Decoding Apatite in Carbonatitic breccia from Mongra, India: Insights to genesis & Rare Earth Element budgets

Abstract:

Carbonatitic breccia is one of the constituent rock types found in Amba Dongar Carbonatite Complex (ADC) situated in western India. These volcanic breccias are collected from Mongra in the north-western margin of ADC mostly found in a series of hillocks associated with nephelinites and stratified tuffs. In the present work, we report the EPMA mineral chemistry of apatites from carbonatitic breccias of Mongra describing the mineral chemistry followed by a comparison to apatite chemistry data sets of previous studies from the main ADC. The comparison of apatite chemistry from these two adjacent areas indicates a possible existence of a secondary eruption centre with distinct variation in the composition of apatites and the probable genesis associated with it.

The Back Scattered Images of apatites show fractures with an interlocking of more than one grain found within a calcitic matrix, and accompanied by grain boundary reactions with distinct zoning. In some apatites the boundaries appear to be fuzzy and transitional, whereas in others distinct grain boundaries are visible. These documented variations in apatite morphology and zoning represent a late-stage process compared to apatites from ADC with a remarkable surge in Heavy Rare Earth Elements (HREE). The LREE/HREE ratio is 9.11 for the Mongra apatites. The apatites from this breccia are Cl-rich fluorapatite with F/Cl = 5.524 compared to ADC with no Chlorine reported. The ∑REE of apatite from Mongra breccia is higher (18221 ppm) in contrast to the ADC apatites (17453 ppm). The concentrations of FeO^T, MnO, SO₃ & Cl are 0.094, 1.381, 0.041 & 0.577 respectively which are devoid in apatites from ADC. Indicator REE ratios sensitive to redox conditions like δCe & δEu form a fairly tight cluster range from 0.44-0.49 and 0.66-1.23 respectively. The zoned apatites inferred from these breccias manifest distinct growth rings along with discontinuous changes in concentration of elements along zoning.

The apatite from breccia is typically depleted in Sr, LREE with (La/Yb)_CN = 2.87 with presence of volatile elements and depletion in δCe from ADC is a strong indicator of hydrothermal genesis. Further, the comparison of Mongra apatite to that of ADC gives a preliminary idea of the presence of a secondary eruption centre which is validated by integrating field and geochemical studies.