Knowledge Management System Of Institute of Oceanology, Chinese Academy of Sciences
|Place of Conferral||中国科学院海洋研究所|
|Keyword||板块俯冲 岩浆活动 资源与气候 新特提斯闭合 太平洋俯冲|
板块俯冲是地表物质输入地球深部的主要途径。俯冲板片通过脱挥发分等过程，触发部分熔融，造成广泛的岩浆活动，促进了元素富集成矿和深部碳再循环，进而对全球资源分布和气候演化产生深远的影响。为探究俯冲带岩浆活动与资源效应的联系，解析其在气候演变过程中的作用，本研究以特提斯和太平洋两大俯冲体系的岩浆活动为研究对象，重点关注新特提斯俯冲闭合和太平洋俯冲过程中的岩浆活动的成矿效应及其与大气二氧化碳含量的耦合效应。在资源效应方面，选取两个典型俯冲构造域发育的优势关键金属Sn-W和Au-Mo成矿区，包括特提斯域的华南南缘-喜马拉雅和太平洋域的胶东半岛，探究其成矿物质来源和富集机制。在气候效应方面，选取新生代从新特提斯碰撞闭合到西太平洋新生代俯冲起始后的弧岩浆活动和关键气候事件为研究对象，重点研究古新世-始新世极热事件(Paleocene-Eocene Thermal Maximum，简称PETM)和始新世长期冷却事件(Long-term Eocene Cooling，简称LTEC)的成因，探讨俯冲带火山活动与关键气候事件的联系及其对全球大气温室气体含量变化的贡献。
(2) 新特提斯闭合阶段喜马拉雅淡色花岗岩形成与稀有金属成矿。二云母花岗岩是喜马拉雅淡色花岗岩带中常见的酸性岩浆岩类型之一，与稀有金属成矿关系密切。一般认为它们是由岩浆分异作用形成的，但淡色花岗质岩浆分异过程中，熔体具体演化和元素迁移过程并不明晰。本论文从微观角度对喜马拉雅淡色花岗岩带中错那地区错那和错那洞两个淡色花岗岩岩体中的电气石主微量元素和B同位素组成开展分析，确定了矿物结晶记录的熔体地球化学成分变化。熔体中挥发分含量与不相容元素正相关，表明挥发分的增加有利于Sr、Zn、Pb等金属元素的富集。相对于错那岩体，错那洞岩体二云母花岗岩中的电气石在岩浆分异过程中结晶更早。电气石B同位素组成表明，淡色花岗质岩浆主要来源于变质沉积岩的部分熔融。由错那洞向错那花岗岩演化，电气石B同位素变轻可能是由于演化过程中流体出溶造成的。错那地区岩浆电气石表现出高Sn和Li等稀有金属元素含量的特征，表明错那地区二云母花岗岩具有成矿潜力，可能与错那地区已经被发现的多金属矿床紧密相关。其中，错那洞热液Sn-W-Be多金属矿床是错那地区典型的淡色花岗岩相关的热液矿床。目前，关于岩浆流体出溶和热液-围岩相互作用对稀有金属元素的富集作用仍不清晰。热液矿床中电气石是主要含B矿物，其地球化学组成和B同位素特征可以记录成矿流体的来源和演化过程。本论文通过对含矿热液石英脉和矽卡岩中的两种热液电气石(Tur-1和Tur-2)进行主微量和B同位素研究，发现Tur-1和Tur-2均具有较高的K和Na含量，属于碱性电气石族。根据电气石的Li-Fe-Mg含量分类，Tur-1属于富Fe的黑电气石，而Tur-2则属于富Mg的镁电气石。Tur-1的δ11B值范围为−13.7至−13.2 ‰，低于Tur-2的δ11B值(−11.1至−9.3 ‰)。岩石接触关系和地球化学组成表明，热液石英脉中Tur-1是由岩浆热液形成的，受围岩影响较小，与错那洞淡色花岗岩有成因联系；而矽卡岩中的Tur-2则有交代结构并混染了大量高δ11B值的大理岩围岩物质。矽卡岩中Tur-2的Sn，Li和Be稀有金属元素含量高于石英热液脉中的Tur-1，且Tur-1和Tur-2中的这些成矿金属元素均高于错那地区二云母花岗岩中的岩浆电气石。因此，本研究认为由变质沉积物部分熔融形成的岩浆经流体出溶，形成富集稀有金属元素的岩浆热液，是错那洞Sn-W-Be多金属矿床形成的必要条件，同时出溶的岩浆热液与大理岩围岩发生的水岩相互作用是促进稀有金属元素进一步富集和沉淀的有利条件。
(3) 太平洋板块俯冲阶段胶东Au，Mo成矿。胶东半岛中生代与花岗质岩浆活动相关的Au，Mo矿床在岩石成因和地球动力学背景上仍存在争议。本论文通过对胶东Mo矿化相关的牙山花岗闪长岩进行锆石U-Pb年龄测定，确定其形成年龄为117.5±1.4 Ma，表明牙山花岗闪长岩属于伟德山期岩浆活动(~123–108 Ma)。同时，牙山花岗闪长岩成岩年龄与牙山Mo成矿年龄一致。该区成岩和成矿紧密的时空相关性，指示它们之间存在成因联系。牙山花岗闪长岩具有低的Sr/Y (48.8–115)和高的(La/Yb)N (23.8–50.4)值和继承锆石，表现出其下地壳来源的埃达克质岩石的特征。其内的基性微粒包体表明了其形成过程中存在基性岩浆交代。同时，锆石中高的Ce4+/Ce3+值表明牙山岩体具有较高的岩浆氧逸度，暗示了在牙山岩体的源区存在板块俯冲物质的参与。因此，本研究认为伟德山期形成的牙山岩体是俯冲物质参与的壳幔相互作用的产物。胶东半岛伟德山期岩浆岩的Dy/Yb、La/Yb和Sr/Y比值明显低于早期的郭家岭期岩浆活动(~136–123 Ma)，表明该时期太平洋俯冲板片回撤过程中，华北克拉通东部岩石圈发生减薄，岩浆熔融深度整体变浅。