Petrogenesis of pegmatites in the Shakhdara−Alichur dome, South Pamir (Tajikistan): Constraints from apatite and monazite U-Pb geochronology, geochemistry, and Sm-Nd isotope analysis

The petrogenesis of pegmatites in collisional orogens provides key insights into crustal melting and reworking. To investigate the age and source of pegmatites in the Shakhdara gneiss dome (South Pamir, Tajikistan), we integrated in situ apatite and monazite U-Pb geochronology, trace-element geochemistry, and Sm-Nd isotope analysis. U-Pb ages reveal pegmatite crystallization in the late Miocene (ca. 14−11 Ma for apatite; ca. 21−10 Ma for monazite) and host gneiss formation in the Eocene−Oligocene (ca. 35−27 Ma for apatite; ca. 28−22 Ma for monazite). The Nd isotopes show that pegmatite apatite [εNd(t) = −32.2 to +3.0] preserves signatures of reworked ancient crust, while monazite in both lithologies [εNd(t) = −20.8 to −4.2] reflects Nd equilibration during high-temperature anatexis. The F-rich, heavy rare earth element (HREE)-enriched, and Sr-depleted signature of the pegmatite apatite indicates crystallization from an evolved felsic melt. This melt was generated via anatexis of ancient basement crust, triggered by asthenospheric heating during lithospheric extension. In contrast, gneiss apatite is Cl-rich and light REE (LREE)-enriched, consistent with metamorphic recrystallization. Similarly, gneiss monazites are homogeneous and LREE-rich, contrasting with the chemically variable pegmatite monazites. Texturally, pegmatite minerals preserve euhedral forms, whereas gneiss grains show embayed margins and patchy zoning indicative of fluid-assisted recrystallization. Collectively, these features define a two-stage, genetically linked model for REE enrichment. Stage A (ca. 12.9−11.0 Ma) involved magmatic differentiation, which enriched the pegmatites in REEs and homogenized the isotopic signatures of the host gneiss. Stage B (ca. 11−10 Ma) featured hydrothermal overprinting by resolved fluids, driving the dissolution−reprecipitation of apatite and monazite. This magmatic−hydrothermal continuum confirms a common anatectic source. While apatite records signatures from ancient sources and fluids, monazite reflects isotopic homogenization, documenting the multistage reworking of the crust of an evolving collisional orogen. This study provides critical constraints on the petrogenesis and mineralization potential of pegmatite−gneiss systems in collisional orogens worldwide.