SHRIMP U-Pb zircon dating of quartz-bearing eclogite from the Sanbagawa Belt, south-west Japan: implications for metamorphic evolution of subducted protolith

摘要

In order to decipher the origin of eclogite in the high-P/T Sanbagawa metamorphic belt, SHRIMP U-Pb ages of zircons from quartz-bearing eclogite and associated quartz-rich rock (metasandstone) were determined. One zircon core of the quartz-rich rock yields an extremely old provenance age of 1899 ± 79 Ma, suggesting that the core is of detrital origin. Eight other core ages are in the 148-134 Ma range, and are older than the estimated age for trench sedimentation as indicated by the youngest radiolarian fossil age of 139-135 Ma from the Sanbagawa schists. Ages of metamorphic zircon rims (132-112 Ma) from the quartz-rich rock are consistent with metamorphic zircon ages from the quartz-bearing eclogite, indicating that eclogite facies metamorphism peaked at 120-110 Ma. These new data are consistent with both the Iratsu eclogite body and surrounding highest-grade Sanbagawa schists undergoing coeval subduction-zone metamorphism, and subsequent re-equilibration under epidote amphibolite facies conditions during exhumation.