Rare-metal enrichment mechanism of leucogranites and pegmatites from Central Lhasa Terrane, Tibet: Constraints from geochronology, geochemistry, and mineral chemistry

Leucogranites and pegmatites are typically considered as high-silica granites, which are valuable economic resources due to their common association with rare-metal mineralization. Recent ore prospecting shows that the leucogranites in the Himalayan terrane of Tibet is emerging as a new world-class rare-metal metallogenic belt. Nevertheless, the research on rare-metal mineralization in the adjacent Lhasa terrane of Tibet is limited. In this paper, we present comprehensive results of petrology, mineralogy, geochronology, and geochemistry analyses for the Yangbajain leucogranites and pegmatites located in the central Lhasa subterrane. Zircon and monazite U-Pb dating revealed the existence of two distinct periods of beryllium mineralization in the Yangbajain area, occurring at ca. 32 Ma and 18 Ma. The leucogranites and pegmatites exhibit high SiO2 and Al2O3 contents, along with low MgO and total iron content. Additionally, the zircon εHf(t) values (−4.6 to +3.6) and apatite Sr-O isotopes (87Sr/86Sr = 0.71308−0.72413, δ18O = 5.01‰−7.04‰) suggest that two periods of leucogranites and pegmatites were derived from partial melting of ancient crust material (ancient lower crust and ancient Lhasa basement). Furthermore, the Li, Be, Rb, Cs, and F concentrations of leucogranitic and pegmatitic magmas have been evaluated based on the trace element compositions of muscovite. The reconstructed granitic and pegmatitic magmas present positive correlations between Rb and Cs contents, indicating they underwent intense fractional crystallization. Notably, decreasing F contents during the pegmatite stage, along with low Be contents (0.2−34 ppm), which are below the beryl saturation value (70 ppm), suggest that undercooling caused by the removal of fluxing components (e.g., H2O and F) from the magma system played a pivotal role in beryl mineralization. Compared with previously published data for Himalayan leucogranites and pegmatites (whole-rock Rb/Sr ratio: 0.2−405, Be: 0.09−440 ppm, total rare earth element [ΣREE]: 0.18−449 ppm), Yangbajain leucogranites and pegmatites (whole-rock Rb/Sr ratio: 3.3−16.6, Be: 3.52−13.16 ppm, ΣREE: 2.27−147 ppm) show a lower degree of differentiation, but they share many similarities in genesis, including similar major formation periods (Oligocene−Miocene) and origination from partial melting of the ancient crust material.