Petrogenesis and implications of jadeite-bearing kyanite eclogite from the Sanbagawa belt (SW Japan)

摘要

Jadeite-bearing kyanite eclogite has been discovered in the Iratsu body of the Sanbagawa belt, SW Japan. The jadeite + kyanite assemblage is stable at higher pressure-temperature (P-T) conditions or lower H_2O activity [a(H_2O)] than paragonite, although paragonite-bearing eclogite is common in the Sanbagawa belt. The newly discovered eclogite is a massive metagabbro with the peak-P assemblage garnet + omphacite + jadeite + kyanite + phengite + quartz + rutile. Impure jadeite is exclusively present as inclusions in garnet. The compositional gap between the coexisting omphacite (P2) and impure jadeite (C2/c) suggests relatively low metamorphic temperatures of 510-620?°C. Multi-equilibrium thermobarometry for the assemblage garnet + omphacite + kyanite + phengite + quartz gives peak-P conditions of ~2.5?GPa, 570?°C. Crystallization of jadeite in the metagabbro is attributed to Na- and Al-rich effective bulk composition due to the persistence of relict Ca-rich clinopyroxene at the peak-P stage. By subtracting relict clinopyroxene from the whole-rock composition, pseudosection modelling satisfactorily reproduces the observed jadeite-bearing assemblage and mineral compositions at ~2.4-2.5?GPa, 570-610?°C and a(H_2O) >0.6. The relatively high pressure conditions derived from the jadeite-bearing kyanite eclogite are further supported by high residual pressures of quartz inclusions in garnet. The maximum depth of exhumation in the Sanbagawa belt (~80?km) suggests decoupling of the slab-mantle wedge interface at this depth.