【作者】 赵英利；

【导师】 刘永江；

【作者基本信息】 吉林大学 ， 构造地质学， 2010， 博士

【摘要】 研究区位于大兴安岭中南部,内蒙古东部草原区,为中亚造山带的东南缘,华北板块北缘同“佳-蒙”地块南缘碰撞拼贴的关键部位,记录着中亚造山带东段(兴蒙造山带)最终消亡的演化过程。前人研究在该区划分出贺根山、交其尔-锡林浩特、索伦山-林西和西拉木伦河等多条缝合带,并趋向于在晚二叠世最终闭合,但各缝合带之间的关系、板块拼贴的过程仍然存在很大争议。本文通过砂岩的骨架成分统计分析、地球化学特征、碎屑锆石LA-ICP-MS U-Pb年代学等物源分析手段,选取大兴安岭中、南段二叠系哲斯组及林西组沉积砂岩作为研究对象,结合现有最新研究成果,探讨该区域古生代的构造演化过程及二叠纪沉积盆地的盆地性质。研究结果表明,该区域内古生代具有6个构造演化阶段:晚寒武-早奥陶世,洋-陆俯冲的开始拉开古亚洲洋南支消亡的序幕;至早志留世末期,洋壳的双向俯冲并在大洋两侧形成安第斯型岩浆弧;晚石炭世末期,“佳-蒙”地块南缘“弧-弧后盆地”的构造环境的形成;早二叠世,贺根山洋的夭折导致弧-陆碰撞过程中贺根山蛇绿岩的就位,在岛弧一侧造成了大石寨组火山岩的喷发;中、晚二叠世,华北板块与“佳-蒙”地块的碰撞拼贴过程的缓慢完结,致使哲斯组至林西组由海相沉积逐渐转入陆相沉积,来自“佳-蒙”地块南缘的苏尼特左旗-锡林浩特-西乌旗南岩浆弧,大石寨组火山岩以及早古生代形成的苏尼特岩浆弧为两组沉积提供物源;直至晚二叠世末期华北板块北缘与“佳-蒙”地块南缘最终闭合,此刻来自华北板块古老基底的物源在林西组沉积中显著增加。更多还原

【Abstract】 The northeastern China and its adjacent area tectonically belongs to the eastern segment of Central Asian Orogenic Belt （CAOB）, located among the Siberia Craton, the North China Craton and the Pacific plate, and it is one of the important members composing the“Trench-Arc-Basin”system along the eastern margin of Asian Continent. NE China has experienced multi-stage tectonic evolution and reconstructions, e.g. this region was not only affected by the subducion and collision between the North China and the Siberia Cratons accompanied by the closing of Paleo-Asian Ocean during the Paleozoic, but also affected by the subsequent overprint and reconstruction related to the Pacific subduction along the eastern margin of Asian Continent. Both the Paleozoic and Mesozoic tectonics well recorded here, the NE China therefore becomes a key area as contribution to better understanding the formation and tectonic evolution of the eastern Asian Continent.The southern margin of“Jiamusi-Mongolia”block, which is developed between the North China and the Siberia Cratons, is traditionally considered to be preserved the evidence responded to the subduction and collision of that two Cratons. However, when and where the final suturing took place is still contentious so far. The current study is mainly focusing on the provenance characteristics of the Permian sandstones exposed in the central and southern Grate Xing’an Ranges, north China, further, the systemically sandstone detrital clasts component analysis, geochemistry, and LA-ICP-MS U-Pb detrital zircon ages have been carried out on the studied Permian sandstones, in order to better constraining the suturing processes between the North China and the Siberia Cratons. The result presented in this paper suggests a possible provenance of the Permian sandstone and its tectonic setting, and also contribute toward the consideration of the regional Paleozoic tectonics of NE China （“Jiamusi-Mongolia”block）.The study areas （including Hexigten Banner （KQ）, Lixin （LX） and Suolun （SL） areas distributing toward to the north） geographically exposed in central and southern Grate Xing’an Ranges, eastern Inner Mongolia. This region is bounded by the Ondon Sum-Xar moron River suture belt to the south and the Eren Hot-Hegenshan suture belt to the northwest, in where the Permian strata are widespread. The sediments of the Mid-Permian Zhesi Formation （P2z） and Late-Permian Linxi Formation （P3l） are mostly classified as feldsparthic litharenite, which consists mainly of quartz （44⁵2%）, lithic clasts （27³2%）, feldspar （15²4%） and micas （<5%）, with a small amount of accessory minerals （<5%）, e.g. magnetite, limonite, pyroxene, titanite and zircon. The lithic clasts are predominately characterized by the volcanic lithic grains, with Lv/L ratio ranging from 0.65 to 0.97. Additionally, the monocrystalline quartz grains are extremely high in the framework of total quartz grains, with Qm/Qt = 0.70⁰.95. The detrital compositions indicate that the sandstones are immature, probably due to the effects of rapid erosion, transport, and diagenetic process and/or nearly supplying derived from the source. The abundance of volcanic