Evolution processes of Ordovician-Devonian arc system in the South-Kitakami Massif and its relevance to the Ordovician ophiolite pulse

摘要: The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop volcanisms period. On northern western margin Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) high-pressure low-temperature metamorphic (Motai rocks) exhumed sometime Ordovician–Devonian periods are distributed. Chronological, geological, petrochemical studies on Hayachine–Miyamori Ophiolite, Motai rocks, other early Paleozoic units reviewed for reconstruction system during Devonian times supplementary new data. suggests a change convergence polarity from eastward- westward-dipping subduction before Late Ophiolite developed above eastward-dipping through three distinctive stages. Two separate stages overriding plate extension inducing decompressional melting minor involvement slab-derived fluid occurred after stage under strong influence fluids. first took place back-arc side forming basin. second immediately exhumation near fore-arc region. We postulate triggered by slab breakoff, which preceded rollback trench-parallel wedge mantle flow non-steady heat transport leaving exceptionally hydrous residual mantle. formation history implies weaker coupling may provide preferential conditions very Very peridotites involved magmatism have not yet been discovered except Cambrian–Ordovician periods, its implications global geodynamics, such as thermal state water circulation