Finding of Miocene mylonites in the Yoshimi Hills to the east of the Kanto Mountains

Abstract

The Yoshimi metamorphic rocks were mylonitized to various degrees. Fine-grained mylonites are often found in shear zones, which are partly overprinted cataclasis. Pelitic mylonites are mainly composed of muscovite, chlorite, clinozoisite, calcite, albite, quartz and opaque minerals. Mylonite foliations are paralell to bedding planes. The thickness of mylonites in the Hinatayama region ranges from 5cm to 1m. The K-Ar bulk rock age is measured for a highly mylonitized pelitic rock which was not deformed cataclastically deformation. The K-Ar age determined is 18.0Ma. Taking into consideration the Miocene tectonics in the northern Kanto Mountains, it is considered that the mylonitization took place during the clockwise rotation of the Kanto Mountains in the middle Miocene time.