Apatite and Whole-Rock Chemistry of Matrix in Volcanic Carbonatite Breccia From Outer Breccia of Amba Dongar Carbonatite Complex, Gujarat, India

Abstract

Volcanic carbonatitic breccia is the most voluminous carbonatite formation found in the Amba Dongar Complex (ADC). These breccias are present in both the inner ring dyke as well as the northwestern region of Mongra-Padvani, referred to as the outer breccia. This study presents the mineral chemistry of apatite along with whole-rock geochemistry of the matrix from volcanic carbonatitic breccia of the Padvani region from ADC. The breccias are matrix-supported with few carbonatitic clasts of ADC. Apatites are found in the groundmass and display a morphology and chemistry that are distinct compared to apatites from the coherent carbonatites of the ADC, and have significantly larger concentrations of rare earth elements. The trace element concentration of the carbonatitic matrix closely resembles that of ADC, with anomalously high Nb (290–960 ppm) linked to a significant presence of pyrochlore supergroup minerals. REE-bearing phases include monazite and pyrochlore group of minerals with minor parisite-Ce. The samples also exhibit high (La/Yb)_CN values (~42), and moderate (La/Sm)_CN values (~7), with similar REE patterns compared to the ADC carbonatites. The integrated study of petrography, whole-rock geochemistry, and mineral chemistry suggests that the Mongra-Padvani region is a secondary eruption centre in the northwestern region of ADC. An emplacement model is presented for the origin of the outer breccia. In addition, the present study highlights the economic relevance of carbonatitic breccia in the ADC, emphasising their potential as hosts for REE mineralisation. The significant concentration levels observed in these rocks suggest promising avenues for mining endeavours.