Island Arc最新文献

{"title":"Characteristics of crustal structures in the Yamato Basin, sea of Japan, deduced from seismic explorations","authors":"Takeshi Sato,&nbsp;Tetsuo No,&nbsp;Shuichi Kodaira","doi":"10.1111/iar.12474","DOIUrl":"10.1111/iar.12474","url":null,"abstract":"<p>Crustal structures around the Yamato Basin in the southeastern Sea of Japan, inferred from recent ocean bottom seismography (OBS) and active-source seismological studies, are reviewed to elucidate various stages of crustal modification involved from rifting in the crust of the surrounding continental arc to the production of oceanic crust in the Yamato Basin of the back-arc basin. The northern, central, and southern areas of the Yamato Basin have crustal thicknesses of approximately 12–16 km, and lowermost crusts with P-wave velocities greater than 7.2 km/s. Very few units have P-wave velocities in the range 5.4–6.0 km/s, which corresponds to the continental upper crust. These findings, combined with previous geochemical analysis of basalt samples, are interpreted to indicate that a thick oceanic crust has been formed in these areas of the basin, and that this oceanic crust has been underplated by mantle-derived magma. In the central Yamato Basin, the original continental crust has been fully breached and oceanic crust has been formed. Conversely, the presence of a unit corresponding to the continental upper crust and the absence of a high-velocity part in the lower crust implies that the southwestern edge of the Yamato Basin has a rifted crust without significant intrusion. The Oki Trough has a crust that is 17–19 km thick with a high-velocity lower crust and a unit corresponding to the continental upper crust. The formation of the Oki Trough resulted from rifting with magmatic intrusion and/or underplating. We interpret these variations in the crustal characteristics of the Yamato Basin area as reflecting various instances of crustal modification by thinning and magmatic intrusion due to back-arc extension, resulting in the production of a thick oceanic crust in the basin.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"32 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12474","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43737512","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sedimentology and biostratigraphy of the upper Triassic carbonates from the Shiriya cape, North Kitakami Belt, Japan","authors":"Giovan Peyrotty,&nbsp;Andrea Fucelli,&nbsp;Camille Peybernes,&nbsp;Tetsuji Onoue,&nbsp;Hayato Ueda,&nbsp;Rossana Martini","doi":"10.1111/iar.12473","DOIUrl":"10.1111/iar.12473","url":null,"abstract":"<p>Relics of the Panthalassa Ocean occur today in accretionary complexes and terranes on the entire Circum-Pacific region. Among them, remains of shallow-water carbonate systems provide valuable information about the ecology and environmental conditions that prevailed in this immense ocean. In this regard, the Shiriya Cape of the North Kitakami Belt (Aomori Prefecture, Honshu Island, Japan) is an essential place to study. Characterized by the presence of numerous limestone deposits embedded in typical accretionary mélange, including pluri-kilometric massive slabs and metric bedded deposits, the cape is indeed a key vestige of carbonate deposition in the Panthalassa. All the limestone outcrop form the Shiriya Cape were sampled and studied in detail for the first time. Despite a general poor preservation, macroscopic sedimentary features such as burrows and large megalodontids patches as well as four microfacies were identified and are presented in detail in this work. On the basis of foraminifers and conodonts biostratigraphy, the age of the limestone deposits is defined as Norian. Similarities with analogous and synchronous systems from Panthalassan and Tethyan realms are extensively discussed. The closest analog to the Shiriya limestone appears to be the Panthalassan Dalnegorsk limestone (Russian Far East), both regarding microfacies and depositional setting. Sedimentary successions similar to Lofer-cycles, well known in Tethyan synchronous systems, were also discovered in the Shiriya Cape. Along with the large limestone slabs, countless limestone-bearing conglomerates also crop out on the Shiriya Cape. Their study and comparison with similar deposits in Japan highlighted their importance in the understanding of the dismantling of mid-oceanic carbonates. The observations and comparison with well-known carbonate systems permitted to establish a hypothetical depositional model of the Shiriya limestone corresponding to a Norian isolated carbonate system developed on an immerged volcanic seamount.