Periods of the igneous activities of the granitic rocks in japan by k-a dating method

Abstract

As a result of the K-A dating of about 300 samples, the ages of the granitic rocks of Japan can be grouped into the following nine periods 1. (1) older than 370 m.y. (million years) 2. (2) 180−170 m.y.; 3. (3) 120−110 m.y.; 4. (4) 100−90 m.y.; 5. (5) 95−75 m.y.; 6. (6) 65−50 m.y.; 7. (7) 40−30 m.y.; 8. (8) 25−20 m.y.; 9. (9) 15−3 m.y. From the distribution of the granitic rocks as grouped in the above periods, several zones are distinguished in Japan. In northeastern Japan four zones are arranged “en echelon”. They successively become younger toward the Japan Sea; the oldest one occupies the outermost side along the Pacific coast. In southwestern Japan, the distribution of the granitic zones is more complicated than that in northeastern Japan. The geographical relation between the 95−75 m.y. and the 65−50 m.y. zones in the area is quite similar to that between the 100−90 m.y. and the 65−50 m.y. zones in northeastern Japan.