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Gláucia Queiroga, Federal University of Minas Gerais (Brazil)
Antônio Carlos Pedrosa-Soares, Federal University of Minas Gerais (Brazil)
Carlos Maurício Noce, Federal University of Minas Gerais (Brazil)
Fernando Flecha Alkmim, Federal University of Ouro Preto (Brazil)
Márcio Martins Pimentel, University of Brasília (Brazil)
Elton Dantas, University of Brasília (Brazil)
Ivo Dussin, Federal University of Rio Grande do Sul (Brazil)
Maximiliano Martins, Federal University of Minas Gerais (Brazil)
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In southeastern Brazil, the Neoproterozoic Araçuaí orogen extends from the eastern margin of the São Francisco Craton to the Atlantic coast, between the 15° and 21° S parallels. The evolution of the Araçuaí Orogen has been subdivided into pre-collisional (630-585 Ma), syn-collisional (582-560 Ma), late collisional (560-530 Ma) and post-collisional (530-490 Ma) stages. The main oceanic record of the Araçuaí orogen is the Ribeirão da Folha ophiolite, a tectonic dismembered rock-assemblage composed of slices of meta-ultramafic and metamafic rocks thrust onto packages of pelitic schists with intercalations of sulfide-bearing metachert, diopsidite and banded iron formations, metamorphosed in the kyanite zone of the intermediate pressure amphibolite facies (550°C; 5,5kbar). The geochemical signatures of the metamafic and meta-ultramafic rocks indicate ophiolite affinity and ocean-floor origin. Sm-Nd isotopic data from both the metamafic and meta-ultramafic rocks yielded positive epsilon Nd values (+3 to +7). Their Sm-Nd whole-rock isochrons, as well as Sm-Nd TDM model ages, suggested development of oceanic lithosphere in Neoproterozoic times. Leucocratic bodies resembling plagiogranite, hosted by medium- to coarse-grained amphibolite (metadolerite to metagabbro), were recognized and show irregular vein-like shapes, ranging in size from millimeters to 50 cm. These leucocratic bodies consist of foliated plagiogranite composed essentially of calcic plagioclase with albitic rims, quartz, hornblende and epidote, with titanite, sulfide, apatite and zircon as the main accessory minerals. The zircon crystals recovered from a plagiogranite sample are euhedral and very clean, and show elongated prismatic (3:1) morphology, suggesting magmatic origin. U-Pb (LA-ICPMS) analyses were carried out on eighteen zircon crystals which yielded concordant results, indicating a crystallization age of 660+/-29 Ma. The spread of some of the analyses along the concordia curve suggests Pb loss due to the amphibolite facies metamorphism at ca. 580 Ma. A new discovery of an ophiolitic sliver is the Santo Antonio Grama amphibolite, a huge (50km along strike) gabbroic body composed of hornblende, calcic plagioclase and pyroxenes, with oceanic chemical and Sm-Nd isotopic (epsilon + 2) signatures. Clean zoned zircon crystals from this metagabbro yielded U-Pb (LA-ICPMS) ages of 595+/-4 Ma for magmatic cores and 577+/-3 Ma for metamorphic rims. The ca. 660 Ma plagiogranite predates the oldest U-Pb ages (ca. 630 Ma) of tonalites of the pre-collisional magmatic arc of the Araçuaí Orogen, but the generation of oceanic crust would have took place even during the evolution of this arc, as suggested by the zircon magmatic ages of the Santo Antonio Grama metagabbro. The metamorphic recrystallization age (ca. 577 Ma) of this oceanic sliver agrees with the time interval of the syn-collisional stage (580-560 Ma).
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