K-A dating on seventeen samples of granitic rocks, one pegmatitic vein in hornblende gabbro and one epidote-amphibolite from the Abukuma massif were carried out. As a result, maximum and minimum values of granitic rocks show 107×106 years and 87×106 years respectively, and these ages of emplacement of granitic rocks are broadly divided into two periods 90×106 and 100×106 years. On the other hand, formerly the ages of intrusion of granitic rocks in the Abukuma massif have been geologically divided into two main periods younger and older. But the results of K-A dating of this time do not accord well with the former geological observations, and even the reverse cases are recognized indicating the minor time difference of these emplacements. It is clarified that the granitic rocks of the Abukuma massif are emplaced in later Cretaceous and younger than that of Kitakami massif about 20×106 years on the average. K-A dating on metamorphic rock from the northern Abukuma by muscovite minerals shows 300×106 years; this indicates that the metamor-phism would have occurred older than Carboniferous age at least.
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Kawano et al. (1965) studied this question.