KMS XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY，CAS
| 塔吉克帕米尔地区 Pshart 杂岩年代学和地球化学特征及其构造意义 | |
| Alternative Title | The Geochronology and Geochemistry of Tajik Pamir Pshart Complex and its tectonic implications |
| Dzhovid Yogibekov | |
| Subtype | 硕士 |
| Thesis Advisor | 肖文交 |
| 2020-06-30 | |
| Degree Grantor | 中国科学院大学 |
| Place of Conferral | 北京 |
| Degree Discipline | 工程硕士 |
| Keyword | 帕米尔 Pshart 增生杂岩 Rushan-Pshart 洋 古特提斯 中特提斯 Pamirs Pshart accretionary complex Rushan-Pshart Ocean PaleoTethys MesoTethys |
| Abstract | 增生型造山带是由洋壳持续俯冲导致的俯冲带之上岛弧、蛇绿混杂岩、 海山/洋底高原等不断拼贴增生而形成。在大洋板块俯冲过程中，洋壳上覆岩石和沉积物从俯冲板块转移到上覆板块，从而形成增生楔，又名增生杂岩。对增生杂岩的解剖是理解增生造山带构造特征的关键所在。增生杂岩记录了大洋板块地层学（OPS）的信息，这些信息保留了大洋板块物质从其在洋中脊诞生到海沟中死亡的整个寿命历史。增生杂岩中的火成岩可能源自不同的构造环境，具有洋脊玄武岩，洋岛玄武岩/海山和大洋高原玄武岩的特征。帕米尔高原被划分为北帕米尔、中帕米尔和南帕米尔，是青藏高原的西向延伸部分，是研究大陆增生、地壳生长的天然实验室。塔尼玛斯（Tanymas） 和鲁尚-普沙尔特（Rushan-Pshart） 缝合带分别是北帕米尔、中帕米尔和南帕米尔之间的界限。其中，鲁尚-普沙尔特缝合带保留了消减的鲁尚-普沙尔特洋盆的残余信息。然而由于该区域缺乏年代学和地球化学数据，导致关于鲁尚-普沙尔特洋俯冲-增生演化历史的几个问题尚未解决：普沙尔特杂岩的物质成分属性， 洋盆的俯冲极性，以及洋盆的闭合时间。本文选择普沙尔特增生杂岩作为研究对象，通过解析增生杂岩中的沉积物和基性岩的结构-构造特征，建立鲁尚-普沙尔特洋的俯冲增生演化模型。具体内容包括（1）厘清普沙尔特增生杂岩中的物质组成，及其构造接触关系；（2）分析沉积物物源和火成岩的构造背景；（3）建立鲁尚-普沙尔特洋俯冲-增生演化模型。本文对普沙尔特地区肯基尔加（Ken Djilga）和卡拉基尔加（Kara Djilga-2）两条河谷分别进行了大比例尺野外岩性-构造地质剖面填图，并采集了相关样品。野外填图显示普沙尔特增生杂岩主要由连续的沉积单元和混杂的基性岩组合构成。连续单元主要由砂岩/粉砂岩基质组成，其中局部夹有不同大小的层状硅质岩，灰岩等，呈现出岩块-基质的结构特征。混杂岩单元主要由玄武岩和辉长岩等基性岩构成。大比例尺野外岩性-构造地质剖面填图揭示了大量岩块-基质的结构特征，以及主要发育向北逆冲的叠瓦扇和双冲构造。本文对所采集样品中的基性岩、沉积岩以及侵入岩分别进行了岩石薄片鉴定、锆石U-Pb 定年，对基性岩和侵入岩开展了地球化学分析。地质年代学 U-Pb 同位素结果表明，沉积岩的年龄分布范围很广，从晚三叠纪到前寒武纪（主要是新元古代，几乎没有太古宙时代的锆石） 。其最小锆石年龄为 212 Ma，为沉积岩提供了最大的沉积年龄，并且该年龄比前人提出的石炭纪-二叠纪年龄年轻。连续沉积单元最下部是变形较弱的变质砂岩，最小锆石年龄为 528 Ma，本文推测它们源自帕米尔高原南部或中部的基底，其年龄范围类似南羌塘的沉积基底。这些沉积物中的三叠纪碎屑锆石主要来自三叠系卡拉库尔-马扎儿（Karakul Mazar） 增生弧复合体（北帕米尔高原）和巴什古姆巴兹（Bashgumbaz） 岩浆弧。 此外， 本文首次报道了 Pshart 杂岩中花岗岩的侵位时间为早白垩世（117.2±3.2 至 123.9±2.3 Ma） 。本文主要用高场强元素（HFSE）（如 Ti、 Zr、 Y、 Nb、 Ta 和 Hf）、 Th 和稀土元素（REE）讨论岩石成因和构造环境，因为这些元素是不活动或者低活动性元素，不容易受海底热液蚀变、俯冲-增生变形和变质作用的影响。 地球化学数据表明，普沙尔特杂岩中的玄武岩具有相对低的 SiO2（44.67~51.76 wt%） 含量， 以及高的 TiO2（1.50~4.61 wt%）、Fe2O3T（11.33~17.38 wt%）和 MgO（4.29~13.30 wt%）含量。 它们的 Mg＃变化于 44.2 至50.9 之间。在原始地幔标准化的微量元素蛛网图上，所有样品都显示出明显的 Nb 和 Ta正异常，与 OIB 型岩石相似，指示他们可能是地幔柱来源的熔体。在 Hf/3-Th-Ta 和 Zr/Y与 Zr 的判别图解中，所有样品均在投在板内玄武岩区域。在 Th/Yb-Nb/Yb 和 Nb/Y-Zr/Y判别图解中，所有样本都投在 OIB 型岩石区域。地球化学结果表明， 普沙尔特杂岩中的玄武岩形成于海山环境，并与砂岩、灰岩和硅质岩等共同产出。白垩纪花岗岩的 SiO2 含量为 70.32 至 74.40 wt%，具有较高的铝饱和指数（A/CNK）[(Al2O3/CaO+Na2O+K2O） mol]（1.56~1.64）。它们的 A/NK 为 1.66~1.83， 所有样品均为高钾钙碱性岩石，并显示出明显强过铝质 S 型花岗岩的地球化学特征。这些花岗岩样品具有中等至相对高的 Al2O3 含量（13.49~15.53 wt%）、低的 TiO2含量（0.05~0.47 wt%）以及较高的 Al2O3/TiO2比值（31.43~310.06）。它们的 K2O 含量为 4.21~5.79 wt%， Na2O 含量为 3.17~3.95 wt%， K2O/Na2O 比值为 1.21~1.83。样品的 Fe2O3T含量变化于 0.48 至 2.81wt%之间， MgO 含量变化于 0.13 至 0.87 wt%之间， Mg＃为 26.3~46.4。在原始地幔标准化的微量元素蜘蛛图上，所有样品均显示出明显的 LREE、 Rb、 U 和 Ta 富集、负 Eu 异常以及 Nb、 Sr、 Eu 和 Th 的亏损（除了样品 Psh-3、 4a 显示出正的 Th 异常）。所有地球化学数据都表明，本文研究的强过铝质花岗岩是由变沉积岩（泥质岩为主）部分熔融产生。辉长-辉绿岩墙是镁铁质的，具有相对较低的 SiO2 含量（49.77~50.71 wt%），高的Fe2O3T含量（8.73~9.48 wt%）以及高的 MgO 含量（7.93~8.58 wt%）。它们的 Mg＃变化于 66.1 至 62.4 之间。样品的 K2O 含量为 1.66~1.80 wt%， Na2O 含量为 2.84~3.22 wt%，K2O/Na2O 比值为 0.51~0.63。