The Ribeira Belt

Abstract

The generation of the NW-verging Ribeira belt that fringes the São Francisco craton to the south resulted from collisional episodes dated at 620–605, 605–565, and 535–510 Ma. During these collisional events, micro-continents and magmatic arcs converged and accreted to a continental margin system formed along the previously amalgamated São Francisco-Paranapanema landmass. The São Francisco-Paranapanema collision at around 640–620 Ma led to the development of the Southern Brasilia belt. The Ribeira orogenic front overprinted the pre-existent southern end of east-verging Brasília belt, thereby creating a tectonically very complex interference zone. The Ribeira belt as whole comprises four tectono-stratigraphic terranes: the Occidental, Paraíba do Sul, Oriental, and Cabo Frio terranes. The Occidental terrane, representing the external sector of the belt, involves the reworked cratonic basement (Archean and Paleoproterozoic orthogneisses and orthogranulites), Mesoproterozoic intra-cratonic basins and a Neoproterozoic passive margin unit (Andrelândia Group). The uppermost unit of the Andrelândia Group is regarded to be deposited in an active margin setting and sourced by a magmatic arc installed in the Paranapanema plate. The Paraíba do Sul terrane, composed of an Archean-Paleoproterozoic basement, Neoproterozoic metasedimentary units, and a continental magmatic arc (the 640–595 Ma Serra da Bolívia complex), accreted to southeastern sector of the craton margin between 620 and 605 Ma. Afterwards, the juvenile to immature magmatic arc of the Oriental terrane (860-620 Ma Rio Negro and Serra da Prata complexes) collided to proto-Ribeira belt. The crustal thickening resulting from these collisions gave rise to widespread generation of I, S and hybrid granites that intrudes both basement and cover units in the most deformed sectors of the belt. The Cambrian (535-510 Ma) docking of the Cabo Frio terrane (an Angola craton fragment) reworked the previous accreted terranes, generating large scale folds and dextral transpresional shear zones that reached the SFC margin. Finally, a vigorous bimodal magmatic event associated with transtensional deformation episodes characterizes the orogenic collapse of the belt and marks its stabilization in the interior of the Gondwana supercontinent.