U-PB DATING OF DETRITAL ZIRCONS FROM THE SANGUN METAMORPHIC ROCKS, KYUSHU, SOUTHWEST JAPAN - AN EVIDENCE FOR 1.9-2.0 GA GRANITE EMPLACEMENTIN THE PROVENANCE

U-Pb and Rb-Sr radiometric dating has been done on zircon samples and mineral concentrates, respectively, separated from psammitic schists o f the Sangun metamorphic rocks in central Kyushu, Japan. Based on thei r shapes and colors, zircon grains were grouped into six. Rounded and short prismatic zircons define, independently of their colors, a commo n discordia with an upper intercept at 1.91 +/- 0.04 Ga. Elongated zir cons define another discordia with an upper intercept at 0.93 +/- 0.06 Ga. A Rb-Sr internal isochron defined by the mineral concentrates giv es 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 wit h that of zircons from the Oki metamorphic rocks (1.96 +/- 0.04 Ga: Ya mashita and Yanagi, 1994). This and the great abundance of 1.91 Ga gra ins in the zircon populations seem to indicate a prominent plutonic ac tivity at 1.9-2.0 Ga in the provenance common for the Sangun and Oki m etamorphic 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 Asi an continent close to the Japanese Islands.