Geochronology, Geochemistry and Petrogenesis of the Ahelangan Granitic Pegmatite， West Kunlun

The Late Triassic pegmatites in the Dahongliutan region of the West Kunlun Orogenic Belt are genetically closely linked to rare-metal mineralization. However, the formation age, genetic type, and tectonic setting of the widespread barren granitic pegmatites hosted within the Dahongliutan pluton still lack systematic constraints. This study focuses on the Ahelangan granitic pegmatites. Based on detailed petrographic observations, we present integrated analyses including LA-ICP-MS zircon U-Pb geochronology, in-situ Lu-Hf isotopic measurements, and whole-rock geochemical analyses.Our results demonstrate that the Ahelangan granitic pegmatites yield a weighted mean zircon U-Pb age of (204.5 ± 1.4) Ma, constraining their formation to the Late Triassic Indosinian period. The rocks are classified as highly fractionated peraluminous granitic pegmatites, geochemically characterized by high SiO₂ and Al₂O₃, extremely low Mg, Fe, and Ti contents in major elements, enrichment in Rb, U, Sr, and Sm, depletion in Ta, Nb, La, Zr, Ti, and Yb, and low abundances of rare-metal elements such as Li, Cs, Ta, and Be. Whole-rock total rare earth element (ΣREE) contents range from 2.090×10⁻⁶ to 9.370×10⁻⁶, with δEu values of 0.881–8.577. Zircon εHf(t) values span from –7.22 to +6.35, corresponding to two-stage Hf crustal model ages (T_DM2) of 834–1695 Ma, indicating that the primary magma was predominantly derived from crustal sources with minor mantle-derived components involved.Integrated with the regional tectonic framework, the Ahelangan granitic pegmatites are interpreted to have formed in a post-collisional extensional setting. Comparison shows that the barren Ahelangan pegmatites, emplaced in a relatively closed system within the interior of the pluton, were derived from parental magmas that underwent more extensive cooling and fractional crystallization, resulting in increased melt viscosity. At the time of emplacement, the magmas had already reached the low-temperature terminal stage of magmatic evolution, which further suppressed the exsolution of ore-forming fluids and the migration of elements. Thus, low efficiency of fluid exsolution is considered one of the key controlling factors for the rare-element depletion in this type of pegmatite.