西准噶尔萨吾尔地区吉木乃组火山岩锆石U-Pb年代学、地球化学特征及构造背景

西准噶尔萨吾尔地区吉木乃组为一套以火山碎屑岩、火山熔岩、沉积岩为主的火山-沉积地层。对该剖面顶部玄武安山岩进行LA-ICP-MS锆石U-Pb测年结果表明，吉木乃组顶部火山岩结晶年龄为（294.0±1.4） Ma，为早二叠世。结合吉木乃组内古生物化石组合，将其时代归属为晚石炭世—早二叠世。该组火山岩w（SiO₂）介于48.10%~54.35%之间，主要为玄武岩、玄武安山岩；w（TFeO）为7.38%~10.92%，w（MgO）为3.35%~5.16%，Mg^#介于41.02~55.05之间，主要为拉斑系列。稀土分配模式呈右倾型，轻稀土富集（（La/Yb）_N为4.00~6.04）且重稀土轻微分异（（Gd/Yb）_N为1.68~2.26），δEu=0.91~1.07，δCe=0.91~1.06。微量元素蛛网图上表现为大离子亲石元素（Rb、Ba、Sr、P）富集，除样品JM2和JM3外均表现为高场强元素（Nb、Ta、Ti）相对亏损的特征。大部分样品（Th/Nb）_N值介于1.39~2.10之间，Nb/La值介于0.39~0.74之间，显示样品受到岩石圈地幔和地壳的轻微混染。吉木乃组火山岩Zr/Y值以及微量元素质量分数较高，结合相关判别图解，该组火山岩形成于后碰撞构造背景，是源于软流圈地幔的玄武质岩浆上涌，在上升过程中受到岩石圈地幔和地壳轻微混染后的产物。

Zircon U-Pb Age,Geochemical Characteristics and Tectonic Settings of Volcanic Rocks in Jimunai Formation,West Junggar Basin

Jimunai Formation is located in Sawuer area of west Junggar basin, which is mainly composed of lava, pyroclastic rocks, and sedimentary rocks. Basaltic andesite from the upper section is (294.0±1.4) Ma by LA-ICP-MS zircon U-Pb dating. The result indicates that the lava underwent crystallization during Early Permian. Combined with local paleontological fossil assemblages, Jimunai Formation was formed in Late Carboniferous or Permian. The SiO₂ content varies from 48.10% to 54.35%, belonging mainly to basalt and basaltic andesite. The total FeO content is between 7.38%-10.92%, MgO content is between 3.35%-5.16%, and Mg^# index is between 41.02-55.05, mainly belonging to tholeiitic series. The rare earth element (REE) distribution pattern is right deviated, and rich in LREE ((La/Yb)_N=4.00-6.04), with slight fractionation of HREE((Gd/Yb)_N=1.68-2.26), δEu=0.91-1.07, δCe=0.91-1.06. The trace element spider diagram shows the enrichment of LILE (Rb, Ba, Sr, P), and moderate depletion of HFSE (Nb, Ta, Ti) except for the samples of JM2 and JM3. The ratios of (Th/Nb)_N are mainly between 1.39 and 2.10, and Nb/La are between 0.39 and 0.74, indicating that the magma was partially contaminated by the lithospheric mantle and the crust. Based on the geochemical characteristics of high Zr/Y ratio, high trace element contents, the volcanic rocks of Jimunai Formation was derived from basaltic magma of asthenospheric mantle upwelling under post-collision tectonic background, and the magma was partially contaminated by lithospheric mantle and crust in the process of ascending.