Island Arc最新文献

{"title":"Thermal Structure Around the Shibukawa Ultramafic Body of the Mikabu Greenstones in the Low-Grade Sanbagawa Schists, Northwest Shizuoka, Central Japan","authors":"Seira Katagiri,&nbsp;Yui Kouketsu,&nbsp;Katsuyoshi Michibayashi","doi":"10.1111/iar.70034","DOIUrl":"https://doi.org/10.1111/iar.70034","url":null,"abstract":"<p>The Cretaceous high-P/T metamorphic complex such as Sanbagawa schists and Mikabu greenstones is located in the outer zone of the Median Tectonic Line in the Shibukawa area, northwestern Shizuoka Prefecture, central Japan. In this area, a 2 × 1 km Shibukawa ultramafic body, a member of the Mikabu greenstones, occurs within the chlorite zone of the low-grade Sanbagawa schists. The upper part of this ultramafic body is covered by pelitic schists (classified as the Upper unit). The peak temperatures estimated from the Raman spectra of carbonaceous material (CM) in the pelitic schists were 277°C–354°C. The temperatures of the Upper unit range from 277°C to 293°C, which are lower than those of the surrounding Sanbagawa schists, ranging from 295°C to 354°C. Considering the elevation and large-scale structures of the body, the Upper unit is inferred to be a shallow, lower-grade Sanbagawa schist unit preserved in the ultramafic body, as it has a geological structure similar to that of the surrounding schists. This suggests that although the occurrence of the Shibukawa ultramafic body has complicated the geological structure of the Sanbagawa schists, the metamorphic temperature increases toward apparently lower structural levels. In addition to constraining metamorphic temperatures, it shows that the Raman CM geothermometer is a powerful tool for revealing structural characteristics based on geological and thermal relationships within low-grade metamorphic rocks in shallow subduction zones.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2025-09-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70034","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145111248","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":"Early Miocene Fore-Arc Magmas Derived From Subcontinental Lithospheric Mantle During the Japan Sea Opening: Geochemistry of the Ishimoriyama and Iritono Volcanic Rocks in the Iwaki District, NE Japan","authors":"Takahiro Yamamoto","doi":"10.1111/iar.70032","DOIUrl":"https://doi.org/10.1111/iar.70032","url":null,"abstract":"<p>During the Miocene opening of the Japan Sea, volcanic activity expanded greatly toward the trench due to the injection of the hot asthenosphere into the mantle wedge. The Ishimoriyama and Iritono volcanic rocks, both erupted at around 17.5 Ma in the Iwaki district on the Pacific coast of NE Japan, are products of this event. Ishimoriyama is a small composite volcano comprising calc-alkaline basaltic to andesitic volcaniclastic rocks. Iritono is a monogenetic volcano composed of low-K aphyric pillow basalts with high TiO₂ contents. The Pb isotopic compositions of volcanic rocks from both volcanoes (²⁰⁶Pb/²⁰⁴Pb = 18.39–18.40, ²⁰⁷Pb/²⁰⁴Pb = 15.61–15.62, and ²⁰⁸Pb/²⁰⁴Pb = 38.53–38.57) are more radiogenic than the Indian MORB-like trend defined by other Miocene volcanic rocks in the fore-arc region of NE Japan and overlap those of the most enriched Japan Sea Miocene basalts. The variation among the Japan Sea basalts can be explained by the mixing of depleted Indian MORB-like mantle and enriched preexisting subcontinental lithospheric mantle (SCLM). Therefore, the Ishimoriyama and Iritono volcanic rocks were derived from SCLM, and the reason why magma was generated by partial melting of the SCLM, which does not normally melt spontaneously, was due to the injection of the hot asthenosphere. The trace element abundances in the Ishimoriyama volcanic rocks match those in normal arc-type volcanic rocks, but their compositional variations suggest the fractionation of large amounts of amphibole from basaltic andesite magma. Because this differentiation process requires a high pH₂O in the parental magma, the source could have been hydrous SCLM. In contrast, the Iritono volcanic rocks are depleted in fluid-mobile elements and have convex rare earth element patterns peaking at Sm. Accordingly, this magma formed when a residual SCLM domain was re-melted by the hot asthenospheric injection. These distinct geochemical differences between two nearly contemporaneous volcanic rocks only 20 km apart indicate that the SCLM was heterogeneous, with adjacent hydrous and residual domains.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2025-08-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70032","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144869977","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":"Slab Tear Beneath the Sunda Strait","authors":"Luhut Pardamean Siringoringo,&nbsp;Benyamin Sapiie,&nbsp;Alfend Rudyawan,&nbsp;I Gusti Bagus Eddy Sucipta","doi":"10.1111/iar.70031","DOIUrl":"https://doi.org/10.1111/iar.70031","url":null,"abstract":"<div>\u0000 \u0000 <p>Slab tearing plays a critical role in subduction zones worldwide, as it facilitates mantle upwelling, influences magma generation, and leads to the formation of tensional structures, which may create favorable conditions for geothermal potential. Understanding its mechanisms and effects is essential for interpreting the complex geodynamics of convergent margins. This research investigates geodynamic processes in the Sunda Strait, focusing on slab tearing and its geological implications. Analysis of earthquake hypocenter data (1906–2025) reveals significant differences in slab depth between Sumatra and Java, supporting the slab tearing hypothesis. These differences arise from variations in the age and density of the subducting oceanic crust, causing sharp contrasts in slab dip angles. High heat flow in the eastern and northeastern Sunda Strait indicates mantle upwelling due to vertical and horizontal slab tearing, respectively. The presence of Sukadana Basalt (OIB-like) in eastern Lampung and normal faults in the Sunda Strait, along the eastern Lampung and western Java coasts further supports this hypothesis. Additionally, shifts in volcanic arc distribution and volcanic activity discontinuities confirm the tectonic effects of slab tearing. This study highlights how slab dip angle variations influence magma genesis and volcanic behavior. It also reveals the unique geodynamics of the transition zone between orthogonal and oblique subduction. The findings enhance understanding of geological processes in the Sunda Strait and provide insights for future research on slab tearing and regional tectonics.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2025-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144869842","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 Amphibole Cumulates at the Base of an Exposed Arc Crustal Section: Perspectives From Fiordland, New Zealand","authors":"Patrick Manselle,&nbsp;Stephen Foley,&nbsp;Nathan Daczko","doi":"10.1111/iar.70030","DOIUrl":"https://doi.org/10.1111/iar.70030","url":null,"abstract":"<p>The formation of amphibole-rich cumulates in arc settings plays a crucial role in influencing the chemistry of ascending arc magmas. However, understanding the processes of formation of these cumulates is challenging due to the subtle nature of their fractionation. This study presents new petrographic and geochemical analyses of amphibole-rich cumulate rocks from outcrops across Fiordland, New Zealand, an exposed section of the lower arc crust. Our findings show two primary styles of formation. Style 1 represents direct fractionation from a melt, and is typified by adcumulate textures, similar rare earth element concentrations, and minor mineral phases. Style 2 amphiboles show greater variations in their rare earth element contents and typically lack adcumulate textures. Our results indicate that direct igneous fractionation from a melt is the primary formation mechanism for amphibole cumulates, with minor contributions from melt–rock interaction, aligning with the amphibole sponge model. This research underscores the significance of amphibole cumulate formation in the evolution of the lower crust in arc systems and illustrates how co-crystallizing minerals and precursor assemblages can influence the chemistry of ascending magmas during metasomatism.