Sequential deformation from serpentinite mylonite to metasomatic rocks along the Sashu Fault, SW Japan

Abstract

The deformation of serpentinites along the Sashu Fault, Saganoseki Peninsula, SW Japan, involved both mylonitization and brecciation. The brecciation was accompanied by metasomatism, and the metasomatized rocks record additional ductile deformation. Serpentinite mylonite is composed mainly of antigorite, with minor magnetite and Cr-spinel. Foliation and lineation in this rock type are defined by the shape preferred orientation of antigorite and the alignment of fine-grained magnetite and carbonate minerals. The antigorite also shows a lattice preferred orientation (LPO), as measured with a universal stage, with c axes oriented near-perpendicular to the foliation and b axes oriented parallel to the lineation. Brecciation is concentrated in the reaction zone along the boundary between serpentinites and surrounding country rocks. Talc and chlorite schists developed in this zone as a result of associated metasomatism and deformation; consequently, they contain composite planar fabrics and drag folds. The breccias themselves are composed of serpentinite fragments cut by veins of calcite and talc. Analyses of fluid inclusions in calcite from the breccia indicate that brecciation occurred at 200–300 °C at depths of 3.8–11.8 km. Metasomatic reactions, particularly the formation of talc, contributed to weakening of this serpentinite-bearing fault zone.