M.C.H. Figueiredo

An outline of Early Proterozoic crustal evolution in the São Francisco craton, Brazil: a review

Abstract The Early Proterozoic evolution of the Sao Francisco craton (SFC) occurred between 1.8–2.5 Ga and includes the Transamazonian cycle (1.9–2.2 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 (2.6–3.2 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 Minciro 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 1.8–1.9 Ga ago, the SFC was fully cratonized.

The Rio Doce Orogeny, Southeastern Brazil

Abstract The Neoproterozoic-Eopaleozoic superposed orogenic system of Southeastern Brazil, which was active during the Brasiliano-Pan-African Cycle during the assembly of this sector of the Gondwana Supercontinent, includes distinct terranes such as the Guanhaes, Curitiba, Apiai-Guaxupe and Serra do Mar microplates and the Juiz de Fora Thrust Belt. These orogenic systems also affected the reworked border of the Sao Francisco Craton. The collisional or ocean plate subduction-controlled “Brasiliano I” Orogeny was responsible for the generation of fold belts along the southeastern border of the Sao Francisco Craton, the accretion of different microplates and the formation of a magmatic arc associated with the roots of a northwestward trending thrust belt. The “Brasiliano I” evolution occurred during the Neoproterozoic and by 600 Ma was already in a post-orogenic stage in the Apiai-Guaxupe Microplate, with the intrusion of rapakivi-like granitoids. The Rio Doce Orogeny is best characterized in the Serra do Mar Microplate by a magmatic arc, active between 590 and 570 Ma, with batholithic calc-alkaline plutonism exhibiting subduction zone components and a chemical zonation indicative of northwestward subduction. The collisional stage (560–530 Ma) accounted for the accretion of the Serra do Mar Microplate to the former orogenic domains. Anatexis of mostly metasediments producing peraluminous migmatites and granites, began at the calc-alkaline magmatic arc stage and culminated during crustal thickening associated with the northwesterly piling-up of large thrust slices. The post-collisional plutonism (520–480 Ma) is characterized by plutons and dikes of mainly alkali-calcic granitoids enriched in incompatible elements.