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-11-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12473","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45086583","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Editorial for the thematic issue, “Orogens, ophiolites, and oceans: A snapshot of Earth's tectonic evolution”","authors":"Yasufumi Iryu,&nbsp;Tatsuki Tsujimori,&nbsp;Naoto Hirano,&nbsp;Yuji Ichiyama","doi":"10.1111/iar.12468","DOIUrl":"10.1111/iar.12468","url":null,"abstract":"&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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, &lt;span&gt;1978&lt;/span&gt;). The “Yakuno Ophiolite” identified by Dr. Ishiwatari has been internationally recognized as an ophiolite with an unusually thick oceanic crust (Ishiwatari, &lt;span&gt;1985a&lt;/span&gt;, &lt;span&gt;1985b&lt;/span&gt;; Ishiwatari, &lt;span&gt;1991&lt;/span&gt;). 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, &lt;span&gt;1985c&lt;/span&gt;) and Russian Far East (Ishiwatari et al., &lt;span&gt;2003&lt;/span&gt;; Ishiwatari &amp; Ichiyama, &lt;span&gt;2004&lt;/span&gt;), as well as on plume-related accreted ocean-island basalts (Ichiyama et al., &lt;span&gt;2008&lt;/span&gt;), and Miocene volcanic rocks formed during the opening of the Japan Sea (Ayalew &amp; Ishiwatari, &lt;span&gt;2011&lt;/span&gt;; Ishiwatari &amp; Imasaka, &lt;span&gt;2002&lt;/span&gt;). He also studied deep-sea igneous rocks as a mo","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12468","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45551345","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Isochronline in chemostratigraphy for global correlation and an exception of the Walther's law of facies","authors":"Wei Wang","doi":"10.1111/iar.12470","DOIUrl":"10.1111/iar.12470","url":null,"abstract":"<p>Chemostratigraphy based on various proxies has been well developed in recent decades for stratigraphic correlation by higher resolution, gap-less sampling, and environmental records. At present, various types of samples are measured quantitatively in higher accuracy and wider application than traditional biostratigraphy. Even the benthonic index fossils are widely replaced by pelagic data for biostratigraphic correlation, fossil gaps also could cause data break in a section. However, chemostratigraphy remains challenge as its data from sediments is to resolve mixtureof local depositional environment and global correlatable signal. How to separate these two parts and extract the local imprints free from the mixture is the subject of this study. An isochronlines is a newly introduced auxiliary line for the globally correlatable signal extraction from the mixture signals. An isochronline is a virtual synchronous line on plane of an equal depositional environment, and can evaluate the global signal unlike the chemostratigraphic data that provides environmental records. Based on the data obtained from lateral stratigraphic investigation along a bed strike, or from grid mapping on a wide-open outcrop of a bedding plane, or on a surface of a lithological unit, the facies-independent chemostratigraphic proxies could be extracted. A set of methods was designed including lateral profiling and grid mapping on the outcrop to extract the facies-independent geochemical signal for global correlation based on isochronlines identification, and an exception of the Walther's law of facies was revealed. Meanwhile, the author presents examples to avoid misleading of the global chemostratigraphic correlation based on isochronline analysis. This method included measurement of two-wing lateral-vertical strata, determination of non-lithological lens from geochemical abnormities of diagenesis, and microhabitat. An example from Laibin Auxiliary boundary Stratotype Section and Point of Lopingian-Guadalupian of Permian was measured by grid mapping, that revealed the isochronlines, extracted facies-independent geochemical proxies for global correlation, and hidden geochemical lenses.