K-Ar ages of the Higo metamorphic belt

Abstract

The Higo metamorphic belt belongs to the Ryoke belt and was metamorphosed by an andaluste-sillimanite type activity. Biotite, hornblende and white mica separated from metamorphic and igneous rocks were dated by K-Ar method. These samples were collected systematically from the belt. The biotite ages from the lower to the higher grade zone of the belt are concentrated in a narrow range of 103-108Ma, indicating no significant correlation between the metamorphic temperature and the age. The biotite and hornblende ages of granite, porphyrite and lamprophyer intruding into the belt exist in the range for metamorphic rocks. The biotite and hornblende from a garnet bearing amphibolite give 104 and 107Ma, respectively, suggesting a cooling rate of about 70°C/Ma from 500°C to 300°C. Thus, the Higo metamorphic belt has had an igneous and metamorphic activity and the subsequent rapid cooling history in a short time at ca.105Ma. The psammitic schist collected from the lowest grade zone, where the Manotani schists containing lawsonite and alkali-amphibole occur, gives 214Ma K-Ar age for white mica, indicating a cooling age of high P/T type of metamorphic rocks in subduction zone. These type of rocks are correlative with the rocks in the Suo metamorphic belt having tectonic contact with the Jurassic accretionary complexes in the eastern Yamaguchi Prefecture.