【作者】 李加好；

【导师】 宋传中；

【作者基本信息】 合肥工业大学 ， 矿物学、岩石学、矿床学， 2013， 博士

【摘要】 秦岭造山带作为中国中央造山带的主要部分,长期以来都是地质学家们关注与研究的热点地区。北秦岭构造带位于秦岭造山带核部,夹持在洛南-栾川断裂带和商丹断裂带之间。它是秦岭造山带变质最强烈和变形最复杂的地区。北秦岭古生代以来造山过程中的构造变形研究对建立华北板块与华南板块之间的最终拼合过程尤为重要。前人在该地区的工作主要集中在北秦岭内部各块体的大地构造属性方面,关于其内部块体与边界断裂的精细构造变形研究尚显不足。因此本文从野外调研入手,主要从构造地质学,变质岩石学和构造热年代学等方面对北秦岭进行综合研究,同时结合区域地质资料探讨了北秦岭古生代构造演化过程,并对其进行了动力学分析。(1)通过野外构造观察与室内显微变形分析,揭示出刘岭群北带内部岩石面理倾向NNE,矿物拉伸线理为倾向线理。其构造变形特征主要表现为带内岩石由北向南逆冲,呈构造岩片叠置,形成轴面南倒北倾的紧闭褶皱。对其内部的含榴云母石英片岩的变质作用研究发现,其峰期变质达绿帘-角闪岩相到低角闪岩相变质条件,T=560℃-588℃,P=0.59～0.65GPa,对应大陆造山带环境。对其内部1个含榴云母石英片岩样品和2个千糜岩样品中的白云母进行40Ar-39Ar法定年,得到其冷却年龄分别为247±1Ma,247±1Ma,249±1Ma。(2)对商丹带进行了详细的构造变形研究,发现不同部位其构造变形特征有所差别。南带发育由北向南的逆冲的断层和紧闭的同斜褶皱,中心部位内岩石具有面理陡倾,线理近水平的特征,并发育枢纽产状与矿物拉伸线理产状一致的A型褶皱。而北带岩石矿物拉伸线理多为向SEE倾伏,倾伏角为20-50°,A型褶皱枢纽产状与矿物生长线理一致。利用糜棱岩化过程中矿物变形温度计和石英C轴组构特征估算商丹带的变形环境主要集中在中一高绿片岩相到低角闪岩相。岩石变形温度主要集中在400-500℃之间,属中高温条件。总体上西段岩石变形温度较东段高。古应力差计算结果显示商丹带的古应力差值较大,达到140MPa。涡度分析表明商丹断裂带的变形以纯剪作用占据主导作用,但包含有简单剪切的分量,在此背景下岩石的有限应变表现为压扁的椭球体。(3)对秦岭岩群进行了详细的构造解析,结果指示秦岭岩群主要经历了两个构造旋回。早期晋宁期变质变形具体表现为沉积地层发生角闪岩相变质成为片麻岩等一系列岩石,并形成紧闭-平卧褶皱,发育透入性的片麻理和向NWW缓倾的矿物生长线理。加里东-海西期间秦岭岩群又发生了一次较为强烈的变质变形作用,使得早期褶皱的秦岭岩群内部岩石产生向SEE倾伏的矿物拉伸线理和A型褶皱。变质作用研究表明秦岭岩群变质峰期达角闪岩相,T=686℃,P=0.63Gpa,其加里东-海西期变质p-T轨迹具有顺时针旋转的特点,指示减压抬升过程。结合上述一系列地质现象,说明秦岭岩群在向上抬升过程可能具有平行造山带向西侧向挤出的运动学分量。(4)对朱夏断裂带两期活动进行了区分,早期由南向北的逆冲活动使得秦岭岩群逆冲到二郎坪岩群之上,其活动时代大约在400Ma左右。后期右行走滑剪切奠定了当今朱夏断裂带的地质外貌,使得早期的由南向北逆冲剪切作用记录很少被保留。该期活动形成的糜棱岩面理高角度顷向SWS,矿物拉伸线理向ESE缓倾,倾角2-15。,宏观或微观变形证据都很好的指示了右行走滑的运动学特征。该期变形的温度在400-550℃之间,根据糜棱岩中白云母的40Ar-39Ar年代学分析指示出该期剪切活动最迟发生在146Ma左右。(5)瓦乔断裂带走向为SEE-NWW,倾向NNE,糜棱岩内矿物拉伸线理既有向NNE的陡倾线理,又有近水平的缓倾线理。研究表明断裂带具有由北向南逆冲的运动学特征,且存在左旋剪切的分量。根据石英C轴组构的优选方位判断其剪切带的变形温度为400-650℃,为中高温变形。通过对断裂带内同构造形成的二云母石英片岩中白云母的40Ar-39Ar定年,首次获得瓦乔断裂带的活动年龄上限为387Ma。经过区域综合构造分析,认为北秦岭构造带现今构造格局形成于古生代构造运动。其主要动力学原因是扬子板块与华北板块之间的挤压和碰撞作用。随着秦岭洋的闭合,两大板块开始碰撞拼合,北秦岭内部各块体也完成了拼贴就位,此时北秦岭地区区域变质作用达到顶峰,朱夏断裂带和瓦乔断裂带在区域变质与热变质作用下启动。更多还原

