Stratigraphy, Structure and Metamorphism of Dalradian Rocks of the Maumturk Mountains, Connemara, Ireland

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 stratigraphy 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.

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by Kilburn et al. (I965) and Edmunds & Thomas (i966). However, the stratigraphy of the overlying highly varied sequence, which includes quartzites, psammites, 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 further 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 carbonates, 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 destroyed other primary sedimentary structures. These grits are in turn overlain by a further grey calcite marble, the Muingboy Member. This sequence, probably

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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

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llhon .o I 1962•

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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 elsewhere 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 semipelitic 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 c. I8% of the Formation's outcrop area). Their concordant but laterally discontinuous form is suggestive of sills. The observed association of amphibolite with marble may reflect ease of intrusion along marble to grit bed boundaries. The occurrence of abundant sills within the Easdale subgroup in Donegal, Islay and Argyll (Harris & Pitcher I975) may indicate that the subgroup in these areas and Connemara was at an optimum depth of burial for sill injection during this phase of igneous activity. Further evidence of volcanic igneous activity in the Easdale Subgroup comes from the Piflochry district of Perthshire, where the Ben Lawers schist is succeeded by basaltic volcanic rocks which there form the top of the Easdale Subgroup (Harris & Pitcher I975, Sturt 196I). If these volcanic and igneous episodes are correlatable it is possible that the Lake Marble Formation may be equivalent to the Easdale Subgroup with the banded amphibolites of the Finnisglin Member forming the top of the subgroup. Crinan Subgroup. The characteristic alumina and iron-rich petites of the Maum- turk Member mark a change to argillaceous deposition. The petites, in the central Maumturk area, grade up into the Gleniska Member by the entry of pebble beds. The pebble beds range from o. 5 m to 2 m in thickness and are often graded with white quartzite pebbles set in a semipelitic matrix. Gritty quartzites, often graded, are interbedded with the petites. The member shows considerable variation along strike. The pebble beds which predominate in the east, gradually give way west- wards to semipelites and psammites. Towards Lough Nacarrigeen in the west these latter two tithologies make up almost the entire formation, the sequence

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typically consisting of rapid alternations between psamrnites a few centimetres thick and muscovite bearing semipelite. The distribution and relationship of the lithologies suggests that the pebbly beds may be the proximal equivalents of tur- bidity flows depositing a distal sequence of alternating psammite and semipelite. The Maumturk and Gleniska Members group readily together to form the Kyle- more Formation which with its return to locally coarse elastics and turbidite- influenced deposition, is not dissimilar to representatives of the Crinan Subgroup outside Connemara. Tayvallich Subgroup and Southern Highlands Subgroup. A 4 m black semipelitic to psammitic rock with abundant graphite disseminated along grain boundaries and as xenomorphic grains, outcrops in the extreme north of the area. The formation, although thin, is very persistent and can be traced for more than 5o km along strike in North Connemara. To the west and especially to the east, the formation becomes more varied with the occurrence of black graphitic marbles. The name Cornamona Formation is taken from the area in East Connemara where the fullest known sequence crops out (B. E. Leake, pers. comm.). The presence of a black graphitic marble and associated sediments overlying a sequence thought to be equivalent to the Crinan, Easdale, Islay and Blair Atholl subgroups allows consideration to be given to the possibility of a correlation between the Cornamona Formation and the Tayvallich Subgroup. In the Maumturks the black psammites of the Cornamona Formation are overlain by grey impure psammites with minor thin semipelites. Again the formation is far better developed to the west, the Bofin Siliceous Granulite and Sili- ceous Grits of Cruse & Leake (1968), and to the east where a thick sequence of impure psammites (never quartzites) overlies the Cornamona Formation. Leake (pers. comm.) has named these beds the Ben Levy Formation and this terminology is used in the Maumturk area. Harris & Pitcher (i975) suggest that this sequence of elastics may be equivalent to the Southern Highlands Group. This possibility is acknowledged in Fig. I but it is also considered that the Cornamona and Ben Levy Formations could be a part of the Crinan Subgroup.

