Dehydration of metasedimentary rocks triggers early water-flux melting of metagranitoids and influences strain localization

Abstract Large melt volumes in metagranitoid lithologies are described in numerous orogens and often play a major role in the strain distribution within the orogenic crust. In a continental crust devoid of free water, melt fractions above 10 vol% in metagranitoid rocks can only be generated by the influx of external water via a fluid phase. One possible origin of these aqueous fluids is the subsolidus dehydration of the surrounding metasedimentary rocks. To test this hypothesis, a path-dependent multi-system phase equilibrium model was developed. This model simulates the amount of aqueous fluid produced by a metasedimentary system in the temperature interval between the solidus of the orthogneiss and that of the metasediment. This amount of aqueous fluid is transferred to the orthogneiss system, and the new melt fraction is calculated. This simulation was carried out along eight prograde Pressure–Temperature (P–T) paths using metapelite/orthogneiss and metagraywacke/orthogneiss associations. The results show that metasedimentary rocks produce between 0.01–0.2 wt.% of aqueous fluid due to the breakdown of staurolite and paragonite, and the minor consumption of muscovite and biotite. Nevertheless, the small amount of aqueous fluid increases the melt fractions of the orthogneiss system by several percent, making orthogneiss the most fertile lithology along most prograde paths at temperatures below 750°C and pressures from 0.5 to 1.5 GPa. If less than half of the crust is composed of orthogneiss, the volume of melt generated in the orthogneiss is sufficient to significantly weaken it, allowing deformation to localize within the orthogneiss bodies. We propose that this strain localization situation creates preferential pathways for additional fluid influxes, which enhances partial melting in metagranitoid lithologies and creates a positive feedback loop. Our results suggest that this process may occur in most prograde P–T paths in orogenic crusts composed of metasedimentary rocks and orthogneisses.