Assessing the budget of water-present melting in a heterogeneous continental crust

Melt fractions higher than 10 vol.% in meta-igneous rocks are reported from many orogenic belts and commonly exert a strong control on strain localization within the crust. The production of this melt requires the addition of external water to increase the degree of partial melting. One potential source of this water is the subsolidus dehydration of adjacent metasedimentary rocks. To evaluate this hypothesis, we developed a path-dependent, multi-lithology phase equilibrium model that simulates the amount of water released by metasedimentary rocks between their solidus and the orthogneiss solidus. The released water is then transferred as external fluid influx to the orthogneiss and the resulting melt fractions simulated. We applied this model to ten prograde pressure–temperature (P–T) paths using metapelite–orthogneiss and metagraywacke–orthogneiss associations.

Our results show that metasedimentary rocks release less than 1.0 mol% H₂O, mainly through the breakdown of staurolite and paragonite, with minor contributions from muscovite and biotite consumption. Despite these limited quantities, the water significantly enhances melt fractions in orthogneiss by several percent, making orthogneiss the most melt-fertile lithology along most prograde paths at temperatures below 750 °C. If orthogneiss constitutes half or less of the crust, the melt fractions generated are sufficient to substantially weaken it and localize deformation. We propose that such strain localization may promote the development of preferential pathways for further fluid influx, thereby enhancing partial melting in meta-igneous rocks and establishing a positive feedback mechanism. These results indicate that this process is likely to operate along most prograde P–T paths in orogenic crusts composed of metasedimentary rocks and orthogneisses.