Metasomatized mantle and diapirism via subducted marine sediments as the source of rare earth element–rich ultrapotassic alkaline igneous systems: Evidence from the postcollisional Fanshan complex, North China Craton

Abstract The rare earth elements (REEs), critical in a range of new technologies, are primarily hosted in alkaline-silicate rocks and carbonatites. Alkaline-silicate rocks, which also contain large ion lithophile elements (LILEs), are typically potassic and miaskitic, and often form in postcollisional settings. However, the processes by which their mantle sources metasomatized remain poorly understood. The Fanshan intrusion, located along the northern margin of the North China Craton, is a typical ultrapotassic alkaline complex enriched in REEs and LILEs, but depleted in high field strength elements (HFSEs), which makes it an ideal setting for investigating these processes. In this study, we present zircon U-Pb age and Hf-O isotopic data, and whole-rock geochemical and Sr-Nd-Pb isotopic analyses, to constrain the petrogenesis of the Fanshan complex. Zircon U-Pb dating yielded concordant ages of 224.1 ± 2.9 Ma and 222.8 ± 2.8 Ma, which correspond to the postcollisional phase of the North China Craton (260–220 Ma). The Sr-Nd-Pb isotopic data are uniform, with initial (87Sr/86Sr) ratios of ~0.7051, εNd(t) values of −5.11 to −5.39, and (206Pb/204Pb)i of 17.54–17.72. Zircon Hf-O isotopic data reveal a 10%–25% contribution of subducted marine sediments to the mantle source, which is linked to the subduction events of the Paleo-Asian Ocean. Based on the petrological and geochemical evidence, as well as the presence of cumulus apatite rocks, we propose that the Fanshan complex formed from multiple melt batches derived from both the previously metasomatized mantle wedge and an underlying apatite-rich mélange. These findings highlight the role of multiple melting events in the genesis of REE-rich alkaline-silicate rocks and provide a valuable reference for REE exploration in a similar geological setting.