羌塘地块晚三叠世—早白垩世沉积地层古地磁研究

发布时间：2018-08-15 17:09

【摘要】：青藏高原属于特提斯构造域东段,羌塘地块是青藏高原的主要地体,研究羌塘地块对其构造演化及运动学、动力学等问题至关重要,其形成演化涉及到古特提斯洋的关闭和新特斯洋的开启等地学热点问题。本次研究以羌塘地块北部的唐古拉山地区的中生代地层为目的层,该剖面地层时代清楚,地层接触关系确定。以期望通过该区中生代地层的古地磁研究为羌塘地块的大地构造演化以及新特提斯洋的形成提供定量约束。本次研究在羌塘地块北部的唐古拉山地区共设置了15个采样点,其中晚三叠世结扎群设置采点9个,采集107块定向标本,早白垩世错居日组设置采点6个,共43块定向标本,共采集150块古地磁定向样品。在室内将其加工成标样,在中科院地质与地球所古地磁与年代学实验室进行了岩石磁学和退磁实验。晚三叠世结扎群沉积岩地层通过其岩石磁学研究发现,主要携磁矿物为赤铁矿、磁黄铁矿、少量钛磁铁矿、极少量的磁铁矿。早白垩世错居日组紫红色细砂岩的主要携磁矿物是赤铁矿,还有少量针铁矿。根据磁性矿物类型,制定了合适的退磁方法和退磁实验步骤,对样品有针对性的进行了系统的热退磁,交变退磁和混合退磁等实验。对剩磁数据分析采用了kirschvink主分量分析法,对采点内和地层组水平上数据统计采用Fisher统计,晚三叠世结扎群沉积岩样品的高温分量通过了McElhiny褶皱检验,早白垩世错居日组细砂岩样品高温分量通过了倒转检验,均可被认为是原生剩磁分量。通过以上实验和分析统计及计算,得出结扎群沉积岩组的高温剩磁分量平均方向为：Ds=4.2°,Is=-29.7°, Ks=24, α95=10.7°;计算出相应的古地磁极位置为：φp=259.4°, λp=71.5°, dp/dm=8.8°;古纬度为Φ古=15.9°N；错居日组的高温剩磁分量平均方向为：Ds=154.8°,Is=-43.2°, Ks=19.2, α95=15.7°;计算出相应的古地磁极位置为：λp=66.3°,λP=346.2°, dpldm=15.4°,古纬度为Φ古=25.2°N。根据本次研究得到的最新古地磁数据,并结合前人对拉萨地块的研究成果,进行了对比讨论,并绘制了羌塘和拉萨的石炭纪到白垩纪的视极移曲线(APWP)图和古纬度变化图。得出以下结论：晚三叠世班公湖-怒江洋缝合带所代表的新特提斯洋扩张到最大,羌塘(Φ古=15.9)和拉萨(Φ古=-16.6)两者纬度最大相差至少32.5°,指示了班怒洋此时的规模至少有3600km,此后该洋盆开始收缩。从APWP可知,从晚三叠世到中晚侏罗世,羌塘板块表现出了较明显的“极移”现象,并且快速的向北漂移,说明该时期是羌塘板块的快速北移时期。中晚侏罗到早白垩世,羌塘地块纬向漂移量显著减少,主要是逆时针旋转期。根据拉萨和羌塘地块在此时的视极移曲线与古纬度的分析,指示中侏罗世拉萨地块的北移,使得其北的班公湖-怒江洋闭合。
[Abstract]:The Qinghai-Tibet Plateau belongs to the eastern segment of the Tethys tectonic domain, and the Qiangtang block is the main terrane of the Qinghai-Tibet Plateau. It is important to study the Qiangtang block for its tectonic evolution, kinematics and dynamics. Its formation and evolution related to the closure of the ancient Tethys Ocean and the opening of the New Tethys Ocean. The Mesozoic strata in Tanggula Mountain area in the north of Qiangtang block are taken as the target layer. The stratigraphic age of the section is clear and the stratigraphic contact relationship is determined. The paleomagnetic study of Mesozoic strata in this area is expected to provide quantitative constraints for the tectonic evolution of the Qiangtang block and the formation of the NeoTethys Ocean. In this study, 15 sampling sites were set up in Tanggula Mountain area in the north of Qiangtang block, of which 9 were collected in late Triassic ligation group, 107 directional specimens were collected, and 6 were collected in early Cretaceous staggered Jourri formation, with 43 directional specimens. A total of 150 paleomagnetic directional samples were collected. The lithomagnetic and demagnetization experiments were carried out in the paleomagnetism and geochronology laboratory of the Institute of Geomagnetism and Geochronology of the Chinese Academy of Sciences. The sedimentary strata of the late Triassic Lianza Group found that the main magnetic carrying minerals are hematite, pyrrhotite, a small amount of ilmenite and a very small amount of magnetite. In the early Cretaceous Miaojuri formation, the main magnetic carrying minerals of the fuchsia fine sandstone are hematite and a small amount of goethite. According to the type of