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Kampa leucogranites have SiO 2 contents ranging from 74.0% to 78.3%. The δ 138/134Ba of most Kampa leucogranites ranges from −1.32 to +0.12‰. This is similar to that of A-type granites but much lower than those of published S-type, I-type granites, highly fractionated I-type granites and the average value of the upper continental crust (~ 0.00 ± 0.04‰). The low δ 138/134Ba of Kampa leucogranites suggests the participation of exotic magmatic fluids exsolved from deep magma reservoirs during magmatic-hydrothermal interactions, indicating potential rare metal mineralization. [Display omitted] As an important post-collisional magmatic product in the orogenic belt, the Himalayan leucogranites are the critical host rocks for a number of rare-metal mineralization (such as Li, Be, Cs, Rb, Nb, Ta, and Sn). However, there is still a lack of good understanding on the formation and evolution of the leucogranites. Particularly, the role of the magmatic fluids in transporting and enriching the rare elements is not clear. Here we measure Ba isotope compositions for leucogranites from the Kampa Dome of the Himalayan belt to understand the fluid activity and behavior of fluid-mobile elements during leucogranite formation. Our results show that the δ 138/134Ba of leucogranites range from −1.32‰ to +0.12‰, much lower than the literature values for S-type granites and various sedimentary materials, suggesting that the Ba isotope compositions of the leucogranites does not reflect the sedimentary source signatures. Instead, their low δ 138/134Ba is accompanied by non-charge-and-radius-controlled (CHARAC) twin-element (such as Nb/Ta) behaviors, clearly showing the involvement of magmatic fluids during magma evolution. Experimental studies suggest that the low δ 138/134Ba of the magmatic fluids most likely results from exsolution from a large deep magma reservoir. Such fluids not only modified Ba isotope compositions of the leucogranites, but also transported many fluid-mobile metal elements which may help form the rare metal ore deposits. Therefore, Ba isotope data provide new insights into formation and evolution of magmatic fluids and exploration of the rare-metal mineralization. [ABSTRACT FROM AUTHOR]