Deformation and of rocks and the evolution of the cordillera

BRUCE W. D. YARDLEY

SUMMARY Dalradian metasediments in the Cur district gradient, since breakdown of the assemblage of north-east Connemara have had a complex staurolite + muscovite + quartz produced an- history of deformation and metamorphism. dalusite at high structural levels, but sillimanite The earliest schistosity present formed during deeper down. D 3 deformation began after the biotite-grade metamorphism and may be initial uplift at the peak of metamorphism and mimetic after a compaction fabric. Subsequent- produced two major northward-facing nappes ly, the rocks attained garnet grade and major thrust over a basal fold-nappe. The nappes D2 folds developed. This deformation involved root to the south of the Corcogemore Moun- an initial buckling followed by a coaxial tains but continued uplift in south Cormemara homogeneous flattening and the structures faultcd out the root zones as later nappcs produced were probably initially upright. developcd. After cooling, broad open D 4 After D2, staurolite, and locally kyanite, folds wcrc formed and the Connemara grew under moderately high pressure con- werc thrust up and to thc south over lower ditions. Continued increase in metamorphic gradc rocks. Uplift of the Conncmara region grade was, however, apparently accompanied may have bccn complcmentary to the sub- by regional uplift and erosion, for staurolite sidcncc of the Mayo Trough to the north, in breakdown occurred at lower pressures than which case the oldest Ordovician rocks in those required to form kyanite. Furthermore, South Mayo may have been deposited at the the highest grade metamorphism was ac- samc time that the peak of metamorphism was companied by a steepening of the thermal attained in Conncmara.

THE CONNEMARA REGION (Fig. I) is composed of moderate to high grade Dalradian metamorphic rocks grading southwards into migmatites with syn- orogenic ultrabasic to acidic intrusives (Leake i97oa , b, Senior i973) and has been intruded by late to post orogenic rocks (Bradshaw et al., i969, Leake i974). Wager & Andrew (I93O) first conclusively demonstrated that the Connemara Schists are distinct from the very low grade lower Palaeozoic rocks of Mayo, immediately to the north, but it was not until the work of Kilburn et al. (i965) that the Connemara Schists were accurately correlated with the Dalradian rocks of Scotland and Donegal. The present paper is based on detailed mapping and structural and metamorphic studies in the Cur region (Fig. I) and was facilitated by access to the unpublished work of Patrick (1967). Detailed structural studies are possible in Connemara since the stratigraphy is well known over much of the succession. Local successions have been described by Kilburn et al. (i965), Edmunds & Thomas (i966), Patrick (i967) , Cruse & Leake (I968), Cobbing (I969), Evans & Leake (i97o) and Badley (1972). Badley (I972) has attempted to collate earlier proposals and to erect a rigorous nomenclature which is similar to that adopted by Harris & Pitcher (1975). There are two tectonic breaks within the succession (Fig. 2) and so far as is known no beds occur within more than one tectonic unit. The lowermost tectonic unit has by far the largest area of outcrop and contains many distinctive and Jl geol. Soc. Lond. vol. x3~, I976, PP. 52x-542, t x figures, Printed in Northern

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continuous marker horizons. The stratigraphy of the upper units is less well known, and there are few marker horizons. Evidence for way-up is found in all three tectonic units. The lower part of the succession is known to be largely Argyll (Middle) Dalradian (Harris & Pitcher i975) from the occurrence of the characteristic Boulder Bed sequence near the base; however, there is no direct evidence of the age of the rocks of the two higher tectonic units. They are assumed to be younger because they occur progressively further north and there is a i • u

M

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/521/4885243/gsjgs.132.5.0521.pdf by guest on 29 September 2021 524 B. IV. D. Yardley x. Structural geology Polyphase deformation has been recognized for many years in Connemara, and there is now broad agreement among many workers on the identity and number of fold phases (Leggo et al., z966 , Cruse & Leake z968 , Moorbath et al., z968, Leake z 97ob, Badley z972 ). Three major periods of folding (D2-D4) are recognized on the basis of the development and subsequent deformation of planar fabrics, and an earlier D I event is inferred from the presence of a pre-S2 fabric preserved as inclusion trails in garnet and plagioclase porphyroblasts. No major D z structures have been recognized by the writer, and the rare D I small scale folds that are occasionally found may be deformed sedimentary structures. Thus there is a possibility that the S I fabric is simply mimetic after bedding. There has been quite extensive post-metamorphic faulting. Major structures are shown on Figs. I, 3. D2 Deformation. The earliest folds recognized in the Cur area have a pronounced axial planar schistosity, designated $2. D2 folds are tight to sub-isoclinal and often plunge steeply to the north-west or north-east. A lineation, L2, parallel to the D2 fold axes is sometimes developed, notably in siliceous metasediments. The $2 schistosity is commonly defined by alignment of micas and tabular flattened quartz and plagioclase grains. There is also generally some incipient segregation of feldspathic or quartzo-feldspathic layers in amphibolites or pelitic to semi-pelitic metasediments. Important information on the nature of the D2 deformation is provided by the relationship of earlier garnet porphyroblasts to the $2 fabric (Fig. 4). These porphyroblasts include a fabric of quartz grains that is sometimes straight and at a high angle to $2, but is often microfolded.

