CARACTERÍSTICAS GEOQUÍMICAS E DE POSICIONAMENTO TECTÔNICO DO MAGMATISMO NEOPROTEROZÓICO AFLORANTE NO PLATÔ DO TAQUAREMBÓ - RS

Abstract

The Neoproterozoic-Cambrian Taquarembo Plutonovolcanic Association, exposed in the Taquarembo Plateau, southern Brazil, is composed of volcanoclastic deposits, lavas and shallow intrusive rocks of silica-saturated alkaline affinity. Metaluminous basalts, basaltic andesites, dacites to trachytes, monzodiorites and rhyolites are associated with peralkaline syenites and rhyolites of comenditic affinity. Basic rocks were discriminated as low and high Ti-P, the former probably related with the metaluminous more evolved liquids and the second one with the peralkaline intermediate and acid rocks. REE, HFS and particularly Zr-P, discriminate the metaluminous and peralkaline rocks, increasing in the second group. Isotope and trace element based geochemical interpretation, suggests that both basic magmas are probably derived from the same EM1 mantle sources, and represent different melt fractions. The comenditic character, Nb- Ta negative anomalies in MORB normalised spidergrams, low Zr/Nb ratios, and LREE enriched patterns are suggestive of mantle sources modified by crustal subduction. Crustal influence was mostly inherited from a previous subduction and the consequent mantle metasomatism. The differentiation of parental magmas was dominantly through mineral crysytallization, involving mainly pyroxene and feldspar, with minor amounts of apatite, zircon and ilmenite. This magmatism was generated during the postcollisional period of Brasiliano Cycle, overlying the foreland of a continental margin arc.