宮下 敦

The Kitomyo Schist from Kurosegawa Belt, Shikoku, has been long considered as the oldest records of subduction metamorphism in Japan, based on an early 1970s K-Ar dating of white mica. The schist consists of mafic and pelitic layers and occurs as a tectonic block within serpentinite. Reappraisal of the schist confirmed the schist is characterized by an epidote-amphibolite peak metamorphic facies. The mafic portion is characterized by zoned amphibole + epidote + chlorite + titanite phengite +/- rutile. The presences of relict rutile surrounded by titanite and the barroisitic cores of zoned amphibole suggest a high-pressure intermediate type metamorphism at the metamorphic peak (P = -0.8-1.5 GPa and T = -500-570 degrees C). The presence of Mn-rich garnet and the lack of biotite, oligoclase and paragonite also support high-pressure intermediate type metamorphism that eliminate the possibility of a typical blueschist-facies metamorphism. New SHRIMP and LA-ICPMS zircon U-Pb geochronology on a pelitic sample show detrital grains of Mesoproterozoic and Early Paleozoic ages, suggesting a maximum deposition age for the trench-fill sediment of -440 Ma. Also the U-Pb data confirmed-360 Ma overgrown rims that might have formed during the subduction zone epidote-amphibolite facies metamorphism. Reappraisal revealed that the Kitomyo Schist is not the oldest high-pressure type schist in Japan and rather comparable to the Late Paleozoic Renge Metamorphic Rocks and their equivalents in the Kurosegawa Belt. The Devono-Carboniferous high-pressure metamorphic rocks in Japan might have been paired with their coeval batholiths along the 'Greater South China' margin that was extensively eroded during later tectonic processes. (C) 2020 Elsevier B.V. All rights reserved.

<p>東北日本の東縁に沿って分布する母体-松ヶ平帯中の古生代の年代を示す山上変成岩類は，高変成度の山上変成岩Ⅰと低変成度の山上変成岩Ⅱに区分されている．このうち，山上変成岩Ⅰは主に泥質片岩と角閃岩からなる．泥質片岩中のルチルにパラゴナイトが含まれることや変成鉱物の組成領域の検討から，山上変成岩Ⅰは蓮華帯の高変成度岩類と同等の低温高圧型変成作用を受けていると推定される．また，泥質片岩と角閃岩中のフェンジャイトK-Ar年代は322-287Maを示し，蓮華帯変成岩類と一致する．変成作用の特徴とフェンジャイトK-Ar年代から見て，山上変成岩類は西南日本の蓮華帯の高変成度岩類に直接対比できる可能性が高いと考えられる．</p>