Xun Kang, Jingqiang Tan, Hans-Martin Schulz, Bin Fu, Biao Chang, Cao Jian, Wenxuan Hu
The oxidation of hydrocarbons, including methane, is part of interrelated hydrogeochemical reactions affecting the carbon budget in Earth’s crust. To investigate these processes in deep siliciclastic strata, we analyzed core samples from Lower Triassic red beds in the Mahu Sag (Junggar Basin, northwest China) by coupling petrological observations with high-resolution in situ secondary ion mass spectroscopy stable carbon and oxygen isotope analyses and clumped isotopes (Δ47) of authigenic calcite. The strata contain variable oil and gas content as well as abundant high-valence Fe and/or Mn oxides. Three sequential generations of cement occur, which are characterized as (1) non-luminescent, early diagenetic calcite (MnO <0.3%, δ13CVPDB [Vienna Peedee belemnite] = −5.6‰ to −4.1‰); (2) bright-orange luminescent late-stage I calcite (0.75%−5.23% MnO, δ13C = −51.4‰ to −25.8‰); and (3) dull-orange late-stage II calcite (4.10%−12.93% MnO, δ13C = −91.4‰ to −30.9‰). Clumped isotopic thermometry reveals that the calcite precipitation temperature increases successively from <40 °C, to 81−107 °C, to finally 107−132 °C, corresponding to three precipitation time periods: before the Late Triassic, from the Early Jurassic to the Early Cretaceous, and from the Early Cretaceous to the present, respectively. δ13C values of −55.7‰ to −25.8‰ indicate that late-stage I calcite is the final product of oxidation of both methane and C2+ hydrocarbons, whereas δ13C values as low as −91‰ indicate that late-stage II calcite is mainly derived from the thermochemical oxidation of methane (δ13C = −46.8‰ to −39.3‰) induced by high-valence Mn and/or Fe oxides. For late-stage I calcite, hydrocarbon oxidation was most likely promoted by high temperatures, although microbial oxidation cannot be completely ruled out. The higher precipitation temperature of late-stage II calcite demonstrates that the oxidation of methane requires higher activation energies than oxidation of C2+ hydrocarbons. We provide reliable geochemical evidence for thermally induced sequential oxidation of hydrocarbons within deep siliciclastic strata.
{"title":"Clumped and in situ carbon and oxygen isotopes of calcite as tracers for oxidation of hydrocarbons in deep siliciclastic strata","authors":"Xun Kang, Jingqiang Tan, Hans-Martin Schulz, Bin Fu, Biao Chang, Cao Jian, Wenxuan Hu","doi":"10.1130/b37326.1","DOIUrl":"https://doi.org/10.1130/b37326.1","url":null,"abstract":"The oxidation of hydrocarbons, including methane, is part of interrelated hydrogeochemical reactions affecting the carbon budget in Earth’s crust. To investigate these processes in deep siliciclastic strata, we analyzed core samples from Lower Triassic red beds in the Mahu Sag (Junggar Basin, northwest China) by coupling petrological observations with high-resolution in situ secondary ion mass spectroscopy stable carbon and oxygen isotope analyses and clumped isotopes (Δ47) of authigenic calcite. The strata contain variable oil and gas content as well as abundant high-valence Fe and/or Mn oxides. Three sequential generations of cement occur, which are characterized as (1) non-luminescent, early diagenetic calcite (MnO <0.3%, δ13CVPDB [Vienna Peedee belemnite] = −5.6‰ to −4.1‰); (2) bright-orange luminescent late-stage I calcite (0.75%−5.23% MnO, δ13C = −51.4‰ to −25.8‰); and (3) dull-orange late-stage II calcite (4.10%−12.93% MnO, δ13C = −91.4‰ to −30.9‰). Clumped isotopic thermometry reveals that the calcite precipitation temperature increases successively from <40 °C, to 81−107 °C, to finally 107−132 °C, corresponding to three precipitation time periods: before the Late Triassic, from the Early Jurassic to the Early Cretaceous, and from the Early Cretaceous to the present, respectively. δ13C values of −55.7‰ to −25.8‰ indicate that late-stage I calcite is the final product of oxidation of both methane and C2+ hydrocarbons, whereas δ13C values as low as −91‰ indicate that late-stage II calcite is mainly derived from the thermochemical oxidation of methane (δ13C = −46.8‰ to −39.3‰) induced by high-valence Mn and/or Fe oxides. For late-stage I calcite, hydrocarbon oxidation was most likely promoted by high temperatures, although microbial oxidation cannot be completely ruled out. The higher precipitation temperature of late-stage II calcite demonstrates that the oxidation of methane requires higher activation energies than oxidation of C2+ hydrocarbons. We provide reliable geochemical evidence for thermally induced sequential oxidation of hydrocarbons within deep siliciclastic strata.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"31 11","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140673313","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.
{"title":"Carbon-rich polyphasic inclusions in postcollisional mafic magmatic rocks from the Dabie Shan, China: Implications for the carbon cycle in continental subduction zones","authors":"Dong-Bo Tan, Yilin Xiao, Yangyang Wang, Deshi Jin, Hai-Ou Gu, He Sun, Lingling Jiang","doi":"10.1130/b37103.1","DOIUrl":"https://doi.org/10.1130/b37103.1","url":null,"abstract":"Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"20 5","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140677655","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Chaoyang Wang, Tao Wang, C. V. van Staal, Zengqian Hou, Shoufa Lin
The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435−420 Ma), syncollision (415−405 Ma), and postcollision (395−370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435−420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −5 to −2; εHf[t] = −3 to −1; δ18O = +6‰ to +8‰). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415−405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −6 to 0; εHf[t] = +1 to +3; δ18O = +5‰ to +6‰). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of >9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395−370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover−Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting.
