Múltiplos episódios térmicos no Orógeno Araçuaí: insights a partir de rochas de diferentes idades e origens (Estado do Espírito Santo, Brasil)

Abstract

The evolution of a collisional orogen concerns a sequence of events capable of producing a large volume of igneous rocks and reaching high-grade metamorphic conditions, and even ultrahigh temperature conditions. The region that comprises the crystalline core of the Araçuaí orogen records a sequence of events between 630 Ma and 480 Ma, including magmatism, metamorphism and anatexis that have been unraveled through geochronological data of zircon and monazite and metamorphic modeling. The study area, located in the central region of the state of Espírito Santo-Brazil, encompasses the back-arc region where igneous rocks related to the pre-, syn- and post-collisional stages and granulitic migmatitic paragneisses of the Nova Venécia complex occur. Systematic monazite dating of a large number of igneous rock samples representing distinct tectonic stages and the migmatitic paragneisses yields three main age groups at ca. 550 Ma, ca. 530 Ma and ca. 500 Ma to 480 Ma. These record late syn-collisional magmatism, early late-collisional magmatism and the post-collisional stage, including the last orogenic magmatism and related hydrothermal fluid activity. Although several samples record more than one event, complete resetting of monazite ages was observed from a granulitic paragneiss close to a post-collisional intrusion. The metamorphic modeling of a contact aureole formed at the contact of the paragneiss with a post-collisional mafic igneous intrusion, combined with zircon ages, establish the P–T–t conditions of ~5.8 kbar – 990°C – 520 Ma for the contact metamorphism peak, while the P–T–t peak at 7.5 kbar – 785°C – 560 Ma was related to the regional metamorphic peak defined in the literature. The pressure-time relationship between these events presents a pressure difference of 1.7 kbar over 40 Ma, corresponding to low decompression and exhumation rates between the collisional stage and the post-collisional stage, reflecting a period of relative tectonic stability in the studied region. The results presented suggest that although cases of textural overprint imposed by post-collisional magmatism are restricted to the development of contact aureoles, the resetting of monazite ages indicates the superposition of a heating event at the final magmatic stage of the Araçuaí Orogen.