Zircon U-Pb ages from the Galway granite and the Dalradian, Connemara, Ireland

Synopsis The zircon U-Pb age of 510±10 m.y. of the Cashel-Lough Wheelaun basic intrusion establishes a point early in the time span of the Dalradian metamorphism in Connemara; the intrusion is post F 1 and late M 1 but pre-F 2 . This is also a maximum age for the base of the Lower Llanvirn and can be linked with Rb-Sr results if the 5.0 × 10 10 y Rb 87 half-life is used. The metasedimentary zircon results are compatible with an isotopic disturbance(s) between 400 and 510 m.y. and suggest that a large proportion of the source rocks were older than 1200 m.y. The U-Pb isotopic relationships of the metasedimentary zircons have similar features to those of cogenetic zircon suites.