KMS XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY，CAS
| 中国西天山晚古生代以来的剥露作用与浅成低温热液矿床保存的研究 | |
| 王艳楠 | |
| Subtype | 硕士 |
| Thesis Advisor | 蔡克大 |
| 2017-12-01 | |
| Degree Grantor | 中国科学院大学 |
| Place of Conferral | 新疆乌鲁木齐 |
| Degree Discipline | 工学博士 |
| Keyword | 中国西天山 吐拉苏盆地 浅成低温热液矿床 磷灰石裂变径迹 剥露历史 |
| Abstract | 浅成低温热液型的金矿是目前世界上最为重要的金矿床类型之一，具 浅成低温热液型的金矿是目前世界上最为重要的金矿床类型之一，具有储量大、埋藏浅、易开采的特点，并且往往与深部斑岩型矿床具有密切的成因联系，一直以来都是国内外矿床学研究的热点。该类型矿床的就位深度极浅（<2 km），很容易受到后期的构造剥蚀，因此，世界范围内的浅成低温热液矿床多数产出于年轻的岛弧或者陆弧环境，其形成时代多数为中-新生代，中生代之前的矿床可能遭受长期的剥蚀而消失殆尽。然而，中国西天山发育了众多古生代大型、超大型浅成低温热液型的金矿床（如：阿希金矿和京希-伊尔曼德金矿），构成了全球罕见的前中生代浅成低温热液成矿省，其保存机制和保存条件是目前尚未解决的重要科学问题。吐拉苏盆地是中国西天山浅成低温热液矿床的重要聚集地，该盆地与其周缘山体的盆山耦合特征涉及到矿床本身的剥露和埋藏作用，是理解其保存机理的关键。磷灰石裂变径迹测年方法具有较低的封闭温度（~110℃），可以有效揭示浅成低温热液矿床自就位以来的热历史。本研究对中国西天山主要山体（科古琴山、乌孙山、那拉提山和婆罗科努山）和赋矿的盆地（吐拉苏盆地）进行了详细的野外地质调查，并系统采集了样品，采用磷灰石裂变径迹年代学的方法研究山体和盆地的构造抬升-剥蚀历史，揭示它们之间的盆山耦合特征，最终讨论浅成低温热液矿床的保存机制和条件。本研究在中国西天山主要山体（科古琴山、乌孙山、那拉提山和婆罗科努山）共采样 28 件，所测得的磷灰石裂变径迹年龄范围为 47.9±9 Ma~182.3±9.9 Ma，平均围限径迹长度为 9.8±0.5~13.7±0.1 μm。裂变径迹数据分析和热历史模拟结果表明，中国西天山主要经历了四个剥露期次，分别对应了不同的构造事件，晚石炭世-晚二叠世（320-250 Ma），随着古天山洋的闭合，塔里木-中天山和北天山发生碰撞拼合，西天山经历了强烈的剥露作用，导致科古琴山、婆罗科努山、乌孙山和那拉提山均隆起地表；晚三叠世-早白垩世（~230-130 Ma），西天山在该阶段经历了慢速的伸展剥露作用，其动力学来源为东部蒙古-鄂霍茨克海的消减闭合；晚白垩世-古新世（~80-50Ma），由于天山西南部科伊斯坦-德拉弧与欧亚大陆南缘的碰撞，西天山经历了剥露作用，但强烈的山体隆升仅集中在南天山，伊犁-中天山和北天山则发生了慢速剥露或者局部的掀斜作用；晚渐新世-早中新世（~30-20 Ma），新生代以来，印度板块与欧亚大陆的持续挤压导致了西天山强烈的变形，科古琴山、婆罗科努山、乌孙山、那拉提山均经历了快速隆升的阶段。吐拉苏盆地及其周缘山体晚古生代以来的盆山演化为最终探讨中国西天山浅成低温热液矿床的保存机理提供了重要的依据。早石炭世晚期（350-320 Ma），由于同火山断裂（走滑断层和正断层）活动致使盆地经历了沉降的阶段，~2.33 km 厚的沉积物（下石炭统阿恰勒河组）覆盖在大哈拉军山组火山岩之上；二叠纪（300-250 Ma），西天山主要山体快速剥露，为吐拉苏盆地提供丰富的物源，~2.50 km 厚的二叠纪沉积覆盖在下石炭统阿恰勒河组之上，浅成低温热液矿床被覆盖，从而避免被剥蚀；晚三叠世-早白垩世（230-100 Ma），西天山主要山体和吐拉苏盆地都经历了慢速剥露的阶段，大约 1.56 km 厚的物质被剥蚀，仍旧有~3.27 km 厚的物质覆盖在浅成低温热液矿床之上；晚渐新世以来（~35 Ma），吐拉苏盆地及其北侧科古琴山经历了快速剥露作用，覆盖在浅成低温热液矿床之上的沉积物被剥蚀，浅成低温热液矿床被剥露至近地表。造山带内部曾经的埋藏作用是古老（前中生代）浅成低温热液矿床得以保存的先决条件。 有储量大、埋藏浅、易开采的特点，并且往往与深部斑岩型矿床具有密切的成因联系，一直以来都是国内外矿床学研究的热点。该类型矿床的就位深度极浅（<2 km），很容易受到后期的构造剥蚀，因此，世界范围内的浅成低温热液矿床多数产出于年轻的岛弧或者陆弧环境，其形成时代多数为中-新生代，中生代之前的矿床可能遭受长期的剥蚀而消失殆尽。然而，中国西天山发育了众多古生代大型、超大型浅成低温热液型的金矿床（如：阿希金矿和京希-伊尔曼德金矿），构成了全球罕见的前中生代浅成低温热液成矿省，其保存机制和保存条件是目前尚未解决的重要科学问题。吐拉苏盆地是中国西天山浅成低温热液矿床的重要聚集地，该盆地与其周缘山体的盆山耦合特征涉及到矿床本身的剥露和埋藏作用，是理解其保存机理的关键。磷灰石裂变径迹测年方法具有较低的封闭温度（~110℃），可以有效揭示浅成低温热液矿床自就位以来的热历史。本研究对中国西天山主要山体（科古琴山、乌孙山、那拉提山和婆罗科努山）和赋矿的盆地（吐拉苏盆地）进行了详细的野外地质调查，并系统采集了样品，采用磷灰石裂变径迹年代学的方法研究山体和盆地的构造抬升-剥蚀历史，揭示它们之间的盆山耦合特征，最终讨论浅成低温热液矿床的保存机制和条件。 |
| Other Abstract | The term “epithermal” was coined to classify those deposits formed in an ore-formingenvironment of low temperature and shallow depth. They usually develop at temperature of 100~320℃ and crustal level of 0~2 km. Since epithermal deposits are an important source of Au, Ag, Cu, Pb, and Zn, and often characterized by large reserve and easy mining, they have always been a hot spot of research around the world. The worldwide epithermal deposits are mainly distributed in the circum-Pacific, Paleo-Central Asian and TethysHimalaya metallogenetic provinces. Because they form so close to the surface, epithermal deposits are frequently obliterated by erosion and reworking, with the result that the majority of these deposits are generally Mesozoic-Cenozoic in age. However, several large and super large epithermal deposits whose