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2025-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70030","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144833088","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":"Large Contact Aureole Overprinting the Ryoke Regional Metamorphic Zoning: Insights From the Takato–Komagane Area, Central Japan","authors":"Takuro Yoshioka,&nbsp;Hideo Takagi,&nbsp;Hiroyuki Nagahama,&nbsp;Tomoki Taguchi","doi":"10.1111/iar.70027","DOIUrl":"https://doi.org/10.1111/iar.70027","url":null,"abstract":"<p>The Ryoke metamorphic belt records the conditions of the middle–upper crust during Cretaceous crustal deformation and magmatism. To understand the development and evolution of the middle–upper crust, it is important to investigate the thermal structure of the Ryoke metamorphic belt. The Ryoke metamorphic rocks crop out widely in the Takato–Komagane area in Nagano Prefecture, central Japan. However, the spatial variations in metamorphic conditions in both areas are still unclear. We therefore investigated the metamorphic zoning and conducted Raman spectroscopy of carbonaceous material (RSCM) thermometry, chemical analyses, and pseudosection modeling of the Ryoke metamorphic rocks. The Bt, Sil–Kfs, and Grt–Crd zones in the Takato area, and the Bt, Kfs–Crd, and Grt–Crd zones in the Komagane area were redefined based on the mineral assemblages in metasedimentary rocks. The metamorphic zoning is different between the Takato and Komagane areas. We revealed that sillimanite occurs throughout both areas, although its occurrence changes gradually from in the matrix to inclusions in cordierite, between the Takato and Komagane areas. Peak <i>P</i>–<i>T</i> conditions of the Sil–Kfs zone are estimated to be 630°C–650°C and 320–380 MPa using RSCM thermometry and pseudosection modeling. Pseudosection modeling, microscopic observations, and thermometry results indicate higher temperatures in the Komagane area than in the Takato area. It is interpreted that part of the regional metamorphic belt underwent thermal overprinting induced by a late-stage pulse of the gneissose Ryoke granitoids, resulting in the formation of a metamorphic zone distinct from that in the Takato area, characterized by the occurrence of sillimanite inclusions.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2025-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70027","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144782258","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":"Microstructural Characteristics of Sheared Gabbroic Rocks From the Mado Megamullion, Shikoku Basin, Philippine Sea","authors":"Kohei Nimura,&nbsp;Katsuyoshi Michibayashi,&nbsp;Yumiko Harigane,&nbsp;Yasuhiko Ohara","doi":"10.1111/iar.70029","DOIUrl":"https://doi.org/10.1111/iar.70029","url":null,"abstract":"&lt;p&gt;This paper investigated the deformation microstructures of gabbroic rocks from the breakaway region of the Mado Megamullion (site YK19-04S-6K-1536), Shikoku Basin, Philippine Sea to constrain the processes that contribute to the mechanical weakening of the lower oceanic crust. The deformed gabbroic rocks are characterized by porphyroclastic textures consisting mainly of coarse plagioclase + clinopyroxene ± amphibole porphyroclasts and a fine-grained matrix of plagioclase + clinopyroxene + amphibole + Fe-Ti oxides ± talc. Based on macroscopic foliation development, the samples are divided into low- and high-strain domains. The low-strain domain shows weakly developed foliations and a granoblastic texture with partially preserved magmatic textures, whereas the high-strain domain shows well-developed foliation defined by monomineralic and mixed mineral layers and porphyroclastic textures. In the low-strain domains, plagioclase grains show similar crystallographic orientations and chemical compositions between porphyroclasts and surrounding matrix, consistent with deformation predominantly via dislocation creep and subgrain rotation recrystallization. Clinopyroxene porphyroclasts and adjacent brown amphibole at porphyroclast rims share similar crystallographic orientations, suggesting topotactic replacement of clinopyroxene by amphibole associated with fluid infiltration. In the high-strain domains, fine-grained plagioclase shows distinct compositions from porphyroclasts and fabric development with [100] aligned to lineation and (001) to foliation, indicating that grain nucleation occurred during deformation via fluid-assisted reactions. It suggests that the deformation could have been accommodated by a combination of dislocation creep and dissolution–precipitation creep. Fine-grained clinopyroxene within