Paleozoic Subduction-related Metamorphism in Japan: New Insights and Perspectives

Abstract

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 meta-morphic rocks provide a petrotectonic constraint on the earliest subduction event in the Japa-nese orogen. The presence of Middle to Late Paleozoic Renge lawsonite-blueschist and glauco-phane-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 ophioli-titic 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. Key words： Japanese Orogen, Paleozoic high-pressure metamorphic rocks, Renge metamorphic rocks, Kitomyo-Fuko Pass metamorphic rocks ： 日本列島造山帯古生代高圧変成作用蓮華変成岩木頭名-普甲峠変成岩 ＊ 岡山大学地球物質科学研究センター・PML ＊ PML, Institute for Study of the Earth's Interior, Okayama University 294-