样品具有钾玄质岩石的地球化学特征，显示出 LILE 和LREE 的富集以及 HFSE 和 HREE 的亏损， 在 K2O-Na2O、 Zr/Y-Zr、 Th/Yb-Nb/Yb、 Hf/3-Th-Ta 和 Nb/Y-Zr/Y 图解中，样品投图点分别位于岛弧玄武岩和板内玄武岩范围内。综上所述，通过野外大比例尺岩性-构造地质填图与构造解析、辅以年代学和地球化学数据分析，取得以下主要创新成果与认识：（1） 野外工作表明东普沙尔特区域沉积混杂带中具有岩块-基质结构的物质组分、结合前人蛇绿混杂岩的报道， 本文厘定了普沙尔特杂岩的俯冲-增生属性。 最年轻的碎屑锆石年龄显示沉积物的最小沉积年龄为晚三叠世， 表明鲁尚-普沙尔特洋在晚三叠世仍然没有闭合。（2） 大量的碎屑沉积物源自北帕米尔卡拉库尔-马扎儿岛弧和巴士古姆巴兹岛弧，而古老的碎屑沉积物源自南帕米尔。野外观察、碎屑锆石记录以及普沙尔特增生中杂岩北部和南部区域里的北向逆冲构造，共同揭示了鲁尚-普沙尔特洋的南向俯冲极性。（3） 花岗岩年龄数据表明其形成于白垩纪，可能为后造山产物。地球化学数据表明普沙尔特碱性玄武岩源自洋岛构造背景，具有 OIB 特征、是海山残片， 在鲁尚-普沙尔特洋彻底闭合前的南向俯冲期间于中-晚三叠世逐步就位至普沙尔特增生杂岩中。（4）普沙尔特增生杂岩形成于中特提斯洋南向俯冲和闭合期间的俯冲-增生过程中。本文建立了鲁尚-普沙尔特洋俯冲期间海山增生的构造演化模型这些成果具有重要理论意义，首次对帕米尔普沙尔特增生杂岩进行了高精度年代学和地球化学研究，有助于解决长期针对古特提斯洋俯冲极性和最终闭合时间的重大争议问题，并示范了通过野外大比例尺构造填图、使用大洋板块地层学来解决古老造山带增生楔构造解析难点的关键作用。 |
| Other Abstract | Accretionary orogens form where the oceanic plates subduct beneath the continental oroceanic crust. During the oceanic crust subducting, it transfers the sediments and rocks from thedowngoing plate to overriding plate, thus forming accretionary prism (accretionary complex).The anatomy of accretionary complexes is the most important part to unravel accretionaryorogeny and reconstruct the tectonic history of the former oceans and adjacent regions. Anaccretionary complex contains a number of fragments of the oceanic plate stratigraphy (OPS),which preserves the history of the OPS material from its birth in the mid-ocean ridge to its deathin the trench. The tectonic slices of the volcanic rocks hosted by the accretionary complexes mayhave formed in different tectonic setting and bear characteristics of mid-ocean ridge basalts,ocean island basalts/or seamounts and ocean plateau basalts.As the western continuation of the Tibetan Plateau, the Pamirs is divided into Northern,Central and Southern part, which is considered to be a nature laboratory for studying crustalgrowth. The PaleoTethys (Tanymas) and MesoTethys (Rushan-Pshart) suture zones mark theboundaries between the Northern, Central and Southern Pamirs terranes, respectively. TheRushan-Pshart suture zone preserves the remnant of the consumed Rushan-Pshart Ocean. ThePshart accretionary complex is the focus of our study. Late Paleozoic-Mesozoic volcanosedimentary rocks, within the Pshart accretionary complex in the Pamirs, bear criticalinformation of the subduction-accretion history of the Rushan-Pshart Ocean (MesoTethys) priorto collision between the Central and Southern Pamirs. However, several issues concerning theevolutionary history of the Rushan-Pshart Ocean remain unresolved because of insufficientgeochronological and geochemical data. Among them, 1) the subduction polarity, 2) closure timeof the ocean and 3) the magma source of the Cretaceous granitoids that intruded into the Pshartaccretionary complex are the most focused issues.Pshart complex is the target of the research objective in this study. By means of detailedstructural, geochronological and geochemical study of a fossil accretionary complex in the PshartRegion, this paper analyzes the evolution and ocean plate stratigraphy of the