Abstract The Himalayan leucogranites represent typical high-silica (SiO2 70 wt.%) granite in continental collisional orogens. Their petrogenesis commonly involves crustal anatexis, fractional crystallization, and magmatic-hydrothermal processes, and is thus crucial for understanding the formation and evolution of the upper continental crust. Recent advances in multiple non-traditional stable isotopes have revealed extreme fractionation in these leucogranites. However, the role of magmatic-hydrothermal processes in shaping such isotopic signatures remains controversial, particularly for fluid-mobile elements like Zn. Here, we characterize the geochemical and Zn isotope signatures of highly evolved Kampa and Qianjingou leucogranites in the Himalayan orogen. These leucogranites exhibit significantly elevated δ66Zn values (0.29‰ to 0.91‰) compared to the average value of upper continental crust, which cannot be attributed to chemical weathering, source heterogeneity, or partial melting. Quantitative modeling shows that the Zn elemental and isotope characteristics of the Himalayan leucogranites also cannot be reproduced by fractional crystallization alone. We propose that a fluid assimilation-fractional crystallization (FAFC) process can well explain the heavy Zn isotope enrichment in the Himalayan leucogranites. Specifically, with the addition of magmatic fluids exsolved from underlying magma reservoirs, the Zn-depletion caused by fractional crystallization is partially compensated, and heavy Zn isotopes are preferentially introduced. This model is supported by the covariation between δ66Zn and both δ138/134Ba and δ87Rb within the Kampa samples, because deep magmatic fluids are simultaneously enriched in light Ba isotopes and heavy Rb isotopes. Analysis of major rock-forming minerals suggests that mica and tourmaline control the main Zn budget in the Himalayan leucogranites. These volatile-rich minerals exhibit δ66Zn values higher than those of coexisting plagioclase, which contradicts theoretically predicted inter-mineral Zn isotope fractionation trends. This phenomenon likely reflects the modification of shallow magma compositions by deep magmatic fluids as well. Meanwhile, ascending magmatic fluids can efficiently scavenge fluid-mobile fluxing components and rare metals to shallow levels of the magmatic system, thereby promoting crystal-melt separation and rare-metal enrichment in highly evolved granites. Therefore, the FAFC process should have made an important contribution to the formation of rare-metal mineralization and the extreme differentiation of the upper continental crust.
Luo et al. (Tue,) studied this question.