The Caledonian fold belt is thought to have formed as a result of the closure of the 'Iapetus' ocean basin during Lower Palaeozoic times. Granitic magmas were intruded into Precambrian and Lower Palaeozoic country rocks between approximately 640 and 390 Ma. Most important, spatially and volumetrically, are the late Caledonian granites, the 'Newer' and 'Last' granites of Read (1961). In general, the late Caledonian granites have the calc-alkaline characteristics typical of plutonic rocks emplaced at destructive plate margins. They range in composition from diorite and tonalite, through granodiorite, to peraluminous granite with chemical variation continuous among the various rock types. Petrographically and chemically, the granitoids span the range from S 'sedimentary' to I 'igneous' types as defined by Chappell and White (1974) with no discernible geographical pattern to their distribution. There have been few sulphur isotope studies on primary sulphides in granites; yet such data have considerable potential in understanding the petrogenesis; that is in terms of granites derived either from igneous or metasedimentary protoliths. From about 50 British and Irish Caledonide granites, including both 'Newer' and 'Older' intrustions, conventional separation techniques have yielded sulphide separates from only 19 granites for delta34S analyses. Mineralogically, the sulphides consist of mainly pyrite and pyrrhotite, although chalco-pyrite is present in a few intrusions. It was noticeable that granites with high delta18O and high initial 87Sr/86Sr yielded very little sulphides. S analyses proved most informative when compared with the following: mol Al2O3/(CaO + Na2O + K2O) , K/Na, Fe2O3/FeO ratios, initial 87Sr/86Sr and delta18O. Granites with high to intermediate delta18O (11 to 10%) and high initial 87Sr/86Sr (> 0.709) are accompanied by g 34S from +5.0 to +16.0%. and are indicative of crustally derived granites (S-types). These are mainly 'Older' Caledonian granites (e.g. the Oughterard granite and the Cashel microgranite sill in Connemara, Ireland; the Aberdeen granite in Scotland). Intermediate to low delta18O (10 to 6%.) and low initial 87Sr/ 86Sr (0.703 to 0.708) correspond to delta34S values between -4.5%. and +4.4%. indicative of granites derived from the mantle or lower crust; these intrusions are mainly 'Newer' Caledonian granites. delta34S analyses also confirm the diverse protolithic derivation of zoned granites; a good example being the Strontian granite in the northwest Highlands of Scotland with a variety delta34S value of -0.1%. and, therefore, considered to be derived from a basic igneous protolith, and a more leucocratic member showing a spread delta34S values between +5.5 and +8.4%. which is considered to be derived from a crustal component. Sulphur isotope analyses of coexisting sulphide minerals are also useful as a geothermometer as long as the sulphide-bearing phases are in equilibrium. The putative temperature of formation of two coexisting mineral pairs from the Oughterard granite has been calculated; 41