SILURIAN SEOiMENTS UNCONFORMITY" semi-petites ,~nd siliceous granufites BENLEVY with mlno~ petite ond pebbly beds FORMATION SLIDE Mg~l~ ,3,,, g.,.hi.,: ...b,. ~.A,*.,'.C gr,~iti, sch,.~, MEMBER | gronulites SEMI-PELITIC7C1 C0RPlAM~NA...... Epldote-banded schists MEM~R FORMATION Pebbly Unit (s~ J -- RENVYLE-BOFIN SLIDE ? u~it of goortzlteS tO semi-pelltes "~ relation to rest of successiml ondear l .~. ~.d. in .i.. r ~"KYLE¥9.e~ • | rIJtINIA I I~JFI pelitn thinly hondlKI ~Ite with *"% interbedded rnetosedimenf$ morble "UPPERMARBLE" omphibolite-- siliceous gronulitn gritty guartzite LAKES / FORMATION

rnorhle "LOWERM.4RBLE I J omphibollte petit,s to sillc,ous gronu,t, STREAMSTOWN FORMATION

moss~ve gtmrtzite BENNABEOLA QUARTZITE F.M. FIG, 2. tililte ond matrix CLEGGAN BOULDER BED FM. Stratigraphy of the Cur district. At creom morble CONNEMARA MARBLE FORMATION least the lower part of the succession OUGHTERARD GRANITE belongs to the Argyll (Middle Dal-

Not to sc(zle radian) Group.

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Often the axial planes of the microfolds are parallel to the external $2, and occasionally the small folds appear to tighten up towards the edge of the garnet. Snowball structure has been found in two samples only. The microfolds appear to represent an early buckling phase in the D2 event, prior to the main develop- ment of $2. The $2 fabric is invariably flattened about the garnets, hence the pronounced flattening characteristic of D2 occurred after garnet growth. No such flattening is present about post-D2, pre-D 3 porphyroblasts. The D2 deformation therefore appears to have consisted of an initial buckling event followed by co- axial homogeneous flattening. This interpretation is supported by the shape of pebbles around rare D2 folds in pebble beds. Pebbles show uniform strong

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FIo. 3. Cross sections of the Cur area (see Fig. 2 for key). Sectior~ I h for the western part of the area shown on Fig. 1 ; section 2 is from the southern Corcogemore Moun- tains to the Failmore Valley, south central part of Fig. I.

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FIG. 4. MP x garnet with Si oblique to the external $2 segregation layering. $2 is broadly flattened about the garnet. Scale bar I mm.

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flattening parallel to $2 throughout the folds, and, since the development of a fold implies a heterogeneous strain, relatively intense subsequent homogeneous strain must have occurred to eliminate initial variation in pebble shapes around the folds. A single major D2 fold, the Lissoughter Antiform, has been recognized in the Cur region and is an extension of a fold mapped to the west by Edmunds & Thomas (1966) and Badley (1972). This structure exerts a fundamental control on facing directions; thus to the north of the trace of the Lissoughter Antiform beds always young northwards along the axial surfaces of D 3 folds. This is true within all three tectonic units and indicates the probable absence of any other major pre-D 3 closures. D 3 Deformation. Moderately tight D 3 folds are the most abundant mesoscopic structures in north Connemara. They strike approximately east-west and plunge gently to the west in the Cur area. Elsewhere they commonly have a gentle easterly plunge. D 3 folds in most rock types are apparently of similar style, but in fact are Type III/Type IC hybrids in the terminology of Ramsay (i967) , and probably result from homogeneous deformation of flexural folds (Huddleston 1973). Pure similar folds occur in some and massive quartzites, and rarely there is textural evidence of simple shear along fabric planes in pelites. In most rocks an S 3 schistosity is developed, and this may largely obliterate $2 in pelites; in more siliceous metasediments S 3 is less finely penetrative than $2. In the sillimanite zone ellipsoidal fibrolite Faserkiesel occur oriented in S 3. Pebbles that were flattened during D2 are crenulated in the hinges of D 3 folds, indicating some shortening perpendicular to $3, but there is no strong flattening of S 3 about pre-D 3 porphyroblasts comparable to that developed during D2. The dominant fabric element of D 3 is a lineation parallel to the fold axes, and this may be defined by orientation of quartz and amphibole grains, elongation of Faserkiesel and pebbles or by rodding. In addition to the folds, a number of slides developed during D 3. These range from structures visible in outcrop to major regional discontinuities. Small pods of serpentinized ultramafic material occur along the traces of large D 3 slides. The tectonic breaks separating the Cornamona Formation and the Benlevy Formation from the rest of the succession (Fig. 2) are D 3 slides and appear to be continuous throughout north Connemara, but in general D 3 slides are most important in the east. In the western part of the Cur region there is a continuous sequence of complex D 3 folds (Fig. 3, section I) ; however, in poorly exposed ground further east certain parts of the sequence appear to be omitted and this has been tentatively attributed to the sliding out of the limbs of some of the folds seen to the west (Fig. 3, section 2). To the south and east of the Cur region Bradshaw et al. (I969) have mapped contacts between Lakes Marble Formation rocks and Bennabeola Quartzite, which in part probably result from D 3 sliding. D 4 Deformation. D 3 structures are refolded by a series of broad open asymmetric D 4 folds, whose axial planes dip steeply south at between 6o ° and 8o °. The northern limbs of the D 4 antiforms dip more steeply than the southern limbs and progressively higher D 3 structural elements are brought down to the north

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(Fig. 3). In view of this, and since mesoscopic D 4 structures on the gently dipping limbs are apparently symmetrical, it is believed that the present form of the D 4 folds is the result of symmetrical folding (with reference to a hypothetical horizontal datum) of originally north-dipping D 3 structures.

