Petrogenesis of the Melilitite–Nephelinite–Carbonatite Association and its Tectonic Implication for Northeastern Tibetan Plateau

Abstract The distribution and cycling of volatile elements are key to understanding intrinsic connections among geodynamic processes, tectono-magmatic evolution, and Earth’s habitability. Deeply sourced SiO2-undersaturated rocks offer valuable insights into volatile-rich melting events and the C-H-O inventory of the deep mantle. In the eastern Tibetan Plateau, Cenozoic alkaline magmatism coincides with the reactivation of sinistral strike-slip fault systems, implying a genetic relationship with deep-seated processes and tectonic extrusion during ongoing India–Asia convergence. This study presents a systematic investigation of the Miocene melilitite–nephelinite–carbonatite association in the West Qinling orogenic belt. We integrated bulk-rock geochemistry, mineral chemistry, and Sr-Nd-Pb-Mg-C-O isotopic data to elucidate the petrogenesis of these CO2-rich alkaline rocks and the controlling role of subduction-modified mantle in plateau outward growth. Geochemical modeling and in situ Sr isotopic analyses of clinopyroxenes reveal that primary melilititic and nephelinitic magmas evolved through fractional crystallization (olivine ± clinopyroxene) coupled with assimilation of isotopically enriched lithospheric materials. Carbonate melts derived from partial melting of metasomatized mantle lithosphere were mixed with ascending SiO2-undersaturated magmas and subsequently, underwent liquid immiscibility at shallow depth. Textural and in situ Sr isotopic evidence indicate that the extrusive carbonatites represent hybrid products formed between externally derived sedimentary carbonates and immiscible carbonate liquids. The varying low δ26Mg values (−0.61 to −0.27‰) of the melilitites and nephelinites reflect a carbonated source in the convecting mantle rather than magmatic differentiation during ascent. Covariations among δ26Mg, 206Pb/204Pb, and trace element ratios (e.g. Hf/Hf*, Ba/Th, and Ce/Pb) implicate contributions from an HIMU-type (high 238U/204Pb ratio or high µ) component sourced from recycled Paleo-Tethyan oceanic crust with carbonate-bearing sediments. Seismic tomography and spatiotemporal correlations between magmatic and tectonic events suggest that the India–Asia convergence reactivated pre-existing lithospheric weaknesses, inducing highly localized decompression melting of carbonated asthenospheric mantle. The resulting low-volume, carbonate-bearing SiO2-undersaturated melts migrated through translithospheric shear zones, highlighting a genetic link between deep C-O recycling and large-scale tectonic processes across the Tibetan Plateau.