lithic and monocrystalline quartz grains significantly reveal the magmatic provenance. In addition, the detrital modal analysis applying the Dickinson’s triangle diagrams also indicate the sediments of Zhesi Formation are mainly derived from recycled orogenic source, while the sediments of Linxi Formation are mostly derived from dissected arc source.The major elements and their radios （especially, Fe_<sub>2O₃/K₂O and Na₂O/K₂O） are closely associated with graywackic lithologies for the sandstone from both the Zhesi and Linxi Formation. Relatively higher Fe_<sub>2O₃/K₂O in the sediment of Linxi Formation may be inferred to the occurrence of more easily weathered iron-rich minerals （e.g. biotite, chlorite）, and it further reveals that more rapid diagenetic process is responsible for the sandstones in Linxi Formation respect to those in the Zhesi Formation. Trace elements in the studied sandstones from the two formations mentioned above are upper crust-normalized patterns, and show the similar trends in the spider diagram, suggesting the similar provenances related to the rocks of upper crust. Furthermore, the obvious negative anomalies of Ta and Nb significantly indicate an affinity of subduction-related magmatic provenance. The rare earth elements and their chondrite-normalized patterns in the investigated sandstones mostly show similar characteristics, with steep LREE and flat HREE （ΣLREE/ΣHREE = 5.95⁸.44） and week negative anomalies of Eu （δEu = 0.67⁰.97）, revealing that the similar provenances derived from the felsic upper continent crustal rocks are reasonable to supplying for the Permian sediments of both the Zhesi and Linxi formations.The indexes of paleo-weathering and chemical alteration （CIA and CIW values） together with the A-CN-K projections have been widely investigated in the sandstone from two formations, and show strong source weathering and potassium- metasomatism in the Mid-Permain Zhesi Formation aspect to the Late-Permian Lixin Formation. The geochemical discrimination diagrams （e.g. Fe_<sub>2O₃+MgO-TiO₂, Fe_<sub>2O₃+ MgO-Al_<sub>2O₃/SiO₂, K₂O/Na₂O-SiO₂, La-Th-Sc, and so on） and F1-F2 discriminant function diagram exhibit the similar tectonic setting corresponded for the provenance of the Permian Zhesi and Linxi formations, which are mainly characterized by the Active Continental Margin （ACM） and Continental Island Arc （CIArc）. Worthy of note is that there are small amounts of provenances from Passive Margin （PM） supplied to the Zhesi sediments and subsequently transferred to the ACM; whereas the tectonic setting of provenance of Lixin sediments tends to transfer progressively from ACM to CIArc.The detrital zircons mostly occur in the form of long columnar euhedral grains with the magmatogenic oscillatory zonings in cathodoluminescence （CL） images, whereas a few subrounded zircon grains present thin metamorphic outmost rims, as well as relicts of core also preserved in some grains suggesting the nature of captured zircons. More significantly, the detrital zircon has the Th/U radio ranging from 0.04 to 1.99 （mainly Th/U > 0.4）, indicating their magmatic origin. The LA-ICP-MS U-Pb dating of detrital zircons from Permian sandstones show several distinct dominant populations of c. 280Ma, c. 310Ma, c. 450Ma and c. 500Ma, respectively, suggesting the multi-stage magmatic events taking place in this region. The ages of c. 280Ma are widespread in central and southern Grate Xing’an Ranges area, and is previously considered as the formation time of Dashizhai Formation; The ages of c. 310Ma, are generally obtained from the Sunidzuoqi and sorthern Xi Ujimqin as a subduction-related magmatic belt （Baolidao magmatic arc）; The ages of c. 450Ma⁵00Ma are mainly represented by the Paleozoic magmatism in the Sunid arc, as well as the post collision-related granites in the Ergun block. Some old ages, e.g. 1800Ma and 