GEOTECTONIC SIGNIFICANCE OF THE NEOPROTEROZOIC GRANITIC MAGMATISM IN THE ARAÇUAÍ BELT, EASTERN BRAZIL: A MODEL AND PERTINENT QUESTIONS

Abstract

The Araçuaí Belt is the Brazilian counterpart of the West Congo Belt. Both belts compose the Neoproterozoic Araçuaí-West Congo Orogen. Remnants of an oceanic lithosphere, formed at about 816 Ma ago, are related to the passive margin stage of the Ara?uaf basin. During the Brasiliano Event (650-500 Ma), widespread melting of a pre-existing crust produced a considerable amount of granitoid magma in the internal tectonic domain of the Ara?uaf Belt. Five different granitoid suites can be distinguished. The Gl syn-tectonic suite constitutes the anatectic nucleus of the orogen. It comprises batholitic bodies of S-type orthogneisses. The G2 suite consists of syn-tectonic, I-type, calc-alkalic granitoids. The G3 suite comprises late-tectonic, S-type peraluminous granitic bodies emplaced mainly in the Gl domain. The G4 suite consists of late- to post-tectonic intrusive bodies of I-type, high-K calc-alkalic granitoids. The G5 suite comprises post-tectonic intrusive plutons composed of S-type granites. We propose a model for explaining the origin of these granitoid suites. The development of the Arafuaf basin was followed by the subduction of the oceanic lithosphere, continental convergence and crustal thickening. The progressive increase in the regional temperature related to magma rising and crustal thickening induced widespread partial melting of the metasedimentary piles. The extensive anatexis under high-H2O-pressure generated Gl granitoids from kinzigitic gneisses and maybe from the Salinas Formation. At a late stage, the release of the regional pressure may have produced the G3 granites. G2 and G4 granitoid suites show a N-S trend and a west to east zoning, suggesting successive stages of maturation of calc-alkalic magmatism. In this scenario, G2 may represent the root of a continental volcanic arc related to an east-dipping subduction zone. G4 granitoids represent the post-collisional, high-K calc-alkalic melts evolved from the lowermost crust, probably with mantle contributions. Finally, the post-collisional, S-type granites of the G5 suite were generated and emplaced in shallow levels of the orogen.