Metamorphic Rocks in the SW Ox Mountains Inlier, Ireland; Their Structural Compartmentation and Place in the Caledonian Orogen

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Metamorphic rocks in the SW Ox Mountains Inlier, Ireland; their structural compartmentation and place in the Caledonian orogen

C. B. LONG & M. D. MAX

SUM MARY Three tectono-sedimentary units having Cale- deepest structural level seen and consists of donian tectono-thermal histories are recognized basic volcanic rock, graphite schist, marble within the metamorphic rocks of the SW Ox quartzite, psammite, and semi-pelite. It Mountains Inlier. The different structural levels forms the envelope to the syn- to late kinematic represented by each of these units were juxta- 5o0 4- 18 Ma Ox Mountains Granodiorite and posed along tectonic contacts late in the indivi- stratigraphically it can best be compared with dual tectono-thermal histories. The highest the Appin and Argyll groups of the Dalradian structural unit is the virtually unmetamor- of Connemara. phosed, probably Southern Highland (Upper) The differing tectono-thermal histories of the Dalradian, Ardvarney Formation which can be different structural levels in this part of the related on a regional scale to the Donegal 'Tay metamorphic Claledonides of western Ireland Nappe' structure. The Raheen Barr succession possibly reflect the diachronous nature of the probably ranges from the Islay to Easdale deformation and metamorphism in the orogen. subgroups of the Middle Dalradian and can be Interaction during juxtaposition must have correlated with the metamorphic rocks of the involved movement related to more than one Nephin Beg succession in NW Co. Mayo. The direction of tectonic transport. Ox Mountains succession represents the

THF SW FLANKS OF THE Ox MOUNTAINS INLIER (Fig. t) are composed mainly of psammitic and semi-pelitic schist, graphite schist, pebbly grit, minor quartzite, marble, calc-silicate schist and basic volcanic rock. The backbone of the Ox Mountains is the Ox Mountains Granodiorite, which has an associated suite of appinite, lamprophyre and granite. Carboniferous rocks rest uncon- formably on the inlier to the south but are mainly in faulted contact to the north. Devonian rocks occur in graben to the west. Part of the area has been described by Currall ( 1956, 1963), Taylor (1966, x968, t969) , Currall & Taylor (i965) and Long (I974, I975), but interpretation of stratigraphic age by others (cf. Dunning & Max i975, Johnstone I975, Harris & Pitcher i975) varies. This paper is based on i :io 56o mapping between x972 and 1975. Each tectono-sedimentary unit (Fig. I) is characterized by a unique structure and metamorphic history, thus suggesting their origin from different structural levels. The Ox Mountains succession occurs north and south of the synkinematic Ox Mountains Granodiorite and extends to the NE from the granodiorite to form the bulk of the Ox Mountains Inlier shown on Fig. I. The succession also occurs immediately to the southwest and west of the Ox Mountains Granodiorite (Fig. 1) where it has not been previously recognized. The Ox Mountains Granodiorite

o71 geol. Soc. Lond. vol. i33, 1977, pp. 4x3-432, 6 figs., 2 tables. Printed in Great Britain.

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exhibits the same intrusive relationships to the Ox Mountains succession along the northern, southern and western margins where there are similar envelope-rock lithologies, successions, structures and metamorphic histories. The Raheen Barr succession which crops out at the western end, and locally along the northern margin of the inlier (Fig. 2), yields evidence for a slightly different tectono-thermal history from the Ox Mountains succession. The Raheen Barr succession can be correlated across Glen Nephin and Glen Hest to the Nephin Beg Mountains in the NW Mayo Inlier (Fig. I) by matching of formations. Metasediments of the Nephin Beg succession (TrendaU & Elwell I963, Max I974) can be followed west to the Corraun Fault (Crow et al. 197 I) and compared with other Caledonian rocks in NW Co. Mayo (cf. Kennedy I969, Sutton I97I, Max I97O, Crow et al. I97i ). The Ardvarney Formation, comprising low-grade metamorphic rocks with a simple tectono-thermal history, occurs in the southwest. Rocks of this formation are probably related to other low-grade rocks outside this area, i.e. the Cloonygowan 'Group' (Taylor I968 ) and the Westport Grits (Kilroe I9O7). The Raheen Barr succession is probably Argyll (Middle) Dalradian and the _A_rdvarney Formation is probably Southern Highland (Upper) Dalradian. Their tectonic contacts with the Ox Mountains succession have been used to infer a pre- Caledonian age for that unit (Phillips 1973), although Shackleton (in Phillips 1973) pointed out that the tectonic juxtaposition of differing structural levels could better explain the relationships. The synkinematic Ox Mountains Granodiorite has yielded a Sr/Rb whole rock isochron of 5oo 4- I8 Ma (Max et al. I976b ) (also see Pankhurst et al. I976 ) and it is consistent both with structural and probably stratigraphic evidence (cf. Long I974, I975) to regard the metasedimentary envelope of the granodiorite as Caledonian in age.

I. Stratigraphy (A) OX MOUNTAINS SUCCESSION (342o m +) The term Ox Mountains succession (Table I) is not an exact synonym for previous interpretative or mapping units (i.e. Slieve Gamph Succession, Taylor I968; Ox Mountains 'Group', Long i975; Ox Mountains Sequence, Phillips et al. I975). Earlier mapping has been incomplete and has given rise to some miscorrelations (cf. Currall I963, Taylor I968 ). (i) Tawnyshane Formation ( 17 ° m ÷). Two poorly exposed members occur north of Lough Anaffrin (Fig. I). The Tawnyshane Marble Member (9 ° m +) consists of white tremolitic marble and bands and patches of green serpentinous carbonate marble. Thin calc-silicate beds occur above the highest marble and a few very thin petitic intercalations form a zone transitional to the overlying petite. The Tawnyshane Pelitic Member (8o m) consists of 2o m of massive to indistinctly banded, brownish-weathering petite and semi- pelite, with quartz segregations throughout and a 5 cm marble band near their top. These pass upward into a transitional zone with interbedded petite and white vitreous quartzite. Immediately below the lowermost quartzite band, rounded

