ABSTRACT

U-Pb and Rb-Sr radiometric dating has been done on zircon samples and mineral concentrates, respectively, separated from psammitic schists of the Sangun metamorphic rocks in central Kyushu, Japan. Based on their shapes and colors, zircon grains were grouped into six. Rounded and short prismatic zircons define, independently of their colors, a common discordia with an upper intercept at 1.91 ± 0.04 Ga. Elongated zircons define another discordia with an upper intercept at 0.93 ± 0.06 Ga. A Rb-Sr internal isochron defined by the mineral concentrates gives an age of 197.7 ± 6.5 Ma, which represents the time of the Sangun metamorphism in central Kyushu. The U-Pb age of 1.91 Ga coincides with that of zircons from the Oki metamorphic rocks (1.96 ± 0.04 Ga: Yamashita and Yanagi, 1994). This and the great abundance of 1.91 Ga grains in the zircon populations seem to indicate a prominent plutonic activity at 1.9–2.0 Ga in the provenance common for the Sangun and Oki metamorphic rocks. The U-Pb ages of zircon from the Sangun metamorphic rocks indicate the episodic growth at 1.91 and 0.93 Ga of the East Asian continent close to the Japanese Islands.