原特提斯北界东段早古生代变形特征与构造演化

发布时间：2018-11-07 14:07

【摘要】：北秦岭地体位于秦岭造山带北部,主要包括宽坪群、二郎坪群和秦岭群,是研究原特提斯洋北界构造演化的关键区域。近年来,地质学家们通过岩石学、地质年代学、地球化学和构造地质学等手段对北秦岭地区进行了深入的研究。但这些研究多集中于造山带内部的各个构造单元及商丹缝合带,而对洛南—栾川断裂研究较少,这使得宽坪洋的演化过程至今仍不清楚,也阻碍了对原特提斯洋北界的深入研究。本文以构造解析为基础,通过筛分北秦岭地体及华北块体南缘不同期次的构造变形样式,建立横跨研究区的地质剖面,并通过收集已有的变质年龄资料和碎屑锆石数据,来探讨宽坪洋的俯冲—碰撞—折返过程。根据详细的构造解析及已有的变质年代学资料,本文将研究区内的褶皱构造变形分为三幕。其中第一幕变形以倾向北北东和南南西方向的透入性面理(S1)为特征,局部残留紧闭的F1褶皱。第二幕变形以紧闭不对称褶皱为特征,其轴面面理(S2)也主要倾向北北东和南南西方向。这两幕变形发生在约440-400 Ma之间,限制了北秦岭地体与华北块体南缘的碰撞和折返过程。之后,这两幕变形被第三幕变形所改造,后者主要以北东东走向的直立宽缓褶皱和广泛发育的膝折带为特征,其发生时间较晚,可能为晚侏罗世—早白垩世。此外,划分北秦岭地体与其邻区及其内部各岩石构造单元的洛南—栾川、官坡—乔端、朱阳关—夏馆和商丹断裂带均具有多期活动的特征,主体表现为早期以韧性剪切作用为主,晚期以逆冲作用为主,且这些断裂活动在平面上都具有分段性,具体表现在不同地区断裂倾向或韧性剪切方向不同,这可能是在区域应力作用下局部发生调节作用的结果。宽坪洋是个个存在于中元古代—早古代期间的大洋,其闭合的古缝合线为洛南—栾川断裂,同时由于该断裂两侧地球化学存在巨大差异,甚至超越了商丹带,因而推测洛南—栾川断裂代表了研究区内原特提斯洋的北界。华北块体南缘长期存在的被动陆缘沉积体系说明宽坪洋不可能向北俯冲到华北块体之下,此外,地质剖面揭示了主体向南倾的边界重大断裂和F2褶皱轴面,收集的前人的变质岩数据说明高压榴辉岩主要出露于秦岭群北部,而高压麻粒岩主要出露于秦岭群南部,这些现象共同说明宽坪洋洋壳向南俯冲到秦岭群之下。此外,本论文将研究区内不同构造单元的碎屑锆石年龄谱进行对比,结果显示最新年龄为-640 Ma的宽坪群的碎屑锆石年龄谱与秦岭群相似,而与华北块体完全不同：而最新年龄为-510 Ma的宽坪群的碎屑锆石年龄谱则与秦岭群和华北块体均有相似性,这说明华北块体在新元古代—早古生代期间随着宽坪洋的持续向南俯冲而与宽坪群逐渐靠近。宽坪洋最终在早古生代晚期完全闭合,表现为华北块体南缘与北秦岭地体的碰撞及随后北秦岭地体的折返过程。
[Abstract]:The North Qinling terrane is located in the northern part of the Qinling orogenic belt, which mainly includes the Kuanping Group, Erlangping Group and Qinling Group. In recent years, geologists have studied the North Qinling area by means of petrology, geochronology, geochemistry and tectonic geology. However, most of these studies focus on each tectonic unit and Shangdan suture zone in the orogenic belt, but less on Luonan-Luanchuan fault, which makes the evolution of the Kuanping Ocean still unclear. It also hinders the further study of the northern boundary of the original Tethys. On the basis of structural analysis, the geological profile across the study area is established by sifting the tectonic deformation patterns of the North Qinling terrane and the southern margin of the North China block, and by collecting the existing metamorphic age data and the data of detrital zircon. To study the subduction-collision-reentry process in the wide Ping Ocean. According to the detailed structural analysis and metamorphic chronological data, the fold tectonic deformation in the study area is divided into three acts. The deformation of the first act is characterized by the permeable surface (S1) in the NNE and west-south directions, and the partially closed F1 folds remain. The deformation of the second act is characterized by closed asymmetric folds and its axial surface (S2) is also mainly oriented to NNE and NW. These two deformations occurred in the range of 440-400 Ma, limiting the collision and reentry process between the North Qinling terrane and the southern margin of the North China block. Later, these two deformations were transformed by the third act deformation, which was characterized by the wide vertical fold and the widely developed knee fold, which occurred in the late Jurassic to the early Cretaceous, and was characterized by the wide vertical fold and the widely developed knee fold in the east and west direction of the latter, which may be the late Jurassic to the early Cretaceous. In addition, the Luonan-Luanchuan, Guanpo, Zhuyangguan-Xiaguan and Shangdan fault zones, which divide the North Qinling terrane and its adjacent areas and their internal rock tectonic units, are all characterized by multi-stage activities. The main body is ductile shearing in the early stage and thrust in the late stage, and these faults are segmented in the plane, which are manifested in the different fracture tendency or ductile shear direction in different regions. This may be the result of local adjustment under the action of regional stress. The Kuanping Ocean is an ocean that existed in the middle Proterozoic to the early Paleozoic. The closed paleo-suture line is the Luonan-Luanchuan fault. At the same time, because of the huge geochemical differences on both sides of the fault, it even transcends the Shangdan belt. Therefore, it is inferred that the Luonan-Luanchuan fault represents the northern boundary of the original Tethys ocean in the study area. The long-existing passive continental margin sedimentary system in the southern margin of the North China block indicates that the Kuanping Ocean cannot submerge under the North China block in a northerly direction. In addition, the geological profile reveals the major faults along the boundary and the F 2 fold axis of the main southward dip. The data of metamorphic rocks collected show that the high-pressure eclogites are mainly distributed in the northern Qinling Group, while the high-pressure granulites are mainly distributed in the southern part of the Qinling Group. These phenomena together indicate that the Kuanping oceanic crust subducts to the south under the Qinling Group. In addition, the zircon age spectra of different tectonic units in the study area are compared in this paper. The results show that the clastic zircon age spectrum of the Kuanping Group with the latest age of -640 Ma is similar to that of the Qinling Group. The age spectrum of the clastic zircon of the Kuanping Group with the latest age of -510 Ma is similar to that of the Qinling Group and the North China Block. This indicates that the North China block subducted to the south during the Neoproterozoic and early Paleozoic with the Kuanping Group gradually approaching along with the Kuanping Ocean. The Kuanping Ocean was finally completely closed in the late early Paleozoic, which was characterized by the collision between the southern margin of the North China block and the North Qinling terrane and the subsequent reentry process of the North Qinling terrane.
【学位授予单位】：中国海洋大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：P542

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