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  • 環伊豆地塊蛇紋岩帯(鮫島輝彦先生追悼論文集)
  • カン イズ チカイ ジャモンガンタイ

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The Circum-Izu Massif Serpentine Belt is a fossil transcurrent plate boundary of the Oligocene-Miocene time beween the Philippine Sea and the Eurasia plates. The Circum-Izu Massif Serpentine Belt encircles the northern end of the Izu Massif, almost parallel to a trace of the present plate boundary, the Sagami Trough and the Suruga Trough. The Circum-Izu Massif serpentinite had been mainly emplaced into or protruded onto the Oligocene-Miocene sediments. The Circum-Izu Massif peridotite, mainly harzburgite, is characterized by the mineral assemblage, olivine (Fo_<90-92>) + opx + cpx + chromian spinel (Cr#, ca. 0.5) +/- plagioclase (An_<90>), and is a low-pressure (ca. 5 kb) restite with or without melt impregnation. It may represent the uppermost mantle of the Shikoku Basin, which had begun opening during Oligo-Miocene, protruded along the transcurrent plate boundary at the northern end of the Philippine Sea plate ( the Shikoku Basin ) , by analogy with the protrusion of abyssal peridotites along the oceanic fracture zone. It had been accreted to the Eurasia plate by the ocean-ward bending of the central Honshu arc during the opening of the Sea of Japan in the Miocene. Detrital chromian spinel grains in the Oligocene-Miocene sediments in the Circum-Izu Massif Serpentine Belt memorize some ancestral peridotites with arc-mantle characteristics which had been protruded in the Belt before the present Circum-Izu Massif serpentinite (peridotite). The ancestral peridotites and dioritic rocks frequently associated with the sepentinites were derived from the deep parts of the Paleo-Izu-Bonin arc, which had been split and disrupted by the Shikoku Basin opening. Picritic basalts of intra-plate type closely associated with the Circum-Izu Massif serpentinite were possibly derived from the mantle plume which had caused the opening..


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