该时期的酸性岩浆锆石中出现了高的Ce4+/Ce3+和Eu/Eu*值，表明俯冲板块来源的流体和/或熔体参与了其形成过程。同时，相比于胶东早期酸性岩浆，伟德山期酸性岩浆锆石Hf-O同位素成分最接近幔源基性岩浆，说明伟德山期岩浆活动期间壳幔相互作用最为剧烈，俯冲交代富集的幔源物质的混入变多。在地球动力学方面，早期板块俯冲缓慢回撤脱水形成含水富Au的岩石圈地幔薄弱带，并且由于俯冲板片脱水，残余板片上富Mo变质沉积物氧逸度升高。在120 Ma左右，俯冲板块快速回撤的过程中，富Au薄弱带被破坏并释放流体进入地壳深大断裂，造成了大量的Au矿化。同时，通过元素比值和同位素证据，本研究认为滞留板片上的富Mo变质沉积物被活化熔融，并随软流圈地幔上涌，与陆壳发生交代相互作用，是形成胶东伟德山期岩浆活动和小规模的Mo矿化的原因。
(4) 新特提斯闭合与古新世-始新世极热事件(PETM)。在新特提斯洋闭合碰撞期间，印度板块边缘有大量的富有机质黑色页岩被带入俯冲带，通过岩浆活动再循环进入大气可能会引起温室气体浓度变化，进而造成气候响应。根据印度大陆边缘沉积层厚度、俯冲速度和俯冲带长度和PETM持续时间建立箱式模型，本研究发现在PETM期间(~55.9 Ma, 持续170 kyr)大陆碰撞俯冲能够输入7.7×103到2.3×104 GtC的轻碳同位素物质到地幔深部。该输入量远超过了PETM所需要的碳输出量(3.2×103到5.5×103 GtC)。如果再考虑大陆碰撞过程中的俯冲脱碳效率和深部碳的积累，陆弧火山的碳释放可能会更多。因此，在PETM期间，陆弧林子宗年波组火山岩的喷发，将释放大量具有轻碳同位素的温室气体，可能对PETM期间地球表层碳含量和同位素的波动有一定的贡献。
(5) 西太平洋俯冲起始与始新世长期冷却事件(LTEC)。在约51Ma新特提斯洋碰撞闭合诱发西太平洋板块俯冲起始，全球俯冲带体系从特提斯型的被动陆缘俯冲碰撞为主导，逐渐转变为了太平洋型的大洋板块俯冲。特提斯被动陆缘沉积物厚，水深在碳酸盐补偿深度(CCD)以浅，而太平洋板块沉积物薄，水深在CCD以深，因此这种转变会让俯冲进入地幔的沉积地层变薄，同时沉积碳输入量也会降低，进而会造成弧火山岩浆活动过程中碳释放量的减少。本研究通过火山喷发体积计算，发现现今西太平洋俯冲带火山年喷发量达到1.11±0.45 km3，全球火山喷发量为2.98±0.30 km3。假设新生代火山和现在一样强烈，那么每年将有2.22×1012 kg的弧火山灰进入海洋。考虑到火山灰快速的化学风化速率，那么弧火山灰每年通过化学风化作用释放的阳离子数量，与现今河流风化输入海洋的阳离子量相当。通过海气界面交换，海水中增加的阳离子会促进固碳反应的发生，使大气碳进入海水和沉积物，从而减少大气中的二氧化碳含量。因此，本研究认为特提斯硬碰撞的开始导致西太平洋俯冲起始，这种俯冲体制的转换可能促进了51Ma之后大气二氧化碳含量的持续下降，进而造成了LTEC事件的发生。
Plate subduction is a crucial mechanism for the transportation of surface materials to the deep Earth, triggering partial melting and magmatic activity through devolatilization processes. This leads to the enrichment of elements and deep carbon recycling, with significant implications for global resource distribution and climate evolution. This study aims to investigate the relationship between subduction-related magmatic activity and resource effects, as well as their role in the process of climate change. Two typical subduction tectonic domains were selected to investigate the enrichment and mineralization mechanisms, including the Sn-W and Au-Mo mineralization areas developed in the South China-Himalaya of the Neo-Tethys domain and the Jiaodong Peninsula of the Pacific domain, respectively. To explore the relationship between subduction-related volcanic activity and key climate events, as well as