Magmatic activity period and estimation of P-T metamorphic conditions of pre-collisional opx-metatonalite from Araçuaí-Ribeira orogens boundary, SE Brazil

Abstract

The southern region of the Espírito Santo State is located in the Araçuaí-Ribeira Orogen System (AROS) limit, where neoproterozoic orthoderived rocks undergone metamorphism at high-grade condition exhibiting partial melting and deformation features. This study focuses on the Pre-collisional Opx-granitoids Unit that shows varied proportions of orthopyroxene and garnet. Petrographic, geochemical, geothermobarometric and U-Pb in zircon studies were carried out. We distinguished two mineral assemblages: (i) consisting of plagioclase, quartz, biotite, amphibole, orthopyroxene and ilmenite; and (ii) with plagioclase, quartz, orthoclase, biotite, amphibole, garnet and ilmenite. The former is related to the opx-metatonalite protolith and the latter to metamorphic paragenesis. The geochemical analyses of the Pre-collisional Opx-granitoids Unit reveal an arc-related environment, similar to the Rio Doce and Rio Negro - Serra da Prata Arcs defined in the Araçuaí and Ribeira orogens, respectively. The temperature conditions obtained for assemblage (ii) were 670–860 °C (granulite facies) and pressure from 5.24 to 10.2 kbar. Four different intervals of concordant ages were yielded in the studied samples. The oldest age of 716 Ma is interpreted as related to the pluton emplacement (E1). The ages of 604 Ma and 595 Ma are associated with the same magmatic event (E2), and the magmatic age of 586 Ma (E3) indicates another magma injection event. The age of 574 Ma (E4) is related to a high-grade metamorphic event whose P-T conditions were obtained in assemblage (ii) from geothermobarometric calculations. These results confirm the extension of Rio Doce magmatic arc toward the south and establish a connection between Araçuaí and Ribeira orogens.