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 before 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 voluminous outpouring of peralkaline and peraluminous rhyolite, basalt, dacite and trachyte flows. The third stage ceased with the outburst of ash flow deposits.
The dominant felsic volcanics are rhyolites and rhyodacites spread over an area of about 31, 000 km2. The other rock types associated with rhyolite are trachytes, dacites, pitchstone, welded tuff, vitric, lithic and crystal ash, ignimbrite, obsidian, pyroclastic slates, agglomerate, volcanic breccia and volcanic conglomerates. Majority of the acid volcanics are high potassic and a few are calcalkaline or low potassic in composition.
Feldspar geothermometry suggests the temperature of equilibrium to be above 650°C. Similar results were obtained by magnetite-ulvospinel geothermometry. Oxygen fugacity is estimated to be about 10−18 under FMQ-Ni-NiO buffer conditions.
Malani volcanism was essentially under terrestrial conditions, although deposition by aqueous conditions are also indicated. The volcanic eruptions have been through fissures, shield volcanoes and central cones. The volcanism was triggered in an extensional tectonic regime of continental crust, where geotherm was raised by the repeated influx of basic magma. The initial basaltic magma was possibly generated at deeper depth by ‘hot spot’ activity. This magma while migrating upwards supplied additional heat for the partial 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.
Write-up by Dr. S.K. Bhushan
Figure from Wei Wang, Manoj K.Pandit, Jun-Hong Zhao, Wei-Terry Chen and Jian-Ping Zheng