Geochronology, Geochemistry and Sr–Nd–Pb–Hf Isotopes of the Alkaline–carbonatite Complex in the Weishan REE Deposit, Luxi Block: Constraints on the Genesis and Tectonic Setting of the REE Mineralization

The Weishan rare earth element (REE) deposit at the southeastern margin of the North China Craton (NCC) is one of the three largest carbonatite-associated REE deposits (CARDs) in China. The REE-bearing carbonatite veins formed within quartz syenite and were controlled by NW- and NE-trending faults. The laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb ages of zircons from the quartz syenite and postmineralization granite porphyry dyke samples of the Weishan REE deposit yield weighted mean Pb-206/U-238 ages of 124.6 +/- 1.5 Ma (n = 10, mean squared weighted deviation (MSWD) = 1.7) and 123.3 +/- 1 Ma (n = 13, MSWD = 1.7), respectively. The quartz syenite and REE-bearing carbonatite in the Weishan REE deposit have similar Sr-Nd-Pb isotope compositions, and their diagenetic and metallogenetic ages are the same within error, indicating that they have a very close genetic relationship and originate from identical material sources. The Sr-Nd-Pb-Hf isotope compositions of both the quartz syenite and REE-bearing carbonatite are within the range of those of the Mesozoic isotopically enriched subcontinental lithospheric mantle (SCLM) beneath the southeastern margin of the NCC. The Weishan REE-bearing carbonatite is extremely enriched in REEs, Sr, and Ba and has a high Ba/Th ratio. A positive correlation between the REE and Ba contents and a negative correlation between the REE content and Sr/Ba ratio are observed in the Weishan carbonatite, indicating that the excessive enrichment of Sr, Ba, and REEs in the Weishan REE deposit might be related to the involvement of CO2-rich fluids from recycled marine sediments. Taking previous results on the Mesozoic tectonic evolution of the NCC into consideration, we infer that the parent metallogenic magma of the Weishan REE deposit formed by the partial melting of the REE-refertilized and isotopically enriched SCLM, which had been previously metasomatized by melts derived from the subducted South China Block (SCB) during continental collision and by high-flux REE- and CO2-rich fluids derived from subducted marine sediments and oceanic crust during the subduction of the Paleo-Pacific Plate.