Chihiro Sakai

Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

Summary The Cretaceous (pre-Japan Sea) Sanbagawa metamorphism affected the Japanese Jurassic complex south of the Median Tectonic Line in the regions now recognized as the Sanbagawa, Mikabu and Chichibu belts. The metamorphic peak (116 Ma) was reached and passed during the tectonic ‘D1’ deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by ‘D2’ (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the ‘D1’ trend. These deformational events established the present thermal structure. The final regional deformation formed upright ‘D3’ folds. The four metamorphic zones based on pelitic assemblages can be enhanced by using basic schists to subdivide the pelitic chlorite zone. Apparent Fe-Mg partition coefficients between chlorite and garnet show an essential regional continuity of metamorphism and that thrust-offsets do not juxtapose elements from different mineral zones. Peak conditions of metamorphism ranging from 250°C and 6 kbars to 600°C and 10 kbars are consistent with simple P-T-t loops which progress at higher pressures and return at lower pressures to the surface.

Pressure-temperature trajectory of the Sanbagawa metamorphism deduced from garnet zoning

Abstract Most garnets in a small volume of the pelitic Sanbagawa schists nucleated simultaneously, grew, and then ceased with their growth, maintaining near-equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism. Chemical trends of zoned garnets systematically shift from the pyrope-poor to pyrope-rich portions in a ternary MnFeMg system in order of ascending metamorphic grade. A ternary (MnFeMg) model system suggests that the higher-grade garnets started to grow at lower pressures and higher temperatures than the lower-grade ones during prograde metamorphism. It follows that high-grade metamorphism took place at lower pressures in the garnet to oligoclase-biotite zones of the Sanbagawa metamorphic terrain in central Shikoku.

Growth history of garnet in pelitic schists of the Sanbagawa metamorphic terrain in central Shikoku

Abstract During the prograde metamorphism of the Sanbagawa metamorphic terrain in central Shikoku, garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote, etc. In the quartz-albite layer, it has grown euhedrally. In the micaceous layer, its crystal morphology varies according to the different minerals in contact and their different orientations. Garnet enclosed in or sandwiched between micas is always ellipsoidal, extended parallel to the basal planes of the micas. However, when the edge of a garnet has intruded at an acute angle into the micas, euhedral garnet has grown. It is concluded that the growth of garnet at the interface with micas ceased when the {110} surface of garnet came into contact with the basal plane of the micas in the tectonic c -axis direction. Other models, such as chloritization-resorption, pressure-solution, and mechanical-erosion or grinding, do not adequately explain the origin of the flattened garnet. During retrograde metamorphism, chloritization and resorption of garnets took place in the matrix, resulting in the rough and irregular surface of garnet. Garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism.