Island Arc最新文献

{"title":"Tectono-Magmatic and Metamorphic Evolution of the Shillong Back-Arc Basin in the Assam-Meghalaya Gneissic Complex, Northeast India","authors":"Deepshikha Borah,&nbsp;Hiredya Chauhan,&nbsp;Bibhuti Gogoi","doi":"10.1111/iar.70023","DOIUrl":"https://doi.org/10.1111/iar.70023","url":null,"abstract":"<div>\u0000 \u0000 <p>The present investigation proposes the first geodynamic model to explicate the intricate relationships among the recently reported high Mg-andesites (HMAs) and rhyolites along with the associated metadolerites and granites related to the Shillong back-arc basin (1900–1400 Ma) of the Assam-Meghalaya Gneissic Complex (AMGC), Northeast India. This model aims to provide critical insights into the complex subduction geodynamics, magmatic processes, and crust–mantle interactions that occurred in the AMGC when the South Indian Block subducted beneath the North Indian Block along the Central Indian Tectonic Zone (CITZ) during the Columbia supercontinent amalgamation. The genesis of the HMAs is attributed to the lithospheric mantle, which underwent metasomatism via an influx of exceedingly higher degree of sediment-derived fluids originating from the subducting slab, while the doleritic melt is sourced from a mixed lithospheric-asthenospheric mantle, where slab-derived fluids with a lower sediment flux facilitated the decompression melting of the upwelling asthenosphere, leading to the generation of mafic magma at deeper mantle depths. The rhyolites are characterized as A-type in nature, giving an average zircon saturation temperature of 842°C. They were generated through underplating of the high-temperature mafic magma beneath the crust, which induced substantial thermal flux, driving crustal anatexis and the generation of A₂-type felsic magma in the back-arc setting. The basin's geodynamic significance is corroborated by its complex metamorphic and polyphasic deformational history. Geothermobarometric calculations involving the metadolerites yielded pressure–temperature estimates of 5.62–8.64 (average 7.02) kbar and 655–701 (average 683)°C, which assign an amphibolite facies metamorphism to these rocks, remnants of their peak metamorphic conditions during the Pan-African tectono-thermal episode (~500 Ma). Our postulated geodynamic model for the Paleo-Mesoproterozoic Shillong Basin of the AMGC, which we consider as the easternmost continuation of the CITZ, carries substantial significance in comprehending the broader geodynamic scenario operative during the creation of the Greater Indian Landmass during the Proterozoic.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144515072","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":"Low-Pressure and High-Temperature Type Metamorphism on the Suo Belt From Ozushima Island, Seto Inland Sea Area, Yamaguchi Prefecture, Southwest Japan: Evidence From Detrital Zircon U–Pb Dating and Mineral Paragenesis","authors":"Zejin Lu,&nbsp;Masaaki Owada","doi":"10.1111/iar.70024","DOIUrl":"https://doi.org/10.1111/iar.70024","url":null,"abstract":"<div>\u0000 \u0000 <p>Metamorphosed pelitic and psammitic rocks with small amounts of granitic stock are found on Ozushima Island, in the Seto Inland Sea area of Yamaguchi Prefecture, Japan. The metamorphosed pelitic rocks exhibit pervasive foliation defined by the preferred orientation of muscovite, biotite, andalusite, and sillimanite. General foliation shows an E–W trend and dips to the north and south, forming upright folds. The mineral assemblages and microstructures reveal that the metamorphic rocks of Ozushima Island can be divided into the Bt zone, And zone, and Sil zone, representing three metamorphic zones and corresponding to three deformational stages. Based on field surveys, the following deformational events were identified. Stage 1 deformation resulted in S1 foliation parallel to the bedding plane (S0). The critical minerals in each metamorphic zone formed the S1 foliation. In Stage 2, the S2 foliation developed along the axial planes of the microfolding of the S1 foliation. A NW-SE striking, NE-dipping thrust passed through the southern part of the island. In Stage 3, S3 foliation was locally present along the thrust plane. The metamorphic conditions of the And and Sil zones were estimated using conventional geothermobarometers at 530°C, 60 MPa and 600°C–710°C, 330–400 MPa, respectively. In addition, these metamorphic conditions plot within the metamorphic field gradient of the Cretaceous Ryoke metamorphic rocks from the Yanai-Iwakuni and Omuta areas. The detrital zircon grains separated from the psammitic rocks exhibited two age peaks at c. 1800 Ma and c. 250 Ma. A younger age indicates an older limit on the depositional age of the protoliths. These findings suggest that the metamorphic rocks of Ozushima Island originated from the Suo Belt and underwent low-pressure and high-temperature metamorphism, probably caused by heat supplied from the Cretaceous intrusive rocks, similar to the Ryoke metamorphic rocks.