A. M. M. G. I. U. B. Athauda, P. L. Dharmapriya, S. P. K. Malaviarachchi, K. Sajeev
The investigation of whole-rock major and trace element geochemical data from metapelites, incorporating analyses from both previous studies and new localities within the Highland Complex (HC) of Sri Lanka and the Trivandrum Block of India (TB), aimed to discern the nature and tectonic setting of their provenance. Examination of chondritic REE distribution and K versus K/Rb diagrams suggests that the geochemistry of the studied metapelites closely resembles typical Post Archaean Australian Shale (PAAS), North American Shale Composite (NASC), and Upper Continental Crust (UCC), indicating minimal modification during high-grade metamorphism. Predominantly, the protoliths of the metasediments appear to be shales and greywackes derived from Proterozoic felsic to intermediate sources. Tectonic discrimination diagrams reveal that most metapelites correspond to active continental margins and continental island arcs. These geochemical characteristics suggest that the majority of studied metapelites in the HC and TB originate from felsic to intermediate sources, likely deposited within a continental arc setting. Subsequently, these sediments likely accreted in an accretionary prism and underwent metamorphism during continental-continental collision. The congruence in geochemical signatures between metapelites in the HC and TB, along with established tectonic, geochronological, petrological, mineralogical, and geophysical correlations, implies that precursor sediments of metasedimentary rocks were likely deposited within a laterally extensive Neoproterozoic sedimentary basin.
对来自元古宙的全岩主要元素和痕量元素地球化学数据进行了调查,其中包括对斯里兰卡高地复合体(HC)和印度特里凡得琅区块(TB)内的先前研究和新地点的分析,目的是查明其来源的性质和构造环境。对软玉 REE 分布和 K 与 K/Rb 图的研究表明,所研究的元粒岩的地球化学与典型的后太古宙澳大利亚页岩(PAAS)、北美页岩复合体(NASC)和上大陆壳(UCC)非常相似,表明在高级变质过程中发生的改变微乎其微。这些变质岩的原岩主要是页岩和灰岩,来源于新生代的长岩到中岩。构造判别图显示,大多数变质岩与活跃的大陆边缘和大陆岛弧相对应。这些地球化学特征表明,所研究的HC和TB地区的大部分元古代沉积岩源自长英岩至中生代,很可能沉积在大陆弧环境中。随后,这些沉积物很可能在增生棱岩中增生,并在大陆-大陆碰撞过程中发生变质作用。HC和TB中的元古代沉积岩在地球化学特征上的一致性,以及已确立的构造、地质年代、岩石学、矿物学和地球物理相关性,意味着元古代沉积岩的前驱沉积物很可能沉积在一个横向扩展的新近纪沉积盆地中。
{"title":"Geochemical signatures of metapelites in the Highland Complex, Sri Lanka and Trivandrum Block, India: Implications for provenance, nature and tectonic setting of their source protoliths","authors":"A. M. M. G. I. U. B. Athauda, P. L. Dharmapriya, S. P. K. Malaviarachchi, K. Sajeev","doi":"10.1111/iar.12529","DOIUrl":"10.1111/iar.12529","url":null,"abstract":"<p>The investigation of whole-rock major and trace element geochemical data from metapelites, incorporating analyses from both previous studies and new localities within the Highland Complex (HC) of Sri Lanka and the Trivandrum Block of India (TB), aimed to discern the nature and tectonic setting of their provenance. Examination of chondritic REE distribution and K versus K/Rb diagrams suggests that the geochemistry of the studied metapelites closely resembles typical Post Archaean Australian Shale (PAAS), North American Shale Composite (NASC), and Upper Continental Crust (UCC), indicating minimal modification during high-grade metamorphism. Predominantly, the protoliths of the metasediments appear to be shales and greywackes derived from Proterozoic felsic to intermediate sources. Tectonic discrimination diagrams reveal that most metapelites correspond to active continental margins and continental island arcs. These geochemical characteristics suggest that the majority of studied metapelites in the HC and TB originate from felsic to intermediate sources, likely deposited within a continental arc setting. Subsequently, these sediments likely accreted in an accretionary prism and underwent metamorphism during continental-continental collision. The congruence in geochemical signatures between metapelites in the HC and TB, along with established tectonic, geochronological, petrological, mineralogical, and geophysical correlations, implies that precursor sediments of metasedimentary rocks were likely deposited within a laterally extensive Neoproterozoic sedimentary basin.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.0,"publicationDate":"2024-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141882510","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}
The Middle Miocene Climatic Optimum (MMCO) formed shallow marine strata on continents and island arcs at ~15 Ma, and some of the MMCO strata later experienced uplift. The amount of uplift corresponded to sea level rise after the MMCO period, but ancient sea level could not be observed. This study uses a deductive approach to estimate the amount of uplift. The elevation of the youngest marine sedimentary layer of the MMCO strata approximates the sea level at the MMCO. The current elevation was reconstructed by adding the amount of denudation from the MMCO to the current elevation of the denuded MMCO strata at the outcrops, considering isostatic compensation. The amount of denudation since the MMCO period was estimated to be 180 m based on the global average denudation rate (Willenbring et al., 2013), and the estimated isostatic rebound was 153 m. By comparing the current, albeit virtual, elevation of the youngest marine sedimentary layer in the MMCO strata with sea level at the MMCO, the amount of uplift was estimated. For the MMCO sea level, a 66 m ice-free line was adopted. This method was applied to the Kibi Plateau in Southwest Japan, where the MMCO strata are distributed. The reliability of the denudation amount of 180 m for the MMCO strata was confirmed by paleo-water depths estimated using benthic foraminifera and mollusk fossils. The largest uplift of 504 ± 27 m occurred at the Jinseki-Kogen since 15 Ma. A positive correlation was observed between the amount of uplift and the current altitude of the Kibi Plateau. The Kibi Plateau's topography most likely evolved after the MMCO. It is concluded that a new perspective on estimating the uplift was obtained by taking denudation into account.
