The geologic evolution of the ancient granite—greenstone terrane of central-southern Bahia, Brazil

Abstract

The granite—greenstone terrain of central-southern Bahia belongs to the São Francisco craton and is a key area for the understanding of the Archaean and lower Proterozoic of South America. In addition, it is in an area of great metallogenic potential.The granitic—migmatitic—gneissic basement complex is intimately associated with low to medium grade metasedimentary and metavolcanic sequences, such as the Brumado, Umburanas and Contendas-Mirante. In all of these sequences the metasedimentary rocks predominate, although acid metavolcanics constitute a significant fraction of the Contendas sequence and ultrabasic rocks occur in several isolated units.Early Archaean granitoid rocks (Rb-Sr isochron ages older than 3 Ga) occur in some regions such as Mata Verde-Boa Vista, Lagoa do Morro and Sete Voltas. Extensive granitic intrusive activity, with associated medium grade metamorphism and metasomatism, occurred in most of the area ∼2.7 Ga ago, and the resulting terrain is taken to be the basement, at least for the Contendas-Mirante sequence. The relationships to the Brumado and Umburanas metamorphic complexes are not clear, and these sequences may be of Archaean age.The Contendas-Mirante sequence was affected by strong lower Proterozoic deformation (∼ 2 Ga ago) and contemporaneous granitic intrusions (like the Gameleira granite). Large, domal structures, interpreted as mantled gneiss domes, such as those associated with the Boa Vista and Sete Voltas granitoid rocks, also formed at this time. A late Proterozoic to Early Palaeozoic thermal event affected the southwestern part of the region at ∼ 0.5–0.7 Ga ago as revealed by mica K-Ar ages.