Island Arc最新文献

{"title":"Petrogenesis and Tectonic Significance of the Paleoproterozoic Ramgarh Mylonitic Granitoids in the Baijnath Klippe, Kumaun Himalaya: Evidence for Active Continental Margin During Columbia Supercontinent Assembly","authors":"Shubham Patel,&nbsp;Mallickarjun Joshi,&nbsp;Alok Kumar,&nbsp;Biraja P. Das,&nbsp;Aman Singh,&nbsp;Aditya Singh","doi":"10.1111/iar.70047","DOIUrl":"https://doi.org/10.1111/iar.70047","url":null,"abstract":"<div>\u0000 \u0000 <p>Paleoproterozoic granitoids are important rock types all along the Lesser Himalayan belt and provide key constraints for understanding the evolution of the northern Indian continental margin and its position within the Columbia supercontinent assembly. The Paleoproterozoic Ramgarh mylonitic granitoids of the Baijnath klippe in the Inner Kumaun Lesser Himalayan Sequence show pervasive mylonitization, with augen-shaped porphyroclasts of feldspar and quartz, which characterize the Ramgarh mylonitic granitoids throughout the Lesser Himalaya. We present petrological and whole-rock geochemical data from the Ramgarh mylonitic granitoids of the Baijnath klippe to evaluate their petrogenesis and geodynamic implications. These monzonitic granitoids are geochemically peraluminous with high K₂O and MgO along with enrichment in LILE (Rb, K, Pb, and Th) and depletion in Ba, P, Nb, and Ti, along with moderately negative Eu anomalies. Geochemically, the Ramgarh granitoids are classified as I-type (Igneous arc-type) granite. Zircon U–Pb geochronology from the Kausani body yields a crystallization age of ~1866 Ma. The granitoids are interpreted to have formed by a high degree of partial melting of underplated lower-crustal material during the early phase of slab rollback or break-off of a subducting slab. This magmatic event likely preceded the emplacement of the post-collisional extensional magmatism, suggesting an evolving subduction–accretion system. The geochemical and geochronological data support an active continental margin setting along the northern Indian plate margin at ~1.8 Ga. This suggests that these oldest granitoids represent a magmatic arc developed in a subduction zone setting during the assembly of the Columbia supercontinent. Later, during the Cenozoic Himalayan orogeny, the granitoids were thrust over the Lesser Himalayan Sequence as nappes and klippen.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-04-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147715072","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":"Determination of Stresses and the Appropriate Number of Stresses From Heterogeneous Calcite Twin Data: A Novel Inverse Method","authors":"Atsushi Yamaji,&nbsp;Kei Wakamori","doi":"10.1111/iar.70045","DOIUrl":"https://doi.org/10.1111/iar.70045","url":null,"abstract":"<div>\u0000 \u0000 <p>The orientations of mechanical twins in calcite have been used in various areas in the world for paleopiezometry. This paper presents a new inversion scheme which determines deviatoric stress tensors normalized by the critical resolved shear stress of twinning (CRSS). The unique advantage of the technique is the ability not only to separate stresses from heterogeneous data but also to determine how many stresses should be determined from them. For this purpose, the present scheme capture clusters of data points in a five-dimensional deviatoric stress–strain space. The accuracy and resolution of the present technique were studied using synthetic data sets. Stress axes were determined with error of a few degrees. Differential stresses normalized by CRSS, <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>D</mi>\u0000 </mrow>\u0000 <annotation>$$ D $$</annotation>\u0000 </semantics></math>, were found to be inaccurate if the twin lamellae in question were formed under stress conditions with <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>D</mi>\u0000 </mrow>\u0000 <annotation>$$ D $$</annotation>\u0000 </semantics></math> greater than <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mo>∼</mo>\u0000 </mrow>\u0000 <annotation>$$ sim $$</annotation>\u0000 </semantics></math> 30. Even in such cases, principal orientations and stress ratios were accurately determined. Separation of stresses is equivalent with classification of data. The present method separated successfully even when two classes of data shared 60%–70% of data. The method was applied to a data set from a calcite vein in an Eocene limestone unit in the southern Ryukyu arc, Japan, and clearly separated two stresses. With examples from both synthetic and natural data sets, it is demonstrated that the plausibility of separated stresses can be estimated by means of the proxies of data and nondimensional deviatoric stress tensors defined in a parameter space.