Rushan-PshartOcean. The specific research contents include: (1) clarifying the components of Pshartaccretionary complex and their tectonic relations, (2) constraining the sedimentary provenance and tectonic settings of basic rocks, (3) building the subduction-accretion tectonic model ofRushan-Pshart Ocean.For this study, we selected two valleys (Ken Djilga and Kara Djilga 2) for geologicalmapping and sample collection. To the north of eastern Pshart zone along Ken Djilga River twomain tectonic assemblages have been observed: the coherent unit and mélange. The coherent unitmostly consists of sandstone/siltstone matrix with different sizes of interbedded chert, limestone,silicified limestone, tuff and interlayered chert with limestone blocks, which resemble blocks inthe sandstone/siltstone matrix. The limestone blocks occasionally have thrust-duplex structure.The mélange is mainly composed of basic rocks, including basalt and gabbro. Both the coherentunit and mélange are strongly deformed and display northward trusting. To the south along theKara Djilga 2 River, several units also display northward tectonic thrusting. Through large-scalegeological mapping and structural analysis within cross-sections of Kara Djilga 2 and Ken Djilga,we clarified vital components of Pshart accretionary complex, and uncovered well developedmélanges with block-in-matrix structure, and duplexes & imbricate fans as major structuralpatterns.In this study, we conducted zircon U-Pb dating of the sedimentary rocks, and basic rocks,and carried out geochemical studies of igneous rocks in the northern and southern parts of theeastern Pshart accretionary complex. The results from the geochronological U-Pb isotopic studyrevealed a wide range of the age distribution for the sedimentary rocks, which ranges from theLate Triassic to Precambrian (mostly Neoproterozoic with few zircons of Archean ages). Theyoungest age of 212 Ma from a single grain provided maximum deposition age for thesedimentary rock. This age is younger than previously proposed Carboniferous-Permian ages forthe sedimentary mélange. Weakly metamorphosed sandstones with the youngest age of 528 Malie on the lowermost part of the sedimentary mélanges, which we consider to belong to thebasement of the Southern or Central Pamirs; they have a similar age as the metasedimentarybasement of the South Qiantang terrane. Detritus of the Triassic age in these rocks wereprimarily sourced from the Triassic Karakul Mazar arc-accretionary complex (Northern Pamirs)and Bashgumbaz magmatic arc, which developed on the northern margin of Southern Pamirs inresponse to the southward subduction of the Rushan-Pshart Oceanic lithosphere beneathSouthern Pamirs terrane. In additional this paper firstly reported the U-Pb zircon ages for graniticsamples of 117.2±3.2 to 123.9±2.3 Ma within the Pshart complex.We used major and trace elements as an indicator of the tectonic setting for the igneous rocks.The elements such as Na, K and the low field strength elements are highly mobile and notsustainable