A UNIDADE METASEDIMENTAR DO OFIOLITO DE RIBEIRÃO DA FOLHA, ORÓGENO ARAÇUAÍ, MINAS GERAIS: PETROGRAFIA, GEOTERMOBAROMETRIA E CALCOGRAFIA.

Abstract

The Neoproterozoic oceanic remnants of the Araçuaí Orogen occur as tectonic dismembered sectionsin the region between 42°-43°W and 17°-21°S, southeastern Brazil. The Ribeirão da Folha ophioliteis one of these oceanic remnants, located in the Minas Novas region, northeastern Minas GeraisState. This paper presents field and microscopic studies, as well as quantitative geothermobarometricresults, for the ophiolitic metasedimentary unit mapped in the Baixa Quente-Ribeirão da Folha area.This metasedimentary unit consists of several varieties of quartz-mica schist, peraluminous micaschist,calc-silicate rock (metamarl), graphite-kyanite schist, graphite schist, metachert, diopsidite andbanded iron formation. Such well-developed, ophiolitic sedimentary pile represents pelagic pelitesand chemical-exhalative sediments, as well as mixtures of chemical, argillaceous and/or organiccomponents. These sediments were metamorphosed in the Brasiliano Orogeny (ca. 580 Ma) and itsregional schistosity (Sn) reached the kyanite zone of the amphibolite facies (530-600 °C and 4,9-5,5kbar), in the focused area. An older foliation (Sn-1), developed in greenschist facies, is locallypreserved. The sulfide-bearing rocks are different types of metachert and diopsidite, with massivesulfide bodies, sulfide-type banded iron formation and graphite schist. Sulfate efflorescence, formedby chemical weathering, is a common feature of sulfide-rich outcrops. Pyrrotite, pyrite, botrioidalpyrite, chalcopyrite and spharelite are the main sulfide phases found in metachert and diopsiditetypes. Covellite, marcasite and arsenopyrite are rare. We suggest three stages of sulfide crystallization:i) precipitation from hydrothermal fluids in oceanic environment; ii) recrystallization during theregional metamorphism, mainly along the Sn foliation; and iii) post-Sn hydrothermal alteration.