组构对糜棱岩流变影响的实验研究

Rock deformation has been widespread observed from middle to lower crust. However, experimental data are scarce for performing to investigate the effect of fabric to rheology of rocks, especially, the creep experiments of anistropic rocks are still an untouched area which is related to crustal extension and the detachment faults development. This project plans to perform creep tests in Griggs-type molten salt medium triaxial machine under high temperature and pressure condition using natural fine-grained felsic mylonite samples, and get the flow laws of anistropic rocks and effect of pre-exist fabric to rheology of narural deformed rocks. To do that, the experimental samples should be drilled and polished as the angle between the compression direction of experiments and the original foliation of samples are 0°, 30°, 45°, 60°and 90° respectively. To study the fabric evolution and deformation mechanisms of major minerals, microstructures, and chemical composition both in starting samples and experimental deformed samples should be analyzed under SEM and microprobe, particularly to analyze the c-aixial changing of quartz during experimental deformation using electron backscattered diffraction (EBSD), which could reveal the information of alteration and replacement to fabric of samples after experimental deformation. The experiment can simulate the process of later deformation alteration to former deformation in developing of detachment faults. Compared with the rheological tests data on isotropic granitic rocks published in recent years, effects of fabric to rheology of mylonite samples can be analyzed. Finally, we hope to give rheological profiles of middle crust in North China Craton (NCC). The project will supply the essential experimental data for further understanding the mechanism of NCC extensional thinning.

地壳深部岩石普遍存在变形组构，组构对岩石流变影响的相关实验非常有限，尤其缺少与地壳伸展和拆离断层形成相关的各向异性岩石流变实验。本申请拟采用具有先存变形组构的花岗质糜棱岩样品，在熔融盐固体介质三轴流变仪上开展高温高压流变实验，获得压缩方向与岩石面理呈0°、30°、45°、60°和90°角度的糜棱岩流变参数；利用扫描电镜研究实验样品主要矿物的变形微观结构与变形机制，利用电子探针分析样品的成分特征；利用电子背散射（EBSD）分析样品中石英C轴定向特征在实验前后的变化，研究实验变形对样品组构的改造和置换作用，探讨拆离断层形成和演化过程中后期变形对前期变形的改造作用。把实验结果与近年来取得的均匀结构的花岗质岩石的流变实验对比，综合分析组构对糜棱岩流变的影响，并利用实验结果研究华北克拉通地区中地壳流变结构。本研究能够为深入认识拆离断层形成和演化规律及完善地壳伸展减薄模型提供必要的实验数据。

华北克拉通减薄是近年来地学界的热点问题，拆离断层形成与地壳伸展减薄是岩石圈减薄的浅部响应，这一过程直接受控于岩石流变状态。对于华北克拉通地壳流变研究，流变实验和天然样品流变分析多数集中在下地壳，对于与拆离断层对应的中上地壳流变实验研究比较少。目前为止，有关于地壳伸展和拆离断层形成相关的各向异性岩石流变实验处于空白状态。.中地壳岩石经历了变质变形作用，发育有强烈的变形组构，具有显著的各向异性特征。为了研究中地壳各向异性岩石流变，选择华北克拉通北部辽东拆离断层中具有变形组构的花岗糜棱岩为实验样品，开展压缩方向与岩石面理呈30°、45°和60°夹角的岩石的高温高压流变实验。实验条件为温度600-850℃，围压800-1200MPa，应变速率1×10-4/s×10-5/s为主，少量10-6/s。用扫描电镜和电子探针研究实验样品主要矿物的变形微观结构与变形机制。.实验结果表明，在600-700℃时，三组样品基本都表现为半脆性流变；在800-850℃时，样品的应力指数为n=3±0.3，岩石以塑性变形为主。实验力学数据显示，花岗糜棱岩在半脆性破裂域，压缩轴与岩石面理呈30°夹角时，岩石强度最小；在塑性流变域，压缩轴与岩石面理呈45°夹角时，岩石强度最小；其它斜交角度的样品强度小于压缩轴垂直或平行岩石面理的样品强度，而压缩轴垂直岩石面理的岩石强度大于压缩轴平行岩石面理的岩石强度。微观结构分析表明，实验变形后的糜棱岩样品在温度800-850℃时，石英和黑云母发生强烈塑性变形，剪切拉长形成新的面理，角闪石和长石表现出微破裂和局部塑性变形特征。角闪石和黑云母出现脱水反应，生成微晶角闪石和黑云母，并伴有局部熔融，受应变局部化控制，脱水熔融主要出现在黑云母、角闪石条带中，熔体成分分布不均匀，受边缘矿物成分与应变控制。脱水熔融与应变具有相互促进作用。实验变形对原有面理不同程度的进行了继承与改造，其中，30-45°夹角的样品，新形成的面理主体沿着原有的云母和石英条带发育，而60°方向的样品，新形成的面理以构造置换为主，只有局部继承了原有面理，形成“转折”或“台阶状”新面理。综合力学数据、流变参数和微观结构得出，组构显著影响岩石强度，但组构对岩石的脆塑性转变、流变参数和变形机制影响不大。这表明组构只影响岩石变形的难易程度，不影响岩石的变形方式。

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