Journal of the Geological Society, London, Vol. 150, 1993, pp. 605-607, 0 figs Printed in Northern

Discussion on the Inishkea Division of northwest Mayo: Dalradian cover rather than pre-Caledonian basement (Ireland)

Journal, Vol. 149, 1992, pp. 167-170

M.D. Max and J.A. Winchester write: Kennedy & Menuge Division is thus very different from that of the younger have proposed that the Inishkea Division in the northwest Dalradian, including those stratigraphical horizons which part of the north metamorphic inlier is part Kennedy and Menuge regard as possible Dalradian. of the Dalradian Supergroup, containing beds as young as Geochemistry has also confirmed that, contrary to the Subgroup. We disagree strongly. assertion of Kennedy and Menuge, Inishkea Division Kennedy & Menuge followed the geological subdivision relicts occur at Spinkadoon, southwest of Head of the Caledonian rocks in the inlier proposed by Max (Winchester & Max 1987). (1970), Sutton (1971a, b), Crow et al. (1971), Crow & Max Nd isotope model ages (Menuge & Daly 1990) show that (1976), Max & Long (1985), Winchester & Max (1988a, b) the Inishkea Division sediments could have been derived and Max et al. (1992), and the results of isotopic dating from the Annagh , or a similar source. There is little summarized by Aftalion & Max (1987) that show the long Nd modal distinction between the lower part of the pre-Caledonian tectonothermal history of the Annagh in north Mayo and the Inishkea Division. Division . Indication of similar sources does not necessarily imply the However, the relationship of the Inishkea Division, same age of sedimentation, as postulated by Menuge & linked with the Annagh Division as part of the Erris Daly. Complex (Winchester & Max 1988a, b), to the Dalradian is A poorly defined pre-Caledonian Rb-Sr age of less clear than the relationship between the Annagh 803 + 123 Ma from the least deformed of the Division and the Dalradian. The Inishkea Division does not Inishkea Division (Winchester & Max 1987) is in the same contain the complex extended early structural history and Precambrian range as the last U-Pb event recorded from the variety of deformed and metamorphosed igneous rocks Annagh Division (Aftalion & Max 1987). We do not regard displayed by the Annagh Division (Sutton & Max 1969). these Rb-Sr data as conclusive because of the imprecision Even the Annagh Division was then regarded by some as of the data points, but equally we do not think that it can containing no rocks of tectonothermal activity older than be simply disregarded. Caledonian (Phillips et al. 1969). Sutton & Max (1969), and Max & Long (1985) regarded Kennedy & Menuge comment that 'no unambiguous all contacts of the Inishkea Division as tectonic, both with sedimentary structures have been recorded from any rocks the older Annagh Division and the structurally higher assigned to the Inishkea Division'. During the course of Dalradian. The strongly deformed Scotchport Schist unit of field mapping in Mayo (Max et al. 1992), graded bedding, the Inishkea Division displays a nearly uniform slump conglomerates, heavy mineral beds and bottom deformational style across its entire width, and shows compaction sedimentary structures have been recognized, pervasively deformed mylonitic textures. At the Dooninanir and are especially clearly seen on the Inishkea Islands contact with the Doonamo Formation, high-strain zone where only slightly tectonized greywackes occur in areas up imbrication (as noted by Kennedy & Menuge), is associated to 250m thick between intense mylonite zones (Max & with small-scale folds of mylonitic banding in the Doonamo Long 1985; Winchester & Max 1987). On the mainland, psammites. The tectonized contact is associated with the relict sedimentary structures are rare and equivocal, but development of mylonitic fabrics and banding for over there is a commonly preserved lithological banding where seven metres into the psammites. At no point is a clear, there are few mylonitic textures. In the absence of evidence recognizable gradation seen. for wholesale element redistribution therefore, this Tectonism of Grampian Group sediments at the lithological banding is regarded as bedding. The chemistry Dooninanir contact is similar in every respect to the of the Inishkea Division is that of greywackes and distinct southern contact of the Annagh Division basement gneisses from that of the Annagh Division (Winchester & Max 1987, with Grampian Group psammites on north Gweesalia. At 1988a). the contact between the Inishkea Division and Grampian Results from a large programme of analyses of the Group rocks at Spanish Cellar on the northern Mullet, the Annagh Division, Inishkea Division and Grampian Group basal Grampian Group and Inishkea Division are rocks of Co. Mayo (Winchester & Max 1987, 1988a) show tectonically imbricated in a 3 m thick zone. In the less well that the chemistry of the Inishkea Division metagreywackes exposed inland areas, wherever the generally unexposed resembles only the lowest formations of the Grampian contact of the Inishkea Division is approached closely, Group, notably the Doonamo Formation and the lower strong planar tectonic fabrics are developed, as noted in at part of the Croaghaun Formation in West Achill (Max et al. least one place by Kennedy & Menuge. Early Inishkea 1992). The latter formation is chemically distinct from fabrics comprising two sets of tectonometamorphic formations equivalent to the Islay fabric-forming events coeval with greenschist facies in Co. Mayo, with which it has been correlated previously , were regarded as pre-Caledonian (Crow (Kennedy 1969). The chemical character of the Inishkea 1973; Max 1973; Crow & Max 1976). The question of the 605

