Controlling factors of prolonged REE mineralization in the Maoniuping REE deposit: Constraints from alkaline granite in the syenite–carbonatite complex

Previous studies have shown that the formation of Maoniuping giant rare-earth elements (REE) deposit is closely related to the protracted magmatic evolution (>2 Myr) of syenite–carbonatite magma. However, the reasons responsible for the protracted magmatic evolution of the Maoniuping deposit has not yet been well constrained. The REE-barren alkaline granite intruding the Cenozoic REE-mineralized syenite–carbonatite complex provides an opportunity to resolve this problem. In this study, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotope analyses of the alkaline granite and syenite were undertaken to elucidate the long timescales of magmatic evolution in the Maoniuping deposit. Zircon U–Pb dating indicates that the alkaline granite formed slightly later (25.5 ± 0.4 Ma) than the syenite (27.2 ± 0.6 Ma). The former is characterized by high SiO2 (72.8–75.8 wt%), Na2O + K2O (10.5–11.6 wt%), and FeOT (2.24–2.87 wt%) contents, low Al2O3 (10.3–11.8 wt%), MgO (0.08–0.20 wt%), and CaO (0.04–0.33 wt%) contents, and negative Eu, Ba, Sr, Ti, and P anomalies. It has characteristics typical of A-type granitoids, such as high 10000 Ga/Al (3.46–5.16) and FeOT/(FeOT + MgO) (0.93–0.97) ratios, and high (Zr + Nb + Ce + Y) contents (650–1065 ppm). The distinct Sr–Nd–Hf isotopic and compositional signatures of the alkaline granite and syenite indicate that the former is likely a product of direct high-temperature partial melting of shallow crust induced by the upwelling of asthenospheric mantle in a transtensional setting. Combining our results with those of previous studies, we suggest that shallow-seated, continuous underplating of mantle-derived magmas and stable regional thermal anomalies were responsible for the prolonged magmatic evolution and REE mineralization of the Maoniuping syenite–carbonatite complex.