(C) ROCKS OF IGNEOUS ORIGIN Additional to the amphibolites described above are two other types of igneous rocks. (i) Talcosed basic bodies. Several lensoidal bodies of highly altered basic rock with a characteristic pitted buff brown appearance, intrude the Gleniska Forma- tion north of the Renvyle-Bofin Slide. The lenses vary from small "pods" a metre or so in length, up to 7° m along the strike and 3 ° m at the widest point. No hornfelsing of the country rock by the basic bodies was observed. The lenses appear devoid of structure but Cruse & Leake (I968) recorded $3 in the more altered bodies on Inishshark. The bodies have been altered to talc and calcite 4- chlorite, but relict olivine indicates their original picritic nature. Leake (i97oa) considered these basic bodies without accompanying hornfelses to resemble typical Alpine-type serpentinites that were intruded together with the major basic basic to ultrabasic intrusions of Connemara between D1 and D2.

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(ii) Late dykes. Lamprophyres and plagioclase porphyries occur throughout the area but rarely exceed a metre in thickness. They were intruded prior to some of the post-D4 faulting. The dykes normally trend parallel to the regional E.-W. strike but there are some N.-S. trending dykes.

2. Mesoscopic structures The range of lithology in the Maumturks gave rise to extreme ductility contrasts during deformation, and produced a very varied range of structural styles. (i) First deformation. Microscopic and macroscopic evidence for D1 is found within the area, yet no clearly recognizable mesoscopic D1 structures were observed. Generally the intensity of deformation during D~ destroyed D1 fabrics. A mica fabric clearly pre-dating S~ is preserved in some of the pelitic rocks and is especially evident in pelites of the Maumturk Member in the F~ Tooreenacoona Sy'ncline hinge zone at Muingboy. (ii) Second deformation. S~ is the dominant fabric element in the area and developed during D~. It is a true axial planar schistosity defined by the parallel alignment of both platy and tabular minerals and shows marked flattening of original grit grains. Several types of lineation are associated with D, such as aligned ag- gregates of quartz grains; alignment of elongate minerals; boudins of pre- or early D~ quartz plagioclase segregations and thin psammite bands in pelite; and intersection lineations. The minor F~ folds are isoclinal, with an axial planar schistosity and no visible fabric passing round the fold hinge. (iii) Third deformation. Fs mesoscopic folds are the most common structures seen in the Maumturks. D8 produced both cleavages and lineations, the type of fabric developed depending on rock type. In petites, the $2 anisotropy was too great for the formation of a new schistosity and $3 is a crenulation or incipient strain-slip refolding S~, but in more psammitic lithologies $3 is a true axial planar schistosity, largely destroying S~. However, in the quartzites $3 is only poorly developed and is generally a strain-slip to crenulation cleavage, the cleavage planes spaced up to 5 mm apart. An Ls lineation formed by the intersection of SS and $3 is common in the quartzites and plunges parallel to the F 3 fold axes. In asymmetric folds L.~ is variable in its occurrence, strongly developed on the shorter fold limb (L~ destroyed) but only poorly developed on the longer fold limb (Lz dominant). In the hinge zone both L3 and L2 are present. Accompanying the F8 folding, extension in the form of boudinage and attenua- tion has affected all lithologies and on small scale, structures formed by D3 extension are evident in practically every outcrop. Within amphibolites, internal boudinage of $2 in zones of up to I m thick is common. Pebbles in conglomerates and grit beds are now prolate ellipsoids plunging close to F3 fold axes. Geometrical analysis of Fs folds, utilizing dip isogons (Ramsay 1967), indicated that the majority of these folds are type III/type Ic hybrids, i.e. flattened flexural folds. However, most of the folds examined invariably possessed hinge zones of type II, probably indicating simpIe shear to have also been an important folding mechanism. Plots of L~ lineation loci, both from individual Fs folds and regionally

