O sistema de Nappes Andrelândia Oriental - proveniência sedimentar e evolução tectônica
Distinct paleogeographic segments were recognized in the São Francisco and Paranapanema paleoplate margins and resulted, during the collision, in the tectonic piles of sin-metamorphic nappes from the Southern Brasília Orogen. Through major, minor and trace elements and Sm-Nd and Rb-Sr isotopic systems in whole-rock, REE and U-Pb and Lu-Hf isotopic systems in zircon, it was possible to evidence distinct sedimentation environments and distinct chemistry characters of the sources from the Santo
... s from the Santo Antônio and Serra da Boa Vista units, Andrelândia Nappe, and from the Liberdade Nappe metasedimentary rocks. The Liberdade Nappe rocks register provenance from a tonian magmatic continental arc, with maximum depositional age of 790 Ma. Older sources also occur from the Mesoproterozoic, of 1.10-1.25 Ga and 1.35-1.50 Ga, and from the Statherian-Orosirian of 1.75-1.85 Ga. Metawackes from the Santo Antônio Unit, with maximum depositional age of 680 Ma, register low index of chemical alteration and origin from depleted Neoproterozoic sources, of the Tonian-Cryogenian, derived from an intra-oceanic island arc system and, subordinately, from its more evolved products. Sources of 1.00-1.26 Ga contributed in minor expression. The upper Andrelândia Nappe Unit, Serra da Boa Vista, shows the highest rates of chemical alteration and does not register zircon detritic crystals from the Neoproterozoic magmatism. This metasediment sources were composed by Mesoproterozoic rocks, of 1.00-1.28 Ga and 1.35-1.48 Ga, and from the Shatherian-Orosirian, of 1.75-1.95 Ga. In minor expression Rhyacian sources, of 2.07-2.15 Ga occur. The predominance of Mesoproterozoic ages suggest sources with Grenville affinities, probably related to a western continent suggesting the correlation between Paranapanema and Rodinia. The Rhyacian zircon detritic crystals in the upper unit, Serra da Boa Vista, allied to the absence of Neoproterozoic arc magmatism register, suggest flysch-type collisional sedimentary deposits, with source duplicity, Rodinia to the west and São Francisco-Africa to the east. Ages of 0.99-1.27 Ga, mainly from depleted rocks, suggest sources with Grenville affinity, defining a western continent and marking a possible correlation between Paranapanema and Rodinia. The retroeclogitic and metavulcanic rocks studied, from the Liberdade Nappe, present igneous ages of 1.46-1.48 Ga, older than the maximum depositional age of the metassedimentary rocks (790 Ma) in which they are inserted. They are admitted as blocks or tectonic lenses fallen from the source-area as olistoliths into the sedimentary basin. The retroeclogitic lenses indicate a long-lived subduction process of continental slices. The zircon crystal metamorphic recrystallizations (between 760 Ma and 670 Ma) are concomitant to the garnet cores growth and suggest the beginning of the decompression. Probably these blocks were tectonically inserted into the subduction channel sediments during the extrusion process. An interval of 35 m.y. is admitted for the collisional migration of the active margin nappe system. It is registered in the U-Pb ages of 635-600 Ma from the zircon crystal overgrowths with low Th/U ratio. The collisional between these segments of Rodinia and Africa occurred between 670 Ma and 635 Ma, as indicated through the distinct metamorphic events and by the Santo Antônio metawackes maximum depositional age.