their contribution to changes in the global atmospheric greenhouse gas content, we researched the arc magmatic activity and climate events from the closure of the Neo-Tethys Ocean to the beginning of the Cenozoic west Pacific subduction, with a particular focus on the causes of Paleocene-Eocene Thermal Maximum (PETM) and the Long-term Eocene Cooling (LTEC) events.
(1) Sn-W mineralization in the southern margin of South China during the subduction stage of the Neo-Tethys. Granite enriched in Sn-W elements is closely related to its high differentiation and volatile contents. However, there is still a debate over whether the ore-forming materials' enrichment is due to inherent enrichment in the source area or a high degree of differentiation during the magmatic process. The Xishan Sn-W deposit in the southern margin of South China has a temporal and spatial association with K-feldspar granite, indicating a close connection with the addition of material from a subducted slab. The K-feldspar granite exhibits the characteristics of light K and Li isotopes and high F and low Cl contents. It suggests that the process of subduction and roll-back of the Neo-Tethys oceanic plate led to the decomposition of F-rich metamorphic minerals like phengite, which underwent crust-mantle interaction with mantle upwelling. The high F content of granite indicates that the addition of F-rich mantle source material is crucial for the enrichment of Sn and W. Moreover, the granite underwent an evolution from the characteristics of light K and Li isotopes, low F, and W-rich to the characteristics of heavy K and Li isotopes, high F, and Sn-rich, reflecting the increasing degree of crust-mantle interactions. This was due to the increase of subduction material input and melting temperature, and the changing of crustal source melting material from W-rich muscovite to Sn-rich biotite. This study highlights the importance of F-rich fluids released during the decomposition of phengite during plate subduction and roll-back. The F-rich fluids in the crust-mantle interaction process and magma activity promoted the enrichment and extraction of Sn and W ore-forming elements in the crust.
