Electrical Conductivity Of Pvroxenites And Felsic Gneisses From The North China Craton

The continental lower crust is characterized by the electrical conductivity as high as10-4-10-1S/m, while the upper-middle crust and upper mantle display a low electrical conductivity. Recent experimental conductivity studies on the major minerals of the lower continental crust indicate that temperature and chemical composition are the two controlling factors of the electrical conductivity. So far conductivity measurements still focus on mafic granulites, which are composed of nominally anhydrous orthopyroxene, clinopyroxene and plagioclase. Due to the heterogeneity of the continental crust, knowledge about electrical properties of pyroxenites and felsic gneisses/granulites is important for interpretation of the electrical variations of the lower continental crust.We collected eclogite facies garnet pyroxenite and granulite facies plagioclase pyroxenite xenoliths from the Hannuoba area, and felsic gneisses from the Neoarchean-Paleoproterozoic Hengshan-Wutai terrane in the North China Craton. The electrical impedance of samples was determined using a piston-cylinder apparatus at1.0-1.2GPa and temperature of300-900℃in the Geophysics Lab of University of Frankfurt. With increasing temperature, the electrical conductivity increases. The temperature(T) dependence of conductivity (a) can be fitted by the Arrhenius equation:σ=aoexp (-△H/kT), where T is in Kelvin, k is the Boltzmann constant,△H is the activation enthalpy, and σo is pre-exponential factor. The values of σo in granulite facies pyroxenites and felsic gneisses are5.5-17.1S/M and2.5-9.8S/M, respectively. The activation enthalpy of in granulite facies pyroxenites and felsic gneisses is also similar, with values of0.76-0.95eV and0.83-0.85eV, respectively. In contrast, eclogite facies garnet pyroxenite is characterized by large σo of97.5S/M and AH of1.27eV. This difference can be attributed to lower water content and iron content in clinopyroxene of the eclogite facies garnet pyroxenite than that of granulite facies plagioclase pyroxenites.Recent magnetotelluric (MT) profiles in the North China Craton show the lateral variations in the electrical structure of the lower continental crust. According to surface heat flow and crustal structures, we calculated geothermal profiles of major tectonic units along the MT profiles. The laboratory-derived electrical conductivity-temperature relationship was used to infer in situ conductivity under different geothermal conditions. Compared with the MT observations, we found that the conductivity difference at the same depth is mainly due to temperature variations. Granulite facies plagioclase pyroxenites and felsic gneisses are indistinguishable in MT profiles, but they are about1-2orders more conductive than eclogite facies pyroxenites. Granulite facies pyroxenites occur at a depth of33-40km in the lower part of the lower crust, while eclogite facies garnet pyroxenites are formed by basaltic magma underplating at a depth of40-45km near the crust-mantle transition zone. Therefore magma underplating will result in a shallow crust-mantle boundary than the petrologic Moho. Our results provide new constraints to trace the crust-mantle interaction beneath continental regions.