Abstract
Isotopic age determinations in Japan have been carried out systematically since 1960, and recently it has been tried to summarize these results to clarify the transition of acid igneous activities in the Japanese Islands. In this paper these summaries are reviewed and some problems are proposed to confirm the ages of late Mesozoic to Paleogene igneous rocks. K-Ar ages of biotite imply either the time when the temperature of the rocks had been lowered enough to retain radiogenic argon in course of upheaval following to emplacement and solidification, or the time when the rocks beeing cooled from the temperature which was brought with secondary thermal effect. K-Ar ages of biotite summarized here show a pattern of distribution resembling to that given by KAWANO and UEDA (1967), and the writers pointed out following problems from the pattern. 1) In the Northeast Japan, a part of granitic rocks shown as early Cretaceous age should be considered to have emplaced in middle Jurassic age. These rocks seem to be rejuvinated by the effect of younger granitic rocks. Such examples are found in the Kitakami intrusive belt, in the Abukma intrusive belt and in the Ryoke belt respectively. 2) None of acid and intermediate volcanic rocks in the inner side of Southwest Japan has been measured on the ages, despite that these rocks are considered to be important constituents of late Mesozoic to Paleogene acid igneous activities. 3) The pattern of zonal arrangement of age groups were reported from the east coast of Australia and from the Sierra Nevada batholiths of western America. In these areas igneous activities are related to some structural movements. Detailed age determinations are now necessary to get reasonable explanation of the pattern in the Japanese Islands.