formation was considered as the result of the Late Devonian to Early Carboniferous arc magmatism have been recognized in the western Chinese Tianshan. Consequently, a rarely seen epithermal metallogenic province in pre-Mesozoic has been formed in this area. Since the region underwent complicated multi-phase Paleozoic accretion-collision evolution and Mesozoic-Cenozoic intracontinental tectonic reactivation,the preservation mechanism of these mineral deposits remains enigmatic. For the western Chinese Tianshan, most of the epithermal deposits are located in the Tulasu basin. Therefore,the basin mountain coupling characteristics of this basin and its surrounding mountains are involved in exhumation and burial of the deposits, which is crucial to understand their preserving mechanisms. With the development of isotope geochronology, tectonothermochronology has become a powerful tool to study the preservation of deposits.Utilizing temperature sensitive radiometric dating techniques to reveal low-temperature,upper crustal processes can elucidate many aspects of deposit genesis, including timing and duration of mineralization processes, rate of exhumation and erosion of ore deposits and comparative preservation potential. In particular, apatite fission track (AFT) dating method is characterized by low closure temperature (~110℃), which can be used to reveal the exhumation and burial of epithermal deposits since their emplacement. Accordingly,systematical geological investigations and AFT analysis were conducted in both the main mountain ranges of the western Chinese Tianshan (Kegusinshan, Wusunshan, Nalatishan and Borohoroshan) and ore-bearing Tulasu Basin to study the long-term basin-mountain evolution of these ranges since the late Paleozoic, and then the preserving mechanisms of epithermal deposits in the ancient orogenic belt would be hereby discussed. Twenty-eight samples were collected from the Keguqinshan, Wusunshan, Nalatishan and Borohoroshan. AFT ages are ranging from 47.9±9 Ma to 182.3±9.9 Ma, with confined mean track lengths (MTLs) vary between 9.8±0.5 μm and 13.7±0.1 μm. AFT analysis, in combination with the thermal history modeling results, allow us to summary four exhumation episodes in the western Chinese Tianshan, including the Late CarboniferousLate Permian (320-250 Ma), Late Triassic-Early Cretaceous (230-130 Ma), Late CretaceousPaleogene (80-50 Ma), and Late Oligocene-Early Miocene (30-20 Ma). The first episode may be caused by the final closure of the paleo-Tianshan Ocean and subsequent assemblage of the Tarim-Central Tianshan and North Tianshan, leading to uplift of these mountain ranges.The second episode was considered as the result of the subduction of the Mongolia-Okhotsk Ocean, which gave rise to the slow exhumation of the western Chinese Tianshan. The third episode was ascribed to the collage between the Kohistan-Dryas arc and southern South Eurasia continent. Strong exhumation was restricted at the South Tianshan, with slow exhumation and local tilting action in the Yili-Central Tianshan and North Tianshan in this period. The fourth episode related to the indentation of the India plate into the Eurasia continent. The western Chinese Tianshan