飛騨外縁帯の地質構造発達史―日本列島地質体最古の太平洋型造山運動の復元― : ―日本列島地質体最古の太平洋型造山運動の復元―

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The geotectonic evolution of the 450-270 Ma old serpentinite mélange of the Itoigawa–Omi area of the Hida marginal belt has been reexamined from the viewpoint of reconstructing the oldest Pacific–type orogenic belt of Japan from a recently collected data set. Initiation of subduction to form a juvenile island arc followed by the accretionary complex of Southwest Japan took place at ca. 520 Ma as indicated by U–Pb zircon ages from metasomatic jadeitite formed by subduction zone fluids. Also, the 520 Ma zircons are indentified from volcaniclastic sediments of the eastern Hida marginal belt. Continued subduction proceeded with serpentinization and amphibolitization of the newly formed mantle wedge peridotite and crust, respectively, and high–P/T type metamorphism reaching eclogite facies older than ca. 380 Ma, as indicated by the EC unit of the Itoigawa–Omi area. This prolonged subduction zone metamorphism since the Cambrian formed essential members of the Oheyama–Renge belt of the Pacific–type orogenic belt of Japan, leading to tectonic erosion by subducting the oceanic plate, thereby the position of the subduction zone remained unchanged. The ca. 300 Ma Renge schists of the greenschist through epidote–amphibolite facies to eclogite facies of the Itoigawa–Omi area resulted from recrystallization probably by a subducting oceanic ridge. This ridge subduction caused exhumation of the Oheyama–Renge belt as a serpentinite belt including various hanging wall rocks with pre–existing subduction zone metamorphic rocks at a later stage. Remobilization of serpentinite mélange of the Hida marginal belt continued to act as a lubricant zone at the past Benioff thrust after the Permian, and defined the structure of the Hida marginal belt up to the present. The Itoigawa–Omi area is a type locality of the oldest Pacific–type orogenic belt of Japan. The Hida marginal belt is thus redefined as a composite tectonic belt including the Oheyama–Renge, and Akiyoshi and Maizuru belts, along with Paleozoic fore–arc basin deposits such as the Hitoegane formation of ca. 500 Ma. Note that the Unazuki metamorphic rocks are not members of the Hida marginal belt developed at the periphery of South China plate, but the Hida metamorphic belt as a collision zone between the North and South China plates.

飛騨外縁帯の地質構造発達史―日本列島地質体最古の太平洋型造山運動の復元― : ―日本列島地質体最古の太平洋型造山運動の復元―

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