"三郡変成帯"の変斑れい岩質岩石の産状とK-Ar年代

概要

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Serpentinite and related metagabbroic rocks sporadically occur as small blocks or lenses throughout the "Sangun metamorphic belt". These can be interpreted as fragments of ophiolite (dismembered ophiolites). Mineral assemblages and textures, as well as field evidence, allow the metagabbroic rocks to be divided into metagabbro and amphibolite, both of which have undergone an initial phase of ocean-floor metamorphism in the epidote-amphibolite to amphibolite facies and a subsequent retrograde phase, the "Sangun metamorphism", in the glaucophane-schist to pumpellyite-actinolite facies. K-Ar ages have been determined on relict hornblende in 11 metagabbroic rocks from the Wakasa, Hokubo-Houkoku, Yamaguchi, Wakamiya-Sasaguri and Yamaga areas in the "Sangun metamorphic belt". The all ages obtained range widely from 220 to 477 Ma (Late Triassic to Middle Ordovician time). In individual cases the age is greater than the radiometric ages of the surrounding schist. None of these K-Ar ages are comparable with the "Sangun metamorphism" and instead they are interpreted as dating the initial ocean-floor metamorphism. These results are also consistent with a recent view that the so-called Sangun metamorphic belt is not a single coherent glaucophanitic terrane, but is comprised of at least three glaucophanitic terranes with different radiometric ages, Chizu Terrane (180 Ma), Suo Terrane (220 Ma) and Sangun-Renge Belt (300 Ma).
日本地質学会の論文
1989-04-26