Blueschist facies metamorphic rocks from the basement of the Izu-Bonin inner trench wall

Abstract

Metamorphic rocks from the Izu-Bonin inner trench wall include predominant epidote-amphibole and mica-epidote-amphibole schists and rare garnet-epidote quartzites and chlorite-amphibole schists recrystallized in the lower amphibolite (T=500-600°C; P=3-5.5 kb) to greenschist (T=400-480°C; P=l-3 kb) facies. In epidote-amphibole and mica-epidote-amphibole schists green calcic amphiboles (magnesio-hornblende and rare tschermakitic or edinitic hornblende) are locally replaced by blue sodic-calcic (winchite and barroisite) and alkali (magnesio-riebeckite) ones. This replacement occurs at low temperature (250-300°C) and high pressure (4-5 kb), corresponding to the conditions of blueschist facies. The blueschist facies assemblages, which are overprinted on the earlier assemblage, may be caused by the combination of the accretion of fragments of the downgoing oceanic plate, the influence of special fluid flux through the forearc region, and the general decreasing of temperature after the initiation of subduction of the Pacific plate beneath the Philippine plate.