Stratigraphy, structure and metamorphism of Dalradian rocks of the Maumturk Mountains, Connemara, Ireland MICHAEL EDWARD BADLEY SUMMARY Appin (lower) and Argyll (middle) Dalradian phism that reached its peak (in the Maum- metasediments and associated igneous rocks turks) between D~ and D s (MPg). The rocks are described and a formal stratigraphy pro- were metamorphosed in the amphibolite facies posed. The deformation history has four main and show an increase in grade from north to phases, D t to D 4. Major "nappe" structures, south across the area. It is suggested that the formed during D 1 and D~, were refolded by prograde MP 2 metamorphism, with coeval two phases of approximately coaxial cross- growth of staurolite and sillimanite, was partly folding developed during D a and D 4. Two of caused by heat emanating from the complex of the major F4 cross-folds, the Connemara Anti- basic, ultrabasic and migmatitic rocks located form and Joyce's Synform, superficially dom- during MP 2 in the upper limb of the F 1 Lis- inate the structure of the area. The main soughter Anticline, structurally above the pres- penetrative fabric in the rocks, an axial planar ent day outcrop in the Maumturks and now schistosity, was developed during D~. The de- occurring mainly in south Connemara. formations were accompanied by metamor- T H ~. F I R ST systematic survey of the area was made by the Geological Survey of Ireland (Kinahan et al. 1878). Starkey (196o) studied the southern half of the area and came to conclusions differing greatly from those of the present study. More recently, Edmunds & Thomas (1966) published an account of the stratig- raphy and structure of the rocks along the southern margin of the area and the western part of the area has also been mapped by R. M. Shackleton. The present study included remapping on a scale of I:iO 56o. The map and cross-sections (Figs. I, 2), illustrate the present interpretation of Maumturk stratigraphy and structure. The sequence of structural and metamorphic events is described, using the method outlined by Sturt & Harris (i 96 i), in terms of MS1, MP1, MS2 and MP2, etc. referring in this example to recrystaUizations that were syntectonic or post-tec- tonic with respect to the first and second deformations (D1 and D,), etc. The various mineral assemblages are characteristic of the amphibolite facies. All metasedimentary formations contain abundant quartz and feldspar segregations. Due to the greatly deformed nature of the rocks, estimates of original sedimentary thickness have not been attempted. I. Stratigraphy and lithology The dark grey pelites and serpentine marbles, stratigraphically overlain by tillites, exposed in the Maumturks and other parts of Connemara, correlate with the Appin (lower) and Argyll (middle) Dalradian successions of Donegal and Scot- land. The lithology and stratigraphy of these beds have been described in detail dl geol. Soc. Lond. vol. x32, x976, pp. 5o9-520, 4 figs. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/509/4885219/gsjgs.132.5.0509.pdf by guest on 26 September 2021 51 o M. E. Badley by Kilburn et al. (I965) and Edmunds & Thomas (i966). However, the stratig- raphy of the overlying highly varied sequence, which includes quartzites, psam- mites, pelites and marbles, has not been presented in the few published accounts that have described parts of this younger sequence (Cobbing I969, Cruse & Leake I968 , Evans & Leake i97o ). This paper presents a formal stratigraphy of the Dalradian exposed in the Maumturk area. The exposed sequence is thought to have representatives of all major formations present in Connemara and so is useful as a framework for the Dalradian stratigraphy of Connemara. Nomencla- ture has been revised to bring the stratigraphy into line with the recent proposals of Harris & Pitcher (I975) in their review of the Dalradian Supergroup. Details of the Maumturk Dalradian stratigraphy and lithology are given in Fig. I. (A) LOWER DALRADIAN Blair Atholl Subgroup. The dark grey banded pelites and serpentine marbles out- cropping in the south of the studied area (together with the overlying tillites) have been described in detail by Edmunds & Thomas (1966) and so are not fur- ther described. With regional correlations firmly established the succession is assigned to the Blair Atholl Subgroup (Harris & Pitcher i975). (B) MIDDLE DALRADIAN IslaySubgroup. The Cleggan Boulder Bed Formation forms the base of the Argyll (middle) Dalradian in the Maumturks and elsewhere in Connemara. Only fragmentary parts of the Boulder Bed sequence can be seen at any one locality, with the outcrop of the formation confined to the core of the F I Lissoughter Anti- cline. In common with other areas calcareous clasts are