Magmatic-hydrothermal evolution of the Xuefengling rare-metal pegmatite deposit in the Western Kunlun (NW China): evidence from mineral geochemistry and Sr-Nd isotopic composition

• The quartz diorite at Xuefengling and the lithium-bearing pegmatites are not cogenetic. The former is an I-type granite with contributions from mantle-derived materials, whereas the latter is derived from an S-type granitic magma enriched in rare metals. • The compositional variations of garnet and apatite in the pegmatite clearly record the highly fractionated magmatic evolution from Li-poor to Li-rich stages. The Xuefengling lithium deposit is an important rare-metal pegmatite-type deposit within the West Kunlun-Karakorum orogenic belt. Based on systematic mineral geochemistry and whole-rock Sr-Nd isotope analyses, this study investigates the magmatic-hydrothermal evolution and genetic mechanisms of the deposit. The results show that the spatially and temporally associated quartz diorite (215 ± 2.1 Ma) is a metaluminous, high-K calc-alkaline I-type granite. Its relatively low εNd(t) values (−6.7–−5.4) and high initial ( 87 Sr/ 86 Sr)i ratios (0.7110–0.7147) indicate that it was derived from partial melting of mesoproterozoic crustal materials with involvement of a mantle component. The Xuefengling pegmatite (208–210 Ma) exhibits peraluminous characteristics and shows marked differences from the quartz diorite in apatite compositions (high F and low Cl vs. high F and high Cl). These differences effectively rule out a direct genetic relationship between the two rock suites and suggest derivation from separate magma sources. The mineral geochemical characteristics effectively record the evolutionary processes of the pegmatites. From the Li-poor pegmatites (QAM and QAT zones) to the Li-rich pegmatites (QS zone), apatite exhibits significant increases in MnO and F contents, while garnet shows progressively higher MnO/(MnO + FeO) ratios from the QAT to the QAM zone. Variations in major and trace elements (e.g., decreasing Y and HREE contents) together with Zr/Hf–Y/Ho systematics indicate that the QAM zone represents a more evolved magmatic stage than the QAT zone and experienced stronger melt–fluid interaction. These evolutionary trends are consistent with the metallogenic characteristics of rare-metal pegmatites across the West Kunlun–Songpan–Ganzi belt, where parental granites are typically strongly peraluminous two-mica S-type granites derived from metapelitic sources associated with Triassic flysch sequences. Integrating mineralogical, geochemical, and regional Sr–Nd isotopic constraints, the rare-metal mineralization at Xuefengling is therefore interpreted to have been primarily controlled by the highly fractionated evolution of an S-type granitic magma sourced from Triassic flysch sediments, emplaced within a Late Triassic syn -collisional tectonic setting.