内蒙古乌拉山-大青山地区变泥质岩的地球化学特征及构造意义

发布时间：2018-09-04 07:41

【摘要】：乌拉山-大青山地区位于华北克拉通西北缘孔兹岩带的中段,该地区出露大面积的古元古代高级变质表壳岩系岩石,例如夕线(堇青)石榴二长片麻岩、石榴黑云二长片麻岩、黑云长英质片麻岩、石榴石英岩、大理岩、磁铁石英岩和钙硅酸盐岩等。其中该地区典型的变泥质岩石(夕线(堇青)石榴二长片麻岩和石榴黑云二长片麻岩)多以似层状产出,夕线(堇青)石榴二长片麻岩含有大量的石榴石、夕线石和/或堇青石,并保留典型的堇青石反应边结构。岩石地球化学研究结果表明上述变泥质岩石样品明显富Al2O_3,属于过铝质岩石,其平均化学成分与上地壳平均值较为接近。样品的稀土配分型式具有中等程度的轻重稀土元素分异,轻稀土明显富集和弱铕负异常的特征。微量元素蛛网图显示变泥质岩石样品富集大离子亲石元素如Rb和Ba,而高场强元素如Zr、Hf含量相对较低,且Nb、Ta、P和Ti呈明显亏损的特征。多种原岩判别图解显示其原岩为一套富铝粘土岩及杂砂岩,且成熟度较低,为近源沉积。多数样品的CIA值低于太古宙后澳大利亚平均页岩和北美页岩的CIA值,反映其源岩经历了中等程度的风化作用。物源以上地壳的长英质成分为主,可能有古老沉积物的加入。多种构造判别图解表明研究区样品形成于有演化岛弧发育的活动大陆边缘环境。结合前人对孔兹岩带出露的变泥质岩石的变质作用研究普遍得到近等温减压型顺时针P-T演化轨迹以及锆石同位素年代学研究结果,综合表明乌拉山-大青山变泥质岩石形成于有演化岛弧发育的活动大陆边缘环境,中元古代物质(2.1~2.0Ga)为其提供主要的沉积物源,随后卷入~1950Ma华北克拉通西部古老陆块之间的碰撞造山作用,经历了大规模麻粒岩相变质作用,并于1920~1850Ma碰撞后折返至近地表。
[Abstract]:The Wula-Daqingshan area is located in the middle of the Kongzite belt on the northwest margin of the North China Craton. A large area of paleoproterozoic metamorphic crust rocks, such as the pomegranate gneiss, the pomegranate black cloud gneiss, are exposed in this area. Feldspar gneiss, pomegranate, marble, magnetite quartzite and calcium silicate, etc. The typical argillaceous rocks (pomegranate gneiss, pomegranate gneiss and pomegranate black cloud gneiss) in this area are mostly stratiform, and the pomegranate gneiss contains a great deal of garnet. Sillimanite and / or cordierite, and retaining the typical cordierite reaction edge structure. The results of petrogeochemical study show that the samples of argillaceous rocks mentioned above are obviously rich in Al2O_3, and belong to peraluminous rocks, and their average chemical composition is close to the average value of the upper crust. The rare earth distribution patterns of the samples have the characteristics of moderate light rare earth element differentiation, obvious enrichment of light rare earth elements and negative anomalies of weak europium. The trace element cobwebs show that the samples of argillaceous rocks are rich in large ion lithophile elements such as Rb and Ba, but the contents of high field strength elements such as Zr,Hf are relatively low, and Nb,Ta,P and Ti are obviously depleted. The original rock is a set of aluminum-rich clay rock and complex sandstone, and its maturity is relatively low, so it is a near-source deposit. The CIA values of most samples are lower than the CIA values of the average Australian shale and North American shale after Archean, indicating that the source rocks experienced moderate weathering. The feldspathic composition of the crust above the provenance is mainly composed of ancient sediments. Various tectonic discriminant diagrams show that the samples in the study area were formed in the active continental margin environment with the evolution of island arc. Combined with previous studies on metamorphism of altered shaly rocks from the Kongzi belt, the results of near isothermal decompression clockwise P-T evolution and zircon isotopic chronology are generally obtained. The results show that the argillaceous rocks of the Wula-Daqingshan Mountains were formed in the active continental margin environment where the island arcs developed, and the Mesoproterozoic material (2.1~2.0Ga) provided the main sediment source for them. The collision orogeny between the ancient continental blocks in the west of North China Craton at 1950 Ma underwent a large-scale granulite facies metamorphism and returned to the near surface after the 1920~1850Ma collision.
【作者单位】：中国地质科学院地质研究所;
【基金】：国家自然科学基金项目(41430210、41502181、41302153) 中国地质调查局地质大调查项目(DD20160121、12120114061901) 科技部“973”项目(2012CB416603) 中国地质科学院基本科研业务费(J1512)联合资助
【分类号】：P588.345

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