The development of a Meso- to Neoarchean rifting-convergence-collision-collapse cycle over an ancient thickened protocontinent in the south São Francisco craton,Brazil |
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| Institution: | 1. Programa de Pós-Graduação em Geologia (PPGL), Instituto de Geociências, Universidade Federal do Rio de Janeiro, Brazil;2. Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil;3. Programa de Pós-Graduação em Geologia, Instituto de Geociências, Universidade de Brasília, Brasília, Brazil |
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| Abstract: | The south São Francisco craton (SSFC) in Brazil is one of the few areas that are key to unveiling the Archean evolution of South America. Despite economic interest since the 18th century, the SSFC was not studied in detail until the beginning of the present decade. The two main greenstone belts in the SSFC, namely, the Rio das Velhas (RVGB) and the Pitangui (PGB), have traditionally been considered to represent a single Archean basin. Here, new geochemistry and geochronology data from both greenstone and TTG-granite rocks are integrated and suggest that these belts evolved as separate domains. These are marked by distinct basal komatiite geochemistry, indicating that the PGB evolved as a back-arc rift on a thick lithosphere section at 2.86 Ga, in contrast to the RVGB basin, which developed near an exotic juvenile TTG terrain. Approximately 100 Ma later, the PGB basin transitioned to a calc-alkaline dominated setting, coeval with the emplacement of two large TTG igneous bodies at the margins of a poly-recycled ancient terrain. This protracted recycling is marked by extremely negative ?Hf values, which are not common for Archean terrains. The dominant strong crustal signature of the SSFC Archean rocks implies the existence of an anomalous overthickened crust or a Meso- to Neoarchean protocontinent. This thickened crust developed by continuous magmatism, delamination and differentiation in a flat-subduction setting. These mechanisms suggest that modern-style plate tectonics or a similar process such as “dripduction” was operating in this area prior to 3.0 Ga, the time at which a thick continental crust regime and horizontal tectonics were thought to have been established. Late TTG magmatism in the PGB suggests the western SSFC experienced an episode of late crustal thickening at ~2.71 Ga in response to collision, while coeval shallower K-rich magmatism in the RVGB corroborates the diachronous evolution of both belts and the surrounding crystalline terrains. |
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