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147715015","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 Silicic Magmas at Kibblewhite Volcano: Implications for Bimodal Volcanism in the Kermadec Arc","authors":"Yasuhiro Hirai,&nbsp;Yoshihiko Tamura","doi":"10.1111/iar.70043","DOIUrl":"https://doi.org/10.1111/iar.70043","url":null,"abstract":"<p>This study investigates the origin of silicic magmas at the Kibblewhite Volcano in the Kermadec arc. Two alternative differentiation pathways are evaluated: fractional crystallization of primary basaltic magmas producing low-Mg andesites, and fractional crystallization of primary andesitic magmas represented by magnesian andesites. Major- and trace-element geochemistry, rare earth element (REE) systematics, geothermobarometric constraints, and thermodynamic modeling demonstrate that the low-Mg andesites represent already evolved melts and cannot generate the observed dacitic to rhyolitic magmas through further crystallization. In contrast, magnesian andesites show close agreement with the silicic magmas in both major-element and REE compositions, indicating that they provide a viable parental magma for the silicic magmas at Kibblewhite Volcano. These results indicate that two distinct differentiation pathways coexist beneath the Kibblewhite Volcano, in which basaltic and andesitic primary magmas evolve independently to produce low-Mg andesites and dacitic to rhyolitic magmas, respectively. When considered in a broader context, parallel differentiation of basaltic and andesitic primary magmas may also contribute to the bimodal SiO₂ distributions characteristic of the Kermadec Arc; however, this arc-scale application should be regarded as a working hypothesis requiring further evaluation at other volcanic centers.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70043","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147714914","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":"Geochronology of Cretaceous to Paleogene Igneous Rocks Along a North–South Traverse in the Western Kinki Region, Southwest Japan","authors":"Daisuke Sato","doi":"10.1111/iar.70044","DOIUrl":"https://doi.org/10.1111/iar.70044","url":null,"abstract":"<p>New zircon U–Pb ages are presented for Cretaceous to Paleogene volcanic and granitic rocks of Awaji Island in the Ryoke Belt, Arima area in the San'yo Belt, and Izushi–Kasumi areas in the San'in Belt. These areas are located on a transect across southwest Japan from the Median Tectonic Line to the Sea of Japan. So this is the first report of the zircon U–Pb ages of the volcano-plutonic complexes aligned across the Japan arc. The obtained U–Pb ages of the Sennan Rhyolite, foliated granite, and massive granite, all in the Ryoke Belt, are 94.0, 89.3–84.1, and 84.3 Ma, respectively. Those of the volcanic rocks of the Arima Group, Rokko Granite, and Nunobiki Granodiorite in the San'yo Belt are 79.9–74.1, 79.6–77.6, and 76.5 Ma, respectively. The volcanic rocks of the Yadagawa Group and Yadagawa Granite of the Izushi–Kasumi areas in the San'in Belt gave the ages of 68.2–64.9 and 64.0 Ma, respectively. The development of foliation and deformation of the foliated granite and the subsequent intrusion of massive granite on Awaji Island occurred at ca. 84 Ma. The volcanic rocks in the Arima and Yadagawa groups have been correlated to surrounding volcanic rocks based on lithological similarities, but the new U–Pb ages reveal that they each represent separate volcanic events. The durations of volcanic and plutonic activity, based on the zircon U–Pb ages, are 94–83 Ma for Awaji Island, 82–74 Ma for the Arima area, and 68–62 Ma for the Izushi–Kasumi areas, indicating a younging in the magmatic activity from south to north. The volcanic activity preceded the plutonic activity in all of the study areas.