Petrogenesis of the Caledonian Shangyou Pluton in Southern Jiangxi: Implications for the Pre-Enrichment of Heavy Rare Earth Elements

Ion-adsorption-type heavy rare earth element (HREE) deposits are predominantly hosted in the weathering crusts of Yanshanian granites, whereas HREE deposits developed from Caledonian granites have rarely been reported. In this study, zircon and monazite U–Pb geochronology, whole-rock geochemistry, zircon Lu–Hf isotope analyses, and scanning electron microscopy (SEM) of REE-bearing minerals were conducted on the Caledonian Shangyou pluton, the parent rock of HREE mineralization, to investigate the mechanism of HREE pre-enrichment in the parent rock. Zircon U–Pb dating of two samples yielded weighted mean ages of 442 ± 3 Ma (MSWD = 0.20, n = 20) and 442 ± 4 Ma (MSWD = 1.15, n = 16), whereas monazite U–Pb dating yielded a Tera–Wasserburg lower-intercept age of 452 ± 7 Ma (MSWD = 2.30, n = 40), indicating that the pluton is Late Ordovician–Early Silurian (Katian–Rhuddanian) granite. The rocks are characterized by high SiO2 (75.90–78.49 wt.%), Al2O3 (11.91–12.88 wt.%), and K2O (4.04–4.95 wt.%) contents, A/CNK ratios of 1.13–1.25, differentiation indices (DIs) of 95–96, and zircon saturation temperatures of 716–740 °C. Their Nb/Ta and Zr/Hf ratios range from 3.70 to 5.91 and from 19.69 to 23.83, respectively, indicating that the Shangyou pluton is a highly fractionated S-type granite. Zircons from the two samples have εHf(t) values of −15.28 to 0.86 and −9.33 to 1.49, corresponding to two-stage Hf model ages (TDM2) of 1375–2392 Ma and 1335–2019 Ma, respectively. These isotopic characteristics suggest that the magma was derived predominantly from Proterozoic metasedimentary crust, with a minor contribution from mantle-derived material. The Shangyou pluton has ΣREY contents of 91.1–208.1 ppm and ΣLREE/ΣHREY ratios of 0.57–1.39, indicating favorable parent-rock conditions for the formation of ion-adsorption-type HREE deposits through weathering. HREE enrichment occurred during the late-magmatic and/or hydrothermal stages following extensive magmatic differentiation. During late magmatic–hydrothermal evolution, F-rich fluids metasomatized relatively weathering-resistant minerals crystallized during the late magmatic stage, producing weathering-prone REE-bearing minerals such as synchysite-(Y). This mineralogical transformation provided a favorable material basis for the subsequent formation of ion-adsorption-type REE deposits during weathering.