【作者】 李小明；

【导师】 谭凯旋； 王岳军；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 构造地质学， 2004， 博士

【摘要】 本论文分两部分,第一部分是裂变径迹方法研究,第二部分是应用裂变径迹方法,结合~40Ar/~39Ar(K-Ar)法研究云开大山印支构造事件与中-新生代的隆升剥露作用。 第一部分首先介绍了裂变径迹定年的原理、分析方法、实验技术及裂变径迹退火动力学的研究现状、进展与发展趋势,并结合笔者长期从事裂变径迹分析工作所获得的一些经验与认识,讨论了裂变径迹实验技术与方法中存在的问题与可能的解决途径。其次简单介绍了裂变径迹热年代学在地质上的应用. 第二部分是以云开大山为实例进行的热年代学研究。云开大山地处华南南缘、紧邻印支地块,西接钦防海槽,是研究华南-印支陆块中-新生代构造演化极富价值的研究区域。本文采用裂变径迹,结合~40Ar/~39Ar热年代学研究云开大山印支期的变形事件与中-新生代的隆升剥露作用.同时为进一步理解印支、华南陆块的变形统一动力学机制及华南中-新生代大地构造演化提供新的约束和启发。 选择云开大山不同岩石类型(糜棱岩、混合岩、花岗岩)的代表性样品进行裂变径迹研究,并从中选择4个代表性的糜棱岩样品进行~40Ar/~39Ar同位素热年代学分析,结合已有的~40Ar/~39Ar(K-Ar)年龄数据,取得如下成果或认识:(1)锆石的裂变径迹年龄集中于97/4-133.0 Ma,峰值年龄为125 Ma±,另一峰值为105 Ma±,说明云开大山早白垩世存在一次抬升冷却事件;磷灰石裂变径迹年龄变化于43.5-68.4 Ma,代表云开大山于早第三纪的另一次抬升冷却事件.磷灰石样品的封闭径迹长度平均值为12.60-14.36μm,大部分样品的标准误差在2μm以上,呈较宽的单峰分布.结合云开大山的地质背景,认为样品的裂变径迹年龄应该是云开大山隆升剥露作用的冷却年龄;(2)选择磷灰石样品的单颗粒裂变径迹年龄与长度分布,借助AFTsolve软件对样品的热历史进行模拟,获得了温度-时间演化曲线与模拟裂变径迹长度分布。模拟的径迹长度峰值在12-15μm之间,呈较宽的单峰分布,与实测值基本一致;(3)采用锆石和磷灰石“矿物对”裂变径迹年龄方法,选择200℃和120℃分别作为锆石和磷灰石裂变径迹的封闭温度。假设云开大山的地表年平均温度为20℃,古地温梯度为35℃/km计算样品的冷却速率和云开大山的隆升速率得出:晚中生代至早第三纪,岩石的平均冷却速率V_c≈1.33℃/Ma,云开大山的平均隆升速率V_u≈O.04mm/a,其隆升幅度大于2km;早第三纪以来,岩石的平均冷却速率为1.82℃/Ma,云开大山的平均隆升速率为0.05mm/a.晚中生代以来,云开大山的隆升幅度至少达5km,云开大山的剥蚀厚度达4km以上.说明云大山块晚中生代以来遭受了较强更多还原

