| The thinning of the lithosphere of North China craton is a focused research topicin recent years. The detachment fault and crustal extension are thought to be theresponse of shallow crust to this process. In the detachment fault, the viscous flow anddeformation mechanism of rocks are controlled by rheology of the middle to lowercrust, where the rocks usually have strongly deformed fabric due to metamorphismand deformation. However, rheological experiments in the literatures have beenperformed using isotropic samples, and experimental data on effects of fabric onrheology of anistropic rocks are still scarce.In order to investigate the effect of preexisting fabric on the rheology of felsicrocks in the middle to lower crust, deformation experiments were performed underhigh temperature and high pressure conditions using strongly foliated, fine-grainedgranitic gneiss and mylonite samples collected from the a detachment fault of easternLiaodong in the North China craton. The samples were drilled from natural graniticgneiss and mylonite with the cylinder axis parallel to the foliation and perpendicularto the foliation. The rheological experiments of the two kinds of rocks were carriedout with the compression direction parallel to the foliation and perpendicular to thefoliation at temperatures of600-890℃, confining pressure of800MPa-1200MPa andstrain rate range of1×10-4/s-2.5×10-6/s. The mechanical data were corrected toeliminate axial dynamic friction, as well as stress difference for changing in thecross-sectional area by assuming a constant volume of the sample. Based on thecorrection, stress-strain data were obtained and the flow law parameters werecalculated. The microstructures and deformation mechanisms were studied underoptical microscope and scanning electron microscopy （SEM）; melt compositionsproduced by dehydration melting of hornblende and biotite were analyzed with thescanning electron microscopy and Energy-dispersive X-ray spectroscopy （EDAX）microanalyses; and the trends of quartz c-axis orientation of starting samples andexperimental deformed samples were measured using electron backscattereddiffraction （EBSD）. To understand the effect of the fabric to rheology of rocks,microstructures and melt characteristics of experimentally deformed quartz dioritesamples with homogeneous texture were also analyzed in this study. The major newresults are obtained as follows:（1） Pre-existing fabric has no effect on the deformation mechanism of the graniticrocks. At temperature from600to800℃，the deformation mechanism of myloniteand granitic gneiss samples is in the semi-brittle deformation regime; at temperaturefrom800to890℃, the deformation mechanism of samples is transformed into plastic deformation.In the semi-brittle deformation regime, the deformation of feldspar grains isaccommodated by brittle fractures, and the quartz grains were deformed by cataclasisand dynamic recrystallization. In the plastic deformation regime, intracrystallinemicro-fractures formed in feldspar grains, and sub-grains formed by dynamicrecrystallization were also found on the grain edge of some feldspar crystals.However, grains of quartz are dominated by sub-grain rotation. With increasingtemperature, more subgrains of quartz appeared, and new quartz subgrain bandsformed in deformed samples, which replaced the original fine-grained quartz bandscompletely. Aggregates of biotite, hornblende and chlorite were elongated and formednew bands. At the high temperature（800℃）, dehydration melting appeared in grainboundaries of biotite and hornblende, and some new micro-crystalline hornblendegrains were found in some localities of the melt. Similar to that, the experimentallydeformed samples of quartz diorite with homogeneous texture underwentbrittle-plastic transition at lower temperature（650℃）, with feldspar dominated bybrittle fracture, and quartz and biotite dominated by dislocation glide. At850℃,intra-granular micro-fractures and mechanical twins were found in feldspar grains,and subgrains were found in quartz. Dehydration melting was seen in grain rims ofhornblende. At900℃-1000℃, the mechanical twins were the major features forfeldspar and subgrains commonly developed in quartz, and most of hornblende andbiotite grains were dehydrated to different extents. Obviously, it was shown thatcharacteristics of the brittle-plastic transition, temperature condition for plasticdeformation and deformation mechanism of the major minerals in foliated graniticgneiss and mylonite are basically similar to that of the uniform samples of quartzdiorite.