Phengite K-Ar ages of High-P type metamorphic rocks of the Hakoishi sub-unit of the Kurosegawa belt in Yatsushiro area, Kyushu.

Abstract

The Hakoishi sub-unit, which suffered lawsonite-blueschist facies metamorphism, is exposed to the west of the Yayamadake limestone in the Kurosegawa belt, Kyushu. The precise radiometric age of the high-P metamorphism of Hakoishi sub-unit has not been determined yet. K-Ar age of 280-299Ma is determined from two phengite bearing metapelites. This age is thought to be the peak age of the high-P metamorphism according to the following reasons: (1) The composition of phengites are Si=3.57-3.68(O=11) meaning metamorphic origin, (2) phengites form main schistosity accompanying with other high-P metamorphic minerals, (3) this area has suffered the peak metamorphism of 350°C, 6-8kbar.