WATER-FLUXED MELTING IN THE ZANSKAR HIMALAYA: HOW IT HAPPENS AND ITS ROLE IN WEAKENING THE CRUST

The Zanskar region, NW India, exposes the High Himalayan Crystalline (HHC) which is a sequence of medium- to high-grade rocks intruded by many granites. Rocks here underwent early dehydration melting that was later overprinted by water-fluxed melting producing extensive migmatites and leucogranites. Each event is recognizable in the field: early dehydration melting has produced leucosomes with peritectic garnet and sillimanite, in contrast to tourmaline-rich, two-mica leucosomes and leucogranites associated with later water-fluxed melting. Potential water source here are the underlying cooler rocks that were heated and dehydrated by the thrusting of the hot rocks of the HHC. This water migrated upwards causing extensive melting of the overlying hot rocks. We conclude that water was driven into hot rocks by melt channels. Up-temperature migration triggers water transfer from the melt to the surroundings promoting the generation of water-fluxed undersaturated melts, which water content is defined by the liquidus curves. This is a process of water-fluxed melting mediated by water-rich anatectic melts. And as these melts are undersaturated in water and are therefore capable of rising to form the leucogranite bodies. Finally, water-fluxed melting may have impacted on the continued development of the orogeny weakening the core of the orogeny and triggering the onset of a period of extension in Zanskar.