【Abstract】 This dissertation is composed of the study on fission track method and application fission track method, combined with 40Ar/39Ar(K-Ar) method to study the Indosinian tectonic events and Meso-Cenozoic uplifting and exhumation of the Yunkaidashan.Firstly, the present status and developing tendencies about the principle, analytical methods, and experimental techniques of fission track dating and fission track annealing kinetics are introduced. Then combined fission track method with some experiences and knowledges learned by the author in the long-term work in fission track analysis, the existing problems and possibly solving approaches about analytical methods and experimental techniques of fission track dating are discussed. Next, the application of fission track thermochronology in geology are briefly introduced.Secondly, a case on the thermochronology of the Yunkaidashan is studied. The Yunkaidashan, situated in the southern edge of South China block, closely adjacent to Indochina block and westward linked to Qinzhou-Fangcheng trough, is the ideal region which is applicated to study the Meso-Cenozoic tectonic evolution of Indochina and South China continental blocks. This paper is combined fission track with 40Ar/39Ar thermochronology to study the Indosinian deformational events and the Meso-Cenozoic uplifting and exhumation of the Yunkaidashan, in the meanwhile, to offer new constraints and elicitations for further understanding the united dynamics mechanism of deformation in the Indochina and South China blocks, and Meso-Cenozoic geotectonic evolution of South China block.The representative samples of various-type rocks (mylonite, migmatite and granite) in the Yunkaidashan were selected to study fission track thermochronology, and four representative mylonite samples from them were selected to study 40Ar/39Ar isotopic dating. And the measured fission track and 40Ar/39Ar data are combined with available 40Ar/39Ar (K-Ar) ages, the main conclusions are obtained as follows:1. The principal zircon fission track ages range from 97.4Ma to 133.0Ma, and their main peak age is 125 Ma±, another peak age is 105 Ma±, indicating that there exists an uplifting and cooling event of the Yunkaidashan in early Cretaceous, and apatite fission track ages range from 43.5Ma to 68.4Ma, indicating that there existsanother uplifting and cooling event in the Paleogene. Average lengths of apatite confined fission tracks range from 12.60 μ m to 14.36 μ m, standard errors of most samples are more than 2 μ m, and distributions of apatite fission track lengths are broad and unimodal. Combined with geologic background of the Yunkaidashan , fission track ages are considered as cooling age of uplifting and exhumation of the Yunkaidashan.2. Grain ages and track length distributions of apatite samples were selected, with the help of AFTsolve software, temperature-time evolvement curves and modeling fission track lengths are obtained. Peak value of modeling track lengths range from 12 μm to 15 μm, which show rather broad and unimodal distributions, and the modeling track lengths distributions are basically identical to the measured data.3. Assuming that the closure temperatures of zircon and apatite fission track are 200 ℃ and 120℃, respectively, the average surface temperature of the Yunkaidashan is considered to be 20℃, and paleogeothermal gradient to be 35℃/km, zircon and apatite (named "the mineral pair") fission track ages are used to calculate the cooling and uplifting rate, and it can be elicited that from late Mesozoic to the Paleogene, the average cooling rate of rocks is Vc≈1.33℃/Ma, and the average uplifting rate of the Yunkaidashan is Vu≈0.04mm/a, and its uplifting amplitude is over 2km; since the Paleogene, the average cooling rate of rocks is Vc≈1.82℃/Ma, and the average uplifting rate of the Yunkaidashan is Vu≈0.05mm/a; since late Mesozoic, uplifting amplitude of the Yunkaidashan is more than 5km, and the exhumation and unroofing thickness is over 4km, indicating that the Yunkaidashan have undergone rather intensive uplifting and exhumation process since late Mesozoic, and the uplifting of the Yunkaidashan since the Paleogene may be greater than that from late Mesozoic to the Paleogene. And during late Mesozoic, the northwestern and southeastern Yunkaidashan have earlier initiated time of uplifting or higher paleographic pattern than that of northeastern and southwestern Yunkaidashan.4. The characteristics of all four 40Ar -39Ar plateau date for biotite manifest that they do not undergo apparently thermal interference, and show considerably identical character. 40Ar/39Ar ages are restricted between 209Ma and 226Ma, indicating that these ages should be a tectono-thermal event age, and represent the age of deformational event in Indosinian epoch. The four 40Ar/39Ar ages combined with partial available 40Ar/39Ar (or K-Ar) ages indicates that strong tectonic-metamorphic-magmatic events of the Yunkaidashan occurs in Indosinian epoch.5. Zircon and apatite fission track thcrmochronology are combined with 40Ar/39Ar thermochronology of the Yunkaidashan, it can be elicited T-t cooling curve of the Yunkaidashan. From late Triassic to early Cretaceous, average cooling rate of samples is about 1.0℃/Ma, from early Cretaceous to the Paleogene, average cooling rate of samples is about 1.33℃/Ma, and since the Paleogene, average cooling rate of samples is about 1.82℃/Ma, all of these show average cooling rate of rocks becomes great step by step.6. The accelerated uplifting in the Yunkaidashan since 25-30Ma modeling from grain ages and track length distributions of apatite samples may be related to the post-effect of the Indian plate subduction and collision with Europe-Asia plate or lateral extrusion of the Indochina block along Ailaoshan-Red River shear system in the Cenozoic.To sum up, the fission track and 40Ar/39Ar thermochronology offers good data for constraining the pattern of tectonic landform, the history of uplift and cooling, and further understanding the uplifting and exhumation process and the mechanism of uplift process in tectono-magmatic belt of South China block during the Meso-Cenozoic.更多还原