岩浆活动随时间的迁移和特征可为了解造山运动的构造演化提供重要信息。我们提出了与纽芬兰阿巴拉契亚山脉东部和南部阿卡迪亚造山周期相关的三个不同阶段的花岗岩岩浆活动的证据。这些解释基于来自 15 个志留纪和泥盆纪花岗岩岩体的 18 个样本的新锆石 U-Pb 年龄、地球化学数据和 Sr-Nd-Hf-O 同位素数据，并结合了以前发表的数据。这三个阶段概述了腹地和前陆导向的迁移趋势，代表了阿卡迪亚造山运动周期中的俯冲（435-420 Ma）、同步碰撞（415-405 Ma）和后碰撞（395-370 Ma）环境。第一阶段的志留纪深成岩（435-420 Ma）主要由石英闪长岩、辉绿岩、花岗闪长岩、单斜花岗岩和正长花岗岩组成，具有高K钙碱性和富集的Sr-Nd-Hf-O同位素组成（例如，εNd[t] = -5 to -2；εHf[t] = -3 to -1; δ18O = +6‰ to +8‰）。据解释，它们记录了阿卡迪亚海道海洋岩石圈的俯冲，该海道将以甘德边缘为代表的复合劳伦西亚前缘与阿瓦隆尼亚分隔开来。第二阶段早泥盆世的长岩（415-405 Ma）含有更多的单斜花岗岩和正长花岗岩；它们具有钙碱性至高钾钙碱性特征、类似金刚石的成分以及更贫化的Sr-Nd-Hf-O同位素成分（例如，εNd[t] = -6 to 0；εHf[t] = +1 to +3；δ18O = +5‰ to +6‰）。这一阶段的岩浆岩主要出现在志留纪花岗岩的西北部，表明岩浆活动以大于9 km/m.y的速度在区域范围内向西北方向（腹地方向）迁移。第三阶段（395-370Ma）的中泥盆世至晚泥盆世岩浆岩主要由单斜花岗岩、正长岩和碱长花岗岩组成，它们是富含硅和碱的花岗岩，具有较大的负Eu异常。这些岩石主要集中在多佛-赫米蒂奇湾断层带两侧，该断层带是阿瓦隆尼亚和复合劳伦西亚的分界线，位于志留纪和早泥盆纪火成岩的东南部。这一阶段的岩浆活动代表了前陆定向（后退）迁移。早泥盆世和中泥盆世至晚泥盆世的岩浆活动被 405 Ma 至 395 Ma 之间的间隙隔开，并记录了从（高钾）钙碱性向碱性成分的演变，这归因于碰撞后环境中阿瓦隆岩石圈地幔的部分脱层。
{"title":"Evolution of Silurian to Devonian magmatism associated with the Acadian orogenic cycle in eastern and southern Newfoundland Appalachians: Evidence for a three-stage evolution characterized by episodic hinterland- and foreland-directed migration of granitoid magmatism","authors":"Chaoyang Wang, Tao Wang, C. V. van Staal, Zengqian Hou, Shoufa Lin","doi":"10.1130/b37336.1","DOIUrl":"https://doi.org/10.1130/b37336.1","url":null,"abstract":"The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435−420 Ma), syncollision (415−405 Ma), and postcollision (395−370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435−420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −5 to −2; εHf[t] = −3 to −1; δ18O = +6‰ to +8‰). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415−405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −6 to 0; εHf[t] = +1 to +3; δ18O = +5‰ to +6‰). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of >9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395−370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover−Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"13 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140673424","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Chen Zhang, Huaguo Wen, Xin Wang, Long Wen, Anjiang Shen, Gang Zhou, Qiqi Wang, Min She, Chao Ma, Zhanfeng Qiao, Da-Quan Liu, Yiquan Ma
Deep core (>4.9 km) from Ediacaran Deng IV Member algal dolomites in the Gaoshiti-Moxi block in the Sichuan Basin, southwest China, reveals multiple generations of dolomite-lined and dolomite-filled opening-mode fractures. Three progressive stages of fracture formation are marked by crosscutting relations visible in the core, by acoustic emission experiments revealing evidence of past stress directions, and by fluid inclusions, U-Pb ages, C-O-Sr-Nd isotope patterns, and rare earth element data for dolomite cements in fractures, which document ages and differing thermal conditions and fluid compositions during fracture. In calcite-filled fractures, U-Pb ages and carbon and oxygen isotope signatures vary greatly, indicating that fractures developed with intensified tectonic activity marked by regional structures and with enhanced diagenetic alteration. In stage I, WNW-striking opening-mode fractures formed that contain dolomite deposits precipitated from basinal fluids between ca. 549 Ma and ca. 532 Ma. At this time, the Sichuan Basin experienced Xingkai taphrogenesis (rifting) from the late Neoproterozoic to early Cambrian. The central Sichuan paleo-uplift was undergoing ENE extension, and preexisting ESE- and nearly E-W−striking faults were oblique to the ENE principal stress orientation. This led to a local stress field favoring dextral shear near fault zones accommodated by the fractures. In stage II, ENE-striking fractures that are younger based on crosscutting relations contain dolomite deposits from basinal fluids with ages from ca. 423 Ma to ca. 411 Ma. Contemporaneous with Xuefeng thrusting, the central Sichuan paleo-uplift was in a NNE-striking transpressional stress field, which likely further generated ENE-striking fractures. In stage III, nearly N-S−striking fractures formed in the Gaoshiti-Moxi block. High-temperature fluids related to the Permian Emeishan large igneous province invaded these fractures from ca. 260 Ma to ca. 256 Ma. At this time, the Sichuan Basin was uplifted under the influence of the Emei taphrogenesis in the late Permian, and the central Sichuan paleo-uplift was subjected to E-W−striking extension. In fractures in these carbonate rocks, micro-computed tomography imaging reveals that macropores (>10 μm, 12.1%−21.8%) and small pores (2−10 μm, 76.6%−86.1%) dominate the dolomite mineral deposits, and that there are few (1.6%−1.8%) micropores or nanopores (<2 μm). Medium-sized throats (1−3 μm) are the main connecting channels. We infer that fractures served as conduits for fluid migration, leading to the dissolution of matrix pores adjacent to the fractures. This secondary porosity not only enhances reservoir storage capacity but also augments reservoir connectivity. Our study shows that in situ U-Pb dating and full-diameter rock acoustic emission data can effectively constrain the timing of fractures. By integrating this information with regional tectonic sequences and fracture diagenetic sequences from combined relat