2500Ma, have obtained from the captured zircons, revealing the nature of basement of the North China Craton. Obviously increasing of captured zircons with old ages in the Linxi Formation suggests the episodes of collision between the North China Craton and“Jiamusi-Mongolia”block had possibly ended in the late-Permian, and therefore the provenance derived from the North China Craton became more predominated in the sediments.Tectonic setting of provenance of the Mid- and Late-Permian sediments and their detrital zircon U-Pb ages in conjunction with the previously reported geotectonic events in this area infer that there are 6-stage evolution contributing to the Paleozoic regional tectonics following as:（1） In Late-Cambrian to Early-Ordovician time, the closing of south branch of Paleo-Asian Ocean between the North China Craton and“Jiamusi-Mongolia”block began, probably causing the north-dipping subduction of oceanic lithosphere beneath the southern margin of“Jiamusi-Mongolia”block, whereas the northern margin of the North China Craton was possibly a passive margin at that time; （2） In Early-Silurian time, the northern margin of the North China Craton evolved into an active margin with the formation of the Bainaimiao arc （c. 430Ma）, further the Sunid magmatic arc （c. 450Ma⁴90Ma） was produced on the southern margin of the“Jiamusi-Mongolia”block; （3） In Late-Carboniferous time, on the northern margin of the North China Craton is still characterized by the Andean-style subduction accompanied by the subduction-related intrusions of 274Ma³24Ma in age, whereas an“arc-back-arc basin”system （possibly resembling the present-day western Pacific margin） was initially active on the southern margin of“Jiamusi-Mongolia”block, and produced Baolidao magmatic arc （co-called Sunidzuoqi-Xilinhot-southern Xi Ujimqin magmatic arc, at c. 310Ma） superimposed on the Sunid arc and subsequently opening of the“Hegengshan”Ocean; （4） In Early-Permian time, the“Hegengshan”Ocean may began collapsing and together with formation of Hegenshan ophiolites in this period, probably due to the continuous northward subduction of Paleo-Asian Oceanic plate. In addition, the widely distributed Early Permian arc-related volcanic rocks （Dashizhai Formation: c. 280Ma） have erupted on the island-arc side. Significantly, the paleo-oceanic crust subducted beneath to the“Jiamusi-Mongolia”block has broken off, causing the uplift of its distal part by the buoyancy derived from the unbalanced gravity. The subduction northward beneath to the“Jiamusi-Mongolia”block thus suddenly becomes slow. In contrast, the subduction beneath to the North China Craton is still an Andean-type; （5） In Mid-Permian time, the Zhesi Formation has been deposited under the conditions associated with the fore-arc basin along the southern margin of“Jiamusi-Mongolia”block, and it is characterized by the relatively constant continental sedimentary environments likely the background of Passive Margin. The provenance supplying mainly from the Baolidao arc, Dashizhai arc-related volcanic rocks and Early-Paleozoic Sunid arc, with minor amounts of provenance from the North China Craton; （6） In the Late-Permian time, the depositing of Linxi Formation has progressively transferred the sedimentary environments from the marine to the continental during the subduction and subsequent collision between the North China Craton and“Jiamusi-Mongolia”block. In this period, the provenance derived from the Baolidao arc, Dashizhai volcanic rocks and Early-Paleozoic Sunid arc were still responsible for the Linxi sediments, additionally, the obvious increase of provenance from the North China Craton respect to the Zhesi Formation. Subsequent to the end of Late-Permian, the final collision in between the North China Craton and“Jiamusi-Mongolia”block had probably ended along the suturing of Ondon Sum-Xar Moron River belt.更多还原