Genetic links between large-scale REE mineralization and intense differentiation of alkaline silicate magmas: Insights from the Weishan syenite-carbonatite complex, China

Mantle-derived alkaline-carbonatite complexes are the primary global source of rare earth elements (REE), yet the important factors promoting REE mineralization remain a subject of debate. In this study, we present an integrated study of the Weishan alkaline-carbonatite complex in North China Craton, combining petrographic observations, whole-rock geochemical and Sr-Nd-Mg isotopic analyses, and in situ mineral chemical analyses. The Weishan complex is mainly composed of Early Cretaceous (~120 Ma) syenite and carbonatite. Syenites form the main intrusive phase, whereas carbonatites occur as veins that intrude both syenites and country rocks. Economic significant REE mineralization is spatially associated with the carbonatite veins. The syenites and carbonatites share overlapping Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7065–0.7086, εNd(t) = -9.7 to -7.7], indicating a common origin from an enriched lithospheric mantle source. In addition, the syenites display pronounced enrichment in light Mg isotopes, with δ26Mg values ranging from -1.60‰ to -0.21‰, implying that the mantle source had been metasomatized by recycled sedimentary carbonates.Geochemically, the syenites are characterized by high SiO2 (62.7‒70.4 wt.%) and total alkali contents (Na2O+K2O: ~10.2 wt.%), coupled with a pronounced depletion in MgO (~0.28 wt.%), Fe2O3T (~1.91 wt.%), TiO2 (~0.17 wt.%), and CaO (~2.00 wt.%). Furthermore, the syenites exhibit high total REE concentrations (~1614 ppm) and elevated (La/Yb)N (50‒565), Zr/Hf (49‒70), and Nb/Ta (35‒73) ratios. They also contain compositionally zoned clinopyroxene, with Ca-rich diopsidic cores grading into Na-rich aegirine-augite rims. These integrated lines of evidence underscore that the syenites are products of highly evolved magmas. Through protracted fractional crystallization, the melt became progressively enriched in alkalis, volatiles, and REE. Such advanced differentiation ultimately promoted the segregation of REE-rich carbonatitic melts, culminating in large-scale mineralization during late magmatic to hydrothermal stages. These results demonstrate that carbonated alkaline melts derived from an enriched mantle source and undergoing advanced differentiation are critical for REE fertility, with broader implications for the genesis of REE deposits in alkaline-carbonatite provinces worldwide.