Brittle-plastic Transition Of Felsic Rocks And Effect Of Deformation On Formation Of Ultra-high Pressure Rocks

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...