Potassic and low- and high-Ti mildly alkaline volcanism in the Neoproterozoic Ramada Plateau, southernmost Brazil |
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| Authors: | Carlos Augusto Sommer Evandro Fernandes de Lima Lauro Valentim Stoll Nardi Ana Maria Graciano Figueiredo Ronaldo Pierosan |
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| Institution: | aInstituto de Geociências, Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves 9500, Cx. Postal 15001, CEP 91501-970 Porto Alegre, RS, Brazil;bCentro de Estudos em Petrologia e Geoquímica/PRONEX/UFRGS, Av. Bento Gonçalves 9500, Cx. Postal 15001, CEP 91501-970 Porto Alegre, RS, Brazil;cIPEN/CNEN-SP, Av. Bento Gonçalves 9500, Cx. Postal 15001, CEP 91501-970 Porto Alegre, RS, Brazil |
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| Abstract: | Ramada Plateau Neoproterozoic volcanism represents a portion of the shoshonitic and mildly alkaline magmatism related to postcollisional events of the Brasiliano/Pan African cycle of southernmost Brazil. It is constituted by shoshonitic basic-intermediate lavas, followed by a bimodal sequence characterized by pyroclastic deposits, lava flows, and hypabyssal rocks with ages of 549±5 Ma. The shoshonitic magmatism presents greater K2O than Na2O2, K2O/Na2O ratios close to 1, and moderate large ion lithophile and high-field strength element contents. The bimodal basic-acid volcanism presents a transitional chemical affinity with features of sodic, silica-saturated alkaline to continental tholeiitic series. Observed basic and acid rocks with contrasting Ti contents are referred to as high- and low-Ti basalt-rhyolites. Another group of acid rocks with higher Nb, Ta, and Rb values was identified as high-Nb rhyolites. The Ramada Plateau magmatism is comparable to associations related to the final stages of orogenic cycles, in which shoshonitic and high- and low-Ti alkaline magmatism reflects the melting of subduction-modified sources, whereas the high-Nb magmas show less influence of subduction-related metasomatism and are closer to magmas produced from anorogenic sources. A model of magma generation in collisional settings involving slab break-off and asthenospheric upwelling is applied to the evolution of magmatism from subduction-related to anorogenic in the Ramada Plateau. |
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| Keywords: | Bimodal volcanism Mildly alkaline magmatism Postcollisional settings Shoshonites |
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