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New Rb–Sr and U–Pb ages for the Carnsore Granite and their bearing on the antiquity of the Rosslare Complex, southeastern Ireland

Published online by Cambridge University Press:  01 May 2009

P. J. O'Connor ,
P. S. Kennan  and
M. Aftalion
P. J. O'Connor
Affiliation:
Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland
P. S. Kennan
Affiliation:
Department of Geology, University College, Belfield, Dublin 4, Ireland
M. Aftalion
Affiliation:
Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, UK
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Abstract

A 12-point whole-rock Rb–Sr isochron for the Carnsore Granite of southeastern Ireland yields an age of 428±11 Ma and initial 87Sr/86Sr ratio of 0.7068±0.0003 (index of goodness of fit, MSWD = 0.65). A similar mean 207Pb/206Pb age of 432±3 Ma from three slightly discordant zircon size fractions from the granite is also indicated, and shows that the pluton was emplaced contemporaneously with the neighbouring Saltee Granite.

Thus, Tuskar Group rocks, which are intruded by the Carnsore Granite, could be as young as Ordovician. While the early history of the Rosslare Complex is undoubtedly Precambrian, the new age for the Carnsore pluton means that the later intrusive and tectonothermal events in the history of this complex are no longer constrained to be Precambrian and may have occurred over a 150 + Ma interval in Cambro-Ordovician time.

The zircons from the Carnsore Granite are slightly discordant, and show no apparent inherited radiogenic lead component. The initial 87Sr/86Sr ratio suggests that the magma may have been derived as a melt in the lower crust. The higher strontium isotope ratios of the Rosslare gneisses would seem to preclude their involvement in the genesis of the Carnsore magma.

Type
Articles
Information
Geological Magazine , Volume 125 , Issue 1 , January 1988 , pp. 25 - 29
Copyright
Copyright © Cambridge University Press 1988

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