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

Leggo, P. J., Compston, W. and Leake, Bernard Elgey 1966. The geochronology of the Connemara granites and its bearing on the antiquity of the Dalradian Series. Quarterly Journal of the Geological Society 122 (1-4) , pp. 91-116. 10.1144/gsjgs.122.1.0091

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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-F4 in the Connemara fold-sequence, and possibly post-F5, this dates the fold phases F1 to F4 and the metamorphisms M5 to M1 as pre-510 ± 35m.y. The older isotopic age of the potash-feldspar gneisses agrees with their field-deduced age of post-F2pre-F3. 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 M1 to M5 and foldings F1 to F4, 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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Publisher: Geological Society of London
ISSN: 0370-291X
Last Modified: 04 Jun 2017 08:16

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