monomineralic layers exhibits a clear (010)[001] crystallographic-preferred orientation indicative of dislocation glide. In contrast, fine-grained clinopyroxene in polymineralic layers shows weaker or nearly random orientations, suggesting enhanced grain boundary sliding at clinopyroxene–amphibole interfaces. This behavior was facilitated by grain-size reduction, abundant phase mixing, and inhibition of grain growth by Zener pinning. Brown amphibole within the high-strain matrix exhibits strong crystallographic and shape preferred orientations parallel to foliation, suggesting syn-tectonic growth and alignment. Ti-rich brown amphibole formed synchronously with deformation through clinopyroxene breakdown at temperatures of 755°C–871°C, whereas Ti-poor green amphibole formed during subsequent lower-temperature (686°C–770°C) hydrothermal alteration. A comparison with the Godzilla Megamullion suggests similar fluid-mediated deformation processes are active in megamullions throughout back-arc basin environments. Our results demonstrate that melt- and fluid-assisted reactions, coupled with grain size–sensitive deformation mechanisms, contribute signifi","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2025-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70029","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144767717","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 Precipitation Change Recorded in Stalagmite Carbon Isotopes and Mg/Ca Ratios in the Pacific Coastal Area of Central Japan","authors":"Akira Murata,&nbsp;Taiki Mori,&nbsp;Hirokazu Kato,&nbsp;Fumito Shiraishi,&nbsp;Kenji Kashiwagi,&nbsp;Akihiro Kano","doi":"10.1111/iar.70025","DOIUrl":"https://doi.org/10.1111/iar.70025","url":null,"abstract":"<p>Stalagmite is an excellent archive for reconstructing climate conditions in temperate-tropical areas. Our previous study reconstructed Holocene temperature changes at high resolution from a well-dated stalagmite (KA01) in the Kiriana Cave in Mie Prefecture, Japan, but could not extract information related to precipitation change. Here, the δ¹³C and Mg/Ca ratio of KA01 were used to discuss the precipitation variability. These proxies reflect prior calcite precipitation (PCP; calcite precipitation before the water reaches a stalagmite), which can be used to evaluate the precipitation. The δ¹³C-based PCP with higher time resolution indicates a humid interval of 9.8–7.2 ka and dry intervals of 12.3–13.3 and 4.0–2.0 ka. These humid and dry intervals broadly correspond to the warm and cold intervals, respectively, which were identified in our previous study. This correspondence may have resulted from the oceanographic change in the northwest Pacific, which is dominant moisture and heat sources for the study area. The humid-warm conditions during 9.8–7.2 ka were unstable, with short-term dry and cold periods. The dry and cold period around 8 ka may correspond to the global “8.2 ka event”. The cold-dry period of 4.0–2.0 ka corresponds to the late Jomon period when the population in the Japanese Islands declined. The cold-dry climate is suggested as one of the possible causes for the population decline, and may have spread across a wide area of the Japanese islands. Due to global warming, the future climate along the Pacific coastline is expected to resemble the conditions of the warmest interval in the early-middle Holocene. Considering the high precipitation during this interval, future warming will be accompanied by an increase in precipitation. (273 letters).</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-07-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70025","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144716563","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 Paleoenvironment Changes Near Lake Funazoko-Ike, East Antarctica, Based on Analyses of Sediments and Microfossils","authors":"Satoshi Sasaki,&nbsp;Koji Seto,&nbsp;Kota Katsuki","doi":"10.1111/iar.70026","DOIUrl":"https://doi.org/10.1111/iar.70026","url":null,"abstract":"<p>Lake Funazoko-Ike was formed by a relative sea level fall due to crustal uplift after the melting of the ice sheet. Today, the lake surface lies approximately 25 m below the present sea level. Evidence of these processes is recorded in sediments deposited from the sill to the lake surface around Lake Funazoko-Ike