Metamorphic Evolution of Basic Rock Series in Dongshengmiao Area, Inner Mongolia
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摘要: 狼山地区位于华北克拉通北缘内蒙古狼山西段，该区构造演化变形强烈，根据基性变质岩系的产出地层时代、产状以及它们与不同期次的变质、变形乃至混合岩化作用的关系，将基性岩系地体按形成时序划分出4个期次。不同产状及期次划分的基性系列变质岩为恢复区域地质演化史提供了重要信息。笔者通过岩相学观察、化学全分析和电子探针等手段对不同期次变质岩的岩石学和矿物学特征进行研究，探讨其温压条件和构造背景。岩石学特征表明，狼山基性变质岩除绿片岩外，原岩都为基性火山岩。第一期斜长角闪岩富集Pb、K、Nb、Ta，存在明显亏损，Hf、Zr、Ti弱亏损，轻重稀土分异较明显，属于岛弧玄武岩；第二、三期斜长角闪岩具有板内和岛弧过渡的特点；第四期脉状斜长角闪岩轻稀土相对较低，表现为产生在板内张裂环境侵位环境。绿片岩Sr、K、Rb、Ba亏损而Th的明显富集具有岛弧或弧后盆地玄武岩的特征。选用斜长石、角闪石共生矿物计算各期斜长角闪岩所经历过变质作用的温度和压力。第一期斜长角闪岩计算得出其属于早期高角闪岩相-麻粒岩相；第二期玄武岩计算得出其代表了早期为角闪岩相晚期向低角闪岩相退变的变质过程；第三期斜长角闪岩计算得出其属于低角闪岩相；第四期斜长角闪岩属于接触变质的绿帘角闪角岩相。Abstract: The Langshan area is located in the western section of the northern margin of North China Craton. This area has experienced complex tectonic evolution, the different occurrence and different stages of basic metamorphic rocks in Langshan area provide important information for the inversion of their regional geological evolution history. The petrological and mineralogical characteristics of metamorphic rocks in various periods have been studied by the means of petrographic observation, chemical total analysis and electron probe. According to the age, occurrence of basic metamorphic rocks and their relationship with the metamorphism, deformation and migmatization of basic metamorphic rocks, these basic metamorphic rocks can be divided into four stages according to their sequence of formation. The petrological results show that the primary rocks except greenschist are basic volcanic rocks. In the first phase of the amphibolite enrichment, there is a significant depletion of Pb, K, Nb and Ta, with weak depletion of Hf, Zr and Ti. The difference between light and heavy rare earths is obvious, belonging to the island arc basalt. The second and third stages of amphibolite are characterized by intraplate and island arc transitions. In the fourth stage, the light rare earth elements of the amphibolite are relatively low, which is manifested by the emplacement environment in the intraplate tension crack environment. The greenschist is depleted in Sr, K, Rb, Ba, but enriched in Th, which is characterized by basalt in island arc or back-arc basin. The plagioclase and amphibole symbiotic minerals have been used to calculate the temperature and pressure of metamorphism experienced by amphibolite in various periods. The first stage of amphibolite calculation shows that the amphibolite belongs to the early high amphibolite facies-granulite facies;The second stage of basalt calculation shows that it represents the metamorphic process from the late amphibolite facies to the low amphibolite facies in the early stage;The third stage of amphibolite has been calculated to be a low amphibolite facies;The fourth stage of amphibolite belongs to the contact metamorphic green-curtain amphibolite facies.
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Key words:
- basic metamorphic rock /
- petrology /
- geologic thermometer /
- Langshan
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