Petrology and Chemistry of Preeambrian Alkaline Rhyolites from Western Bahia (Brazil)

Abstract

The petrology and chemistry of alkaline rhyolites from Western Bahia State are investigated. Original mineralogy and bulk chemistry are modified by secondary processes (cataclastic metamorphism, hydrothermal leaching). The overall chemistry suggests a pattern consistent with derivation from material of near quartz-feldspar cotectic composition. This may represent a pathway for crustal fractionation and for partial melting within the continental crust of material of potassium-rich granitic composition. Ancient basemental biotite-rich gneisses are considered to be the most probable starting material. Possible correlations of volcanism with the main tectonics of the area and of the whole Brazilian shield are discussed.