ABSTRACT

The Kurosegawa structural belt, linearly aligned along the outer zone of S. W. Japan from Kyushu and Shikoku to the Kinki district of Honshu Island contains the oldest rock of Japan both from the paleontological (Silurian fossiliferous limestone) and the geochronological (Rb-Sr age of biotite from the Mitaki granite) points of view. The Rb-Sr age of the biotite is 428±4 m.y. and that of potassium feldspar in the Kurosegawa belt is 288 ± 19 m.y. The latter age seems to coincide with the age of upheaval of the belt. As is revealed by microscopic examination, potassium feldspar is characterized by alteration and replacement at low temperature.