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-03-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70044","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147579803","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":"Non-Marine Diatom Biostratigraphy Based on Evolutionary Changes of the Genus Praestephanos in the Pliocene Lacustrine Deposits of the Kameyama Formation, Tokai Group, and Its Correlation With the Kobiwako Group, Central Japan","authors":"Takahiro Kojima,&nbsp;Megumi Saito-Kato,&nbsp;Keiji Hattori,&nbsp;Taisuke Ohtsuka,&nbsp;Yasufumi Satoguchi","doi":"10.1111/iar.70042","DOIUrl":"https://doi.org/10.1111/iar.70042","url":null,"abstract":"<div>\u0000 \u0000 <p>To gain new insights into non-marine diatom biostratigraphy, we provided taxonomic descriptions and examined the morphological evolution of the freshwater diatom genus <i>Praestephanos</i> from the Pliocene lacustrine deposits of the Kameyama Formation, Tokai Group in Mie Prefecture, Japan, which were deposited in Paleo-lake Anoutsu-ko. We also compared these results with the evolutionary record of <i>Praestephanos</i> from the contemporaneous lacustrine deposits of the Ueno Formation, Kobiwako Group, reported in a previous study, to establish a biostratigraphic correlation between the two formations; the deposits of the Ueno Formation accumulated in Paleo-lake Oyamada, which is thought to have been hydrologically connected to Paleo-lake Anoutsu-ko. In the Kameyama Formation, <i>P. tokaiensis</i> and <i>P. miensis</i>, originally described from the Ueno Formation, occur in intervals of approximately 3.9–3.8 Ma and 3.8–3.6 Ma, respectively. Around 3.8 Ma, during the transition between the two species, changes in the number of valve face fultoportulae were observed, suggesting a morphological shift from the ancestral <i>P. tokaiensis</i> to the descendant <i>P. miensis</i>. This stratigraphic distribution and morphological evolution were consistent with patterns observed in the Ueno Formation, indicating that the same evolutionary changes in <i>Praestephanos</i> occurred isochronously in both lakes. Therefore, the species turnover horizon can be regarded as a biostratigraphic datum for both the Kameyama and Ueno Formations. Notably, the two species were likely endemic to this paleo-water system. Although the biogeographic provinciality of freshwater diatoms can often be a limitation in biostratigraphic research, the results of this study demonstrate that biostratigraphic correlation based on evolutionary changes is possible between formations in regions belonging to the same paleo-water system, even for freshwater diatoms with strong provinciality. Furthermore, the finding that the same diatom species exhibited isochronous morphological evolution with an identical pattern in both regions within the same paleo-water system suggests that comparative analyses of diatom evolutionary changes and stratigraphic distributions between areas can potentially be developed into indicators for reconstructing paleo-water systems.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147566781","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":"A Study on the Pore Evolution Characteristics and Main Control Factors of the Gulong Shale Reservoirs","authors":"Junhui Li,&nbsp;Yangxin Su,&nbsp;Xiuli Fu,&nbsp;Daming Niu,&nbsp;Pingchang Sun,&nbsp;Shi Fang,&nbsp;Kunning Cui,&nbsp;Yue Bai,&nbsp;QingXia Xu","doi":"10.1111/iar.70041","DOIUrl":"https://doi.org/10.1111/iar.70041","url":null,"abstract":"<div>\u0000 \u0000 <p>Gulong shale oil in the first member of the Qingshankou Formation, Songliao Basin, has considerable exploration potential, yet the evolution of its reservoir remains poorly constrained. Clarifying this evolution is essential for evaluating reservoir properties and identifying prospective intervals. In this study, Gulong shale core samples from the first member of the Qingshankou Formation were collected from multiple wells across a maturity range from immature to highly mature. Pore evolution was investigated using N₂ adsorption, high-pressure mercury intrusion (MICP), and argon ion beam milling scanning electron microscopy. Results reveal that porosity evolves non-monotonically, decreasing from the immature to mature stages, increasing at the highly mature stage, and then slightly declining at the highest maturity. Concomitant with this evolution is a shift in the main control factors, with inorganic pores governing reservoir characteristics at immature–mature stages and organic pores becoming the primary control at high maturity. Overall, thermal maturation, diagenesis, and hydrocarbon generation and expulsion processes jointly govern reservoir quality, suggesting that, under favorable preservation and pressure conditions, highly mature shale may hold exploration potential.