{"title":"Estimating the amount of uplift from the current elevation of strata at the Middle Miocene Climatic Optimum: A case study of Kibi Plateau in Southwest Japan","authors":"Yo-ichiro Otofuji","doi":"10.1111/iar.12531","DOIUrl":"10.1111/iar.12531","url":null,"abstract":"<p>The Middle Miocene Climatic Optimum (MMCO) formed shallow marine strata on continents and island arcs at ~15 Ma, and some of the MMCO strata later experienced uplift. The amount of uplift corresponded to sea level rise after the MMCO period, but ancient sea level could not be observed. This study uses a deductive approach to estimate the amount of uplift. The elevation of the youngest marine sedimentary layer of the MMCO strata approximates the sea level at the MMCO. The current elevation was reconstructed by adding the amount of denudation from the MMCO to the current elevation of the denuded MMCO strata at the outcrops, considering isostatic compensation. The amount of denudation since the MMCO period was estimated to be 180 m based on the global average denudation rate (Willenbring et al., 2013), and the estimated isostatic rebound was 153 m. By comparing the current, albeit virtual, elevation of the youngest marine sedimentary layer in the MMCO strata with sea level at the MMCO, the amount of uplift was estimated. For the MMCO sea level, a 66 m ice-free line was adopted. This method was applied to the Kibi Plateau in Southwest Japan, where the MMCO strata are distributed. The reliability of the denudation amount of 180 m for the MMCO strata was confirmed by paleo-water depths estimated using benthic foraminifera and mollusk fossils. The largest uplift of 504 ± 27 m occurred at the Jinseki-Kogen since 15 Ma. A positive correlation was observed between the amount of uplift and the current altitude of the Kibi Plateau. The Kibi Plateau's topography most likely evolved after the MMCO. It is concluded that a new perspective on estimating the uplift was obtained by taking denudation into account.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.0,"publicationDate":"2024-07-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141777071","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}
Long Leng, Zhongjie Xu, Kunning Cui, Shiyao Gao, Ningchen Sun
During the Mesozoic, the tectonic evolution of the southeastern margin of the South China Block was mainly influenced by the subduction of the Palaeo-Pacific Plate. The sedimentary basins along the southeastern margin of the South China Block have preserved the sedimentary record of this process. In this paper, the sedimentary rocks of the Lishan Formation and Zhangping Formation in the Zhangping area of the southeastern margin of the South China Block are studied in terms of sedimentology, petrography, lithogeochemistry, U–Pb chronology of detrital zircon, and zircon trace elements. The study shows that the detrital zircon U–Pb age of the Lishan Formation is 2965–202 Ma, with two major age groups: 253–220 Ma and 1925–1799 Ma; the detrital zircon U–Pb age of the Zhangping Formation is 2587–190 Ma, with two major age groups: 246–234 Ma and 1929–1861 Ma; the ancient sediments in the samples are affected by recycle, and the Early Jurassic source area in the study area is an acidic magma zone, while the Middle Jurassic source area is located in a moderately acidic magma zone and recycle body; the Early Jurassic Lishan Formation and Middle Jurassic Zhangping Formation are mainly derived from the Wuyi Massif. In the upper section of the Lishan Formation, the secondary age groups of ca.448 and ca.773 Ma occur, and the detrital zircons in this section are mainly from the Jiangnan orogenic belt. The ongoing subduction of the Paleo-Pacific Plate led to the formation of diverse back-arc basins within the region; during the Early Jurassic, it was a back-arc compressional basin, transitioning into a back-arc extensional basin during the Middle Jurassic.