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psammitic pebbles up to 5 cm in diameter occur within a massive pelitic matrix. Granitic spheroids were also recognized but might be boudinaged granitic veins rather than granitic boulders. Individual quartzite bands are thicker nearer the top, where pelites are fewer and the highest (0- 5 m thick) pelite marks the top of the member. (ii) Leckee Q.uartzitic Formation (i 75 ° m) The Leckee Quartzitic Formation includes the Garraghill Formation (Long I974, 1975) and most of the Leckee Quartzitic 'Group' (Taylor i968), but Taylor's (1968) Lough Brohly Quartzite, Ballycong Limestone and associated psammite are excluded. There are no distinct members in this formation although sparse striped amphibolite and calc-silicate layers have been mapped (Long 1975). There is a gradation from a more quartzitic base with rare semi-pelite to flaggy, thinly bedded, commonly feldspathic psammite with thin micaceous partings in the upper part. The basal part is a flaggy, muscovitic, white-weathering quartzite, about 6o m thick north of Lough Anaffrin, in which quartzite and rarer vitreous orthoquartzite predominate over psammite. Quartzite beds up to 2o cm thick are internally banded, and thin, infrequent heavy mineral bands are restricted to the lower part of the formation. Rare cross-lamination and possible graded bedding have been used to determine way-up. Beds range from o. 5 cm to a few metres but most are commonly between 2 and 2o cm thick. Bed differentiation is good and occasional transitions within individual beds have been regarded as grading. Pink K-feldspar is common in some horizons, and normally rare, thin semi-pelite and pelite bands are common just south of Foxford, (12833 30225). Quartzitic grits are rare, but not absent (e.g. I i882 29722; 1 i887 29724). Calc-silicate schist occurs at two localities. The I O m + thick Kilmore Limestone (12813 3o14o) contains minor carbonate and is about 65o m below the top of the Leckee Quartzitic Formation (Taylor 1968 , p. 73). Another band o.8 km NNE of the Kilmore Limestone is interbedded with psammite. Calc-silicate bands outside this immediate area (Taylor i968 , Long 1974, p. 464) are uncommon. Rare, discontinuously striped amphibolite may represent volcanic rocks similar to the more positively identified basic material in the Cappagh Formation. (iii) Cappagh Formation ( 1500 m +) The Cappagh Formation (Long I974, 1975) includes the entire semi-pelitic sequence overlying the Leckee Quartzitic Formation in this area. The term Cappagh Formation therefore includes the Ummoon, Lower Lismoran and probably at least a part of the Upper Lismoran 'formations' (Taylor 1968); and the Shanvoley Semi-Pelitic and the Derryharriff Semi-Pelitic 'Groups' (Currall x963). The mainly semi-pelitic schists of the Attymass 'Group' (Taylor 1968) north of the Ox Mountains Granodiorite are also included in the Cappagh Formation and, although amphibolite occurs at several horizons, the only member here defined is the <8o m Corradrishy Member (Corradrishy 'Formation' of Taylor I968 ) which is more psammitic than the other rocks north of the granodiorite and contains

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pale green quartzite. Calcareous schist with epidote is generally more common north of the granodiorite than to its south. The precise stratigraphic position and way-up of these rocks is uncertain, but as they cannot be clearly distinguished from rocks of the Cappagh Formation elsewhere and the Cappagh Formation may be expected to reappear north of the Lough Anaffrin anticline (Fig. i) we consider them best included in this formation. East of the Knockaskibbole Fault the lowermost part of the formation is composed of thinly bedded pelitic and semi-pelitic schist with minor psammite, and west of the Knockaskibbole Fault there are additionally rare lenses of marble up to o'5 m thick. This formation generally consists of a monotonous succession of commonly feldspathic, usually thinly bedded, semi-pelite, pelite, psammite and, less commonly, quartzite and graphitic schist. Semi-pelite is the commonest lithology and bands are usually o- I to I o cm thick. The thickest beds are psammitic and these are up to 2 m thick. Many beds are richly feldspathic, with plagioclase porphyroblasts commonly 2-3 mm across. Tourmaline often occurs and quartz segregations are common. Graphite schist is more common west of the Knock- askibbole Fault but is locally conspicuous to the east (e.g. at i x536 29316) where a i6m thick zone of several graphite schist bands occurs. West of the Knock- askibbole Fault the lithological banding is frequently less conspicuous than to the east. Grits (e.g. at i i i56 29ioi ) are rare. At no more than x3o m from the base of the formation is the Newantrim Member and occurring mainly above this but inter- fingering with its top is the Meelick Member. Beds above the Newantrim Member are generally a little less pelitic than those below. The Newantrim Member consists of amphibolites which crop out in a zone which decreases from 5 ° m wide immediately to the east of the Knockaskibbole Fault to 2 m SSE of Foxford. It is an important marker band and can be traced along the entire length of the Cappagh Formation, even west of the Knockaskibbole fault. A thick amphibolite, best exposed in the vicinity of Slievenagark Lough, has relic volcanic bombs or pillows up to 2o cm in diameter and thin, (several cm) locally garnetiferous striped amphibolite bands both above and below. This amphibolite is fine-grained and schistose and is interpreted as metamorphosed tuff and hyaloclastitic rock with interbedded sediment. Similar amphibolite is known elsewhere (ii443 299o5 and II577 3ox43), and I km southeast from the latter at several exposures on the hillside in Greenans townland where it probably forms part of the same member (Fig. I). A chloritic bed (I I539 29746) is no more than 5 cm thick. The Meelick Member comprises a series of richly garnetiferous pelitic bands, each up to 2 m thick, with garnets commonly up to I cm across. These pelites have a distinctive pale greenish grey matrix, and are interbedded with semi-pelite and psammite. Staurolite is restricted to the garnetiferous pelite, though absent in beds having the largest garnets. Chloritoid, rather than staurolite, occurs with large garnets west of the Knockaskibbole Fault. Three lines of evidence support the extension of the Ox Mountains succession to the west of the Knockaskibbole Fault (Fig. x). The metasediments and their succession are the same on either side of the Knockaskibbole Fault and the structural and metamorphic sequences are very similar. Although metamorphic

