Metamorphic Evolution of the Granulite Rocks of Pedrão village, Jequié Block, São Francisco Craton, East central of Bahia, Brazil

Abstract

The studied granulite are inserted in the São Francisco Craton, more specifically in the Jequié Archean Block. Three different types of granulits were identified: augen-charnoenderbitic-charnockitic granulites, para-derived heterogeneous granulites and heterogeneous ortoderivatives granulites, the latter being the main target of this study, along with boudinage mafic enclaves associated with these rocks. Petrographically the ortoderivatives granulits are constituted by plagioclase, quartz, mesoperthite, pyroxene, amphibole, biotite, mimerquite, apatite, zircon and opaque minerals. The mafic enclaves are composed of amphibole, clino and orthopyroxene, biotite, feldspar, quartz and opaque minerals. Ortopyroxene were classified as ferrosilite, enstatite and in ferrosilite/enstatite transition, the clinopyroxenes as augite and diopside (only in enclaves and contact). The plagioclase crystals are of andesite compositions, except for the enclaves that have been classified as labradorite/andesine. The amphibole crystals are calcic, and are located in the field of edenite (enclaves and contact) and biotite crystals classified as eastonite. The presence of orthopyroxene, clinopyroxene and mesoperthite mark the peak of granulite metamorphism. The amphibole and biotite crystals, associated with the edges of pyroxenes, characterize regressive metamorphism during the orogenic uplift process of Jequié Block, though amphibole and biotite crystals (with high TiO₂ values) ocurr in the granulite progressive metamorphism.