Cooperative Research: Petrogenesis of HP and UHT Granulites from the Brasilia Fold Belt in Minas Gerais and Goias, Brazil

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BrownEAR-0227553This project, with counterparts in Brazil, investigates the relationship between high-pressure (HP; P 1.0 GPa, T 700EC) and ultrahigh-temperature (UHT; T 900EC) granulites and 'common' granulites in the core of the Brasilia Fold Belt. The orogenic core of the Brasilia Fold Belt is composed of a metamorphic complex characterized by granulite facies rocks, and the recent discovery of HP and UHT granulites within the southern (Minas Gerais; kyanite-K-feldspar bearing granulites) and central (Goias; sapphirine-quartz and spinel-quartz bearing granulites) sectors of the belt provides the principal motivation for this research. We will map the distribution of HP (Minas Gerais) and UHT (Goias) granulite types within the metamorphic complex of the core of the Brasilia Fold Belt, and establish their spatial, temporal, structural and metamorphic relationship with 'common' granulites. We will investigate the role of melt loss and melt retention in the preservation and/or retrogression of mineral assemblages formed under conditions of extreme crustal metamorphism. Melt loss will be evaluated using distribution of leucosome, bulk rock chemistry, and pseudosections in multi-element model systems that allow the investigation of subsolidus and suprasolidus phase equilibria. We will characterize the microstructural relationships and mineral assemblages within HP and UHT granulites for a range of bulk compositions, particularly in samples that preserve disequilibrium reaction microstructures such as symplectites and reaction coronae. Peak and post-peak segments of P-T paths will be established using these data and detailed mineral chemistry in rocks with suitably low variance. Of general interest is the relationship between the development, preservation and mode of exhumation of HP and UHT granulites to the tectonics of convergent (subduction - collision) orogens. The Brasilia Fold Belt displays tectonic elements that developed due to ocean basin evolution and subduction that culminated with continent-continent collision in Neoproterozoic time. We will evaluate the results of our research within the framework of current plate tectonics theory, and propose a model that is consistent with the data and which accounts for the evolution of the Brasilia Fold Belt.