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.

作为造山带重要的后碰撞岩浆产物，喜马拉雅隐色花岗岩是多种稀有金属（如Li、Be、Cs、Rb、Nb、Ta、Sn）成矿的关键赋矿岩。然而，人们对淡色花岗岩的形成和演化还缺乏很好的认识。特别是岩浆流体在输送和富集稀有元素方面的作用尚不清楚。在这里，我们测量了来自喜马拉雅带 Kampa Dome 的淡色花岗岩的 Ba 同位素组成，以了解淡色花岗岩形成过程中流体活动和流动元素的行为。我们的结果表明δ ^138/134淡色花岗岩的 Ba 范围为 -1.32‰ 至 +0.12‰，远低于 S 型花岗岩和各种沉积物质的文献值，表明淡色花岗岩的 Ba 同位素组成不能反映沉积源特征。相反，它们的低δ ^138/134 Ba 伴随着非电荷和半径控制 (CHARAC) 双元素（如 Nb/Ta）行为，清楚地表明岩浆演化过程中涉及岩浆流体。实验研究表明，低δ ^138/134Ba 的岩浆流体很可能是由大型深层岩浆储层的溶出作用产生的。这种流体不仅改变了淡色花岗岩的 Ba 同位素组成，而且还输送了许多可能有助于形成稀有金属矿床的流体移动金属元素。因此，Ba同位素数据为岩浆流体的形成演化和稀有金属成矿勘探提供了新的见解。