The Ryoke metamorphic rocks in the western area of Tsu City are mostly composed of metamorphic rocks derived from sandstone and mudstone. Three metamorphic zones in the area are recognized by andalusite-sillimanite transition and disappearance of muscovite. Types of plagioclase twinning in pelitic rocks are determined by optical methods. Plagioclase twins having a composition plane (010) are less frequent (23-74%) in muscovite-bearing rocks than in muscovite-free rocks (63-93%). These observations suggest that stronger shear stress was exerted in the lower grade metamorphic zones than in the higher grade zones, because the experimental results show that pericline twin is generally produced by mechanical deformation under high temperature.

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Petrology and Rb-Sr chronology were studied for Otagiri two-mica granite, one of the younger intrusives in the Ryoke belt, which are exposed in the Ina district of the Ryoke belt. Three groups, northern, central and southern granite bodies, were identified in the field, with the former two intruding litic and psammitic gneisses of sillimanite zone, and the last intruding a foregoing granite, the Ichida granite. Distinct Rb-Sr whole rock isochron ages, 71.1 ± 6.8 Ma, 99.0 ± 10.9 Ma, 92.7 ± 13.8 Ma were obtained for the northern, central and southern granite bodies, respectively. Because the initial Sr and Nd isotopic ratios are also different among different granite bodies, those three groups were probably derived from distinct magmas at different ages. The intrusive relations between these granite bodies and the surrounding gneiss indicate that the main stage of metamorphism and deformation of the Ryoke belt were older than 100 Ma. Likewise, Ichida granite and Inagawa granite were 90 Ma or older. Granitic rocks equivalent to those bodies are widely distributed in the Ryoke belt in central Honshu, thus suggest a widespread intrusion of the 90-100 Ma granites in the southwestern parts of the Cretaceous granite terrain including the Ryoke belt.

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K-Ar ages were determined for nine andesite dykes in the Hida region, central Japan. The obtained ages are classified into three groups; ca.100 Ma (one sample), 53-61 Ma (five samples) and ca.16 Ma (three samples). Three dykes yielding ca.16 Ma are in close relation to the Miocene volcanic activities which produced the volcanic rocks of Iwaine Group in the northern margin of the Hida region. Other dykes of ca.100 Ma and 53-61 Ma are distinctly older than those estimated by Nozawa et al. (1975, 1981).

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