EVOLUÇÃO DAS BACIAS PROTEROZÓICAS E O TERMO-TECTONISMO BRASILIANO NA MARGEM SUL DO CRÁTON DO SÃO FRANCISCO

Detailed mapping associated with the application of sequence stratigraphy, structural and petrographic analyses, permitted a reconstruction of the Meso to Neoproterozoic geologica evolution south of the craton. A major unconformity separates two main rock units, a pre l.8 Ga basement and its cover. The basement is composed of high grade gneisses, greenstpne belts and intrusives of variable composition. Four depositional cycles were recognised in the cover successions. In stratigraphic order these are: the Tiradentes and Lenheiro cycles, predominantly quartzitic, the Carandai cycle with abundant pelites and carbonates, and the Andrelândia cycle with meta-sandstones and pelites. The analysis of these depositional cycles or sequences allowed the recognition of three depositional basins, the Sao Joao dei Rei basin, the Carandai basin and the Andrelândia basin. The first two are intracontinental basins, whereas the last represents a passive continental margin of the Sao Francisco paleo-continent, that, at the time, extended futher southeast with relation to the present limit of the craton. A continental collision around 600 Ma, here considered equivalent to the collision that resulted in the Brasilia belt further north, produced extensive thrusting with nappe formation and top to the east tectonic transport. The associated metamorphism (M B ) was of relatively high pressure type, as testified by the local occurrence of retro-eclogites. It reached the granulite facies in the stability field of kyanite. A second tectonic event, related to the Ribeira belt, at about 570 Ma, produced crustal shortening with tectonic transport to the NNW in the southern part of the region. The related metamorphism (M R ), characterised by sillimanite and cordierite, is of lower pressure type and produced a large number of anatectic granite bodies. Finally, important NE-SW transpressive dextral shear zones were generated in response to renewed E-W shortening.