Fenitized pre-carbonatite Deccan basalts in and around the carbonatite diatreme of Amba Dongar, Gujarat, Western India

The late Cretaceous carbonatite-nephelinite diatreme at Amba Dongar area in western India intrude into the sedimentary Bagh beds overlain by Deccan basalt sequence. Being situated on the western periphery of the Deccan flood basalt, the thickness of basalt cover is thin. Basalt (pre-carbonatite) covers the large part of the ring complex in the outer periphery while in the central part of the ring structure thin basalt cover of the pre-doming topography has been preserved. Fragments of this basalt are caught up in the carbonatite breccia rimming central basalt. Large number of post-carbonatite basalt and dolerite dikes intrude in to central basalt, carbonatite breccia and sövite. Pre-carbonatite Deccan basalt shows widespread fenitization effects induced by the emanation from carbonatite magma which resulted into replacement of pyroxene by calcite in these basalts. As a result, the basalt has acquired a composition of ‘plagioclase-calcite’ rocks. Chemically the fenitized basalts show increase in Si, K, Fe, Fe and CO2, Ba, Sr, Rb, and REE and decrease in Mg and Al. Na decreases in some fenitized basalts wherein plagioclase has been replaced by calcite. With help of major, trace and REE geochemistry the changes brought about in the pre-carbonatite basalt are presented and discussed.