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