Identifying recycled oceanic components and volatiles in the Lizhuang carbonatite-associated REE deposit in southwest China: New constrain from in situ Hf–O isotopes

Abstract

Carbonatite-associated rare earth element (REE) deposits (CARD) are mainly found at the edges of cratons in the rift or post-collisional settings, but only a minority carbonatite can form large to giant REE deposit. Fluids rich in volatiles (such as F, Cl, S) are critical for the extraordinary enrichment of REE, but the manner in which volatiles and REE concentrate in the mantle source of CARD remains unclear. This study focuses on the Lizhuang syenite from the Mianning–Dechang REE belt, southwest China, to investigate its petrogenesis, source characteristics, and volatiles evolution, through in situ Hf–O isotope analysis, coupled with mineral chemistry, bulk-rock geochemistry, and geochronology. The Lizhuang syenite was generated in a post-collisional setting at about 27 Ma, resulting from partial melting of an enriched lithospheric mantle, followed by fractional crystallization and immiscibility. When contrasted with barren carbonatite-syenite complexes worldwide, the syenite in REE deposit has a relatively higher content of F and S, with no significant difference in Cl content, and there is no clear correlation between magma oxygen fugacity and REE mineralization. The Hf–O isotopic signatures of zircon and apatite, along with bulk-rock Sr–Nd isotopic modeling, reveal that the mantle source of the Mianning–Dechang CARD experienced metasomatic processes related to the melts/fluids derived from both marine sediments and altered oceanic crust during subduction. The cratonic root, enriched in volatiles and REE, is reactivated during the upwelling of asthenosphere, and the released volatiles are able to participate in the formation of CARD.