Metamorfismo de fundo oceânico e alto grau em meta-peridotitos ofiolíticos neoproterozóicos, Faixa Brasília Sul, Minas Gerais

Abstract

In the southern region of the Minas Gerais State, Brazil, among Sao Vicente de Minas, Andrelândia e Liberdade, in the southern border of the Sao Francisco Craton, southern portion of the neoproterozoic Brasilia fold-thrust Belt, there are several ultramafic bodies among oceanic, turbiditic and pelagic to hemipelagic metasediments, with high to medium grade metamorphism, positioned along deep thrust faults associated to granulites, eclogites, retroeclogites and basement slivers. These ultramafic rocks, of controversial and probable ophiolitic origin we denote the ultramafic rocks of Morro do Corisco and Fazenda da Roseta, both in the Liberdade City. These ultramafics correspond to serpentinized cumulatic meta-harzburgite which presents primary serpentinization textures (ocean floor?), Lithogeochemical diagrams, for discrimination of tectonic environments, positioned these meta-peridotites in the specific fields of ophiolitic, orogenic and/or abyssal peridotites. Analytical data for minor and trace elements when compared to primitive mantle values show an enrichment in lithophile elements, and values close to those of the mantle for siderophile elements, similar to those rocks of oceanic origin, submitted to serpentinization process and/ regional metamorphism.Structural and petrologic data show an evolution essentially in 4 episodes which are: 1) an earlier episode related to formational of primary ultramafic cumulatic rocks, in a probable crustal-subcrustal oceanic environment 2) primary process of ocean floor metamorphism and/or hydrothermalism which reach amphibolite facies (ocean floor spreading process?); 3) a progressive metamorphism related probably to exumation of these bodies, which reaches granulite facies conditions and it is probably related to early subduction and later obduction processat last, 4) a final retrometamorphism which reached greenschist facies.