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-10-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49002488","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Post-collisional alkaline lamprophyre magmatism in northern Iran: Implications from whole-rock geochemistry and mineral compositions","authors":"Hadi Omrani,&nbsp;Christina Günter,&nbsp;Gholam Hossein Shamanian,&nbsp;Mehdi Omrani","doi":"10.1111/iar.12469","DOIUrl":"10.1111/iar.12469","url":null,"abstract":"<p>The Shanderman lamprophyre dykes crop out in the western part of the Alborz Mountains (Talesh). These rocks are classified as camptonites, composed of primary olivine, Ti-rich diopside, kaersutite, biotite, plagioclase, K–feldspar, and minor Ti–rich spinels, magnetite, pentlandite–pyrrhotite/chalcopyrite, and powellite–scheelite. Secondary analcime–wairakite, serpentines, and prehnite are common minor minerals within the studied rocks. Olivine, Ti-rich diopside, spinel, and amphibole show distinct chemical zoning. Spinels display a core-to-rim decrease in Cr₂O₃, MgO, and Al₂O₃ concentrations and an increase in TiO₂ and FeO^T (total Fe as FeO), reflecting the oxidation state increase due to hydrothermal fluid influx. Low SiO₂ contents (&lt;42 wt%), high MgO (12.44 to 13.98 wt%), and Fe₂O₃^T (12.76 to 13.43 wt%), Cr (318–537 μg/g) and Ni (231–327 μg/g) contents indicate the ultrabasic nature of the rocks. The samples show potassic character (2.1–2.8 wt% K₂O), along with elevated LREE and LILE, and also exhibit minor positive Eu anomalies (Eu/Eu* = 1.09 to 1.20). Olivine–spinel geothermometry indicates a maximum crystallization temperature of 1227 °C (<i>ave.</i> 988 °C ± 65 °C). Exsolution of pentlandite–pyrrhotite/chalcopyrite solid solutions occurred during magma cooling and crystallization. At lower temperatures, analcime–wairakite and prehnite partially replaced plagioclases. The geochemical modeling of the rocks indicates the Shanderman lamprophyre magmas were derived from low-grade melting (&lt;5%) of amphibole–bearing garnet lherzolite source without or with very few phlogopites. The primary magma of Shanderman lamprophyres was derived from a depth of ~135 km by partial melting of a metasomatized mantle source in a post-collisional environment.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-10-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43486830","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"An automatic peak deconvolution method for Raman spectra of terrestrial carbonaceous material for application to the geothermometers of Kouketsu et al. (2014)","authors":"Shunya Kaneki,&nbsp;Yui Kouketsu","doi":"10.1111/iar.12467","DOIUrl":"10.1111/iar.12467","url":null,"abstract":"<p>Carbonaceous material undergoes changes in its maturity mainly in response to its thermal history. Among the various geothermometers based on Raman spectra of carbonaceous material, those proposed by Kouketsu et al. (2014) have been employed to investigate the temperatures experienced by terrestrial rocks that undergo low- to medium-grade metamorphism. Although their geothermometers use spectral parameters that can be determined only by peak deconvolution, criteria for setting the initial conditions for nonlinear least-squares fitting have not been specified, which may generate unnecessary errors in the estimated temperatures owing to differences in fitting results among data analysts. To address this issue, we developed a code that automatically calculates the parameters required for the geothermometers of Kouketsu et al. (2014); that is, the widths of the D1- and D2-bands. Our code yields parameter values consistent with those of Kouketsu et al. (2014) for the same Raman data and is applicable to datasets other than those used to develop it. The geothermometers based on the obtained results show a comparable performance to Kouketsu et al. (2014). Errors in the estimated temperatures caused by variation in Raman systems and the subjective selection of the fitting method are smaller than the uncertainties of the geothermometer itself, so long as the temperature is calculated by following the procedures of the present study. Although this study focused only on geothermometers that require peak deconvolution, the development of those based on fitting-free Raman parameters and on the reaction kinetics of carbonaceous material should be investigated in future studies.