Mineral-scale insights into the petrogenesis of the 3.30 Ga rhyolite in the Contendas-Mirante region, northern São Francisco Craton, Brazil : implications from results of plagioclase and biotite analyses.

Abstract

The 3.30 Ga high-silica volcanic system of the Gavião Block, São Francisco Craton, represents the remnants of within-plate magmatism related to an intracontinental rift. However, the petrogenetic processes that may have taken place in the relatively shallow primitive continental crust has not been fully constrained due to a scarce record. Petrographic and chemical analyses in biotite, as well as in-situ Sr isotope ratios in plagioclase, were used to trace petrogenetic processes and physicochemical conditions of the magmatic system. The subvolcanic rock has a well-preserved primary volcanic feature represented by magma flow textures, euhedral to subhedral plagioclases, rapakivi microstructures, and glomerocrysts. Plagioclase populations formed at two distinct stages recorded by trace elements and Sr isotope. Plagioclase phenocrysts and rapakivi phenocrysts have a slight enrichment of light rare earth elements (LREE), Sr/Ba ratio, and slight variation of Sr isotopes composition. Meanwhile, other phenocrysts and rapakivi crystals have low LREE, Sr/Ba, and a limited variation of Sr isotope ratio. Mineral chemistry evidence points to country rock assimilation during plagioclase formation and a crustal source for primary biotites under oxidized conditions.