common in lower parts of the formation. The dllites pass upwards, though a transitional 15 m flaggy psammite into the thickly bedded, white to light grey feldspathic quartzites and subordinate, laterally discontinuous, grits, schists and calcareous beds of the Bennabeola Quartzite Formation. The formation crops out over a substantial part of the area and forms the Maumturk Mountains south of Gowlaunard. Heavy mineral bands are common throughout, defining planar and cross-strati- fied beds. Where not tectonically removed, the top 15 m of the formation becomes graphidc and grades into the basal graphidc psammite of the Streamstown Forma- tion. The reddish brown, thickly bedded psammites, interbedded semipelites and thinly bedded quartzites comprising the Streamstown Formation continue the dominantly clasdc non-calcareous sequence of deposition that was terminated by deposition of carbonates. The base of these carbonates forms a logical upper limit to the Islay Subgroup in the Maumturks. Easdale Subgroup. The change in depositional conditions heralded by the car- bonates, now metamorphosed to the granoblasdc grey calcite marbles of the Sruffaunbaun Member, continued with the influx of turbiditic grits forming the Luggartarriff Member. The grits and associated pebble beds are commonly graded and show marked lateral variations in grain size. Deformation has de- stroyed other primary sedimentary structures. These grits are in turn overlain by a further grey calcite marble, the Muingboy Member. This sequence, probably Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/509/4885219/gsjgs.132.5.0509.pdf by guest on 26 September 2021 FiQ. I OF THE MAUMTURK MOUNTAINS AND INAGH VALLEY CONNEMARA, IRELAND 9•39·w KEY ,,..._,, LITHOLOGICAL BOUNDARY ,-./ INTERPOLATED LITHOLOGICAL BOUNDARY DIP FOLIATION S2 SCHISTOSITY F2 FOLD L2 LINEATION S3 SCHISTOSITY F FOLD 3 L3 LINEATION F 4 SCHISTOSITY F4 FOLD L4 LINEATION FAULT ...,,··"" SLIDE LIMIT SUPERFICIAL DEPOSITS ANTICLINE ANTIFORM SYNCLINE SYN FORM NUMBER ON FOLD TRACE TO INDICATE FOLD PHASE ll HEIGHT IN FEET 1 x~ -il LINE OF SECTION I Km IML£ STRATIGRAPHY I! J ..... -.ouFF GROUP • LOUGH MASK, KILBRIDE ANO GOWLAUN Fms. " SOUTHERN HIGHLANDS? ? GROU~ CR I NAN GLENISKA Mb. SUBGROUP KYLE MORE Fm. MAUMTURK Mb. --? FINNISGLIN Mb. EASOALE ARGYLL SUBGROUP ~~~ LAKES MARBLE MUINGBOY Mb. GROUP Fm. LUGGARTARRIFF Mb. SRUFFAUNBAUN Mb. STREAMSTOWN Fm. ISLAY Fm. SUBGROUP Fm. ~~-= BARNANAURAN Fm. APPIN BLAIR GROUP ATHOLL - - ·1-------- CONNEMARA MARBLE Fm. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/509/4885219/gsjgs.132.5.0509.pdfSUBGROUP ~ by guest on 26 September 2021 =:_~- - CllFDEN Fm. - - --- Loug h lnagh ••• ALPINE-TYPE ULTRABASIC ~ DYKE Fm. Formation Mb. Member LOCATION MAP KEY E5!J Ct'lrtlorltftrO\.IS Q Silur•on a OrdoYJC•On D ConJWn.O '- e Sch1!it' ~ Ccnnemoro Mtgmot.re o• llhon .o I 1962• Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/5/509/4885219/gsjgs.132.5.0509.pdf by guest on 26 September 2021 Dalradian rocks of Maumturks, Connemara 5II produced in a low energy environment during carbonate deposition invaded by turbidites, is similar to the Easdale Subgroup in other areas. Throughout the Maumturks and in other parts of Connemara, marbles of the Muingboy Member are frequently overlain by striped or banded amphibolites. These are particularly well developed at Finnisgtin and Muingboy and else- where are tectonically thinned or removed. The striping varies from a milli- metre up to 2o cm thick. The commonest bands are composed of diopside, epi- dote and plagioclase with accessory sphene and calcite. Less common are semi- pelitic bands with biotite, plagioclase and garnet sometimes with quartz and these have gradational contacts often comprising a zone of biotite-bearing am- phibotite. The contact with petites of the overlying Maumturk Member is also gradational being marked by more frequent phlogopitic biotite and hornblende- bearing pelitic intercalations. The gradational contact with the Maumturk Mem- ber, the stratigraphical continuity and areal extent, all indicate that the banded amphibolites are metamorphosed ash or tuff beds. They are designated the Fin- nisglin Member, a significant time marker horizon in Connemara. The banded amphibolites together with the underlying marbles and grits form a distinctive lithological assemblage, the Lakes Marble Formation. Within this formation, non-banded laterally discontinuous, amphibolites up to 4 ° m thick, are common as concordant bodies and are ~specially closely associated with the marbles in their occurrence. Although also present in the Bennabeola to Kylemore Forma- tions the amphibolites attain by far their greatest abundance within the Lakes Marble Formation, (constituting