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The Ulgii Khiid carbonatites show recycled C-O isotopic compositions (Fig. a), suggesting the incorporation of the subducted sedimentary carbonates into their mantle source. The Ulgii Khiid complex has relatively lower (87Sr/86Sr) i and higher ε Nd(t) values than the other carbonatite-alkaline complexes (e.g., Lugiin Gol, Mushgai Khudag, and Bayan Khoshuu) within the Southern Mongolian Alkaline Province. It is proposed that these rocks were derived from the underlying lithospheric mantle sources, which were heterogeneously metasomatized by the subducted materials due to the closure of the Paleo-Asian Ocean. [Display omitted] • Carbonatite is a product of liquid immiscibility from a carbonated silicate melt. • Subducted sedimentary carbonates incorporated into the carbonatite mantle source. • Southern Mongolian Alkaline Province source was modified by subducted materials. The Ulgii Khiid carbonatite-alkaline rock complex is located within the well-known Alkaline Province of Southern Mongolia, Central Asia. However, the complex's origin is poorly constrained, and its tectonic environment remains controversial (i.e., mantle plume vs. subduction-related post-collision). Here, we report apatite U-Pb dating, major and trace elements, and C-O-Sr-Nd isotopic data to constrain the origin and mantle source of the carbonatite and associated alkaline syenite rocks. The apatites yield an age of 166 ± 25 Ma, coeval to the related alkaline syenite's age reported previously. The carbonatite show higher P 2 O 5 , Sr, and REE but lower alkalis and HFSE (Nb, Ta, Zr, and Hf) contents. The carbonatite and alkaline syenite show restricted and similar Sr [(87Sr/86Sr) i : 0.7041–0.7049 and 0.7044–0.7048] and Nd [εNd(t): 4.1–4.8 and 4.2–4.7, respectively] isotopic compositions, indicating a common mantle source. Combined with the previous melt inclusion data, it is, therefore, suggested that the Ulgii Khiid carbonatite was formed by carbonate–silicate liquid immiscibility from a carbonated mantle source. The δ13C VPDB and δ18O VSMOW values of the carbonatites vary from −2.6 to −9.4‰ and 10.6 to 23.2‰, respectively and show a positive C-O linear correlation between the mantle and sedimentary carbonates, which imply the incorporation of the subducted sedimentary carbonates into the mantle source. We, therefore, propose that the Ulgii Khiid, and other carbonatite complexes in the Alkaline Province of Southern Mongolia, were produced via low-degree melting of the underlying carbonated lithospheric mantle, which is heterogeneously modified by the subducted Paleo-Asian Ocean. [ABSTRACT FROM AUTHOR]