Editorial for the thematic issue, “Orogens, ophiolites, and oceans: A snapshot of Earth's tectonic evolution”

Abstract

Integrated studies of orogens, ophiolites, and oceans (OOO) provide a snapshot of Earth's dynamic evolution. As a key proxy of lithospheric plate underflow, the rocks in orogenic belts and oceans have been intensively studied from various scientific viewpoints and at different spatial–temporal scales. The rock record in OOO helps to better understand past plate-tectonic processes and thereby to interpret ongoing geodynamic processes in Earth's outer shell where we dwell. However, challenges remain with respect to obtaining a more comprehensive understanding of such processes through interdisciplinary exchange.

This thematic issue contains presentations given at the international symposium “Orogens, Ophiolites, and Oceans: A Snapshot of Earth's Tectonic Evolution” held in Sendai, Japan, in late February 2020. This thematic issue also honors Dr. Akira Ishiwatari for his important contribution to ophiolite studies. Dr. Ishiwatari played a pioneering and pivotal role in developing ophiolite studies in Japan based on field geology and petrology. The 2-day international symposium, miraculously held in person just before the coronavirus outbreak, focused on recent progress in petrochemical–tectonic studies of orogens, ophiolites, and oceans (oceanic crusts and oceanic deposits), especially in the vicinity of the Japanese Islands. Participants and presenters (including Dr. Ishiwatari) at the symposium were geoscience professionals and students in the areas of petrology, geochemistry, geochronology, and marine geology related to OOO (Figure 1). We trust that this thematic issue will serve as a worthy tribute to the research and contribution of Dr. Akira Ishiwatari.

We dedicate this thematic issue to Dr. Akira Ishiwatari for his important contribution to our understanding of ophiolites, especially the petrological classification of worldwide ophiolites and the geological features of Japanese ophiolites. His early studies in the late 1970s and early 1980s focused on the geology of the so-called “Yakuno Intrusive Rocks” in SW Japan, leading to the first recognized ophiolite sequence in Japan (Ishiwatari, 1978). The “Yakuno Ophiolite” identified by Dr. Ishiwatari has been internationally recognized as an ophiolite with an unusually thick oceanic crust (Ishiwatari, 1985a, 1985b; Ishiwatari, 1991). As a skilled field geologist with a strong background in petrology, Dr. Ishiwatari worked not only on the Yakuno Ophiolite but also on ophiolites in the French Alps (Ishiwatari, 1985c) and Russian Far East (Ishiwatari et al., 2003; Ishiwatari & Ichiyama, 2004), as well as on plume-related accreted ocean-island basalts (Ichiyama et al., 2008), and Miocene volcanic rocks formed during the opening of the Japan Sea (Ayalew & Ishiwatari, 2011; Ishiwatari & Imasaka, 2002). He also studied deep-sea igneous rocks as a modern ophiolite analog (Ishiwatari, 1992; Ishiwatari et al., 2006). Throughout his career, Dr. Ishiwatari made numerous cooperative academic exchanges with Chinese and Russian geoscientists (Figure 2). His international networking led to the success of the ophiolite session at the 29th IGC in Kyoto, Japan, in 1992, the key presentations in the session were collated in a book entitled “Circum-Pacific Ophiolites” (Ishiwatari et al., 1994). His collaborative works on coesite-bearing ultrahigh-pressure metamorphic rocks in eastern China, blueschist–ophiolite associations in the Sikhote-Alin (Russian Far East), and the Japanese Hida Belt resulted in a novel proposal termed the “Yaeyama Promontory Hypothesis” (Ishiwatari & Tsujimori, 2003).

Dr. Ishiwatari served as Editor-in-Chief of the journal Island Arc during 2004–2007 and as a co-chair of the Scientific Planning and Evaluation Panel of the Integrated Ocean Drilling Program during 2008–2010. He also served as the president of the Geological Society of Japan from 2012 to 2014. He increased his profile in Japan as a Commissioner of the Nuclear Regulation Authority, Japan, from 2014 to the present.

This thematic issue contains 11 papers on studies of OOO from different perspectives. The papers focus on past geodynamic processes, as recorded in on-land and submarine rocks at different scales from mineral-equilibrium to plate-tectonic levels.

The two papers by Hiroi et al. (2020) and Harada et al. (2021) present new observations concerning middle-crustal metamorphic processes in or near zones of continent–continent convergence. Hiroi et al. (2020) report felsite inclusions within granulite-facies garnets from continental-collision orogens (Sri Lanka, East Antarctica, Canada, and India). Those authors focus on euhedral quartz phenocrysts in the felsite inclusions. Zoning of cathodoluminescence (CL) emission attributed to trace amounts of titanium indicates crystal growth in supercooled felsic melt. The CL zoning of quartz phenocrysts in felsite inclusions implies that the cooling rates of studied granulites from collision orogens are one to two orders of magnitude higher than presumed to date. Harada et al. (2021) document a COSr isotope geochemical investigation into upper-amphibolite-facies marble and carbonate–silicate rock from the Hida Belt, which was once part of the crustal basement of the East Asian continental margin. The calcite CO isotopic compositions show wide variation in δ¹³C [VPDB] and δ¹⁸O [VSMOW] values (from −4.4‰ to +4.2‰ and from +1.6‰ to +20.8 ‰, respectively). In particular, the carbonate–silicate rock shows low δ¹³C values (from −4.4‰ to −2.9‰). Those authors conclude that the low δ¹³C values can be explained by decarbonation (CO₂-releasing) reactions.

