泥質変成岩の部分融解におけるアルカリ長石と斜長石の花崗岩質メルトへの溶解過程 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 SiO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub>. Relatively voluminous melts form at feldspar-quartz boundaries, where the melt thickness increases with the run duration. In low-SiO<sub>2</sub> domains located relatively far from quartz grains, Al<sub>2</sub>O<sub>3</sub> mineral grains form around and in feldspar grains. The melts surrounding feldspars are enriched in Al<sub>2</sub>O<sub>3</sub>. In addition to the Al<sub>2</sub>O<sub>3</sub> 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 SiO<sub>2</sub> from quartz but also by the growth of Al-rich minerals and the interface reactions.
岩石鉱物科学 37(4), 96-109, 2008-07-30