北祁连山早古生代岛弧火山岩地质特征及构造意义

发布时间：2018-06-02 03:02

本文选题：北祁连山 + 早古生代　；参考：《中国地质大学(北京)》2017年硕士论文

【摘要】：北祁连山是阿拉善地块和中祁连-柴达木地块之间的古生代造山带,也是我国洋内俯冲增生作用发育最充分、岩石记录保存最完整的区域之一。本文选择沿北祁连早古生代缝合带出露的火山岩进行了野外地质、岩石学、矿物学、元素地球化学、LA-ICP-MS锆石年代学研究工作,进而深入探讨了古祁连洋俯冲汇聚地质过程和演化历史。研究区位于青海省祁连县野牛沟乡一带,早古生代火山岩、次火山岩的岩石类型包括:流纹岩、英安岩、石英斑岩、安山岩、玄武岩和凝灰岩、火山碎屑岩等,中酸性火山岩和中基性火山岩常见互层状产出;火山岩层中还见有硅质岩、碳酸盐岩夹层,玄武岩常发生不同程度的蚀变。通过对研究区火山岩的元素地球化学特征研究分析,揭示岩石K_2O含量存在明显差异,依据中酸性岩K_2O2 wt%、中基性岩的K_2O1 wt%,可将研究区的火山岩划分出富K火山岩系列,其它(中酸性岩K_2O2 wt%、中基性岩的K_2O1wt%)归属于低K火山岩系列。所有火山岩均呈现出不同程度Nb、Ta、Zr、Hf和Ti亏损的岛弧火山岩特征,揭示它们均形成于岛弧环境,但低K火山岩系列中的流纹岩、玄武岩具有基本一致的近平坦型稀土配分型式和近平坦的微量元素蛛网图,显示主要为大洋岩石圈部分熔融形成熔浆的结晶产物;富K火山岩系列中的玄武岩和流纹岩则具有基本一致的右倾稀土配分型式和右倾微量元素蛛网图,其中LREE和大离子亲石元素明显富集,显示有大陆岩石圈物质的加入。通过综合对比研究,祁连洋早古生代俯冲-汇聚-碰撞过程中存在多期次幔源岩浆上侵、喷发作用,由于不同阶段岩石圈结构和组成的差异,母岩浆成分明显不同,形成了高K和低K两套、均含有中基性和中酸性火山岩的岛弧火山岩系列,两个系列的火山岩沿主缝合带叠加产出。通过LA-ICP-MS锆石U-Pb年代学研究,获得了富K火山岩系列玄武岩的成岩年龄,为482±2.7Ma。综合研究认为北祁连地区弧火山岩包括:(1)富K岛弧火山岩系列,其形成于被动大陆边缘的洋-陆板块俯冲汇聚阶段,有明显的大陆岩石圈边缘物质加入,该期火山喷发年龄~482 Ma,也指示晚寒武末-早奥陶世早期由洋-陆俯冲汇聚引发的岩浆活动达到了峰期;(2)低K岛弧火山岩系列,岩石出露规模相对较小,推测其成岩时代稍晚,在晚奥陶世末-早志留世祁连洋闭合-碰撞造山(445Ma~428Ma)阶段,已俯冲至大陆岩石圈之下的洋壳残片因碰撞挤压作用发生了进一步部分熔融,形成的低K富Na熔浆基本沿原火山弧位置喷发,陆缘地壳增生加厚。
[Abstract]:The northern Qilian Mountains is an Paleozoic orogenic belt between the Alashan massif and the Qilian Qaidam block. It is one of the most fully developed regions of the intra oceanic subduction and proliferation and the most complete rock record preservation. In this paper, the volcanic rocks exposed along the early Paleozoic suture zone in northern Qilian were selected for field geology, petrology, mineralogy, and elemental earth. Chemistry, LA-ICP-MS zircon chronology, and further study the geological process and evolution history of the subduction and convergence of the ancient Qilian ocean. The study area is located in the area of buangou, Qilian County, Qinghai, and the volcanic rocks of the early Paleozoic, and the subvolcanic rocks include rhyolite, British anite, quartz porphyry, andesite, basalt and tuff, and volcanic crater. The intermediate acid volcanic rocks and middle base volcanic rocks are common interbedded rocks, and there are also siliceous rocks, carbonate intercalation and basalt in different degrees of alteration in the volcanic rocks. Through the analysis of the geochemical characteristics of the elements of the volcanic rocks in the study area, it is revealed that there are obvious differences in the K_2O content of the rocks, which are based on the K_2O2 wt% in the middle acidic rock. The K_2O1 wt% of the middle base rock can divide the volcanic rocks in the study area out of the rich K volcanic series, and the other (intermediate acid rock K_2O2 wt%, the K_2O1wt% of the medium base rock) belong to the low K volcanic series. All the volcanic rocks show the characteristics of the island arc fire rocks of varying degrees Nb, Ta, Zr, Hf and Ti loss, revealing that they are formed in the island arc environment, but low in fire. The rhyolite in the series of rocks and basalts have the basic and nearly flat rare-earth distribution pattern and the near flat trace element cobweb diagram, showing that the major oceanic lithosphere is partially fused to form the crystallization products of the melts, and the basalt and rhyolite in the rich K volcanic series have the basic right leaning Rare Earth Distribution pattern and right deviation. Trace element cobweb, in which LREE and large ion lithospheric elements are obviously enriched, showing the inclusion of continental lithosphere material. Through a comprehensive comparison, there are many magma invading and erupting during the early Palaeozoic subduction collisions in the Qilian ocean, and the composition of the lithosphere in different stages, the composition of the lithosphere in different stages, and the composition of the mother magma. Obviously different, the two sets of high K and low K are formed, all of which contain medium and medium acidic volcanic rocks, and two series of volcanic rocks are superimposed along the main suture zone. Through LA-ICP-MS zircon U-Pb chronology, the Cheng Yannian age of the basalt in the series of rich K volcanic rocks is obtained, and the northern Qilian region is considered as 482 + 2.7Ma. comprehensive study. The arc volcanic rocks include: (1) the rich K island arc volcanic series, which formed in the subduction and convergence stage of the oceanic and continental plates on the passive continental margin, with the obvious continental lithospheric marginal substances added, and the volcanic eruption age of ~482 Ma, indicating that the magma activity caused by the oceanic continental subduction and convergence early in the late Cambrian Early Ordovician reached the peak period; (2) low K The series of volcanic rocks in the island arc is relatively small, and it is presumed that the late Ordovician period of the late Ordovician Early Silurian Qilian ocean closed collision orogenic (445Ma~428Ma) stage, the crust fragments of the oceanic crust subducted to the continental lithosphere have been partially melted because of collision and extrusion, and the low K rich Na melts formed basically along the original fire. The position of the mountain arc is erupted, and the crustal hyperplasia of the continental margin is thickened.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P534.4;P588.14

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