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grade changes slightly from east to west, the unit as a whole can be contrasted with the Raheen Barr succession from which the Ox Mountains succession is tectonically separated. Most importantly, an intrusive contact with the Ox Mountains Granodiorite has been identified to the west of the fault, approximately 2.2 km E of LevaUy Lough and contact relationships and rock types are indistinguishable from those seen along both flanks of the Ox Mountains Granodiorite (Max et al. I976b). The Knockaskibbole Fault, which has been mapped through the granodiorite and traced into the major fault along the north side of the Ox Mountains Inlier, does not everywhere separate major structural units; it disrupts only a single tectono-sedimentary unit along its northern end. The introduction of the term 'Knockaskibbole Fault' followed the recognition by Long (I974, x975) that the Conloon and Sole thrusts (Currall I963) are a single high angle fault.

(B) ~AI~EEN BAI~I~ SUCCESSION (284O m+) This includes all metamorphic rocks in the inlier west of the Raheen Barr Fault and the Sheeans Thrust, and locally along the north side of the inlier east of Lough Conn (Fig. I). The metasediment north of glens Nephin and Hest (Fig. I ) forms part of the same tectono-sedimentary unit. Graded bedding mainly in the Glenis- land Formation provides poor way-up evidence. (i) Raheen Barr Quartzite Formation (I24 mq-) The lowest part of this oldest formation is the poorly exposed Raheen Barr Graphite Schist Member ( >46 m) (io956 29584; io968 29567) which is composed of both banded and massive graphitic semi-pelitic schist. Above this member lies a sequence, of white and grey sometimes gritty orthoquartzite and quartzite interbedded with dark-grey graphitic quartzite and orthoquartzite. About 6 m below the top of the formation a graphitic semi-pelite occurs. Above this, semi-pelite and psammite are interbedded with quartzite in a transitional upper unit. White and very light grey, poorly schistose, well jointed, locally brecciated and iron-stained quartzite, orthoquartzite and minor psammite in beds < x m thick form the lowest part of the formation seen in the northeastern part of Raheen Barr Hill. Locally, thin graphitic grey quartzite bands are gradational with non- graphitic types. Two schistose grit horizons I2o and 3o cm thick (I I 17 ° 2947 o) occur above in a zone of alternating dark and light-grey or white quartzite. These contain white and opalescent bluish-grey flattened quartz clasts up to 3 cm across. The uppermost part of the formation contains grey graphitic quartzite with rare thin lenses and small irregularly shaped clasts of graphite schist, and gritty bands (I I I73 2944 ° and 11 I58 2942o) up to o. 5 m thick. Graphite-free, sometimes opalescent, quartz clasts in these grits are up to I cm across. (ii) Raheen Barr Albite Schist Formation (Ioo m) This is best exposed along the eastern and southern sides of Raheen Barr Hill and to the west (Io938 2958o). Feldspathic psammite with pink albite porphyroblasts up to 3 mm in diameter are characteristic. More semi-pelitic lithologies locally contain conspicuous epidote. Marker bands are lacking. Poorly developed

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cross-lamination (i 1211 29477) and chloritic and epidotic, possibly tuffaceous, green beds (i I214 29477; 1 I2oo 29464) occur locally. (iii) Glenisland Formation (2612 m+) The Glenisland Formation is a pebbly grit in its lower part and there is a gradual upward transition through finer grit and gritty feldspathic psammite with smaller and more dispersed clasts, to feldspathic semi-pelite. At the southeastern end of Raheen Barr Hill, quartzitic and psammitic pebbly grit occurs with conspicuous, sometimes opalescent bluish-grey quartz and quartzite clasts up to x cm across. West of Raheen Barr Hill, four (I 4-35 m) bands of gritty psammite, grit and pebbly grit are interbedded with amphibolite. The grit and pebbly grit bands are up to 3 m thick. The commonest clasts, which are of white or opalescent bluish- grey quartz, are elongate and locally flattened. They range up to I "5 cm across in the coarser grit. A psammitic, often feldspathic, matrix is most common. Grit bands are poorly graded and only rarely indicate younging (e.g. 1 lO78 2942o). The Raheen Barr Volcanic Member (amphibolite 855 m; grit I I I m) occurs at the base of the formation. Amphibolite is interbedded with grit and pebbly grit. The lower contact of the Glenisland Formation is not exposed but is placed at the base of the lowermost grit or pebbly grit bed. Locally, the lowermost amphibolite band is taken as the base if grit is absent. At least four distinct amphibolite bands (6-685 m) probably represent submarine basaltic lava, tuff and volcanic ag- glomerate. The amphibolite is dark-green, generally fine-grained, porphyritic, and throughout shows little conspicuous variation. Pillows < i m across (i I21I 294 62), a vague banding defined by variation in size and abundance of plagioclase porphyroblasts, and a hyaloclastite texture (lO87O 29536), are the only certain retie igneous features. Currall (i 956) recorded features which may be infilled vesicles. Marble is interbedded with amphibolite on the eastern side of Raheen Barr Hill (I 1184 29428 and I I I87 29433), and in Derryharriff North (i i 119 2936o) a x5 m calciferous white marble (the Derryharriff Marble, Currall I963) is regarded as part of this member. Above the Raheen Barr Volcanic Member, about I-4 km WSW of Lough Ben, lithologies are generally psammitic with a few interbedded grit and semi-pelite layers. Twelve thin grit bands occur here in a 2o cm thickness. Chloritoid-bearing semi-pelite is conspicuous in lenses at this level and these persist upward to the basal part of the more generally semi-pelitic rocks (Long 1975, pl. III). Chloritoid is restricted to this Cashel Member (Fig. I). The upward passage of this member to dark, feldspathic semi-pelite is generally gradational, but locally there is an abrupt change. Thin gradational banding and quartz segregations are common throughout the semi-pelite and there are several lenses and possibly bands of coarse grit and gritty psammite. (iv) The Sheeans area Here (Fig. I, 2a) the Raheen Barr succession is represented chiefly by amphibolite, grit, psammite and semi-petite of the Glenisland Formation. The Barna- geehy Limestone (Currall 1963) is the main marble band within the volcanic rocks,

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but other thin calcareous bands are also found within the main amphibolite. The sequence, with two marble and orthoquartzite bands near the northern end of the Sheeans area (Fig. 2c), is part of the Raheen Barr succession but its exact stratigraphic position is obscured by extensive faulting.

