Condições físicas de formação de associações metamórficas do cinturão ultramáfico Carrancas-Liberdade, MG

Abstract

Allochthonous ultramafic bodies occur associated with metasediments at Andrelândia Depositional Cycle in southern Minas Gerais. They cross regions belonging to the domains of the Brasília Belt and the interference tectonic zone between this and the Ribeira Belt. This paper discusses the differences in the metamorphic evolution of ultramafic rocks of both domains, based on equilibrium reactions, textural analysis and application of geothermometers. The high-grade rocks of a Ribeira Belt Tectonic Domain (meta-pyoxenites) show equilibrium conditions under higher temperatures (700 to 900°C) regarding to their congeners in the Brasília Belt Tectonic Domain (maximum of 640°C). Besides these contrasting temperatures the areas show different styles of deformation. In the Brasília Belt, the metamorphic peak is associated to pyroxene and olivine growth, followed by a deformation that affects the high grade rocks only in a discreet form. The main structures in this region are associated to a retro-metamorphic phase, represented by formation of hydrated rocks over the highest grade assemblages. In the domain of the Ribeira Belt, the formation of the highest grade minerals is concomitant to an intense deformation, indicating that the metamorphic peak coincides with the development of the dominant structures of ultramafic bodies.