against alterations. In order to avoid misinterpretations, we used the high fieldstrength elements (HFSE) (Ti, Zr, Y, Nb, Ta, Hf), Th and rare earth elements (REE), which areessentially immobile or have fairly low mobility during the seafloor-hydrothermal alteration,subduction-accretion deformation and metamorphism. All our geochemical analyzed maficsamples of Permian-Triassic are alkaline-basalts. The samples are characterized by relativelylarge range of SiO2 (44.67 to 51.76 wt%), high TiO2 (1.5 to 4.61 wt%), Fe2O3T(11.33 to 17.38wt% ) and MgO (4.29 to 13.30 wt%) with Mg# varying from 44.2 to 50.9. On primitive-mantlenormalized trace element spider diagrams, all samples have typical hump-shaped patterns, whichare inherent in the OIB, with positive anomalies of Nb and Ta, which are indicative of a plumederived melt. In the Hf/3-Th-Ta and Zr/Y versus Zr diagrams, all samples plot in the within-platefield. In the Th/Yb-Nb/Yb and Nb/Y-Zr/Y diagrams, all samples plot in the OIB field. Thegeochemical results show that the mafic rocks of the Permian and Triassic ages are formed in anocean island setting. It is consistent with their rock association (sandstone, siliceous shale andchert), which has been described in many accretionary complexes around the world inassociation with ocean island basalts.The Cretaceous granites have 70-74 wt% SiO2, with high alumina saturated index of A/CNK[Al2O3/(CaO+Na2O +K2O) mol)] and A/NK (Al/ Na2O +K2O) mol.), ranging from 1.56 to 1.64 and1.66 to 1.83, respectively. In SiO2 vs K2O diagram the samples classified as high-K calc-alkalinerocks. The granites have a pronounced strongly peraluminous S-type granite affinity. Thesamples show moderate to relatively high content of Al2O3 ranging from 13.49 to 15.53 wt% andlow content of TiO2 from 0.05 to 0.47 wt%, with Al2O3 /TiO2 ratio in the range from 31.43 to310.06. The K2O ranges from 4.21 wt% to 5.79 wt% and Na2O in the range from 3.17 wt% to3.95 wt%, with K2O/ Na2O ratio from 1.21 to 1.83. The Fe2O3T ranges from 0.48 to 2.81 wt%and MgO ranges from 0.13 to 0.87 wt% with Mg# between 26.3 and 46.4. On the primitivenormalized spidergram, significant LREE enrichment and a negative Eu anomaly, as well asdepletion in Nb, Sr, Eu and Th (conversely the samples Psh-3, 4a show positive anomaly on Th)and enrichment in Rb, U and Ta. All geochemical features display that the highly peraluminousgranites from our study were originated from the partial melting of metasedimentary rocks(pelite-dominant source).The gabbro-diabase dikes relatively low SiO2 (49.77 to 50.71 wt%), high content of Fe2O3T(8.73 to 9.48 wt%) and high MgO content (7.93 to 8.58 wt%), with Mg# between 66.1 to 62.4.The K2O content ranges from 1.66 to 1.80 wt% and Na2O ranges from 2.84 to 3.22 wt%, withK2O/ Na2O ratio ranging from 0.51 to 0.63. The samples are exhibit shoshonitic affinity andshow enrichment in LILE and LREE and depletion in HFSE and HREE with negative Ta and Nbanomalies. In the K2O vs Na2O, Zr/Y versus Zr, Th/Yb vs Nb/Yb and Hf/3 vs Th vs Ta, andNb/Y