Experimental Study On The Rheology Of Intermediate Granulite At High Temperature

The lower continental crust is composed of granulite Laboratory-derived flow laws for granulate provide important constraints on its rheological behavior. However, most of experimental studies focused on creep of hot-pressed single and two phase aggregates of plagioclase and pyroxene, as well as natural samples of gabbro and diabase constituted by plagioclase and pyroxene. There is few data covered to creep of granulite, especially it is scarce for rheological tests of intermediate and felsic granulite.In this study, samples of natural intermediate granulite were deformed in a gas medium (Paterson) apparatus to evaluate the flow strength of the lower crust. We performed 40 creep tests on 11 samples at 300 MPa confining pressure, temperatures between 900℃ and 1200℃, and axial stresses of 12-764 MPa, resulting in strain rates between 6.25×10-6s-1 and 5×10-5s-1 and the total strain up to 7.8-15.1%. The samples were collected at Wayaokou village, located in Huai’an, Hebei province, China. Composition of the sample was about ～52 vol% plagioclase,～40 vol% pyroxene,～3 vol% quartz,～5 vol% magnetite and ilmenite, with mean grain sizes of 294μm,282μm,97μm and 109μm for plagioclase, pyroxene, quartz, magnetite and ilmenite seperately. Water content of samples was ～0.17±0.05wt% measured by a Fourier transform infrared spectrometer.The sample strength decreased with increasing of temperature and decreasing of strain rate under experimental conditions.Based on creep data of samples for natural intermediate granulite, the stress exponent n is between 8.1-12.9 at the temperatures between 900℃ and 1000℃, and 4.8-5.8 at the temperatures between 1050℃ and 1150℃. We obtained an activation energy of Q of 975 kJ/mol by the strain rate data for temperatures-step creep test between 1050℃ to 1100℃.Microstructural observations on thin sections parallel to the sample axis using optical microscopy, SEM and EDS show that deformed samples are different from the starting materials. At 900℃, grains are enlongated and a shape preferred orientation develops in the direction perpendicular to the compression direction of minerals, sample shows the dislocation slip characteristics with intragranular microcracks significantly. At the temperatures between 950℃ and 1000℃, grains in the upper and lower part of the sample are enlongated, a shape preferred orientation develops in the direction 45° to the compression direction of minerals. Grain boundaries of most minerals in the central part became round showing dislocation climb characteristics. Melt films appeared mainly at some of grain boundaries of pyroxene and magnetite. There are some melt film between grain boundaries of plagioclase, but chemical analysis show that plagioclase is not involved in partial melt, which implied that melt was diffused and migrated from grain boundaries of pyroxene and magnetite to grain boundaries of plagioclase during the deformation process.At the temperatures between 1050℃ and 1200℃, the phenomenon of mineral deformation is not apparent, plagioclase is also involved in melting, the content of melt increases with increasing of temperature and the duration of the experiment (strain), melt distributes in mineral edges, and the diffusion migration of melt is significant. The melt composition strongly dependent on the components of mineral involved in the melting, and most of the melts are rich in iron. At the temperatures between 1100℃ and 1200℃, the reaction from pyroxene to olivine happened, the reaction production of fine-grained olivine was distributed around rims of pyroxene grains. There are some reaction transition zone in rim of pyroxene grains, which show solid phase reaction and diffusion.Microstructure and composition analysis show that dislocation slip and micro-fracture deformation is the main deformation mechanism of granulite at 900℃. The main deformation mechanism is dislocation climb at 950-1000℃. At the temperatures between 1100℃ and 1200℃, dislocation climb is one of major deformation mechanism, but partial melt and mineral reaction induced diffusion process. So melt and reaction assisted diffusion and dislocation climb controlled the rheology of granulite at high temperature.