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12467","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42427278","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Zircon trace-element compositions in Miocene granitoids in Japan: Discrimination diagrams for zircons in M-, I-, S-, and A-type granites","authors":"Yusuke Sawaki,&nbsp;Hisashi Asanuma,&nbsp;Shuhei Sakata,&nbsp;Mariko Abe,&nbsp;Takeshi Ohno","doi":"10.1111/iar.12466","DOIUrl":"10.1111/iar.12466","url":null,"abstract":"<p>Detrital zircons demonstrate high resistance to alteration and, as such retain information about their formation ages and parental magmas for a long period of time. Geochemical researchers have proposed a wide variety of discrimination diagrams applicable to detrital zircons. In our research, we focused on the conventional classification scheme for granites (Mantle, M; Igneous, I; Sedimentary, S; and Alkaline, A types) and sought to characterize zircon trace-element compositions that are sensitive to differences among these granite types. To accomplish this, we examined trace-element compositions of zircons extracted from granitoids in the Ohmine granitic rocks and the Ashizuri plutonic complex in southwestern Japan. The zircons showed systematic differences in Nb/P, Ta/P, Ce/P, Ce/Nd, Y/P, Th/U, and Sc/Yb ratios and the Eu anomaly. Zircons in A-type granite are rich in Nb, Ta, Ce, and Y, and their signatures clearly reflect those elements in their parental bodies. Sc/Yb ratios of zircons in A-type granites are &lt;0.1, which is similar to those of ocean-island-type zircons. Despite their high abundance at the whole-rock level, zircons in S-type granite are characterized by low Nb/P, Ta/P, and Th/U ratios. This is attributable to the depletion of Nb, Ta, and Th in the magma by ilmenite and monazite prior to zircon crystallization. In general, S-type granitic magmas exhibit reducing environments, which decrease the proportions of Ce⁴⁺ and Eu³⁺. These effects lead to a low Ce/Nd ratio and a large negative Eu anomaly in S-type zircons. On the basis of these findings, we recommend the combined use of Nb/P–Ce/P or Ta/P–Ce/P crossplots and of Sc/Yb ratios to discriminate zircons in M-, I-, S-, and A-type granites. Although the crossplots are created using data from Miocene granitoids in Japan, the discrimination diagrams are based on the general features of each type of granite.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-10-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41450936","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The nature of the Moho beneath fast-spreading centers: Evidence from the Pacific plate and Oman ophiolite","authors":"Yoshihiko Tamura,&nbsp;Mathieu Rospabé,&nbsp;Gou Fujie,&nbsp;Akane Ohira,&nbsp;Kentaro Kaneda,&nbsp;Alexander R. L. Nichols,&nbsp;Georges Ceuleneer,&nbsp;Tomoki Sato,&nbsp;Shuichi Kodaira,&nbsp;Seiichi Miura,&nbsp;Eiichi Takazawa","doi":"10.1111/iar.12460","DOIUrl":"10.1111/iar.12460","url":null,"abstract":"<p>It is common knowledge that the Moho is the boundary between the crust and the Earth's mantle. Here we show along several seismic profiles through the Pacific Plate that a correlation exists between the strength of Moho reflections, crustal thickness, and water depth. Where the Moho can be detected clearly, the overlying oceanic crust is systematically thicker and the water depths are shallower. We suggest that two end-members of oceanic crust exist in fast spreading environments: one thick, underlain by a clear Moho; the other thinner, without a Moho; with all intermediate situations. In the Oman ophiolite, the best-preserved on-land analogue of fossil oceanic lithosphere created by fast-spreading, the boundary between the mantle peridotites and the lower crustal gabbros mainly consists of a dunitic transition zone (DTZ) ranging from a few meters to a few hundred meters in thickness. A sudden influx of seawater down to the base of the crust at the mid-ocean ridge (MOR) results in the hydrous (re-)melting of mantle peridotites, producing a dunitic residue at the crust–mantle boundary that represents the most reflective Moho. At the same time, the hydrous melting, in addition to the normal decompression melting, beneath the MOR, increases the thickness of the oceanic crust by enhancing magma production. In the absence of hydrous melting, the DTZ is thin or absent at the crust–mantle boundary, and instead the uppermost mantle harzburgite is intruded by gabbros, and/or the overlying crustal gabbro is intruded by numerous wehrlite bodies, which will be seismically gradational.