magnetic minerals, the appropriate demagnetization method and demagnetization experimental steps were established, and systematic experiments such as thermal demagnetization, alternating demagnetization and mixed demagnetization were carried out on the samples. Kirschvink principal component analysis was used to analyze the residual magnetic data, Fisher was used to statistics the data in the production point and stratigraphic group, and the high temperature component of the sedimentary rock samples of the late Triassic ligation group passed the McElhiny fold test. The high temperature components of the early Cretaceous Miaojuri formation fine sandstone have passed the inversion test and can be considered as primary remanent magnetic components. Based on the above experiments, statistics and calculations, it is obtained that the mean direction of the high-temperature remanent magnetic components of the sedimentary rocks in the Lianzhang Group is: 1: Dsl 4.2 掳Is-29.7 掳, KsN 24 掳, 伪 9510 7 掳, and the corresponding paleomagnetic pole positions are: 蠁 p0 259.4 掳, 位 p0 71.5 掳, dp/dm=8.8 掳, and 桅 paleo (15.9 掳N). The mean direction of the high temperature remanent magnetic component in the staggered group is: 1 DsN 154.8 掳Is-43.2 掳, Ksl 19.2 掳, 伪 95N 15.7 掳, and the corresponding paleomagnetic pole positions are calculated as 位 PU 66.3 掳, 位 PU 346.2 掳, dpldm=15.4 掳, and paleolatitude 桅 paleo 25.2 掳N. Based on the latest paleomagnetic data obtained in this study and combined with the previous research results of Lhasa block, this paper makes a comparative discussion, and draws the (APWP) diagram of the Carboniferous to Cretaceous apparent pole shift curve and the paleolatitude variation map of Qiangtang and Lhasa. The following conclusions are drawn: the Neo-Tethys ocean represented by the late Triassic Bangong Lake-Nu River ocean suture zone expanded to the largest extent. The maximum latitudes difference between Qiangtang (桅 Guji15.9) and Lhasa (桅 Guji-16.6) is at least 32.5 掳, indicating that the scale of Banru Ocean is at least 3600km at this time, and then the basin begins to shrink. According to APWP, from the late Triassic to the Middle and late Jurassic, the Qiangtang plate showed a more obvious "pole shift" phenomenon, and the rapid northward drift, indicating that this period is a period of rapid northward migration of the Qiangtang plate. From the middle late Jurassic to the early Cretaceous, the zonal drift of Qiangtang block decreased significantly, mainly in the counterclockwise rotation period. According to the analysis of the apparent pole shift curve and paleo latitude of Lhasa and Qiangtang blocks at this time, it is indicated that the northward migration of Lhasa block in the Middle Jurassic resulted in the closure of the Bangong Lake and the Nujiang River in the north of Lhasa and Qiangtang blocks.
【学位授予单位】：西北大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P318.44;P534.5

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