(2) Formation of Himalayan leucogranites and rare metal mineralization during the closure stage of the Neo-Tethys. Two-mica granite is a common magmatic rock type in the Himalayan leucogranite belt and is closely related to rare metal mineralization. The process of magma differentiation and element migration during the formation of leucogranitic magma and its associated rare metal mineralization is not fully understood. In this study, the major-trace elements and B isotopes compositions of tourmaline from two leucogranite bodies, Cuona and Cuonadong, in the Cuona area of the Himalayan leucogranite belt were measured. The geochemical composition changes of the melt recorded by tourmaline were determined. This study finds that the volatile content in the melt is positively correlated with incompatible elements, which suggests that the increase in volatiles may contribute to the enrichment of metal elements such as Sr, Zn, and Pb. The Cuonadong tourmaline crystallized earlier during magma differentiation compared to the Cuona tourmaline, and the tourmaline B isotope composition indicates that the leucogranitic magma is mainly derived from partial melting of metamorphic sedimentary rocks. The light of tourmaline B isotope may be caused by fluid exsolution during the evolution from Cuonadong to Cuona leucogranites. The high content of rare metal elements such as Sn and Li in the magmatic tourmaline in the Cuona area indicates that the two-mica granite in the Cuona area has mineralization potential and may be closely related to the discovered polymetallic deposit in the Cuona area, and the Cuonadong hydrothermal Sn-W-Be polymetallic deposit is a typical hydrothermal deposit related to leucogranite in the Cuona area. However, the mechanism of rare metal element enrichment through magma fluid exsolution and water-rock interaction remains unclear. This study examines two types of hydrothermal tourmaline (Tur-1 and Tur-2) from mineralized hydrothermal quartz veins and skarn to clarify the source and evolution process of ore-forming fluids. Both Tur-1 and Tur-2 have high K and Na contents and belong to the alkali tourmaline group. Based on the tourmaline Li-Fe-Mg content, Tur-1 belongs to the Fe-rich schorl, while Tur-2 belongs to the Mg-rich dravite. The δ11B values of Tur-1 range from −13.7 to −13.2 ‰, which are lower than those of Tur-2 (−11.1 to −9.3 ‰). The study finds that Tur-1 in the hydrothermal quartz veins was formed by magmatic-hydrothermal fluids with little influence from the surrounding rocks and is related to the Cuonadong leucogranite. In contrast, Tur-2 in the skarn has a metasomatic structure and is mixed with a large amount of high-δ11B value marble material. The rare metal element contents of Sn, Li, and Be in Tur-2 from the skarn are higher than those in Tur-1 from the quartz veins, and both Tur-1 and Tur-2 have higher contents of these ore-forming metals than the magmatic tourmaline in the two-mica leucogranite in the Cuona area. Therefore, this study suggests that the fluid exsolution enriched in rare metal elements from the magma, which formed by partial melting of metasediment, is a necessary condition for the formation of the Cuonadong Sn-W-Be polymetallic deposit. Additionally, the water-rock interaction between the exsolved fluid and the marble promotes further enrichment and precipitation of rare metal elements.
(3) Gold, molybdenum mineralization in Jiaodong Peninsula during the subduction of the Pacific plate. The origin and geodynamic background of Mesozoic gold-molybdenum deposits related to granitic magmatism in the Jiaodong Peninsula are still a subject of debate. This study aimed to shed light on the issue by conducting zircon U-Pb dating of the Yashan granodiorite associated with molybdenum mineralization in Jiaodong. The results show that the Yashan granodiorite was formed during the Weideshan period magmatic activity (~123–108 Ma) with a formation age of 117.5±1.4 Ma, which is consistent with the Mo mineralization age. The Yashan granodiorite has low Sr/Y (48.8–115) and high (La/Yb)N (23.8–50.4) values and inherited zircons, suggesting its origin from low crust-derived adakitic rocks. Additionally, mafic magmatic enclaves in the Yashan granodiorite indicate the involvement of basic magma during its formation. The high Ce4+/Ce3+ values in the zircons suggest a high magma oxygen fugacity, implying the involvement of subducted materials in the source of the Yashan pluton. Thus, this study suggests that the Yashan pluton formed through the crust-mantle interaction during the Weideshan period, which involved subducted materials. The Dy/Yb, La/Yb, and Sr/Y ratios of the Weideshan period magmatic rocks in the Jiaodong Peninsula are significantly lower than those of the Guojialing period magmatic activity (~136–123 Ma), indicating that the lithosphere of the eastern North China Craton was thinned during the roll-back of the subducting Pacific plate, and the depth of source melting became shallower. The high zircon Ce4+/Ce3+ and Eu/Eu* values in the Weideshan period granitic rocks suggest the involvement of subduction-related fluids and/or melts. The zircon Hf-O isotopic compositions of the Weideshan period granitic rocks are closer to those of the mantle-derived basic magmas, indicating that the more intense crust-mantle interaction happened in the Weideshan period, and more subducted materials were involved. The early slow dehydration of the subducting Pacific plate happened during the slow roll-back stage, which formed a weakened layer in the lithospheric mantle enriched with water and gold. Due to the dehydration of the subducting plate, the oxygen fugacity of the molybdenum-rich metamorphic sediments on the residual plate increased. Around 120 Ma, during the rapid roll-back of the subducting plate, the gold-rich weakened layer was disrupted and released fluids into the deep crustal faults, resulting in abundant gold mineralization. Based on elemental ratios and isotopic evidence, this study suggests that the molybdenum-rich metamorphic sedimentary rocks on the stagnant slab were activated and melted, and then upwelled along the asthenosphere mantle, interacting with the continental crust, which led to the formation of the Jiaodong Weideshan period magmatic activity and small-scale molybdenum mineralization.