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144503294","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":"Zircon Crystallization Timings of Granitoids in the Aoyama Area, Ryoke Belt, Southwest Japan","authors":"Fumiko Higashino,&nbsp;Tetsuo Kawakami,&nbsp;Shumpei Kudo,&nbsp;Takafumi Hirata,&nbsp;Shuhei Sakata","doi":"10.1111/iar.70022","DOIUrl":"https://doi.org/10.1111/iar.70022","url":null,"abstract":"<div>\u0000 \u0000 <p>We performed U–Pb zircon dating using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for gneissose and massive granitoids from the Aoyama area, Ryoke belt, Southwest Japan. This is the first comprehensive report of U–Pb zircon ages from the Aoyama area, a key region to discuss an along-arc variation of the lower crustal magma generation in the Ryoke belt. Solidification timings of the Kabuto Granodiorite, Ao Granite, Misugi Tonalite, Joryu Tonalite, and Kimigano Granodiorite are considered to be ca. 80, ca. 72, ca. 74, ca. 88, and ca. 112–99 Ma, respectively. These timings are consistent with published intrusion relationships. Kimigano Granodiorite characteristically gives old magmatic ages, showing the magmatic activity predating the regional high-temperature metamorphism in the Aoyama area. The scattered U–Pb dates indicate either a long duration of zircon crystallization or rejuvenation of U–Pb dates of preexisting zircon grains. Magmatic activities are continuously observed for ~15 Myr, following the ~10 Myr “lull” after the solidification of the Kimigano Granodiorite at ca. 112–99 Ma. There is an ~8 Myr age gap between the youngest gneissose granitoid of the Joryu Tonalite and the oldest massive granitoid of the Kabuto Granodiorite. Zircon grains of the Kimigano Granodiorite and the Joryu Tonalite could be, respectively, affected by regional Ryoke metamorphism and by contact metamorphism due to the intrusion of the adjacent Ao Granite.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-06-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144472848","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":"Correction to “Origin and Evolution of the Paleo-Kuril Arc Inferred From Detrital Zircon U–Pb Chronology in Eastern Hokkaido, NE Asia”","authors":"","doi":"10.1111/iar.70018","DOIUrl":"https://doi.org/10.1111/iar.70018","url":null,"abstract":"<p>Nanayama, F., Yamasaki, T., Kanamatsu, T., Iwano, H., Danhara, T., and Hirata, T. (2022). “Origin and Evolution of the Paleo-Kuril Arc Inferred From Detrital Zircon U–Pb Chronology in Eastern Hokkaido, NE Asia.” <i>Island Arc</i> 31, no. 1: e12458. https://doi.org/10.1111/iar.12458.</p><p>In the Supporting Information, Table S4 and Table S5 contained errors. The data intended for Table S5 was mistakenly uploaded in place of Table S4. This has been corrected by restoring the original Table S4 and updating Table S5 with the correct data.</p><p>We apologize for these errors.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70018","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144308802","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":"Changes in the Redox State of the Nishinoshima Magmatic System During and After the 2020 Explosive Eruption","authors":"Kenta K. Yoshida,&nbsp;Noriko Tada,&nbsp;Tomoki Sato,&nbsp;Erika Tanaka,&nbsp;Hidemi Ishibashi,&nbsp;Yohei Mori,&nbsp;Fukashi Maeno,&nbsp;Yoshihiko Tamura,&nbsp;Shigeaki Ono","doi":"10.1111/iar.70021","DOIUrl":"https://doi.org/10.1111/iar.70021","url":null,"abstract":"<p>Intermittent volcanic activity has continued at Nishinoshima volcano in the Izu–Bonin arc in the western Pacific since 2013, characterized mostly by effusive Strombolian eruptions (Episodes 1–3). Subsequently, the eruption style changed suddenly to explosive violent Strombolian in mid-June 2020 (Episode 4), after which the volcano entered a period of quiescence. In 2021, volcanic activity restarted at Nishinoshima, although the eruptions were small (Episode 5). The change in eruption style between Episodes 1–3 and 4 was controlled mainly by a change in magma composition. However, the details of the redox state with respect to magma composition remain unclear. We aimed to understand the change in magma composition during Episode 4 and the subsequent activity, focusing on the redox state. Fe–K edge X-ray absorption near edge structure (XANES) analysis and petrographic observations were performed on volcanic glass in seafloor and subaerial samples from Episodes 4 and 5. The results show that the explosive eruption of Episode 4 was characterized by the intrusion of oxidized basaltic magma into a reduced andesitic magma, which is a similar tendency to another large-scale eruption in the Izu-Bonin arc (Fukutoku-Oka-no-Ba). Episode 5 shows different redox changes, involving remnants of the basaltic andesite magma from Episode 4, whose <i>f</i>_O2 had been reduced. The groundmass glass in Episode 5 subaerial ejecta has oxidized and differentiated (SiO₂-rich) composition possibly due to the subaerial cooling process. The observed geochemical and petrographic characteristics suggest that the small Episode 5 eruption occurred due to the continuous feed of the magma similar to Episode 4, but the magma plumbing system in the volcano made the provided high <i>f</i>_O2 magma reduced, which may mitigate the explosivity of the future eruption.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70021","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144299863","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":"Pumice Drifting from Ioto Volcano in the Izu–Bonin Arc to the Nansei Islands, Japan","authors":"Reona Hiramine,&nbsp;Daisuke Ishimura,&nbsp;Masashi Nagai,&nbsp;Takahiro Miwa,&nbsp;Haruka Nishikawa,&nbsp;Tatsu Kuwatani,&nbsp;Tomoki Sato,&nbsp;Kenta Yoshida","doi":"10.1111/iar.70020","DOIUrl":"https://doi.org/10.1111/iar.70020","url":null,"abstract":"<p>Pumice is a pyroclastic material of low density and is occasionally buoyant in water. When it erupts from submarine volcanoes, it can drift long distances and wash ashore at remote beaches. Pumice from the eruption of Fukutoku-Oka-no-Ba volcano in the Izu–Bonin arc in October 2021 drifted for about 2 months over a distance of &gt; 1300 km onto the Nansei Islands of Japan, disrupting shipping, fishing, and coastal ecosystems. In March 2024, large amounts of dark brown to black pumice washed ashore on these islands, and subsequently in the Kanto area of Japan. Their geochemical and petrographic characteristics were examined, and Ioto volcano, which is one of the most active volcanoes in Japan, was identified as a possible source based on these characteristics. Small amounts of similar pumice clasts drifting ashore in the Nansei Islands during 2022–2023 were also identified as originating from Ioto volcano. Since July 2022, volcanic activity involving magma eruption has occurred off the southeast coast of Ioto, where pumice rafts generated by the eruptions have been observed. Drift simulations indicate that pumice ejected from Ioto could reach the Nansei Islands, where it is widely distributed. The observation of pumice drifting along the coast of these islands thus provides information on the dispersal of pumice to other parts of Japan, with sources effectively identified through their chemical composition. The results contribute to our understanding of the eruption histories of submarine volcanoes in the Izu–Bonin arc.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70020","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144292364","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":"When Did a Giant Peak Collapse in the Annapurna Himalaya—Medieval or Latest Pleistocene? Geological Evidence From Debris Avalanche and Debris Flow Deposits","authors":"Harutaka Sakai,&nbsp;Achyuta Koirala,&nbsp;Jörg Hanisch","doi":"10.1111/iar.70019","DOIUrl":"https://doi.org/10.1111/iar.70019","url":null,"abstract":"<div>\u0000 \u0000 <p>A giant summit collapse in the Annapurna Himalaya was detected by Lavé et al. in 2023 and proposed to have happened at approximately 1190 AD. These authors concluded that the collapse transformed into a debris avalanche and subsequent debris