Decoding apatite in volcanic carbonatitic breccia from Mongra, NW of Amba Dongar Carbonatite Complex, Gujarat, India: insights to genesis and rare earth element budgets

Chemical variations in apatite in context to carbonatites and trace element partitioning between apatites and carbonatite-rich liquids are important in assessing the petrogenesis and evolution of the carbonatites as well as the associated carbonatitic breccia owing to apatite's sensitivity to the surrounding magma composition. Volcanic carbonatitic breccia is one of the constituent rock types found outside the ring structure of the Amba Dongar Carbonatite Complex (ADC) situated in western India. In the present work, we report the mineral chemistry of apatites from the carbonatitic breccias of the Mongra region (ADC outer core) and compare them with apatites from ADC carbonatites. Apatite chemistry from Mongra displays a larger concentration of rare earth elements, manganese and chlorine when compared with those of ADC carbonatites. The morphology and distinct zoning of these apatites represent late-stage magmatic processes with high heavy rare earth element concentrations (high lutetium), followed by the interaction of fluids from the surrounding alkaline rocks. Variation in sulfur concentration in the apatites of this study indicates crystallization under mildly reducing conditions. Integrated field observations, petrography and apatite mineral chemistry from the Mongra region allow for an understanding of the genesis of apatites in the ADC outer core, with possible implications for late-stage mineral–melt interactions.