STRUCTURE OF THE CUR REGION IN RELATION TO THE REST OF CONNEMARA With the aid of maps from other parts of Connemara the structural elements recognized in the Cur area have been extended to the east and west, and an attempt (Fig. 5) has been made to relate these structures to those found in south Connemara by Leake (I97oa , b). The cross section for the western part of the Cur area (Fig. 3) matches closely the structure found in the Maumturk mountains and Inagh Valley by Badley (1972) apart from some differences in the ages to which folds are ascribed. Badley located a major fold, the Tooreenacoona Syncline, that lies in the same part of the structural succession as the Maumeen Syncline. The Tooreenacoona Syncline is refolded over the D 4 Connemara Antiform and since structures plunge to the east in the Inagh Valley this yields an axial trace that is the mirror image of the trace of the Maumeen Syncline (Fig. I), the intervening ground constituting a plunge depression. The two structures are therefore believed to constitute a single fold. Badley described the Tooreenacoona Syncline as a D2 fold on the basis of fabric relationships in the hinge region at Finnisglin; however, there is no evidence that it is refolded in D3, and lesser D B folds in fact appear to be parasitic to the structure. Fabric relationships in the hinge elsewhere appear to indicate a D 3 age (Tanner, pers. comm. 1973), and since the better exposed Maumeen Syncline is undoubtedly a D 3 fold the Tooreenacoona Syncline must also be D 3. In this case the earlier Lissoughter Antiform might be a D2 or a D I structure, but there is no evidence to suggest that it is earlier than D2. Cruse & Leake (1968) described a break in the stratigraphy to the north of that is directly analogous to that found in the Bealanabrack Valley in this study. They ascribed the break to a major D 3 slide, the Renvyle- Bofin Slide, and the slide in the Bealanabrack Valley is believed to be a continu- ation of this structure.

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The easternmost part of the Connemara outcrop has been mapped by Bradshaw el a/. (I 969). Here too the D 3 structures plunge to the east resulting in the exposure of higher parts of the structure that have been removed from the Cur area. Despite poor exposure and extensive intrusion of Oughterard Granite it is apparent from the outcrop pattern of Lakes Marble Formation lithologies that the steeply dipping D 3 folds seen at Cur, notably the Cur Synform, are refolded over the Connemara Antiform; they appear to be nappes rooting in the complex belt of steeply dipping Streamstown and Lakes Marble Formation rocks that extends from Oughterard to . The structure of the 'steep belt' appears to involve numerous slides, producing frequent repetition of the succession, and rather few folds (Leake & Senior, pers. comm.). To the south of the 'steep belt' the structure has been described by Leake (i97oa , b) who reported that the D 3 folds at Cashel (Fig. i) face upwards. D 3 folds immediately to the south of the Lissoughter Antiform face downwards and since there is no refolding of the D 3 structures between the Lissoughter Antiform in the southern Corcogemores and Cashel, the two regions must be separated by a pre-D 3 fold closure. This might be the refolded Lissoughter Antiform (as indicated in Fig. 5) but could be another complementary D2 closure. Such a structure has never been reported in the field and could be largely sheared out amongst the complex slides of the 'steep belt'.

2. Metamorphism of the Connemara pelitic schists There is a continuous increase in metamorphic grade southward across the strike in Connemara, apart from a small region south of Kilbride Lough in the far north-east (Leake I97ob), and the grade of metamorphism also increases eastward along the trace of an~" D 3 structure in north Connemara. Thus the trace of the isograds is oblique to the D 3 fold axial surfaces (Fig. 6). The sequence of assemblages developed in pelites during progressive metamorphism is apparently uniform throughout Connemara, indicating similar P-T paths of metamorphism. However, staurolite schists from the Cornamona region in the east are usually finer grained than those from the Cur region, and still coarser grained pelites occur at the same grade further west. This may be the result of variation in the rates of heating during staurolite-grade metamorphism. All the Connemara Schists have probably been metamorphosed to at least the amphibolite facies, apart from a small region of chloritoid schists described by Cruse & Leake (1968) from Renvyle (Fig. I). However staurolite-grade metamorphism has not been proved for the upper part of the Cornamona Formation, nor for the Benlevy Formation, in the Cur area, although staurolite has been found in these units a short way to the east by Leake (pers. comm.). In part of the Cornamona Formation north of the Bealanabrack River there has been extensive greenschist-facies retrogression.

METAMORPHIC ZONES AND THEIR CHARACTERISTIC ASSEMBLAGES Steurolite zorn. This zone is defined here as the region in which staurolite, muscovite and quartz coexist in pelites in the absence of sillimanite. Biotite,

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quartz, plagioclase, apatite and tourmaline are ubiquitous in staurolite-bearing rocks. Muscovite is normally present where staurolite occurs, but is absent from some Fe-rich rocks. Garnet is almost always present but occasionally chlorite occurs instead, probably as a function of the Fe:Mg ratio (cf. Carmichael i97o , fig. 6). The major opaque phases are ilmenite, rutile and pyrite, all three often coexisting, and small amounts of chalcopyrite are always present. (Opaque mineral identifications have been confirmed by microprobe analyses at the

~~ CHLORITE-MUSCOVITE- ALBITE SCHISTS

STAUROLITE ZONE

STAUROLITE-SILLIMANITE TRANSITION ZONE

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FIo. 6. Metamorphic zones in the Cur area. Zones are based on assemblages in pelitic rocks, except for the sillimardte zone which is also defined by the assemblage ~;~-feldspar + dllimanite in other non-calcareous metasediments. Structural ir~or- ;nation simplified from Fig. I.

