Stable strontium isotope fractionation by fluid-melt interaction recorded in the Himalayan leucogranites

The evolution of granitic magmas represents a crucial process in the compositional differentiation of the continental crust. Stable Sr isotopic composition (δ⁸⁸Sr), which may undergo significant fractionation during fractional crystallization, has been recognized as an effective geochemical tracer for investigating granitic magma evolution. However, the behavior of stable Sr isotopes during fluid-melt interaction remains poorly understood. In this study, we present Sr isotope data for the fluid-interacted Kampa leucogranite from the Tethyan Himalaya Sequence in the Himalayan orogenic belt. Our results reveal distinct δ⁸⁸Sr variations (−1.51 ‰ to 0.31 ‰). The extremely low δ⁸⁸Sr values of the leucogranites cannot be attributed to partial melting, inheritance from the sedimentary source, or fractional crystallization. Petrographic evidence such as metasomatic textures demonstrates that the Kampa leucogranite experienced significant fluid interaction. The relationships between δ⁸⁸Sr and multiple fluid activity indicators (i.e., Nb/Ta, Y/Ho, and TE_1,3) suggest that lighter stable Sr isotopic compositions are associated with intense fluid activity. The interaction between deep-derived Cl-rich fluids with light Sr isotopic signatures and evolved granitic melts is likely the primary cause of the low δ⁸⁸Sr values. Therefore, stable Sr isotopes can provide key constraints on late-stage evolution processes and fluid-melt interaction in granitic systems. Furthermore, the correlations observed among stable Sr, Ba, and Rb isotopes indicate that combined isotopic tracing of fluid-mobile elements may provide further insight into fluid activity.