A Rhyacian continental arc during the evolution of the Mineiro belt, Brazil: Constraints from the Rio Grande and Brumado metadiorites

Abstract

The Mineiro belt corresponds to a sequence of Siderian to Rhyacian arcs added to the Archean continental margin of the São Francisco Craton by soft-collision episodes. The study of the Rio Grande and Brumado metadiorites constrains processes of crust generation during the Rhyacian evolution of the Mineiro belt. The plutons contain autoliths of hornblendites and decrease in MgO, Fe2O3total, and CaO contents with the increase of SiO2 that points to fractional crystallisation. Calc-alkaline signature along with negative NbTa, P, and Ti anomalies indicate a magmatic arc setting. The age of the Rio Grande metadiorite between 2128 ± 24 and 2102 ± 33 Ma and of the Brumado metadiorite as 2124 ± 6 Ma correlate these plutons to the Rhyacian Ritápolis arc. The Rio Grande metadiorite displays slightly negative εNd(2.1 Ga) values and low 87Sr/86Sr(2.1 Ga) ratios that point to a mantle-derived magma. The presence of inherited zircon with age 2348 ± 11 Ma in this pluton indicates the contribution of the Siderian Resende Costa arc (2.35–2.31 Ga). The Brumado sample shows higher values of εNd(2.1 Ga) and 87Sr/86Sr(2.1 Ga) that suggest crustal contribution. The model for the generation and evolution of these plutons involves a subduction setting with the hydrous melting of the overlying mantle from fluids of the slab and consequent generation of dioritic magma. This magma ascended into the Resende Costa arc and the Rio das Mortes metavolcanic-sedimentary sequence. Crystallisation started by the generation of cumulates of magnesio-hornblende followed by their destabilisation and assimilation by a second pulse of dioritic-tonalitic magma and continuous fractional crystallisation.