Rb-Sr whole-rock isochron studies of the Barnesmore and Fanad plutons, Donegal, Ireland

✍ Scribed by P. J. O'Connor; C. B. Long; J. A. Evans

• Publisher: John Wiley and Sons
• Year: 1987
• Tongue: English
• Weight: 750 KB
• Volume: 22
• Category: Article
• ISSN: 0072-1050
• DOI: 10.1002/gj.3350220103

✦ Synopsis

Results of R b S r 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 x7Sr/x6Sr ratio of 0.7063 * 04005 (MSWD 10.2) while the Fanad pluton yields an age of 402 t 10 My and initial X7Sr/ %r ratio of 0.7050 t O~OOO1 (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 R b S r 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 dedicated zones within the pluton: the granite of such zones may exhibit elevated *'SdR6Sr ratios.