Santa Bárbara Formation (Early Paleozoic, Caçapava do Sul, southern Brazil): Petrographic and Sm–Nd isotopic provenance parameters |
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Authors: | Andr Weissheimer de Borba Ana Maria Pimentel Mizusaki Anderson Jos Maraschin Diogo Rodrigues Andrade da Silva |
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Institution: | aUniversidade Federal do Rio Grande do Sul, Instituto de Geociências (IG-UFRGS), Av. Bento Gonçalves, 9500, P.O. Box 15.001, Porto Alegre (RS), Brazil;bMinistério Público do Estado do Rio Grande do Sul, Divisão de Assessoramento Técnico, Rua Andrade Neves, 9, 4° andar (DAT/MP-RS), Porto Alegre (RS), Brazil |
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Abstract: | Integrated petrographic and Sm–Nd isotopic data were applied in order to constrain the provenance of the Early Paleozoic Santa Bárbara Formation, Sul-rio-grandense Shield, southern Brazil. This unit comprises continental sandstones, conglomerates and siltstones deposited under semi-arid climate in a rift or pull-apart basin. Samples were collected within a stratigraphic framework composed of three sequences, in which the two basal ones present northeastwards paleoflow, and the third one marks the inversion of basin filling. Samples from sequence I show, in the south, a strong influence of intermediate volcanic (Hilário Formation) sources, and a significant increase in quartz and metamorphic fragments upsection. In the northern deposits, there is a possible influence of juvenile units (Cambaí/Vacacaí), and a more significant input of Paleoproterozoic-sourced sedimentary rocks (e.g. Maricá Formation) upsection. Samples collected from the topmost deposits of sequence II present a clear increase in the amount of volcanic fragments (mostly acidic), reflecting denudation of the “Caçapava high”. Data obtained in sequence III (Pedra do Segredo) show a progressive decrease in quartz content and a significant increase in feldspathic, plutonic fragments. A more evolved phase of denudation of the “Caçapava high”, exposing leucogranitoids of the Caçapava do Sul complex, is proposed for this interval. |
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Keywords: | Sedimentary provenance Petrography Sm-Nd isotopes Santa Bá rbara Formation Sul-rio-grandense Shield |
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