Abstract
Plagioclase twinning laws were investigated for the pelitic metamorphic rocks of the Hidaka Metamorphic Belt in Hokkaido. The frequency of pericline twinning law is low in greenschist to low amphibolite facies and low to high in high amphibolite to granulite facies. Tonalitic mylonite and sheared metamorphic rocks show high frequency of pericline twinning law. These are explained by the experimental data that the pericline twinning is resulted from the mechanical deformation under high temperature.
