Crustal shearing induced significant magmatic fractionation of Himalayan leucogranites

Himalayan leucogranites, some of the best-known high-silica granites in the world, are associated with abundant rare-metal mineralization, particularly lithium pegmatites in the Higher Himalaya. Although structural deformation has been proposed to play a major role in magmatic fractionation, how it enhances this process remains poorly understood. Here, we examine the Gyirong leucogranite, a representative Higher Himalayan leucogranite adjacent to the South Tibetan Detachment System (STDS), through detailed analyses on minerals and whole-rock samples along the transect. Mineral microstructures reflect the granites experienced high-temperature melt-present deformation, with typical differential stresses of the STDS. Systematic geochemical variations are observed, with increasing Li, Rb, and Cs and decreasing La, Sr, and Zr toward the STDS, reflecting a clear magmatic fractionation trend. The muscovite compositional zonings indicate that fractionated melt is concentrated at the top of the pluton without magmatic recharge signatures. According to the systematic geochemical composition, we divide the Gyirong leucogranite into cumulate and residual melt parts. The calculated crystallinity of cumulate part is higher than the maximum packing fraction, reflecting deformation must play a role. Forward modeling shows that at least 200 kyr is required to achieve the observed geochemical profile, longer than the cooling duration estimated by thermal modeling. Thus, we propose that crustal shearing could increase the efficiency of crystal-melt segregation, leading to significant magmatic fractionation of high-silica granitic magmas over short periods of time. Such a process is intrinsically linked to the Himalayan uplift, providing a critical mechanism for rare-metal elements enrichment.