山西吕梁杂岩岩浆混合作用及界河口群形成背景

发布时间：2018-08-06 15:32

【摘要】：近年来,关于华北克拉通中部造山带在古元古代时期的构造演化尚存在很多争议。吕梁杂岩体是华北中部造山带的重要组成部分,发育新太古代末到古元古代末的多期花岗类岩石,基性岩侵入体和火山-沉积岩系。本文对其中发生的岩浆混合作用区域岩石和界河口群进行了研究。吕梁杂岩岩浆混合作用发生的时代与背景如何;对于反映中部带构造演化有何指示意义;以及界河口群的形成时代和物质来源,是本文要讨论的主要问题。本文关于岩浆混合作用的研究区域岩体主要为斜长角闪岩,片麻岩,对岩浆岩的锆石进行LA-ICP-MS U-Pb定年表明,斜长角闪岩和片麻岩样品的形成年龄分别为2104Ma和2147Ma,这两个样品的采样点均为明显的岩浆混合作用区域。据此我们推断,岩浆混合作用发生于～2.1Ga。地球化学分析表明,基性岩样品呈平坦的稀土配分模式,总稀土含量以及轻稀土富集程度很低,无明显负Eu异常;岩石相对富集Cs、Rb、Ba、K等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素,Zr、Hf有负异常,为板内亚碱性拉斑玄武岩。酸性岩样品轻重稀土分馏明显,Eu异常呈现正异常与负异常两种模式。样品富集Rb、Th、U、Pb、Zr和Hf,而亏损Nb、Ta、P、Ti等高场强元素,为A1型花岗岩,指示华北克拉通中部带在～2.1Ga为大陆裂谷环境。界河口群变质沉积岩以石英云母片岩与变质砂岩为主,三个样品的碎屑锆石最小年龄峰分别为2000Ma,2048Ma和2071Ma。我们根据它们得到的加权平均年龄～2.0Ga限定了界河口群变质沉积岩原岩的最大沉积年龄。地球化学分析表明,变质沉积岩样品相对富集Cs、Rb、K等大离子亲石元素,亏损Ba、Nb、P、Ti等元素,大多数样品形成于大陆岛弧或活动大陆边缘构造背景。我们认为很有可能为弧后盆地环境。将界河口群沉积岩与西部陆块孔兹岩带的碎屑锆石年龄数据进行投图对比分析,得到了相似的年龄分布。而与TNCO和东部陆块碎屑锆石具有明显差别。因此我们倾向于界河口群与东部陆块和中部造山带不是同源的,而是与西部陆块孔兹岩带相同起源,俯冲方向应为向东。界河口群为表壳岩,普遍遭受了角闪岩相的变质作用,我们在庞泉沟剖面未观察到上述表壳岩,从所测试的样品看,无论是片麻岩还是斜长角闪岩,其锆石都有明显的岩浆锆石特征。特别是片麻岩样品,锆石形态与花岗岩中的锆石十分相似,未见具搬运特征的锆石。两个基性岩的形成年龄分别为～1.91Ga、～1.94Ga,而片麻岩样品14PQG03的核部年龄为～2.12Ga,边部年龄为～1.92Ga,前者侵入后者,在～1.92Ga经历一期变质作用。综合上述特征,我们认为,庞泉沟地区不存在代表表壳岩系的界河口群。华北中部在～2.1Ga处于大陆裂谷环境,在此之后逐渐转变为向东的俯冲岛弧环境,并且最早在～2.0Ga,在吕梁地区的弧后盆地环境开始了界河口群碎屑物质的沉积。两个基性岩墙样品指示了俯冲所形成的弧后盆地。
[Abstract]:In recent years, there are still many controversies about the tectonic evolution of the central North China Craton orogenic belt during the Paleoproterozoic period. The Lv Liang complex is an important part of the orogenic belt in the central part of North China. It develops multi-stage granitic rocks from the late NeoArchean to the late Gu Yuan, basic rock intrusions and volcanic-sedimentary rock series. In this paper, the rock and estuarine group in the magmatic mixing area are studied. The main problems to be discussed in this paper are the age and background of magmatic mixing in the Lv Liang complex, the indicative significance for reflecting the tectonic evolution of the central belt, and the formation age and material source of the boundary estuary group. In this paper, the magmatic mixing study area is mainly composed of amphibolite and gneiss. The LA-ICP-MS U-Pb dating of zircon from magmatic rocks shows that, The age of formation of amphibolite and gneiss samples is 2104Ma and 2147 Ma.The sampling points of these two samples are both magmatic mixing zones. Based on this, we infer that magmatic mixing occurred at 2.1 Ga. Geochemical analysis shows that the basic rock samples show a flat REE distribution pattern, the total REE content and the LREE enrichment degree are very low, and there is no obvious negative EU anomaly. There is negative anomaly in the depleted NbTa-Ta-Pa-Ti isoprestic field strength element, which is subalkaline tholeiitic basalt in the plate, and there is a negative anomaly in the zirconium HF. The fractionation of heavy and heavy rare earth elements in acid rock samples shows positive and negative EU anomalies. The samples are rich in RbPU UPbN Zr and HF, but depleted in high field strength elements such as NbCU TaPnti, which indicate that the central North China Craton belt is a continental rift environment at 2.1Ga in the North China Craton zone, which is characterized by a high field strength element, such as A 1 type granite. The metamorphic sedimentary rocks of Jiegou Group are mainly quartz mica schist and metamorphic sandstone. The minimum age peaks of zircon for the three samples are 2000 Ma 2048 Ma and 2071 Ma respectively. According to their weighted mean age of 2.0 Ga, we define the maximum sedimentary age of the metamorphic sedimentary rocks of the boundary estuary group. Geochemical analysis shows that metamorphic sedimentary rocks are relatively enriched in large ion lithophile elements, such as Csln Rbnk, and depleted in elements such as BaNbPU Ti. Most of the samples were formed in continental island arcs or active continental margin tectonic setting. We believe that it is very possible for the back-arc basin environment. The age data of detrital zircons from the sedimentary rocks of the estuarine group and the korzite belt of the western continental block are compared and analyzed, and a similar age distribution is obtained. But it is different from TNCO and the eastern continental block detrital zircon. Therefore, we tend to suggest that the boundary estuary group is not homologous with the eastern continental block and the central orogenic belt, but with the same origin as the western continental block khontzite belt, and the subduction direction should be eastward. The Jiegou Group is a epidermal rock and has generally suffered from the metamorphism of amphibolite facies. We have not observed the above-mentioned epidermal rocks in the Pang Quangou section. From the samples tested, whether they are gneiss or amphibolite, The zircon has obvious magmatic zircon characteristics. Especially for gneiss samples, the morphology of zircon is very similar to that of zircon in granite, and there is no zircon with transport characteristics. The age of formation of the two basic rocks is 1.91 Ga / 1.94 Ga, while the nuclear age of gneiss is 2.12 Ga and the edge age is 1.92 Ga. The former intruded into the latter and underwent one stage metamorphism at 1.92 Ga. Based on the above characteristics, we believe that there is no estuarine group in Pangquangou area, which represents the epidermal rock series. The central part of North China is located in the continental rift environment of 2.1Ga, then gradually transformed into an eastward subduction island arc environment, and the depositional depositions of clastic materials of the boundary estuarine group began to occur in the back-arc basin environment of Lv Liang area as early as 2.0 Ga. Two base-wall samples indicate the back-arc basin formed by subduction.
【学位授予单位】：南京大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.11;P542

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