Abstract
K-Ar age determination of 21 samples of high P/T metamorphic rocks were carried out on phengite from the Omi, Hakuba-Happo, Gamata and Ise areas of the Hida Gaien belt. K-Ar ages of 19 samples range from 283 to 338Ma. Though these rocks occur as isolated small geological units at present, the age range confirms that the Hida Gaien belt diagnostically includes high P/T metamorphic rocks of ca. 300Ma subduction zone metamorphic belt as the Renge belt (Nishimura, 1998) in the Southwest Japan. Absence of ca. 200 Ma subduction zone metamorphic rocks, the Suo belt (Nishimura, 1998), discards the idea that the Hida Gaien belt is a simple eastward extension of the pre-Jurassic geologic units in the Chugoku province of the Southwest Japan.
U-Th-Pb EMP dating was carried out on zircon, monazite and thorite in samples from 3 granitic bodies of the Gamata, Hakuba-Happo and Naradani areas. Serpentinite in the Gamata and Hakuba-Happo areas was metamorphosed thermally by the Horadani and the Ariake granites, respectively. The Horadani granite shows ca. 100 Ma and the latter granite does ca. 60 Ma, suggesting that the serpentinite was exhumed after the ages of the contact metamorphism. Serpentinite also occurs as olistoliths in the Permian accretionary complex in the Hida Gaien belt, and also as a basement of the Kuruma-Group of Jurassic age. The exhumation of serpentinite thus took place at various stages from the Permian to at least Paleogene because of its low density and plasticity in nature.
The Hida Gaien belt is characteristic of the association of high P/T subduction zone metamorphic rocks with age of ca. 300 Ma and serpentinite.