(v) Ballycong Unit In discontinuous exposures along the northern boundary of the inlier (Fig. I, 2b) are marble, quartzite, orthoquartzite and psammite with minor semi-pelite, which can best be compared with the Raheen Barr succession. This sequence, which includes the Lough Brohly Quartzite and the Ballycong Limestone of Taylor (i 968), is here called the Ballycong unit and it is separated from the Ox Mountains succession by a fault with associated breccia and mylonite. (vi) The Lahardaun exposures That part of the NW Mayo Inlier (east of Nephin) discussed here (Fig. I) can be regarded as a structural, lithological and metamorphic continuation of the Raheen Barr succession, as the lithostratigraphy can be matched and both areas have suffered a similar tectono-thermal history. In the east semi-pelite with occasional grit layers constitute the upper part of the Lahardaun Formation, which is equivalent to the Glenisland Formation. The gritty horizons contain opalescent bluish-grey quartz clasts. Amphibolite with relic pillow structure (Trendall & Elwell 1963, p. 223), and interbedded gritty quartzite and psammite bands with bluish-grey opalescent quartz clasts occur to the west. The amphibolite is referred to the Raheen Barr Volcanic Member. The Lahardaun Limestone (Trendall & Elwell 1963) is associated with the amphibolite and is stratigraphically comparable with the Derryharriff Marble Member of Raheen Barr, and the Barnageehy Limestone in the Sheeans area. Further to the west semi-pelite and psammite occur; the latter locally contains conspicuous pink plagioclase porphyroblasts, and can be correlated with the Raheen Barr Albite Schist Formation. At the foot of Nephin on the eastern side are sub-outcrops and much debris of dark grey graphitic quartzite resembling the upper part of the Raheen Barr Quartzite Formation. The Lahardaun Schists were correlated by Trendall & Elwell (1963) with the Lough Feeagh Schists to the west and were collectively referred to as the Grey- wacke Group. It is with this 'Group' that we, in accord with CurraU (1963, p. I63), suggest correlation of the Raheen Barr succession.

(C) ARDVARNEY FORMATION (850 to 300 m). Semi-pelitic grit, pebbly grit, flaggy phyllite and graphite schist are best exposed in the Clydagh River between Ardvarney and Conloon. Downstream, the section has few pebbly grit bands and a single zone of graphite schist within finely banded greenish-grey phyllite having alternating (0.5 to 5 ram) micaceous and quartzose bands. Upstream are alternating pebbly grit and phyllite beds. An isolated out- crop of amphibolite (i 1433 29367) is regarded as part of this formation. Poor exposure and absence of marker bands make structural interpretation and esti-

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mation of thickness difficult. The formation represents a metagreywacke turbidite sequence. (D) STRATIGRAPHIC CORRELATION In terms of lithology and metamorphism the Ardvarney Formation is similar to and probably lithostratigraphically equivalent to the Westport Grits (Kilroe 1907) and Cloonygowan 'Group' (Taylor 1968). Lithologically, it is probably Southern Highland (Upper) Dalradian and broadly equivalent to the Inishowen Head Grits of Donegal (Roberts 1973). The Raheen Barr succession, on the basis of its lithostratigraphy, probably represents Appin (Lower) and Argyll (Middle) Dalradian, high Blair Atholl to high Easdale subgroups (Harris & Pitcher 1975). The Ox Mountains succession has been thought older than the Raheen Barr succession, though not necessarily much older, and Long (1975) placed the Raheen Barr succession just above the Ox Mountains succession. We now believe that these two successions may well be lithostratigraphically equivalent as their similarities are more significant than their differences (Table I).

2. Metamorphic and deformational history Each of the three tectono-sedimentary units has a characteristic tectono-thermal history. The detailed mineralogy has been presented elsewhere (Currall i956 , 1963, Taylor 1966, i968 , Long i975). The Ox Mountains succession has been the most highly metamorphosed, and in places a low almandine amphibolite-facies mineral assemblage is recorded; the Raheen Barr succession has reached high greenschist facies, while the Ardvarney Formation reached only low greenschist facies (Fig. 3)- The metamorphic history and sequence of deformation in each unit (Fig. 3 & Table 2) differ significantly. Terminology follows that of Sturt & Harris (1961) and Fleuty (1964). Use of SI, $2 etc. does not imply equivalence of structures in the different structural units.

(A) OX MOUNTAINS SUCCESSION Metamorphism reached a peak during MS2 to MP2 and was roughly coeval with intrusion of the late-D20x Mountains Granodiorite. The entire Ox Mountains succession was affected similarly, but metamorphism shows local variation (Figs. 3 & 4a, 4b). Although S I has been largely obliterated by the pervasive MS2 mineral growth probably it was a penetrative schistosity axial planar to small scale FI folds. Remnants of fine-grained MSI minerals are preserved in porphyroblasts of garnet and plagioclase which are commonly zoned; the MP2 zone is often the most conspicuous. East of the Knockaskibbole Fault co-existing oligoclase and garnet indicate that the staurolite-almandine sub-facies had been attained (except northwest of Foxford). Staurolite is confined to the Meelick Member, while chloritoid is present in this member west of the Knockaskibbole Fault. From MS3 times onwards development of porphyroblasts was restricted to low greenschist-facies minerals; feldspar became sericitized and saussuritized, staurolite altered to mica, and garnet, biotite and hornblende were altered to

