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

Abstract

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.

Cretaceous to Miocene NW Pacific Plate Kinematic Constraints: Paleomagnetism and Ar–Ar Geochronology in the Mineoka Ophiolite Mélange (Japan)

“…Modern discrimination schemes by Vermeesch (2006aVermeesch ( , 2006b) also suggest a N-MORB and IAB geochemical characteristic (Figures 4e and 4f). The presence of plagioclase-bearing peridotites suggest a back-arc setting (e.g., Kimura et al, 2020;Ozawa et al, 2015). In general, the plagioclase peridotites are uncommon among the global abyssal peridotite suites, and have been reported from slow-spreading ridges (e.g., Dick, 1989;Warren, 2016).…”

Eocene Basalt of Summit Creek: Slab breakoff magmatism in the central Washington Cascades, USA

“…53 and 45 Ma, a wave of dynamic/ RESEARCH thermal uplift (Smith et al, 2014) and magmatism (Tepper, 2016) migrated southwestward (trenchward) across the northwestern United States. This activity is attributed to rollback of the Farallon slab following the accretion of Siletzia, and in other arcs, rollback has been identified as a precursor to slab breakoff (Lucente and Margheriti, 2008;Ozawa et al, 2015).…”

Slab‐fluid metasomatism in the Early Paleozoic forearc mantle deduced from the Motai serpentinites, South Kitakami Belt, northeast Japan

“…The Kitakami Belt, northeast Japan, comprises a series of Early Paleozoic to Jurassic geological terranes (Figure 1). Particularly, the South Kitakami Belt contains Early Paleozoic complexes, the Hayachine-Miyamori Ophiolite (Kimura et al, 2020;Ozawa, 1988;Ozawa et al, 2015;Ozawa & Shimizu, 1995) and the Motai metamorphic rocks, (serpentinite with metamorphic rocks; Maekawa, 1981) and is considered to be one of the oldest geological terranes in Japan, as well as the Oeyama Ophiolite and Kurosegawa Belt in southwest Japan (cf. Ehiro et al, 2016;Ishiwatari et al, 2016).…”

“…Ehiro et al, 2016;Ishiwatari et al, 2016). Ozawa et al (2015) interpreted the Hayachine-Miyamori Ophiolite as fragments of an Early Paleozoic arc-backarc system and also regarded the Motai metamorphic rocks as subducted oceanic materials in the forearc regions of the Hayachine-Miyamori subduction zone.…”

“…According to the model of Ozawa et al (2015), the South Kitakami Belt preserves the fragments of a cross-section from forearc to backarc in a subduction zone and therefore plays an essential role in understanding the evolution of a subduction system on an Early Paleozoic East Asian continental margin. Although the field occurrence and petrographic description of the Motai serpentinites were given by Shimazu (1954aShimazu ( , 1954b, Maekawa (1981), and Machida and Ishiwatari (2013), detailed petrological characteristics of the Motai serpentinites are lacking from the viewpoint of forearc mantle wedge materials.…”