Abstract
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 207 Pb/ 206 Pb 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 −14 Ma and a very uncertain upper intercept age of 1807 +646 −570 Ma. 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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