Timescale of material circulation in subduction zone: U-Pb zircon and K-Ar phengite double-dating of the Sanbagawa metamorphic complex in the Ikeda district, central Shikoku, southwest Japan

摘要

We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U-Pb zircon and K-Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high-P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92-81 Ma for the trench-fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74-72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi-type deposits formed at mid-ocean ridge suggest that the 60-Myr-old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench-fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76-74 and 64-62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U-Pb and K-Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high-P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).