Existence of multiple units with different accretionary and metamorphic ages in the Sanbagawa Belt, Sakuma-Tenryu area, central Japan

摘要

U-Pb ages of detrital zircons and white mica K-Ar ages are obtained from two psammitic schists from the western and eastern units of the Sanbagawa Metamorphic Belt located in the Sakuma-Tenryu area. The detrital zircons in the sample from the western unit (T1) show an age cluster around 95 Ma, and the youngest age in the detrital zircons is 94.0 ± 0.6 Ma. The detrital zircons in the sample from the eastern unit (T5) show a main age cluster in the Late Cretaceous with some older ages, and the youngest age in the detrital zircons is 72.8 ± 0.9 Ma. The youngest zircon ages restrict the older limit of the depositional ages of each sample. White mica K-Ar ages of T1 and T5 are 69.8 ± 1.5 Ma and 56.1 ±1.2 Ma, respectively, which indicate the age of exhumation and restrict the younger limit on the depositional age of each sample. The results show that the western and eastern units were different in their depositional and exhumation ages, suggesting the episodic subduction and exhumation of the Sanbagawa Belt in the Sakuma-Tenryu area. These results also suggest simultaneous existence of subduction and exhumation paths of metamorphic rocks in the high-P/T Sanbagawa Metamorphic Belt.