K-AR AGES OF METAMORPHIC ROCKS FROM THE TSUKUBA DISTRICT, IBARAKI PREFECTURE

抄録

K-Ar dating on micas from six metamorphic rocks and two granitic rocks in the Tsukuba district were carried out. The ages of the metamorphic rocks are 58-62 m. y. showing no regular relationship to their metamorphic grade. Dating of the granitic rocks gives 53-63 m. y. The results, together with geological and petrological facts, support the idea that these metamorphic rocks were influenced by thermal effects of the associated mass of biotite granite.
With regard to the existence of plutonism and metamorphism in early Tertiary time the Tsukuba metamorphics resemble the Ryoke terrane.