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from areas homogeneous to $3, approximate to great circle distributions--a characteristic of lineations deformed by simple shear and also of lineations on flexural folds subjected to intense homogeneous flattening (Ramsay I967, p. 44:6). Sub- horizontal E.-W. trending F3 "a" directions calculated from L2 lineation/S3 plots are only significant if simple shear was the dominant folding mechanism. (iv) Fourth deformation. D4 structures consist of open folds and a variably developed crenulation cleavage, $4 and intersection lineation L4. $4, with an E-W. strike and average dip 64°S is the only fabric with consistent attitude throughout the Maumturks; it is common in pelites and amphibolites but rarely observed in quartzites in which frequently the only mesoscopic structure produced by D, is a widely spaced delicate lineation. (v) Later deformations. Minor structures, without accompanying major folds, clearly indicate by their refolding of earlier fabric elements that they occurred late in the deformation sequence. A sub-horizontal crenulation cleavage inclined at up to 2o ° southwards is quite common in the petites and amphibolites but has not been observed in the more competent lithologies. A later set of conjugate kink bands (one set orientated 3200-35 °0 , the other 0200--060 ° ) is present in all lithologies. 3. Macroscopic structures Two major folds, the Connemara Antiform to the south and Joyce's Synform to the north are largely responsible for the present day orientation of pre-Dl structures. The cross-sections in Fig. 2 illustrate the spatial relationships between macroscopic structures in the Maumturks. (i) First deformation. The facing and primary distribution of the stratigraphical units in the area are due to a major F 1 fold, the Lissoughter Anticline, the axial trace of which occurs in the south at Cloonnacartan, Lissoughter and Derryveala- wauma. Based on exposures at present day structural level, the Lissoughter Anticline, an isoclinal fold of large but unknown amplitude, appears to close in a northerly direction over Connemara. The fold is upward-facing to the south but downward-facing to the north of the Connemara Antiform. The rocks outcrop- ping in the Maumturks east of the Lough Inagh fault are on the lower limb of the Lissoughter Anticline. Evidence of its F1 age has been provided by mapping further west (Tanner I967) , where, in the vicinity of Waterloo Bridge, near Clifden, the fold is refolded by a major F2 fold. In Connemara, the Bennabeola Quartzite forms a convenient envelope by which the Lissoughter Anticline may be recognized as all older units are restricted to the core of the fold. Mapping by the author has revealed the existence of a large fold, parasitic on the Lissoughter Anticline, with an axial trace passing north through Cloonnacarton and a hinge in the Bennabeola Quartzite a few hundred metres to the north. In light of the present study, the Cloonnacartan Slide (Edmunds & Thomas 1966 ) is inconsistent with the recognized structure. If the Lissoughter Anticline is accepted as an F1 fold, it seems likely that the F~ folds of Edmunds & Thomas on Illion mountain are in reality the continuation across the Maumahoge South Fault of the Lissoughter Anticline, where at this higher structural level it is refolded by F3 folds.

