Evolução metamórfica/metassomática paleoproterozóica do Complexo Saúde, da Bacia Jacobina e de leucogranitos peraluminosos na parte norte do Cráton do São Francisco

Abstract

The Saude Complex and the Jacobina Basin, located in the northern part of the Sao Francisco Craton, are constituted by metasedimentary rocks, metamorphosed during the paleoproterozoic orogeny (≈ 2.08-1.88 Ga), and are associated to leucogranites. The kinzigites and the non-garnetiferous paragneisses of the Saude Complex, the aluminous schists of the Jacobina Basin and the leucogranites contain mineral assemblages related to the orogenic collision. The garnet in the kinzigites and staurolite in the paragneisses, associated to sillimanite + quartz + biotite, constitute mineral assemblages formed during the progressive metamorphic path whereas cordierite and plagioclase crystallized during the retrometamorphism associated to the orogenic decompression. In the aluminous schists, garnet and staurolite crystallized during the metamorphic progressive path, but andaluzite and plagioclase record the retrometamorphism. The non-garnetiferous paragneisses, the andaluzite-schists and the leucogranites have fibrolite-quartz-muscovitebearing microstructures formed from the circulation of reducing fluids that lixiviated mainly base-cations of the preexisting silicates. Those microstructures characterize an event of metasomatism that occurred during and/or after the orogenic decompression with the leucogranites acting as thermodynamic triggers in the mobilization of the crustal fluids. The metasomatic event was responsible for almost isobaric inflections (≈ 3.0 kbar) in the regressive P–T path of the non-garnetiferous paragneisses and of the aluminous schists, toward temperatures near 600oC. The absence of fibrolite and muscovite in the kinzigites besides geothermometry estimations, placed between the low-granulite and the high-amphibolite facies, suggest allochthonous positioning of those rocks originated from the orogen infracrust.