三郡結晶片岩の同位体年代  [in Japanese] Isotopic ages of the Sangun crystalline schists, Southwest Japan  [in Japanese]

K-Ar and Rb-Sr age determinations were carried out on the so-called Sangun crystalline schists, which are of glaucophanitic type and sporadically distributed in the Inner Zone of Southwest Japan. Excluding some K-Ar ages partly rejuvenated by the thermal effect of granite intrusion, K-Ar and Rb-Sr ages are grouped into three as follows. 1. About 300 Ma for schists from the Wakasa (Shitani Formation), Toyoga-dake and Wakamiya areas, correlated to those of the Hida Marginal Belt, and the distribution area is collectively named the Sangun-Renge Belt. 2. About 220 Ma for schists from the western region (Yamaguchi Prefecture to north Kyushu), the area is named the Suo Terrane. 3. About 180 Ma for schists from the eastern region (Tottori, Okayama and Shimane Prefectures), the area is named the Chizu Terrane. Each of these age groups represents the time of the so-called Sangun metamorphism. Difference in rock facies and age of sedimentation is noted between the Suo and Chizu Terranes. Whole-rock samples of crystalline schists from the Nishiki-cho and Wakamiya areas give Rb-Sr isochron ages of 218.8±8.1 Ma and 308.4±19.0 Ma, respectively. These ages are nearly equal to ages of white micas and represent the respective times of metamorphism.