Experimental Studies On The Electrical Conductivity Of Partial Melting Granite And Its Implication For The Crust High-conductivity Anomaly In Qinghai-tibet Plateau

The uplift and material migration of the Qinghai-Tibet Plateau is one of the focal issues concerned by many scientists.The magnetotelluric(MT)sounding results of several electromagnetic profiles on the Qinghai-Tibet Plateau play an important role in understanding this important scientific problem.The results show that there exist high conductivity layers in the middle and lower crust of Qinghai-Tibet Plateau,and their conductivity values can reach ~0.01 S /m order of magnitude.However,the cause of the high crustal conductivity anomaly in the Qinghai-Tibet Plateau remains unclear.The partial melting of rocks may be one of the important reasons for the formation of high conductivity in the middle and lower crust of the Qinghai-Tibet Plateau.Although some results on electrical conductivity of the granite has been previously reported,more measurements on electrical conductivity of granite collected from Tibet Plateau are still needed to constrain the electromagnetic anomalies in this region.The Qinghai-Tibet Plateau has special geological structure,which makes the rocks in this area also have special characteristics.Therefore,the research on the electrical properties of rocks in Qinghai-Tibet Plateau is of great significance for understanding the electrical structure of the crust and the genesis of the high conductivity layer in this region.In this dissertation,taking the leucogranite collected from the southern Tibet as the samples,we explored the relationship between the conductivity of leucogranite during partial melting and the crustal high conductivity layer of Qinghai-Tibet Plateau by means of high temperature and high pressure experiment and partial melting conductivity calculation method.Firstly,under the conditions of 0.5~2.0 GPa and 773~1373 K,the impedance of the leucogranite samples from south Tibet was measured by Solartron-1260 Impedance/Gain-Phase Analyzer,and the effects of temperature,pressure and melt on the conductivity w ere also discussed.Secondly,combined with the calculation method of partial melting conductivity model,the HS+ upper boundary model,which is suitable for the calculation method of partial melting of the experimental samples,was selected.Finally,the possible genesis of the crustal high conductivity layer in Qinghai-Tibet Plateau were suggested by analyzing the conductivity experimental results of leucogranite,HS+ upper boundary model calculation results and MT sounding results.The main innovative conclusions are summarized as follows:(1)Temperature effects on the electrical conductivity of leucogranite are obvious.The conductivity of the sample increases with temperature increasing,and the relationship between the reciprocal of the temperature and the logarithm of the conductivity show s a linear negative correlation,which satisfies the Arrhenius relationship.The activation enthalpies of the samples at 773~1223 K are 1.01~1.09 e V,and the activation enthalpies at 1223~1373 K are 2.16~2.97 e V.The change of activation enthalpy in different temperature ranges may be related to the dehydration and melting of biotite in granite samples.The conduction mechanism of granite during partial melting is ion conduction,and Na+ is dominated charged carrier.(2)The effect of pressure on conductivity is relatively weak.With the pressure increasing,the electrical conductivity of the samples decreases slightly.The effect of pressure on electrical conductivity can be characterized by the activation volume.The activation volume,(1,at 773~1223 K is 1.63 cm3/mol,and the activation volume,(1,at 1223~1373 K is 6.64 cm3/mol,showing that the effect of pressure on conductivity in the low temperature stage is weaker than that in the high temperature stage.(3)The conductivity values for the partial melting leucogranite falls within the conductivity range for the high-conductivity layer found by the MT.According to the calculation of the HS+ upper boundary solid-solution two-phase mixed model,the melt volume fraction of 10% to 15% can be used to explain the value of the high conductivity layer observed by the magnetotelluric layer.The cause of the crustal high-conductivity layer on the Qinghai-Tibet Plateau may be related to the partial melting of leucogranite in the deep part of the crust in this region.The estimated melt volume is about 10% to 15%.