Malani rhyolites - A review

The late Proterozoic Malani bimodal volcanics constitute the largest suite of anorogenic acid volcanics in India. The volcanism took place during 745/- 10 Ma ago, succeeding the granitic activity of Abu pluton and ceased bef ore the onset of Marwar sedimentation. On the basis of field evidences, three stages of igneous activity have been recognised. Volcanics of the first stage are mostly basalt with occasional andesite or trachybasalts. These are subsequently covered by the voluminou s outpouring of peralkaline and peraluminous rhyolite, basalt, dacite and t rachyte flows. The third stage ceased with the outburst of ash flow deposit s. The dominant felsic volcanics are rhyolites and rhyodacites spread over an area of about 31,000 km(2). The other rock types associated with rhyolite a re trachytes, dacites, pitchstone, welded tuff, vitric, lithic and crystal ash, ignimbrite, obsidian, pyroclastic slates, agglomerate, volcanic brecci a and Volcanic conglomerates. Majority of the acid volcanics are high potas sic and a few are calcalkaline or low potassic in composition. Feldspar geothermometry suggests the temperature of equilibrium to be above 650 degrees C. Similar results were obtained by magnetite-ulvospinel geoth ermometry. Oxygen fugacity is estimated to be about 10(-18) under FMQ-Ni-Ni O buffer conditions. Malani volcanism was essentially under terrestrial conditions, although dep osition by aqueous conditions are also indicated. The volcanic eruptions ha ve been through fissures, shield volcanoes and central cones. The volcanism was triggered in an extensional tectonic regime of continental crust, wher e geotherm was raised by the repeated influx of basic magma. The initial ba saltic magma was possibly generated at deeper depth by 'hot spot' activity. This magma while migrating upwards supplied additional heat for the partia l melting of lower sialic crust resulting in the generation of felsic magma . The crustal extension has helped in the upward advancement of the felsic magma.