中国西南部四川盆地高石梯-磨溪区块埃迪卡拉纪邓四世藻白云岩的深部岩芯（大于4.9千米）揭示了多代白云岩衬砌和白云岩填充的开裂模式断裂。岩芯中可见的横切关系、声发射实验揭示的过去应力方向的证据，以及断裂中白云岩胶结物的流体包裹体、U-Pb年龄、C-O-Sr-Nd同位素模式和稀土元素数据，都标志着断裂形成的三个渐进阶段。在方解石填充的断裂中，U-Pb年龄和碳氧同位素特征差异很大，表明断裂是在以区域构造为标志的构造活动加剧和成岩蚀变增强的情况下发育的。在第一阶段，形成了向西北方向冲击的开口模式断裂，其中含有约 549 Ma 至约 532 Ma 之间基底流体沉淀的白云岩沉积物。此时，四川盆地经历了新元古代晚期至寒武纪早期的兴凯断裂。四川中部的古隆起正经历着ENE向的延伸，原有的ESE向和近E-W向断层与ENE向的主应力方向相斜。这导致局部应力场有利于断裂所容纳的断层带附近的右旋剪切。在第二阶段，根据横切关系，ENE-条纹断裂的年龄较小，其中含有来自基底流体的白云岩沉积物，其年龄约为423Ma至约411Ma。与雪峰推覆同时，四川中部古隆起处于NNE向转压应力场中，这很可能进一步产生了ENE向断裂。在第三阶段，高石梯-磨溪地块形成了近N-S向的断裂。与二叠纪峨眉山大型火成岩带有关的高温流体在约260Ma至约256Ma期间侵入了这些断裂。此时，四川盆地在二叠纪晚期峨眉山大火成岩带的影响下发生隆起，川中古隆起带受到了东西向的伸展。在这些碳酸盐岩的断裂中，显微计算机断层扫描成像显示，白云岩矿物沉积以大孔隙（>10 μm，12.1%-21.8%）和小孔隙（2-10 μm，76.6%-86.1%）为主，微孔或纳米孔（<2 μm）很少（1.6%-1.8%）。中等大小的节理（1-3 μm）是主要的连接通道。我们推断，裂缝是流体迁移的通道，导致裂缝附近的基质孔隙溶解。这种次生孔隙度不仅提高了储层的存储能力，还增强了储层的连通性。我们的研究表明，原位铀-铅年代测定和全直径岩石声发射数据可以有效地确定裂缝的时间。通过将这些信息与区域构造序列和裂缝成因序列相结合，结合相对时间、地球化学和岩石力学证据，我们阐明了控制裂缝形成的因素。
{"title":"Formational stages of natural fractures revealed by U-Pb dating and C-O-Sr-Nd isotopes of dolomites in the Ediacaran Dengying Formation, Sichuan Basin, southwest China","authors":"Chen Zhang, Huaguo Wen, Xin Wang, Long Wen, Anjiang Shen, Gang Zhou, Qiqi Wang, Min She, Chao Ma, Zhanfeng Qiao, Da-Quan Liu, Yiquan Ma","doi":"10.1130/b37360.1","DOIUrl":"https://doi.org/10.1130/b37360.1","url":null,"abstract":"Deep core (>4.9 km) from Ediacaran Deng IV Member algal dolomites in the Gaoshiti-Moxi block in the Sichuan Basin, southwest China, reveals multiple generations of dolomite-lined and dolomite-filled opening-mode fractures. Three progressive stages of fracture formation are marked by crosscutting relations visible in the core, by acoustic emission experiments revealing evidence of past stress directions, and by fluid inclusions, U-Pb ages, C-O-Sr-Nd isotope patterns, and rare earth element data for dolomite cements in fractures, which document ages and differing thermal conditions and fluid compositions during fracture. In calcite-filled fractures, U-Pb ages and carbon and oxygen isotope signatures vary greatly, indicating that fractures developed with intensified tectonic activity marked by regional structures and with enhanced diagenetic alteration. In stage I, WNW-striking opening-mode fractures formed that contain dolomite deposits precipitated from basinal fluids between ca. 549 Ma and ca. 532 Ma. At this time, the Sichuan Basin experienced Xingkai taphrogenesis (rifting) from the late Neoproterozoic to early Cambrian. The central Sichuan paleo-uplift was undergoing ENE extension, and preexisting ESE- and nearly E-W−striking faults were oblique to the ENE principal stress orientation. This led to a local stress field favoring dextral shear near fault zones accommodated by the fractures. In stage II, ENE-striking fractures that are younger based on crosscutting relations contain dolomite deposits from basinal fluids with ages from ca. 423 Ma to ca. 411 Ma. Contemporaneous with Xuefeng thrusting, the central Sichuan paleo-uplift was in a NNE-striking transpressional stress field, which likely further generated ENE-striking fractures. In stage III, nearly N-S−striking fractures formed in the Gaoshiti-Moxi block. High-temperature fluids related to the Permian Emeishan large igneous province invaded these fractures from ca. 260 Ma to ca. 256 Ma. At this time, the Sichuan Basin was uplifted under the influence of the Emei taphrogenesis in the late Permian, and the central Sichuan paleo-uplift was subjected to E-W−striking extension. In fractures in these carbonate rocks, micro-computed tomography imaging reveals that macropores (>10 μm, 12.1%−21.8%) and small pores (2−10 μm, 76.6%−86.1%) dominate the dolomite mineral deposits, and that there are few (1.6%−1.8%) micropores or nanopores (<2 μm). Medium-sized throats (1−3 μm) are the main connecting channels. We infer that fractures served as conduits for fluid migration, leading to the dissolution of matrix pores adjacent to the fractures. This secondary porosity not only enhances reservoir storage capacity but also augments reservoir connectivity. Our study shows that in situ U-Pb dating and full-diameter rock acoustic emission data can effectively constrain the timing of fractures. By integrating this information with regional tectonic sequences and fracture diagenetic sequences from combined relat","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"26 11","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140674692","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Deciphering the contribution of crustal materials to generation of mafic arc igneous rocks at different subduction stages is of great significance to unravel the fate of the subducted paleo-oceanic crust. Here we present an integrated geochemical study on two types of early Mesozoic mafic arc igneous rocks from the East Kunlun Orogen. Zircon U-Pb isotopic analyses yield ages of 252−248 Ma for lamprophyres and 239−238 Ma for diorite porphyries. All the samples display arc-like trace element distribution patterns, high zircon δ18O values, and variably low zircon εHf(t) values. However, significant geochemical distinctions exist in terms of trace element concentrations, radiogenic isotopes, and other geochemical