TIME CONSTRAINTS ON SUBDUCTION OF THE CRETACEOUS SANBAGAWA HP ROCKS IN SW JAPAN: ZIRCON U-PB AGE AND GEOCHEMISTRY BY LA-ICP-MS

To resolve issues associated with the tectono-metamorphic evolution of the Cretaceous Sanbagawa HP metamorphic rocks in Japan, we performed LA-ICP-MS zircon U-Pb and trace elements analyses on mafic and pelitic-psammitic schists collected from the Besshi–Asemi-gawa region, central Shikoku. Zircon U-Pb dating of the Tonaru eclogite suggests that igneous events from ca. 200 to 180 Ma were involved in the protolith formation. In addition, the trace-element compositions of zircons are enriched in U and Sc, and depleted in Nb, suggesting that the parental magmas related to the Tonaru eclogite were formed at an oceanic arc setting. Data obtained from the Iratsu eclogite also suggest that the protolith was formed by oceanic-arc magmatism at ca. 195 Ma, and the eclogite facies metamorphism occurred during ca. 120-90 Ma. On the other hand, detrital zircons from the Seba eclogites shows ages from ca. 2000 to 100 Ma, and the metamorphic zircons show ca. 90 Ma with HREE depletion and without Eu anomalies. In addition, detrital zircons from the pelitic-psammitic schists in the Asemi-gawa area have young ages of ca. 100–90 Ma regardless of their metamorphic grade. Thus, the metamorphic rocks in the Seba and Asemi-gawa areas underwent the prograde metamorphism after ca. 90 Ma. A combination of our results with previous geochronological data for the Sanbagawa HP rocks indicates that the Sanbagawa metamorphic belt in Shikoku is composed of three metamorphic units with different deposition/accretion and metamorphic ages. Moreover, our results indicate that the oldest metamorphic unit was stagnated at the depth of the eclogite-facies condition during ca. 120-90 Ma. This was probably caused by the detachment of the oceanic-arc material from subducting slab, and the trench movement toward ocean side at the time.