中生代时期,华南地块东南缘的构造演化主要受古太平洋板块俯冲作用的影响。华南地块东南缘的沉积盆地保存了这一过程的沉积记录。本文从沉积学、岩相学、岩石地球化学、碎屑锆石U-Pb年代学和锆石微量元素等方面对华南地块东南缘漳平地区的黎山层和漳平层沉积岩进行了研究。研究表明,里山组的锆英石U-Pb年代为2965-202Ma,分为253-220Ma和1925-1799Ma两大年龄组;漳平组的锆英石U-Pb年代为2587-190Ma,分为246-234Ma和1929-1799Ma两大年龄组:样品中的古沉积物受循环影响,研究区早侏罗世源区为酸性岩浆带,中侏罗世源区位于中酸性岩浆带和循环体中;早侏罗世里山组和中侏罗世漳平组主要来源于武夷山地块。在里山地层上段,出现了约 448 Ma 和约 773 Ma 的第二年龄组,该段的碎屑锆石主要来自江南造山带。古太平洋板块的持续俯冲导致该地区形成了多种多样的弧后盆地;早侏罗世为弧后压缩盆地,中侏罗世过渡为弧后伸展盆地。
{"title":"Subduction of the Paleo Pacific Plate at the southeast edge of the South China Block during the Early to Middle Jurassic: Sedimentary records from Fujian Province","authors":"Long Leng, Zhongjie Xu, Kunning Cui, Shiyao Gao, Ningchen Sun","doi":"10.1111/iar.12530","DOIUrl":"https://doi.org/10.1111/iar.12530","url":null,"abstract":"<p>During the Mesozoic, the tectonic evolution of the southeastern margin of the South China Block was mainly influenced by the subduction of the Palaeo-Pacific Plate. The sedimentary basins along the southeastern margin of the South China Block have preserved the sedimentary record of this process. In this paper, the sedimentary rocks of the Lishan Formation and Zhangping Formation in the Zhangping area of the southeastern margin of the South China Block are studied in terms of sedimentology, petrography, lithogeochemistry, U–Pb chronology of detrital zircon, and zircon trace elements. The study shows that the detrital zircon U–Pb age of the Lishan Formation is 2965–202 Ma, with two major age groups: 253–220 Ma and 1925–1799 Ma; the detrital zircon U–Pb age of the Zhangping Formation is 2587–190 Ma, with two major age groups: 246–234 Ma and 1929–1861 Ma; the ancient sediments in the samples are affected by recycle, and the Early Jurassic source area in the study area is an acidic magma zone, while the Middle Jurassic source area is located in a moderately acidic magma zone and recycle body; the Early Jurassic Lishan Formation and Middle Jurassic Zhangping Formation are mainly derived from the Wuyi Massif. In the upper section of the Lishan Formation, the secondary age groups of ca.448 and ca.773 Ma occur, and the detrital zircons in this section are mainly from the Jiangnan orogenic belt. The ongoing subduction of the Paleo-Pacific Plate led to the formation of diverse back-arc basins within the region; during the Early Jurassic, it was a back-arc compressional basin, transitioning into a back-arc extensional basin during the Middle Jurassic.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.0,"publicationDate":"2024-07-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141584005","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}
Serpentinite mélanges occur as thin horizons up to 350 m thick within coherent schists of the Nishisonogi unit of the Nagasaki Metamorphic Complex located in western Kyushu, which represent serpentinite-hosted exhumation channels within a Cretaceous subduction complex. This study gives the petrography of coherent schists and various tectonic block types embedded in serpentinite or chlorite-actinolite schist of the mélange. The P–T conditions recorded in these rocks were compared, to clarify a possible exhumation process and style of the Nishisonogi unit. In the mélange, two types of tectonic blocks were recognized based on peak temperature conditions: a lower temperature type (400–590°C) and a higher temperature type (780–830°C), the latter of which shows temperatures higher than those of the coherent schists (370–450°C). The garnet glaucophanite in the coherent schists shows a prograde pressure increment from 0.9 to 2.3 GPa. The serpentinite mélange comprises tectonic blocks with peak temperatures higher than those of the coherent schists, indicating that blocks from the deep structural levels of the lower-plate underwent tectonic mixing into the mélange during exhumation. Ultrahigh-pressure (UHP) conditions have not been recorded either in the coherent schists or in the serpentinite mélanges, although microdiamonds have been reported from the Yukinoura mélange located at the western margin of the Nishisonogi unit (Nishiyama et al., Scientific Reports, 2020, 10, 11645). A possible correlation between the Nishisonogi unit and other HP metamorphic rocks or belts (Renge rocks, Suo Belt and Sanbagawa Belt) was examined to conclude that the Nishisonogi unit is a unique paleo-subduction complex in terms of the coexistence of HP subduction complex with intercalated serpentinite mélanges and a UHP mélange derived from the slab-mantle interface.
{"title":"Significance of serpentinite-hosted exhumation channels in a palaeo-subduction complex, Nishisonogi unit, Nagasaki Metamorphic Complex: P–T trajectories of mélange blocks and coherent schists","authors":"Tadao Nishiyama, Yosuke Moribe, Yasushi Mori, Miki Shigeno","doi":"10.1111/iar.12527","DOIUrl":"https://doi.org/10.1111/iar.12527","url":null,"abstract":"<p>Serpentinite mélanges occur as thin horizons up to 350 m thick within coherent schists of the Nishisonogi unit of the Nagasaki Metamorphic Complex located in western Kyushu, which represent serpentinite-hosted exhumation channels within a Cretaceous subduction complex. This study gives the petrography of coherent schists and various tectonic block types embedded in serpentinite or chlorite-actinolite schist of the mélange. The <i>P–T</i> conditions recorded in these rocks were compared, to clarify a possible exhumation process and style of the Nishisonogi unit. In the mélange, two types of tectonic blocks were recognized based on peak temperature conditions: a lower temperature type (400–590°C) and a higher temperature type (780–830°C), the latter of which shows temperatures higher than those of the coherent schists (370–450°C). The garnet glaucophanite in the coherent schists shows a prograde pressure increment from 0.9 to 2.3 GPa. The serpentinite mélange comprises tectonic blocks with peak temperatures higher than those of the coherent schists, indicating that blocks from the deep structural levels of the lower-plate underwent tectonic mixing into the mélange during exhumation. Ultrahigh-pressure (UHP) conditions have not been recorded either in the coherent schists or in the serpentinite mélanges, although microdiamonds have been reported from the Yukinoura mélange located at the western margin of the Nishisonogi unit (Nishiyama et al., Scientific Reports, 2020, 10, 11645). A possible correlation between the Nishisonogi unit and other HP metamorphic rocks or belts (Renge rocks, Suo Belt and Sanbagawa Belt) was examined to conclude that the Nishisonogi unit is a unique paleo-subduction complex in terms of the coexistence of HP subduction complex with intercalated serpentinite mélanges and a UHP mélange derived from the slab-mantle interface.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.5,"publicationDate":"2024-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141430316","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}
Eocene to Lower Miocene coal-bearing formations in northern and western Kyushu, northern Ryukyu arc, are folded, and the horizontal compression has been attributed to the opening of the Japan Sea or to the significant movement along the Median Tectonic Line and its southwestern extension. However, the timing and implication of the folding are not well understood. To deal with these issues, we studied the Amakusa region where the folded Eocene strata with a total thickness of a few kilometers. Paleomagnetic directions of Middle Miocene intrusions and of Late Miocene lavas were measured in this study to apply the fold test to judge the relative timing of the folding and magmatism. As a result, the concentration of the directions was improved by the tilt-correction, indicating the folding younger than the magmatism. Our detailed geological mapping revealed that the folding is older than a horizontally-lying basaltic lava which yielded a K–Ar age of 6.8 Ma, because folded Eocene formations were truncated and unconformably blanketed by the lava. In addition, we found that some of the normal faults trending perpendicular to the folds were reactivated as transfer faults after the normal faulting which also postdated the magmatism. This reactivation is concordant with the above-mentioned relative timing. Synthesizing geological data from surrounding regions, we conclude that the folding was probably contemporaneous with the Taiwan-Shinji fold belt which grew in the Ryukyu and southwest Japan backarcs. Since the simultaneous compression affected not only these regions but also northeast Japan, the compression possibly resulted from the resumed subduction of the Philippine Sea Plate in the Serravallian–Tortonian time.
