Role and origin of water-fluxed melting in the generation of High Himalayan leucogranites

Abstract

Knowledge of the role water has played in the evolution of Earth’s continents is crucial for improving our understanding of how its layers (e.g. crust and mantle) interact with each other and co-evolved. Orogenic belts are the preferred pathway for material circulation, heat transfer, and water cycling. The Himalayan orogen preserves a large number of syn- and post-collisional rocks, of which the Himalayan leucogranites are key to studying the role of water in crustal reworking. Although some geochemical proxies support water-fluxed melting in the source of Himalayan leucogranites, the timing of water addition, the scale of water-fluxed melting, and how water affects or controls the chemical and physical properties of the partial melts are not well constrained. Here, we filtered the published data on High Himalayan leucogranites to ensure that they represent the geochemical characteristics of primary melts. Our results show that the primary leucogranites with high Th/U and low Rb/Sr ratios were produced by water-fluxed melting as a result of the dissolution of monazite, melting of plagioclase, and formation of peritectic K-feldspar. Furthermore, thermodynamic modeling demonstrates that water-fluxed melting generates a high water content and a large volume of felsic water-unsaturated melt with chemical compositions similar to those of the High Himalayan leucogranites. We suggest that the external water is released by the breakdown of hydrous crustal minerals during dehydration melting and prograde metamorphism.