DISSERTAÇÃO APRESENTADA AO INSTITUTO DE GEOCIÊNCIAS DA UNIVERSIDADE ESTADUAL DE CAMPINAS PARA OBTENÇÃO DO TÍTULO DE MESTRA EM GEOCIÊNCIAS, NA AREA DE GEOLOGIA E RECURSOS NATURAIS

This work examines field relationship and petrography of migmatites at the northeastern part of the Carajás Domain, Amazon Craton, and presents new geochemical and geochronological data in order to contribute to understanding the nature and significance of anatexis during the mesoarchean evolution of Carajás Domain. Regionally, in the northeastern Carajás area, the Xingu banded Orthogneiss corresponds to the highest metamorphic-grade rock with minimum leucosome content among the studied litotypes. The mafic layers are clinopyroxene-bearing amphibolite with calcium-rich plagioclase in paragenesis, suggesting at least upper amphibolite facies metamorphism. The migmatites of the Xingu Complex are well exposed in two localities, the Britalider and the Britamil quarries. These migmatites are spatial and temporally related though individually form a complex relationship spectrum from metatexites to diatexites, in which the constituent parts varies according to the sourcerock and melting rates. The dominant rock types at the Britalider Quarry are amphibolite and metatonalite to trondhjemite structured as i) patch metatexite; ii) stromatic metatexite; iii) schollen diatexite and iv) schlieren diatexite. The predominant rock types at Britamil quarry are a light-gray tonalite gneiss, a metagranodiorite, and an amphibolite, which are structured as i) stromatic metatexite and ii) nebulitic to schlieren diatexite migmatites. In comparison to the Britalider Quarry, the Britamil migmatite reveals lower melting rates, as amphibolite paleosome remained unmelted during anatexis and melt accumulation as diatexites are less expressive. In the Britamil quarry, a high-temperature event at ca 2.88 Ga is registered in zircon grains of a Hercynite-Cordierite–bearing gneiss, which might represent a retrogressed granulite. The petrological diversity produced in these migmatites occurrences provide examples of how the water content and strain rates are as determinant as the protolith composition and temperature in melting of high-grade rocks. Field relationships and detailed petrography strongly suggest that the Xingu banded Orthogneiss most likely represent a parental gneiss of the Xingu migmatites at Britalider Quarry. The comparison of geochemical igneous fingerprints of the mafic layers of the Xingu banded Orthogneiss and the Britalider quarry amphibolite mesosome is not conclusive, though suggests a petrogenetic link between these units. The Xingu migmatites at Britamil quarry preserve the tonalite gneiss mesosome, suggesting that the metatexite closest protolith was a composite gray gneiss. The Tonalite gneiss mesosome presents severe HREE depletion, high La/Yb ratio and low Y, which indicate a TTG series affinity for its igneous protolith. Emphasis was given to detailed petrography and microstructural analysis, which in addition to whole rock geochemistry and U-Pb zircon geochronology data provide evidence that the Carajás mesoarchean basement rocks underwent partial melting at ca 2.86 Ga.