日本列島の誕生場―古太平洋の沈み込み開始を示す飛騨外縁帯の５２０Ｍａの熱水活動―

Abstract

This paper examines the early stage of the geotectonic history of the Japanese Islands on the basis of finding hydrothermal jadeitite including zircons of ca. 520 Ma in serpentinite mélange of the Itoigawa-Omi area of the Hida-Gaien belt, Central Japan. Hydrothermal jadeitite contains euhedral jadeite in natrolite veins and patches, and consists of jadeite-albite and jadeite-natrolite without quartz. These minerals were crystallized from an aqueous fluid phase at the low-pressure and high-temperature side of the reaction boundary of albite = jadeite + quartz in the system NaAlSiO4-SiO2-H2O. The occurrence of rounded relict hornblende mantled by omphacite rimmed by fine-grained aggregates of jadeite in the matrix of jadeite and albite suggests a pervasive hydrothermal fluid flow, through which metabasite was extensively replaced by jadeitite. This rather high-temperature hydrothermal activity of ca. 520 Ma did not occur in an ordinary subduction zone but in a newly-formed mantle wedge suffering severe hydration from a subducting slab. Recently accumulated U-Pb ages of zircon of ca. 450-500 Ma from paleozoic sediments and granitic rocks of the Hida-Gaien belt were due to initiation of subduction followed by subduction zone magmatism. Protolith of serpentinite in the Hida-Gaien belt includes highly depleted harzburgite, thus requiring tectonic setting of a high-temperature-rift zone rather than a low-temperature-slow spreading ridge. Subduction was initiated at ca. 520 Ma along the boundary between low-density harzburgitic rift zone peridotite and lherzolitic spreading ridge peridotite with a slightly higher density, resulting in the common occurrence of harzburgitic serpentinite in the oldest part of the accretionary complex of Southwest Japan. An area including the Japanese Islands was born around the Yangtze block by the breaking up of the Rodinia supercontinent, because the oldest K-Ar age of biotite actinolit rock of 672 Ma (Matsumoto et al., 1981) and the subduction initiation of ca. 520 Ma are in accord with the paleogeographic history of the Yangtze block, and because ca. 300 Ma Renge schists of the Hida-Gaien belt did not suffer the ca. 280-200 Ma collision-type metamorphism of the Hida metamorphic belt that is an eastern extension of the suture between the Sino-Korea and Yangtze blocks.