The Sanbagawa metamorphic rocks in the Shibukawa area are mainly composed of Sanbagawa schists with minor amounts of Mikabu greenstones. The Sanbagawa schists consist of pelitic and mafic schists, with minor amounts of siliceous and psammitic schists. The Mikabu greenstones are weakly metamorphosed mafic rocks that were subducted before the Sanbagawa schists.
The Mikabu greenstones are coupled with ultramafic–mafic complexes consisting of hornblendite, hornblende gabbro, and ultramafic rocks and are structurally located above the Sanbagawa schists bounded by faults. The specificity of these lithologies has been reported in several studies, as follows. Some of these ultramafic–mafic complexes, particularly the large Kichijosan and Joyama complexes, are thought to have undergone ocean-floor metamorphism before Sanbagawa metamorphism. In recent years, jadeite has been reconfirmed within a veinlet cutting dunite in an ultramafic body located on the eastern side of the Joyama complex.
The Sanbagawa schists in this area have been previously studied and described, as follows. Seki et al. divided them into four zones based on the mineral assemblages of the mafic and metasedimentary rocks. Isogai focused on mafic schists bearing quartz and albite and defined eight zones based on mineral assemblages. The metamorphic zones in both studies did not coincide. They reported the occurrence of characteristic metamorphic minerals in schists, such as jadeite, lawsonite, and sodic amphibole. Although no quantitative metamorphic P–T conditions have been determined in this area, the main reported mineral assemblage corresponds to the chlorite zone of Higashino, the lowest grade of Sanbagawa metamorphic rocks.

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