山西五台地区峨口花岗岩地球化学特征及大地构造意义

发布时间：2018-09-12 05:53

【摘要】：山西五台地区峨口主要出露酸性花岗岩,形成时间约在太古界晚期,岩体边缘片麻状较发育,峨口花岗岩对华北克拉通的形成有着比较特殊的意义,本文基于对该地区侵入岩的野外调查之上,通过对峨口花岗岩的岩相学(薄片鉴定)、LA-ICP-MS锆石U-Pb年代学以及全岩地球化学等方面的研究,讨论山西峨口花岗岩的岩浆来源、成因及形成的构造环境。两组高精度LA-ICP-MS锆石U-Pb定年实验测得峨口花岗岩侵入体的形成年龄为2507±53Ma和2420±2.5Ma;形成时间出入不大,大概在太古代晚期形成,与五台运动时间接近。地球化学特征显示侵入岩体属于亚碱性系列准铝质-弱过铝质钙碱性花岗岩,LREE/HREE=1.52~9.72,平均值为5.21,岩石重轻稀土元素分馏较强;LaN/YbN=0.7~10.53,平均值为3.94;表明轻稀土较富集,重稀土元素分馏较小;δEu为0.08~1.67,平均值为0.48,呈现强烈的负Eu异常,表明岩浆源区残留有一定量的斜长石。微量元素蛛网图可以得出,岩石富集Rb,Th,K等大离子亲石元素,明显亏损Ba,Nb,Sr,P,Ti等高场强元素,花岗岩体具有显著Nb、Ta槽以及亏损P、Ti等高场强元素,这就说明了岩浆原岩是在岛弧火山岩环境下形成的。根据以上地球化学结论判断,总体来讲峨口岩体具有I型花岗岩地球化学特征。结合该岩体地球化学特征和构造背景,表明峨口岩体形成于大陆边缘活动带岛弧环境下,其源区是中下地壳玄武岩与洋壳沉积物熔融混合物。洋壳俯冲作用形成洋弧,在进一步俯冲作用下,阜平陆块和岛弧间碰撞发育同斜-平卧褶皱、推覆型韧性剪切带及冲断构造,伴随有峨口岩体的侵入,发育绿片岩相-角闪岩相低温动力变质作用。华北克拉通总体布局为阜平陆块和恒山陆块及两者之间所夹的一系列狭长火山弧地质体,这是由于陆块间洋壳发育的洋弧随造山作用向SE方向推覆叠加,形成走向NNE向的狭长条带。这是华北克拉通最早的造山运动,是华北克拉通基底形成的初始阶段。通过以上对峨口花岗岩岩体的研究,表明峨口岩体形成由于大陆边缘活动带岛弧环境的I型花岗岩,是华北克拉通中部不可或缺的组成部分。
[Abstract]:In Wutai area, Shanxi Province, Ekou mainly exposed acidic granites, which formed in the late Archaean period, and the margin of the granite was relatively developed. Ekou granite has a special significance for the formation of the North China Craton. Based on the field investigation of intrusive rocks in this area, this paper discusses the magmatic origin of Ekou granite in Shanxi Province by studying the lithofacies (thin section identification) of the Ekou granite in U-Pb chronology of zircon and the geochemistry of the whole rock. Origin and tectonic setting of formation. The age of formation of the Ekou granite intrusion is 2507 卤53Ma and 2420 卤2.5Ma. the formation time of the two groups of high-precision LA-ICP-MS zircon U-Pb dating is 2507 卤53Ma and 2420 卤2.5Ma.The time of formation is similar to that of Wutai movement, and is probably formed in the late Archean. The geochemical characteristics show that the intrusive rocks belong to the sub-alkaline series of quasi-aluminous and weakly peraluminous calc-alkaline granites, LREE / HREEN 1.52U 9.72, with an average value of 5.21, and the rocks are characterized by strong fractionation of heavy and light rare earth elements, with an average of 3.94, indicating that LREE is relatively rich. The fractionation of heavy rare earth elements is small and 未 Eu is 0.08 ~ 1.67 with an average value of 0.48, showing a strong negative Eu anomaly, indicating that there is a certain amount of plagioclase remaining in the magmatic source area. The trace element cobweb diagram shows that the rocks are enriched with large ion lithophile elements such as Rb,Th,K, and the high field strength elements such as Ba,Nb,Sr,P,Ti are obviously depleted, and the granite body has significant Nb,Ta trough and depleted high field strength elements such as Pu Ti. This shows that the magmatic proto-rock was formed in the island arc volcanic environment. According to the above geochemical conclusions, the Ekou rock mass has the geochemical characteristics of type I granite. In combination with the geochemical characteristics and tectonic setting of the rock mass, it is suggested that the Ekou rock body was formed in the island arc environment of the continental margin active zone, and its source area is the melting mixture of basalt and oceanic crust sediments in the middle and lower crust. The oceanic crust subduction formed an ocean arc. Under the further subduction, the collision between the Fuping continental block and the island arc developed the syncline-horizontal fold, the nappe ductile shear zone and the thrust structure, accompanied by the intrusion of the Ekou rock mass. Development of greenschist facies-amphibolite facies low temperature dynamic metamorphism. The general layout of the North China Craton is a series of long and narrow volcanic arc geological bodies between the Fuping and Hengshan landmasses, which is due to the oceanic arcs developed in the intercontinental oceanic crust as a result of the orogeny nappe and superposition in the direction of SE. A long strip towards NNE is formed. This is the earliest orogenic movement of the North China Craton and the initial stage of the formation of the North China Craton basement. Based on the study of the Ekou granitic body above, it is shown that the Ekou granite formed an I type granite in the island arc environment of the continental margin active belt, which is an indispensable part of the central part of the North China Craton.
【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】：P588.121;P548

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