Abstract
The alkaline acid volcanism of the Southern part of the Taquarembo Plateau, Dom Pedritro, RS, is located at the Western portipn of the Sul-rio-grandense Shield. This Neo-Proterozoic unit, named Acampamento Velho Formation, is represented by effusive and pyroclastic rock succession, referred as Acid Volcanic Sequence (SVA). The volcanic activity was characterized by pulse successions, effusive and explosive, as registered by the presence of lava flows, flow and surge pyroclastic deposits. Re-worked volcanoclastic deposits, with abundant juvenile clasts and volcanogenic sedimentary rocks are also observed. The SVA stratigraphy shows an early trachytic followed by rhyolitic volcanism. Themain geochemieal characteristics of the SVA are the high values of SiO 2 , FeO t , Na 2 O, K 2 O, Rb, Zr, Nb, Y, ETR, high FeO t /FeO t +MgO ratio and low values of Al 2 O 3 , MgO, CaO, Sr and Ba, which characterize this magmatism as of alkaline affinity and peralkaline character (comendiitic). The magmatic evolution was dominated by mineral fractionation, involving mainly plagioclase, alkali-feldspar, pyroxene and magnetite. The REE pattern and the tectonic environmental discriminants diagrams (e.g. Nb x Zr) classify this volcanism as typical post-orogenic magmatism, associated with the later stages of the Brasiliano Cycle at the Sul-rio-grandense Shield.
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