| Natural Rocks contain radioactive heat-producing elements,the decay heat of which is an important source of the geothermal energy.The elements of U,Th and40K are characterized by relatively higher abundance,high heat production and long half-life period?similar to the formation time of the Earth?,which can satisfy the significance of the research of geothermics.Zhangzhou area is located in the Southeastern China,it is not only a important research area for seismology,but also one of the most representative geothermal zones in China's land area,which has the highest temperature record of hot springs in the southeast coastal zone.The thermal conductivity of rock,the rock density and the radioactive heat production elements U,Th and K content analyses have been carried out in this study,the analytical results of 51 granites show that U has a average content of 6.39ppm with a range of 1.4412.15 ppm,Th has a average content of 23.85 ppm with a range of 8.2938 ppm;K has a average content of 1.75%with a range of 1.32.31%,respectively.The radioative heat rate of all granite samples have a range of 1.134.97?W/m3with a mean value of 3.32?W/m3.The fluctuation is more turbulent with the increase of vertical depth for the borehole granitic rock samples,and the overall pattern is serrated with an average value of 2.75?W/m3with a rang of 1.133.57?W/m3.The thermal conductivity of the regional granite samples is between 1.724.54 W/mK,with an average of 2.83 W/mK.and the overall granite sample density test results were 2.312.81 g/cm3,and the mean value was 2.61 g/cm3,.The thermal conductivity and density of granite which are similar to the average value of the granite in the world.Through this study and combined with Geophysics,geology and geothermics studies which obtain radioactive enrichment layer,lower part of upper crust,middle crust and the lower crust are 3.43?W/m3,1.03?W/m3,0.40?W/m3and 0.17?W/m3,respectively.The crust heat flow?Qc?and mantle heat flow?Qm?in the region are 30 mW/m2 and 44 mW/m2respectively,namely Qm/Qc>1,which is the typical thermal structure of the"cold crust-hot mantle"type.Based on geology,geochemistry,geophysics and geothermal data,this paper simulates the thickness of hot lithosphere in this area at 77-78km,which is consistent with previous research results.Through comparing with sufficient data form other southeastern China regions,we further explore the differences in the characteristics of lithosphere structure between Nanling region and the southeast coastal area.The research results of our team show that the average radioactive heat production ratio in Nanling region is 5.18?W/m3,which belongs to the high heat production granite?HHPG?,whereas the granites from the southeast coastal area in the current study exhibit average heat production rate of 3.08?W/m3.The Nanling area and southeast coastal area have crustal heat contributions of55%60%and 40%45%for the whole surface heat flow respectively,indicating that the lithosphere thermal structures of the two regions of“hot crust-cold mantle”and“cold crust-hot mantle”types.The integrated radioactive geochemistry,lithosphere thermal structure study,together with previous data,shows that the southeast coastal areas in the late Mesozoic is influenced by the subduction of ancient Pacific plate.The area is effected by the invasion of magmatism from the bottom of mantle,which is accompanied the process of the earth's crust remelting.In crust-mantle layer,there is not only material exchange exists,but also the energy is conversed,so the deep mantle source become one of the important sources of heat in the region,which is accordance with the geophysical features of thin Moho depth and low speed-high consistent values.This paper considers that the Zhangzhou area has a good geothermal background,the mantle heat has a large proportion in the surface thermal energy,and the lithospheric thermal structure is different from the adjacent area.Therefore,during the study of the traditional geothermal energy and non-traditional dry hot rock geothermal resources in the region,it is necessary to consider the thermal distribution features,the deep thermal structure characteristics and the differences of lithospheric thermal structure between this region and that of its adjacent areas. |
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