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-09-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12460","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62641163","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Petrogenesis of two types of gabbro from Neoproterozoic Fuchuan ophiolite complex, Jiangnan Orogen: Implication for a Japan Sea-like back-arc basin","authors":"Wentao Hong,&nbsp;Minggang Yu,&nbsp;Zhentao Song,&nbsp;Pingli Chu,&nbsp;Kai Liu,&nbsp;Xiaohua Zhou","doi":"10.1111/iar.12465","DOIUrl":"10.1111/iar.12465","url":null,"abstract":"<p>The Neoproterozoic Fuchuan ophiolite complex (FCO) represents the lithospheric remnant of the back-arc basin in the southeastern margin of the Yangtze Craton. However, the nature and development of this back-arc system are still confusing. This study focuses on the tholeiitic gabbros in the FCO, which can be divided into enriched and depleted types according to their geochemical characteristics. Enriched gabbros (843 ± 5 Ma) in the FCO are characterized by left-sloping rare earth element (REE) patterns and relatively low ε_Hf(t) (mostly in the range of 4.6–7.8). In contrast, depleted gabbros are slightly younger (838 ± 5 Ma) and isotopically more depleted (ε_Hf(t) mostly in 7.7–11.8) than enriched gabbros, exhibiting flat REE patterns. Based on geochemical variations and numerical modeling, the primary magmas of enriched gabbros were generated by a low degree (~14 %) partial melting of the melt-modified depleted mantle. In contrast, the more depleted isotopic composition and variable Ba/Th and V/Yb ratios of depleted gabbros suggest that their magmas have originated from a higher degree (~14–26 %) partial melting of the fluid-modified residual depleted mantle. Considering that the most ~860–830 Ma magmatism in the eastern Jiangnan Orogen is calc-alkaline, the Neoproterozoic back-arc basin where the FCO formed could be sialic and built on the thinning continental crust, resembling the Japan Sea. The occurrences of tholeiitic gabbros in the FCO thus provide crucial insights into the opening of the back-arc basin in the southeastern margin of the Yangtze Craton. The changes of mantle source nature and metasomatic agent reflect the upwelling of the depleted asthenosphere and oceanward retreading of the continental arc during the back-arc opening. Consequently, the early continental arc in the southeastern Yangtze Craton may have been transformed into a new island arc, resembling the formation of the northeast Honshu arc during the Miocene.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-09-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46870717","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Overprint of secondary Du folding in the Sambagawa metamorphic belt, SW Japan: Implications for strain ellipsoids and Paleogene tectonics of the east-Eurasian margin","authors":"Mutsuki Aoya","doi":"10.1111/iar.12463","DOIUrl":"10.1111/iar.12463","url":null,"abstract":"<p>Two contrasting results of strain analyses, constriction and flattening, are recognized in the Sambagawa high-pressure/temperature metamorphic belt, SW Japan. An unverified proposal to account for this situation is that the constrictional strain ellipsoids develop only in areas where there is strong overprinting by a secondary Du-phase folding after a penetrative Ds-phase deformation. Field studies in the Hibihara district, central Shikoku, which is located between a southern constrictional region and a northern flattening region, reveal there is a map-scale contrast in the effect of Du: outcrop-scale Du upright folds are common in the southern region while they are rare in the northern region. Field measurements show that overall orientation of Ds strain is characterized by E–W stretching and vertical shortening, while that of Du strain is characterized by N–S shortening and vertical extension. The shortening caused by Du in the southern high-Du-strain regions estimated by the fold-curve tracing method are down to about 70%–56% and by removing this Du shortening most of the constrictional strain ellipsoids are restored back to the flattening field with only a few exceptions, suggesting that pre-Du strain states of the Sambagawa metamorphic rocks were dominantly of the flattening type. These results support the previous ideas that invoke differences in the strength of Du overprinting for the two different types of strain ellipsoids observed in the Sambagawa belt. From a tectonic point of view, the semi-penetrative occurrence of Du folds throughout the Sambagawa belt, which stretches about 800 km in SW Japan, suggests that the Du phase can be related to some ancient plate movement. The significant shortening subnormal to the orogen that characterizes Du may reflect a shift to orogen-subnormal subduction of the Pacific plate beneath the Eurasian plate at around 60 Ma.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43758351","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}