Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

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Abstract
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 ‘D 1 ’ deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by ‘D 2 ’ (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the ‘D 1 ’ trend. These deformational events established the present thermal structure. The final regional deformation formed upright ‘D 3 ’ 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.