{"title":"Serravallian–Tortonian (Miocene) folding in the Amakusa region, northern Ryukyu arc: Possible subduction resumption of the Philippine Sea Plate","authors":"Kentaro Ushimaru, Atsushi Yamaji, Naoto Ishikawa","doi":"10.1111/iar.12528","DOIUrl":"https://doi.org/10.1111/iar.12528","url":null,"abstract":"<p>Eocene to Lower Miocene coal-bearing formations in northern and western Kyushu, northern Ryukyu arc, are folded, and the horizontal compression has been attributed to the opening of the Japan Sea or to the significant movement along the Median Tectonic Line and its southwestern extension. However, the timing and implication of the folding are not well understood. To deal with these issues, we studied the Amakusa region where the folded Eocene strata with a total thickness of a few kilometers. Paleomagnetic directions of Middle Miocene intrusions and of Late Miocene lavas were measured in this study to apply the fold test to judge the relative timing of the folding and magmatism. As a result, the concentration of the directions was improved by the tilt-correction, indicating the folding younger than the magmatism. Our detailed geological mapping revealed that the folding is older than a horizontally-lying basaltic lava which yielded a K–Ar age of 6.8 Ma, because folded Eocene formations were truncated and unconformably blanketed by the lava. In addition, we found that some of the normal faults trending perpendicular to the folds were reactivated as transfer faults after the normal faulting which also postdated the magmatism. This reactivation is concordant with the above-mentioned relative timing. Synthesizing geological data from surrounding regions, we conclude that the folding was probably contemporaneous with the Taiwan-Shinji fold belt which grew in the Ryukyu and southwest Japan backarcs. Since the simultaneous compression affected not only these regions but also northeast Japan, the compression possibly resulted from the resumed subduction of the Philippine Sea Plate in the Serravallian–Tortonian time.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.5,"publicationDate":"2024-06-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141424930","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}
Chunjiao Wu, Zhenyi Wang, Jin Zhang, Jianfeng Liu, Beihang Zhang, Rongguo Zheng, Jun Ma, Jie Hui, Heng Zhao
Multiple branch oceans existed in the Paleo-Asian Ocean (PAO), but their closure times are in dispute and unclear, which constrains our understanding of the final closure time of the PAO and the tectonic evolution of the Central Asian Orogenic Belt (CAOB). This study focuses on the Permian plutons of the northern Alxa, which is located in the middle segment of the southern CAOB that recorded the final subduction history of the PAO. We performed the 1:50000 mapping, whole-rock geochemistry, geochronology, and Sr-Nd-Hf isotopic analysis and compiled the Sr-Nd-Hf isotopic compositions and whole-rock geochemical data of igneous rocks from the northern Alxa. LA-ICP-MS zircon U–Pb dating reveals the study plutons emplaced in the Early Permian (285–296 Ma). Whole-rock geochemical data show the intrusion belongs to medium-K calc-alkaline peraluminous highly fractionated I-type granite, enriched in Rb, K, Th, Pb, and depleted in Nb, Ta, Ti, Sr, and P elements, which suggest a subduction arc-related setting and metaluminous to weak peraluminous parental magma. The weak negative εNd(t) (from −2.3 to −1.2), relatively high ISr (0.704772–0.708037) and depleted mantle model ages TDM (1.14–1.49 Ga), combining with weak negative to slightly positive εHf(t) (from −2.0 to +4.1) and crustal model ages TDMC (1.18–1.43 Ga), indicate that the parental magma might originate from remelting of the Mesoproterozoic lower crust and mixing with mantle-derived materials. The field occurrence, deformation, and geochemical features, integrating with the compiled data and regional geology, show that the igneous rocks formed before or after the late Early Permian show different features in terms of deformation, zircon saturation temperatures, crustal thickness, potassium contents, and εHf(t) values. This might relate to the closure of the Yagan branch ocean of the PAO in northern Alxa.
