Late sodic metasomatism evidences in bimodal volcanic rocks of the Acampamento Velho Alloformation, Neoproterozoic III, southern Brazil

Abstract

A mineralogical study was carried out in mafic and felsic volcanic rocks of the Acampamento Velho Alloformation at Cerro do Bugio, Perau and Serra de Santa Bárbara areas (Camaquã Basin) in southern Brazil. The Acampamento Velho bimodal event consists of two associations: lower mafic at the base and upper felsic at the top. Plagioclase and alkali-feldspar were studied using an electronic microprobe, and magnetite, ilmenite, rutile, illite and alkalifeldspar were investigated through scanning electron microscopy. The rocks were affected by a process of late sodic autometasomatism. In mafic rocks, Ca-plagioclase was transformed to albite and pyroxenes were altered. In felsic rocks, sanidine was partially pseudomorphosed, generating heterogeneous alkali-feldspar. In this association, unstable Ti-rich magnetite was replaced by rutile and ilmenite. In mafic rocks, the crystallization sequence was: (1) Ti-rich magnetite (?), (2) pyroxene and Ca-plagioclase, (3) albite (alteration to Ca-plagioclase), (4) sericite, chlorite and calcite (alteration to pyroxene), and kaolinite (alteration to plagioclase/albite). In felsic rocks: (1) zircon, (2) Ti-rich magnetite, (3) sanidine, (4) quartz. The introduction of late Na-rich fluids, generated the formation of (5) heterogeneous alkalifeldspar, (6) ilmenite and rutile from the Ti-rich magnetite, (7) albite in the spherulites. Finally, alteration of sanidine, vitroclasts and pumice to (8) illite.