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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 alkali-feldspar 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 alkali-feldspar, (6) ilmenite and rutile from the Ti-rich magnetite, (7) albite in the spherulites. Finally, alteration of sanidine, vitroclasts and pumice to (8) illite.
Highlights
The Camaquã Basin developed during the final stages of the Brazilian-Pan-African Orogeny (700 m.y–540 m.y.) in the Sul-rio-grandense Shield
During the late stages of Brazilian / Pan-African Orogeny (Neoproterozoic III), the Camaquã Basin was gradually filled by the bimodal volcanic rocks of alka
Line composition of the Acampamento Velho Alloformation (AVAf) (Paim et al 2000). It has been traditionally considered as exclusively acid in composition, but detailed geological mapping at the Cerro do Bugio, Cerro do Perau and Santa Bárbara area revealed the existence of a basalts/andesite unit at the base and a felsic unit at the top (Zerfass and Almeida 1997, Zerfass et al 2000, Almeida et al 2002), that leads to the existence of a bimodal alkaline volcanism: mafic at the base and a felsic unit at the top
Summary
The Camaquã Basin developed during the final stages of the Brazilian-Pan-African Orogeny (700 m.y–540 m.y.) in the Sul-rio-grandense Shield. The rhyolitic flow samples are siliceous, with SiO2 average of 77.2%, low CaO (average of 0.17%) and normal alkalinity, these rocks are more sodic than the pyroclastic ones (average of Na2O = 2.12% and K2O = 5.34). The increasing values of REE and a marked negative Eu anomaly are common in the felsic rocks (UFA) associated with mafic one (LMA).
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