Age of felsic volcanism and the role of ancient continental crust in the evolution of the Neoarchean Rio das Velhas Greenstone belt (Quadrilátero Ferrífero, Brazil): U-Pb zircon dating of volcaniclastic graywackes

Volcaniclastic graywackes are characterized by an abundance of detritic/xenocrystic zircon grains displaying a polymodal age spectrum ranging from 2.8 to 3.4 Ga. Older zircon grains must be derived from magma contamination, and/or post-eruptive sedimentary reworking and mixing with felsic detritus from other sources. It may be concluded that felsic magmas were in contact with older granitoid crust, and that this basement was also exposed as source for detritic material. In the Quadrilátero Ferrífero, the ancient complexes of TTG gneisses and migmatites make up most of the exposed Archean terrains, and its present juxtaposition with the supracrustal assemblage suggests an authochthonous to parauthocthonous development for the Rio das Velhas Greenstone belt in a continental or near-continental tectonic setting.