A sulphur isotope study on selected Caledonian granites of Britain and Ireland

A sulphur isotope study was carried out to assess the value of such isotopic data in constraining the contributions of igneous and sedimentary protoliths for British and Irish Caledonian granites. Conventional separation techniques yielded sulphides from only 19 of the 50 selected granites, including both Newer and Older intrusions. These sulphides are dominated by pyrite, with lesser pyrrhotite, and rare chalcopyrite. Two groups emerge from consideration of δ34S in conjunction with δ18O, initial 87Sr/86Sr (ISr) and age; Group I have δ18O generally < + 10·5‰, ISr in the range 0·703 to 0·708, and are younger than 435 Ma. δ34S of this group ranges from −4·5‰ to +4·4‰ x̄ = +0·7 ± 2·6‰ (n=40:1σ). Group II have δ18O generally > + 10·5‰, are generally older, ISr between 0·710 and 0·7208, with δ34S in the range +6·2‰ to +16‰: x̄ = +9·5 ± 3·3% (n = 13:1σ). The source region for group II granites contained a significant sedimentary component. Since sulphides in Lower and Middle Dalradian metasediments are characterized by unusually high δ34S values, between +11‰ and +17‰. we suggest that group II granites contain a significant proportion of sulphur derived from these rocks