Evolution of fluid from the ultrahigh temperature lower crust to shallower levels: Constraints from silicate–oxide–sulphide–sulphate assemblages of mafic granulites of the Eastern Ghats Belt, India

Abstract

Mafic granulites from key localities of the Eastern Ghats Province preserve Fe-Ti oxides, Cu-Fe sulphides and traces of sulphate minerals along with silicate phases. Two different varieties of mafic granulite exhibit slightly contrasting mineral assemblages. While the massive type of mafic granulite contains minerals assemblage orthopyroxene + clinopyroxene + plagioclase + magnetite + ilmenite + pyrite + pyrrhotite, the migmatitic variety contains garnet as an additional phase. Both oxide and sulphide minerals show contrasting textural characters. Textural analysis and construed mineral reactions imply that the variation of oxide-silicate, oxide-sulphide and sulphate relations is linked to variation of f O 2 during the pre-peak, peak and post-peak stages of metamorphism. The calculated f O 2 values range up to +4 log units relative to the QFM (quartz-fayalite-magnetite) buffer among the samples, except for one sample which shows lower values (−10 log units relative to the FMQ (fayalite-magnetite-quartz) buffer). The consistently high f O 2 condition at the lower crust could result from several factors, but the role of the externally derived fluid appears to be plausible. Hot brine solution with CaCl 2 species can explain the oxidation as well as local metasomatism of the mafic lower crust even though its presence is not verified from direct characterisation like fluid inclusion analysis.