MAFIC ALKALIC MAGMATISM IN CENTRAL KACHCHH, INDIA: A MONOGENETIC VOLCANO FIELD IN THE NORTHWESTERN DECCAN TRAPS

Magmatism in Kachchh, in the northwestern Deccan continental flood basalt province, is represented not only by typical tholeiitic flows and dykes, but also plug-like bodies, in Mesozoic sandstone, of alkali basalt, basanite, melanephelinite and nephelinite, containing mantle nodules. Their volcanological context is poorly understood. Based on new and published field, petrographic and geochemical data, we identify this suite as an eroded monogenetic volcano field. The plugs are very shallow intrusions (necks, sills, dykes, sheets, laccoliths), there are local peperites reflecting mingling between magmas and soft sediments, and a well-developed phreatomagmatic vent, made up of lapilli tuff rich in mafic lithic blocks, and injected by mafic alkalic dykes. The lapilli tuff contains basalt fragments, glass shards, and detrital quartz and microcline, and the peperites and the phreatomagmatism are consistent with existing evidence for a humid subtropical climate in Kachchh at ~65 Ma. The Central Kachchh monogenetic volcano field has >30 individual structures and possibly many more if compositionally identical igneous intrusions in northern Kachchh are proven by future dating work to be contemporaneous. The field has important tectonic implications (low-degree, diffuse crustal extension and low-degree mantle melting, unlike significant directed extension and vigorous mantle melting under the Deccan proper region of flood basalts themselves). The field attests to regional volcanological, compositional, and tectonic variability within flood basalt provinces, and adds the Deccan Traps to the list of such provinces containing monogenetic- and/or hydromagmatism, namely the Karoo-Ferrar and Emeishan flood basalts, and plateau basalts in Saudi Arabia, Libya, and Patagonia.