【Abstract】 As the main part of central orogenic belt of China, The Qinling orogenic belt is always the interested research area among geologists. The north Qinling tectonic belt is located in the core of Qinling orogenic belt, which is bounded by Luonan-Luanchuan fault (LLF) in the north and Shangnan-Danfeng fault (SDF) in the south, and have undergone strong metamorphism and deformation. Therefore, deformation in the orogenesis of the Qinling orogen since the Palaeozoic is very important to reconstructing the last assembly process of the North China Plate and South China Plate. Previous research work in the region is mainly concentrated in the tectonic attribute of blocks in the north Qinling tectonic belt, but lack of fine structure analysis of internal block and boundary faults. In this paper, we present original results of our field investigation, and structural geology, metamorphic petrology and geochronology studies on the north Qinling tectonic belt. Combining with regional geologic data, the tectonic evolution process in Palaeozoic and geodynamics of the north Qinling tectonic belt are discussed.The detailed field investigation and microstructure analysis reveals that the rocks in the north belt of Liuling group develop NNE-dipping foliation and dipping stretching lineation. The structural deformation of the north belt of Liuling group is characterized by rocks thrusting toward south, tectonic slices superimposed and forming N-verging tight folds. Metamorphism study on garnet-bearing mica quartz schist suggests the peak metamorphism reaches epidote-amphibolite facies to low amphibolite facies of560℃～588℃and0.59～0.65GPa,and it is similar to the environment of continental orogenic belt. Muscovites from one sample garnet-bearing mica quartz schist and two samples of phyllonite give40Ar/39Ar plateau agesrespectively247±1Ma,247±1Ma, and249±1Ma.Fine structural analysis suggests that different parts of Shangdan fault zone have different structural deformation characteristics. South belt develops faults thrusting towards south and N-verging tight homoclinal folds. However mylonites are exposed in the center of Shangdan fault belt with dipping foliations and nearly horizontal stretching lineations, and A-type folds whose hinges occurrence is parallel to the stretching lineation are well developed. To the north, the mineral stretching lineation pitch to SEE with plunge angle of20-50°, and also A-type folds whose hinges occurrence is parallel to the stretching lineation are common to see.Mineral deformation thermometer during mylonitization and quartz C axis fabric analysis are carried out to estimate the deformation environment of Shangdan fault zone. The result shows the deformation temperature is mainly between400℃～500℃, and indicates mylonites formed in the middle-high greenschist facies to low amphibolite facies metamorphic conditions. In generally the deformation temperature of west part is higher than theeast part.Differential stress calculation show that ancient stress difference of the Shangdan fault zone is up to140Ma. The kinematic vorticity analysissuggests that the deformation wasmainly dominate by the pure shear, but contains a simple shear component. Under the background of the finite strain of rocks shows the flattened ellipsoid.A detailed structural analysis indicates the Qinling Group has undergone two major tectonic cycles. The early sedimentary strata has been subjected metamorphismin Jinning period and formed a metamorphic rock association of schist-gneiss series. The deformation is characterized byformation oftight-recumbent folds and development of penetrativegneissosity and NNW-dipping mineral lineation.During the Caledonian-Hercynian the Qinling Group once again suffered strongly metamorphism and deformation, forming SEE-dipping stretching lineations and A-type folds in early deformed Qinling group. The peak metamorphism of Qinling group reaches amphibolite facies conditions of686℃and0.63Gpa. The p-T path of the Caledonian-Hercynian tectonometamorphiccycle shows clockwise, indicating decompression during rapid uplift.The above all geological phenomena demonstrate the Qinling Group has experienced orogen-parallel westward uplift withboth vertical and horizontal displacement components.The Zhuxia fault zone has experienced two tectonic events. Early thrust shearing made the Qinling group thrust over the top of the Erlangping group at about400Ma.Later tectonic event made the early structural deformation seldom preserved. The foliation of mylonite steeply dips to SWS, and stretching lineation plunges ESE at2-15°. Macro and micro deformation evidences strongly suggested that the later tectonic event hasthe kinematics characteristics of dextral shear. The deformation temperature is mainly between400-550℃. The40Ar-39Ar plateau age of muscovitegrains from mylonite suggests the dextral shear occurredno later than146Ma inWNW-ESE direction.The Waqiao fault zone is trending SEE-NWW, dips to NNE. Mineral stretching lineationsin mylonite are both the steeply NNE-dipping lineation, and nearly horizontal lineation. The Waqiao Fault zone has the kinematics characteristics of the thrusting from north to southand containing sinistral shear component. Quartz C-axis preferred orientation reflected the deformation temperature of the shear zone is between400-650℃, which belongs to mid-high temperature deformation. A40Ar-39Ar plateau age of muscovitegrains from the mica schistformed in syntectonic period reveals that the Waqiao ductile shearing occurred no later than387Ma.On thebasisof tectonic analysis,we proposed that the main component and thepresent tectonic framework of north Qinling tectonic belt were formed during the Palaeozoic orogeny. The main geodynamic mechanism is compression and collision tectonic background between the Yangtze block and the North China block. It is the results of northward subduction of the Qinling ocean. As the Qinling ocean closed, followed by two plates collision, all the blocks in the north Qinling are in place, at this time the regional metamorphism reached its peak. The Zhuxia fault zone and Waqiao fault zone are initiated by the regional metamorphism and thermal metamorphism.更多还原