Multi-stage enrichment in the mantle source region of cratonic lamproites: Geochemical and Sr-Nd-Hf-Os isotopic insights from the Mesoproterozoic lamproites of Bastar Craton, India

Deep cratonic mantle-derived magmatic rocks such as lamproites provide significant insights into the composition and evolution of the sub-continental and sub-lithospheric mantle. This study presents SHRIMP U-Pb geochronology of rutile and bulk-rock geochemistry including Sr-Nd-Hf-Os isotopes on lamproite dykes from the diamondiferous Nuapada Lamproite Field (NLF) at the tectonic contact between Bastar Craton and Eastern Ghat Mobile Belt, India. The lamproites sampled from the Darlimunda cluster of NLF exhibit inequigranularporphyritic texture with phenocrysts of olivine pseudomorphs in a groundmass rich in phlogopite. The abundance of low-temperature secondary phases such as vermiculite, chlorite, and carbonates in these rocks indicate significant post-magmatic alteration. The presence of quartz crystals and micro-veins along with zircon megacrysts indicate crustal assimilation to some extent but a strong enrichment in high-field strength elements buffers against significant crustal contamination. Rutile U-Pb geochronology reveals an emplacement age of 1009 +/- 121 Ma, which is consistent with the (i) previously reported whole-rock Ar-40/Ar-39 age of lamproites from the NLF, and (ii) widespread similar to 1.05 Gyr lamproite, kimberlite, and ultramafic lamprophyre magmatism in the Eastern Dharwar and Bastar Cratons, India. Moderate initial Sr-87/Sr-86 (0.703-0.706) and unradiogenic initial Nd-Hf isotopic (epsilon Nd of -5.96 to -8.08 and epsilon Hf of -7.25 to -9.02) composition is similar to the global cratonic lamproites and enriched mantle (EM I type) mantle reservoir. Moderately radiogenic and restricted initial 187Os/188Os (0.189-0.257) with low Os content, unlike that of the kimberlites and sub-continental lithospheric peridotites, show similarity to the Carboniferous orogenic Variscan lamproites from Bohemian massif and Neogene to Quaternary orogenic ultrapotassic mafic rocks from Italian peninsula. Based on the Sr-Nd-Hf-Os isotopic results, we propose that the cratonic lithospheric mantle source of these rocks record the evidence of a long-term enrichment in lithophile elements as a result of an ancient recycled crustal component. Elevated high-field strength elements in the lamproites and kamafugites of the NLF, similar to those of the global cratonic lamproites and ocean island basalts, represent an overprinting of ancient subduction-related metasomatized deep sub-continental lithospheric mantle sources by small-scale carbonatitic melts derived from the convecting sublithospheric mantle prior to their melting. Vein-plus-wall-rock mantle as a result of multi-stage mantle enrichment appears to play an important role in the origin of cratonic lamproites.