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

Abstract

A felsic eruptive event within the Rio das Velhas Greenstone belt is mainly represented by volcaniclastic and resedimented volcaniclastic rocks, besides rare lava flows of dacitic composition. The volcaniclastic rocks comprise four lithofacies: (1) monomictic and polymictic breccias, (2) conglomerate-graywacke, (3) graywacke-sandstone, and (4) graywacke-argillite. SHRIMP and ID-TIMS U-Pb zircon dating of three graywacke samples dated the eruptive event at 2792 ± 11, 2773 ± 7 and 2751 ± 9 Ma, indicating a range of about 40 Ma for the felsic magmatism. This time range may also encompass the whole magmatic activity within the belt, because of the association of the felsic rocks with the more extensive basaltic flows. 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.