Metamorphic History of the Yoshimi Metamorphic Rocks, Kanto Mountains, Japan: Implication From Rutile Exsolution Textures

The Yoshimi Metamorphic Rocks are composed mainly of mafic metamorphic rocks (e.g., garnet-amphibolite, garnet-clinopyroxenite, and orthopyroxene-clinopyroxene granulite) and fault-bounded pelitic gneiss. Peak metamorphic temperatures of mafic metamorphic rocks are estimated to have been approximately 800°C, based on garnet-clinopyroxene, garnet-hornblende, and orthopyroxene-clinopyroxene thermometers. Subsequently, the rocks were retrogressed and hydrated under the amphibolite facies at 0.6–0.8 GPa and around 600°C using a combination of several geothermobarometers. In mafic metamorphic rocks, exsolved rutile is found in garnet, orthopyroxene, and clinopyroxene. Such rutile probably formed during cooling from granulite to amphibolite facies. On the other hand, rutile exsolution is not observed in garnet from the pelitic gneisses, which only record a single stage of amphibolite facies metamorphism at 600°C–700°C and 1.1–1.5 GPa, as determined by garnet-biotite geothermometers and a garnet-biotite-muscovite-plagioclase-quartz geobarometer. Dating of zircon in garnet-amphibolite indicates that the protolith formed at ca. 120 Ma, with retrogressive amphibolite facies metamorphism at ca. 68 Ma. Zircons in the pelitic gneisses yield ages of ca. 66 Ma as an upper limit on the timing of original sedimentation and 63 Ma for metamorphism. The differences in the ages and metamorphism between the mafic metamorphic rocks and the pelitic gneisses clearly indicate they experienced different metamorphic histories, and that they were brought together during the exhumation process.