variations between them. The Early Triassic lamprophyres are characterized by significant enrichment in fluid-mobile trace elements and weakly enriched whole-rock Sr-Nd-Hf isotopes, whereas the Middle Triassic diorite porphyries show high contents of light rare earth elements, Th, Zr, and Hf, and more enriched Sr-Nd-Hf isotopes. Furthermore, the lamprophyres exhibit remarkably higher ratios of Ba/Th, Ba/La, K/La, and Sr/Nd and slightly higher ratios of La/Sm, Th/Yb, and Th/La than mid-oceanic-ridge basalt (MORB), while the diorite porphyries display higher La/Sm, Th/Yb, Th/Nd, and Th/La ratios compared to normal MORB but closer to those of seafloor sediments. Taken together, these differences can be attributed to the incorporation of two distinct slab liquids into their mantle sources, including oceanic slab-derived aqueous solutions and minor sediment-derived hydrous melts for the formation of the lamprophyres, and sediment-derived hydrous melts for the formation of the diorite porphyries. As a result, we suggest the lamprophyres were generated during the Early Triassic subduction of the Paleo-Tethyan oceanic crust, while the diorite porphyries may be generated due to rollback of the subducting oceanic slab in response to the closure of the Paleo-Tethyan Ocean basin. Therefore, the studied Early−Middle Triassic mafic igneous rocks provide important evidence for the recycling of the Paleo-Tethyan oceanic slab at different stages.
{"title":"Recycling of Paleo-Tethyan oceanic crust: Geochemical record from Early−Middle Triassic igneous rocks in the East Kunlun Orogen in western China","authors":"Ye-Dan Hu, Li-Tao Ma, Li‐Qun Dai, Zi‐Fu Zhao, Guo-Chao Sun, Bing Gong","doi":"10.1130/b37375.1","DOIUrl":"https://doi.org/10.1130/b37375.1","url":null,"abstract":"Deciphering the contribution of crustal materials to generation of mafic arc igneous rocks at different subduction stages is of great significance to unravel the fate of the subducted paleo-oceanic crust. Here we present an integrated geochemical study on two types of early Mesozoic mafic arc igneous rocks from the East Kunlun Orogen. Zircon U-Pb isotopic analyses yield ages of 252−248 Ma for lamprophyres and 239−238 Ma for diorite porphyries. All the samples display arc-like trace element distribution patterns, high zircon δ18O values, and variably low zircon εHf(t) values. However, significant geochemical distinctions exist in terms of trace element concentrations, radiogenic isotopes, and other geochemical variations between them. The Early Triassic lamprophyres are characterized by significant enrichment in fluid-mobile trace elements and weakly enriched whole-rock Sr-Nd-Hf isotopes, whereas the Middle Triassic diorite porphyries show high contents of light rare earth elements, Th, Zr, and Hf, and more enriched Sr-Nd-Hf isotopes. Furthermore, the lamprophyres exhibit remarkably higher ratios of Ba/Th, Ba/La, K/La, and Sr/Nd and slightly higher ratios of La/Sm, Th/Yb, and Th/La than mid-oceanic-ridge basalt (MORB), while the diorite porphyries display higher La/Sm, Th/Yb, Th/Nd, and Th/La ratios compared to normal MORB but closer to those of seafloor sediments. Taken together, these differences can be attributed to the incorporation of two distinct slab liquids into their mantle sources, including oceanic slab-derived aqueous solutions and minor sediment-derived hydrous melts for the formation of the lamprophyres, and sediment-derived hydrous melts for the formation of the diorite porphyries. As a result, we suggest the lamprophyres were generated during the Early Triassic subduction of the Paleo-Tethyan oceanic crust, while the diorite porphyries may be generated due to rollback of the subducting oceanic slab in response to the closure of the Paleo-Tethyan Ocean basin. Therefore, the studied Early−Middle Triassic mafic igneous rocks provide important evidence for the recycling of the Paleo-Tethyan oceanic slab at different stages.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"221 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140693205","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The clockwise rotation and southeastward extrusion of the southeastern Tibetan Plateau have played important roles in accommodating the uplift and deformation of the plateau. Numerous paleomagnetic studies have suggested post−late Eocene clockwise rotation of the southeastern Tibetan Plateau along the eastern Himalaya syntaxis, whereas few researchers have addressed the specific Eocene deformation, leading to ambiguous interpretations of the tectonic evolution in the region. Herein, we conducted a paleomagnetic study of the Yunlong Formation in the Yunlong Basin, which is Late Cretaceous to early Paleocene in age. In total, 386 oriented samples were collected. Rock magnetic, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) analyses revealed detrital carriers, such as hematite and some magnetite. In thermal demagnetization processes, 332 characteristic remanent magnetizations were isolated, which yielded positive reversals and tilt tests, providing a site-mean direction of declination (Ds) = 56.0° ± 2.6°, inclination (Is) = 34.3° ± 3.8°, α95 = 2.7°, k = 91.0, and N = 31 after tilt correction. Magnetostratigraphic