古亚洲洋(Pao-Asian Ocean,PAO)存在多个分支洋,但其关闭时间存在争议且不明确,这制约了我们对PAO最终关闭时间和中亚造山带构造演化的理解。本研究的重点是位于中亚造山带南部中段的阿拉善北部的二叠纪长岩,该地区记录了 PAO 的最终俯冲历史。我们进行了1:50000测绘、全岩地球化学、地质年代学和Sr-Nd-Hf同位素分析,并汇编了Alxa北部火成岩的Sr-Nd-Hf同位素组成和全岩地球化学数据。LA-ICP-MS锆石U-Pb年代测定显示,所研究的岩浆岩形成于早二叠世(285-296 Ma)。全岩地球化学数据显示,该侵入体属于中K钙碱性高铝质高分馏I型花岗岩,富含Rb、K、Th、Pb,而Nb、Ta、Ti、Sr和P元素贫乏,这表明该侵入体与俯冲弧有关,母岩为金属铝质至弱高铝质岩浆。弱负εNd(t)(从-2.3到-1.2)、相对较高的ISr(0.704772-0.708037)和贫化地幔模型年龄TDM(1.14-1.49 Ga),结合弱负至轻微正的εHf(t)(从-2.0至+4.1)和地壳模型年龄TDMC(1.18-1.43 Ga),表明母岩浆可能来源于中新生代下地壳的重熔以及与地幔衍生物质的混合。野外发生、变形和地球化学特征,结合资料汇编和区域地质,表明早二叠世晚期前后形成的火成岩在变形、锆石饱和温度、地壳厚度、钾含量和εHf(t)值等方面表现出不同的特征。这可能与阿拉善北部PAO雅干支洋的关闭有关。
{"title":"Geochronology and geochemistry of granitoids from northern Alxa, northwest China: Petrogenesis and tectonic implications","authors":"Chunjiao Wu, Zhenyi Wang, Jin Zhang, Jianfeng Liu, Beihang Zhang, Rongguo Zheng, Jun Ma, Jie Hui, Heng Zhao","doi":"10.1111/iar.12525","DOIUrl":"https://doi.org/10.1111/iar.12525","url":null,"abstract":"<p>Multiple branch oceans existed in the Paleo-Asian Ocean (PAO), but their closure times are in dispute and unclear, which constrains our understanding of the final closure time of the PAO and the tectonic evolution of the Central Asian Orogenic Belt (CAOB). This study focuses on the Permian plutons of the northern Alxa, which is located in the middle segment of the southern CAOB that recorded the final subduction history of the PAO. We performed the 1:50000 mapping, whole-rock geochemistry, geochronology, and Sr-Nd-Hf isotopic analysis and compiled the Sr-Nd-Hf isotopic compositions and whole-rock geochemical data of igneous rocks from the northern Alxa. LA-ICP-MS zircon U–Pb dating reveals the study plutons emplaced in the Early Permian (285–296 Ma). Whole-rock geochemical data show the intrusion belongs to medium-K calc-alkaline peraluminous highly fractionated I-type granite, enriched in Rb, K, Th, Pb, and depleted in Nb, Ta, Ti, Sr, and P elements, which suggest a subduction arc-related setting and metaluminous to weak peraluminous parental magma. The weak negative ε<sub>Nd</sub>(t) (from −2.3 to −1.2), relatively high I<sub>Sr</sub> (0.704772–0.708037) and depleted mantle model ages T<sub>DM</sub> (1.14–1.49 Ga), combining with weak negative to slightly positive ε<sub>Hf</sub>(t) (from −2.0 to +4.1) and crustal model ages T<sub>DM</sub><sup>C</sup> (1.18–1.43 Ga), indicate that the parental magma might originate from remelting of the Mesoproterozoic lower crust and mixing with mantle-derived materials. The field occurrence, deformation, and geochemical features, integrating with the compiled data and regional geology, show that the igneous rocks formed before or after the late Early Permian show different features in terms of deformation, zircon saturation temperatures, crustal thickness, potassium contents, and ε<sub>Hf</sub>(t) values. This might relate to the closure of the Yagan branch ocean of the PAO in northern Alxa.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.5,"publicationDate":"2024-05-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141073689","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}
Igneous rocks are fractured during cooling from magma to form cooling joints, which are typically columnar joints in volcanic rocks, while orthogonal joints are considered typical for plutonic rocks. We performed a 3D study of joint systems in a granitic batholith of the Okueyama granite in western Japan, which has its roof and its internal structures from the roof to 1000 m downward exposed. We used an unmanned aerial vehicle (UAV) to observe the joints in outcrops from various angles. Based on our study, we propose a schematic model for joint systems in a granitic pluton. A granitic pluton has zones of rock columns below the roof and next to the wall. The rock column zone below the roof is as thick as 300 m, and its higher portions form steep cliffs, probably because of increased resistance to weathering. The axes of the rock columns are nearly vertical below the roof and gently plunge next to the walls, with high intersection angles with the wall. The distribution of columnar joints near only the roof and walls suggests that the granite cooled more rapidly near the roof and walls than in the core of the pluton. When the granite was jointed by parallel joints during cooling, the rock slabs between the parallel joints near the roof and the walls are subdivided into columns with polygonal cross-sections. This suggests that the granite was fractured by parallel joints at a temperature immediately below the solidus, after which the rock slabs were subdivided into rock columns during further cooling.
