Abstract
K-Ar, Rb-Sr and fission-track age determinations were carried out on rocks along the Median Tectonic Line (MTL) in the Kayumi area, Mie Prefecture. K-Ar ages on the Ryoke granitic rocks range from 44.1 to 84.3 Ma. Within about 1000m of MTL, the ages for K-feldspars and biotites decrease toward MTL, probably caused by the hydrothermal alteration associated with cataclasis, whereas the ages for homblendes increase toward MTL. The cooling rate of the granitic rocks, calculated from K-Ar ages and closure temperatures of minerals, is 31-56°C/Ma.
K-Ar ages for muscovites from pelitic schists in the Sanbagawa belt are 73.6-75.6 Ma; decrease in age is not observed even near MTL. Rb-Sr ages for muscovites from the schists are 72.2-77.5 Ma, nearly the same as the K-Ar ages. These age data are assumed to indicate the time of maximum temperature for the Sanbagawa schists in this area.
Fission track ages for zircons from the Ryoke granitic rocks range from 52.4 to 59.5 Ma, approximating to K-Ar ages of K-feldspars for the same rocks. Fission-track ages for zircons from the Sanbagawa schists are 59.0 and 61.2 Ma, nearly 15 Ma younger than the K-Ar ages of muscovite for the same rocks.
The fine fractions (<2μm) separated from MTL fault gouges at the Nishitani River and Kayumi give K-Ar ages of 41.8 and 51.4 Ma, respectively. The former age probably represents the time of hydrothermal alteration following the major fault movement of MTL associated with cataclasis.
