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K-Ar dating on micas from twelve metamorphic rocks and one granitic rock in the Hitachi district, southern Abukuma plateau, were carried out. Measured ages of metamorphic rocks including so-called Tamadare and Nishidohira gneisses have a range from 101x106 years to 120x106 years, These obtained ages by K-Ar method are roughly correlated to the Middle Cretaceous. On the basis of K-Ar age determinations on metamorphic and granitic rocks, some considerations on the relation bet ween them in the Abukuma plateau are briefly noted.

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