintervals.
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al. (1980). The three calibrated thermobarometers are: I"""/ KMg3AISi3OIn(OH),+ Fe,AIZSi,O,, =
KFe,AlSi,O,,(OH), + Mg,A1,Si30,, (1)
3CaA1,Siz0, = Ca,Al,Si,O,, + 2AIzSi0, + SiO, (2)
3CaAl,Si,O, + KFe,AlSi,O,,(OH), =
Fe3A1,Si,0,, + Ca3A1,Si,0,, + KAI,Si,Oln(OH)z (3)
Forreaction (1) we use the experimentalcalibration of Ferry & Spear (1978) adaptedfor garnet non-ideality as suggested by Hodges & Spear (1982); for reaction (2) we use the calibration of Newton & Haselton (1981) based on experimental determination of the end-member reaction by Goldsmith (1980). Reaction (3) was empirically calibrated temperature/ "C by Ghent & Stout (1981), but we have used the more Fig. U. P-T fields estimated from simultaneous solution of refined calibration of Crowley & Hodges(see Hodges & equilibria (l), (2) and (3). See Fig. 2 for specimen localities and Royden 1984) although,for consistency, grossular and Table 4 for representative mineral compositions. Kyanite- anorthite activities and the partial molal volume of grossular sillimanite equilibrium from Holdaway (1971). were estimated using the procedures of Newton & Haselton (1981). The equilibrium coefficient equations,and the Evidently the calculated ranges of P andT are very activity-composition relationships used,are given in the large.Some of this variation may reflect compositional papers cited above. Representative mineral compositions factorsnot incorporated in the calibrations(e.g. unrecog- (oneset for each sample) are given in Table 4, andthe nized Fe3+ in biotite and garnet). However, it seems likely results of simultaneously solving equilibria (1)-(3) are that most of the scatter is due to element partitioning being shown in Fig. 12. Equilibrium (1) is (almost)pressure- seriously disturbed by contactmetamorphism within the independent and a temperature estimate is represented on Omeygranite aureole.Note that the only three samples Fig. 12 by a (nearly) vertical line; the ranges shown result outside the aureole produce narrow temperature ranges and from two or more garnet-biotitepairs being analysed for show a progressive increasein temperature from the most samples. The two geobarometersare strongly sillimanite-in 'isograd' (19701) to nearthe staurolite-out temperature-dependentand the pressure ranges shown 'isograd' (19072). The large pressure range for all samples reflect the application of both equilibria (except for sample (except 19701) results because equilibrium (2) always yields 19701) as well as therange of mineral compositions. The a pressure estimate 1-2 kbar greater than (3) for the same P-T quadrilaterals(Fig. 12) would obviously be larger if garnet-plagioclase pairs, even though in other areas the two estimated errors microprobein analysis, or in the geobarometers give similar results (Hodges & Royden calibrations, were taken into account. 1984). We feel that equilibrium (2) is inherently unsuitable
Table 4. Representative mineral compositions used to construct P-T estimates shown on Fig. 12.
Pk. garnet biotite muscovite
X,, xgr xpy Xalm xsp X"" Xph X, X", X,,
19043 S 0.282 0.151 0.054 0.762 0.033 0.523 0.227 0.960 0.040 0.923 19056 S 0.216 0.114 0.079 0.777 0.030 0.489 0.326 0.892 0.108 0.897 19072 S+K 0.379 0.112 0.114 0.700 0.073 0.454 0.364 0.935 0.065 0.850 19596 S 0.347 0.156 0.057 0.639 0.148 0.439 0.332 0.959 0.041 0.924 19633 S+K 0.145 0.076 0.101 0.724 0.099 0.483 0.300 0.956 0.044 0.927 19686 K 0.254 0.148 0.108 0.703 0.042 0.470 0.308 0.953 0.047 0.934 19701 - 0.8840.0980.9020.3300.4800.022 0.775 0.095 0.108 0.247 19740 S 0.194 0.064 0.114 0.760 0.062 0.465 0.329 0.870 0.130 0.917 31632 S+K 0.258 0.150 0.112 0.683 0.056 0.433 0.373 0.873 0.127 0.916 4278 S+K 0.312 0.134 0.074 0.664 0.128 0.417 0.341 0.881 0.119 0.924 4296 S+K 0.340 0.166 0.119 0.682 0.033 0.417 0.371 0.899 0.101 0.924
Localities: see Fig. 2.Al,SiO, phaseindicated as S (sillimanite) and/or K(kyanite). Microprobe determinations: University of Cambridge (samples prefixed 19, analyst S. I. AI-Ameen) and University of Washington, Seattle(others, analyst B. W. D. Yardley). Garnetand plagioclase compositions are rim analyses, excluding Mn-rich edges in garnet. Mica compositions are matrix grains parallel to schistosity.