(4) The closure of the Neo-Tethys Ocean and the Paleocene-Eocene Thermal Maximum (PETM). As the Neo-Tethys Ocean closed, a significant amount of organic-rich black shale was subducted at the continental margin of the Indian Plate. Magmatic activity recycled this material into the atmosphere, potentially causing changes in greenhouse gas concentrations and subsequent climate responses. Using a box model based on sedimentary layer thickness, subduction velocity, subduction zone length, and PETM duration at the Indian continental margin, this study found that during the PETM period (~55.9 Ma, lasted 170 kyr), continental subduction could have introduced 7.7×103 to 2.3×104 GtC of light carbon isotope material into the deep mantle. This input greatly exceeds the carbon output required for causing the PETM (3.2×103 to 5.5×103 GtC). When considering subduction decarbonization efficiency and deep carbon accumulation during continental collision, the carbon release from arc volcanoes may be even greater. Therefore, during the PETM period, the eruption of volcanic rocks in the continental arc, such as the Nianbo Group of Linzizong volcanic rock, may have released a large amount of greenhouse gases with light carbon isotopes, potentially contributing to fluctuations in surface carbon content and isotopes during the PETM.
(5) West Pacific subduction initiation and the Long-term Eocene Cooling (LTEC). The closure of the Neo-Tethys Ocean triggered the subduction initiation of the west Pacific plate at around 51 Ma. The global subduction system changed from the passive margin subduction of the Neo-Tethys type to the oceanic plate subduction of the Pacific type. The sediment thickness of the passive margin in the Neo-Tethys type was thick, and the water depth was shallower than the carbonate compensation depth (CCD). In contrast, the sediment thickness of the Pacific plate was thin, and the water depth was deeper than the CCD. Therefore, this transition caused the subduction of sedimentary layers to become thinner, and the subducting carbon decreased. This, in turn, led to a reduction in the amount of carbon released during arc volcanic activity. This study calculated the volume of volcanic eruptions in the west Pacific subduction zone and the whole world, and found that the annual eruption volumes are 1.11±0.45 km3 and 2.98±0.30 km3, respectively. Assuming that the intensity of volcanic activity in the Cenozoic era was similar to the present, approximately 2.22×1012 kg of arc volcanic ash would enter the ocean each year. Considering the rapid chemical weathering rate of volcanic ash, the amount of cations released through chemical weathering of arc volcanic ash is equivalent to the amount of cations input into the ocean through river weathering. The increase in cations in seawater through sea-air exchange promotes carbon fixation, which causes atmospheric carbon to enter the ocean and sediment, thereby reducing atmospheric CO2 content. Therefore, this study suggests that the west Pacific subduction initiation caused by the hard Neo-Tethys collision, and this transition of global subduction patterns may have contributed to the sustained decrease in atmospheric CO2 content after 51 Ma, which led to the occurrence of the LTEC.
In conclusion, the subduction of the Neo-Tethys Ocean facilitated the interaction between the crust and mantle, leading to mineralization in various regions. F-rich fluids released from the subducting plate promoted Sn-W mineralization in the southern margin of South China during the subduction stage. In the closure stage, passive margin sedimentary layers underwent decarbonation reactions, contributing to the occurrence of the PETM. The Himalayan leucogranitic rocks, which were derived from metasediments, were gradually enriched in Sn, Li, and Be elements through processes such as fractional crystallization, fluid exsolution, and water-rock interaction, promoting rare metal mineralization during the magmatic-hydrothermal activities. During the Pacific subduction stage, melting of the residual high oxygen fugacity and Mo-rich metasediments and crust-mantle interactions resulted in the formation of the Weideshan period magmatic rocks and small-scale Mo mineralization in the Jiaodong Peninsula. The transition of global subduction patterns after the west Pacific subduction initiation led to a reduction in carbonate input and carbon release from arc volcanoes. Arc volcanic ash, on the other hand, enhanced carbon sequestration, leading to a sustained decrease in atmospheric CO2 and the occurrence of the LTEC.
|MOST Discipline Catalogue||理学::海洋科学|
|谢国治. 板块俯冲带岩浆活动的资源与气候效应——西太平洋与新特提斯俯冲带典型地区对比[D]. 中国科学院海洋研究所. 中国科学院大学,2023.|
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