K-Ar ages of the Kamuikotan metamorphic rocks in Hokkaido, Japan

The Kamuikotan metamorphic rocks are divided into two units, a unit of high-pressure type dominated by mafic rocks and that of high-pressure intermediate group dominated by pelitic and psammitic rocks, respectively. Fifteen muscovites and one hornblende separated from the metamorphic rocks were dated by K-Ar method. The age data show that the rocks of high-pressure type (101-145 Ma) are significantly older than the rocks of high-pressure intermediate group (51-107 Ma). The epidote amphibolite blocks in serpentinite are 123-126 Ma in age, suggesting that the blocks have been partly recrystallized by retrograde metamorphism of the high-pressure type. -from English summary