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University of Washington.) Minute grains of pentlandite, contained in (and apparently exsolved from) pyrite have been found in some samples. Kyanite has not been found in the Cur region but it is sometimes present in the staurolite zone (as defined above) in western Connemara (Cobbing I969, C. C. Ferguson, pers. comm.) and near Cornamona in the east (Leake, pers. comm.). Andalusite and cordierite are locally developed in the staurolite and staurolite-sillimanite transition zones; their distribution and occurrence will be described later. Small amounts of graphite are often present in staurotite zone petites, but graphite becomes rare, or possibly absent, at higher grades. Staurolite-sillimanite transition zone. Sillimanite first appears in muscovite-rich staurotite schists as fibrolite growing on biotite, often in the immediate vicinity of garnets. (Sillimanite fibres may appear in quartz-plagioclase veins at lower grades, but, since such sillimanite is not produced by the same reactions that occur in the host petite, these occurrences are not related to the sillimanite isograd in petites.) In the lower grade portion of the transition zone sillimanite is simply an additional phase in staurolite-zone assemblages and typical petites have staurolite + sillimanite + garnet + muscovite + biotite + quartz + plagioclase + accessories. At higher grades muscovite is often absent though siUimanite, staurolite, garnet and quartz still stably coexist. Less commonly, garnet has completely reacted out in the upper transition zone, being pseudomorphed by fibrolite with manganiferous ilmenite and vestiges of biotite. In this case some muscovite may remain. Thus in general, staurotite, sillimanite, muscovite and garnet only coexist in the lower grade part of the transition zone but there are intercalations with apparently higher grade muscovite-free assemblages pre- venting a rigid subdivision of the zone. These intercalations may result from variations in the H~O content of the fluid phase, or in X~g of the rock, but a further discussion is beyond the scope of this paper. There is a distinct region within the transition zone, lying in the hinge of the Cur Synform at Cur and Teernakill, in which petites display a different pattern of metamorphism from that just described. Sillimanite and muscovite are abundant and staurolite is absent except as rare armoured relicts. Muscovite is produced from staurolite breakdown instead of reacting out with the staurolite. These petites are contiguous with 'normal' staurolite and staurolite-sillimanite schists to the west but are of different composition. Petites with similar compositions and assemblages have been described by Badley (i97~) from the hinge of the Tooreenacoona Syncline at Finnisgtin, again in the transition zone, but are otherwise unknown from Connemara. They are believed to result from syntectonic in the hinge of major D 3 structures where they intersect particular isograds (Yardley I975). SiUimanite zone. Petite assemblages in the siltimanite zone are more uniform than at lower grades and have fewer phases. The most common assemblage is sillimanite + garnet + biotite + quartz -+-plagioclase + accessory minerals as at lower grades. Pyrite and rutile are the principal opaque minerals, with ilmenite often virtually absent. Minor late muscovite may also be present and corroded staurolite relicts occur in plagioclase porphyroblasts. K-feldspar is only rarely

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present in the Fe-rich pelites of the Kylemore Formation but K-feldspar + sillimanite, usually also with muscovite, is widespread in Streamstown Formation rocks in this zone and this assemblage is also sometimes developed in the Benna- beola Quartzite. Andalusite and cordierite in the zonal sequence. Andalusite and cordierite are locally developed in the higher grade part of the staurotite zone and in the staurolite- sillimanite transition zone. Both phases replace and include staurolite, but they have never been found to occur together. A late garnet growth, also including staurotite relicts, is also present where cordierite occurs. Cordierite is rare, and appears to be restricted to muscovite-free petites with moderately high Niggti mg values in the range o.45 to o.5o. Andalusite is more common though very patchily developed. It is found both in petite assemblages and in pegmatites. The pegmatites consist of inclusion-free andalusite with quartz and muscovite, and appear to have developed in a later event distinct from the growth of anda- lusite in the host. In the host rocks andalusite occurs as large porphyroblasts, commonly an order of magnitude larger than other porphyroblasts, that have clearly grown by constant volume replacement of matrix minerals, particularly quartz and plagioclase, while tending to preserve biotite, opaques, garnet and staurolite as corroded inclusions with their original textural relationships (Fig. 7A). There is extensive segregation of Al~SiOs in the growth of these porphyroblasts, changing the local composition of the rock. Although andalusite is sometimes incipiently replaced by fibrolite, no pseudomorphs after andalusite are found in the sillimanite zone, neither are there any corroded relicts of andalusite por- phyroblasts nor any traces of the chemical segregation involved in their growth. It is therefore concluded that andalusite was never developed in the schists of the sillimanite zone in the Cur region. Post-staurolite metamorphism north of the Bealanabrack River. There is a region of chlorite-muscovite-albite-magnetite schists with associated late chlorite- albite-quartz veins north of the Bealanabrack River (Fig. 6). Relict staurolite is occasionally preserved in feldspar-free samples. Garnet is sometimes present and there is at least one band in which large (2 cm) cordierite poikiloblasts occur, with chlorite, muscovite, albite and magnetite. These cordierites are quite distinct from those found in staurotite-bearing rocks, both in texture and in their mineral associations, and are invariably completely pinitized. Coarse prismatic andalusite is occasionally present, growing in staurolite-rich zones found at the margins to some quartz veins. These rocks appear to have a very complex history. For example similar cordierites are found to the east, between Maam and Cornamona, in muscovite schists that have not been so extensively retrogressed and contain a green biotite and a more calcic plagioclase (oligoclase). Thus the growth of the cordierite is believed to be due to an earlier event distinct from the more widespread retrogression to chlorite-muscovite-albite schists. The retro- gression involves metasomatic exchange of Na for Ca, and also oxidation, since the pelitic schists associated with the distinctive epidote-banded schists (Figs. 1,2) in the Cur region are enriched in Na, depleted in Ca and more highly oxidized than the staurotite zone petites that occur at the same stratigraphic position a

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few miles to the east of Cornamona (Yardley I974). Cordierite growth in these rocks also appears to result from oxidation. 3. Sequence of metamorphism and deformation The sequence of mineral growth episodes has been worked out, chiefly using reaction relationships and the standard criteria of Rast (z 958) and Zwart (196o), and is summarized in Fig. 8. The details of the criteria used in dating each mineral are discussed by Yardley (z974). The age of staurolite growth is critical to the rest of the interpretation because a number of other minerals grow by replacing staurolite. The ages of formation of the aluminium silicate polymorphs

A, ~'~ : ~9~,~ B

.~, ~'-3~.~.,,. ~. ~ -'-~'~...