{"title":"A zone of columnar joints beneath the roof of a granitic pluton: The Okueyama granite, southwestern Japan","authors":"Masahiro Chigira, Hironori Kato","doi":"10.1111/iar.12524","DOIUrl":"https://doi.org/10.1111/iar.12524","url":null,"abstract":"<p>Igneous rocks are fractured during cooling from magma to form cooling joints, which are typically columnar joints in volcanic rocks, while orthogonal joints are considered typical for plutonic rocks. We performed a 3D study of joint systems in a granitic batholith of the Okueyama granite in western Japan, which has its roof and its internal structures from the roof to 1000 m downward exposed. We used an unmanned aerial vehicle (UAV) to observe the joints in outcrops from various angles. Based on our study, we propose a schematic model for joint systems in a granitic pluton. A granitic pluton has zones of rock columns below the roof and next to the wall. The rock column zone below the roof is as thick as 300 m, and its higher portions form steep cliffs, probably because of increased resistance to weathering. The axes of the rock columns are nearly vertical below the roof and gently plunge next to the walls, with high intersection angles with the wall. The distribution of columnar joints near only the roof and walls suggests that the granite cooled more rapidly near the roof and walls than in the core of the pluton. When the granite was jointed by parallel joints during cooling, the rock slabs between the parallel joints near the roof and the walls are subdivided into columns with polygonal cross-sections. This suggests that the granite was fractured by parallel joints at a temperature immediately below the solidus, after which the rock slabs were subdivided into rock columns during further cooling.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.5,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12524","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140844701","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}
The Early Cretaceous Takanokura Formation in the eastern part of the Abukuma Mountains consists of a lower felsic ignimbrite and upper intermediate lavas and volcaniclastic rocks, representing the initial arc-type in northeast Japan. In this study, I analyzed the major and trace element contents and Sr-Nd-Pb isotopic ratios of these eruptive products; then, I discussed their magma genesis based on their geochemical properties. Although igneous rocks of the same period in other localities of northeast Japan are characterized by the occurrence of adakites, these volcanism are composed of non-adakitic high- to medium-K andesite to rhyolite that are rich in large-ion lithophile elements and poor in high-field-strength elements and have low Sr/Y values and flat heavy rare earth element patterns. Furthermore, these rocks have high radiogenic Pb isotopic ratios. The rhyolite and dacite have been thought to derive from crustal melting, whereas the andesite formed by the mixing of crustal felsic melts and mafic magmas generated by melting of the lithospheric mantle. Although previous studies attributed the formation of the Early Cretaceous adakites to the hot subduction of a mid-ocean ridge, recent global plate motion reconstructions reject this model. To generate magma from a cold slab and lithospheric mantle that does not spontaneously melt, the mantle wedge under northeast Japan must have experienced heating. During this period, the volcanic province along the eastern margin of Eurasia expanded rapidly toward the trench, forming grabens. Therefore, I concluded that the advance of the hot asthenosphere into the forearc region that led to this expansion, which caused the retreat of the subduction boundary of the paleo-Pacific plate to retreat and ultimately converted northeast Japan from an accretionary complex into a volcanically active region.
{"title":"Arc volcanism initiated on the eastern margin of Eurasia during the Early Cretaceous: Geochemistry of the Takanokura volcanic rocks in the Abukuma Mountains, Northeast Japan","authors":"Takahiro Yamamoto","doi":"10.1111/iar.12523","DOIUrl":"https://doi.org/10.1111/iar.12523","url":null,"abstract":"<p>The Early Cretaceous Takanokura Formation in the eastern part of the Abukuma Mountains consists of a lower felsic ignimbrite and upper intermediate lavas and volcaniclastic rocks, representing the initial arc-type in northeast Japan. In this study, I analyzed the major and trace element contents and Sr-Nd-Pb isotopic ratios of these eruptive products; then, I discussed their magma genesis based on their geochemical properties. Although igneous rocks of the same period in other localities of northeast Japan are characterized by the occurrence of adakites, these volcanism are composed of non-adakitic high- to medium-K andesite to rhyolite that are rich in large-ion lithophile elements and poor in high-field-strength elements and have low Sr/Y values and flat heavy rare earth element patterns. Furthermore, these rocks have high radiogenic Pb isotopic ratios. The rhyolite and dacite have been thought to derive from crustal melting, whereas the andesite formed by the mixing of crustal felsic melts and mafic magmas generated by melting of the lithospheric mantle. Although previous studies attributed the formation of the Early Cretaceous adakites to the hot subduction of a mid-ocean ridge, recent global plate motion reconstructions reject this model. To generate magma from a cold slab and lithospheric mantle that does not spontaneously melt, the mantle wedge under northeast Japan must have experienced heating. During this period, the volcanic province along the eastern margin of Eurasia expanded rapidly toward the trench, forming grabens. Therefore, I concluded that the advance of the hot asthenosphere into the forearc region that led to this expansion, which caused the retreat of the subduction boundary of the paleo-Pacific plate to retreat and ultimately converted northeast Japan from an accretionary complex into a volcanically active region.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.5,"publicationDate":"2024-04-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/iar.12523","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140814301","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}
Trace elements in igneous zircon crystals exhibit variability within single crystals or among populations of crystals, demonstrating heightened sensitivity to changes in melt composition. The three distinct types of zircons in the Nuqara caldera complex (659 ± 16 Ma andesites, 602.3 ± 4.4 Ma rhyolites, and 589.4 ± 6.1 Ma rhyolite porphyry; A, B, and C, respectively) signify a collective geological history influencing the multistage magmatic evolution. Significantly, the studied zircons demonstrate growth rate and variable length-to-width ratios that progressively increase from A to C. Ti-in-zircon geothermometer (TTi-in-zrc = 924°C) along with the internal structure and geochemistry of type A zircons, such as very weak cathodoluminescence (CL) brightness, zoning, and higher concentrations of some trace elements content, suggest their formation during the early, hotter, and less-evolved melt stage of volcanic activity. Type B zircons exhibit TTi-in-zrc (833°C) and commonly display resorption with an absence of singular dark CL, indicating substantial reheating of the magma reservoir. The interaction between the incoming evolved magma and the resident magma results in the formation of zircon rims during the magma cooling, featuring significant overlaps in zircon trace elements. This final phase in the Nuqara caldera complex marks the complete hybridization of the initially distinct magmas, culminating in a gradual cooling process. The newly formed zircons (type C) are characterized by light CL features with weak zoning occurring either as the rim of the oscillatory zoned zircon or as an individual zircon grain. Their less evolved chemical signature and TTi-in-zrc (708°C) highlight the significance of this final stage in shaping the overall geochemical and thermal evolution. The obtained zircon data, spanning from the initial crystallization to the subsequent recharge, mixing, and hybridization stages, delineate the discernible phases in the formation of the Nuqara caldera, providing insights into the transitions from subduction to collision-related geological processes.
