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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 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/5/413/4885498/gsjgs.133.5.0413.pdf by guest on 28 September 2021 4~4 C. B. Long ~ M. D. Max ! .- ~ .- --~ o.:.= . i~ Z z ~~ ;~-~ g o - = o= ~oo:.oo. =E o_ I/) u o u ~, g~ t/) -~ -- Z Z ,,- := .- "~ ~ :E ~ - (" .,-t =) v~ Z .< o uJ ~ Z = z o ~ -~ '-" ~ ~ z z z • ~ E~. o ._ : : ® ;® O~ ._-; :- .~u .~-~ .~ ~., . _e z ~ ~ ~-~,. e .~ .,s- o .-, -~ o~ ~.. ~. ... u ~ ~~ " ., z ~ -~ o o 3 .~ .~ U • z (/) ._~ .., .-- ~{ .., I,hl z z 0 :£ Z = _o X 0 Z 0 g~ A =ID ® =• 0 o.• -6 .= ~ 0 "- E Z Z "~ o "~ 0 o o • e z 8la.. 0 ~- uJ ~ ,, = ° 0 Z o z o Z o 0 ..-?. i-- u ,( g~ z a o z ~: < 0., z o 2 u. -- v') '- Z =E o Z z " =E u. 0 ...4 U • u. U d nO~lO llAO~I¥ dno~o Nldd¥ Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/5/413/4885498/gsjgs.133.5.0413.pdf by guest on 28 September 2021 Ox Mountains metamorphic rocks 415 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 pro- bably stratigraphic evidence (cf. Long I974, I975) to regard the metasedimen- tary 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/5/413/4885498/gsjgs.133.5.0413.pdf by guest on 28 September 2021 4x6 C. B. Long & M. D. Max 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 ortho- quartzite 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).
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