西秦岭北缘道帏地区晚古生代—早中生代沉积地层地质特征与物源分析

发布时间：2018-11-02 09:21

【摘要】：西秦岭北缘分布有二叠纪甘家组和早三叠世隆务河组,其详细的沉积地层划分、沉积环境、沉积物源和原型盆地研究仍较为薄弱。本文以西秦岭北缘循化道帏地区出露的二叠纪甘家组、早三叠世隆务河组为研究对象,在野外观察、系统剖面测制、面积性地质填图基础上,进行详细的岩石地层单位划分,综合岩石学、沉积学、碎屑骨架组分、砂岩岩石地球化学、碎屑锆石U-Pb年龄信息分析,对沉积时代、沉积特征、沉积相与沉积环境、盆地充填序列、沉积物源、盆地构造属性及原型等进行了论述,在此基础上尝试恢复了西秦岭北缘晚古生代-早中生代构造演化过程。取得了以下主要进展和认识:1、西秦岭北缘道帏地区晚古生代-早中生代沉积地层主要包括二叠纪甘家组和早三叠世隆务河组。甘家组由下到上可划分为4段:一段(Pg~1)为粗碎屑岩夹板岩段,由扇三角洲平原沉积向上过渡为碳酸盐岩台地潮间带沉积。二段(Pg~2)为细碎屑岩夹砂岩段,为浅海陆棚边缘盆地沉积。三段(Pg~3)为生物礁灰岩段,为浅海陆棚碳酸盐岩台地潮间带沉积。四段(Pg~4)为细碎屑岩夹灰岩段,为半深海—陆棚边缘盆地沉积。隆务河组由下到上可划分为5段:一段(T1l~1)为粗碎屑岩夹板岩段,属浊积扇内扇沉积。二段(T1l~2)为细碎屑岩夹薄层状砂岩、灰岩段,属浊积扇中扇沉积(无水道部分)。三段(T1l~3)为薄层板岩、中-中薄层砂岩段,属浊积扇中扇沉积(有水道部分)。四段(T1l~4)为板岩夹中-中厚层砂岩段(互层),为浊积扇中扇沉积,可能位于大陆斜坡中下部。五段(Tll5)为粉砂质(泥质)板岩段,属浊积扇外扇沉积,为浊积扇远端与深水盆地交接地带。2、在甘家组一段的砂岩样品中获得的最小碎屑锆石U-Pb年龄为299Ma,代表甘家组的沉积下限。结合古生物化石资料、地层叠覆关系将甘家组地层沉积时代厘定为二叠纪。隆务河组砂岩中最小碎屑锆石U-Pb年龄为258Ma,为地层沉积下限,侵位于隆务河组的古夷闪长岩结晶年龄为246.7±2.1Ma,为地层沉积上限,从而将隆务河组沉积时代限定在258~247Ma。综合前人古生物化石资料,将研究区隆务河组沉积时代厘定为早三叠世。3、甘家组源岩风化程度较低,属第一次旋回沉积,主要物质来源为祁连造山带早古生代切割型岩浆弧,以酸性岩类为主,还有少量来自于祁连造山带中元古代热事件和新元古代响应Rodinia超大陆汇聚、裂解的物质记录。沉积盆地原型为被动大陆边缘浅海陆棚。4、隆务河组为第一次旋回的沉积物,源区化学风化较弱,古水流方向为NWW-SEE方向,物源包括祁连造山带的早古生代切割型岩浆弧、祁连造山带南缘经历古特提斯洋初始裂解至俯冲过程中的系列记录,以及祁连造山带的变质基底,特别是研究区出露的化隆岩群可能是其直接物源之一。源岩以酸性岩为主,还有少量基性岩,为两种特征岩性的混合分布的区域,可能是受到基底隆升影响的结果。沉积盆地原型应为弧后盆地。5、综合本文资料、区域地质资料及前人研究将西秦岭北缘及邻区晚古生代-早中生代的构造演化过程划分为三个阶段:1)泥盆纪-中二叠世古特提斯洋裂解-拉张阶段;2)中二叠世晚期-早三叠世洋壳北向俯冲,弧后裂谷盆地扩张阶段;3)中晚三叠世弧后盆地关闭,碰撞及后碰撞阶段。
[Abstract]:In the north margin of west Qinling, there are Permian Gan jia Formation and Early Triassic Longwu River Formation, and the detailed sedimentary stratum division, sedimentary environment, sediment source and prototype basin research are still relatively weak. Based on field observation, system profile measurement and area geological mapping, detailed rock formation unit division, comprehensive petrology and sedimentology are carried out on the basis of field observation, system profile measurement and area geological mapping. The sedimentary age, sedimentary characteristics, sedimentary facies and sedimentary environment, basin filling sequence, sediment source, basin structure attribute and prototype are discussed in this paper. On this basis, the evolution of late Paleozoic-early Mesozoic tectonic evolution in the northern margin of west Qinling is attempted. The following major progresses and understandings have been made: 1. The Late Paleozoic-Early Mesozoic sedimentary formation in the northern margin of the West Qinling Mountains mainly includes the Permian Gump Formation and the Early Triassic Longwu River Formation. The Ganjia Formation can be divided into 4 segments from below: one segment (Pg ~ 1) is the coarse clastic rock sandwiched slate section, and the transition from the fan delta plain to the carbonate platform. The second section (Pg ~ 2) is a fine clastic rock sandwiched sandstone section, which is deposited in the marginal basin of shallow sea continental shelf. Three sections (Pg ~ 3) are reef limestone sections, which are deposited in shallow sea shelf carbonate platform. The fourth section (Pg ~ 4) is a fine clastic rock sandwiched limestone section, which is deposited in the marginal basin of the semi-deep sea basin. The Longwu River Formation can be divided into 5 sections from below: one section (T1L ~ 1) is coarse clastic rock and slate section, which belongs to fan deposit in turbidite fan. The second stage (T1l ~ 2) is thin layer sandstone and limestone section of fine clastic rock, and belongs to fan deposit in turbidite fan (no water channel part). Three sections (T1l ~ 3) are thin layer slate, middle-middle layer sandstone section, and belongs to the fan deposit in the turbidite fan (with water channel section). The fourth section (T1l ~ 4) is the middle-middle thick layer sandstone section (mutual layer) in the slate clip, which is fan-deposited in the turbidite fan and may be located on the continental slope. The fifth section (Tll5) is a silt (mud) slate section, which belongs to the turbidite fan outside fan deposit, which is the transition zone of the turbidite fan far end and the deep water basin. 2. The minimum debris flow rate U-Pb obtained in the sandstone sample of the Ganjiagang Formation is 299Ma, representing the lower limit of the deposition of the Ganjia Formation. In this paper, the stratigraphic sedimentary age of Ganjia Formation is determined to be Permian in combination with terrestrial fossil data. The average age of U-Pb in the sandstone of Longmenhe Formation is 258Ma, which is the lower limit of the formation, and the crystallization age of the Paleogene in Longwu River Formation is 246. 7Mt. 2.1Ma, which is the upper limit of the formation, so that the sedimentary age of Longwu River is limited to 258 ~ 247Ma. The sedimentary age of Longwu River Formation in the study area is determined as early Triassic. The weathering degree of the source rocks in the Ganjiashan Formation is lower than that of the first cycle. There is also a small amount of the material records from the middle and late PROTEROZOIC THERMAL events in Qilianshan Belt and the new PROTEROZOIC-response Rodinia supercontinent. The sedimentary basin prototype is passive continental margin shallow sea shelf. 4, Longwu River Formation is the first cycle sediment, the source area chemical weathering is weak, the paleocurrent direction is NWW-SEE direction, and the object source includes the Early Paleozoic cutting type magma arc of Qilianshan belt. It is probably one of the source sources of the metamorphic basement of Qilianshan Belt and the metamorphic basement of Qilianshan belt, especially in the research area. The source rocks are dominated by marble, and a small amount of basic rocks, which are the mixed distribution of the two characteristic lithologies, may be the result of the influence of the uplift of the substrate. The sedimentary basin archetype should be an arc-back basin. 5. According to the data of this paper, the regional geological data and the previous studies, the tectonic evolution of the Late Paleozoic-Early Mesozoic in the northern margin of the West Qinling and the adjacent region is divided into three stages: 1) Triassic-Middle Permian Gutertitian lysis-tension stage; 2) Middle Permian late-Early Triassic oceanic crust north-to-dive, post-arc rift basin expansion stage; 3) middle-late Triassic arc back basin closing, collision and post-collision stage.
【学位授予单位】：长安大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P512.2;P534

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