during the middle to late Holocene. A total of 123 sediment samples were collected from outcrops of the Holocene-raised beach around Lake Funazoko-Ike in Skarvsnes, Antarctica, during geomorphological surveys by the 38th and 46th Japanese Antarctic Research Expeditions (JARE38 and JARE46). The samples were analyzed for grain size, carbon, nitrogen, and sulfur (CNS) contents, as well as microfossil assemblages, including arenaceous foraminifera and siliceous microfossils. Radiocarbon dating of marine fossils yielded ages ranging from approximately 4100 to 800 cal. yr. BP. From six samples, we identified at least 54 species belonging to 29 genera of siliceous microfossils. Arenaceous foraminifera were identified in one additional sample. Three sea-ice indicator diatom taxa were dominant in all six samples. These results suggest that Lake Funazoko-Ike was in an open marine environment from approximately 4100 to 1200 cal. yr. BP. The lake became isolated from the sea sometime between 800 and 1200 cal. yr. BP. This timing aligns with the isolation of the coastal margin in northwest Skarvsnes, providing evidence to understand spatial and temporal variations in ice sheet dynamics during the Holocene.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-07-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70026","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144716564","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":"Ages and Geochemistry of Igneous Rocks in the Northern Sierra Madre-Caraballo Mountains, Philippines: Implications for Subduction Reconstruction Beneath Northern Luzon","authors":"Yikai Yang,&nbsp;Mengming Yu,&nbsp;Graciano P. Yumul Jr.,&nbsp;Karlo L. Queaño,&nbsp;Carla B. Dimalanta,&nbsp;Peijun Qiao,&nbsp;Lei Shao","doi":"10.1111/iar.70028","DOIUrl":"https://doi.org/10.1111/iar.70028","url":null,"abstract":"<div>\u0000 \u0000 <p>Subduction tectonics related to Late Paleogene island-arc magmatism in Northern Luzon, the Philippines, has been debated for some time. New field data, combined with zircon U–Pb dating and Hf isotopic studies, whole-rock element and Nd isotopic data from the Caraballo Formation in the Northern Sierra Madre reveal two suites of island-arc magmatic rocks: (1) ~34 Ma igneous rocks characterized by low-K₂O (≤ 0.6 wt.%), low Nb/Yb (0.11–0.26), and high ¹⁴³Nd/¹⁴⁴Nd (εNd_[t] = 9.5–9.9), and (2) ~31 Ma igneous rocks exhibiting progressive enrichments of K₂O (~0.8 wt.%), Nb/Yb (0.37–0.76), and ¹⁴³Nd/¹⁴⁴Nd (εNd_[t] = 8.2–9.1). These differences reflect a shift in subduction influences, transitioning from slab fluids of an old subducted slab to slab melts with adakitic characteristics of young subducted slab between 34 and 31 Ma. The ~46 Ma Dupax Batholith in the Caraballo Mountains shows similar geochemical and isotopic signatures as the ~34 Ma igneous rocks of the Caraballo Formation (K₂O: ~0.7 wt.% vs. ≤ 0.6 wt.%; Nb/Yb: 0.24 vs. 0.11–0.26; zircon εHf_[t]: 13.8–15.8 vs. 13.7–15.7). This suggests a similar mantle source and subduction influx in both cases. When considered in conjunction with available geochronological and geochemical data from the Northern Sierra Madre, the Caraballo Mountains, and the Central Cordillera, this suggests that arc growth of Northern Luzon may have started as early as the Middle Eocene. The eastern and western ranges of Northern Luzon comprised the Proto-Luzon Arc before the intra-arc rifting started in the Late Oligocene–Early Miocene. Unlike a Neogene cessation of island-arc magmatism in the Northern Sierra Madre, there was a persistent arc growth in the Central Cordillera. It suggests that the Central Cordillera always situated in the frontal arc while the Northern Sierra Madre evolved from a Late Paleogene back arc to a Neogene remnant arc. This supports a continuously east-dipping subduction along the western plate boundary of Northern Luzon. We conclude that the change of subduction influence in the Northern Luzon Arc during the Middle Eocene to Early Oligocene is linked to the migration of the subducted slab from the Mesozoic Eastern Neo-Tethys to the Eocene Celebes at ~31 Ma.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2025-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144705491","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":"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}