Post-collisional Ediacaran volcanism in oriental Ramada Plateau, southern Brazil

Abstract

Ediacaran volcanic sequences in southernmost Brazil are related to intense post-collisional magmatism of the Brasiliano Orogeny. A portion of this volcanism occurs in the oriental Ramada Plateau located in the center part of the Rio Grande Sul State and is correlated with Hilário and Acampamento Velho formations. The first one is represented dominantly by lava flows and dikes of shoshonitic andesitic composition, besides of volcanogenic sedimentary deposits. The acid rocks of the Acampamento Velho Formation are expressive in the area, comprising high-silica ignimbrites, usually densely welded. Dikes and domes are common too and rhyolitic lava flows occur at the top and intercalated to ignimbrites in the middle of the sequence. The acid rock association has a sodic alkaline affinity. In this unit we mapped a subvolcanic sill of trachyte showing evidence for magma mixing with the rhyolitic magma. It has sodic alkaline affinity, and FeOt/FeO + MgO ratios and agpaitic index lower than those recorded in the rhyolites/ignimbrites. The Acampamento Velho Formation includes in this area, subordinately, basalts as àà flows and dikes intercalated with acid rocks. They have sodic alkaline nature and characteristics of intraplate basic rocks. New zircon U-Pb dating indicates crystallization age of 560 ± 2 Ma in a densely welded ignimbrite, 560 ± 14 Ma for a mafic trachyte and 562 ± 2 Ma for a subvolcanic rhyolite. The sodic alkaline rocks in this region evolved by fractional crystallization processes and magma mixing with major crustal contribution at approximately 560 Ma. The chemical characteristics are similar to those of A-type granites associated with Neoproterozoic post-collision magmatism in the Sul-rio-grandense Shield.