flows, which reached the Pokhara Basin 60 km downstream to form the Pokhara Formation. Our geological investigations of the sediments in the Sabche Cirque and the valley-fill sediments in the Pokhara Basin demonstrate, however, that the Pokhara Formation is not equivalent to the mountain collapse deposit in Sabche Cirque. To the contrary, the Tallakot Formation—the oldest valley-fill formation, which is composed of a monomictic breccia of cataclastic texture, is equivalent to the sediments in the Sabche Cirque consisting entirely of the sediments derived from the Tethys Himalayan Sequence. The Ghachok Formation, which overlies the Tallakot Formation, is a wide-spread well-consolidated debris flow deposit also consisting of the debris derived from the Tethys belt. Several dating studies on the samples collected from the Ghachok Formation and overlying Phewa Formation, the dammed-up lacustrine deposits yielded¹⁴C IntCal20 ages between 15 and 10 ka, the oldest of which originates from a layer of humic soil at the base of the Ghachok Formation. These findings indicate that the series of events from the giant summit collapse to debris flows occurred at 15–14 ka. This timing coincides with the deglaciation period in the latest Pleistocene; it suggests that the melting of glaciers and permafrost weakened the rock strength and supported the mountain collapse. The main triggering agent of the collapse is attributed to an E-W extensional, normal fault-type earthquake that occurred in the Tethys belt. Unlike the Tallakot and Ghachok Formations, the Pokhara Formation is a polymictic heterometric almost nonconsolidated deposit that unconformably overlies the Ghachok Formation and is dated to be approximately 1250 AD.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-06-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144256322","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":"Origin of the Sandstone Dikes Intruding Into the Miocene Shidara Group, Southwest Japan Based on Sandstone Composition and Detrital Zircon U–Pb Ages","authors":"Sakurako Yabuta,&nbsp;Makoto Takeuchi,&nbsp;Yoshihiro Asahara,&nbsp;Qi Li","doi":"10.1111/iar.70017","DOIUrl":"https://doi.org/10.1111/iar.70017","url":null,"abstract":"<p>Geological conditions and fluid migration records that lead to the formation of sandstone injectites, including sandstone dikes, are attracting attention from various perspectives, including resource development. It is crucial to know the direction of intrusion from the parent sand body and the type of dike to understand the formation mechanism of sandstone dikes, while the location of the parent sand body is often unknown. Although the parent sand body of sandstone dikes intruding the Miocene Hokusetsu Subgroup of the Shidara Group in Southwest Japan is also unknown, this study has identified its parent sand body by comparing the sandstone composition and detrital zircon U–Pb age spectra of the sandstone dikes with those of each formation of the Hokusetsu Subgroup. The sandstone of the sandstone dikes are feldspathic sandstones similar to those of the lower part of the Hokusetsu Subgroup. Furthermore, the detrital zircon U–Pb ages of the sandstone dikes, which are characterized by a major cluster at about 95 Ma and minor clusters at 73–72 Ma, constrain its parent sand body among the lower Hokusetsu Subgroup. It resembles the zircon age spectrum of the lower Umedaira Sandstone Member, the uppermost horizon of the lower part of the Hokusetsu Subgroup. This means that the sandstone dikes originated from the lower part of the Umedaira Sandstone Member. This fact suggests that comparing the sandstone properties, such as sandstone composition and detrital zircon U–Pb age spectra of sandstone dikes with those of the host rock, is an effective method for identifying the parent sand body.