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for rocks containing both sillimanite and kyanite (5 of the 11 the Connemara Antiform, and north of the migmatite belt) samples), especially when contact andalusite is also present. the combined effect of these deformations is so intense that Thus, although equilibrium (3) is empirically calibrated, it is theBennabeola Quartzite (equivalent tothe Islay-Jura in part calibrated against (1) and (2) in areas free from the Quartzite in Scotland) is thinned to Fig. 13. Geometry of some major F4 (Connemara Antiform and Synform) and F, axial surfaces, constructed from cross-sections in Tanner (1981). No vertical exaggeration. Horizontal and vertical scales the same in E and W (i.e. no size reduction for perspective effect) although the E-W distance, about 40 km, is foreshortened for ease of presentation. Average plunge of Connemara Antiform is about 10" to E. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/237/4893018/gsjgs.143.2.0237.pdf by guest on 25 September 2021 250 FERGUSON C. C. & S. I. AL-AMEEN A and C could be mainly due to difference in structural level aries. The isopleths shown in Fig. 14 are consistent with with respect to the major F3 folds. That the two areas are Harte & Hudson’s at the S + C = A + B univariant curve, now found at about the same elevation is due mainly to F4 and with our own analyses (and Baker’s calibration) in the folding;in particular, ignoring minor perturbations,the kyanite field. Connemara Antiformplunges gently (- 100) westwards throughout the region and this corresponds to about 7 km P-T-Time paths for area A vertical difference over the 40 km separating areas A and C. The proposedpaths (labelled Al, A2, A3 in Fig. 14) are Tectonometamorphic synthesis consistent with a metamorphic peak around 630-660 “C and 8 kbar, and assume maximum X,, in biotite around 0.43. In Figure14 attemptsto integrate the major metamorphic the north of the area (north of the sillimanite ‘isograd’, Fig. constraints in Connemara with the structural model outlined 2) thepath (Al) is always down-temperature of the 0.43 in Fig. 13. We assume thatthe important continuous isoplethand no AI,SiO, phase can develop.South of the reaction in the staurolite-sillimanite (kyanite)transition ‘isograd’ thepath (A2)intersects the 0.43 isopleth in the zone is sillimanite field but not in the kyanite field, so that only the 6 Fe-staurolite + 4 muscovite + 8.5 quartz+ former can develop. Farther south path A3 intersects the 0.43 isopleth in the kyanite field; kyanite will develop but 4 annite + 31 Al,SiO, + 6 H,O. (4) l will be metastable once the path crosses into the sillimanite Harte & Hudson (1979) attempted to construct isopleths of field. This provides a simple explanationfor the biotite X,, in this assemblage but, following Chinner (1980) sillimanite-in ‘isograd’ being north (and down-temperature) and Baker (1985), we infer from AS and AV of the reaction of the kyanite-in ‘isograd’. Notethat fibrolite iswidely that isopleths must refract to steeper slopes as they cross the developed in the south in rocks with biotite X,, much less andalusite-sillimanite and sillimanite-kyanite phase bound- than 0.43. This is not a product of staurolite breakdown but of the reaction garnet + muscovite- biotite + sillimanite + quartz, (5) the product assemblage of which is found commonly as Faserkiesel formed around highly corroded garnet cores. Yardley et al. (1980) argued for early growth of kyanite from chl + musc + gt+ bte + staur + ky + qtz + plag + H20,(6) areaction producing a