Jadeite-garnet glaucophane schists in the Bizan area, Sambagawa metamorphic belt, eastern Shikoku, Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite-garnet glaucophane schists occur as lenticular or sheet-like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X-Jd 0.46-0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote-blueschist to eclogite facies. Metamorphic P-T conditions estimated using pseudosection modelling are 580-600 degrees C and 18-20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite-garnet glaucophane schists have low CaO (4.4-4.5 wt%) and MgO (2.1-2.3 wt%) bulk-rock compositions. The P-T-X-Ca(Rock) pseudosections show that low X-Ca bulk-rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X-Ca bulk-rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low-T type and subsequent high-T type eclogitic metamorphic events. The jadeite-garnet glaucophane schists experienced low-T type eclogite facies metamorphism, and the P-T path is similar to lawsonite-bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low-T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high-T type eclogitic metamorphism within a single subduction zone.