analysis was performed, and a depositional age of 79−61 Ma for the section was obtained, which is consistent with the previous paleontological and detrital zircon ages. Compared with the Eurasia reference pole of the period, the data revealed a 45.2° ± 5.1° clockwise rotation of the Yunlong area since 79−61 Ma. The integrated regional paleomagnetic results suggest the occurrence of ∼20° of clockwise rotation of the Lanping-Simao terrane during the Eocene, which is similar (in terms of magnitude and time of occurrence) to that of the Gonjo Basin in the eastern Qiangtang terrane. Integrated with other lines of geologic evidence, we propose a new deformation model in which the entire southeastern Tibetan Plateau experienced ∼20° of rigid clockwise rotation during the Eocene, followed by subsequent oroclinal bending.
{"title":"Moderate magnitude clockwise rotation of the Yunlong Basin: Implications for synchronous Eocene rotation of the southeastern Tibetan Plateau","authors":"Wanlong Xu, Dawen Zhang, M. Yan, Weilin Zhang, Zhenbei Zhang, Zunbo Xu, Yuwei Zhang, Chunhui Song, Xiaomin Fang","doi":"10.1130/b37395.1","DOIUrl":"https://doi.org/10.1130/b37395.1","url":null,"abstract":"The clockwise rotation and southeastward extrusion of the southeastern Tibetan Plateau have played important roles in accommodating the uplift and deformation of the plateau. Numerous paleomagnetic studies have suggested post−late Eocene clockwise rotation of the southeastern Tibetan Plateau along the eastern Himalaya syntaxis, whereas few researchers have addressed the specific Eocene deformation, leading to ambiguous interpretations of the tectonic evolution in the region. Herein, we conducted a paleomagnetic study of the Yunlong Formation in the Yunlong Basin, which is Late Cretaceous to early Paleocene in age. In total, 386 oriented samples were collected. Rock magnetic, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) analyses revealed detrital carriers, such as hematite and some magnetite. In thermal demagnetization processes, 332 characteristic remanent magnetizations were isolated, which yielded positive reversals and tilt tests, providing a site-mean direction of declination (Ds) = 56.0° ± 2.6°, inclination (Is) = 34.3° ± 3.8°, α95 = 2.7°, k = 91.0, and N = 31 after tilt correction. Magnetostratigraphic analysis was performed, and a depositional age of 79−61 Ma for the section was obtained, which is consistent with the previous paleontological and detrital zircon ages. Compared with the Eurasia reference pole of the period, the data revealed a 45.2° ± 5.1° clockwise rotation of the Yunlong area since 79−61 Ma. The integrated regional paleomagnetic results suggest the occurrence of ∼20° of clockwise rotation of the Lanping-Simao terrane during the Eocene, which is similar (in terms of magnitude and time of occurrence) to that of the Gonjo Basin in the eastern Qiangtang terrane. Integrated with other lines of geologic evidence, we propose a new deformation model in which the entire southeastern Tibetan Plateau experienced ∼20° of rigid clockwise rotation during the Eocene, followed by subsequent oroclinal bending.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"53 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140699244","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Guoqiang Li, Xiaoyan Wang, He Yang, Ming Jin, Caixin Qin, Yixuan Wang, Tara N Jonell, Long Pan, Chunzhu Chen, Wenwei Zhao, Xiaojian Zhang, David B. Madsen
Understanding the mechanisms driving hydrological change in arid Central Asia over a range of time scales is crucial for making predictions for future changes in fragile desert-lake ecosystems. As of yet, the drivers of hydrological changes in lake systems of arid Central Asia over the Holocene remain largely unexplored. Aibi Lake, fed by rivers originating from the glaciated Tianshan Mountains and terminating in the arid Junggar Basin of northwestern China, presents a perfect natural laboratory to explore lake evolution in context to Holocene climate evolution in arid Central Asia. Here, a single-grain K-feldspar dating method was used to effectively date 20 paleolake shorelines with poorly bleached sediment to constrain lake level evolution over the past 18 k.y. Results indicate that Aibi Lake experienced a rapid increase in water levels, reaching a peak of ∼36 m during the early to mid-Holocene period (10−7 ka). Subsequently, the lake level may have shown a general decline during the middle Holocene (7−4 ka), with the lake reaching a low level of less than 10 m at ca. 4 ka. In the late Holocene, lake levels fluctuated by 10−30 m above modern levels during 4−1 ka, with generally low levels of <9 m after 1 ka. The evolution of Aibi Lake underlines a clear out-of-phase relationship between Central Asian lake evolution and Westerlies precipitation changes, where Holocene lake changes were instead more directly controlled by the flux of glacial meltwater from the Tianshan Mountains, driven by change in Northern Hemisphere summer insolation. Glacier meltwater, in combination with variable delivery of Westerlies and East Asian summer monsoon precipitation, are responsible for asynchronous lake evolution trends across Central to East Asia.
