Summary: The Langtang section records strongly contrasting fluid and deformation histories between the upper and lower levels of the HHC. In general, the proportion of melt and the length and complexity of both the deformation and the fluid histories all increase with distance from the base of the section.
Fluid-absent conditions probably prevailed during peak metamorphism, with anatexis occurring via dehydration-melting reactions. Evidence for fluids during the cooling history, as an aid to both closed- and open-system oxygen-isotope exchange, appears to relate to the presence of melt material, suggesting that melting has been an important mechanism in the redistribution of aqueous fluids within the HHC. Although a small fluid influx could have been important in triggering melting, fluid transport has largely been a passive response to melt movements. An exccption to this is the local influx of surface-derived fluids late in the exhumation history at the top of the HHC, associated with brittle deformation.
The relationship between anatexis and deformation is much more difficult to define. In the sillimanite-zone, the onset of migmatisation during peak metamorphism may have aided distributed syn-metamorphic deformation, in contrast to the unmelted, undeformed kyanite- grade rocks beneath. The fact that pervasive, internal deformation ceased soon after peak metamorphic conditions (and hence melt crystal- lisation) supports this idea. However, deformation may merely record the tectonic process by which the rocks passed into a pressure-temperature regime suitable for melting, or may have provided an energy input necessary to initiate melting (e.g. Hand and Dirks, 1992).