HR: 0800h
AN: T51A-0296    [Abstracts]
TI: The youngest blueschist belt in SW Japan
AU: * Aoki, K
EM: kazumasa@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
AU: Itaya, T
AF: Research Institute of Natural Sciences, Okayama University of Science, 1-1, Ridai-cho, Okayama, 700-0005, Japan
AU: Masago, H
AF: Center for Deep Earth Exploration, Japan Agency for Marine-Earth Science and Technology, 3173-25, Showo-machi, Kanazawa-ku, Yokohama, Kanagawa, 236-0001, Japan
AU: Terabayashi, M
AF: Department of Safety Systems Construction Engineering, Kagawa University, 2217-20, Hayashi-cho, Takamatsu, Kagawa, 761-0396, Japan
AU: Kaneko, Y
AF: Department of Sciences and Technology, Meisei University, 2-1-1, Hodokubo, Hino, Tokyo, 191-8506, Japan
AU: Maruyama, S
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
AB: We report discovery of the youngest blueschist belt in SW Japan and describe exhumation process of the high- P/T metamorphic belt. The Cretaceous Sanbagawa belt is distributed in SW Japan and is one of the typical high- P/T metamorphic belts in the world. In the Oboke area, central Shikoku, schists are widely distributed and were considered as a part of the Sanbagawa belt. However, recent zircon chronological study has revealed the older limit of the depositional age of this area, which indicates that this area belongs not to the Sanbagawa belt but to the Northern Shimanto belt (Aoki et al., 2007). The Northern Shimanto belt is known for a Cretaceous-Tertiary accretionary complex in Japan. The redefinition of the attribution of the Oboke schists to the Northern Shimanto belt raised an importance of studies from view points of metamorphic petrology and geochronology in the area, those are essential for understanding the tectonic evolution of the of the Northern Shimanto belt. In this study, tectonic evolution of the Northern Shimanto belt was examined based on petrological and geochronological studies in the Oboke area. Some basic schists in this area contain Na-amphibole (magnesioriebeckite close to crossite and winchite). The peak metamorphic P-T condition with Na-amphibole-bearing basic schist and Na- amphibole-free basic schists are estimated to P = 4-4.5 kbar and T = 240-270° (12-15 km depth), which corresponds to the transition among blueschist, greenschist and pumpellyite-actinolite facies. K-Ar ages of phengite from schists showed 64.8 +/- 1.4 Ma and 64.4 +/- 1.4 Ma for the basic schists, and 65.0 +/- 1.4 Ma, 61.4 +/- 1.3 Ma and 63.6 +/- 1.4 Ma for the pelitic schists. Metamorphic temperature in the Oboke area is below the closure temperature of the K-Ar phengite system. Hence, the K-Ar ages (66-61 Ma) in this area represent the peak metamorphic age of the Northern Shimanto belt. In the broad sense of definition of blueschist facies, the highest- grade part of the Northern Shimanto metamorphic rocks belongs to the blueschist facies metamorphism, according to Miyashiro (1973). These results suggest that the Northern Shimanto belt is the youngest blueschist belt in Japan. Integrating the results of this study with previous studies, the tectonic evolution of the Northern Shimanto belt and the Sanbagawa belt are as mentioned below. The Northern Shimanto metamorphic rocks were formed as an accretionary complex by a subduction of the oceanic plate between 100-65 Ma (e.g., Taira et al., 1982). At that time, the Sanbagawa metamorphic rocks had already begun exhumation, and juxtaposed onto the Northern Shimanto metamorphic rocks at 12-15 km depth, associated with an extensive fluid infiltration from the underplated Northern Shimanto belt between 88 and 72 Ma. After that time, both the Sanbagawa and the Northern Shimanto metamorphic rocks were exhumed to the surface by domal up-lift until 52-42 Ma (Isozaki and Itaya, 1990), and at which time the Sanbagawa schists were eroded to be transported to fore arc-basins.
DE: 1115 Radioisotope geochronology
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3651 Thermobarometry
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2007 Fall Meeting