Apatite fingerprints the magmatic-hydrothermal processes of the Wajilitag alkaline‑carbonatite complex, NW China

Apatite serves as a sensitive proxy of magmatic and fluid processes, thereby providing critical insights into the genesis of alkaline‑carbonatite complexes, which host globally significant rare earth elements (REE) and niobium (Nb) resources. We present an integrated textural, compositional, and isotopic study of apatite from the Wajilitag alkaline‑carbonatite complex (Tarim Large Igneous Province, NW China), and propose a genetic model involving multiple stages of magmatic-hydrothermal processes. Evolving compositional trends and consistent Sr-Nd isotopic signatures across the complex indicate that extensive fractional crystallization, dominated by clinopyroxene followed by Fe-Ti oxides and apatite, generated a continuous differentiation series from mafic–ultramafic rocks through alkaline to peralkaline syenites. Apatite from the alkali feldspar syenite (Ap-afs, ca. 283 Ma) records derivation from a depleted, mantle-derived alkaline magma, consistent with the early magmatic stage. Apatite crystals from the peralkaline syenites, i.e., aegirine nepheline syenite (Ap-ans, ca. 277 Ma) and nepheline syenite (Ap-ns, ca. 275 Ma), formed in volatile-rich (F, Cl) evolved magmas that had reached fluid saturation. They have progressive LREE and Sr enrichment, abundant fluid inclusions, and homogeneous Sr-Nd isotopic compositions recording the transition from magmatic to hydrothermal stages under relatively closed-system conditions. Apatite from the dolomite carbonatite (Ap-dc, ca. 264 Ma) occurs as fracture-filling aggregates with exceptionally low LREE and F but extreme Sr enrichment. Its decoupled Sr-Nd isotopic compositions are well modeled by binary mixing between mantle-derived magmatic fluids and Tarim basement-derived crustal fluids. In summary, these four apatite groups spanning from 283 Ma to 264 Ma demonstrate that apatite geochemistry can effectively resolve the temporal and petrogenetic relationships within alkaline‑carbonatite systems, from early magmatic differentiation through volatile exsolution to late-stage open-system fluid mixing.