A SULFUR 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 delta-S-34 in conjunction with delta-O-18, initial Sr-87/Sr-86 (I(Sr)) and age; Group I have delta-O-18 generally < + 10.5 parts per thousand, I(Sr) in the range 0.703 to 0.708, and are younger than 435 Ma. delta-S-34 of this group ranges from -4.5 parts per thousand to +4.4 parts per thousand: xBAR = +0.7 +/- 2.6 parts per thousand (n = 40:1 sigma). Group II have delta-O-18 generally > + 10.5 parts per thousand, are generally older, I(Sr) between 0.710 and 0.7208, with delta-S-34 in the range +6.2 parts per thousand to +16 parts per thousand: xBAR = +9.5 +/- 3.3 parts per thousand (n = 13:1 sigma). 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 delta-S-34 values, between +11 parts per thousand and + 17 parts per thousand, we suggest that group II granites contain a significant proportion of sulphur derived from these rocks.