Contrasting oxygen fugacity of I- and S-type granites from the Araçuaí orogen, SE Brazil: an approach based on opaque mineral assemblages

This study presents mineralogical characterization of opaque assemblages from I- and S-type granites from the Araçuaí orogen, southeastern Brazil that belong respectively, to the pre- and syn-collisional stages of the orogeny. Although these granites are geochemically well-characterized, with a robust geochemical, isotopic and geochronological database, their opaque minerals have not yet been investigated, and they provide important information about the oxygen fugacity and temperature conditions of their magmas. I-type granites (G1 Supersuite) consist of biotite hornblende granites and their opaque assemblage is ilmenite + pyrite + pyrrhotite ± magnetite ± Fe-Ti oxides ± chalcopyrite. S-type rocks (G2 Supersuite) are biotite muscovite sillimanite granites with ilmenite + graphite + pyrrhotite + pyrite as opaques. Our results combined with literature data show that ranges for oxygen fugacity (fO₂) are: I-type granitoids containing magnetite and free of pyrite and phyrrhotite likely crystallized under fO₂ between 10⁻¹⁵ bars and 10^–8.5 bars, whereas magnetite free rocks containing pyrite and pyrrhotite should have crystallized with fO₂ higher than 10⁻¹⁸ bars and lower than 10⁻¹⁵ bars. Regarding S-type granites, they must have crystallized under fO₂ lower than 10⁻¹⁸ bars.