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

Publisher Website
Google Scholar
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.