Fie. 7. Textures in pelitic rocks, scale bars I ram. A. Texture within a large andalusite porphyroblast. Large twinned MP2 staurolite (high relief) with corroded margins transgresses an $2 fabric of biotite (ruled) which has not been deformed around the staurolite. MP2 biotite is also present. Neither quartz nor plagioclase occur, even as inclusions; the matrix is part of a single andalusite crystal. Sample I84, Teernakill. B. MP2 staurolites (high relief) in pelite with an $3 fabric of moderately-aligned biotite (ruled) and quartz-plagioclase layers (white). Mats of fibrolite (represen- ted by cross hatching) tend to be aligned in S 3. Sample 175, south limb of the Cur synform, west of Cur. C. Detail in the hinge of a small D3 fold, sillmanite zone. Mats of fibrolite with minor biotite are aligned parallel to the fold axial surface, (shown with exag- gerated high relief), and cut across the $2 segregation layering. Present in the rock are biotite (ruled), quartz and plagioclase (white), and fibrolite (cross hatched). Sample 223, Maumeen. D. Typical cloudy MPI/MS2 garnet with numerous inclusions overgrown by a rim of clear, pink garnet (MP2). The composite garnet is enclosed in a mat of fibrolite with biotite (cross hatched). Biotite (ruled), quartz and plagioclase (white) are also present. Sample 284, eastern Failmore Valley.

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are also discussed below because they are critical to an understanding of the changing conditions of metamorphism with time. The terminology used is that of Sturt & Harris (z96I); MP2 denotes static growth after D2, MS2 denotes syntectonic growth during D2. Pronounced flattening late in D2 has resulted in deformation of the $2 schistosity about MPi/early MS2 porphyroblasts (Fig. 4). The absence of such flattening about staurolites and many other porphyroblasts is taken to indicate that they grew after D2. The MP2 age of staurolite is confirmed by relationships between S~ and S,; the external fabric is $2 in many instances and St is contiguous with S,. Where there has been extensive transposition of $2 to form an S 3 fabric, the Si in staurofite is oblique to the external fabric (Fig. 7B). Very rarely there is some marginal MS3 staurolite growth. The structural dating of fibrolite is often problematical. Fibrolite Faserkiesel are flattened in the axial planes of D3 folds and elongated in L3, but this may be due to deformation of an initially spherical mass offibrolite (Dalziel & Brown 1965) or to syntectonic growth. There is good evidence for both MP2 and MS3 silli- manite in the present area, and the Faserkiesel textures probably reflect the last, MS3, growth. Fibrolite in the siUimanite zone schists of the Failmore Valley appears to have grown in MP2 because small staurolite relicts occur in large plagioclase porphyroblasts that are MP2 in age with MS 3 margins (on the basis of included biotite fabrics). Assuming that sillimanite growth was coeval with breakdown of staurolite, much of it must have occurred prior to D 3. In contrast fibrolite from the unusual rocks in the hinge of the Cur Synform appears to be largely MS 3. Large andalusite porphyroblasts from pelites in the fold hinge include relict staurolite and are incipiently replaced by sillimanite. No staurolite remains in the sillimanite and muscovite-rich matrix outside the andalusites and the growth of andalusite must therefore have predated that of siIlimanite.

MS1 MP1 MS2 MP2 MS3... MP3 MS4 R.AC.4OCLASE ~---- ~ ....

K-FELDSPAR BIOTITE qq~l)" -- - ....

MUSCOVITE -.Am, • .mm.~- • CHLORITE •,,ql~ o v GARNET .-? STAUROLITE SILLIMANITE ~,qm mmb-,_- ANDALUSITE ,4~- v" KYANIT~ CORDIERITE ANTHOPHYLLITE

Not loon# in Cer oreo V Occurrencesin veinsonly N North of the Beelanabrack River only

Fin. 8. Summary of structural ages of mineral growth or recrystallization across the Cur area for pelitic rocks.

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However, occasional open crenulations in the biotite fabric included in the anda- lusites can be related to D 3 crenulations in the matrix and indicate that the andalusite is MP2 to early MS 3 in age. Hence the sillimanite growth is MS3. Commonly long fine needles of sillimanite grow out along the S 3 fabric (Fig. 7 B) and even appear to cut across $2 micas (Fig. 7C). It is difficult to imagine such textures developing from rotation of MP2 fibrolite, so MS 3 fibrolite growth or recrystaUization was probably widespread. Kyanite in thin sections shown to the writer by Leake and Ferguson exhibits the same textural relationships as staurolite with which it appears to coexist stably (cf. Atherton & Brotherton I972 ). Kyanite is therefore believed to have grown at about the same time as staurolite, and certainly prior to the main breakdown of staurolite and growth of sillimanite and andalusite. It is apparent from Fig. 8 that there is an increase in metamorphic activity in the higher grade zones prior to D 3. 4- Relationships between metamorphic zone boundaries and structures The metamorphic zone boundaries (Fig. 6), apart from that defining the occurrence of chlorite-albite-muscovite schists, are isograds recording the MP2 to early MS 3 breakdown of staurolite and growth of sillimanite. Thus they relate to a thermal structure that developed prior to D 3 and was apparently not much displaced relative to the beds during the initial stages of D 3. However, whereas bedding is intricately folded around the D 3 folds, the isograds appear relatively undeformed (Figs. 6, 9). From this it is inferred that the D 3 fold axial surfaces developed at only a small angle to the isograd surfaces, whereas both these surfaces were initially at a high angle to bedding. Since the isograds are known to trend east-west across Connemara, somewhat oblique to the D 3 structures, the initial intersection of the isograd surfaces with bedding would not have been parallel to the D3 fold axes. The isograd pattern indicates that the relict thermal structure between Cur and Maumeen is inverted, as was first suggested by Badley (i972). Since the bedding here is also inverted due to the effects of D 3 and D 4 folding, this is in accordance with the notion that the thermal structure was imposed prior to D 3. Within the present area the southern limit to the occurrence of andalusite porphyroblasts is not known with any precision; however, similar andalusite occurs in the same part of the zonal sequence throughout most of north Connemara and it is inferred that the southern limit of andalusite is approximately parallel to the other isograds.

