MICROTECTONIC FEATURES AND EVOLUTION OF THE YUSHIGOU PERIDOTITE ROCK FROM THE QILIANSHAN MOUNTAIN
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摘要: 橄榄岩作为上地幔的主要成分，影响着上地幔的流变学行为，其显微构造记录了岩石形成发展过程中所经受的构造事件。通过对祁连山玉石沟橄榄岩样品的镜下显微构造观察、岩石组构测定及TEM位错分析，探讨了与变形相关的温度、围压、含水性和应变速率等因素，总结该岩石的变形机制和变形历史，并推断其形成发展时大地构造环境。研究表明，玉石沟橄榄岩产自上地幔，其变形改造经历了上地幔演化和脆-韧性变形2个阶段。上地幔演化阶段橄榄石发育明显的A型原生组构，电子背散射衍射技术（EBSD）测定的橄榄石显微组构表明，该地区橄榄岩形成于高温（>1200℃）、低应力（ < 350 MPa）、低应变速率、低含水量的地幔浅部环境条件下；脆-韧性变形阶段叠加改造了原生A型组构，而发育明显的D型次生组构。橄榄石变形主控因素为动态恢复作用，普遍发育亚晶粒、消光带和扭折显微构造等相关组构，并与透射电镜下所观察到的位错排对应，同时还发育以微破裂为主的一套脆性变形组构，表现出两组共轭剪破裂和另一组张性破裂等现象，揭示出与韧性动态恢复现象一致的应力场方向指示。Abstract: As a main component of upper mantle, peridotite affects the rheological behavior of the upper mantle. The microstructures contain records of tectonic events experienced during the development process of the specimen. Rheological parameters, such as temperature, confining pressure, factors of water and strain rate, are defined according to microtectonic observations, EBSD data and TEM results. Deformation mechanisms, deformation history of the Yushigou rock samples and geotectonic environment are summarized. Problems of petrogenesis and evolution history of the Yushigou peridotite body are finally concluded as:Ⅰ, the evolution stage in the upper mantle, which is indicated by the A-type original fabric of olivine. The fabric of olivine lattice-preferred orientation (LPO) is determined by electron backscatter diffraction (EBSD) analysis. Our peridotite sample shows some upper mantle features, such as high temperature (>1200℃), low stress ( < 350 MPa), low strain rate and low water content environmental conditions. Ⅱ, the ductile-brittle deformation stage, showing D-type fabric which superimposed and transformed the A-type original fabric. Dynamic recovery is the main controlling factor for olivine deformation. Sub-grains, extinction bands and kink bands are generally observed by polarized-light microscopy, and are also shown as dislocation rows in the transmission electron microscopy (TEM) graphs. Brittle microcrack fabric including micro fractures with different dynamic nature is also analyzed as two groups of conjugate shear ruptures and another group of tension fractures. One same stage of paleostress field is shown by both microcrack and the ductile dynamic recovery phenomena.
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Key words:
- Yushigou /
- Qilianshan Mountain /
- peridotite /
- microfabric /
- deformation mechanism /
- deformation history
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图 1 典型橄榄石组构及其与滑移系和应力、含水量关系（据Karato等[3]，略有修改）
Figure 1. Typical olivine fabric and the relationship with dominant slip systems, stress and water content
图 2 祁连山—柴北缘地区地质简图
1—板岩夹灰岩、灰岩、板岩及中基性火山岩；2—辉长岩；3—滑石菱镁片岩等；4—纯橄榄岩；5—方辉橄榄岩；6—蛇纹岩；7—白云岩、石英片岩；8—UHP terrane；9—北祁连缝合带；10—断裂；11—构造边界；12—取样点
Figure 2. Geological sketch map of Qilianshan to northern margin of Qaidam regions
图 4 玉石沟超基性岩体A-B地质剖面示意图（剖面位置见图 1）
1—北岩体（方辉橄榄岩）；2—中岩体（方辉橄榄岩）；3—小岩体（纯橄榄岩）；4—南岩体（纯橄榄岩）
Figure 4. Schematic cross section of ultrabasic rock from Yushigou
图 5 玉石沟橄榄岩的显微图像
a—方辉橄榄岩中残斑结构；b—橄榄石中剪/张破裂；c—橄榄石中消光带、亚晶粒；d—斜方辉石中扭折带；e—斜方辉石中出溶叶理；f—斜方辉石中出溶现象；g—方辉橄榄岩中铬尖晶石
Figure 5. Photomicrograph of peridotites from Yushigou
图 6 橄榄岩共轭剪裂面及其受力模式图（里德尔剪切）
Figure 6. The conjugate shear fracture surfaces and their force mode in peridotite (Riedel Shear)
图 8 橄榄石晶体在样品中简易定向图
①—镁橄榄石矿物晶体形态及光率体主轴与结晶轴关系示意图；②—橄榄石晶体简易图；③—橄榄石晶体在样品中的定向；④—岩石样品定向薄片磨制
Figure 8. Simple directed graph of olivine crystals in the sample
图 9 透射电镜下橄榄岩位错组态及晶体弯曲
Figure 9. TEM images of dislocation configuration and crystal bend in olivine
表 1 五种LPO类型特征及其形成环境
Table 1. Features and formation conditions of five LPO-types
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