Experimental Semibrittle-plastic Deformation And Microstructure Of Granite And Mafic Granulite

Shallow earthquakes commonly occur in depth close to the brittle–plastic transition zone.Seismicity data show that earthquakes normally occur in the middle crust,but strong earthquakes may also occur in the lower crust.The brittle–plastic transition and experimental studies on rheological properties of the major rocks that compose the middle and lower crust are important for our understanding of tectonic deformation of the continental lithosphere,the strength and deformation mechanism of faults and the generation of strong earthquakes.Previous experimental study of brittle-plastic transition and rheology mainly focused on single crystal aggregates of quartz,plagioclase and pyroxene,two-phase aggregates of plagioclase and pyroxene,as well as gabbro and diabase which are mainly composed of plagioclase and pyroxene.The experimental study of natural granite and granitic mylonite show that the deformation characteristics of natural granite are different from that of quartz-plagioclase aggregates,and the brittle-plastic transition and deformation mechanism are obviously different between K-feldspar and plagioclase.Natural granulite is mainly composed of plagioclase,clinopyroxene and orthopyroxene.Experimental studies on three-phase assemblage and hot-pressed natural mafic granulite show that the deformation characteristics of three-phase assemblages are more complex than that of two-phase assemblage.There are few experimental data that have covered to multiphase assemblages,thus the data set is still in the accumulation stage.The experimental data of natural granulite are especially much fewer,thus more experimental studies are needed.In order to study the semi-brittle deformation and rheological characteristics of natural rocks composed of three-type minerals,the experimental samples of this study are prepared from natural granite and mafic granulite that represent the compositions of the middle and lower crust respectively.All experiments are performed under high temperature and high pressure using gas medium triaxial deformation apparatuses,and the semi-brittle and rheological characteristics of the two kinds of natural rock have been studied.The major advances of this paper are as follows:（1）.Rebuilding of the gas medium triaxial high temperature and high pressure creep apparatusThe temperature and confining pressure of the initial version of this apparatus was not able to achieve the designed specifications.We rebuilt the loading,heating and control system:（1）we setup the control system which achieved high-precision digital control of the apparatus and real-time observation and logging of the experimental data by using an industrial computer and high precision sensors,combined with reasonable control program;（2）the high pressure vessel can work well while load to 300MPa confining pressure,the error of confining pressure is achieved≤±0.5 MPa by using well-designed control program and control algorithm;（3）axial load is achieved maximum load of 500KN by hydraulic cylinders,the error of axial load is achieved≤±0.2 MPa by using a high-precision load cell and well-tuned control parameters embedded in the control code;（4）the heating system is controlled by a digital controller,providing maximum temperature of 1300℃with an error of±1℃;（5）an inner axial load cell is designed for measuring axial load in the vessel which avoids the influence of seal friction.（2）.An experimental study on the brittle–plastic transition during deformation of graniteThe deformation experiment of granite was carried out using the Paterson type creep apparatus at the German Earth Science Research Center（GFZ）.The test samples were prepared from a light-colored fine-grain granite collected from Luding,Sichuan,china.The granite is composed of about36 volume percent quartz,²6%plagioclase,³4%microcline,³%muscovite,and¹%chlorite,free of biotite and amphibole.There three set of deformation experiments were performed:under850℃to 1050℃and compression strain rate and shear strain rate were 10^-5s^-1 and 1.8×10^-5s^-1were:（1）five compression experiments were carried out under conditions of 300MPa,with strain rate of 10^-5s^-1 and temperatures of 850℃,900℃,950℃,1000℃,1050℃;（2）two compression experiments were performed under temperature of 950℃,strain rate of 10^-5s^-1,and stress of100MPa,at 1atm confining pressure;（3）a torsion test was conducted under confining pressure of400 MPa,temperature of 950℃and shear strain rate of 1.8×10^-5s^-1.The microstructures of undeformed and deformed samples were observed by optical microscope and scanning electron microscope.Chemical composition of the minerals in deformed samples was analyzed by EDS.EBSD analysis were conducted on fine-grained quartz aggregates of both the starting material and deformed samples for identifing the quartz grain fabrics developed during the experimental deformation.The stress–strain curves for all but two samples display weakening.The two exceptions are the sample that deformed with steady-state creep under a CP of 100 MPa and the sample that displayed brittle fracture under a CP of 100 kPa.For the other samples,peak strengths decreased with increasing temperature or lower CP.Microstructures show that samples underwent a brittle–plastic transition with increasing