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Rb‐Sr whole‐rock isochron studies of the Barnesmore and Fanad plutons, Donegal, Ireland
Author(s) -
O'Connor P. J.,
Long C. B.,
Evans J. A.
Publication year - 1987
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350220103
Subject(s) - pluton , isochron , geology , geochemistry , petrogenesis , devonian , magma , batholith , radiogenic nuclide , isochron dating , petrology , mantle (geology) , tectonics , paleontology , volcano
Results of Rb‐Sr whole‐rock isochron studies of the Barnesmore and Fanad plutons of Co. Donegal are presented. The Barnesmore pluton yields an age of 397 ± 7 My and initial 87 Sr/ 86 Sr ratio of 0·7063 ± 0·0005 (MSWD 10·2) while the Fanad pluton yields an age of 402 ± 10 My and initial 87 Sr/ 86 Sr ratio of 0·7050 ± 0·0001 (MSWD 1·6). These studies supplement recent Rb‐Sr dating of the Donegal granitoid batholith by Halliday et al. (1980) and O'Connor et al. (1982), and confirm that the entire suite was emplaced during a short interval spanning mid‐Silurian to earliest Devonian time. The relatively low and restricted range of initial Sr isotope ratios suggests small, but significant, differences in the composition of the source region of these ‘Caldedonian I‐type’ magmas. The Rb‐Sr data indicate that contamination processes have not played a significant role in the petrogenesis of the Fanad rocks and their intermediate character and high Sr content are considered primary features of the parent dioritic magma. At Barnesmore, late stage devolatilization of evolved granitic magmas has resulted in the development of desilicated zones within the pluton: the granite of such zones may exhibit elevated 87 Sr/ 86 Sr ratios.