High-silica volcanism of the Tupanci Region, Northwestern portion of the Sul- -Rio-Grandense Shield: faciology, petrography and lithochemistry

Abstract

Volcanic and hypabyssal acid rocks occur in the Tupanci area, NW portion of the Sul-Rio-Grandense Shield. These rocks are stratigraphically correlated to the Acampamento Velho Formation (~550 Ma), at the Camaquã Basin. This region has the northernmost exposure of this volcanic episode, which comprises effusive/ hypabyssal and pyroclastic rocks, with dominantly acid composition and sodic-alkaline affinity, whose genetic processes are linked to the post-collisional stages of the Brasiliano/Pan-Africano orogenic cycle. Acid volcanic rocks occur mainly as effusive deposits and also as pyroclastic deposits, in two Cerros (Hills): Tupanci and Picados. Cerro Tupanci defines an elongated (N-S) sub-volcanic intrusion of porphyritic rhyolites with phenocrysts of alkali feldspar and quartz surrounded by equigranular-fine-grained to aphanitic quartz-feldspar matrix, with a strong flow foliation on border regions. Cerro dos Picados shows texturally similar rhyolites, but with aphanitic to glassy matrix and presence of biotite; and pyroclastic deposits, characterized by rhyolitic ignimbrites. Ignimbrites occur in two facies: lithic-rich, with few devitrified and poorly elongated pumice and crystal fragments; and rheomorphic, with abundance of devitrified pumices with detachable eutaxitic texture, crystal fragments and rarely lithic fragments. Geochemical behavior allows to classify the magmatism as silica oversaturated, similar to the systems with high-silica, alkaline affinity and a metaluminous to slightly peralkaline trend with similar characteristics to “A” type granites. Petrographic and lithochemical data indicate a genetic linkage with the Acampamento Velho Formation magmatism