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Deformation and Metamorphism of Dalradian Rocks and the Evolution of the Connemara Cordillera

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Deformation and Metamorphism of Dalradian Rocks and the Evolution of the Connemara Cordillera Deformation and metamorphism of Dalradian rocks and the evolution of the Connemara 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 Schists 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 Ireland Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/521/4885243/gsjgs.132.5.0521.pdf by guest on 29 September 2021 522 B. IV. D. Yardley 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 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 5~3 tendency for beds to young northwards within each of the tectonic units. Because of the uncertainties of correlation between tectonic units the Kylemore 'Group' and Cornamona 'Group' of Harris & Pitcher (x975) have been redefined so that the Kylemore 'Group' (renamed Kylemore Formation) does not include both rocks above and below the lower tectonic break. Minor details and problems of the stratigraphy have been discussed in Yardley (i 974). 0 0 tx0 ."44.a o ~: ~:~ ...iii!iiiiiii:: iiiiiii!ii!i!ii!i:i:i:::': ~i~ ~" .':~ ~:.'~..... ............ ~, ".&." .°.. .... .....~°.+:.~...~..+~°~+.+~... o,,~ iii iiiiiii!i!iii I) r~ ...:,-+~.:+:+~.~+....~+.• O "O t~ "d ¢9 o r~4.) 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 MARBLE / 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. 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 525 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 •.::;:i::i:!::!,:~! ....... "" ....... :":~i~ili~! ~!~::~, ...:.:+:..........,.... .... ~ / e~r or I :.~:!: ~: " ~ .::::!:i:i::: :::::::::::::::::::::::::::::::: ~ii:~ O ~e~UN WOOD 0 ~ M.e ./~ YOUNGING SECTION I • ,.::::~,~::~:,~i~::~:i~iii:i~:.:.,,-.7-. ~ .. .~::i~ .~...:.....:.:~..: .................... ~,~ .-.:~:::i::~ ....:.:.;. -'.6~; .......... • .:.:.: ..... ;,:...:.......:...:.... p,. ..... :.:.:.:. ~ ~%'. G~ ""'":'""'"" "i0~ " " +:"'"'"'""" "'" " " ':"" ..... "'""'"'"" "" " e8 " "'"'"''" .... ,.:.... .,:.:.:.'.:.-.:.'....:.:.-.:.....,...-.:.::-.-.-.'.:,;.-..-... ....... ...... ..... ...:. ,.....................,.:,:.,:,.;.::;.:...........:.:....,..............
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