Significado tectônico das sequências metavulcanossedimentares pré-estaterianas da Serra do Espinhaço em Minas Gerais

Abstract

Vulcanosedimentary sequences are of great importance for understanding the dynamics of the crust since they represent the sedimentary record resulting from the interaction between tectonic plates, hydrosphere, and atmosphere. In the Southern Espinhaço Ridge, two sequences have been the subject of studies in recent decades: the metavulcanosedimentary sequences Pedro Pereira and Costa Sena. Despite efforts, many questions remain open, to some extent due to the complexity of the fragmented and polycyclic geology to which they were subjected. To contribute to the positioning of sequences in geological time and an evolutionary model, these sequences were revisited in this doctoral thesis. Zircons extracted from metariolite interlayered in a banded iron formation of the Pedro Pereira Group define the crystallization age of the volcanic rock in 2.77 Ga, an age coincident with those found for the greenstone belts Rio da Velhas and Pitangui, in the Quadrilátero Ferrífero. Geochemical analyses of the banded iron formation associated with metariolite imply the existence of locally oxygenated pockets as early as the beginning of the Neoarchean era, more than 250 Ma before the GOE. Similar results were found in the Pitangui greenstone belt. Thus, the southeastern region of the San Francisco Craton records some of the oldest evidence of the onset of ocean oxygenation. Geochronological and geochemical analyses were also performed in the Costa Sena metavulcanosedimentary sequence. The concordant age spectra of detritic zircons for schist and parametaconglomerate were collected and, jointly with geochemical data, suggest deposition in a collisional geotectonic environment, in a foreland basin, between 2.0 and 2.2 Ga. The geochemical signatures of part of the schists indicate a contribution from sources extremely enriched in light ETR, compatible with values for calc-alkaline rocks. Other schists have ETR signatures compatible with the values of the local basement, Gouveia leucogranite, its most likely dominant source. Schists with kyanite-rich levels, which may constitute true kyanitites, would represent the metamorphism of paleosols generated during the frequent episodes of subaerial exposure of the forebulge region of the basin in hot and humid climate conditions, together with precipitation of evaporites rich in boron and phosphorus. The isotopic data of detritic zircon from the metabreccia sample of the Bandeirinha Formation in its type-area produced a younger statistical peak at 2093 ± 9 Ma (n=98), given that the age of the youngest spot was 2000 ± 40 Ma. These data, compatible with the originally defined positioning of the unit, strengthen the Paleoproterozoic origin conception of the sequence and suggest the maintenance of the Bandeirinha Formation within the Costa Sena Group.