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长英质岩石的脆塑性转化与变形对超高压岩石形成的影响
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摘要
柯石英是超高压岩石中的标志性矿物,研究石英-柯石英相变的形成条件对讨论超高压岩石的形成条件具有重要意义。目前对超高压岩石中石英的变形以及在差应力条件下石英-柯石英的相变研究程度低。虽然已经认识到了差应力对石英-柯石英转化有显著影响,但由于缺少对实验细节的了解,对于在差应力条件下石英-柯石英转化的条件是否具有普遍性还缺乏全面的认识。另外,超高压岩石能够承受的差应力究竟有多大,也并不十分清楚。为此,本文根据高温高压实验结果,研究了石英岩的脆塑性转化与在差应力的条件下石英-柯石英相变,并初步研究了石英闪长岩的半脆性-塑性流变,进而根据流变实验结果估计了大陆俯冲带的流变强度。
     所有实验都是在中国地震局地质研究所地震动力学国家重点实验室的高温高压固体介质三轴实验系统上完成的。实验样品之一为长城系石英岩,石英含量超过99%,极少量白云母和斜长石,粒度均匀。实验条件为:温度500-1000℃,围压在1.2-1.4GPa,应变速率为5×10~(-5)/s、2.5×10~(-4)/s,部分实验采用等应力控制。另一样品周口店石英闪长岩的实验条件为:温度650-1000℃,围压为1000-1100MPa,应变速率为1×10~(-4)/s、2.5×10~(-5)/s、6.25×10~(-6)/s。斜长石70%,石英5-10%,角闪石15-20%,黑云母<5%,绿帘石<1%。
     根据实验结果,本文得到以下结论:
     (1)实验结果表明,从500-1000℃,随温度的增加,石英显示了三种力学特征半脆性破裂、半脆性流动和塑性变形。在半脆性破裂域,强度对温度的依赖性不明显,而且样品的强度较高,在1500MPa以上;应力-应变曲线出现应力降。在半脆性流动域,温度对强度的影响明显增加,且强度随温度的增加而有大幅度下降;应力-应变曲线与半脆性破裂域有明显的差别,样品基本表现出屈服特征,部分实验样品出现应变弱化特征。在塑性变形域,强度随温度的增加而降低,应力-应变曲线具有典型的稳态塑性流变特征,即应力-应变曲线超过屈服点后应力趋于平稳,不再随应变的增加而增大,甚至出现应变弱化特征。从半脆性破裂,半脆性流动到塑性变形域,随温度的增加,应力-应变曲线的弹性应变量有逐渐减小的趋势,弹性变形阶段曲线的斜率下降。
     (2)利用偏光显微镜和拉曼光谱研究实验变形样品时,发现有部分实验样品中出现了柯石英,柯石英出现在样品的一侧靠近活塞的地方。柯石英出现的条件:温度950-1000℃围压在1.3-1.35GPa,差应力在1.5-1.78GPa之间,应变量大于40%,样品属于塑性变形。这表明在差应力存在的条件下,在较低的压力(<2.0GPa)下就有稳定的柯石英出现,所以,差应力对石英-柯石英的相变至少在某些特定条件下有很大的影响,而静岩高压力不是产生柯石英的唯一条件。
     (3)通过在1000-1100MPa围压条件下的石英闪长岩的流变实验,得到的稳态流变方程:在800-900℃时,属于半脆性流变域:
     (?)=0.2σ~(6.2)exp(-435/RT)
     在950-1000℃时,属于塑性流变域:
     (?)=5.6×10~(13)σ~(3.6)exp(-624/RT)
     其中,(?)为应变速率,R为气体常数,T为温度。
     (4)根据石英闪长岩的流变实验结果,计算得到的大陆俯冲带流变强度随温度的升高而降低。当俯冲带应变速率为10~(-12)s~(-1)时,在600-700℃的温度范围内进行外推,大陆俯冲
    
    n
    带差应力极限为100一1500MPa。800一900℃区间,它在20一100MPa之间。当俯冲带应变速率
    为10一145一,时,600一700℃范围内,大陆俯冲带差应力极限为50一80伽Pa,800一900℃是
    10一50MPa。
     (5)利用石英闪长岩的流变参数计算得到的大陆俯冲带流变强度是超高压岩石能承受
    的差应力极限的下限,约为几十兆帕到几百兆帕的范围内,甚至达到一千多兆帕。利用与
    超高压岩石相关的单斜辉石、榴辉岩和辉长岩的流变参数来计算大陆俯冲带地壳的流变强
    度,得到的是超高压岩石能承受的差应力的上限,约在几百兆帕到一千多兆帕之间。在柯石
    英榴辉岩中,石英和柯石英的含量非常少,它的强度以榴辉岩的强度为主,所以差应力约在
    几百兆帕到一千多兆帕,它在超高压柯石英榴辉岩的形成过程中可能起到了重要的作用。
    ;
Coesite is one of the indicative minerals in the ultra-high pressure rocks. The study on formation conditions of quartz-coesite phase transition is of important to understanding the generation setting of ultra-high pressure rocks. Up to now there has been few studies on deformation of quartz in ultra-high rocks and quartz-coesite phase transition at differential stress. Although it has been realized that differential stress has notable influence on this transition, the question whether the condition for quartz-coesite transition are ubiquitous is open because of lacking knowledge on the detail of relevant experiments. Moreover, it is not known how much the differential stress that ultra-high pressure rocks can support. This thesis attempts to address these questions based on results of rock experiments at high pressure and temperature. The research involves the brittle-plastic transition of quartzite and quartz-coesite phase transformation at differential stress. It also studies semibrittle-plastic rheology of quartz diorite, and