Abstract
Metamorphosed intrusive rocks (metagabbro and metatonalite), and nonmetamorphosed aplite dyke, all of which intrude into the coarse-grained biotite-hornblende gneiss, are exposed in the Urushiyama area along the Takahara River, the main part of the Hida metamorphic belt. RbSr mineral isochron ages of 216±10 Ma and 222±2 Ma are given for the coarse-grained biotitehornblende gneiss and metagabbro, respectively. On the contrary, the whole rock isochrons of the metagabbro and metatonalite show 415±144 Ma and 332±74 Ma, respectively. These whole rock ages correspond to the ages of intrusion of the gabbro and tonalite and they were re-equilibrated with respect to the Rb-Sr system approximately 220 Ma by the regional metamorphism. Whole rock samples of aplite dyke yield an isochron age of 172±7 Ma. The Hida gneiss in the Urushiyarna area seems to have been formed by the main stage of metamorphism and the deformation having been already finished before the intrusion of gabbro.
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