柴北缘达达肯乌拉山地区闪长岩锆石LA-ICP-MSU-Pb年龄、岩石地球化学特征及其构造意义

LA-ICP-MS zircon U-Pb age and petrogeochemical characteristics of diorite from Dadakenwulashan area on the northern margin of Qaidam basin and their tectonic implications

达达肯乌拉山位于柴达木盆地北缘中东部,区内发育闪长岩体,以小岩株形式产出。通过对达达肯乌拉山闪长岩岩石LA-ICP-MS锆石U-Pb定年和地球化学特征的研究,获得闪长岩体的年龄为(240.5±1.7) Ma,属早印支期。闪长岩体的w(K_2O)较高,为2.03%～2.30%,w(Na2O)含量较低,为3.2%～3.44%,为一套准铝质及弱过铝质高K、低Na的高钾钙碱性型I型花岗岩。岩石稀土元素总量为158.35×10^(-6)～200.41×10^(-6),球粒陨石标准化曲线向右倾斜,重稀土元素曲线呈水平状,轻稀土元素富集,但负铕异常不明显,岩浆分异程度不高。岩石的La/Ta(54.32～68.51)、Sm/Nd(0.18～0.19)及Rb/Sr(0.11～0.19)比值特征反映该岩体岩浆具有壳幔混合的特点。通过构造判别,反映达达肯乌拉山岩体可能形成于俯冲陆壳断离、幔源岩浆底侵的地球动力学背景,在中央造山带早中生代统一的板块碰撞与挤压构造体制下,以滩间山蛇绿岩带为基础,发生陆壳俯冲和断离作用,并诱发幔源岩浆的底侵和下地壳物质的部分熔融,沿火山机构侵位形成达达肯乌拉山岩体。

Dadakenwulashan in the eastern part of the northern margin of the Qaidam Basin had frequent magmatic ac- tivity in late Devonian epoch, and intrusive rocks exist in the form of small stocks or dikes. Through the study of the LA-ICP-MS zircon U-Pb dating and petrogeochemical characteristics of diorite rock from Dadakenwulashan, the authors obtained the age of diorite rock mass of （240.5 ＋ 1.7）Ma, suggesting a product of Early Indosinian pe- riod. Diorite rock has high w （K2O）content of 2.03%-2.30% and low w （Na20）content of 3.2%-3.2%, being I type granite composed of a set of prospective aluminum and weakly peraluminous and high K and low Na type high potassium calc-alkaline rock masses. The total amount of rare earth elements of rocks is 158.35×10^-6-200.41×10^-6, the chondrite standardized curve is tilted to the right, the heavy rare earth elements are in horizontal curve shape, the light rare earth elements are concentrated, the negative Eu anomaly is not obvious, and the mag- matic differentiation degree is not high. The La/Ta （54.32-68.51）, Sm/Nd （0.18-0.19） and Rb/Sr（0.11-0.19） ratio characteristics of the rock imply that the formation of the magmatic rock had the characteristics of crust-mantle contamination. Structural discrimination shows that diorite rock was probably formed in a geodynamic back- ground of continental crust breakup and mantle-derived magma underplating invasion. Under the tectonic regime of unified plate collision and extrusion in Early Mesozoic in the central orogenic belt and on the basis of Tanjian- shanophiolite belt,continental crust subduction and breakup took place, which induced mantle-derived magma un- derplating invasion and partial melting of the lower crust material and formed Dadakenwulashan rock mass along the volcanism emplacement.