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-05-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70017","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144140343","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":"Holocene Terraces Along the Tsailiao River, Western Foothills, Southwest Taiwan: Lithofacies, Chronology, Fossil Assemblages, and Neotectonic Implications","authors":"Kodai Iwasaki,&nbsp;Ai Kawamura,&nbsp;Haengyoong Kim,&nbsp;Yoshinari Kawamura,&nbsp;Chun-Hsiang Chang,&nbsp;Muneki Mitamura","doi":"10.1111/iar.70016","DOIUrl":"https://doi.org/10.1111/iar.70016","url":null,"abstract":"<p>Understanding the geohistory of fossil-bearing strata is essential for paleontological research. The Tsailiao area in the Tsochen District of Tainan City is one of the most important terrestrial fossil sites for Quaternary mammalian paleontology in Taiwan because it has yielded the most abundant and diverse mammalian fossils. However, the stratigraphy and depositional processes of the fossil-bearing terrace deposits in the area have not been well understood because previous research is insufficient for the precise distribution of the terraces and lithological and chronological descriptions of their representative deposits. Therefore, we studied terrace deposits, focusing on their distribution and the fossils they contain, from geomorphological, stratigraphic, chronological, and sedimentological perspectives. The terraces in the study area were grouped into three different heights, T1, T2, and T3, in descending order. Meandering rivers formed the T1, T2, and T3 terraces during the early, middle-late, and late Holocene, respectively. Each terrace deposit contained a basal gravel bed that yielded diverse fossils. However, most invertebrate fossils are inconsistent with a freshwater river environment and show signs of abrasion. Thus, they are considered derived fossils from underlying basement rocks. The mammalian fossils, including those of extinct species, were also found in the terrace deposits. Since the ages of the fossils do not correspond to those of the deposits, these fossils are likewise considered derived fossils. The results of our study contribute to the understanding of the terraces in the area. Our study of the terraces is expected to provide additional knowledge for geomorphological and tectonic studies in and around the area.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70016","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144074517","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":"Geotectonic Identity of Cretaceous-Paleogene Granitoids in the Tsukuba Igneous Complex, Japan: A New Multi-Proxy Reassessment","authors":"Wataru Fujisaki,&nbsp;Kanta Sato,&nbsp;Kazuma Iwata,&nbsp;Mariko Abe,&nbsp;Hisashi Asanuma,&nbsp;Masahiko Sato,&nbsp;Yusuke Sawaki,&nbsp;Kaoru Sugihara","doi":"10.1111/iar.70015","DOIUrl":"https://doi.org/10.1111/iar.70015","url":null,"abstract":"<div>\u0000 \u0000 <p>To reassess the geotectonic identity of the Tsukuba Igneous Complex (TIC) granitoids, we compiled a comprehensive data set of the granitoids and associated microgranular enclaves (MGEs) using zircon U–Pb geochronology, magnetic susceptibility (MS) analysis, and whole-rock geochemistry. The TIC granitoids comprised high-K, calcic to calc-alkaline, and peraluminous I-type granite. SiO₂ values were relatively high, and enrichment in Pb was observed while Ti and Nb were both depleted. The trace element signatures are diagnostic indicators for rocks formed in subduction-related settings. The MGEs in the Kabasan granitoid body were almost coeval with the host granitoids at ca. 79–76 Ma. Taking into consideration the mineral and geochemical compositions between the MGEs and the host granitoids, it was concluded that the MGEs originated from diorite xenoliths. Moreover, our new zircon U–Pb dating of TIC granitoids and MGEs showed that the TIC emplacement ages are divided into two groups; that is, ca. 80–76 Ma and ca. 70–61 Ma. This fact clearly demonstrates that TIC magmatism occurred two times during the Late Cretaceous to Early Paleogene. On the other hand, the data of TIC MS is two or three orders of magnitude lower than that of San-in granitoids MS, and the TIC granitoids belong to the ilmenite-series. The MS differences between the TIC and San-in granitoids can be explained by the amount of involved sediment, indicating that the zircon U–Pb dating is the most appropriate proxy for the reassessment of the geological identity of TIC granitoids. Accordingly, we propose that the TIC granitoids have two separate origins based on the emplacement age: that is, the Late Cretaceous TIC granitoids belong to those of the Ryoke or San-yo belts, whereas the Early Paleogene TIC granitoids are considered as the eastern extension of the San-in belt.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143939124","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}