Anorogenic Acid Volcanic rocks in the Kundal area of the Malani Igneous Suite, Northwestern India: geochemical and petrogenetic studies

Acid volcanic rocks from the Kundal area of the Neoproterozoic (725±7 Ma) Malani Igneous Suite (MIS) are predominantly represented by rhyolite (porphyritic and non-porphyritic) with minor amounts of trachyte, welded tuff and volcanic ash beds. The non-porphyritic rhyolite is peralkaline (acmite normative;>1 AI and A/CNK ratio <1) whereas porphyritic rhyolite is peraluminous and metaluminous (corundum and anorthite normative; <1 AI). Acid volcanic rocks are characterized by low CaO, MgO, Fe2O3(t), Cr, Ba, Sr and Eu and high SiO2, NaO+K2O, Fe/Mg, Zr, Nb, Y and REEs (except Eu), indicating their A-type affinity. Negative anomalies of Ti, P, Sr and Ba in the multi-element spider diagrams indicate retention of plagioclase and accessory minerals in the source rock during partial melting. Rhyolite was probably derived from relatively low degrees of partial melting as they are enriched in LREE and incompatible elements compared to trachyte. Underplating of basaltic magmas at the base of the crust may have provided the heat source for extensive crustal melting. Petrogenetic modeling suggest that these acid volcanic rocks may have been derived from a melasyenite source by partial melting (5-20%) at ∼8 kbar pressure and ∼1000 °C temperature. An anorogenic setting for these acid volcanic rocks are also suggested which supports a rift-related tectonic environment for MIS in the Trans-Aravalli blocks of the Indian Shield.