Sulphur isotope systematics of granitoids and associated rocks from the Avila - La Alberca area (western Sistema Central, Spain)

Résumé

A reconnaissance sulphur isotopic study has been carried out on acid-soluble and non acid-soluble sulphides separated from late Hercynian granites and their host-rocks 'in the western area of the Sistema Central Español. Of the different rocks studied (amphibole-bearing biotite granites, biotite granites, cordierite-bearing biotite granites, nebulites and Complejo Esquisto Grauváquico host-rocks) only the Complejo Esquisto Grauváquico shales, the nebulites and the cordierite-bearing granites (and their enclaves) have yielded enough sulphur for analysis. Petrographic examination, in polished thin sections, of the samples analyzed identified the acid-soluble sulphide as pyrrhotite, and the non acid-soluble sulphide as pyrite, with minor contributions from chalcopyrite. Mass-balance corrected δ34S values range between -3.9 and +4.8‰ (average: -0.8 ±3.8; 1σ, n=4) for cordierite-bearing granites. The nebulites with which these granites are sometimes in gradual contact yielded exclusively positive values between δ34S = +1.1 and +10.6‰ (av.: +4.1 ±3.4; lσ, n=6). The Complejo Esquisto Grauváquico shale-type rocks gave both positive and negative δ34S values of + 7‰ and around -5‰. Intermediate to basic enclaves gave δ34S values mainly close to O‰, except for one anomalous acid-soluble sulphide in a quartzdioritic enclave that has a value of δ34S = + 13.4‰. Most of the acid-soluble 1 non acid-soluble pairs follow a straight line, whose slope approaches 1, in a δ34Sacid-soluble VS δ34Snon acid-soluble plot, and this is interpreted to indicate reequilibration at high T; therefore an origin for the sulphur in the granites is suggested to be external to the granite magma system, consistent with assimilation of country-rocks by the granite magma.