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(ii) Second deformation. A major tight isoclinal F, fold, the Tooreenacoona Syn- cline, refolds the lower limb of the northerly-closing Lissoughter Anticline. The fold is best seen 6oo m S.E. of Tooreenacoona Bridge, where a perfectly developed S, schistosity intersects gently folded inverted graded pebble beds of the Gleniska Formation at high angle. Pelites of the Maumturk Member, well developed on the upper fold limb west of Muingboy Stream, are very thin or absent from the lower limb. The Maumturk Member occupies the hinge zone of the fold at Fin- nisglin with banded amphibolite of the Finnisglin Member on both fold limbs. To the S.E. of Finnisglin the hinge zone of the fold trace is difficult to locate but is thought to pass through Illion West where its presence is indicated by banded amphibolite and (possibly) Maumturk Member pelites. South of the Connemara Antiform the axial trace is thought to lie in the poorly exposed ground near Derryvoreada Village and Lough Lehanagh and west to the Lough Inagh Fault. West of Cloonnacartan the fold can be traced towards Clifden, where, in the vicinity of Waterloo Bridge it refolds the Lissoughter Anticline (Tanner I967). The Tooreenacoona syncline is not seen east of the Maumturkmore-Maumahoge Faults, the level of present day exposures being structurally too high. (iii) Third deformation. Although D 8 developed the most common macro- and mesoscopic folds in the Maumturks, they are not fundamental to the overall structure, their facing directions being controlled by earlier folds. A major D8 slide in north Connemara was first recognized by Cruse & Leake (1968) who named it the Renvyle-Bofin slide. At Renvyle the slide occurs on the north limb of the F3 Tully Mountain Synform. This fold can be traced eastwards and correlates with the Barnaheskabaunia Synform (Tanner pers. comm.). A slide is present immediately to the north of the F3 Barnaheskabaunia synform bringing together the Streamstown and Kylemore Formations and it is likely that this slide is equivalent to the Renvyle-Bofin slide and will be referred to by this name hereafter. The course of the Renvyle-Bofin slide is shown in Fig. i. Three major F8 folds, from north to south, the Barnaheskabaunia Synform, Kylemore River Antiform and Knockpasheemore Synform have been mapped on the northern limb of the Connemara Antiform south of the Renvyle-Bofin slide. They have their simplest form to the west of the Mauminagh Fault where rocks of the Bennabeola Formation occupy the hinge zones of the two synforms. Al- though minor F3 folds are lacking in the quartzites, the variation in orientation of L, across the Barnaheskabaunia synform, and convergent s and z patterns of F3 folds in the Streamstown Formation on the fold limbs, provide good evidence of an F8 age for the synform. East of the Mauminagh Fault, at a lower structural level, the three major F3 folds are more complex. Numerous satellite folds are developed with axial planes dipping steeply north and fold axes plunging east at approximately 2o ° . The fold traces are dextrally displaced 2 km on the Maumturkmore Fault. Here and further east the folds are exposed at progressively lower structural levels and plunge west at about 15 ° . The Knockpasheemore Synform at lower structural level east of the Maumi- nagh Fault develops five satellite folds (KS1 to KS6 respectively on Fig. I) defining the synform. Traced eastwards across the Maumahoge Fault, Bennabeola

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Quartzite occupies the fold cores with the trace of the southern most synform passing close to Slievenaroy Summit. A further complementary antiform to the south is present in the St Patrick's Well area where it is refolded by the Connemara Antiform. Indications of this F3 antiform are found in a similar, though slightly lower structural level at Illion West. North of the Renvyle-Bofin slide a major F3 antiform, indicated by minor structures, is thought to lie beneath the Silurian sequence. (iv) Fourth deformation. The influence of the Connemara Antiform and Joyce's Synform on the present day attitude of structures has been mentioned. The axial planes of major, mesoscopic and minor F4 folds strike E.-W., dip south at about 64 ° and plunge at low angle. In the vicinity of St Patrick's Well the Connemara Antiform refolds the non-parallel limbs of a major F3 fold, resulting in a series of E.-W. trending enenchelon folds. (v) Faults. All of the mapped major faults appear to post-date D4 and are us- ually well defined by fault scarps and or depressions, with associated brecciation common in quartzites. The Mauminagh Fault zone, trending N 4 °o E has a size- able down-throw west with great change in structural level indicated both by stratigraphy and contrasting simple and complex fold geometry of F3 folds on either side of the fault. West of the fault all pre-D4 structures dip north at low to moderate angles but east of the fault corresponding structures dip north at a much steeper angle; $4 and later structures have a similar geometry on both sides of the fault. The variability of early fabric geometry is probably due to a com- bination of change in structural level across the fault and spatial relationship of the areas relative to the Connemara Antiform. Further east the Maumturkmore-Mauminagh fault system traverses the area from NNW. to SSE. with both dextral and normal movement evident. West of the fault line major F3 and F4 folds plunge east, whilst to the east these same folds plunge west, demonstrating post-D4 tilting. In the south the South Mauminagh Fault has a dextral movement of I. 5 km; further north the Maumturkmore, Mid-Gowlaunlee and North Maumahoge Faults have a similar combined dextral displacement. Post-Silurian movement on the Mid-Gowlaunlee Fault was noted by McKerrow & Campbell (x 96o). Other major faults exhibit dextral movements but the amount of vertical displacement on any of these faults is difficult to deter- mine due to the complexity of folding. 4- Metamorphism and disposition of isograds Evidence for the earliest recognized metamorphism, MSI has only been observed in petites of the Maumturk Member. The metamorphism, thought to have been syntectonic with D~, produced a fine grained phyltitic mica fabric, now preserved as heticitic inclusions (often tracing out crenulations) within later porphyroblasts and quartzose domains of the matrix. MP1 metamorphism is indicated by a static porphyroblastic growth of plagioclase and garnet. MS2 produced S,, the main penetrative fabric developed in the Maumturks, and is the earliest fabric recognized in amphibotites with hornblende and plagioclase aligned parallel to $2. In pelites and semipetites idioblastic biotite and muscovite crystallized in the