（2） The strength of samples and flow law parameters indicate that pre-existingfabric of samples has a significant effect on the strength of samples and activationenergy, but it has a negligible effect on the stress exponent. That means pre-existingfabric only controls the degree of difficulty for rock deformation, but probably has noapparent effect on macroscopic deformation modes and deformation mechanisms ofrocks.In the plastic deformation regime, averaged stress exponent is3for both of thetwo groups of mylonite, and it is2for two groups of granitic gneiss. However, theactivation energies of mylonite samples with compression direction perpendicular（PER） to the foliation and parallel to the foliation （PAR） are438kJ/mol and193kJ/mol, respectively. The activation energy values of granitic gneiss samples withPER and PAR to the foliation are respectively380.0kJ/mol and246.4kJ/mol. Evidently the activation energy of experimentally deformed samples with PER isapparently higher than that of samples with PAR. So the flow strength of myloniteand granitic gneiss samples with PER is stronger than that with PAR under identicalstrain rate and temperature conditions.（3） The new fabric bands developed in the deformation process replaced theoriginal foliation of samples. The quartz bands and aggregates of biotite, hornblendeand chlorite bands which formed during experimental deformation transformed theoriginal foliation of samples. In the deformation process, the original foliation of themylonite and granitic gneiss samples with PER were totally destroyed and replaced bythe new foliation. The deformed zone of the samples with PAR followed the originalfoliation and the shear deformation enhanced the deformation bands which led to thelower flow strength of samples with PAR compared to that of PER samples. In otherwords, the samples with PAR are easier to deform. In the deformation experiments onquartz diorite, some samples contain large feldspar grains of preferred orientation.The orientation of long axis in coarse feldspar grains is nearly perpendicular tomaximum principal stress. Mechanical twinning and bending were found in the largefeldspar grains. It suggests that this kind of structure is similar to mylonite andgranitic gneiss samples with PER. Obviously, the fabric pattern has a significanteffect on the strength of rock. This means that when the foliation of rock isperpendicular to the orientation of maximum principal stress, the deformation of rockand detachment fault development are not favored, while in homogeneous rock or inrocks of foliation with a small angle to the orientation of maximum principalstress, the deformation of rock and detachment fault develop easily.（4） C-axis of the new quartz grains formed in the deformation experiment has newlattice preferred orientation, which transformed the original c-axis fabric of preferredorientation in quartz. EBSD measurements showed that the C-axis of quartz instarting mylonite and granitic gneiss samples are localized within Z-max domain forbasal <a> slip, suggesting the basal slip in low temperature deformation. Theexperimental deformed samples of mylonite show apparent change in quartz fabric.The quartz fabrics in samples with PER are basal<a> slip, prism<c> slip and prism<a> slip respectively at the temperatures of800℃,840℃and850℃. The quartzfabrics in samples with PAR are prism<a> slip（rhomb<a> slip）, prism<c> slip andprism<c> slip for temperatures of840℃,850℃and890℃separately. Theresults are consistent with the well-known dominant slip systems under moderatetemperature. The C-axis of quartz in experimental deformed granitic gneiss sampleswith PER localized near the X-axis, indicating prism<c> slip. In the samples withPAR, the C-axis of quartz is localized near Z-axis, accompanied by small amount of C-axis localized near X-axis, indicating basal <a>slip and prism<c> slip. These factsshow the quartz fabric in experimentally deformed PER samples experienced morecomplete structural replecement than that of PAR samples.（5） The dehydration melting of hornblende and biotite have a weak influence onthe strength of samples. The composition of melt shows that dehydration melting islocalized, heterogeneous and non-equilibrium partial melting. The stress-strain curveof the granitic gneiss sample shows strain-softening at840℃. Similar softeningoccurred in mylonite samples in the temperature from840℃to850℃. Thesephenomena indicate that with the increase of strain and accumulation of melt content,the dehydration melting has the effect of work softening during deformation ofsamples under high temperature. The melt dehydrated from hornblende is localized inthe grain rims of hornblende, while the melt dehydrated from biotite is localized aspoint-like and thin films at the grain boundaries between biotite, feldspar and quartz.The distribution of partial melt of hornblende and biotite shows strong heterogeneity.The contents of main oxides in the melt show that the composition of melt iscontrolled by the minerals which participated in the partial melting, showingcharacteristics of non-equilibrium partial melting. The compostion of melt near biotiteis exactly similar to that of biotite, showing melt mainly came from grains of biotite.However, the composition of melt in the grain margin of hornblende is different fromthat of hornblende, and it seems that melt came not only from the grains ofhornblende, quartz, feldspar, but also from ilmenite. |
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