Evolution of the archean/proterozoic crust in the southern São Francisco craton near Perdões, Minas Gerais, Brazil: petrological and geochemical constraints

Abstract

Detailed geochemical and petrographic analyses of a lower crustal bimodal sequence near Perdões/Minas Gerais contribute to the reconstruction of the Archean–Proterozoic evolution of the southern São Francisco craton. Trace element patterns of enderbites with tonalitic/trondhjemitic compositions (felsic members) indicate an origin by a two-phase partial melting process from a mantle source at depths, where garnet exists as a residual phase. Gabbroic rocks and mafic–ultramafic cumulates (mafic members) occur as lens-shaped elongated bodies exposed in the felsic complex. The gabbroic rocks are tholeiitic in composition, with rare earth element (REE) patterns indicating generation through relatively high percentages of partial melting from mantle material. Trace element ratios indicate a slightly depleted mantle source for the gabbroic rocks. Charnockites, which occur in the same area, are granitic/granodioritic in composition. The REE patterns indicate magma generation by intracrustal partial melting of tonalitic/trondhjemitic material, which must have occurred at depths of less than 40 km, where plagioclase is a stable residual phase. The whole-rock sequence shows a uniform PT path. The rocks were generated in granulite facies conditions. During exhumation, they underwent a strong retrograde amphibolite facies metamorphism, followed by a weak greenschist facies overprint. The granitic rocks of Perdões represent a rock association that is characteristic of the transition between Archean and Proterozoic times, during which the continental crust evolved from Archean tonalitetrondhjemitegranodiorite (TTG) suites towards an increased lithologic diversity in PostArchean time, with greater amounts of more evolved and more potassium-rich granitoids (granodiorite/granite/monzogranite (GGM) suites).