Calcium and molybdenum isotopes trace eclogitic crust recycling in formation of unique carbonatite-hosted Mo-REE-U-Nb deposits in orogenic belt

Carbonatites in orogenic belts provide an exceptional natural laboratory for investigating crust-to-mantle recycling processes and for understanding the fluxes of carbon and other materials in subduction zones. The diverse nature of subducted materials contributes significantly to mantle heterogeneity and imparts varied geochemical signatures on associated carbonatites. However, the role of these materials in controlling specific types of carbonatite-hosted mineralization remains poorly understood. Carbonatites in the Qinling Orogenic Belt host unique Mo-REE-U-Nb mineralization and are characterized by enrichment of heavy rare earth elements (HREE), which contrasts with most global carbonatites that mostly show light rare earth element (LREE) and Nb enrichment. This study presents whole rock geochemical and Sr-Nd-Ca-Mo isotope data on a suite of carbonatite samples from the Huanglongpu Mo-REE and Huayangchuan U-Nb-REE deposits in the Qinling Orogenic Belt to constrain the role of crustal recycling in mantle heterogeneity and genesis of unique carbonatite-hosted mineralization.

The Huanglongpu and Huayangchuan carbonatite dykes consist predominantly of unaltered calcite, and exhibit geochemical features typical of primary igneous carbonatite. The absence of coeval alkaline silicate rocks, along with the lack of geochemical co-variations (e.g. Ca-Mo isotope ratios versus CaO, MgO and Nb/Th) suggest limited crustal contamination, and that isotopic fractionation during melt immiscibility and fractional crystallization processes was minimal. The light and variable Ca-Mo isotope ratios in the Huanglongpu type I carbonatite samples (δ⁴⁴/⁴⁰Ca = 0.72‰ to 0.86‰, δ⁹⁸/⁹⁵Mo = -1.08‰ and -0.61‰) and Huayangchuan carbonatite samples (δ⁴⁴/⁴⁰Ca = 0.55‰ to 0.80‰, δ⁹⁸/⁹⁵Mo = -0.76‰ to -0.13‰) are likely source controlled. The light Mo isotopic signature precludes the involvement of marine carbonate (δ⁹⁸/⁹⁵Mo = 0.07‰ to 2.51‰) in the mantle source, but points towards deeply subducted and dehydrated eclogitic rocks as the light Mo isotope reservoir. Binary mixing models of Mo-Sr and Ca-Sr isotopes confirm mixing processes between depleted mantle and eclogitic rocks including the subducted and metamorphosed continental crust (Dabie eclogite), as well as oceanic crust. The deeply subducted continental crust of the South China Craton with high Mo content, and the eclogitic sediments and oceanic crust of the Mianlue ocean with high REE, Nb and U provide the essential metal sources for the unique carbonatite-hosted Mo-REE mineralization at Huanglongpu and U-Nb-REE mineralization at Huayangchuan in the Qinling Orogenic Belt. Slab break-off during the Triassic post-collisional stage triggered asthenospheric mantle upwelling and partial melting of the refractory eclogite and lithospheric mantle to generate the metal endowed carbonatite melt. Our results suggest that deep subduction and recycling of eclogitic oceanic crust and continental crust are essential process for generating mantle heterogeneity and unique carbonatite-hosted Mo-REE-U-Nb mineralization in orogenic belts.