Detrital zircon geochronology of modern rivers in the eastern Pamir syntaxis and its implications on formation of the Pamir accompanying growth of the Tibetan Plateau

We have collected three new sand samples from Taxkorgan River in eastern Pamir syntaxis . Zircon separation was carried out from ~10 kg samples using conventional magnetic and density techniques to concentrate the non-magnetic, heavy fractions. Representative selections of zircons were then extracted by handpicking under a binocular microscope. Then, ~300 zircon grains were randomly selected and then cast into an epoxy mount and polished to obtain an even surface. And the mount was sanded down, polished and cleaned prior to isotopic analysis.
U‐Pb geochronology of zircons was conducted by laser ablation multicollector inductively coupled plasma mass spectrometry (LA–MC–ICPMS) at the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. The analyses involve ablation of zircon with Resolution M50-LR ArF excimer laser ablation system using a spot diameter of 28 microns, and it is necessary to set the integration time of isotopes to be measured, including 15 ms for 204Pb, 206Pb and 208Pb, 30 ms for 207Pb, 10 ms for 232Th and 238U, and 6 ms for other trace elements using Agilent 7900 quadrupole inductively coupled plasma mass spectrometer (Li et al., 2019). Every set of ten samples analyses was followed by analysis of two zircons standard 91,500 and one zircon standard GJ-1, and the glass standard (Jackson et al., 2004). Common Pb was corrected using the method proposed by Andersen (2002). The experimental results were tuned by the standard program Glitter 4.0, and U‐Pb geochronology with concordance less than 90% were excluded. U‐Pb geochronology were output used by 206Pb/238U and 1σ if less than 1.2 Ga, unless use 207Pb/206Pb and 1σ (Nemchin and Cawood, 2005).

本研究从帕米尔高原东部塔什库尔干河流域采集了三份新的沙样。通过从约10公斤的样品中采用传统的磁性和密度技术分离非磁性重质组分，实现了锆石的分离。随后，在双目显微镜下手工挑选出具有代表性的锆石。接着，随机选取约300颗锆石颗粒，将其浇筑在环氧树脂模具中并抛光以获得均匀的表面。在 isotopic analysis 之前，模具经过打磨、抛光和清洁处理。锆石的 U-Pb 同位素年代学分析是在国家地震动力学重点实验室、中国地震局地质研究所进行的激光剥蚀多道电感耦合等离子体质谱法（LA-MC-ICPMS）。分析过程中，使用 M50-LR ArF 激光剥蚀系统，以28微米的点直径进行锆石的剥蚀，并需要设置要测量的同位素的积分时间，包括 204Pb、206Pb 和 208Pb 的 15毫秒，207Pb 的 30毫秒，232Th 和 238U 的 10毫秒，以及其他微量元素的 6毫秒，使用 Agilent 7900 四极杆电感耦合等离子体质谱仪（Li et al., 2019）。每组十个样品的分析之后，紧接着是对锆石标准样品 91,500 和 GJ-1 以及玻璃标准样品（Jackson et al., 2004）的分析。采用 Andersen (2002) 提出的方法对普通铅进行校正。实验结果通过 Glitter 4.0 标准程序进行调整，U-Pb 同位素年代学分析中，当一致性低于 90% 时予以排除。输出 U-Pb 同位素年代学数据时，若年龄小于 1.2 Ga，则使用 206Pb/238U 和 1σ，否则使用 207Pb/206Pb 和 1σ（Nemchin and Cawood, 2005）。