We estimated the protolith age and peak metamorphic temperature of the Yokokawagawa metamorphic rocks (YMR) east of the Itoigawa-Shizuoka Tectonic Line using detrital zircon U-Pb dating and Raman carbonaceous material geothermometry, respectively. U-Pb dating of a psammitic rock yielded a youngest age of ~100 Ma, which corresponds to the protolith age.Raman carbonaceous material geothermometry results for five pelitic rock samples give metamorphic temperatures of ~350-380 °C. The protolith age is consistent with those of the Sanbagawa metamorphic rocks (SMR), strongly indicating that the YMR are an extension of the SMR. The increase in peak temperature toward intrusive rocks along the eastern margin of the YMR may indicate that the relatively young ages yielded by previous K-Ar dating of the YMR reflect thermal resetting due to contact metamorphism.

Needle-shaped rutile inclusions occur in garnet within the quartz-eclogite at Mt. Gongen in the Sanbagawa belt, central Shikoku. They are approximately 5-25 µm along the long axis and are typically oriented along three directions, each intersecting at 120°. This indicates that the needle-shaped rutile is a lamella exsolved from the garnet. Garnet with needlerutile inclusions is restricted to the quartz-poor domain of the quartz eclogite sample, which consist of quartz, garnet, omphacite, phengite, epidote, kyanite, and hornblende. Garnet grains with rutile lamellae show a composition of the almandine-pyrope series with 14-21 mol% grossular content. Rutile exsolution lamellae were concentrated in the range of 27-34 mol% pyrope of garnet crystals. The garnet host with rutile lamellae has a higher TiO2 content (TiO2 = 0.06-0.19 wt%) than those in rutile-free areas. These chemical compositional characteristics suggest that Ti was incorporated into the crystal structure during garnet growth and subsequently partially exsolved as rutile lamellae during the retrograde stage. Rutile lamellae in garnet have generally been regarded as indicators of ultrahigh-pressure metamorphism, but the present report from quartz-eclogite of the Sanbagawa belt, where no coesite has been found, provides evidence in a natural sample that the appearance of rutile exsolution lamellae is not necessarily under ultrahigh-pressure conditions.