Metamorphic processes at convergent plate margins are also discussed in the following two papers. Nishiyama et al. (2021) investigate gneiss-hosted metaperidotite bodies with spinifex-like texture from the Higo Metamorphic Rocks (HMR), Kyushu, Japan, and propose a new petrogenetic mechanism. According to their new results, the metaperidotites might have formed by high-pressure dehydration of antigorite-dominant serpentinite, possibly in a relatively deep root of mantle wedge under metamorphic conditions of P = ~1.6 GPa and T = ~740–750°C. The exotic occurrences of small bodies of high-pressure metaperidotites within the medium-pressure HMR gneisses suggest a tectonic juxtaposition of these two contrasting lithologies at a convergent plate margin. Fukushima et al. (2021) present a study on trace-element zoning patterns in prograde-zoned garnets in Group-C, low-temperature eclogites from Syros (Greece) and South Motagua Mélange (Guatemala). Analysis of Y + HREE profiles of the garnet porphyroblasts enables to discuss porphyroblast growth rates and cation diffusivities in the eclogitic matrices. Those authors also modify a previous diffusion-limited rare-earth element uptake model. Their new garnet porphyroblast growth model should contribute to unraveling cation-diffusion processes and thus lead to a better understanding of the geochemical kinetics in subduction zones.

Two pieces of research in NE Japan by Uchino (2021) and Okamoto et al. (2021) provide new insights into the tectonic evolution of Japanese Paleozoic oceanic plate convergence. Uchino (2021) use detrital zircon U–Pb ages to newly recognize an Early Triassic accretionary complex in the Nedamo Belt. This recognition is used as a basis for a new subdivision of the Nedamo Belt, namely, the Early Triassic Takinosawa and early Carboniferous Tsunatori units. The study also establishes a northeastward-younging trend of accretionary-orogen growth in the Kitakami Mountains. The detrital age spectra suggest that the Takinosawa Unit is comparable to Early Triassic fragments in the Kurosegawa Belt in Shikoku, which supports a pre-Jurassic geotectonic correlation between SW and NE Japan. Okamoto et al. (2021) investigate the petrological and geochemical natures of the early Paleozoic Motai serpentinites in the South Kitakami Mountains. The study finds that the geochemical characteristics of primary mantle minerals and secondary hydrous minerals are similar to those of serpentinites from the Mariana forearc. Those authors conclude that the protoliths of the Motai serpentinites were depleted-mantle peridotites that developed beneath the forearc region of a subduction zone located in a convergent margin of the early Paleozoic proto-East Asian continent.

The remaining papers focus on ophiolites and oceans, including modern sea floors and seamounts. The papers by Tamura et al. (2022) and Machida et al. (2021) deal with the crustal structure and surface environment of the Pacific Plate. Tamura et al. (2022) present an overview of the relationship between crustal thickness and seismic survey Moho reflection structure in a broad area of the Pacific Plate (the western–northwestern part and the East Pacific Rise) and examine several crust formation processes, as inferred from geochemical variation in magmas at mid-oceanic ridges. Those authors conclude that a relatively thick oceanic crust with thick dunite layers is plausibly caused by hydrous melting below mid-oceanic ridges, implying that seawater penetrates to the mantle along faults that develop as the crust forms. Machida et al. (2021) investigate ferromanganese nodules from the western North Pacific seafloor around Minamitorishima Island. The lack of some growth layers in ferromanganese nodules is interpreted as being due to a delayed supply of the nucleus material of nodules or a growth hiatus of Fe–Mn layer(s); in addition, missing sublayers in the ferromanganese nodules are inferred to regulate nodule size. This study demonstrates that multidimensional compositional mapping of ferromanganese nodules is a highly effective approach for reconstructing the submarine environment.

The three papers by Miyata et al. (2020), Aftabuzzaman et al. (2021), and Hirano et al. (2021) address the topic of seamounts. Miyata et al. (2020) examine shallow-water carbonates collected from the eastern slope of Hahajima Seamount, located at the junction between the Izu–Bonin and Mariana forearc slopes. Despite numerous previous studies, the origin of the Hahajima Seamount remains uncertain. On the basis of sedimentological and chronological analyses of the Hahajima carbonates, which are dominated by floatstone with numerous mollusks, those authors report lithological similarities and similar Sr isotope ages (Cretaceous) of carbonates between the Hahajima Seamount and the Ogasawara Plateau located east of the former and on the opposite side of the Izu–Bonin Trench (Pacific Plate). This study indicates that shallow-water carbonates of the Hahajima Seamount were not deposited in situ but instead originated from the Ogasawara Plateau. Thus, the eastern section of this seamount is interpreted as an accretionary wedge. Aftabuzzaman et al. (2021) present sedimentological, geochemical, and chronological analyses on carbonate rock samples collected from the western slope of Minamitorishima (Marcus Island) and determine the history of this seamount as follows. After deposition of the Cretaceous shallow-water carbonates, including the mollusk-rich limestone, Minamitorishima was inundated, and its top was covered with a pelagic cap. Late Eocene–early Oligocene volcanism caused episodic uplift and returned the top of Minamitorishima to a shallow-water environment. After the early Oligocene phosphatization of the pelagic cap, coral reefs flourished on the top of this island. The reef limestone was dolomitized during the Tortonian–Messinian. Hirano et al. (2021) report an investigation into the basement basalts of the Minamitorishima volcanic edifice. Those authors report a Paleogene volcanic edifice with an ocean-island-basalt-like geochemical affinity. The Paleogene volcanic activity might have locally overprinted Early to mid-Cretaceous volcanic edifices, at least on the Ogasawara Plateau and the Minamitorishima volcanic edifice. The presence of Cretaceous reefal limestone reported by Aftabuzzaman et al. (2021) strongly supports this scenario. As reported by Miyata et al. (2020), such seamount-capping carbonates that were deposited on the Pacific Plate prior to its subduction can provide highly valuable records that allow comprehensive geological interpretations to be made of the Izu–Bonin–Mariana forearcs.