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chlorite. Mica flakes were buckled while fracture cleavages dislocated most minerals and quartz, in particular, suffered mechanical granulation. Except for the hinge of the major probably FI Lough Anaffrin Anticline (Fig. I), other F t folds are minor; they fold bedding and rare, thin, pre-Fi quartz segregations (Table 2). Isoclinal to tight F2 folds refold uncommon FI folds and, since the axial planes of both sets are parallel, type 3 interference patterns (Ramsay 1963) are produced. F2 folds are upward-facing, at least south of the Lough Ana- ffrin Anticline. The main penetrative schistosity throughout is $2. Intrusion of the appinitic suite and Ox Mountains Granodiorite took place during late MS2 times before completion of the F2 folding (Fig. 5). Late-D2 stress produced the main (first) foliation and schistosity in the granodiorite and in all the hypabyssal intrusions of the igneous sequence. This stress also caused open to close folding

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FIo. 4. Micrographs. Scale applies to all sections. A. MP2 staurolite/garnet/plagioclase from the Meelick member. Cappagh 74-o718. Staurolite (St), garnet (G) have darkened margins. Tourmaline (T), quartz (Q,), Plagioclase--open stipple. The trend of the largely phyllosilicate groundmass is shown by dashed lines. B. MP2 chloritoid/plagioclase. North of Shanvoley 73-4oo2. Chloritoid close lines, plagioclase--open stippled and the trend of the generally phyUosilicate groundmass is shown by dashed lines. C. MPx albite and garnet (G) porphyroblasts overprinting S x quartz and phyUo- silicate fabric. Raheen Barr Albite Schist. Raheen Barr 74-o597. D. Ardvarney Formation. S I augening feldspar and quartz clasts. Conloon 73-3 x67.

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(Long x975, figs. VII. 6, VII. 7) which is best displayed by granitic veins in the country rock. $2 may be composite locally (of. $2-$3 in part of Donegal described by Pitcher & Berger (I972 ; p. 8I) ), but we regard the composite nature of $2 as a reflection of local planar fabric adaptations during a lengthy D2. Extremely rare upright flexural folds of $2 are thus regarded as being only locally significant.

T AB ,.~. 2: Summary of deformational history Note that the structural nomenclature within cessationof deformation in each of the units is each of the three units is restricted and no not known, with the exception that D 3 in the equivalency (e.g. DI:DI) is proposed. The Ox Mountains succession may be roughly precise relationship between the initiation and equivalent to DI in the Ardvarney Formation.

OX MOUNTAINS SUCCESSION RAHEEN BARR SUCCESSION ARDVARNEY FORMATION

SO Sedimentation with SO Sedimentation with SO Sedimentation with syn - sedimentary basic syn - sedimentary basic virtually no syn - sedimen- vulcanicity. Earliest vulcanicity. tary basic vulcanicity. quartz segregations.

DI FI Isoclinal folds (upward D1 F1 Close to tight main folds D1 F1 Tight to isoclinal folds. facing Lough Anaffrin (downward facing Raheen Anticline). Barr 5ynform ). $I Included fabric. 51 Penetrative schistosity Penetrative schistosity, axial planar to F1. locally passing into D3 cataclastic zone of Ox Mountains succession. Partial granulation of clasts.

D2 F2 Tight to isoclinal main D2 F2 Close buckle folds. D2 F2 Minor open folds. folds. 52 Penetrative schistosity, 52 Crenu~tion deavage. S2 Local crenulation cleavage. axial planar to F2. Intrusion of 500-+18 Ma Ox Mountains Granodiorite and associated appinites cross cutting F2 folds, but predating final F2 tightening. Late F2 open to close folds in igneous types. Main foliation in igneous types is coplonar with and equivalent to regional $2.

D3 F3 Close buckle folds. D3 Minor flexuHng, kink- D3 Minor flexurmg, kink- $3 Crenulation cleavage, also and bands and fracture cleavages. and bands and fracture prominent throughout the later Thrusts (Sheeans Thrust) and later cleavages. Faults. granodiorite, locally faults ( serpentinite at cataclastic (e. g. Lough Lough Darrdun). Rusheen Cataclastic Zone). Relative age of feldspar Boudin development in porphyries uncertain, but Cappagh Formation. probably fairly late.

D4 Minor flexuring, kink- and bands and fracture Dater cleavages. Faults, with final location of serpentinites. Minor intrusion of olivine dolerities in Cappagh Formation.

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F 3 minor, close, buckle folds are related to a crenulation cleavage which transects the granodiorite, and in the Cappagh Formation prominent boudins, generally with sinistral displacement, have their long axes parallel to F 3 fold axes (Currall & Taylor 1965, fig. 2, Long 1975, fig. VII. 3)- F3 buckle folds, often associated with sliding and local mylonitic fabrics parallel to the flat-lying axial planes, occur northeast of Sheeans. Locally in the metasediments S 3 intensifies, becoming penetrative and cataclastic, as in the Lough Rusheen cataclastic zone, the Com- mauns and Shanvoley area, and the western end of the Glenawoo cataclastic zone (Fig. i) (Glenawoo Slide, D2 of Taylor I969, and D 4 of Phillips et al. I975), which is near the eastern edge of this area (Long I975, fig. VII. 8). The Lough Rusheen Fault was possibly initiated as early as D 3 because it is spatially associated with and genetically related to a cataclastic zone. D 4 and younger deformations gave rise to minor flexuring of S 3 and development of kink-bands and fracture cleavages.

(B) RAHEEN BARR SUCCESSION Metamorphism of this succession differs from that of the Ox Mountains succession in that the metamorphic peak was during MS i and MPI (Figs. 3 & 4c). Quartz- albite-epidote-almandine sub-facies was attained throughout, although this is uncertain in the Ballycong Unit owing to cataclasis and lack of pelitic lithologies. MS I minerals are found both within MPI porphyroblasts and between them. Fairly coarse MSI micas form the only penetrative schistosity. MPI porphyroblasts of albite, some possibly with a little peripheral oligoclase, garnet, epidote, clinozoisite and chloritoid, which is controlled by bed composition, are conspicuous. The Raheen Barr Synform is the largest of the downward-facing, close to tight FI folds with an axial planar SI schistosity (Fig. i). Although regarded here as F i, these folds may possibly have either been overturned by later folding or developed in strata already overturned by pre-FI folding. F2 minor folding produced a crenulation cleavage and was followed by the development of F 3 or later kink-bands and fracture cleavages, and subsequent thrusting and faulting (Table 2).

