Cr-Spinels from Brazilian Mafic-Ultramafic Complexes: Metamorphic Modifications

Chemical compositional data on the Cr-spinels from selected Brazilian mafic-ultramafic complexes are presented and compared. The massive stratiform chromitites from the Pium-Hi and Serro complexes and massive podiform chromitites from the Abadiânia and Morro Feio bodies were strongly deformed and metamorphosed from greenschist to epidote-amphibolite facies. The massive stratiform chromitites from Campo Formoso, Medrado-Ipueiras, and Luanga were metamorphosed to the upper amphibolite facies and variably deformed. The large Barro Alto, Niquelândia, and Canabrava (and the small Americano do Brasil, Mangabal-I and -II, and Caraíba) stratiform complexes were polydeformed and metamorphosed to granulite and amphibolite facies. Regardless of their metamorphic and deformational changes, the chromitites still retained some primary features, such as: (1) texture, (2) structures, and (3) original chemical composition. It was observed that the chemical changes imparted to the Cr-spinels during metamorphicdeformational events have led to distinctive compositions. However, only the cores of grains in massive chromitites retained some original chemical features, although these could be strongly deformed and/or metamorphosed; the cores of the accessory and disseminated Cr-spinels retained their original chemical features only under low metamorphic and/or deformational intensities. The last-formed, outermost Cr-spinel composition and the path followed by the chemical changes were the direct result of the interaction of: (1) country-rock composition; (2) chromite composition; (3) textures and structures; (4) mass-transfer mechanisms; (5) ƒ(O2); (6) temperature and pressure; (7) fluid composition; and (8) rock/fluid ratio.