Abstract
We have been measuring the K-Ar ages of the granitic rocks taken from the all granitic areas in Japan, since 1964. The number of granitic samples measured, at present, amounts to 400 and they are representative of each granitic area of Japan.
The ages of the granitic rocks of Japan can be grouped into the following nine periods: 1) Older than 370 m.y. (Pre-Devonian), 2) 170-180 m.y. (Early Jurassic), 3) 110-120 m.y. (Late stage of Lower-Cretaceous), 4) 90-100 m.y. (Early stage of Upper-Cretaceous, ) 5) 75-95 m.y. (Middle stage of Upper-Cretaceous), 6) 50-65 m.y. (Late Cretaceous-Early Tertiary), 7) 30-40 m.y. (Late Eocene-Early Oligocene), 8) 25-25 m.y. (Miocene), 9) 3-13 m.y. (Pliocene).
Some groups of the granitic rocks belonging to the above periods show zonal distribution.
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 zonal distribution of the granitic rocks is more complicated than that in northeastern Japan.