(G) ARDVARNEY FORMATION Only low gTeenschist facies (quartz-albite-muscovite-chlorite sub-facies) was attained (Fig. 3 d) and the rocks lack the conspicuous development of porphyroblasts found in the other two tectono-sedimentary units. Generally there has been compaction of the matrix around rodded clasts, accompanied by an incipient granulation of quartz clasts, and an S I, possibly mimetic, development of chlorite and white mica along bedding planes. Locally, in the Lough Rusheen cataclastic zone, S I is a cataclastic fabric. Later minor folding has no associated S-fabric except rarely where there is a crenulation cleavage involving the growth of late chlorite. FI isoclinal to tight folding was accompanied by rodding of clasts and the development of a fine-grained penetrative S i schistosity. Locally this is co-planar with, and passes into, the Lough Rusheen cataclastic zone D 3 fabric of the Ox

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Mountains succession but this does not unequivocally equate the Ox Mountains succession D 3 with Dz of the Ardvarney Formation. Fz major folds have not been identified. F2 minor folding, sometimes with a crenulation cleavage, was followed by minor flexuring and the development of kink-bands and fracture cleavage.

3. Intrusive igneous rocks The Ox Mountains Granodiorite, which is the major part of the Slieve Gamph Igneous Complex of Taylor (x966 , 1968), contains a composite basic to acid suite of igneous rocks and has sheeted, deformed margins. It is banded and foliated, and is thoroughly deformed. Diorite, quartz diorite, granodiorite, adamel- lite, trondhjemite, pegmatite, aplite and granitic veins occur, and microcline- microperthite, zoned plagioclase megacrysts, quartz, white-mica, biotite and minor hornblende, epidote, allanite, sphene, iron-ore and apatite are common minerals in the igneous assemblages.

~!~ , II ..~" ~, ~ / I \ ',J ,P" 7/ , i k;l !I"; /" l! I -,, \ \, ',, ', / i j, J~ '~!l'./.z,/..~\~ I i ",, /Z / • I1,'. It I I I l ;A("! ~i II I B ~,i ',',! ~r~ l I l / I I I , I | ! ~ I ~ iJ 2^ ~ I ! I~ ,I $2 , i 5cm II 1 I

F zo. 5. Structural relationships between selected intrusive rocks and the planar fabrics seen in the Ox Mountains succession. A. Pegmatite dyke in semi-pelite with composite S t/$2. (I z9o2-29483); B. Microgranite in semi-pelite (1 x9o3-29495). C. Folded (F2) psammite band in semi-pelite with younger microgranite dyke (i 19o2-29483). Dashed lines indicate the trend of $2. D. Micrograph of the contact relationships between granodiorite dyke and the Leckee Q uartzitic Formation southeast of Foxford. This granodiorite margin is from sample 73-388o which lies on a 5oo 4- t8Ma isochron (Max et al. I976). For the Ox Mountains Granodiorite location (x345- 3o35), the minerals defining $2 in part augen and 'penetrate' the foliated granodiorite; there would appear to be a petrofabric equilibrium between the sediments and the intrusive rocks. The heavy dashed line marks the granodiorite's contact. Igneous feldspars are stippled. Quartz is unshaded. Phyllosilicates follow the quartz fabric directions.

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Appiniti¢ Suite. Probably post-D1 and certainly pre-D 3 appinite, granite and minor lamprophyre bodies of a dioritic series occur as lenses up to 4"5 km long mainly in a o.8 km wide zone south of the Ox Mountains Granodiorite (Fig. I). There has been either a sequence of intrusions with increasing acidity, or reaction of basic magma with younger permeating acid fluids. Late acid veins have locally agmatized the appinites which are locally podded (e.g. I2795 3o177). Coarse- grained hornblendites with I cm stumpy green hornblendes packed with abundant, regularly arranged iron-ore granules occur as patches within poikilitic amphibole. Pyroxene is generally absent, but the largest appinite (e.g. i 17o5 29488) locally contains augite, which when enclosed by plagioclase is mantled by uralite, hypersthene, green hornblende, plagioclase (andesineulabradorite) and acces- sories. Dioritic hornblendite, diorite and quartz diorite occur with complex veining and back-veining relationships in the appinites. Some dioritic types are associated with reaction with granodiorite (e.g. 12800 3o 18o). Granodiorite often intrudes the appinites and metasediments as discrete bodies. Pegmatite, strongly schistose lamprophyre (kersantite) with garnet, and minor granitic veins also occur. Within metasediments in the appinitic zone are patches of hornfelsed psammite largely devoid of sedimentary textures and psammite which locally has been intimately pervaded by granodiorite (Long 1975). The appinites are approximately coeval with the Ox Mountains Granodiorite because they have the same structural relationship to the country rock as the granodiorite and pods of appinite within the granodiorite (e.g. 11688 29750 ) suggest a co-magmatic relationship not dissimilar from that of the granites and appinites in Donegal (cf. French 1966 ). Serpentinite. Serpentinite pods derived from basic and ultrabasic rocks occur along the southern part of the Knockaskibbole Fault, and east of Lough Darrdun. Fibrous chrysotile is the major constituent, with antigorite, dolomite, serpophite, sericite, iron-ore, fragmented chrome-spinel and a little quartz. Serpentinite bodies mainly occur along or south of the Fair Head-Clew Bay Line in Ireland (Max & Riddihough 1975, Lemon i966 , Phillips et al. 1975). Amphibolite. One exposure east of Lough Darrdun resembles massive parts of the Raheen Barr Volcanic Member, but its origin is uncertain. It also resembles intrusive amphibolite on the south side of Clew Bay and it may be related to the adjacent serpentinite. Feldspar Porphyry. East of Raheen Barr Hill (i I 196 29455) are three small pods of saussuritized feldspar porphyry of uncertain age. Olivine Dolerite. Minor intrusions of olivine dolerite in the Cappagh Formation (ii93o 29539 , ii537 29347 ) are probably related to other (Tertiary) olivine dolerite intrusions in the nearby Carboniferous rocks.

