U-Th/He Thermochronology: Method And Its Application In The Cenozoic Uplift Of The Qinling Range

As a thermochronological method,U-Th/He dating have been developed as a useful technique for geological studies.But the process of U-Th/He data processing in the laboratory is cumbersome when calculating the U-Th/He age,which reducing the work efficiency.Therefore,this paper compiled a U-Th/He data processing software with computing age and data visualization in C#.At the same time,this work utilized apatite U-Th/He method for the granite samples of the Taibai Mountain in the Qinling Range for dating.The data revealed the timing,magnitude,and velocity of the uplift of the Qinling Range.Combining with the geological data of the surrounding areas,and this paper discussed dynamical mechanism of Cenozoic uplifting in the Qinling Range.1.Compiling of U-Th/He data processing softwareWhen calculating the U-Th/He age,the signal value of the sample measured by Alphachron He and ICP-MS needs to firstly convert to the mother and daughter elemental content in moles(operating in Microsoft Excel).And then using Helio Plot,He Fty to calculate the original age and alpha correction,respectively.This data processing process is cumbersome and reduces the work efficiency of the laboratory.Secondly,Helio Plot,He Fty,and other age calculation software do not have the function of calculating other key data of the sample(such as ppm concentration of element,effective U,helium-uranium ratio,etc.).For the later analysis of age outliers,these calculating software is difficult to visualize multiple data.Therefore,this paper used C# to compile a U-Th/He experimental data processing software which can directly calculate the age starting from original test data,and can visualize each calculation result.Through visualizing and linear fitting data,users can view the potential relationships between various data(such as Age-Radius,Th/U-Age).Standard deviation between the various data is transferred by using the standard deviation transfer formula,and finally it passed to the standard deviation of U-Th/He age.These standard deviations include isotope ratios in standard solutions,isotope ratios in diluents,U,Th,He elemental signals,and pipette corrections.2.The application of apatite U-Th /He method in the Cenozoic uplift of the Qinling RangeThe Qinling area,which stretching 1000 km from west to east,locate in the central part of China and sandwich between the Tibetan Plateau and the Pacific Plate.To the north,lies the Han River.To the south,lies the the Weihe Basin which bounded by the north qinling margin fault.The Qinling orogenic belt is a complex collisional orogen that was formed by collision between the Yangtze plate and the North China plate in the Triassic period.After experiencing strong uplifting in the Cenozoic time,eventually the Qinling Range formed and it became the natural boundary which separate the geology,environment and climate of the North and South China.Privious studies utilized apatite fission tracks(AFT)and other method to stdudy the timing,magnitude and velocity of uplift of the Qinling Range in the Cenozoic time.However,the thermochronological thermometer which used in these studies is slightly insufficient for extracting high-precision time and magnitude of uplift of mountain.Apatite U-Th/He dating has the characteristics of low closure temperature(75±5?)and can reflect the tectonic activity of crustal depth of 2-3km,which make it suitable for the study of the uplift of the Qinling Range in the Cenozoic time.This study can help to precisely determine the timing,magnitude,and velocity of the uplift of Qinling Range in the Cenozoic time and further understand the impact on the Pacific Plate's westward subduction.Main production and results are as follows:(1)This work have tested 14 granite samples collected from the elevation profile of the Taibaishan Mountain for apatite U-Th/He dating.Sample ages ranges from 15.6 to 65.2 Ma.In the age-elevation relation plot,He ages generally increase with elevation.We find the marked inflection point at ca.1968 m.Above the inflection point,the slope of the fitting curve is relatively shallow and ages gradually increase from 29.3 Ma to 65.2 Ma.The corresponding age at 1968 m is 29.3 Ma,which may represent the corresponding time of tectonic activity.Between 1968 m and 1080 m above sea level,the slope of the curve becomes sharp,all ages vary in the range of 26.2 to 29.3 Ma.These focused He ages suggest an intensified cooling event initial at 26.2-29.3 Ma.A second inflection point is observed at ca.1000 m.The rapid cooling phase is defined between the two inflection point.Placing the first infelction point(1968m)as the upper limit and the second infelction point(1035m)is setted as the lower limit,and the magnitude of uplift of 933 m is derived.In the rapid cooling phase,the elevations and corresponding ages of the two infelction points were taken as the end members(1832m-29.3Ma,1080m-26.7Ma).We figure out the uplift rate of the Taibaishan mountain is 0.3km/Ma.(2)Thermochronological results of Taibaishan Mountain reveal a rapid cooling event of Taibaishan Mountain occurred at the 26.2-29.3Ma and the exhumation reached approximately 1000 m,the velocity of uplift is 0.3km/Ma.Combining provious results of research,this paper relate the uplift of Taibaishan Mountain occurred at 26.2-29.3Ma to westward subduction of the Pacific plate under Asia continent.This study precisely determined the timing,magnitude,and rate of the uplift of Taibaishan Mountain in the Cenozoic time,and provided valuable basic geochronological data for further understanding of the uplift history of Qinling Range in the Cenozoic time and of its geodynamic mechanism.