vs Zr/Y diagrams, the samples plot in the shoshonitic, island arc basalts fields and withinplate field respectively. The combination of major and trace element diagrams suggests that therocks were formed in an extension setting and possess island arc signature.Through large-scale geological mapping and structural analysis, together with comprehensivelaboratory study the main innovative conclusions and achievement were obtained as follows:1. Our field study reveals that block-in-matrix components within some sedimentarymélanges are the main characteristics of the East Pshart area. Together with some previouslydefined ophiolitic mélanges, we firstly define the Pshart AC. The ages of detrital zircons fromthe sedimentary mélange indicate that the age of deposition of the sedimentary rocks was at 212Ma. Combined with previous fossil evidence, the sediments in the study area vary from LatePaleozoic to Late Triassic, indicating that the Pshart AC may have existed and the Rushan-PshartOcean remained open in the Late Triassic.2. Most detrital sediments were derived from the North Pamir/Karakul Mazar Arc andproximal Bashgumbaz Arc and older detritus from the South Pamir. Field observations combinedwith detrital zircon data document northward thrusting in the northern and southern parts of thePshart accretionary complex, suggesting a southward polarity for the subduction of the RushanPshart Ocean.3. The granite ages indicate it may be produced in the Cretaceous. Geochemical data ofPshart alkaline basaltic rocks suggest it formed in an oceanic island, suggesting a typical OIBaffinity, which we interpret as fragments of seamounts. They were incorporated into theaccretionary complex during southward subduction of the Rushan-Pshart Ocean in the Middleand Late Triassic, before the complete consumption of the Rushan-Pshart Ocean.5. The Pshart AC formed by subduction-accretion processes during the southward subductionand closure of the Meso-Tethys Ocean and the final docking of the Central and South Pamir after the Late Triassic. The tectonic evolution of seamount accretion during Rushan-Pshart subductionwas reconstructed.These results have profound significance, which help to provide more evidence and data forresolving the long-standing controversies with respect to the subduction polarity and the timingof the final closure of the Tethys Ocean, and play a pivotal role in solving the difficulties in theanatomy of accretionary complex in an ancient orogen through large-scale field mapping andOPS study. |
| Subject Area | 地球探测和信息技术 |
| Language | 英语 |
| Document Type | 学位论文 |
| Identifier | http://ir.xjlas.org/handle/365004/15390 |
| Collection | 中国科学院新疆生态与地理研究所 研究系统 |
| Affiliation | 中国科学院新疆生态与地理研究所 |
| First Author Affilication | 中国科学院新疆生态与地理研究所 |
| Recommended Citation GB/T 7714 | Dzhovid Yogibekov. 塔吉克帕米尔地区 Pshart 杂岩年代学和地球化学特征及其构造意义[D]. 北京. 中国科学院大学,2020. |
| Files in This Item: | There are no files associated with this item. | |||||
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.
Edit Comment