4. Structural relationships

(A) LOCAL The contacts between the three main tectono-sedimentary units (Figs. I & 6) are of the greatest importance because determination of their relationships has an

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important bearing on regional geological interpretation. The contact between the Ox Mountains succession and the Raheen Barr succession is locally either a high- angle fault or a low-angle post-metamorphic thrust. The thrust is seen east and south of Sheeans Hill (Fig. 6b). South of Laugh Brohly, brecciation and locally developed mylonites within and adjacent to the Ballycong Unit suggest a fault contact and possible thrusting (Fig. 6a). In the Raheen Barr area the contact is probably a high-angle fault (Fig 6c). In general the Raheen Barr succession, representing the tectonic unit of metamorphic rocks to the northwest, appears to have been thrust to the south and southeast over the Ox Mountains succession. The contact post-dates the development of all metamorphic fabrics of both successions, and no age relationship between the fabrics of both successions can be proposed on structural grounds. The contact between the Ox Mountains succession and the Ardvarney Forma- tion is the Knockasldbbole Fault to the east and the Laugh Rusheen Fault with an associated broad zone of probably D 3 cataclasis, to the north and west. The Raheen Barr succession is nowhere seen in contact with the Ardvarney Formation. The northern and southern contacts of the synkinematic Ox Mountains Granodiorite are both sheeted intrusive contacts, with sheeting occurring as far as 3 km from the contact (e.g. NE of Castlebar) ; the zone of sheeting is normally narrower, however, especially along the northern contact. The granodiorite and its many marginal sheets post-date isoclinal to tight F2 folds in the metasediments but are themselves schistose and foliated by $2 and closely folded coaxially by late stage D2 stress (Fig. 5c). Development of the main (first) foliation in the granodiorite is contemporaneous with the latest stages of $2 and F2 development

,w A SE NW B SEJ main North Ox Mtns. i i i :.ll!//.(./ "./ c ..!:.:;.~:;i ...... (~(~

...... :.:.:.:.::.:..... I .-:.:-:.:v.v.." I ...... '.'.'.'.'.'.'l.'.Z'.'" ' ...... ~!:.ii!!.iii~!:~.....:.:.:.:.'.'" Granodiorito sheets I ,.:..:..::.;:..:::.::..~,.:...:::.:..:::...::.:..:,..:...*...... " are common near contact

...-.:.:::!:~.:~-~:ii:/~,:t . . ii i

N* C SE DEVONIAN / CARBONIFEROUS Rahesn Barr Hill ""

~:'i~'-::..:.... ARDVARNEY Formation

RAHEEN BARR succession r ...... ii ".'.'.'.'." ...... III OX MOUNTAINS succession °.... .oo

F I o. 6. Diagrammatic cross sections. See Fig. x for true scale. A. Across the main north Ox Mountains Fault (also see Fig. 2B). B. Across Sheeans Hill (also see Fig. 2A) C. Across the Ardvarney Formation and Raheen Barr Hill.

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in the metasediments. This suggests that only S I and perhaps the earlier stages in the $2 imprint predate 500 + 18 Ma (Max et al. 1976b ). As there appears to have been only a single main heat pulse (Fig. 3) during regional metamorphism, the development of S I and $2 was almost certainly during the Caledonian orogeny. The S 3 crenulation cleavage passes into and through the granodiorite. The envelope rocks were undergoing deformation at or close to amphibolite- facies conditions throughout most of the area during emplacement of the Ox Mountains Granodiorite (Phillips et al. 1975, p. 320, table 11; Long 1975). This fact is central to our proposal that Caledonian rocks, all probably Dalradian but with different tectono-thermal histories, have been juxtaposed fairly late in the tectono-thermal history. In none of the three tectonic units have we found evidence for deformation or metamorphism older than that which we regard as Caledonian.

(B) REGIONAL The North Ox Mountains Fault, which passes westwards into the Newport Fault and then probably into the Achill Beg Fault (Trendall & Elwell 1963, Crow et aI. i974) , marks the trace of an important structural line in Co. Mayo. Although much of the movement on the North Ox Mountains Fault is post-Carboniferous, slump conglomerates and a complex sedimentary history locally in the Carboni- ferous rocks indicate some seismic activity along the fault during Upper Palaeozoic times. Throw on the fault in pre-Upper Palaeozoic times was in the opposite sense to its movement since Devonian times; metamorphic rocks in Achil] Beg and the Devonian rocks in the eastern end of the Clew Bay Graben are thrown down to the south. Upper Palaeozoic movements renewed displacement along this line. The Knockaskibbole and other arcuate faults of this set which form the western boundary faults to the inlier are all splays of the North Ox Mountains Fault (Fig. I). Other similar arcuate faults in northwest Murrisk and the major E-W faults on Clare Island (Cole et =I. 1914, Phillips I973) can all be related to the splay set on the south side of Clew Bay or to E-W faults within the Clew Bay graben. Clare Island must be regarded as lying entirely within the Clew Bay graben area bounded to the north by the Achill Beg-Newport-North Ox Mountains Fault, and the south and east by major fault splays which converge eastwards in the North Ox Mountains Fault. In this area the North Ox Mountains Fault is coincident with the geophysical Fair Head-Clew Bay Line (Max & Riddihough I975) which may reflect a major early structure. This structural line separates the coherent tectono-thermal Caledonian province of north Co. Mayo from the area of more complex relationships between different structural levels to its south. South of the line are metamorphosed Dalradian rocks in Connemara, and probably in the Ox Mountains succession, and weakly deformed and essentiallyunmetamorphosed Cambrian and probable Upper Dalradian rocks. The main age of deformation within the orogen to the south of this line can be regarded as pre-Arenig or Tremadoc when the evidence of the Lower Palaeozoic rocks in Murrisk and the Dalradian in Conne- mara is invoked (Max et aL I976a ). The age of the deformation to the north of the line is not directly known but by comparison with the evidence from Scotland may

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be post-Arenig in age. Metamorphic rocks common to the northern side of the Fair Head-Clew Bay Line occur in two main localities in this area to the south of this trace along the extreme NW margin of the Ox Mountains Inlier, and there is a tectonic contact. The Sheeans Thrust dips at a low angle to the northwest. The essential nature of the juxtaposition of the two main tectono-sedimentary units is one of overthrusting of the Raheen Barr succession. Recognition of the tectonic contact between the Raheen Barr succession, Nephin Beg succession and the Ox Mountains succession at isolated localities along the NW margin of the Ox Mountains Inlier supports the interpretation of Max & Riddihough (1975) concerning the importance of the Fair Head-Clew Bay Line and its possible reflection of large-scale structural compartmentation within the metamorphic Caledonides.