estimates the rheological strength of the continental subduction zones.All the experimental were performed at the state Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. One rock sample is the quartzite of Grest Wall system, of which the 99% content is quartz and the other includes muscovite and clinfedspar of trace amount .Its grain size is uniform. The experimental conditions are: temperature 500-1000 癈, confining pressure 1.2-1.4GPa, strain rates 5X10-5/s,and 2.5X10-4/s, and constant stress control for some experiments. Another rock sample is the quartz diorite from Zhoukoudian, southwest Beijing. It contains 70% plagioclaseis, 5-10% quartz, 15-20% hornblend, biotite lessthan 5%, and epidote less than 1%. Experimental conditions are: temperature 650-1000 ,confining pressure 1000-1100MPa, strain rates 1 X 10-4/s 2.5 X 10-5/s and 6.25 X 10-6/s.The result of this thesis is summarized as follows:(l)The experiments show that the quartzite exhibits three mechanical behaviors :semibrittle faulting, semibrittle flow and plastic flow when the temperature increase from 500 to 1000. In the semibrittle faulting regime, its strength is not much dependent upon temperature and has a large value over 1500MPa.A stress drop occurs on its stress-strain curves. In the semibrittle flow regime, its strength is influenced by temperature remarkable and decreases greatly with growing temperature. Its stress -strain curves is much different from that in the semibrittle regime. The sample is mainly characterized by yielding. Some portions of it have features of strain weakening. In the plastic deformation regime, the strength of the sample declines with increasing temperature. Its stress-strain curves are typical for steady state plastic flow. It means that the stress tends to be constant when the curves passes through the yielding point, and the stress does not increase with growing strain, and even strain weakening occurs. With the rising temperature, the sample transits from semibrittle faulting to semibrittle flow then to plastic flow regime. Meanwhile the elastic strain on the stress-strain curve tends to be reduced, with a declining slope in the elastic stage.(2) When observing the deformed samples by polarized microscope and the Raman spectrum, it was found that coesite appeared near the piston in some samples, where the temperature was 950-1000,confining pressure 1.3-1.35GPa,differential stress 1.5-1.78GPa, and the strain is greater than 40%, implying plastic deformatioa Under such a condition of differential stress, stable coesite formed at low pressure (<2.0GPa). This means that differential stress has important effect on quartz-coesite transition under some conditions. And the high hydrostatic pressure is not the unique condition for generation of coestie.
    
    (3)Through experiments of quartz diorite at confining pressure 1000-1100MPa, steady state flow laws are obtained. At temperature 800-900 ,it is semibrttle flow:At 950-1000, it is plastic flow:Where, s is the
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