Correlation between melting, deformation and fluid interaction in the continental crust of the High Himalayas, Langtang Valley, Nepal

Combined metamorphic, stable-isotope and structural studies from the High Himalayan Crystalline sequence in the Langtang Valley of north-central Nepal reveal a strong positive correlation between distance above the base of the section (the Main Central Thrust), the amount of melt material and evidence of prolonged fluid and deformation histories, thus suggesting that these processes are strongly interdependent. Kyanite-grade rocks at the base of the section are unmelted and have undergone little syn- or post-metamorphic internal deformation and little prograde or retrograde fluid-rock interaction. By contrast sillimanite-grade rocks higher in the section contain progressively larger volumes of melt, have suffered increasingly complex syn- and post-metamorphic deformation and show increasing evidence for the presence of fluids. Although the factors that initiated these processes remain problematic, it is suggested that fluid distributions within the Langtang section have been passively controlled by the movement of melts. These melts may have provided a primary control on deformation during both the magmatic stage and, subsequently, through the exsolution of exsolved aqueous fluids.