【作者】 宋博；

【导师】 闫全人；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2013， 硕士

【摘要】 凭祥中三叠世盆地位于印支地块与华南地块碰撞拼合的滇—琼缝合带内,记录了沿华南地块南缘展布的古特提斯分支洋盆俯冲闭合、印支地块与华南地块碰撞拼合的过程。本文通过大比例尺实测地质剖面,详细测量了凭祥盆地沉积相序及其组合变化,分析了不同相序的沉积环境及其物源,并结合岩相学和碎屑岩地球化学特征对盆地构造属性进行了探讨。结果表明,凭祥盆地主要充填有深水盆地相、浊积扇相和三角洲相等不同环境的沉积物。中三叠世期间盆地呈现为一系列因碰撞拼合作用形成的构造高地间夹深谷的岩相古地理面貌。深水盆地相以发育大套深灰色泥页岩和裹夹碎屑流沉积为特征,碎屑流沉积发育于构造高地陡坡一侧。浊积扇相以发育槽模、正粒序、爬升波纹层理、包卷层理、双向交错层理、透镜状层理、平行层理为特征。三角洲相以发育大型板状交错层理、潮沟、厚层透镜状砂体和砖红色泥岩为特征,类似发育于俯冲汇聚环境下的牙买加型扇三角洲,可能发育于构造高地缓坡一侧。沉积作用分析表明,主要存在碎屑流、浊流等重力流作用,并识别出底流作用。古水流分析表明存在向南和向北两个方向的物质搬运。岩相学特征表明盆地砂岩成分成熟度和结构成熟度均较低,物源为再旋回造山带或碰撞造山带。碎屑岩地球化学特征表明,凭祥中三叠世盆地中的砂岩和泥岩样品SiO2含量为61.71-74.85%(平均为66.61%),其含量接近于上地壳的平均值。Al2O3含量为14.47-22.37%(平均为18.32%),Fe2O3t含量为6.28-9.13%(平均为7.24%),具有高的K2O/Na2O为6.50-0.51(平均为3.86%)和高的(Fe2O3t+MgO)为7.29-10.31%(平均9.12%),Fe2O3t/K2O比值为2.05-5.54%(平均2.93%),矿物稳定性较差。稀土元素标准化配分曲线呈现出轻稀土富集、重稀土平坦和明显Eu、Ce负异常特征,类似于上地壳和典型太古代页岩,具有海相沉积特征。砂岩风化蚀变指数高CIA(71-88),Th/U比值为3.68-9.53,表明砂岩和泥岩经历了较强的风化作用,砂岩物源区判别图研究表明凭祥盆地物源区为酸性岛弧源区。砂岩源区构造环境判别图解及特征指数分析表明,中三叠世砂岩源区主要形成于大洋岛弧和活动大陆边缘。空间上与之相配套的是盆地周缘早三叠世岛弧型流纹斑岩。这些特征综合表明,凭祥盆地可能是一个伴随古特提斯分支洋盆闭合而被构造改造的残余弧前盆地。该弧盆系统可能于晚二叠世末开始发育,中三叠世末结束沉积充填,暗示印支地块和华南地块最终于中三叠世碰撞拼合。更多还原

【Abstract】 The Middle Triassic Pingxiang basin, located in the suture zone between the Indochina and South China blocks, records the processes of the closure of the Paleo-Tethys and the collision of the Indochina and South China blocks. Based on careful observations along a single transect across the Pingxiang basin and detailed analysis of sedimentation and provenance of this basin, its tectonic setting are systematically studied in this paper. The results presented in this paper show that the sedimentations in the Pingxiang basin are dominated by deep-water basin facies and turbidite fan facies, with minor amounts of deposits of delta facies. In particular, tectonic rises and deep valleys were developed during the Middle Triassic associated with the subduction and collision. The deep-water basin is dominated by dark-grey mudstone facies with the interbeds of debris flows, is formed on the steep slope of the tectonic rises. The turbidite fan facies is characterized by flute cast, normal-graded bedding, climbing ripple lamination, convoluted lamination, bidirectional cross bedding, lenticular bedding and parallel bedding. Commonly, the delta facies is spatially associated with the tectonic rises, and characterized by large tabular cross bedding, tidal creek deposits, thick lenticular sandbody and brick red mudstone, in great agreement with the Jamacia-type or active-type fan delta that is always developed in the subduction-related tectonic settings and occurs on the gentle slopes of the tectonic rises. Sedimentation mechanism of the deposits in the Pingxiang basin includes debris flow, turbidity current, and bottom current. Paleocurrent analysis performed within the Pingxiang basin indicated northward and southward directed paleo-sediment dispersal patterns. Petrography features suggest that the sandstones, which have low compositional and textural maturity, were derived from recycled orogen or collisional orogen. The geochemistry of fine-grained sandstones and mudstones are characterized by moderate S1O2contents (61.71%-74.85%) that are close to the upper crust, high K20/Na20ratio (6.50-0.51) and high Fe2O3t+MgO contents (7.29%-10.31%). The Fe2O3t/K2O ratio (2.05%-5.54%) shows low stabilities of materials for these rocks. The REE patterns of analyzed sediment rocks in the Pingxiang basin are similar to those of the upper crust and typical Archean shales, and show enriched LREE and flat HREE with obvious Eu-and Ce-anomalies. The CIA index (generally71-88) and Th/U ratios (3.68-9.53) indicate that the sediments in the basin may have undergone high degree of weathering. Geochemistry demonstrates that the detritus in the Pingxiang basin were derived predominantly from felsic volcanic rocks source, and an active continental margin setting in which the Pingxiang basin developed. In summary, the Pingxiang basin is a tectonically modified remnant forearc basin, temporally and spatially associated with a volcanic arc (mapped as the Beisi Formation and dominated by rhyolites). This arc-forearc basin configuration presents supporting evidence of northward subduction of the oceanic lithosphere of the Paleo-Tethys that started in the late Permian and finally terminated in the Middle Triassic when the Indochina block collided with the South China block.更多还原