FAILMORE RIVER ..... VL, e'^e,UL~ FaG 9. o =~mFo~ ~un o/~, rvnnq~c, ~k| Generalized cross section across the '0- KNOCKRI~HEEMORE ,~ ! ~O~L ... Cur district showing probable rela- d'..'~_~"_~,,,o,.~'~~ S'YNFORM,I \ I/ :~ \ k/~-"~_A ~/~eneronzeo ]bedding tions between isograds, structures and "~'-,,~':2~¢ .~ ~, ~.~"surfoce bedding cf. Figs. 3, 6. The isograds -. . are at the margins of the staurolite- S'~,._[ "'~'i'" ~ sillimanite transition zone and are Zone iransmone ..... Zone shown by heavy lines.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/521/4885243/gsjgs.132.5.0521.pdf by guest on 29 September 2021 Dalradian deformation & metamorphism, Connemara 535 5. Conditions of metamorphism A fuller account of the metamorphic reactions and processes will be published elsewhere. Here, gross changes in mineral abundances are noted and compared to experimentally determined equilibria, but no attempt will be made to describe the detailed geometry of the phase diagrams or to consider the details of the chemo- graphic relationships between phases. MPI to early MP2 metamorphism. The oldest assemblages identified are those of the staurolite zone from which andalusite and cordierite are absent. Such rocks have not undergone any further reactions during the late MP2/MS 3 meta- morphism, and are taken to represent an early stage in the development of the assemblages now seen at higher grades. There are two lines of evidence to indicate that the staurolite grade metamorphism occurred at moderately high pressures (Barrovian facies series). Firstly the occurrence of kyanite indicates higher pressure conditions than during subsequent metamorphism when andalusite developed. The rarity of kyanite appears to result from the unfavourable com- position of most of the Connemara pelites. Atherton & Brotherton (1972) have shown that kyanite in the Barrovian kyanite zone is restricted to pelites having an MgO/(MgO + FeO) ratio (M/FM ratio) greater than o-54. In contrast 37 pelites from Cur and Cornamona gave a mean M/FM ratio ofo.25 (Yardley 1974). The only pelites in the Cur region that have been found to have an M/FM ratio greater than o.54 are the retrogressed rocks from north of the Bealanabrack River that have suffered late oxidation. Since known kyanite occurrences are widely spaced across Connemara it is quite likely that Barrovian kyanite grade conditions were developed over much of the staurolite zone, as defined here, and that unfavourable bulk compositions prevented the more frequent development of kyanite. Secondly there is independent evidence that the kyanite-free staurolite- zone petites were indeed metamorphosed along a Barrovian facies series from the partitioning of Ca between garnet and plagioclase. Kepezhinskas (1973) showed by a statistical treatment of published analyses of coexisting garnet and plagio- clase from staurolite-zone rocks that it was possible to separate pairs from terrains in which kyanite was regionally developed from pairs from lower pressure anda- lusite-bearing terrains, using a plot of CaO in garnet versus wt.% An in plagio- clase. This is because there is a reaction relationship between grossular and anorthite when quartz and an aluminous mineral (such as staurolite or sillimanite) are also present. For the staurolite zone pelites the equilibrium is: grossular + staurolite + quartz = anorthite + pyrope-almandine + H20 (in garnet) (in plagioclase) (in garnet) Unfortunately Kepzhinskas did not consider the effect of temperature on this partitioning (Lambert I959) , but his plot appears to be valid for the relatively narrow temperature range of the staurolite zone. Compositions of coexisting garnet and plagioclase (averaged from analyses of the core and rim of each grain) are plotted on a Kepzhinskas diagram in Fig. IO, and clearly fall in the high pressure field. The samples plotted all contain staurolite and quartz in addition to garnet and plagioclase.

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Late MP2 to MS3 metamorphism. The principal reactions that occurred at this time involve breakdown of staurolite with growth of sillimanite or, less commonly, andalusite or cordierite. To a first approximation the initial reactions that form silliminate are: garnet + muscovite ~ sillimanite + biotite + quartz staurolite + muscovite + quartz ~ sillimanite + biotite + H~O 2 The new biotite appears to have rapidly homogenized with preexisting biotite. In most Connemara pelites these reactions continue until all the muscovite has been consumed, further breakdown of staurolite then occurs through the reaction: staurolite + quartz --~ garnet + sillimanite + H,O 3 Chinner (z 965) and Kwak (z 974) have argued that this reaction is probably rare or unknown in nature, but there is good textural evidence in the present instance: sillimanite-zone pelites (containing neither muscovite nor K-feldspar) have composite garnets consisting of an irregular, corroded and inclusion-riddled core, often demonstrably MPz to MS~ in age, overgrown by a clear inclusion-free rim of pink garnet (Fig. 7D). These rims are not found where appreciable staurolite remains. Reactions x- 3 are divariant, since the compositions of the participating phases may vary. This is unlikely to cause very large variations in the equilibrium conditions of reactions 2 and 3 since they liberate water and hence have relatively large entropies. Furthermore the phases do not in fact show large variations in composition. Reaction x will however be very sensitive to the composition of the garnet, and pelites with virtually uncorroded garnets are found immediately adjacent to others in which the garnet is very largely destroyed. Reaction 3 appears to have occurred at about the same grade as the reaction: muscovite + quartz ~ K-feldspar + sillimanite + H~O 4 seen chiefly in Streamstown Group and Bennabeola Quartzite metasediments. In the field there are broad zones in which both the reactants and the products of these reactions coexist. In addition to the effects of variation in mineral compositions, the equilibrium assemblages of the dehydration reactions may buffer PrT,o over a range of temperatures to give rise to the broad transition zones (Evans & Guidotti I966 , Guidotti 1974). This may be achieved through variations in the fluid composition.