火成岩锆石晶体中的微量元素在单个晶体内部或晶体群之间表现出变异性,显示出对熔体成分变化的高度敏感性。努卡拉火山口复合体中三种不同类型的锆石(分别为 659 ± 16 Ma 安山岩、602.3 ± 4.4 Ma 流纹岩和 589.4 ± 6.1 Ma 流纹斑岩;A、B 和 C)表明了影响多级岩浆演化的集体地质历史。值得注意的是,所研究的锆石显示了从 A 到 C 逐渐增加的生长率和不同的长宽比。A 型锆石的 Ti-in-zircon 地球温度计(TTi-in-zrc = 924°C)以及 A 型锆石的内部结构和地球化学特征,如非常弱的阴极发光(CL)亮度、分带和较高浓度的某些微量元素含量,表明它们是在火山活动的早期、较热和较低演化的熔融阶段形成的。B 型锆石显示出 TTi-in-zrc (833°C),并且通常显示出再吸附现象,没有奇异的暗 CL,这表明岩浆储层进行了大量的再加热。进入的演化岩浆与驻留岩浆之间的相互作用导致在岩浆冷却过程中形成锆石边缘,其特点是锆石痕量元素明显重叠。努卡拉火山口复合体的这一最后阶段标志着最初不同岩浆的完全混合,并在逐渐冷却过程中达到顶峰。新形成的锆石(C 型)具有轻 CL 特征,其弱分带既可作为振荡分带锆石的边缘,也可作为单个锆石晶粒。它们的化学特征演化程度较低,在锆石中的钛度(708°C)凸显了这一最后阶段在塑造整体地球化学和热演化过程中的重要性。所获得的锆石数据跨越了从最初的结晶到随后的补给、混合和杂化阶段,勾勒出了努卡拉火山口形成过程中的可辨阶段,为了解从俯冲到碰撞相关地质过程的过渡提供了见解。
{"title":"Deciphering the multistage magmatic processes in the Nuqara caldera, the northern Egyptian Nubian-Shield: Insights from zircon morphology and geochemistry","authors":"Osama K. Dessouky","doi":"10.1111/iar.12522","DOIUrl":"https://doi.org/10.1111/iar.12522","url":null,"abstract":"<p>Trace elements in igneous zircon crystals exhibit variability within single crystals or among populations of crystals, demonstrating heightened sensitivity to changes in melt composition. The three distinct types of zircons in the Nuqara caldera complex (659 ± 16 Ma andesites, 602.3 ± 4.4 Ma rhyolites, and 589.4 ± 6.1 Ma rhyolite porphyry; A, B, and C, respectively) signify a collective geological history influencing the multistage magmatic evolution. Significantly, the studied zircons demonstrate growth rate and variable length-to-width ratios that progressively increase from A to C. Ti-in-zircon geothermometer (T<sub>Ti-in-zrc</sub> = 924°C) along with the internal structure and geochemistry of type A zircons, such as very weak cathodoluminescence (CL) brightness, zoning, and higher concentrations of some trace elements content, suggest their formation during the early, hotter, and less-evolved melt stage of volcanic activity. Type B zircons exhibit T<sub>Ti-in-zrc</sub> (833°C) and commonly display resorption with an absence of singular dark CL, indicating substantial reheating of the magma reservoir. The interaction between the incoming evolved magma and the resident magma results in the formation of zircon rims during the magma cooling, featuring significant overlaps in zircon trace elements. This final phase in the Nuqara caldera complex marks the complete hybridization of the initially distinct magmas, culminating in a gradual cooling process. The newly formed zircons (type C) are characterized by light CL features with weak zoning occurring either as the rim of the oscillatory zoned zircon or as an individual zircon grain. Their less evolved chemical signature and T<sub>Ti-in-zrc</sub> (708°C) highlight the significance of this final stage in shaping the overall geochemical and thermal evolution. The obtained zircon data, spanning from the initial crystallization to the subsequent recharge, mixing, and hybridization stages, delineate the discernible phases in the formation of the Nuqara caldera, providing insights into the transitions from subduction to collision-related geological processes.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.5,"publicationDate":"2024-04-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140649522","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}
C. Perumalsamy, S. Vijay Anand, R. Nagarajan, Bappa Mukherjee
The previous studies revealed the I-type Ladakh magmatism in the Andean-type southern margin of the Ladakh batholith (LB) was related to the subduction of the Neotethyan Ocean and India-Eurasia collision. However, LB's S-type granitic magmatism and associated mafic microgranular enclaves (MMEs) are poorly constrained. Here, we present the new data for S-type Ladakh granite (LG) and associated monzodiorite MMEs in the Andean-type orogeny in the southern margin of the Eurasian plate. The low SiO2 (47.4–53.9 wt%), high K2O (1.56–3.21 wt%), Mg# (52–65), continental-arc tracer patterns, and slightly depleted to evolved Sr-Nd isotopic composition ((87Sr/86Sr)i = 0.7047–0.7166; ℇNd (t = 50 Ma) = (+1.40 to −8.92)) for MME suggest that they were derived from the phlogopite-bearing deep lithospheric mantle-source at a depth of 