Abstract
Heavy rare earth elements (HREEs) are critical metals in advanced technologies. Here, we report unusual REE patterns and O-Hf-Nd isotopic compositions of composite granites from the Zhaibei HREE deposit. The complex consists of two Mesozoic granites with ages of 188 and 170 Ma. The older granites are characterized by higher HREE contents and negative Ce anomaly and contain cerianite veinlets. Their REE-rich minerals also show strong Ce depletion, indicating a high oxidation state. Magmatic zircons have variables delta O-18 (6.1-9.4 parts per thousand) and epsilon Hf-(t) (-6 to 0), towards a depleted mantle contribution. Lower delta O-18 (similar to 5.4 vs 6.6 parts per thousand) and higher epsilon Nd-(t) (approximately -3 vs -11) values in the altered apatite rim compared to the core and primary phase result from subduction-derived fluids. In contrast, the younger granites have lower REE contents and lack a Ce anomaly. They show mantle-derived Nd (apatite epsilon Nd-(t) = 1-2), Hf (zircon epsilon Hf-(t) = 3-8), and O (zircon and apatite delta O-18 = 4.6-5.7 parts per thousand) isotopic compositions. Therefore, we infer that a REE-rich fluid migrated from the flat subducting slab and flushed the subsolidus granitic mush in the Early Jurassic. This caused volatile saturation and degassing, leading to the generation of highly oxidized fluids. These processes may cause temporary oxidizing conditions in post-magmatic hydrothermal stages, resulting in the oxidation of Ce3+ to Ce4+. The resulting Ce fractionation from other REE3+ promoted HREE enrichment in the granites.