SUCCESSIONS OF THE MISSISSIPPIAN NANDUAN FORMATION IN THE “MIDDLE-UPPER DEVONIAN” AND THE “PERMIAN LABA FORMATION” OF THE CHANGNING-MENGLIAN BELT, WESTERN YUNNAN, CHINA AND ITS GEOLOGICAL SIGNIFICANCE

The U-Pb dating analysis of zircon was completed at Beijing Kehui Testing Technology Co., Ltd. The instrument used for zircon dating analysis is AnalyticJena PQMS type ICP-MS and the accompanying RESOlution 193nm excimer laser ablation system. The beam diameter used for laser ablation is 24 μ m. The frequency is 6 Hz, the energy density is about 5 J/cm2, and He is used as the carrier gas. LA-ICP-MS laser ablation sampling adopts a single point ablation method. Before testing, the instrument is adjusted using zircon standard GJ-1 to achieve the optimal state. Zircon U-Pb dating was performed using standard GJ-1 as the external standard, and trace element content was quantitatively calculated using NIST 610 as the external standard and Si as the internal standard. During the testing process, two zircon standard samples GJ-1 were repeatedly measured before and after every 10 samples to calibrate the samples, and one zircon Plesovice was measured. The status of the instrument was observed to ensure the accuracy of the test. The data processing was carried out using LADR_1.1.07 (Norris et al, 2018) software program. The zircon age harmony plot was obtained using the Isoplot 4.15 program. The detailed experimental testing process can be found in Hou Kejun et al. (2009). During the sample analysis process, the analysis result of Plesovice standard sample as an unknown sample was 336.6 ± 1.2 Ma (n=24,2 σ）， The recommended age value is 337.13 ± 0.37 Ma (2 σ） (Slama et al, 2008), the two are completely consistent within the error range.