Late Proterozoic bimodal volcanic assemblage of Siwana subsidence structure, western Rajasthan, India

Neoproterozoic Malani bimodal volcanism occurs in a cauldron subsidence str ucture of a shield volcano at Siwana. This collapse caldera has exposed 45 volcanic flows comprising six of basalt, 20 of rhyolite, alkali rhyolite an d rhyodacite, 15 of trachyte, one of dacite and three of trachyandesite com position. Major and trace element geochemistry of lavas shows a bimodal cha racter in the mafic and felsic components. The lower felsic flows are alkal ine and highly enriched in HFS elements while the upper flows are peralumin ous in composition. Alkalinity has been reintroduced in the uppermost flows . Wide variation in incompatible trace element ratios indicates a nonfracti onal crystallization relationship between the mafic and felsic magma. Malan i volcanism was triggered in an extensional tectonic setting where geotherm was raised by the repeated influx of basaltic magma. Crustal extension inh ibited the bouyant rise of silicic magma.