了解一系列时间尺度上中亚干旱地区水文变化的驱动机制，对于预测脆弱的沙漠-湖泊生态系统未来的变化至关重要。迄今为止，中亚干旱地区湖泊系统在全新世水文变化的驱动因素在很大程度上仍未得到探索。艾比湖的水源来自于冰川化的天山山脉，终点位于中国西北干旱的准噶尔盆地，它为探索中亚干旱地区全新世气候演变背景下的湖泊演变提供了一个完美的自然实验室。研究结果表明，艾比湖在全新世早中期（10-7 ka）经历了水位的快速上升，并达到了 36 米的峰值。随后，在全新世中期（7-4 ka），湖泊水位可能出现了总体下降，大约在 4 ka 时，湖泊水位降到了 10 m 以下。4 ka.在全新世晚期，4-1 ka 时的湖面水平比现代水平高出 10-30 米，1 ka 后湖面水平普遍较低，<9 米。艾比湖的演变凸显了中亚湖泊演变与西风降水变化之间明显的非同步关系，全新世湖泊的变化更直接地受北半球夏季日照变化驱动的天山冰川融水通量的控制。冰川融水与西风降水量和东亚夏季季风降水量的变化共同作用，导致了中亚至东亚地区湖泊演变趋势的不同步。
{"title":"Asynchronous Holocene lake evolution in arid mid-latitude Asia is driven by glacial meltwater and variations in Westerlies and the East Asian summer monsoon","authors":"Guoqiang Li, Xiaoyan Wang, He Yang, Ming Jin, Caixin Qin, Yixuan Wang, Tara N Jonell, Long Pan, Chunzhu Chen, Wenwei Zhao, Xiaojian Zhang, David B. Madsen","doi":"10.1130/b37288.1","DOIUrl":"https://doi.org/10.1130/b37288.1","url":null,"abstract":"Understanding the mechanisms driving hydrological change in arid Central Asia over a range of time scales is crucial for making predictions for future changes in fragile desert-lake ecosystems. As of yet, the drivers of hydrological changes in lake systems of arid Central Asia over the Holocene remain largely unexplored. Aibi Lake, fed by rivers originating from the glaciated Tianshan Mountains and terminating in the arid Junggar Basin of northwestern China, presents a perfect natural laboratory to explore lake evolution in context to Holocene climate evolution in arid Central Asia. Here, a single-grain K-feldspar dating method was used to effectively date 20 paleolake shorelines with poorly bleached sediment to constrain lake level evolution over the past 18 k.y. Results indicate that Aibi Lake experienced a rapid increase in water levels, reaching a peak of ∼36 m during the early to mid-Holocene period (10−7 ka). Subsequently, the lake level may have shown a general decline during the middle Holocene (7−4 ka), with the lake reaching a low level of less than 10 m at ca. 4 ka. In the late Holocene, lake levels fluctuated by 10−30 m above modern levels during 4−1 ka, with generally low levels of <9 m after 1 ka. The evolution of Aibi Lake underlines a clear out-of-phase relationship between Central Asian lake evolution and Westerlies precipitation changes, where Holocene lake changes were instead more directly controlled by the flux of glacial meltwater from the Tianshan Mountains, driven by change in Northern Hemisphere summer insolation. Glacier meltwater, in combination with variable delivery of Westerlies and East Asian summer monsoon precipitation, are responsible for asynchronous lake evolution trends across Central to East Asia.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"328 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140703703","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
T. Schwartz, S. Wyld, Joseph P. Colgan, Douglas W. Prihar
Fluvial strata of the Upper Triassic Chinle Formation and Dockum Group, exposed across the Western Interior of North America, have long been interpreted to record a transcontinental river system that connected the ancestral Ouachita orogen of Texas and Oklahoma, USA, to the Auld Lang Syne basin of northwestern Nevada, USA, its inferred marine terminus. Fluvial strata are well-characterized by existing detrital zircon data, but the provenance of the Auld Lang Syne basin is poorly constrained. We present new detrital zircon U-Pb and Hf isotopic data that characterize the provenance of Norian siliciclastic strata that dominate the Auld Lang Syne basin. Mixture modeling of Auld Lang Syne basin data identifies the Alleghany−Ouachita−Marathon belt of eastern Laurentia as a dominant source of sediment, but the presence of Triassic detrital zircon grains in Auld Lang Syne basin strata indicates that at least one peri-Laurentian arc segment had to have also contributed sediment. A comparison of new Hf isotopic data with those characterizing various peri-Laurentian volcanic arcs demonstrates that although multiple arc segments may have simultaneously contributed zircons to the Auld Lang Syne basin, the west Pangean arc of northern Mexico stands out as a unique source of highly evolved Permian to Triassic detrital zircon grains in samples from the Auld Lang Syne basin. Altogether, our data and analyses demonstrate source-to-sink connectivity between the Late Triassic (Norian) Cordilleran margin and remnant late Paleozoic highlands of southern to eastern Laurentia, which ultimately framed a Mississippi River−scale, transcontinental watershed that traversed the topographically subdued Laurentian continental interior.
