Orosirian (ca. 1.96 Ga) mafic crust of the northwestern Sao Francisco Craton margin: Petrography, geochemistry and geochronology of amphibolites from the Rio Preto fold belt basement, NE Brazil

摘要

The bulk of the Atlantic shield basement in the eastern South American platform is made of Rhyacian calc-alkaline magmatic rocks (similar to 2.2-2.15 Ga) affected by granulite-facies metamorphism at around 2.06 Ga, as part of a Paleoproterozoic orogenic system which joined Archean crustal fragments together. Orosirian (2050-1800 Ma) rocks, on the other hand, are scarce in all four major provinces of eastern South America (the Borborema, Sao Francisco, Tocantins and Mantiqueira provinces). Recently, however, sparse occurrences of Orosirian-aged rocks have been described, particularly in the northern Sao Francisco Craton margin and adjoining fold belts that compose the Borborema Province further north. In this paper, we present new geochemical, geochronological and isotopic data of amphibolites which compose the basement of the Rio Preto fold belt, in the northwestern Sao Francisco Craton margin. Geochemical data suggest tholeiitic gabbro protoliths intruded in a subduction-related setting, as suggested by tectonic discrimination diagrams and differentiation trends. U-Pb analyses of magmatic zircon crystals yielded a 1958.3 +/- 16 Ma Concordia age. Slightly negative to positive epsilon Nd-(1.96) = (-0.3 to +1.0) and epsilon Hf-(1.96) from -0.76 to -4.55 suggest variable mixing of a depleted mantle source and older continental crust. We interpret the amphibolite body as a remnant of a subduction-related crustal fragment developed in the Orosirian, around 1.96 Ga ago, possibly in a continental back-arc setting. This crustal fragment was further reworked as part of the basement of the Rio Preto fold belt during the Brasiliano Orogeny (similar to 600-550 Ma). The Cristalandia do Piaui Complex gneisses further north might represent fragments of a coeval continental magmatic arc. These rocks could be the first well documented representatives of an Orosirian tectono-magmatic event which has been, up to now, broadly ignored within this portion of West Gondwana, and thus present major implications for the Proterozoic crustal evolution of South America and for Paleoproterozic supercontinent reconstructions in general. (C) 2015 Elsevier Ltd. All rights reserved.