【作者】 龙晓平；

【导师】 孙敏； 袁超；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 地球化学， 2007， 博士

【摘要】 中国阿尔泰造山带位于我国新疆自治区北部，北邻萨彦—蒙古造山带，南接准噶尔盆地，沿南东—北西向展布2000余公里，是中亚造山带的重要组成部分。现有研究表明，中亚造山带是一个复杂大洋演化的产物，该大洋类似现今的西太平洋，其中存在不同时代的岛弧、海山和微陆块等，是一典型的显生宙增生型造山带。但对于我国新疆阿尔泰造山带，虽然研究显示陆壳的垂向增生是该造山带地壳生长的重要方式之一，但有关该造山带的基底形成时代和构造演化模式的争论仍旧没有解决。传统观点认为阿尔泰造山带存在前寒武纪结晶基底，该造山带的构造演化模式以开合构造演化为主。新疆阿尔泰造山带西北和南缘广泛分布了一套古生代的浅变质及未变质的碎屑沉积岩，主要岩性为砂岩、粉砂岩、泥岩，以及浅变质的千枚岩、云母片岩和绿泥石片岩等。由于该套碎屑沉积岩被广泛出露的古生代花岗岩侵入并受到断层系统的切割，因而连续性差且区域对比困难。目前，对于该套碎屑沉积岩的沉积时代及其构造背景存在不同认识。一种观点认为，阿尔泰西北碎屑沉积岩形成时代为震旦一早寒武世，沉积于被动陆缘构造环境；而阿尔泰南缘的浅变质碎屑沉积岩形成时代为早—中泥盆世，沉积于被动大陆边缘拉张裂谷环境。阿尔泰西北的哈巴河群碎屑沉积岩和南缘的康布铁堡组及阿勒泰组碎屑沉积岩的主量元素含量变化大，而微量元素含量比较一致。哈巴河群的砂岩和角闪石片岩具有相近的主量元素含量（SiO₂=68-77wt％，Al₂O₃=9-14wt％，Fe₂O₃=2.3-5.7wt％，MgO=0.7-4.6wt％，CaO=0.3-3.5wt％，Na₂O=0.7-3.1wt％；K₂O=0.3-4.0wt％）。与之相比，泥岩和黑云母片岩含有较低的SiO₂（58-64wt％）和较高的Al₂O₃（15-19wt％）、Fe₂O₃（6.4-11.5wt％）和MgO（3.3-5.0wt％）含量。该群所有碎屑沉积岩具有比较一致的稀土配分特征，轻稀土富集（La_N／Yb_N=3-13），重稀土平坦型，并伴有明显的Eu负异常（Eu／Eu^*=0.43-0.80）。阿尔泰南缘的康布铁堡组绿泥石片岩主量元素组成（SiO₂=61-64wt％，Al₂O₃=15-18wt％，Fe₂O₃=7.5-8.0wt％，MgO=3.3-3.7wt％）与哈巴河群的泥岩和黑云母片岩一致，而该组云母片岩及千枚岩则（SiO₂=63-77wt％，Al₂O₃=11-17wt％，Fe₂O₃=3.5-6.2wt％，MgO=0.8-3.4wt％）类似于哈巴河群的砂岩和角闪石片岩。康布铁堡组碎屑沉积岩中大离子亲石元素Sc、Th和U及高场强元素（如Nb、Ta、Zr、Hf、Y）含量与后太古代澳大利亚页岩（PASS）接近，而大离子亲石元素Rb和Sr含量低于PASS。除个别云母片岩外，该组碎屑沉积岩稀土配分模式与哈巴河群碎屑沉积岩类似，轻稀土富集（La_N／Yb_N=2.88-8.24），并具有明显的Eu负异常（Eu／Eu^*=0.45-0.89）。阿尔泰组碎屑沉积岩主体与哈巴河群的砂岩和角闪石片岩类似，具有相似的主量元素组成（SiO₂=72-79wt％，Al₂O₃=10-13wt％，Fe₂O₃=3.4-5.5wt％，MgO=2.3-2.9wt％）和稀土配分特征（La_N／Yb_N=4.29-9.90，Eu／Eu^*=0.46-0.65）。阿尔泰造山带西北及南缘的碎屑沉积岩中的碎屑锆石比较自形，整体磨圆度较差，以岩浆锆石成因为主，其²⁰⁶Pb／²³⁸U年龄主要介于463-542Ma之间，此外还有一定量的元古代及个别太古代碎屑锆石（～30％），其中检获的最古老的碎屑锆石为一粒比较谐和的太古代岩浆锆石(²⁰⁷Pb／²⁰⁶Pb年龄3087±20Ma，Th／U=0.36)，确认了源区中有太古代物质的加入。寒武纪—早奥陶世的碎屑锆石多具有正的或近于零的εHf（t）值。通过对阿尔泰古生代碎屑沉积岩详细的地球化学及其碎屑锆石年代学和Hf同位素组成研究，得出以下主要认识：1、阿尔泰西北哈巴河群与阿尔泰南缘的康布铁堡组和阿勒泰组碎屑沉积岩的岩石学和地球化学特征相似，经历了相对较弱的化学风化作用及相对简单的沉积演化过程，其源区物质以中—酸性岛弧火成岩为主，形成于活动大陆边缘构造环境，而不是传统观点所认为的被动大陆边缘构造环境。2、哈巴河群沉积于中奥陶世之后，早泥盆世之前，并非形成于前寒武纪；康布铁堡组碎屑沉积岩沉积于早志留世之后，与前人给出的晚志留—早泥盆世年龄一致；对于阿勒泰组，出露于塔尔浪及哈拉苏南的该组碎屑沉积岩形成时代应早于早泥盆世，介于中奥陶世（～470Ma）和早泥盆世（～412Ma）之间，既不是早期所认为的中泥盆世，也不是后来所认定的早元古代，说明阿勒泰组不同地段的形成时代可能不同。3、阿尔泰西北与阿尔泰南缘的碎屑沉积岩中的碎屑锆石均以比较自形的寒武纪—早奥陶世（～460-540Ma）的岩浆锆石为主，多数具有正的εHf（t）值，说明二者的碎屑沉积物均主要来自寒武纪—早奥陶世形成的岩浆弧，并具有相同的主要物质源区，可能为相同构造环境下不同时期沉积的产物。4、阿尔泰西北与阿尔泰南缘的碎屑沉积岩源区物质均以早古生代为主，并含有一定量的新元古代物质和极少太古代碎屑物质，不同于华北板块碎屑沉积物源区物质组成，可能与西伯利亚板块具有亲缘性。5、阿尔泰南缘的阿勒泰组碎屑沉积岩的角闪岩相变质作用发生在中泥盆世（389Ma），与此同时，阿尔泰西北哈巴河群碎屑沉积岩发生了混合岩化变质作用（384±6Ma），该期区域变质作用是阿尔泰早古生代一期重要变质事件。结合区域二叠纪的变质事件，说明阿尔泰造山带存在多期变质作用。6、阿尔泰存在前寒武纪结晶基底的证据不足，需要进一步深入研究。阿尔泰碎屑沉积岩系，如哈巴河群、康布铁堡组和阿勒泰组，均形成于中奥陶世之后，不能代表阿尔泰造山带基底。更多还原

【Abstract】 The Chinese Altai orogen is situated in the northern Xijiang Uygur Autonomous, between the Sayan and associated belts to the north and the Junggar basin to the south, extending northwestwards for more than 2000 km. The Chinese Altai orogen is a part of the Central Asian Orogenic Belt （or Altaids）, which is one of the largest Phanerozoic accretionary type orogens in the world, with both lateral and vertical accretion of juvenile materials contributing to the crustal growth. As for the Chinese Altai orogen, recent studies have shown that the continental vertical accretion is an important aspect of the crustal growth in