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147653357","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":"Geochemistry and U–Pb Geochronology of Volcaniclastic Rocks From the NW Lesser Himalayan Sequence: Implications for Arc-Related Tectonic Setting at Northern Proto-Indian Continental Margin During the Neoproterozoic","authors":"Subhojit Saha,&nbsp;Kaushik Das,&nbsp;Shivani S. Choudhari,&nbsp;Abhishek Kumar Pandey,&nbsp;Shruti Ranjan Mishra,&nbsp;Sumit Kumar Ghosh,&nbsp;Alok Deep,&nbsp;Partha Pratim Chakraborty","doi":"10.1111/iar.70040","DOIUrl":"https://doi.org/10.1111/iar.70040","url":null,"abstract":"<div>\u0000 \u0000 <p>The protracted tectonic history concerning the northern edge of the pre-Himalayan proto-Indian plate margin during the Neoproterozoic time has been a long-standing subject of debate, with various hypotheses enduring over several decades. Although a continental rift to passive margin setting has been proposed for the deposition of Neoproterozoic rocks of the Outer Lesser Himalayan Sequence (OLHS), the similarity in detrital zircon age peaks and Nd isotope values of OLHS rocks with those of the Higher Himalayan Crystalline Sequence (HHCS) led to the proposition of an arc-related setting. So far, there is no such report of Neoproterozoic arc-related magmatism from the Lesser Himalayan Sequence. In the present study, an interbedded volcaniclastic horizon has been identified from the basal stratigraphic level of the Nagthat Formation, OLHS. Geochemistry and U–Pb zircon geochronology are used to understand the provenance and tectonic setting of the Nagthat volcaniclastic rock. The angularity of the framework minerals, moderate degree of weathering (avg. Chemical Index of Alteration value 62), and immature nature (Index of Compositional Variability 1–1.1) suggest that a contemporaneous volcanic province was probably the main source. Whole rock geochemical analysis shows high Th/Sc (1.67–3.40, avg. 2.71), La/Sc (4.74–8.21, avg. 7.14), and low Cr/Th ratios (1.78–5.45, avg. 3.02). It supports that the volcaniclastic was supplied from an acid to intermediate source rocks, such as rhyolite and dacite/andesite. U–Pb detrital zircon analysis yielded a major age peak around 775 Ma. However, comparison of the U–Pb zircon age data with published datasets from the Lower (Chandpur Formation) and upper (Nagthat Formation) stratigraphic horizons suggests a timing of volcanism between 800 and 700 Ma. Geochronological and geochemical characteristics of the volcaniclastic rock suggest a signature transitional from passive to active continental margin setting, indicating that at least subduction was ongoing during the Tonian (800–700 Ma) at the northern proto-Indian plate margin.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146256270","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":"Unraveling the Tectonic Evolution of the Eastern Jiangnan Orogen, South China: Constraints From Basalts of Trace Elements, Nd Isotope, and Geochronology","authors":"Jiandong Sun,&nbsp;Xuehui Zhang,&nbsp;Jinsong Lv,&nbsp;Haili Li,&nbsp;Shiru Lin","doi":"10.1111/iar.70039","DOIUrl":"https://doi.org/10.1111/iar.70039","url":null,"abstract":"<div>\u0000 \u0000 <p>The Jiangnan Orogen has long been recognized as an ideal location for Precambrian geologists to study Neoproterozoic tectonic movements. However, the paleogeographic location of the South China Craton within the Rodinia supercontinent and the associated geodynamic mechanisms remain contentious. This research presents results from Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) zircon U–Pb dating, whole-rock geochemistry, and Nd isotopes of early Neoproterozoic Jinshan basalts from the Zhangcun Formation. The basalts, dated at approximately 844 Ma, display a tholeiitic affinity, characterized by high MgO (8.19–13.70 wt.%), moderate TiO₂ (0.89–1.40 wt.%), and positive <i>ε</i>_Nd(<i>t</i>) values (+ 1.35 to +3.53). Normalized rare earth element (REE) patterns and primitive mantle-normalized trace element distributions indicate a slight enrichment of light rare earth elements (LREE) and a significant enrichment of large ionic lithosphere elements (LILEs), comparable to E2 basalts from the East Scotia Ridge back-arc basin. It is suggested that the Jinshan basalts originated from a slightly depleted asthenospheric mantle, combined with recycled slab melt. When integrated with existing regional data, these basalts from the eastern Jiangnan Orogen exhibit notable geochemical anomalies, including elevated LREE and incompatible element ratios, as well as lower <i>ε</i>_Nd(<i>t</i>) values, in contrast to back-arc basin basalts from the central Jiangnan Orogen. This study proposes that the observed geochemical disparities between the eastern and central Jiangnan Orogen are attributed to varying degrees of slab rollback within the Yangtze Block during the early Neoproterozoic. Furthermore, evidence from prolonged arc magmatism and extensional tectonics related to slab rollback supports the conclusion that the South China Craton is positioned at the periphery of the Rodinia supercontinent.