Dissolution processes of alkali feldspar and plagioclase in granitic melts during partial melting of a metapelitic rock

Partial melting experiments of migmatitic pelitic gneiss were performed at 850 °C and 200 MPa with varied run duration up to 1008 hrs. Granitic melts form at almost all grain boundaries between various minerals including alkali-free pairs such as sillimanite-quartz. The melts exhibit compositional heterogeneity, particularly in SiO2 and Al2O3. Relatively voluminous melts form at feldspar-quartz boundaries, where the melt thickness increases with the run duration. In low-SiO2 domains located relatively far from quartz grains, Al2O3 mineral grains form around and in feldspar grains. The melts surrounding feldspars are enriched in Al2O3. In addition to the Al2O3 mineral, euhedral An-rich plagioclase newly forms around alkali feldspar, typically at the alkali feldspar-plagioclase boundaries. On the other hand, pre-existing plagioclase is partially modified to form ‘reaction zones’ composed of fine-grained An-rich plagioclase and interstitial melt along twin surfaces and cracks. The An-rich plagioclase at the outermost part of these reaction zones subsequently dissolves in the melts. These results indicate that the dissolution processes of alkali feldspar and plagioclase in peraluminous granitic melts are controlled not only by diffusive supply of SiO2 from quartz but also by the growth of Al-rich minerals and the interface reactions.