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plane of $2 together with the porphyroblastic growth of syntectonic rotated gar- nets and plagioclase. More generally a general matrix coarsening (Harte & John- son I969) affected all of the rocks. The metamorphic peak was attained during MP~ with coeval growth of staurolite and siUimanite in the petites and hornblende and oligoclase-andesine in amphibolites. The variation in mineral assemblages produced during MP2 pre- sumably delineate a simple thermal structure relating to temperature and pres- sure gradients acting in the area at that time. Staurolite, occurring as stumpy sub- hedral poikiloblasts is extensively developed and characteristic of pelites of the Maumturk Member north of Finnisglin. Sillimanite first appears in petites of the Maumturk Member at Luggartarriff as minute fibrolites nucleated epi- taxially on MS~ biotite laths. The percentage of sillimanite in the mode increases from here southward reaching 15% at Finnisglin where staurolite is almost absent. Static growth of biotite, muscovite, andesine, oligoclase, garnet and tour- maline accompanied that of staurolite and sillimanite during MP2. An MP2 chlorite growth in Maumturk Member petites at Maumturkmore in the north is indicated by a marked decrease in the mode of chlorite southwards although tex- tural evidence is equivocal. Fig. 3 summarizes variations within the Maumturk Member of the modal abundance of the principal minerals in an approximate north to south traverse across the northern half of the area. Locally, prograde metamorphism may have continued into MSs, as is indicated by the restricted occurrence of siltimanite faserkiesel, apparently axial planar to F 3 folds, in petites and semipelites of the Streamstown Formation at St. Patrick's Well. The replace- ment of sillimanite, staurolite and micas by andalusite in petites of the Maumturk Member at Muningboy and the not uncommon quartz, andalusite, muscovite, plagioclase segregation veins in petites throughout the area may also date from MS3. Yardley (1974) details evidence for continued prograde metamorphism into MS3 in the Cur area to the immediate east.

DISPOSITION OF ISOGRADS The climax of metamorphism during MP2 produced a simple thermal structure now manifest in the spatial distribution of the MP, mineral assemblage. Recogni- tion of MP, isograds is based mainly on assemblages occurring in the Maumturk Member. An MP2 isograd defined by the first appearance of sillimanite is located in the Luggartarriff outcrops. Pelites at Finnisglin are considered to be at or beyond the upper limit of staurolite stability for the particular bulk chemistry of the rocks. Thus, the MP, isograd for the breakdown of staurolite is probably located here or to the immediate south. The MP2 isograds strike approximately east to west and are dextrally displaced by at least i-5 km across the Maumturkmore, Maumahoge faults. Due to this southwards displacement and the general absence of the pelitic rocks south of Finnisglin little is known from the Maumturk area of further mineralogical change with increasing grade. It is apparent that MP, isograds will have been folded by both F 3 and F4 folds, although the plunge of the folds and topography in the Maumturks are not sufficient to allow demonstration of this in the field. It can be deduced from the