</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-01-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146096457","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":"Reexamination of the Neogene Paleostress Field Using Fault-Slip Data From Jogashima, Miura Peninsula, Central Japan","authors":"Toru Sakai,&nbsp;Hideo Takagi","doi":"10.1111/iar.70038","DOIUrl":"https://doi.org/10.1111/iar.70038","url":null,"abstract":"<div>\u0000 \u0000 <p>The Neogene Misaki Formation, which occurs on the southwestern part of Jogashima Island at the southern tip of Miura Peninsula, Japan, is cut by numerous minor faults. Using fault-slip data, we reconstructed the paleostress field by stress inversion analysis. Two significant stresses, A and B, were detected. Stress A is an E–W tensional (normal-faulting) stress that is close to axial compression, and Stress B is an E–W compressional (reverse-faulting) stress that is close to axial extension. These stresses occurred after the formation of large-scale geological structures, such as the Kenzaki Anticline and Jogashima Syncline. In addition, the dip-slip faults are cut by a large dextral fault that is subvertical and strikes WNW–ESE. Therefore, this dextral fault has been active in a stress field similar to that of the present day (i.e., <i>σ</i>₁-axis oriented NNW–SSE and <i>σ</i>₃-axis oriented ENE–WSW). (143 words).</p>\u0000 </div>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"35 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2026-01-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146007808","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":"Timing of Mylonitization Based on Zircon U–Pb Ages of the Ryoke Plutonic Rocks in the Akaiwadani and Taraki Areas, Eastern Kii Peninsula, Southwest Japan: Implications for Uplifting of a Late Cretaceous Volcanic Arc on the Asian Continental Margin","authors":"Makoto Takeuchi,&nbsp;Sakurako Yabuta,&nbsp;Qi Li,&nbsp;Yoshihiro Asahara","doi":"10.1111/iar.70036","DOIUrl":"https://doi.org/10.1111/iar.70036","url":null,"abstract":"<p>The timing of mylonitization of the Ryoke Plutonic Rocks in the Tsukide area, eastern Kii Peninsula, Southwest Japan, which is part of a Late Cretaceous volcanic arc on the Asian continental margin, was investigated based on geological relationships and zircon U–Pb dating. The mylonite zone is located north of the Median Tectonic Line (MTL) and has accommodated a large displacement between the low-<i>P</i>/<i>T</i>-type Ryoke Metamorphic Complex, including the Ryoke Plutonic Rocks, and the high-<i>P</i>/<i>T</i>-type Sanbagawa Metamorphic Rocks. The mylonitization in this area was previously dated to 74–67 Ma, as determined from the K–Ar cooling ages of hornblende, biotite, and alkali feldspar. However, these ages might represent the most recent cooling ages that reflect thermal resetting by later intrusions. In this study, the timing of mylonitization was constrained by zircon U–Pb ages of a deformed granitoid and undeformed granitoid that intrudes it. Three stages of mylonitization, M1–M3, were recognized in this area. M1 is high-<i>T</i>-type mylonitization that occurred between 87.5 ± 1.7 and 83.2 ± 1.2 Ma. M2 and M3 are medium- to low-<i>T</i>-type mylonitization that occurred at ca. 77.5 Ma and after ca. 70 Ma, respectively. Top-to-the-west sense shearing occurred after M2 along shear zones that converge obliquely with the present-day MTL. The shear zone became narrower than that of M1, due to uplift and cooling, and eventually transitioned into discrete faults. M1 coincided with the change in the subducting plate beneath proto-Japan from the Izanagi Plate to the Kula Plate at 85 Ma, and M2 occurred during subduction of the Kula Plate.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":"34 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2025-11-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.70036","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145625942","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}