[S-MP29] Oceanic and Continental Subduction Processes

[SMP29-P01] Zircon U-Pb ages of metasedimentary rocks in the Hakogase (Hida Marginal) area, Fukui Prefecture.

*Atsushi MIYASHITA¹, Tatsuki Tsujimori², Daniel Pastor-Galán³, Yi Keewook⁴ (1.Seikei University, 2.Tohoku University, 3.Salamanca University, 4.Korea Basic Science Institue)

Keywords:high P/T metamorphic rocks, Zircon U-Pb ages, Phengite K-Ar ages

Combined chronological study using detrital zircon U-Pb dating and phengite K-Ar dating enables to understand the tectonic evolution of the high P/T metamorphic belt. The youngest age of detrital zircon U-Pb ages of psamites in the Hida marginal metamorphic rocks (HMMR = Renge belt) approximate its depositional ages (Tsutsumi et al., 2009). The closure temperature of the phengite K-Ar ages are controlled by deformation and recrystallization during exhumation of the metamorphic rocks(Itaya and Tsujimori, 2015). Therefore, the gap between the youngest age of detrital zircons and phengite K-Ar ages are the age difference between subuduction and uplift of metamorphics.
The Hakogase (Nigure-gawa, Kuzuryu, Fukui Prefecture) metamorphic rocks belong to HMMR member as the olistolith in the Cretaceous Tedori Group. It consists mainly pelites with small amount of basites and mables. The main index mineral assemblage of metasediments are garnet + chlorite, and the phenigite K-Ar ages are 287.8 - 283.6 Ma (Kunugiza et al., 2004). Relic Lawsonite was also found in the basaites and psamites (Miyakawa, 1982).
The detrital Zircons U-Pb ages in the two pelites are obtained by the SHRIMP II of KBSI. The youngest U-Pb ages of detrital zircons are 307Ma. The gap of the sedimentation between uplift of the Hakogase metamorphics is estimated 20 Ma. This value means the migration speed of the HMMR metamorphic rocks are 1/10 of plate motion. This rapid migration speed is common for the high P/T metamorphic rocks.