The Application Of Inductively Coupled Plasma Mass Spectrometry In Geological Samples Test: Small The Qinling Too Huaqun Gray Gneiss Geochemistry And Zircon Geochronology,

As we known, the accurate and precise analytical data is the basis of geological research, especially of geochemistry. Since I will work in the Key Laboratory of Continental Dynamics after graduation, this paper contains two fields of studies: geochemical analyses and geology. I investigated and improved some methods related to Solution Nebulization Inductively Coupled Plasma Mass Spectrometry (SN-ICPMS) and Excimer Laser Ablation ICP-MS (ELA-ICPMS); on the other hand, I carefully studied the geochemical character and single zircon geochronology of grey gneisses in the Taihua high terrain, Yiyang area based on the accurate analytical data. Some conclusions are listed as following:1 The tungsten carbide mortars are widely used to grind geological sample, but the degree of element contamination is not well known. Compared with agate pot, tungsten carbide has no apparent contamination to grind geological samples for measured trace elements except Co. Co contamination is more serious in granitic rocks than others.2 The rotating speed of peristaltic pump of ELAN ICP-MS affects the results of solution nebulization apparently. While the rotating speed of peristaltic pump speeding up, intensity of Indium increases at the beginning and decreases when the speed is higher than 11 rpm, and the oxide ratios of CeO/Ce always increases, however, the Ba2+/Ba+ keeps steady. The 11 rpm is the optimum rotating speed to satisfy lower oxides ratio, double ion and the higher sensitivity.3 Zircon is the most widely used mineral for U-Pb dating, but we can not obtain the information both age and related trace elements to quite small and complex zircons. Although Doctor Yuan developed two-stage ablation method, this method is difficult to obtain both REEs and U,Pb isotopic compositions when the zircon goes smaller than 30μm. Agilient 7500a ICP-MS with Shield Torch, just installed in Key Laboratory of Continental Dynamics of Northwest University, has much higher intensity in all mass range. With this instrument, we developed a new method to obtain in situ zircon age and trace element information simultaneously, accurately and conveniently.4 Compositionally, the grey gneisses in the Taihua high terrain, Yiyang area contains SiO2>56%, Al2O3>15%, NaaO/K2O>2, Sr>300ppm, Y<18ppm, Yb<1.8ppm, (La/Yb)M>10, Sr/Y>20, and typically strongly fractionated REE patterns. It is similar to the REE distribution patterns of Post-Archean TTG. The grey gneisses should be the tonalite-granodiorite series.5 There is an argument about the age of Taihua group: Late Archaean or neoproterozoic? According to my research, the main reason of this situation probably is the different dating methods by different researchers. For the early Precambrian rock, the zircon dating method is superior to other isotope dating methods because zircon is resistant to geological weathering and it can obtain three ages, namely, 206Pb/238U,207Pb/235U and 207Pb/206Pb.The different of the three ages can showwhether the system is disturbed or not. So the U-Pb dating of zircon is more reliable than other geochronology methods. I firstly obtained the TTG gneisses single zircon U-Pb age in the Taihua high terrain, Yiyang area, and the single zircon geochronology suggests that they are derived from granitoid plutons which were emplaced around 23Ga, in another words, there is a magma activity at 23Ga.