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


Dr Kazuaki Okamoto. Present address: Institute of Earth Sciences, Academia Sinica, PO Box 1-55 Nankang, Taipei 115, Taiwan, R.O.C.


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.