[S-MP27] Deformed rocks, Metamorphic rocks and Tectonics

[SMP27-03] Thermobaric structure of the Ryoke metamorphic complex in Yanai-Hikari area, SW Japan, buffered by dehydration melting

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

Keywords:dehydration melting reaction, thermobaric structure of volcanic-plutonic system, Ryoke metamorphic complex

The Ryoke metamorphic complex, SW Japan, occurs in a huge region of volcanic and plutonic activities in the Cretaceous. The high-grade area of the complex in Yanai-Hikari area is divided into three zones, K-feldspar-Cordierite (Kfs-Crd), Sillimanite-K-feldspar (Sil-Kfs) and Garnet-Cordierite (Grt-Crd) zones. Characteristic mineral assemblage of Grt-Crd zone is stable on the higher-temperature side of the dehydration melting reaction, sillimanite + biotite + quartz = garnet + cordierite + melt, whereas those of Kfs-Crd and Sil-Kfs zones are stable at lower and higher pressures on the lower-temperature side of the reaction, respectively. The estimated pressure-temperature conditions agree with this feature, and those of Grt-Crd zone roughly coincide with the dehydration melting reaction. These features suggest that maximum temperatures at middle crustal depths were buffered by the dehydration melting. The heat supplied by emplacement of magmas raised neighboring temperature up to the dehydration melting point, and further heat supply increases melt fraction in migmatites of this zone. The sequential magma emplacements and their pauses may cause temporal fluctuation of the melt fraction in Grt-Crd zone. This may be responsible for the pulse-like growth of zircon synchronous with the plutonic activities. Retention of the latent heat at the middle crustal depths also enabled the subsequent magmas to migrate to shallower levels and erupt on the earth's surface in volcanic-plutonic system.