Geochronology and C-O-Sr-Nd isotopic study of the Ulgii Khiid carbonatite from the Southern Mongolian alkaline Province and its implications for the mantle sources

Abstract

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 P2O5, 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 δ13CVPDB and δ18OVSMOW 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.