西藏日土南部阿依拉杂岩体的地球化学 年代学特征及其构造意义

Geochemical and chronological characteristics of the Ayila complex, Rutog, Tibet, China, and its tectonic significance

冈底斯岩基西段的阿依拉杂岩体含有大量的镁铁质包体,在1∶25万区域地质填图的基础上,系统开展了该杂岩体内的包体和寄主岩石的岩石学、主量元素、微量元素地球化学、锆石年代学的研究工作。确定阿依拉杂岩体的包体具火成结构,包体与寄主岩石存在地球化学差异,但同时二者主量元素、微量元素在比值-比值协变图中呈双曲线或直线变异关系,在共分母比值-比值成分变异图中则呈直线变异特点,包体εNd(t)值在1.6～4.4之间,寄主花岗岩εNd(t)值为-10.2～-1.1,反映了壳幔混源的特征。锆石测年获得眼球状花岗闪长岩的结晶年龄为69.8Ma±0.3Ma,根据杂岩体内强烈的塑性构造变形和杂岩体主要岩性在R1-R2图解、微量元素洋脊花岗岩标准化图解上的判别结果,确定该杂岩体属同碰撞花岗岩,69.8Ma±0.3Ma代表了青藏高原西构造结一带印度大陆和欧亚大陆最初碰撞的时间,与两大陆碰撞有关的底侵作用是青藏高原地壳生长和岩石圈结构改变的重要因素之一。

The Ayila complex in the westem segment of the Gangdise bathohth contains many mafic microgranular enclaves. On the basis of 1：250000 regional geological mapping, the authors have systematically performed petrological, major and trace element geochemical and zircon chronological studies of enclaves and their host rocks in the complex and revealed that the microgranular enclaves in the complex have an igneous fabric and that, in spite of the geochemical difference, the major elements and trace elements of the microgranular enclaves and their host granite show a hyperbolic or rectilinear variation relation in the ratio-ratio covariation diagram and the features of rectilinear variation in the SiO2 oxides scatter （such as MgO,CaO, K2O,TiO2） and bi-element scatter and co-denominate ratio-ratio compositional variation diagram. The εNa（t） values of the enclaves and host granite are 1.6 to 4.4 and -10.2 to -1.1 respectively, implying a crust-mantle mingling source. Single-zircon U-Pb dating yields a crystallization age of 69.8±0.3 Ma for the augen granodiorite in the Ayila complex. The strong plastic tectonic deformation of the complex and the results of discrimination of the main rocks in the R1-R2 diagram and ocean ridge granite （OILG）-normalized trace element patterns all indicate that the complex is syncollision granite. The age 69.8±0.3 Ma represents the time of initial India-Eurasia collision and the underplating related to the collision between the two continents was one of the important factor for the crustal growth and change of the lithospheric structure beneath the Qinghai-Tibet plateau.