関東山地東方吉見丘陵の変成岩類とそのＫ‐Ａｒ年代

Abstract

Various metamorphic rocks such as amphibolites (garnet-cpx amphibolite, schistose amphibolite, kyanite-bearing amphibolite), pelitic and basic schists (epidote amphibolite), and ultramafic rocks (spinel lherzolite, pyroxenite, hornblendite) are exposed in the Yoshimi Hills, to the east of the Kanto Mountains. Schistose amphibolites, which have the largest exposure in the Yoshimi Hills, and pelitic schists were moderately suffered mylonitization. The chemical composition of metamorphic minerals such as garnet, cpx and hornblende in the garnet-cpx amphibolite, and garnet in the schistose amphibolite suggests that these rocks were formed under the amphibolite to the granulite facies. The garnet-biotite temperature after Pigage and Greenwood (1982) estimated on the pelitic shcists is more than 800°C, considerably higher than the temperature (610°C) estimated on the pelitic schists in the highest metamorphic grade zone of the Sanbagawa belt in the Shikoku region. K-Ar ages of hornblende are 63.5 Ma for the garnet-cpx amphibolite and 61.0 Ma for the kyanite-bearing amphibolite, whereas K-Ar age of muscovite for the pelitic schist is 34.1 Ma. Form these age data, even though taking a possibility of rejuvenation into consideration, it is difficult to correlate the metamorphic rocks of the Yoshimi Hills to the metamorphic rocks of the Kurosegawa belt. Considering the occurrence of tectonic blocks of various amphibolite masses in the Sambagawa belt in the Shikoku region, it is possible to correlate the metamorphic rocks of the Yoshimi Hills to those in the Sambagawa belt. However, further study is necessary before we come to decide the belongings of the metamorphic rocks in the Yoshimi Hills.