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Evolution processes of O rdovician– D evonian arc system in the S outh‐ K itakami M assif and its relevance to the O rdovician ophiolite pulse
Author(s) -
Ozawa Kazuhito,
Maekawa Hirokazu,
Shibata Ken,
Asahara Yoshihiro,
Yoshikawa Masako
Publication year - 2015
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/iar.12100
Subject(s) - geology , geochemistry , ophiolite , metamorphic rock , basement , pyroclastic rock , subduction , seismology , tectonics , volcano , civil engineering , engineering
The S outh K itakami M assif is one of the oldest geological domains in J apan having S ilurian strata with acidic pyroclastic rocks and O rdovician– S ilurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the S ilurian period. On the northern and western margin of the S outh K itakami M assif, an O rdovician arc ophiolite ( H ayachine– M iyamori O phiolite) and high‐pressure and low‐temperature metamorphic rocks ( M otai metamorphic rocks) exhumed sometime in the O rdovician– D evonian periods are distributed. Chronological, geological, and petrochemical studies on the H ayachine– M iyamori O phiolite, M otai metamorphic rocks, and other early P aleozoic geological units of the S outh K itakami M assif are reviewed for reconstruction of the S outh K itakami arc system during O rdovician to D evonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the L ate D evonian period. The H ayachine– M iyamori O phiolite 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 H ayachine– M iyamori O phiolite 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 C ambrian– O rdovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.