CORRELATION BETWEEN MELTING, DEFORMATION AND FLUID INTERACTION IN THECONTINENTAL-CRUST OF THE HIGH HIMALAYAS, LANGTANG VALLEY, NEPAL

Combined metamorphic, stable-isotope and structural studies from the H igh Himalayan Crystalline sequence in the Langtang Valley of north-cen tral 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, t hus suggesting that these processes are strongly interdependent. Kyani te-grade rocks at the base of the section are unmelted and have underg one little syn- or post-metamorphic internal deformation and little pr ograde or retrograde fluid-rock interaction. By contrast sillimanite-g rade 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. A lthough 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 m ay have provided a primary control on deformation during both the magm atic stage and, subsequently, through the exsolution of exsolved aqueo us fluids.