The high-P/low-T Sanbagawa Metamorphic Belt that traverses SW Japan, has been subdivided into two belts thought to have been metamorphosed at ca. 120 Ma and at ca. 65 Ma (‘Sanbagawa Metamorphic Rocks’ and ‘Shimanto Metamorphic Rocks’). The subdivision was based on the assumption that metamorphism occurred at ca. 116 Ma, largely based on an early Rb-Sr isotope study and zircon data obtained for the eclogite unit of the Sanbagawa Belt, whereas in some parts of the belt detrital zircons of late Cretaceous age (90-80 Ma) were discovered. Analysis of detrital zircons sampled from two sites within the area considered to expose the older ‘Sanbagawa Metamorphic Rocks’, including the area investigated by the Rb-Sr study, reveals the presence of zircons younger than 95 Ma in all samples and some grains as young as 80 ± 4 Ma. It is therefore concluded that the Sanbagawa Belt is one single tectonic entity that formed in the Late Cretaceous though it contains older components, including fossiliferous clasts, older basic meta-volcanics and eclogite units that may record earlier metamorphic events.
U-Pb zircon dating
Sanbagawa Metamorphic Belt