The geochronology of the Connemara granites and its bearing on the antiquity of the Dalradian Series

Abstract

Thirty-eight whole-rock samples of Connemara granites give the following Rb–Sr whole-rock ages: Galway granite, 384 ± 1 m.y.; Omey granite, 3884 ± 17m.y.; Roundstone granite, 3954 ± 80m.y.; Inish granite, 404 ± 8m.y.; Oughterard granite, 510 ± 35 m.y.; potash-feldspar gneisses, 725 ± 175 m.y. As the field evidence shows that the Oughterard granite was intruded into the Dalradian metasediments post- F 4 in the Connemara fold-sequence, and possibly post- F 5 , this dates the fold phases F 1 to F 4 and the metamorphisms M 5 to M 1 as pre-510 ± 35m.y. The older isotopic age of the potash-feldspar gneisses agrees with their field-deduced age of post- F 2 pre- F 3 . Muscovite, potash feldspar, and whole-rock Rb–Sr analyses show that the Oughterard granite was metamorphosed about 444 ± 7 m.y. ago, which is close to the K–Ar age obtained from three hornblendes separated from potash-feldspar gneiss. These results indicate that the post-500 m.y. dates obtained from the Connemara Dalradian rocks, and therefore presumably the Scottish Dalradian also, are the dates of late metamorphisms. While the possibility of Pre-Cambrian metamorphism and folding in the Dalradian is apparent, the results are equally in agreement with a Cambrian age for the main metamorphisms M 1 to M 5 and foldings F 1 to F 4 , especially in view of the present uncertainty about the age of the Cambrian period. The base of the Arenig would seem to be younger than 510 ± 35m.y.