kyanite-staurolite ratio of around 3 : 1 to 4: l. The textural evidence cited-“composite kyanite grains with small amounts of epitaxially intergrown staurolite” (Yardley et al. 1980 p. 384)-was interpreted as dueto simultaneous growth. Reaction (6) isdifficult to reconcile with all kyanite localities being found south of the sillimanite ‘isograd’, even though staurolite is abundant farther north. The textural argument is also unconvincing; kyanite-staurolite composite grains are common but stauroliteoften forms the largerpart of thegrain. The ‘epitaxial’ textures are equally consistent with topotactic replacement of staurolite by kyanite, a form of replacement expectedbetween minerals with such close structural similarity. Other texturalfeatures provide compelling evidence forkyanite growth by staurolitereplacement (Ferguson & Harvey 1979), consistent with (4) but not (6). P-T-Time paths for area C I 500 600 7 00 The paths (labelled Cl, C2, C3 in Fig. 14) are constrained ternperature/”C by P-T estimates for lower staurolite-sillimanite transition zone (LTZ), sillimanite-K-feldspar zone (SKZ), migmatite Fig. 14. Possible P-T-time paths for area A (curves Al, A2, A3) zone (MZ) and migmatitic leucosomes (ML) as determined and area C (curves Cl, C2, C3). Schematic isopleths for X,, of et & biotite in assemblages A-S-B (positive slopes) and A-C-B by Yardley al. (1980) and Barber Yardley (1985). (negative slopes) derived, in part, from Harte& Hudson (1979) and Maximum X,, of biotite in LTZ is about 0.41 (Yardley et al. Baker (1985). Invariant points generated by intersection of 1980), thus precluding growth of kyanite. On entering the equilibrium (7) and minimum melting curve (shown for XHZO= 0.6. andalusite field along path Cl, eitherandalusite or 0.7, 0.8 and 1.0) from Kerrick (1972). P, T estimates for E cordierite should develop with the latter expected only in Connemara from lower staurolite-sillimanite transition zone the most Mg-rich or Fe-poor rocks. Kyanite in area C is (LTZ), sillimanite-K-feldspar zone (SKZ), migmatite zone (MZ), expected only in rocks with anomalously high X,,, which and migmatite leucosomes (ML) are from Yardley et al. (1980) and should therefore grow later cordierite; the only kyanite Barber & Yardley (1985). found so farin the area (sample BL 3157, Yardley et al. 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 DALRADIANPELITES FROM CONNEMARA, IRELAND 25 1 1980) occurs as a single relict grain in late cordierite. Rocks temperature for reaction (4)-in effect shifting X,, isopleths with lower X,, could also developkyanite if they follow on Fig. 14 to lower temperatures-thus allowing kyanite P-T-time paths into the migmatite field (e.g. path C3); the growth in rocks which otherwise would have been slightly only other kyanite occurrence known in E Connemara is a too Fe-rich. Unfortunately there is no compelling evidence single relict grain in migmatites from the Shannavara district to test this model otherthan the very restricted spatial to the south of area C (A. Senior, pers. comm.). Paths C1 distribution of K-S-B pelites. to C3 are consistent with a metamorphicpeak occurring Another difficulty with the model is that the metamor- towards the end of D, deformation at a structural level some phic peakin Connemara is usually interpreted as MP2 9-11 km higher than in area A. The later( Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/2/237/4893018/gsjgs.143.2.0237.pdf by guest on 25 September 2021 252 C. C. FERGUSON & S. I. AL-AMEEN folds and, as they were closed up by distributed shortening, GHENT,E. D. & STOUT,M. 2. 