Geochronological and Sr-Pb-C-O isotope constraints on the genesis of carbonatites from the polychronous Mundwara alkaline complex (north western India)

The genesis of carbonatites is commonly related to either direct melting of carbonate-bearing mantle rocks or derivation from CO₂-rich silicate melts via either igneous fractionation or liquid immiscibility. Testing these hypotheses requires accurate geochronological and isotopic constraints on carbonatite and silicate rocks from the same igneous complex. The polychronous Mundwara alkaline complex offers an excellent setting to examine the origin of carbonatites because it contains a range of silicate rocks (including alkali-gabbros, syenites and other alkaline lithologies) and their ages (110–68.5 Ma) and radiogenic isotopic compositions have been previously constrained. However, the age and isotopic composition of the Mundwara carbonatites are currently unknown. To further our understanding of carbonatite magmatism at Mundwara, new apatite U-Pb ages are combined with petrographic observations, bulk‑carbonate C-O isotope analyses and in-situ determinations of trace element contents and Sr-Pb isotopic ratios for calcite and apatite. The Mundwara carbonatite dykes consist of calcite cumulates along with accessory apatite, pyrochlore, alkali feldspar, Fe-oxides and biotite. A range of REE-bearing phases including bastnäsite, parisite, and monazite is also present, although in accessory amounts. Cumulitic and seriate texture along with high Sr contents (>1 wt%), attest to the primary igneous nature of the calcites. The apatites are magmatic as demonstrated by their pill shape, low Sr content and strongly fractionated chondrite-normalized REE patterns, which clearly distinguishes 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 (²⁰⁷Pb/²⁰⁶Pb = 0.820–0.829; ²⁰⁸Pb/²⁰⁶Pb = 2.065–2.088) and Sr isotopic compositions of the calcites are broadly intermediate between enriched mantle (EM) and HIMU (high ²³⁸U/²⁰⁴Pb) compositions. 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 excluding significant contamination or post-magmatic alteration as well as contribution by subducted carbon. Apatite defines an accurate, although imprecise U-Pb age of 100 ± 20 Ma, which closely matches the 110–102 Ma Ar-Ar ages of nepheline syenites from this complex. This observation coupled with overlapping Sr-Pb isotope compositions of the carbonatites and alkaline mafic rocks from Mundwara implies a close genetic link between carbonatites and silicate melts, which is best explained by either liquid immiscibility or differentiation of CO₂-bearing ultramafic melts. The age and isotopic features of the Mundwara carbonatites and related silicate rocks are best explained by melting of metasomatised lithosphere or upper convective mantle affected by deeply subducted material in response to the extensional stress regime associated with the early stages of India-Madagascar separation.