temperature.Samples fractured by cataclastic flow at 850°C with CP=300 MPa and at 950°C with CP=100 kPa.Deformation mechanism of microcline is cataclastic flow at 900–1050°C with CP=100–400 MPa,and accompanied by dislocation glide at temperatures of 1000°C and 1050°C.At 900–1050°C with CP=100–400 MPa,plagioclase displayed bulging recrystallization and grain boundary migration recrystallization and quartz deformed by subgrain rotation recrystallization.Diffusion rims were observed between quartz,plagioclase,and microcline grain boundaries at 900–1050°C with CP=100–400 MPa.Partial melting appeared around plagioclase,microcline,and quartz grains at 1000°C and 1050°C.Exsolution of albite from microcline took place in a sample deformed at 950°C and CP=100 MPa.Electron backscatter diffraction analysis showed that quartz has a lattice preferred orientation in both undeformed and experimentally deformed samples.Basal<a>slip of the quartz grains was found in undeformed samples.The c-axis of quartz is mainly close to the rhomb<a>slip with a secondary orientation in the basal<a>slip in samples deformed at 900°C and 950°C with CP’s of300 MPa and 100 MPa respectively.The c-axis of quartz changes to prism<a>slip in samples deformed at 950–1050°C with CP=300 MPa which is identical to the sample deformed with torsion shear at 950°C with CP=400 MPa.Microstructures show that plagioclase and microcline deform differently.Microcline is a stronger phase compared with quartz and plagioclase.Therefore,microcline is important for the brittle–plastic transition in granitic rocks and the depth of that brittle–plastic transition in the middle crust will be greater where the granitic rocks in the crust contain large amounts of microcline.（3）.An experimental study on the semibrittle–plastic deformation of mafic granuliteThe deformation experiment of mafic granulite was carried out using the gas media creep apparatus at our laboratory.The experimental material was collected from late Archean terrane granulite at Shuigoukou,Hebei Province,China.The granulite is composed of 57%plagioclase（Pl）,24%clinopyroxene（Cpx）,14%orthopyroxene（Opx）,5%magnetite and ilmenite,free of quartz.The mean grain sizes of minerals are plagioclase 513μm,pyroxene 468μm,magnetite and ilmenite265μm.The stress exponent of experimental samples were calculated by testes carried out on different samples which were deformed under confining pressure of 300MPa at the temperatures of950℃-1150℃,and each sample was compressed with variant strain rates of 10^-5s^-1-10^-7s^-1.The activation energy was calculated using the mechanical data of one sample deformed under different temperatures.The microstructures of undeformed and deformed samples were observed by optical microscope and scanning electron microscope.Chemical composition of the minerals during deformation was analyzed by EDS.The mechanical data show that the strength of deformed samples are independent of temperature for temperatures≤1075℃,and the strength decreases with temperature at≥1100℃.The results of microstructure observation show that the deformation mechanism of plagioclase is dislocation creep at≥1000℃,the deformation mechanism changes from dislocation slip to dislocation climb and recrystallization at 1075℃.The deformation characteristics of pyroxene grains present more complex features than that of plagioclase in the deformed samples.The deformation mechanisms of pyroxene grains change from brittle fracture,intragrain cracking to creep under conditions where plagioclase grains present plastic deformation,therein the plastic deformation of pyroxene occurs at temperatures≥1075℃,and the deformation mechanism changes from dislocation slip to dislocation climb at 1150℃.Clinopyroxene develops intracrystalline cracks at 1100℃,and recrystallization occurs at 1150℃.Orthopyroxene develops intracrystalline cracks at 1150℃.The results of natural mafic granulite experiments and microstructure observation show that brittle fracture is the main deformation mechanism at<1000℃.The deformation mechanism is semi-brittle at 1000℃-1075℃,with the stress exponent ranging from 4.3 to 6.2 and the mean value of 5.5,the activation energy of 1871kJ/mol,and the preexponential factor of 10^56.1MPa^-ns^-1.The deformation mechanism is dislocation creep at 1100℃-1150℃,with the stress exponent ranging from 4.9 to 7.2 and the mean value of 6.1,the activation energy of 966kJ/mol,and the preexponential factor of 10^20.5MPa^-6.1s^-1.Plastic deformation of natural mafic granulite is normally accompanied by partial melting in laboratory conditions,partial melting process usually induced by ilmenite and magnetite.Needle and columnar-shaped new mineral grains are crystallized when MgO content is rich enough in the melt,and the composition of new mineral closes to that to pyroxene.The strength of mafic granulite decreases significantly when the melt is connected along the grain boundaries.The flow stresses of the lower continental crust estimated using the exponents which is obtained by our experimental data of mafic granulite is between of that estimated using the parameter of clinopyroxene and diopside,and it is much higher than that estimated using the parameter of plagioclase and granulite reported in previous studies.Then,the rheological strength estimated by this study represents the upper-bound strength of the lower crust.