Tuesday, December 29, 2015

Evidence of a Rhyacian Paleoproterozoic Supercontinent From Brazil

Crustal evolution of the northern Brasília Belt basement, central Brazil: a Rhyacian orogeny coeval with a pre-Rodinia supercontinent assembly


de Sousa et al


The Brasília thrust and fold belt developed in the course of Neoproterozoic Brasiliano orogeny, during the convergence of São Francisco, Amazonian and Paranapanema cratons, in an early phase of Gondwana agglutination. A Paleoproterozoic terrane cropping out in its northern segment -represents the basement of the belt and corresponds to a magmatic arc. The arc developed during Rhyacian, possibly on the margin of an Archean-Paleoproterozoic block that is today represented by the São Francisco craton. The Paleoproterozoic arc evolution starts following a calc-alkaline trend between 2.30 and 2.16 Ga, changing from a typical juvenile to crustal contaminated terrane with slightly negative ɛNd(t) values, as revealed by tonalites and granodiorites of Conceição do Tocantins Suite. By c. 2.17-2.15 Ga, a collisional event took place, triggering crustal melting and generating peraluminous granites. These syn-collisional granites present strongly negative ɛNd values and belong to Aurumina Suite. Natividade, Almas-Conceição do Tocantins and Cavalcante-Arraias domains within the basement do not represent distinct tectonic blocks. The first two are areas where mainly plutonic arc rocks crop out and in the last one, syn-collisional granites are the most common lithology. Syn-collisional granites characterize granitogenesis produced by collision of another landmass against São Francisco landmass. The arc development is contemporary with collisional orogens that culminated in a conjectural Paleoproterozoic pre-Rodinia supercontinent. Since Rhyacian, these rocks are part of the landmass that would become the São Francisco craton and compose Brasília Belt basement.

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