The geology and petrology of the Itoigawa–Omi area of the Hida Gaien belt have been reexamined with pioneering metamorphic zonal mapping of glaucophane schist facies done by Banno (1958). A geologic study shows that crystalline schists of ca. 262-381 Ma, 450 Ma garnet amphibolite, amphibolite, 520 Ma jadeitite, albitite, and rodingite, reaching 10 km long and 2-5 km wide, occur as tectonic blocks of serpentinite forming the Omi serpentinite melange (OSM).
Along the Omi–River of the eastern side of the OSM, chlorite, garnet, and biotite zones of pelitic schists are again recognized across several tectonic blocks of schist, indicating the slightly broken nature of a large metamorphic unit that resulted probably from exhumation of the serpentinite mélange belt. Kfs and chlorite are common throughout pelitic schists along the Omi–River. Thus, biotite- and oligclase-forming reactions may have been K-feldspar + chlorite → muscovite + biotite + quartz + H2O, and muscovite + clinozoisite + quartz → K-feldspar + anorthite + H2O, respectively.
Glaucophane described by Banno (1958) in the chlorite zone of the Omi River has been recognized in composite grains consisting of hornblende core, gluacophane mantle, and actinolite rim. In contrast, glaucophane appears in a few samples along with eclogitic assemblages as plurifacial minerals before, during, and after eclogite facies in the pelitic schists of the western side of OSM (EC unit). Thus, the regional metamorphism described along the Omi–river of OSM (EA unit) is not a retrogressive metamorphism of the EC unit, but a regional hydrothermal recrystallization just before or during exhumation in the serpentinite mélange belt.
