AN OUTLINE OF EARLY PROTEROZOIC CRUSTAL EVOLUTION IN THE SAO-FRANCISCO CRATON, BRAZIL - A REVIEW

The Early Proterozoic evolution of the Sao Francisco craton (SFC) occurred between 2.5-1.8 Ga and includes the Transamazonian cycle (2.2-1.9 Ga), one of the main periods of crustal growth affecting the South American continent. This synthesis of the available data is largely concerned with the Early Proterozoic terranes of the SFC and presents a modeled crustal evolution integrated with that of the West Congo craton. Interpretation of radiometric ages, geochemical and structural data, metamorphism (P, T conditions), and lithostratigraphical correlations leads to the characterization of three Early Proterozoic belts in the SFC, named: the Correntina-Guanambi, Itabuna and Mineiro belts. These belts are here described following a geographic scenario as given by their representatives (medium- to high-grade sequences, low-grade supracrustal sequences and intrusive igneous rocks). The anatomy and regional structures of these belts indicate that they developed along the margin of a large Archean protocontinent, although characteristic older mantled gneiss domes are also found within the Itabuna and Mineiro belts. Available isotopic evidence suggests an episodic growth of the continental crust during the Late Archean (3.2-2.6 Ga) with the formation of microcontinents that gradually coalesced around 2.6 Ga ago. After the Archean-Proterozoic transition, the tectonic evolution of the Proterozoic mobile belts evolved by mantle accretion/differentiation and crustal reworking. The main Early Proterozoic magmatic arc of the SFC is represented by the Itabuna mobile belt, whereas the Mineiro belt is chiefly ensialic. The nature of the Salvador-Correntina belt is still poorly constrained by geochronological and geochemical data. By the end of the Transamazonian cycle, around 1.9-1.8 Ga ago, the SFC was fully cratonized.