北美西部内陆出露的上三叠统钦勒地层和多库姆组冲积地层长期以来一直被解释为记录了一个横贯大陆的河流系统，该系统将美国得克萨斯州和俄克拉荷马州的祖先瓦奇塔造山带与美国内华达州西北部的奥德朗-锡恩盆地（其推断的海洋终点）连接起来。现有的碎屑锆石数据已经很好地描述了冲积地层的特征，但对 Auld Lang Syne 盆地的产状却没有很好的解释。我们展示了新的碎屑锆石 U-Pb 和 Hf 同位素数据，这些数据描述了主导 Auld Lang Syne 盆地的 Norian 硅碎屑地层的来源特征。Auld Lang Syne 盆地数据的混合建模确定劳伦提亚东部的 Alleghany-Ouachita-Marathon 带是沉积物的主要来源，但 Auld Lang Syne 盆地地层中存在的三叠纪碎屑锆石颗粒表明，至少有一个近劳伦提亚弧段也提供了沉积物。将新的 Hf 同位素数据与描述各种近劳伦伦火山弧特征的数据进行比较后发现，尽管多个火山弧段可能同时为奥德朗-锡恩盆地提供了锆石，但墨西哥北部的西泛大陆弧是奥德朗-锡恩盆地样本中高度演化的二叠纪至三叠纪碎屑锆石颗粒的独特来源。总之，我们的数据和分析表明，晚三叠世（纪）科迪勒拉山边缘与劳伦提亚南部至东部的残余晚古生代高地之间存在源-汇联系，最终形成了密西西比河规模的跨大陆分水岭，横穿地形低缓的劳伦提亚大陆内部。
{"title":"Late Triassic paleogeography of southern Laurentia and its fringing arcs: Insights from detrital zircon U-Pb geochronology and Hf isotope geochemistry, Auld Lang Syne basin (Nevada, USA)","authors":"T. Schwartz, S. Wyld, Joseph P. Colgan, Douglas W. Prihar","doi":"10.1130/b37454.1","DOIUrl":"https://doi.org/10.1130/b37454.1","url":null,"abstract":"Fluvial strata of the Upper Triassic Chinle Formation and Dockum Group, exposed across the Western Interior of North America, have long been interpreted to record a transcontinental river system that connected the ancestral Ouachita orogen of Texas and Oklahoma, USA, to the Auld Lang Syne basin of northwestern Nevada, USA, its inferred marine terminus. Fluvial strata are well-characterized by existing detrital zircon data, but the provenance of the Auld Lang Syne basin is poorly constrained. We present new detrital zircon U-Pb and Hf isotopic data that characterize the provenance of Norian siliciclastic strata that dominate the Auld Lang Syne basin. Mixture modeling of Auld Lang Syne basin data identifies the Alleghany−Ouachita−Marathon belt of eastern Laurentia as a dominant source of sediment, but the presence of Triassic detrital zircon grains in Auld Lang Syne basin strata indicates that at least one peri-Laurentian arc segment had to have also contributed sediment. A comparison of new Hf isotopic data with those characterizing various peri-Laurentian volcanic arcs demonstrates that although multiple arc segments may have simultaneously contributed zircons to the Auld Lang Syne basin, the west Pangean arc of northern Mexico stands out as a unique source of highly evolved Permian to Triassic detrital zircon grains in samples from the Auld Lang Syne basin. Altogether, our data and analyses demonstrate source-to-sink connectivity between the Late Triassic (Norian) Cordilleran margin and remnant late Paleozoic highlands of southern to eastern Laurentia, which ultimately framed a Mississippi River−scale, transcontinental watershed that traversed the topographically subdued Laurentian continental interior.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"276 9","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140703813","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Min Liu, Shao-cong Lai, Shuai Ma, Fang-yi Zhang, R. Zhu, Yu Zhu, Jiangfeng Qin, Bo Xu, Hang Yang
Continental arc magmatism is crucial in producing juvenile continental crust of andesitic to dacitic composition, yet its impact on the formation of the modern mature continental crust, which is more enriched in potassic and granitic components, remains poorly understood. We examined a suite comprised of norite gabbro, gabbroic diorite, high-Mg diorite, dioritic enclave, and medium-K to high-K granites (the Urad Houqi suite) from the Langshan arc in the southern Central Asian Orogenic Belt (CAOB). Zircon U-Pb geochronology indicates that the Urad Houqi suite was formed ca. 278−268 Ma by a continuous magmatic event, marking the final episode of the late Paleozoic continental arc magmatism in the southern CAOB induced by the subduction of the Paleo-Asian Ocean. The suite exhibits a more potassic and fertile composition compared to typical continental arc magmas, and shows a continuous increasing K2O/Na2O ratio and incompatible element concentrations, including K, Rb, Th, and U, with increasing SiO2 content. The geochemical evolution of the suite offers valuable insights into the maturation process of the juvenile arc crust. By combining zircon Hf and bulk-rock Sr-Nd-Pb isotope studies, we demonstrated that the various lithologies within the Urad Houqi suite recorded a three-stage enhancement of the geochemical fertility of the juvenile Langshan arc crust: (1) generation of the initially fertile primary melt from the mantle wedge metasomatized by the recycled subducted sediments; (2) trans-crustal open-system hybridization processes, including peritectic reaction and magma mixing, that further fertilized the derived melts in addition to fractional crystallization; and (3) diverse petrogenetic processes that contributed to the fertility of the granitic magmas, including polybaric fractionation of the primary magma, anatexis of ancient crust, and extraction of high-silica melts from the shallow mush system. The three-stage maturation of the juvenile Langshan arc crust was typically coupled with the accelerated subduction of the Paleo-Asian Ocean since the earliest Permian. Our study finds that the Urad Houqi suite can serve as an example of modern continental crust maturation at continental arc settings.
