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).

我们从东帕米尔构造结（eastern Pamir syntaxis）的塔什库尔干河采集了3件全新砂样。对约10kg的样品采用常规磁选与密度分选技术，富集其中的非磁性重矿物组分以分离锆石（zircon）。随后在双目显微镜（binocular microscope）下通过手选法挑取具有代表性的锆石颗粒。随机选取约300颗锆石颗粒，将其镶嵌于环氧树脂靶（epoxy mount）中并抛光以获得平整表面；在开展同位素分析前，需对该靶材进行打磨、抛光与清洗处理。

锆石U-Pb年代学分析在中国地震局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法（laser ablation multicollector inductively coupled plasma mass spectrometry，LA–MC–ICPMS）。本次分析使用Resolution M50-LR ArF准分子激光剥蚀系统对锆石进行剥蚀，剥蚀斑束直径为28微米；搭配Agilent 7900四级杆电感耦合等离子体质谱仪（Agilent 7900 quadrupole inductively coupled plasma mass spectrometer），设定各待测同位素的积分时间如下：²⁰⁴Pb、²⁰⁶Pb与²⁰⁸Pb为15ms，²⁰⁷Pb为30ms，²³²Th与²³⁸U为10ms，其余微量元素为6ms（Li等，2019）。每完成10件样品的分析后，需依次分析2件锆石标样91500（zircon standard 91500）、1件锆石标样GJ-1（zircon standard GJ-1）以及玻璃标样（glass standard，Jackson等，2004）。普通铅校正采用Andersen（2002）提出的方法。实验结果通过Glitter 4.0标准程序进行校正，剔除谐和度低于90%的U-Pb年代学数据。对于年龄小于1.2Ga的样品，采用²⁰⁶Pb/²³⁸U年龄及其1σ误差；反之则采用²⁰⁷Pb/²⁰⁶Pb年龄及其1σ误差（Nemchin与Cawood，2005）。