Zircon U-Pb Ages of the Late Jurassic Diorite from the Central Qilian Mountains of Suliarea and Its Geological Significance
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摘要: 对中祁连西段苏里地区的闪长岩所进行的时代及地球化学研究发现，该闪长岩形成时代为（155.1±1.4） Ma。岩石地球化学分析结果显示，常量元素Al2O3含量为13.37%~13.55%；Fe2O3含量为5.37%~5.90%；FeO含量为5.64%~6.20%；MnO含量为0.11%~0.16%；MgO含量为1.85%~2.16%，CaO含量为5.15%~5.67%，Na2O含量为3.13%~3.58%，K2O含量为1.39%~1.54%。具富铁富钙贫硅贫铝的特点，A/CNK=0.75~0.83，σ=1.32~1.41，为准铝质钙碱性系列，稀土总量较高，ΣREE=291.44×10-6~319.87×10-6，明显具有壳源岩浆岩特征。LREE/HREE=5.06~6.11表现为轻稀土强烈富集的特点，δEu=0.86，（La/Yb）N值大于1，为5.33~7.2。微量元素表现出明显的Ba、K、Sr、Ti等元素的负异常，而Rb、Th、Ce、Nd等元素相对富集的特点。综合岩石地球化学特征和构造环境判别图解可以确定闪长岩体发生在碰撞后的张弛阶段，为板块拉伸环境下的产物。Abstract: The formation age and geochemistry of the diorite, was firstly found in Suli area of western section of Central Qilian by 1:50000 regional geological survey, have been studied and analyzed in this paper. The zircon U-Pb dating shows that this diorite was formed in Late Jurassic, with an age of (155.1+1.4) Ma. The geochemical data suggests that Al2O3, Fe2O3, FeO, MnO, MgO, CaO, Na2O and K2O contents are 13.37%~13.55%, 5.37%~5.90%, 5.64%~6.20%, 0.11%~0.16%, 1.85%~2.16%, 5.15%~5.67%, 3.13%~3.58% and 1.39%~1.54%, respectively, with a geochemical characteristics of rich Fe and Ca, but poor Si and Al. Its σ value and A/CNK ratio range from 1.32 to 1.41 and from 0.75 to 0.83, respectively, showing the feature of quasi-aluminum cal-alkaline series. Its ΣREE value, LREE/HREE ratio and δEu content vary from 291.44×10-6 to 319.87×10-6, from 4.93 to 5.06 and from 0.79 to 0.86, respectively, having obvious characteristics of crust source magmatic rocks. Its trace elements have obvious negative anomalies of Ba, K, Sr, and Ti, while Rb, Th, Ce and Nd are relatively enriched. Combined with the lithogeochemical characteristics and the discrimination diagrams of tectonic setting, the results can determine that this diorite was formed in the relaxation phase after the collision, serving as the product of plate tensional environment. The determination of zircon U-Pb age and genesis of this diorite provide an important geological information for the further study of the tectonic evolution of the Qilian orogenic belt.
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Keywords:
- diagenesis age /
- diorite /
- tectonic environment /
- western section of Central Qilian /
- Donggangou
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