C-Axis fabrics and microstructures in quartz schist from the Sambagawa metamorphic belt, central Shikoku, Japan

Abstract

An integrated study on c-axis fabric and microstructure of plastically deformed and dynamically recrystallized quartz in quartz schist from the Sambagawa metamorphic belt in central Shikoku, Japan has revealed a correlation between c-axis fabrics and three-dimensional grain shape. Recrystallized quartz grains with prolate ( k > 1), oblate (0.2 < k < 1) and uniaxially oblate ( k < 0.2) shapes exhibit c-axis fabric patterns of cleft girdles around the principal finite elongation direction ( X), type I crossed girdles, and small circle girdles around the principal finite shortening direction ( Z), respectively. Fabric intensity calculated from the eigenvalues of orientation tensors of the c-axis fabrics increases with increasing magnitude of the apparent finite strain inferred from the shape of recrystallized quartz grains. These results indicate that the development of recrystallized grain shape as well as c-axis fabrics is primarily controlled by finite strain during progressive deformation. However, the finite strain estimated from three-dimensional grain shape is systematically deviated toward the flattening field compared to that estimated from the c-axis fabrics. We investigated several possible causes for the discrepancy, and propose that a scenario of coupled deformation and dynamic recrystallization in the quartz schist is responsible for the formation of these microstructures.