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early fabrics in the Inishkea Division cannot be lightly Scotchport and there is no evidence for any dismissed by Kennedy and Menuge. pre-Caledonian fabric. On the mainland, we accept that at The presence of extra early fabrics may not be conclusive Kinrovar the unexposed contact of the Kinrovar Schist may evidence for pre-Caledonian tectonothermal activity, as be tectonic, but at Kildun there is a clear stratigraphic continental slope greywackes in certain tectonosedimentary contact with possible Appin Group psammites to the west. environments can develop multiple fabrics which closely The Kinrovar Schist underlies rocks of very uncertain simulate those formed at higher temperatures. Conversely, age, hence their tentative assignment to the Ballachulish, the complexity of Caledonian mylonite generation within Blair Atholl or Islay Sub-Groups (fig. 2, Kennedy & the strongly tectonized margins of the Kinrovar wedge may Menuge 1992). We do not state, as suggested by Max & have been underestimated, and all the early fabrics could Winchester, that there are necessarily rocks of Islay be Caledonian. Sub-Group age present in the Kinrovar Schist. However, we Except for the Kinrovar wedge in the southern part of would not rule out this possibility on the grounds of the area, the Inishkea Division maps out everywhere sediment geochemistry since, as Max & Winchester point structurally below the lowest formation of the Grampian out, 'indication of similar sources.., does not necessarily Group, which is chemically akin to and probably the local imply the same age of sedimentation' and nor does equivalent of the Corrieyairack Subgroup of the Dalradian indication of dissimilar sources imply different ages of Grampian Group of (Winchester & Max 1987, sedimentation. 1988b; Glover & Winchester 1989). Inishkea Division Regarding the other points made by Max & Winchester, schists were recognized by Trendall & Elwell (1963) as the we accept that there may be localities where clear lowest unit in the area, but they regarded them as a product sedimentary structures can be seen in Inishkea Division of Caledonian tectonothermal activity rather than as rocks. We repeat that there is no unambiguous evidence for basement, and did not recognize any tectonic contacts even pre-Dalradian structural or metamorphic events in these with the Annagh Division. rocks. The two geochronological constraints cited by Max & On the mainland the Kinrovar wedge is bounded by Winchester are not only imprecise but both may also be ductile slides and cuts across Grampian and Appin Group inaccurate. The 803 + 123 Ma Rb-Sr isochron for Inishkea Dalradian (Max et al. 1992). These slides and the Grampian Division greywackes has an MSWD of 57 (Winchester & Group rocks bounding the Annagh Division on the Max 1987) and this date may reflect provenance and mainland (Max 1970; Max et al. 1992) are omitted without metamorphic resetting as well as, or instead of, sedimenta- explanation from the generalized map shown by Menuge & tion/diagenesis. The c. 800Ma ages derived from the Daly. Hence the Kinrovar schists in the southern part of the Annagh Gneisses are U-Pb zircon lower intercepts obtained inlier only have structural relationships with the Dalradian from bulk fractions of unabraded zircons (Aftalion & Max succession, and do not possess the stratigraphical 1987) and may reflect continuous, rather than episodic, lead significance suggested by Kennedy & Menuge. loss such that the apparent ages are meaningless. Hence we There is no incontrovertible evidence that the Inishkea regard these isotopic data as offering no worthwhile age Division was deposited and deformed prior to the constraints in the present context. deposition of the Grampian Group. Geochemical evidence We reiterate our conclusion that all rocks previously shows that the Inishkea Division resembles only the lowest assigned to the lnishkea Division are part of the Dalradian Grampian Group formations in northwest Mayo. It has no Supergroup. It follows that the terms Inishkea Division and chemical comparability with, and is quite clearly not Erris Complex should be abandoned and the Annagh equivalent to any formation in the Appin or Argyll Groups. Division renamed the Annagh Gneiss Complex (Kennedy & Even if the Inishkea Division turns out to be part of the Menuge 1992; Menuge & Daly in press). lowest Grampian Group unit in the northwest Mayo inlier, and was not affected by a major pre-Caledonian 31 July 1992 tectonothermal event, it forms a coherent body of greywackes that structurally and stratigraphically underlies References the lowest unequivocal Grampian Group rocks in the inlier. AFTALION, M. & MAX, M. D. 1987. U-Pb zircon geochronology of the Precambrian Annagh Division gneisses and the Termon Granite, NW 3 July 1992 County Mayo, Ireland. Journal of the Geological Society, London, 144, 401-406. CROW, M. J. 1973. Geology of Metamorphic Rocks in part of north-west County Mayo, Ireland. PhD Thesis, Trinity College, Dublin. MJ. Kennedy and J.F. Menuge reply: Max & Winchester -- & MAX, M. D. 1976. The Kinrovar Schist. Scientific Proceedings of the disagree with our main conclusion (Kennedy & Menuge Royal Dublin Society, 5, 429-441. 1992) that the rocks previously assigned to the Inishkea & Suxa'ON, J. S. 1971. Structure and stratigraphy of the Division of the pre-Dalradian Erris Complex are in fact part metamorphic rocks in part of northwest County Mayo, Ireland. Journal of the Geological Society, London, 127, 579-585. of the Dalradian. They claim that all contacts between GLOVER, B. W. & WINCHESTER, J. A. 1989. The Grampian Group: a major Inishkea Division rocks and undoubted Dalradian rocks are Late Proterozoic elastic sequence in the Central Highlands of Scotland. structural. However, as previously stated, we have observed Journal of the Geological Society, London, 146, 85-96. that two such contacts are stratigraphical (Kennedy & KENNEDY, M. J. 1969. The structure and stratigraphy of the Dalradian rocks of north , County Mayo, Ireland. Journal of the Geological Menuge 1992). On the at Doonaneanir Society, London, 125, 47-81. (between Scotchport and Spinkadoon) there is a gradational -- & MENUGE, J. F. 1992. The Inishkea Division of northwest Mayo: change over a few metres from semipelites of the Scotchport Dalradian cover rather than pre-Caledonian basement. Journal of the Schist northwards into psammites of the Doonamo Geological Society, London, 149, 167-170. Formation. The deformation style and intensity in this MAX, M. D. 1970. Stratigraphy of some metasediments in part of northwest County Mayo, Ireland. Scientific Proceedings of the Royal Dublin region is similar to that seen elsewhere within the Society, 29A, 303-317.

