Late Proterozoic Bimodal Volcanic Assemblage of Siwana Subsidence Structure, Western Rajasthan, India

Neoproterozoic Malani bimodal volcanism occurs in a cauldron subsidence structure of a shield volcano at Siwana. This collapse caldera has exposed 45 volcanic flows comprising six of basalt, 20 of rhyolite, alkali rhyolite and rhyodacite, 15 of trachyte, one of dacite and three of trachyandesite, composition. Major and trace element geochemistry of lavas shows a bimodal character in the mafic and felsic components. The lower felsic flows are alkaline and highly enriched in HFS elements while the upper flows are peraluminous in composition. Alkalinity has been reintroduced in the uppermost flows. Wide variation in incompatible trace element ratios indicates a nonfractional crystallization relationship between the mafic and felsic magma. Malani volcanism was triggered in an extensional tectonic setting where geotherm was raised by the repeated influx of basaltic magma. Crustal extension inhibited the bouyant rise of silicic magma.