Abstract
•Mesoproterozoic carbonatite has moderate REE, high Nb contents, and mantle-derived C-O-Sr-Nd isotopes.
•C-rich mantle was formed by mixing between subducted carbonates and upwelling of plume.
•Paleozoic tectothermal-induced fluid alteration enriched REE and re-distributed Nb.
Mantle-derived carbonatites provide the most rare earth element (REE) and Nb metal raw materials for the global market. Among them, the Bayan Obo in China is a huge carbonatite body containing the world’s largest REE and second-largest Nb resources. However, neither the origin of the superlarge carbonatite nor of the REE and Nb mineralization are known. Here, we report the first Bayan Obo primary carbonatites with monazite and columbite U-Pb ages of ca. 1300 Ma. They show moderate REE and relatively high Nb contents and mantle-derived depleted 87Sr/86Sri (0.70255–0.70298), εNdt (0.2–2.2) isotopic values, distinct from the (ca. 433–408 Ma REE mineralization) banded rocks that have radiogenic Sr isotopes (up to 0.7060). Many REE and Nb minerals are veinlets or overprints in the banded rocks. Two types of δ13C (∼−4.2 ‰ and −0.16 – 0.29 ‰) isotopes with less radiogenic Sr (∼0.7030) isotopic values were identified in the carbonatites. They may represent different C sources involving interactions between a subducted C-bearing mantle source and an upwelling Mesoproterozoic plume. Therefore, we infer that the Paleoproterozoic assembly and Mesoproterozoic breakup of the Columbia supercontinent favored precursory mantle carbon enrichment and subsequent melting to generate a giant carbonatite with initial REE and Nb mineralization. External fluid alteration re-enriched REE and remobilized Nb.