underwent rapid exhumation process in this stage.Large-scale geological mapping and systematical AFT analysis were carried out in the Tualsu basin. In total ten samples were collected from the Carboniferous volcanic rocks,Mesozoic sandstone and Paleozoic granite. AFT ages of these samples range from 54.7±6.3 Ma to 272.3±8.7 Ma, and the MTLs are varying between 12.1±0.2 μm and 13.5±0.1 μm. AFT analysis, together with thermal history modeling, allow us to document five evolutionary stages of the Tulasu basin since the late Paleozoic. In the Late Devonian-Early Carboniferous (380-360 Ma), volcanic rocks were formed in this period. Along with the eruption of volcano, the left strike-slip faults were developed. During the late Early Carboniferous (350-320 Ma), the Tulasu Basin underwent important burial process owing to the syn-volcanic faults subsidence which may be caused by retreat of the Junggar Ocean. In the Late Carboniferous (320-300 Ma), intense hydrothermal activities were developed in the Tulasu basin, most of the epithermal deposits have been formed in this stage. During the Permian (300-250 Ma), the Tulasu Basin underwent another significant burial process because of the strong exhumation of neighboring mountain ranges (e.g. Keguqinshan). In the Late Triassic-Early Cretaceous (230-100 Ma), the basin experienced stage of slow exhumation. Since the Late Oligocene (~30 Ma), strong exhumation and deformation have occurred in the Tulasu basin.The coupling characteristics between the Tulasu basin and its neighboring mountains since the late Paleozoic provide the significant basis for preservation of the old epithermal deposits. In both late Early Carboniferous (350-320 Ma) and Permian (280-250 Ma), the Tulasu basin underwent burial processes owing to the syn-volcanic faults subsidence and strong exhumation of the Keguqinshan, respectively. There have been ~2.33 km thick Lower Carboniferous Aqialehe Formation and ~2.50 km thick Permian sediments covering on the ore-bearing Dahalajunshan Formation, which protect the epithermal deposits from erosion.During the Late Triassic-Early Cretaceous (230-100 Ma), both the Tulasu basin and its neighboring mountain ranges underwent slow exhumation process (0.012 m/y), the corresponding ~1.56 km thick sediments were eroded. Given the greater sedimentary pile (~4.83 km), there are still ~3.27 km thick material covering on the epithermal deposits. Since the Late Oligocene (~30 Ma), rapid exhumation occurred in both the Tulasu basin and main mountain ranges of the western Chinese Tianshan. Nearly ~3.27 km thick sediments that covered on the ore-bearing Dahalajunshan formation have been eroded with the exhumation rate of ~0.057 mm/y, epithermal deposits were exposed to the sub-surface again. The former burial function within orogenic belt is the key to preservation of the old (pre-Mesozoic)epithermal deposits. |
| Subject Area | 地球探测与信息技术 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.xjlas.org/handle/365004/14768 |
| Collection | 研究系统_荒漠环境研究室 |
| Affiliation | 中国科学院新疆生态与地理研究所 |
| Recommended Citation GB/T 7714 | 王艳楠. 中国西天山晚古生代以来的剥露作用与浅成低温热液矿床保存的研究[D]. 新疆乌鲁木齐. 中国科学院大学,2017. |
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