1981. Geobarometry and geothermometry of the hinges probably rolled forward (to the South)assisted by plagioclase-biotite-garnet-muscovite assemblages. Contributions to distributerd shear at a depth related to renewed southward Mineralogy and Petrology, 76, 92-7. GOLDSMITH,J. R. 1980. Melting and breakdown reactions of anorthite at high progress of the allochthon.This provides a simple pressures and temperatures. American Mineralogist, 65, 272-84. explanation for why almost all the andalusite porphyroblasts HARRIS,A. L. & PITCHER,W. S. 1975. TheDalradian Supergroup. In: occur north of theConnemara Antiform axial trace;the HARRIS,A. L., SHACKLETON,R. M,,WATSON, J., DOWNIE,C., HARLAND, commonly observedpartial replacement of andalusite by W. B. & MOORBATH,S. (eds) A Correlation of the Precambrian Rocks in the British Isles. Special Report Geological Society, London, 6, 52-75. sillimanite could then be due to downward transport in the HARTE, B. & HUDSON,N. F. C. 1979. Pelite facies series and the wake of the rolling hinge, thus pushing rocks back into the temperatures and pressures of Dalradian metamorphism. In: HARRIS,A. sillimanite field. L., HOLLAND,C. H. & LEAKE, B. E.(eds) The Caledonides of the British Isles-Reviewed. Special Publication of the Geological Society, London, 8, 323-37. Financial support for field work was provided by the University of HODGES,K. V. & ROYDEN,L. 1984. Geologic thermobarometry of Nottingham when C. C. Ferguson was in theDepartment of retrograded metamorphic rocks: an indication of the uplift trajectory of a Geology.Geochemical analyses by S. I. AI-Ameenwere made portion of thenorthern Scandinavian Caledonides. Journal of possible by the technical supportand advice of B. P. Atkinand Geophysical Research, 89, 7077-90. P. K. Harvey (XRF,University of Nottingham) and J. V. P. Long and -& SPEAR,F. S. 1982. Geobarometry and the Al,SiO, triple point at Mt. Moosilauke, New Hampshire. American Mineralogist, 67, 1118-34. P. J. Treloar (Microprobe, University of Cambridge). Some of the HOLDAWAY,M. J. 1971. Stability of andalusiteand the aluminium silicate interpretation and writing was done when C. C. Ferguson was on phase diagram. American Journal of Science, 271, 97-131. ‘study leave’ from the University of Kansas; the U.S. Department HOLLAND,J. G. & WINCHESTER,J. A. 1983. The use of geochemistry in of Justice(Immigration & Naturalization Service) unwittingly solving problems in highly deformedmetamorphic complexes. In: providedthis opportunity,and P. E. Baker generously offered AUGUSTITHIS,S. S. (ed) The Signijicance of Trace Elements in Solving facilities andsupport. We havebenefitted fromthe advice and Petrogenetic Problems & Controversies. Theophrastus Publications, Athens, 263-74. criticism of N. F. C.Hudson and from reviews by B. E. Leake, KERRICK, D. M.1972. Experimental determination of muscovite and quartz P. J. Treloar and B. W. D. Yardley, and the Journal’s referees. Our stability with P,,, < PTota,.American Journal of Science, 272, 946-58. sincere thanks to all these individuals and institutions. KILBURN, C.,PITCHER, W. S. & SHACKLETON,R. M. 1965. The stratigraphy and origin of the Portaskaig Boulder Bed series (Dalradian). Geological Journal, 4, 343-60. LEAKE, B. E., TANNER,P. W. G. & SENIOR,A. (compilers) 1981. The References Geology of Connemara (map, scale 1 :63,360), University of Glasgow, Glasgow. ANDERTON,R. 1979. Slopes,submarine fans, and syn-depositional faults: -, SING,, D. & HALLIDAY,A. N. 1983. 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