Isotopic ages of rocks and intrafault materials along the Median Tectonic line - an example in the Bungui-toge area, Nagano Prefecture ( Japan).

K-Ar and Rb-Sr age determinations were carried out on rocks along the Median Tectonic Line (MTL) in the Bungui-toge area, central Japan. K-Ar ages for the Ryoke granitic and metamorphic rocks range from 26.2 to 72.3 Ma. Within about 300 m of MTL the ages decrease toward MTL. The cooling rate for the Hiji tonalite and the Katsuma quartz diorite, calculated from K-Ar ages and closure temperatures of minerals, is 28oC/Ma and 27oC/Ma, respectively. The age of emplacement for the 2 granitic masses is estimated to be about 80 Ma, whereas the mylonitization is assumed to have been completed at about 70 Ma. The Sambagawa crystalline schists, which are in direct contact with the Ryoke rocks at MTL, give K-Ar and Rb-Sr muscovite ages of 57.8-65.9 Ma and 57.9-65.9 Ma, respectively. The decrease in muscovite ages of schists is observed only within about 20 m of MTL. Mica ages for rocks of the Ryoke and Sambagawa belts far away from MTL are about 65 Ma and similar to each other. Secondary K- feldspar from veinlets in a pelitic schist near MTL gives a K-Ar age of 20.7 Ma. Seven samples of pelitic rocks give a Rb-Sr whole-rock age of 69.3 + or - 10.3 Ma, which probably represents the final stage of metamorphism.-from English summary