Boundary between subbasal I and subbasal II fields of quartz deformation in geological conditions

Abstract

The Sangun crystalline schists of the Hirose district, Yamaguchi Prefecture, Southwest Japan, show progressive metamorphism from the pumpellyitechlorite facies, through the pumpellyite-actinolite facies, to the epidoteglaucophane-schist facies. The quartz fabrics produced during the main phase of metamorphism have been examined in this paper. Quartz microstructures in the low-temperature zone of the pumpellyite-chlorite facies are characterized by undulatory extinction and deformation lamellae which are referred to as type I subbasal lamellae, suggesting that the quartz deformation occurred in the subbasal I field. In the pumpellyite-actinolite facies, the epidote-glaucophane-schist facies and the high-temperature zone of the pumpellyite-chlorite facies, the quartz fabrics are characterized by small quartz grains produced by syntectonic recrystallization of coarsegrained quartz and crossed-girdle orientations of c-axes whose inclination angles are between 17° and 25°, suggesting that the quartz deformation occurred in the subbasal II field. The boundary between subbasal I and subbasal II fields of quartz deformation during the main phase of metamorphism appears to have been placed in the pumpellyite-chlorite facies. The temperature and confining pressure at the boundary are inferred to be at ca. 250 °C and 5–7 kb respectively.