[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-P08] Phase equilibrium of high-grade gneisses of the Ryoke metamorphic complex in Yanai area, SW Japan

*Takeshi Ikeda¹, Kazuhiro Miyazaki², Risa Hayashi¹, Yu Sugawara¹ (1.Department of Earth and Planetary Sciences,Graduate School of Scinece, Kyushu University, 2.Geological Survey of Japan (AIST))

Keywords:Ryoke metamorphism, phase equilibrium, Yanai area

Three zones, K-feldspar-cordierite (KC), sillimanite-K-feldspar (SK) and garnet-cordierite (GC) zone, are defined based on pelitic mineral assemblages in the high-grade region of the Ryoke metamorphic complex in Yanai area. Andalusite and/or sillimanite in KC zone occur in cordierite-bearing rocks, whereas sillimanite in SK zone occurs in garnet-bearing rocks. Garnet coexists with cordierite in GC zone. These features in the pelitic assemblages and the pressure(P)-temperature(T) conditions of the three zones are mutually consistent in terms of the traditional AFM approximation (K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O system with excess quartz, K-feldspar and aqueous fluid). However, the Mg/(Fe+Mg) values of garnet in SK zone are the same as or lower than those in KC zone, and the Mg/(Fe+Mg) values of biotite in SK zone are lower than those in KC zone. This feature is inconsistent with the prediction of the AFM approximation that garnet and biotite would have higher Mg/(Fe+Mg) in SK zone of high pressure than KC zone of low pressure. The spessartine content of garnet in KC zone is higher than that in SK zone. Addition of MnO to the system increases garnet stability, where the maximum Mg/(Fe+Mg) of garnet increases with increasing spessartine content. This also increases Mg/(Fe+Mg) of the biotite that coexists with the garnet. The above discrepancy is explained by the addition of MnO to the system. We conclude that the AFMMn system can describe the phase equilibrium of pelitic and siliceous gneisses of the study area.