40, / Field for andalusite~ / bearing terrains / / + / + E 20 / ._ /+ Field for kyanite,',- bearing terrains 10, / Fio. I o. / ( Kepezhlnskas diagram showing averaged composi- tions of coexisting garnet and plagioclase from the Wt. % CoO in garnet stauro]ite zone.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/521/4885243/gsjgs.132.5.0521.pdf by guest on 29 September 2021 Dalradian deformation & metamorphism, Gonnemara 537 Andalusite-forming reactions. The growth of large andalusite porphyroblasts involves extensive local diffusion metasomatism on a scale of several metres in places. However, there is no evidence that the andalusite-bearing layers have undergone wholesale addition of A1 from an outside source. If this is true then the chemical potential gradients would have been controlled internally within the layer, and the formation of andalusite can be considered as resulting from simple thermal instability of the initial assemblage. Hence the growth of andalusite in the host appears to result from a reaction analogous to reaction 2: staurolite + muscovite + quartz ~ andalusite + biotite + H,O 5 The curve for this reaction on a P-T diagram (Fig. z z) is simply the extension of the curve for reaction 2 across the andalusite-sillimanite boundary, though with a slightly different slope. The curves intersect at point I that is invariant if the compositions of the phases are specified.

EVALUATION OF THE CONDITIONS OF METAMORPHISM Curves for the reactions discussed above, together with possible boundaries in the A12SiO5 system, are shown in Fig. I I. The experimental curves for the reactions involving ferromagnesian minerals will not apply exactly to natural assemblages with phases of different compositions; however, variations in the composition of the fluid phase are potentially a greater source of error in the location of the curves. This has been studied experimentally for reaction 4 by Kerrick (I972). Fluid inclusions in syn-metamorphic quartz veins near Cur clearly contain more than one phase, but because the exact fluid composition is unknown it has been assumed in constructing Figure I I that P~,o -- Pnul~ -- Ptotaa. The effect of a lower Pa,o will be to shift the dehydration curves to lower temperatures at any given Pf~uia. Ghent (i975) has calculated that Pmo may be at least o.9 Ptot~ for a graphite staurolite-kyanite schist from British Columbia; however, this estimate is dependent on the value assumed for Ptotaa which was not accurately known. For these reasons, and because of the considerable uncertainty in the andalusite-sillimanite boundary, it is not possible to specify a location for

O kb ~a ,,,Lowerlimite KY'/ilI.

FIG. I I. P-T diagram illustrating metamorphism of N. Connemara pelites. Kyanite stability field from Possible early Is /. e/ //_ Holdaway (I97i) and Richardson et al. (x969) ; andalusite-sillimanite boundary compiled from Weill (I966), Holdaway (x97x) and Althaus (x 967). Lower limit of staurolite is from Richard- son (I968) and Hoschek (I969). Reactions 5 and 2 are from Hoschek (i969) , reaction 4 is from • ~/ And~ Evans (i965) and reaction 3 is inferred from field relationships, but converges with the experimental curve of Richardson (I968) above 400' 500 " 600 ' 700 ' BOO 5 kb. I is an invariant point when "~3/-stsuroltte~'M g ilill..i--a T °C are specified.

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the invariant point I, but when considering a layer of fairly uniform composition the location of ! is unlikely to vary by much within a single fold belt. 6. Discussion It has been shown earlier that andalusite was never developed in the southern part of the area, and it is thus inferred that the rocks in the sillimanite zone south of Cur were metamorphosed through the temperature for staurolite to break down at a pressure higher than that of point ! (Fig. r I, path A-A'). Andalusite-bearing pelites were metamorphosed at lower pressures (path B-B' in Fig. 1 I). Andalusite in staurolite-sillimanite transiton zone rocks is incipiently replaced by sillimanite indicating increasing temperature or pressure during MS3 staurolite breakdown. Since the breakdown of staurolite in the rocks of the sillimanite zone occurred at higher pressures than in the andalusite-bearing rocks of the transition zone, and began earlier, it is concluded that the breakdown of staurolite during late MP2/MS3 metamorphism resulted at least in part from a steepening of the thermal gradient so that the rocks underwent a component of isobaric temperature change. Thus there is no unique P-T path for the entire succession. According to this model the southern limit of andalusite found in the field is an isobar representing the deepest level at which rocks were metamorphosed through the temperature for the breakdown of staurolite + muscovite + quartz with andalusite as the stable form of AI~SiOs, i.e. the pressure at point L Now since as far as is known this mapped isobar is approximately parallel to the isograds, these must initially have been approximately horizontal. Bedding and $2 foliation are, however, at a high angle to the isograds, and by unfolding from Fig. 9 they are found to have been dipping steeply to the north immediately prior to D3, while the D3 folds must initially have had fold axial surfaces dipping gently to the south. It was argued from the symmetry of the D 4 folds that the D 3 fold axial surfaces dipped north immediately prior to D 4. Hence there would appear to have been a northward rotation of the D 3 folds between D3 and D 4. It is also apparent from Fig. 11 that the pressure at point I is less than that required for the formation of kyanite + staurolite; this is true whatever values for the andalusite-sillimanite boundary are taken except those of Richardson et al., (I 969), even after reasonable allowance has been made for the effects of PH,o < Proton. Furthermore the kyanite at Cornamona occurs about 2 km to the north of the southern limit of andalusite, and the rocks in which it occurs would therefore have been at a lower pressure than I during the MP2/MS 3 event. Hence it is probable that there was actual uplift of the Connemara Schists, with erosion of part of the overburden, between the early MP2 kyanite-staurolite metamorphism, and the growth of andalusite late in MP2. The uplift may have been as much as 5 km, and coincided with the rise in temperature in those rocks that now contain late MP2/MS 3 assemblages. These events may be related to the emplacement of magmatic gneisses in south Connemara. Rocks high in the pile would have been cooled at this time, thereby preserving earlier assemblages. The distinction between an early Barrovian metamorphism and a later lower pressure event is not taken to imply any cooling or appreciable time interval between them. Rather meta- morphism is believed to have been continuous.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/521/4885243/gsjgs.132.5.0521.pdf by guest on 29 September 2021 Dalradian deformation & metamorphism, Connemara 539 7. Summary of the evolution of the Connemara rocks It is now possible to review the history of metamorphism and deformation in Connemara in the light of the present study and previous work (e. g. Moorbath et al. 1968, Leake 1970a, Senior 1973). I. Possible D I deformation, followed by Barrovian metamorphism. 2. Development of D2 folds by buckling, followed by a strong coaxial homogeneous flattening. Folds were probably initially upright. The em- placement of the basic and ultrabasic bodies in south Connemara may have occurred early in D2 or between the 2 phases of D2. 3. Continued static Barrovian metamorphism with growth of staurolite and occasionally kyanite. 4. Onset of regional uplift and increased heat flow in the north Connemara rocks, leading to staurolite breakdown. Emplacement of progressively more acid magmatic gneisses in the south. 5. Development of northward-facing recumbent D 3 nappes. Successive thrust-nappes were piled over a basal fold-nappe, while continued uplift centred in south Connemara or in the 'steep belt' resulted in the faulting out of the roots of earlier nappes during progressive deformation. High grade metamorphism at moderately low pressures in the lower part of the pile. 6. Continued uplift in the south resulted in northward rotation of D 3 folds. ? Emplacement of the Oughterard Granite at structurally high levels. 7. Open D 4 folding after the rocks had largely cooled. Also after cooling the high grade rocks of south Connemara appear to have been thrust up and to the south over low grade schists now exposed in the Delaney Dome in the south west (Leake i97ob ). It is not clear how this relates in time to D4. Subsequent differential vertical movements appear to have been responsible for the doming of the thrust plane (Fig. 5). 8. Emplacement of post-tectonic granites. Bradshaw et al. (I969) believed that the Oughterard Granite was post-D 4. The evidence for this, that the granite outcrops in the hinge region of the Connemara Antiform and therefore 'cuts' the D 4 structure, is clearly inadmissible. No unequivocal relations have been observed between the Oughterard Granite and any D 4 folds, but in view of the chemical similarities found by Senior (I 973) between the Oughterard Granite and the south Connemara migmatites the writer prefers to consider the Oughterard Granite older than has hitherto been supposed.