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-- 1973. Caledonian metamorphism in part of northwest County Mayo, County Mayo, Ireland. Scientific Proceedings Royal Dublin Society, 4A, Ireland. Geological Journal, 8, 375-386. 121-136.

-- & LONG, C. B. 1985. Pie-Caledonian basement in Ireland and its cover -- 1971b. The stratigraphy and structure of the Moinian and Dalradian relationships. Geological Journal, 20, 341-366. metasediments of the Mullet Peninsula, Co. Mayo, Ireland. Scientific ~, -- & MACDERMOT, C. V. 1992. The Bedrock Geology of North Proceedings Royal Dublin Society, 4A, 1-13.

Mayo. (1 : 100,000) Geological Survey of Ireland. 1 sheet. -- & MAX, M. D. 1969. Gneisses in the north-western part of Co. Mayo, MENUGE, J. F. & DALY, J. S. 1990. Proterozoic evolution of the Erris Ireland. Geological Magazine, 106, 284-290. Complex, Northwest Mayo, Ireland: neodymium isotope evidence. In: TRENDALL, A. F. & ELWELL, R. W. D. 1963. The metamorphic rocks of GOWER, C. F., RIVERS, T. & RYAN, B. (eds) Mid-Proterozoic north-west Mayo. Proceedings of the Royal Irish Academy, 62B, Laurentia-Baltica. Geological Association of Canada Special Paper 38, 217-247. 41-51. WINCHFSTER, J. A. & MAX, M. D. 1987. The pie-Caledonian Inishkea

-- & -- The Annagh Gneiss Complex. In: HARRIS, A. L. & GiaaONS, F. Division of NW Co. Mayo, Ireland; its geochemistry and probable A. (eds) A Correlation of Precambrian Rocks in the British Isles, 2nd stratigraphic position. Geological Journal, 22, 309-331.

edition. Geological Society, London, Special Report (in press). -- & -- 1988a. 12. Pre-Dalradian rocks in NW Ireland. In: WINCHESTER, PHILLIPS, W. E. A., KENNEDY, M. J. & DUNLOP, G. M. 1969. Geologic J. A. (ed.) Late Proterozoic Stratigraphy of the Northern Atlantic comparison of western Ireland and northeastern Newfoundland. In: Regions. Blackie & Sons, Glasgow, 131-145. KAY, M. (ed.) North Atlantic Geology and Continental Drift. Memoir of -- & 1988b. 14. The Erris Group, Ireland.In: WINCHESTER, J. A. (ed.) Late the American Association of Petroleum Geologists, 12, 194-211. Proterozoic Stratigraphy of the Northern Atlantic Regions. Blackie & StrrroN, J. S. 1971a. The pre-Caledonian rocks of the Mullet Peninsula, Sons, Glasgow, 162-176.

M. D. MAX, SACLANT Underwater Research Centre, Viale San Bartolomeo, 400, 1-19138 La Spezia, Italy. J. A. WINCHESTER, Department of Geology, University of Keele, Keele, Staffordshire ST5 5BG, UK. M. J. KENNEDY & J. F. MENUGE, Department of Geology, University College, Belfield, Dublin 4, Ireland.

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