{"title":"An enhanced chemical maturation of juvenile arc crust recorded by the Urad Houqi intrusive suite in Langshan arc, Central Asia","authors":"Min Liu, Shao-cong Lai, Shuai Ma, Fang-yi Zhang, R. Zhu, Yu Zhu, Jiangfeng Qin, Bo Xu, Hang Yang","doi":"10.1130/b37465.1","DOIUrl":"https://doi.org/10.1130/b37465.1","url":null,"abstract":"Continental arc magmatism is crucial in producing juvenile continental crust of andesitic to dacitic composition, yet its impact on the formation of the modern mature continental crust, which is more enriched in potassic and granitic components, remains poorly understood. We examined a suite comprised of norite gabbro, gabbroic diorite, high-Mg diorite, dioritic enclave, and medium-K to high-K granites (the Urad Houqi suite) from the Langshan arc in the southern Central Asian Orogenic Belt (CAOB). Zircon U-Pb geochronology indicates that the Urad Houqi suite was formed ca. 278−268 Ma by a continuous magmatic event, marking the final episode of the late Paleozoic continental arc magmatism in the southern CAOB induced by the subduction of the Paleo-Asian Ocean. The suite exhibits a more potassic and fertile composition compared to typical continental arc magmas, and shows a continuous increasing K2O/Na2O ratio and incompatible element concentrations, including K, Rb, Th, and U, with increasing SiO2 content. The geochemical evolution of the suite offers valuable insights into the maturation process of the juvenile arc crust. By combining zircon Hf and bulk-rock Sr-Nd-Pb isotope studies, we demonstrated that the various lithologies within the Urad Houqi suite recorded a three-stage enhancement of the geochemical fertility of the juvenile Langshan arc crust: (1) generation of the initially fertile primary melt from the mantle wedge metasomatized by the recycled subducted sediments; (2) trans-crustal open-system hybridization processes, including peritectic reaction and magma mixing, that further fertilized the derived melts in addition to fractional crystallization; and (3) diverse petrogenetic processes that contributed to the fertility of the granitic magmas, including polybaric fractionation of the primary magma, anatexis of ancient crust, and extraction of high-silica melts from the shallow mush system. The three-stage maturation of the juvenile Langshan arc crust was typically coupled with the accelerated subduction of the Paleo-Asian Ocean since the earliest Permian. Our study finds that the Urad Houqi suite can serve as an example of modern continental crust maturation at continental arc settings.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"319 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140703382","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Smellie, Adam P. Martin, Dougal B. Townsend, G. Di Vincenzo
Research into volcanism in southern Victoria Land, Antarctica, within the West Antarctic rift system, has focussed historically on the geochemistry and chronology of its volcanic centers. However, volcanoes in the West Antarctic rift system have also been dramatically influenced by the prevailing eruptive environment. Mason Spur is a middle Miocene to Pleistocene volcanic complex in the Erebus Volcanic Province. The deep interior of the complex is revealed and is used to assess its development under different environmental conditions. Many of the volcanic sequences erupted within ice and are thus ultraproximal compared with marine sediments, which are the usual source of environmental information. The volcanic rocks provide a unique, well-dated record of the terrestrial environmental conditions independent of the marine record, to which they are an important counterbalance. Evidence is provided for conditions varying between ice-free, alpine ice, and regional ice sheets, and direct comparisons are made with the marine record. The first contemporary ice thicknesses are also deduced for the Erebus Volcanic Province. The results significantly advance our understanding of the middle Miocene−Pleistocene Antarctic environment in the coastal Victoria Land region.
{"title":"Linking the terrestrial environmental record at Mason Spur volcanic complex with the middle Miocene−Pleistocene Ross Sea marine record, Antarctica: A history of subaerial (ice-free) eruptions and glaciovolcanism under variable ice thicknesses","authors":"J. Smellie, Adam P. Martin, Dougal B. Townsend, G. Di Vincenzo","doi":"10.1130/b37364.1","DOIUrl":"https://doi.org/10.1130/b37364.1","url":null,"abstract":"Research into volcanism in southern Victoria Land, Antarctica, within the West Antarctic rift system, has focussed historically on the geochemistry and chronology of its volcanic centers. However, volcanoes in the West Antarctic rift system have also been dramatically influenced by the prevailing eruptive environment. Mason Spur is a middle Miocene to Pleistocene volcanic complex in the Erebus Volcanic Province. The deep interior of the complex is revealed and is used to assess its development under different environmental conditions. Many of the volcanic sequences erupted within ice and are thus ultraproximal compared with marine sediments, which are the usual source of environmental information. The volcanic rocks provide a unique, well-dated record of the terrestrial environmental conditions independent of the marine record, to which they are an important counterbalance. Evidence is provided for conditions varying between ice-free, alpine ice, and regional ice sheets, and direct comparisons are made with the marine record. The first contemporary ice thicknesses are also deduced for the Erebus Volcanic Province. The results significantly advance our understanding of the middle Miocene−Pleistocene Antarctic environment in the coastal Victoria Land region.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"13 27","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140711974","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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