5.4–10.5 km depth with 810–870°C, 1.4–2.8 kbar, and enriched by sediment-melts addition into the mantle-wedge from subducting Neotethyan Oceanic slab. The mantle-derived ascending hot mafic magma mixing with felsic magma of the ancient northern Indian margin-derived, generates monzodiorite MME by assimilation and magma mixing processes. Plagioclase, amphibole, and biotite chemistry support the magma mixing processes. LG are characterized by high SiO2 (63.4–75.0 wt%), K2O (3.93–5.67 wt%), CaO/Na2O ratio of >0.3, differentiation index (90.27–97.46), normative corundum (1.0–2.8), A/CNK values (1.00–1.18), hypersthene (0.7–5.7), and low Al2O3, MgO, TiO2, Fe2O3. They also exhibit peraluminous, variable tracer elemental abundances, variable (87Sr/86Sr)i ratios (0.6967–0.7191), and high whole rock ℇNd (t = 50 Ma) values of −4.15 to −11.92) and ancient two-stage Nd model age of 1160 and 1858 Ma. These features suggest that S-type Ladakh granites were derived from the melting of ancient metagreywacke-dominated metasedimentary rocks of the northern Indian margin by a large amount of mafic magma underplating after subducted Neotethyan slab-rollback. The formation of LG and MMEs related to the Andean-type orogeny in the southern margin of the Eurasian plate.
{"title":"Petrogenesis of S-type Ladakh granite and mafic microgranular enclaves in the southern margin of Ladakh batholith: An evidence of crust–mantle interaction during the collision between Indian and Eurasian plates","authors":"C. Perumalsamy, S. Vijay Anand, R. Nagarajan, Bappa Mukherjee","doi":"10.1111/iar.12520","DOIUrl":"https://doi.org/10.1111/iar.12520","url":null,"abstract":"<p>The previous studies revealed the I-type Ladakh magmatism in the Andean-type southern margin of the Ladakh batholith (LB) was related to the subduction of the Neotethyan Ocean and India-Eurasia collision. However, LB's S-type granitic magmatism and associated mafic microgranular enclaves (MMEs) are poorly constrained. Here, we present the new data for S-type Ladakh granite (LG) and associated monzodiorite MMEs in the Andean-type orogeny in the southern margin of the Eurasian plate. The low SiO<sub>2</sub> (47.4–53.9 wt%), high K<sub>2</sub>O (1.56–3.21 wt%), Mg<sup>#</sup> (52–65), continental-arc tracer patterns, and slightly depleted to evolved Sr-Nd isotopic composition ((<sup>87</sup>Sr/<sup>86</sup>Sr)i = 0.7047–0.7166; ℇ<sub>Nd</sub> (<i>t</i> = 50 Ma) = (+1.40 to −8.92)) for MME suggest that they were derived from the phlogopite-bearing deep lithospheric mantle-source at a depth of 5.4–10.5 km depth with 810–870°C, 1.4–2.8 kbar, and enriched by sediment-melts addition into the mantle-wedge from subducting Neotethyan Oceanic slab. The mantle-derived ascending hot mafic magma mixing with felsic magma of the ancient northern Indian margin-derived, generates monzodiorite MME by assimilation and magma mixing processes. Plagioclase, amphibole, and biotite chemistry support the magma mixing processes. LG are characterized by high SiO<sub>2</sub> (63.4–75.0 wt%), K<sub>2</sub>O (3.93–5.67 wt%), CaO/Na<sub>2</sub>O ratio of >0.3, differentiation index (90.27–97.46), normative corundum (1.0–2.8), A/CNK values (1.00–1.18), hypersthene (0.7–5.7), and low Al<sub>2</sub>O<sub>3</sub>, MgO, TiO<sub>2</sub>, Fe<sub>2</sub>O<sub>3</sub>. They also exhibit peraluminous, variable tracer elemental abundances, variable (<sup>87</sup>Sr/<sup>86</sup>Sr)i ratios (0.6967–0.7191), and high whole rock ℇ<sub>Nd</sub> (<i>t</i> = 50 Ma) values of −4.15 to −11.92) and ancient two-stage Nd model age of 1160 and 1858 Ma. These features suggest that S-type Ladakh granites were derived from the melting of ancient metagreywacke-dominated metasedimentary rocks of the northern Indian margin by a large amount of mafic magma underplating after subducted Neotethyan slab-rollback. The formation of LG and MMEs related to the Andean-type orogeny in the southern margin of the Eurasian plate.</p>","PeriodicalId":14791,"journal":{"name":"Island Arc","volume":null,"pages":null},"PeriodicalIF":1.5,"publicationDate":"2024-04-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140544463","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}