Rb‐Sr isochron study of the Thorr and Main Donegal Granites, Ireland

1 October 1982

journal article
research article
Published by Wiley in Geological Journal

Vol. 17 (4) , 279-295
https://doi.org/10.1002/gj.3350170403

Abstract

A Rb‐Sr whole‐rock isochron study indicates that the entire Donegal granite suite was emplaced into orthotectonic Caledonian (Dalradian) rocks over a short interval during mid‐Silurian to earliest‐Devonian times. The Thorr pluton, probably the earliest member of the suite, yields an age of 418 ± 26 Myr and initial⁸⁷Sr/⁸⁶Sr ratio of 0·7055 ± 4, while the latest member, the Main Donegal pluton has an age of 407 ± 23 Myr and initial⁸⁷Sr/⁸⁶Sr ratio of 0·7063 ± 5 (Λ⁸⁷Rb = 1·42 ± 10⁻¹¹yr⁻¹). Errors on both the age and initial Sr isotope ratios incorporate botha prioriand geological scatter components and are quoted at the 2‐sigma level. The low and restricted range of initial Sr isotope ratios suggests small but significant differences in the composition of the parental granitic magmas which were derived from a low Rb/Sr, low⁸⁷Sr/⁸⁶Sr source.

Keywords

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