8. Relationships between lower Palaeozoic events in Connemara and in adjoining regions The movement picture proposed above for the Connemara Schists has considerable bearing on their relationships to the lower Palaeozoic rocks to the north and south. The peak of metamorphism in Connemara was attained in late Cambrian or

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early Ordovician times. The most reliable date for the main metamorphism is probably the zircon U-Pb age of 5 IO ± io Ma obtained by Pidgeon (I 969) on the Cashel body. Other old ages from Connemara are less precise (Leggo et al., 1966 , 1969). Low grade Ordovician and Silurian rocks from the Mayo Trough to the north are in direct contact with high grade Connemara Schists, and for this reason Wager & Andrew (193o) supposed that the metamorphism of the Connemara Schists was entirely pre-Ordovician. This view has been supported by Dewey (I 961) and accepted in many subsequent studies. However, although Silurian rocks clearly overlap on to Dalradian rocks (Fig. i), Ordovician and Dalradian strata are invariably separated by large normal faults on which the movement may amount to many thousands of feet (Dewey et al. 197o ). Thus there is no direct stratigraphic evidence for a pre-Ordovician age for the meta- morphism in Connemara. Phillips (1973) has suggested that at Clew Bay there may be a continuous succession from Dalradian to Ordovician. Dewey et al. (197o) presented evidence that indicated that during Ordovician times the Connemara Schists underwent uplift and erosion, contributing sediment to the Mayo Trough to the north; the subsidence of the Mayo Trough was complementary to the uplift of the Connemara Cordillera. The present study suggests that the uplift of the Connemara Cordillera began even before the peak of metamorphism was attained, and it therefore seems reasonable to suppose that complementary subsidence and deposition was also occurring in the Mayo Trough at this time. No base is seen to the Ordovician succession in south Mayo; however, the oldest rocks seen are of probable upper Tremadocian age (Skeving- ton 1971, pers. comm. 1975) which is probably the same as the age for the peak of metamorphism in Connemara, within the errors of the radiometric methods. In particular, it is suggested that the change in conditions of metamorphism between D2 and D 3 in Connemara marks the emergence of the Connemara region as an area of uplift. The metamorphic history recorded by Max (1973) from the Dalradian rocks of northwest Mayo is very similar to the proposed history of the Connemara Schists up to the time of the post-D2 uplift, and it is possible that Connemara and Mayo behaved as a single unit to that time. In summary, it is suggested that the initiation of uplift of the Connemara Cordillera, and complementary subsidence of the Mayo Trough, occurred after the D2 event in Connemara, and while metamorphism was proceeding in the Connemara rocks. The highest grade metamorphism was probably related to the emplacement of magmatic gneisses in south Connemara and is believed to have occurred during uplift and while sedimentation was proceeding in Mayo. The faults marking the southern margin of the Mayo Trough make up a major suture zone that was initiated after the D2 deformation in Connemara. It is unreasonable to correlate later events across this zone.

ACKNOWLEDGEMENTS. This work was carried out at the University of Bristol. Electron probe anal- yses for Fig. i o were made at the University of Durham with the assistance of Dr C. H. Emeleus. I am indebted to B. E. Leake, S. C. Matthews, A. Senior, C. C. Ferguson, M. E. Badley and P. W. G. Tanner for discussion, criticism and, unpublished information; to Floyd Bardsley for pre- paring the diagrams, to N.E.R.C. for a Research Studentship, and to the Harkness Fellowship for funds that enabled this paper to be prepared.

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Received 27 October x975; revised typescript received x9 January x976.

BRUCE WILLIAM DAVIDSONYARDLEY, Department of Geological Sciences, University of Washington, Seattle, Washington 98195, U.S.A.

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