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Journal of the Geological Society

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U-Pb zircon ages of basic rocks and gneisses intruded into the Dalradian rocks of Cashel, Connemara, western Ireland

¹ Deparment of Geology, University of Glasgow, Glasgow G12 8QQ,, U.K. (now at Exploration & Production Division, Shell International Maatschappij B. V., Carel van Bylandtlaan 30, The Hague, The Netherlands)
² Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland (now at Accoustics Division, Naval Research Laboratory, Washington, DC 20375, U.S.A.)
³ Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, UK
⁴ Department of Geology, University of Glasgow, Glasgow G12 8QQ,, UK

New isotopic ages are presented from the syntectonic Cashel-Lough Wheelaun basic intrusion and an associated K-feldspar granite gneiss of the Connemara gneiss complex from Lettershinna Hill. The linear, normally discordant distribution of data-points from the Cashel-Lough Wheelaun metagabbro indicates that the zircons in this intrusion have no significant inherited radiogenic Pb. They yield an upper concordia intercept age of 477⁺²⁵_–6 Ma, and a more precise ²⁰⁷Pb/²⁰⁶Pb age of 490 ± 1 Ma. In contrast, zircons from the K-feldspar granite gneiss show distinct reverse discordance. The non-magnetic fractions show a good linear arrangement yielding a lower intercept age of 454⁺⁶ _–14Ma and a very uncertain upper intercept age of 1807⁺⁶⁴⁶_–570Ma. The lower intercept age is younger than the 477 ± 9 Ma K-Ar age of hornblendes in gneisses from Lettershinna Hill (Elias et al. 1988) and therefore reflects a Pb loss episode younger than igneous crystallization. The imprecise Proterozoic upper intercept age indicates contamination by older inherited and only partially reset zircons.

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