Apatite-calcite U-Pb geochronology, trace element and C-O-Sr-Pb isotope geochemistry from the polychronous Mundwara alkaline complex: New evidence of mid-Cretaceous Pre-Deccan carbonatite magmatism in northwestern India.

The polychronous Mundwara alkaline complex displays a range in ⁴⁰Ar-³⁹Ar ages between 68.5-110 Ma. It has been previously correlated to three distinct tectonomagmatic events: (i) the Deccan Large igneous Province associated with the Reunion plume, (ii) Indo-Madagascar breakup triggered by the Marion plume, and (iii) Rajmahal-Sylhet Traps linked to the Kerguelen plume. However, the age of carbonatites from the Mundwara complex was previously unknown and based on apatite U-Pb dating is now constrained at 100 ± 20 Ma. To further our understanding of carbonatite magmatism at Mundwara, this age is supplemented with petrographic observations, bulk-carbonate carbon and oxygen isotope analyses and in-situ determinations of trace element contents and Sr-Pb isotopic ratios for calcite and apatite. The Mundwara carbonatites consist of calcite cumulates and accessory apatite, pyrochlore, albite, orthoclase, Fe-oxides, and biotite. A range of REE-bearing phases is also present, including bastnaesite, parisite, and monazite. Cumulitic and seriate texture and high Sr contents (>1 wt%) attest to the primary igneous nature of the calcites. The apatites are magmatic, as demonstrated by their euhedral shape, low Sr content, and chondrite-normalized REE patterns, distinguishing them from typical hydrothermal apatite elsewhere. The apatite grains yield a weighted mean ⁸⁷Sr/⁸⁶Sr of 0.70447 ± 0.00003 (n = 24), indistinguishable from those of the carbonates analyzed in the same samples (⁸⁷Sr/⁸⁶Sr = 0.70446 ± 0.00001; n = 54). Lead (²⁰⁶Pb/²⁰⁷Pb = 0.820- 0.289; ²⁰⁶Pb/²⁰⁴Pb = 18.53-19.20) and Sr isotopic compositions of the calcites are broadly intermediate between enriched mantle (EM) and HIMU (high 238U/204Pb) compositions and signal a source that experienced geochemical enrichment by either metasomatism or addition of subducted material. The bulk- carbonate δ13C and δ18O data of the Mundwara carbonatites have a narrow range from -6.2‰ to -6.8‰ and from +6.3‰ to +7.3‰ respectively, showing typical mantle values and excludes significant contamination or post-magmatic alteration as well as contribution by subducted carbon. The mid-Cretaceous U-Pb age of the magmatic apatite overlaps with both the pre-breakup of the Indo-Madagascar event at ~88 Ma and the Kerguelen plume-induced magmatism (117 Ma) in the north-eastern parts of the Indian shield. Although this magmatic event cannot be assigned to a specific tectonic episode, this new temporal constraint and previously reported ages for other alkaline rocks from north-western India ascertains a pre-Deccan alkaline magmatic flare-up in this region.