U-Pb geochronologic analyses from Taxkorgan River

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).本研究从东帕米尔构造结的塔什库尔干河采集了3件全新砂样。采用常规磁选与密度分选技术对约10 kg的砂样进行锆石（zircon）分选，以富集非磁性重矿物组分。随后在双目显微镜下通过人工挑拣获取具有代表性的锆石样品。随后随机选取约300颗锆石颗粒，将其镶嵌于环氧树脂靶中并抛光以获得平整表面。在进行同位素分析前，对该环氧树脂靶进行打磨、抛光与清洁处理。 锆石U-Pb地质年代学分析在中国地震地质研究所地震动力学国家重点实验室完成，采用激光剥蚀多接收器电感耦合等离子体质谱（LA–MC–ICPMS）技术。本次分析采用Resolution M50-LR ArF准分子激光剥蚀系统对锆石进行剥蚀，剥蚀光斑直径为28 μm；同时搭配Agilent 7900四极杆电感耦合等离子体质谱仪设置待测同位素的积分时长：204Pb、206Pb与208Pb为15 ms，207Pb为30 ms，232Th与238U为10 ms，其余微量元素为6 ms（Li等，2019）。每完成10件样品的分析后，依次分析2颗锆石标准样品91500、1颗锆石标准样品GJ-1以及玻璃标准参考物质（Jackson等，2004）。普通铅校正采用Andersen（2002）提出的方法。实验数据通过标准程序Glitter 4.0进行校正处理，剔除谐和度低于90%的U-Pb地质年代学数据。当样品年龄小于1.2 Ga时，采用206Pb/238U年龄及其1σ误差作为结果输出；若年龄大于等于1.2 Ga，则采用207Pb/206Pb年龄及其1σ误差（Nemchin和Cawood，2005）。