日本列島に記録された古生代高圧変成作用 : ―新知見とこれから解決すべき問題点―
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概要
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The Japanese islands were formed during a long-sustained active Pacific-type orogen resulting from subduction, oceanward-accretion, and landward-erosion, which began in the early-Paleozoic. The earlier stage of the accretion of oceanic and continental materials took place between 500-300 Ma, reflecting the convergence of two or more paleo-Pacific plates and the stable, non-subducted Middle and Late Proterozoic continental lithosphere. In Japanese Paleozoic geo-tectonic units, two different high-pressure metamorphic rocks with the Pacific-type protoliths are associated with serpentinite bodies of Early Paleozoic Oeyama ophiolite: blueschist-facies pelitic and mafic schists with phengite K-Ar ages of 350-280 Ma (Renge) and high-pressure epidote-amphibolite-facies amphibolite and pelitic schist with 480-400 Ma hornblende and phengite K-Ar age (Kitomyo-Fuko Pass). The Early Paleozoic Kitomyo-Fuko Pass high-pressure metamorphic rocks provide a petrotectonic constraint on the earliest subduction event in the Japanese orogen. The presence of Middle to Late Paleozoic Renge lawsonite-blueschist and glaucophane-eclogite provides evidence of a cold geotherm in the paleo-subduction zone. Metamorphic and geochronologic data provide important constraints on tectonic development during the earliest stages of orogenic growth associated with the subduction of the paleo-Pacific oceanic plates. The lack of Paleozoic batholith belt and fore-arc sediments coeval with either Renge or Kitomyo-Fuko Pass metamorphism and the presence of blueschist-facies metamorphosed fore-arc ophiolititic materials (fragments of the Oeyama ophiolite) in the Renge metamorphic rocks suggest that a significant landward subduction erosion has occurred since early-Paleozoic time; the eroded material must have been recycled back into the mantle during subduction of the paleo-pacific plate. Thus, the early history of subduction-related orogenesis—after the dramatic tectonic conversion from a passive to active convergent margin—in the Japanese islands is comparable to the modern island arc system occurring worldwide. To further our understanding of the continuous paleo-subduction record in the Japanese Paleozoic geotectonic units, a more detailed and comprehensive approach to geology, petrology, and geochronology of high-pressure metamorphic rocks and associated rocks is required than that documented in previous studies.