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generally high angle between the L~ and La lineations that F~ and F 3 folds are not coaxial. The low plunges of F 8 and F4 folds therefore indicate that the F~ folds were originally flat lying. As the metamorphic grade increases from north to south, the MP~ isograds must be at an angle to the F~ axial planes. It is possible to en- visage either originally northerly or southerly dipping isograds (Fig. 4.). On the present day structure of Connemara the former model would imply that after an initial increase, metamorphic grade should decrease southwards. With the scale and tightness of the F3 and F4 folds, such a decrease would be anticipated not too many miles south of the Maumturks and is quite contrary to the continued southwards increase in grade towards the migmatites and to the very southern limit of Dalradian outcrop in Western Ireland. It is suggested that originally southerly dipping isograds are more in accord with the regional geology of Connemara. Implicit in this southerly dipping isograd model is a reversal of the geothermal gradient during MP~ metamorphism, with the heat source above. This heat source is thought to have been associated with the ultrabasic, migmatite complex located during MP2--structurally above present day outcrop in the Maumturks--in rocks of the Kylemore and younger formations on the upper limb of the F1 Lissoughter Anticline.

5. Conclusions The known geology of the area began with deposition of the Dalradian sediments (Kilburn et al. I965) , followed by intrusion of early dolerite sills and then deformation (D1) and metamorphism (MS1). Between D1 and D2 the basic and ultrabasic rocks of Connemara were intruded, hornfelsing the previously metamorphosed rocks (Leake i97ob ). It is likely that the talcosed basic bodies seen north of the Renvyle-Bofin slide in the Maumturks were also intruded at this time. Leake (I97oa) considers that the basic and ultrabasic rocks in both north, east and south Connemara belong to one period of magma injection. The basic and ultrabasic bodies are not found intruded into rocks older than those of the Kylemore Formation and according to the present structural interpretation would have been emplaced as a flat lying sheeted igneous complex, exhibiting cumulate crystallization, into the upper limb of the then flat-lying F1 Lissoughter Anticline. The basic and ultrabasic rocks together with the metasediments were folded, deformed and metamorphosed during D2. D~ was dominantly a flattening deformation that produced the main penetrative fabric in the rocks with the major F1 and F2 folds originally flat lying and isoclinal. Following D~, the basic and ultrabasic bodies and associated migmatites would still have been located at a higher structural level than rocks now exposed in the Maumturks (Fig. 4). The heat emanating from this complex may have resulted in a reversal of the geothermal gradient across the area at that time (as would have been required to produce originally southerly dipping MP2 isograds) and have been the partial cause of the MP~ metamorphism. A direct link between metamorphic processes operating in the South Connemara migmatite complex and the metasediments of the Maum- turks during MP1 may be indicated by a potassium metasomatism common to both areas. Basic rocks and migmatitic quartz-diorite gneisses to the east of Cashel

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were affected by an MP~ potassium metasomatism (Leake i97ob), while further north, similar MP, metasomatism, although less marked, affected the pelites of the Maumturk Formation at Finnisglin (Badley I972 ). During D3 the rocks were further folded and deformed but in contrast to the metamorphic history established in east and south Connemara, evidence of significant prograde MS3 metamorphism in the Maumturks is generally lacking. This is probably due to the present level of exposure in the Maumturks relative to the heat source during MP2 and later. As the gross structure of Connemara plunges east, rocks of a higher structural level are progressively exposed in that direction. The complex of migmatites, basic and ultrabasic rocks on the upper limb of the Lissoughter Anticline is thought to be present in the extreme east beneath the Carboniferous beyond the outcrop of the Dalradian. Leake (i97oa) considers the ultrabasics of Cannaver Islands in the Lough Corrib to be part of this complex. It is probable that areas in the east, relatively closer to the postu- lated heat source than areas further to the west, had a longer thermal history allowing prograde metamorphism to continue during and after D3. By D3 in the Maumturks and areas further to the west, the temperature was already too low for prograde metamorphism to continue. D4, the last major deformation in Connemara, produced large upright open folds with E.-W. trending axes. Although D4 produced relatively simple structures the F4 Connemara Antiform superficially dominates the structure of Conne- mara.

ACKNOWLEDGMENTS. I thank Drs W. A. Cummins and P. K. Harvey for reading the manuscript, Professor B. E. Leake for extensive improvements and Dr P. W. G. Tanner for much helpful dis- cussion. The work was supported by N.E.R.C.