this orogen. However, controversies on its tectonic evolution and timing of its basement still exist. The traditional viewpoints proposed that the basement of the Chinese Altai formed in Precambrian and the Open-closing model was the probable model for the geological evolution of this orogen.In northwestern and southern Chinese Altai orogen, thick Palaeozoic clastic sedimentary rocks are extensively distributed. These rocks mainly consist of sedimentary rocks and metasedimentary rocks, such as sandstone, siltstone, pelite, schist and phyllite, which were intrude by widely distributed Paleozoic granites and cut by complex fault system. As for the deposition time and tectonic setting of the clastic sedimentary rocks, most studies suggested that the clastic sedimentary rocks in northwestern Altai formed in Sinian to Early Cambrian and deposited on a passive continental margin, while the clastic sedimentary rocks in the south margin of the Chinese Altai formed in Early to Middle Devonian and deposited in rift-related tectonic setting on a passive continental margin.The clastic sedimentary rocks of the Habahe Group in Northwest Altai are different to those rocks of the Kangbutiebao Formation and the Altay Formation in South Altai in the major element contents, but similar in their REE and trace element contents. In comparison with the sandstones and hornblende-bearing schists of the Habahe Group （SiO₂= 68-77 wt%, Al₂O₃= 9-14 wt%, Fe₂O₃= 2.3-5.7 wt%, MgO= 0.7-4.6 wt%, CaO= 0.3-3.5 wt%, Na₂O= 0.7-3.1 wt%; K₂O= 0.3-4.0 wt%）, the Habahe pelites and biotite-bearing schists possess slightly higher Al₂O₃ （15-19 wt%）, Fe₂O₃ （6.4-11.5 wt%） and MgO （3.3-5.0 wt%） and lower SiO₂ （58-64 wt%） contents. However, all clastic sedimentary rocks from the Habahe Group show similar REE patterns with distinct LREE enrichments （La_N/Yb_N= 3-13） and negative Eu anomalies （Eu/Eu*= 0.43-0.80）.In the south margin of the Chinese Altai, the chlorite-bearing schists from the Kangbutiebao Formation have similar compositions of the major elements （SiO₂= 61-64 wt%, Al₂O₃= 15-18 wt%, Fe₂O₃= 7.5-8.0 wt%, MgO= 3.3-3.7 wt%） to the Habahe pelites and biotite-bearing schists, while the mica-bearing schists and phyllites （SiO₂= 63-77 wt%, Al₂O₃= 11-17 wt%, Fe₂O₃= 3.5-6.2 wt%, MgO 0.8-3.4 wt%） are similar to the Habahe sandstones and hornblende-bearing schists. The Kangbutiebao clastic sedimentary rocks show similar ranges of the HFSEs （e.g. Nb, Ta, Zr, Hf, Y） and some LILEs （e.g. Sc, Th, U） contents to those of Post-Archaean Australian Shale （PASS）, with lower Rb and Sr contents. Except for few mica-bearing schist, most Kangbutiebao clastic sedimentary rocks display similar REE patterns to Habahe clastic sedimentary rocks, with distinct LREE enrichments （La_N/Yb_N= 2.88-8.24） and negative Eu anomalies （Eu/Eu*= 0.45- 0.89） too. The geochemical characteristics of clastic sedimentary rocks from the Altai Group are consistent with those of the Habahe sandstones and hornblende-bearing schists. The Altai clastic sedimentary rocks posssess high SiO₂ （72-79 wt%） and low Al₂O₃（10-13 wt%）, Fe₂O₃ （3.4-5.5 wt%）, MgO （2.3-2.9 wt%） contents, also show