H-, C-, and a-isotopes studies have been applied to characterize the nature and role of the fluids asssociated with the High Himalaya leucogranitic magmatism, its source rocks and the Main Central Thrust (MCT). 290 isotopic analyses of whole rocks, mineraIs and fluid inclusions coming from the sedimentary formations beneath (Midlands) and above (Tibet an Slab : the source of the Manaslu granite) the MCT and from the Manaslu granite define some of the metamorphic and magmatic processes. The O-isotope variations in the Midlands preclude major infiltration and circulation of fluids. As most of the rocks are very poor in carbon, the C02 -rich fluids observed in inclusions were generated at a deeper level. Dehydration of the schists with 2 to 5 wt% H2 O, liberated 20 to 40% of the initial water content in the region of the MCT where metamorphism was at a maximum, and &D decreased by about 15%0. At deeper levels, near the shear zone, devolatilization was probably more intense. For the granite, &D muscovite varies from -70 to -85%0 and &18 O whole rock from 10.9 to 12.8%0. H- and O-isotope fractionations among mineraIs are consistent with equilibrium at high temperatures. A few samples, especially from the Chhokang dyke swarm, have &D = > -180 %0 and disequilibrium muscovitebiotite fractionations. These rare samples are related to chloritization and late exchange with meteoric waters. Formation 1 gneisses of the Tibetan Slab (source of the granite) have &180 = 12 to 14.3%0, confirming its relationship with the granite. The &D values of Formation 1 are however about 20%0 higher than those of the granite. This difference is related to (1) magmatic degassing associated with a H-isotope fractionation, and (2) the infiltration of D-depleted fluids into the zone of anatexis. Fluids liberated from the underlying Midlands Formation, many of whose initial &D were <-80, are the probable source. The combined chemical and isotopic (O, Sr and Nd) data indicate that the isotopic variations in the Manaslu granite were inherited from those in Formation 1. In addition, these data on Formation 1 can be related to the nature of the sediments and, for O and Nd, to the proportions of detritic and sheet silicate minerais.