5. Conclusions In terms of structure and metamorphism the metasedimentary rocks of the area fall into three main units, each of which displays a characteristic range of structures and metamorphic minerals. They range from greenschist to amphibolite facies in grade. Stratigraphically, the rocks of the three tectonic units can be regarded as Appin (Lower) to Southern Highlands (Upper) Dalradian equivalents, although a complete and uninterrupted sequence is absent. It is most likely that the Ardvarney Formation is equivalent to the Southern Highlands Group and that the Raheen BaIT and Ox Mountains successions are equivalent to Blair Atholl (in part), Islay and Easdale subgroups. Large-scale movements within the orogen are indicated.

6. Discussion Currall's (i963) succession adopted by Harris & Pitcher (i975, fig. I2) is regarded by us, on the evidence of grading in pebble beds in the Glenisland Formation, to be inverted, and the downward-facing Raheen BaIT Synform has now been recognized. The tectonic relationship of the Ardvarney and Cappagh formations has been identified and the Ox Mountains succession has been identified exten- sively to the west of the Knockaskibbole Fault. This fault is a fairly straight, single high-angle fault (cf. Currall i956 ) and is neither 'paired thrusts' (Currall i963) nor an unconformity (Geikie i893 , Kilroe I9o7). We consider that the main erenulation fabric in the metasediments and igneous rocks relates to a regional D 3 event and not D 4 (Phillips et al. 1975) and implies a locally developed, compound $2. We find no need, regionally, to separate a late D2 tightening from the early 132. In regarding the Ox Mountains succession as Dalradian (Long 1974, t 975, Max et al. x976b), this work conflicts with the views of Johnstone (x975), Phillips et al. (1975) and Harris & Pitcher (I975) , but agrees with Taylor (t966, I968 ). The formations of the Ox Mountains succession form a coherent tectono- sedimentary succession. The unity of this group has been questioned by Phillips et al. (i975, p. 321) and the D2 Lough Talt Slide (DI of Taylor 1969) was held to

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break the succession between the Leckee Q.uartzitic Formation and the Ummoon Formation, though further to the east the slide is supposed to lie within the Leckee Q.uartzitic Formation. Phillips et al. (1975) show that the Ummoon Formation is cut out by the Lough Talt Slide west of Foxford. We find, however, no direct evidence which suggests that any significant portion of the succession has been slid out tectonically as both the Newantrim and Meelick members vary little in their distance from the contact throughout the area and the structural and metamorphic histories above and below their proposed slide are the same. The interpretation of the diopside-microcline mineralogy of the Kilmore Limestone by Phillips et al. (1975, p. 321) as high amphibolite or granulite facies, and the suggestion that the Lough Talt Slide separates this assemblage from a low amphibolite facies assemblage in the Cappagh Formation is difficult to support as neither diopside nor microcline are diagnostic (cf. Turner & Verhoogen 196o ). There is also no direct evidence that the Ardvarney Formation was originally unconformable on the Ox Mountains succession as suggested by Phillips et al. (1975). We find the granodiorite northern margin to be a sheeted intrusive contact and structurally the same as the western and southern margins. Taylor (1966 , p. 133 ) regarded emplacement of the granodiorite as largely post-metamorphic (i.e. post- D2, pre-D3). This was also suggested by Currall& Taylor (i965), whilst Phillips et al. (1975), with reservations, suggest it to be late MP2, pre-D 3. The granodiorite is regarded by us as syn-to late-kinematic (i.e. as early/late D2). Pankhurst et al. (1976) conclude, as we do, that the granodiorite was emplaced into country rocks which were at amphibolite facies, and their Rb/Sr whole-rock isochron of 487 + 6 Ma (or 494 + 9 Ma for specimens I- 4 only) accords fairly well with that of5oo + 18 Ma by the same method (Max et al. i976b ). Relatively late tectonic juxtaposing of the Ardvarney and Cloonygowan formations with the Ox Mountains succession in the southwestern part of the inlier and the marked lithostratigraphic similarities of the Ox Mountains succession to known Dalradian rocks are, however, rejected by Pankhurst et al. (1976) partly because significance is given, by them, to a possible 70o Ma step-heating Ar39/Ar *° age (Phillips et al. 1975) for a metabasite in the northeastern part of the inlier, well outside this area. This may possibly have little bearing on the age of the granodiorite and envelope rocks because it may pertain to a completely different structural unit (cf. Dunning & Max 1975, Phillips et al. 1975). We favour a more simple history ofjuxtaposifion. It is clear regionally that with increasing distance from the NW margin of the Caledonian orogen, and by reference to the significance of the Highland Boundary Fault line (Max & Riddihough 1975) , there appears to have been structural compartmentation and large-scale movement which juxtaposed different tectono- thermal sub-units within the orogen. This juxtaposition would appear to have involved the southward movement of the Donegal/SW Highlands unit, as inferred by Pitcher & Berger (1972), over the Ox Mountains succession at least as far as the Ardvarney Formation.

ACKNOWI~DO~.MEN'I~This paper is published by permission of the Director of the Geological Survey of Ireland.

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Received 3oth April t976; revised typescript received 6 October I976.

CHARLES BARRY LONG, Geological Survey of Ireland, MICHAEL DAVID MAX, 14 Hume Street, Dublin 2, Ireland.

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