Jadeite–bearing metaigneous rocks from the Northern Chichibu belt, SW Japan: implications for the lowest–grade Sanbagawa metamorphism

Abstract

Multiple generations of Na–Ca and Na pyroxenes (aegirine–augite, aegirine and jadeite) were found in a metamorphosed monzonitic dike from a coherent mafic layer of the Northern Chichibu belt in central Shikoku, Japan. The mafic layer was derived from alkaline basalt magma and its derivatives, and belongs to the Kamiyoshida unit (Middle Jurassic accretionary complex). The earliest–stage sodic pyroxene (jadeite–free Ti–rich aegirine–augite to aegirine) in the dike probably crystallized during a post–magmatic hydrothermal stage. Igneous Ti–rich augite phenocrysts were almost completely pseudomorphed by chlorite + phengite + Al–OH–rich titanite + sodic pyroxene (aegirine–augite with jadeite component) during early stages of subduction metamorphism. The earliest–stage sodic pyroxene and pseudomorphed Ti–rich augite were further overgrown by jadeite/aegirine fringes during high–P/T metamorphism. Jadeite/aegirine pyroxene also occurs as small (~ 50 µm long) neoblasts within pseudomorphs (albite + phengite + pumpellyite ± Ba–rich K–feldspar) after igneous plagioclase. Individual grains of the fringe/neoblastic pyroxene are zoned with a jadeite core (up to 98% jadeite content) and an aegirine rim (down to 25% jadeite content). Jadeite–rich pyroxene (up to 92% jadeite content) was also found in basaltic rock from the mafic layer. The absence of quartz and the alkaline affinity of the protoliths suggest that the jadeite formation can be explained by the reaction analcime = jadeite + H₂O. If H₂O fluid was present, the decrease in jadeite content in the fringe/neoblastic pyroxene implies decompression from ~ 0.6–0.7 GPa at 300 °C to ~ 0.4 GPa at 210–260 °C. The formation of the jadeite–to–aegirine pyroxene during exhumation probably resulted from the lowest–grade Sanbagawa metamorphism.