东昆仑东段香日德地区察汗陶勒盖正长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义
LA-ICP-MS zircon U-Pb dating of the Chahantaolegai syenogranites in Xiangride area of East Kunlun and its geological significance
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摘要: 东昆仑造山带东段察汗陶勒盖地区出露一套肉红色正长花岗岩,用LA-ICP-MS技术测得的锆石U-Pb年龄为239.9±0.7Ma,表明该岩体形成于中三叠世。地球化学特征表明,察汗陶勒盖正长花岗岩属于过铝质高钾钙碱性系列,岩石具有高SiO2、Al2O3、K2O和低TiO2、MnO、CaO的特征,A/CNK为1.05~1.13(平均值为1.10),K2O/Na2O值介于0.95~1.16之间,里特曼指数σ为1.56~2.24,CIPW标准矿物计算的刚玉质量分数大于1%,矿物组合中出现白云母等富铝矿物,显示S型花岗岩特征。稀土元素总量为61.47×10-6~145.23×10-6,球粒陨石标准化配分曲线显示,轻稀土元素富集((La/Yb)N=15.83~27.11),且有明显的负Eu异常(δEu=0.26~0.49)。原始地幔标准化蛛网图显示,岩石富集Rb、La、Th,亏损Ba、Ta、Sr、P、Ti等元素。微量和稀土元素特征显示出明显的壳源特征,其源岩可能为以上地壳杂砂质岩为主的沉积岩。结合区域资料认为,察汗陶勒盖正长花岗岩体具有过渡型花岗岩的特征,形成于俯冲向同碰撞转化阶段,是东昆仑南缘青山-阿尼玛卿洋壳俯冲晚期的产物。Abstract: A suite of syenogranites is exposed in the Chahantaolegai area along the eastern section of East Kunlun Orogenic Belt. LA-ICP-MS zircon U-Pb weighted average ages is 239.9±0.7Ma, suggesting the formation of syenogranites in Middle Triassic. According to the geochemical characteristics, it is suggested that Chahantaolegai syenogranites belong to peraluminous high-potassic calc-alkaline series. They are characterized by high SiO2, Al2O3, K2O and low TiO2 and Na2O values. A/CNK=1.05~1.13 (average A/CNK=1.10), K2O/Na2O ratios are about 0.95~1.16, and Rittmann index σ=1.56~2.24; the content of corundum exceeds 1%, as shown by CIPW standard mineral calculation. The existence of rock-forming minerals such as muscovite suggests that the syenogranites are of S-type. The values of rare earth elements are 61.47×10-6~145.23×10-6. The chondrite-normalized REE patterns of the samples show enrichment of LREE((La/Yb)N=15.83~27.11)and strong negative Eu anomalies(δEu=0.26~0.49). The primitive mantle-normalized trace element diagrams show enrichment of Rb, La and Th and depletion of Ba, Ta, Sr, P and Ti. Characteristics of trace and earth elements suggest that syenogranites were from the earth's crust. It is suggested that the syenogranites resulted from melting of the sedimentary rocks of the upper crust. Combined with regional studies, the authors hold that Chahantaolegai syenogranites are of the transitional type formed in the transformation from subduction to syn-collision, being a product of the late stage of the subduction of Buqinshan-A'nyemaqen Ocean on the southern margin of the East Kunlun in Middle Triassic.
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