6. References BADLEY, M. E. 1972. The geology of the Maumturk Mountains and Inagh Valley, Connemara. Ph.D. thesis, Univ. Nottingham (unpubl.). COBBINO, E.J. 1969. The geology of the district NW of Clifden, Co. Galway. Proc. R. It. Acad. B67, 302-25. CRUSE, M. J. B. & LEAKE,B. E. 1968. The geology of Renvyle, Inishbofin and Inishark, North- West Connemara, Co. Galway. Proe. R. It. Acad. B67, 1-36. EDMUNDS,W. M. &; THOMAS,P. R. 1966. The stratigraphy and structure of the Dalradian rocks north of Recess, Connemara, Co. Galway. Proc. R. It. Acad. B64, 517-28. EVANS, B. W. & LEAr,E B. E. I97O. The Geology of the Toombeola District, Cormemara, Co. Galway. Proc. R. Ir. Acad. BTo, 105-39. HARRIS, A. L. & PITCHER, W. S. i975. The Dalradian Supergroup. In Harris, A. L., et al. (eds) A correlation of Precambrian rocks in the British Isles. Geol. Soc. Lond. Spec. Rept. 6. HARTE, B. & JOHNSON,M. R. W. I969. Metamorphic history of Dalradian rocks in Glen Cora, Esk and Lethnot, Angus, Scotland. Scott. Geol. J. 5, 54-80. KILBURN, C. PITCHER, W. S. ~; SHACKLETON,R. M. I965. The stratigraphy and origin of the Portaskaig Boulder Bed Series (Dalradian). Geol. J. 4, 343-60. KINAHAN, G. H., NOLAN,J., LEONARDH. & CRUSE, R.J. I878. Mem. Geol. Surv. Ireland. LEA~, B. E. x97oa. The fragmentation of the Connemara basic and ultrabasic intrusions. In Newall, G. and Rast, N. (eds) Mechanisms of Igneous Intrusion. Geol. J. Spee. Issue 2. I97ob. The origin of the Connemara Migmatities of the Cashel district, Connemara, Ire- land. Q. Jl. geol. So¢. Lond. x25, 2 I9-76.

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LEAKE, 1972. Garnetiferous striped amphibolites from Connemara, Western Ireland. Min. Mag. 38, 649-65. McKERROW, W. S. & Ca~PB~T L, C.J. 196o. The Stratigraphy and Structure of the Lower Pale- zoic rocks of north west Galway. Sci. Proc. R. Dublin Soc. A, x, =7-51. RamSAY, J. G. I967. Fracturing and Folding of Rocks. McGraw-Hill, New York. STAR~Y, J. x96o. Studies on the geology and mineralogy of the Maumturk area of Connemara, Eire. Ph.D. thesis, Univ. Liverpool (unpubl.). STURT. B. A. 196 r. The geological structure of the area south of Loch Tummell. Q. Jl geol. Soc. Lond. xx 7, x31-56. & HARMS, A. L. I96I. The metamorphic history of the Loch Tummel area, central Perth- shire, Scotland. Lpool Manchr Geol. J. 2, 689-7II. TANNER, P. W. G. I967. The Dalradian of Connemara, Eire. Ins. African Geol. Univ. Leeds x xtk Ann. Rept. 26-8. YARDLEY, B. W. D. 1974" The deformation, metamorphism and metasomatism of the rocks of the Cur area, Connemara, Eire. Ph.D. thesis, Univ. Bristol (unpubl.).

Received 3 March x975; revised typescript received 29 March x976.

MICHAEL EDWARD BADLEY~ Deminex, Dorotheen Str. z, 43 Essen I, West Germany. Errata. Fig. x: black ornament N. of Muingboy represents Finnisglin Mb., not Alpine-type ultrabasic light stipple o.36 km NW. of Luggartaxtaxiff and o.7 km WSW. of Knockaunbaun should be heavy stipple of Luggartariff Mb.

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