LREE enrichments （La_N/Yb_N= 4.29-9.90） and negative Eu anomalies （Eu/Eu*= 0.46-0.65）.The detrital zircons separated from these rocks have oscillatory compositional zoning, possess high Th/U ratios, and are enhedral to subhedral, showing short transportation from an igneous provenance. Their predominant ²⁰⁶Pb/²³⁸U ages were between～460 and～540 Ma, mostly with positiveεHf values. Except for this predominant group, a small population of Proterozoic and few Archean zircons also existed. Moreover, a concordant detrital zircon with Archean ²⁰⁷Pb/²⁰⁶Pb age （3087±20 Ma） had been discovered, which confirmed the involvement of Archean material in the source.Based on our geochemical data of the Palaeozoic clastic rocks in the Chinese Altai, U-Pb ages and Hf isotope data for the detrital zircons from these rocks, the following major conclusions can be drawn:（1） The clastic sedimentary rocks in northwestern and southern Chinese Altai orogen have similar characteristics of petrology and geochemistry. Their source materials experienced relatively weak chemical weathering and simple sedimentary recycling, and were dominated by intermediate to acidic arc sources. These rocks probably deposited on active continental margins, instead of passive continental margins proposed by previous reserchers.（2） The Habahe Group was deposited in the Middle Ordovician or later, not in the Precambrian. The deposition age of the Kangbutiebao Formation was later than the Early Silurian, which is consistent with the upper Silurian to lower Devonian age assigned to this formation. The clastic sedimentary rocks in Tarlang and southern Halasu from Altay Formation deposited before Early Devonian （470 Ma to 412 Ma）, probably indicating that the clastic sedimentary rocks from this formation have different deposition ages in different areas.（3） The detrital zircons separated from these clastic sedimentary rocks was predominated by Cambrian- Early Ordovician magmatic zircons with enhedral to subhedral shape and positiveεHf values, suggesting similar sources mainly derived from Cambrian to Early Ordovician magmatic arcs. These clastic sedimentary rocks were different sedimentary products in same tectonic settings. （4） The source materials of these rocks were dominated by Early Palaeozoic clastic sediments, with subordinate Proterozoic and minor Archean crustal materials. These characteristics are different to the composition of clastic sedimentary rocks from North China Plate, indicating close affinities to Siberian Plate.（5） The migmatite from the Habahe Group and the garnet-sillimanite gneiss from the Altay Formation both underwent strong metamorphism in the Devonian. Together with the Permian regional metamorphism, these metamorphism events show a multiple thermal history for the Chinese Altai.（6） The evidences were not enough to prove the existence of Precambrian basement in the Chinese Altai orogen, inviting further detailed study. The clastic sedimentary rocks, such as the Habahe Group, Kangbutiebao Formation and Altay Formation, deposited after Middle Ordovician and could not represent the basement of the Chinese Altai orogen.更多还原