新疆西天山阿吾拉勒山那拉提地区石英二长岩LA-ICP-MS锆石U-Pb年龄及其构造意义

Zircon U-Pb LA-ICP-MS Dating and Tectonic Significance of the Quartz Monzonite from the Nalati Area of Awulale Mountain,Western Tianshan Mountains,Xinjiang

位于伊犁微板块北缘阿吾拉勒山那拉提地区的石英二长岩侵位于上石炭统伊什基里克组,LA-ICPMS锆石U-Pb加权平均年龄为307.0±2.5 Ma,时代归属于晚石炭世,代表了岩浆的侵位年龄。岩石地球化学数据显示岩石中Si O_2、Al_2O_3、碱含量较高,里特曼指数σ为3.09~3.94,A/CNK=0.75~0.91,属准铝质钙碱性—碱性花岗岩系列;稀土元素总量(ΣREE)较高,稀土配分模式呈轻稀土富集的右倾型[(La/Yb)_N=2.86~6.14],轻稀土分异程度大于重稀土,Eu具有较为明显的负异常(δEu=0.46~1.02,平均为0.69);强烈富集大离子亲石元素(LILE)Rb、Th、K,亏损高场强元素(HFSE)Nb、Ta、Ti、P;综合表明那拉提石英二长岩具有A_2型花岗岩的特征。高的Th含量(8.68×10^(-6)~14.62×10^(-6))及低的Nb/Ta值(12.23~29.90,均值为18.13)指示岩浆源区主要为地壳,但受到了地幔混染作用的影响。在Nb—Y—Ce、Y/Nb—Ce/Nb及Rb—(Nb+Y)等构造判别图解中样品大多数落入造山后或晚造山花岗岩区,故笔者等认为该岩体应为造山向造山后或晚造山转换时期的产物,属造山后或晚造山岩浆作用。结合区域地质背景,本文认为伊犁微板块在307.0±2.5 Ma前已与准噶尔微板块碰合,北天山洋盆闭合时限早于晚石炭世。这些新资料为西天山区域大地构造演化研究提供了新的约束。

The Nalati quartz monzonite located at Awulale mountain, northern margin of the Ili tiny Plate, intruded in the Upper Carboniferous Yishenjilike Formation, The weighted average age dated by LA-ICP-MS U-Pb method of zircons is 307.0±2.5 Ma, representing its crystalline and intrusive age. Petrogeochemical analyses suggest the quartz monzonite belongs to calc alkaline-alkaline metaluminous granite series, which is characterized by high silica, aluminum and alkaline, σ is 3.09~3.94, A/CNK is 0.75~0.91;the rocks have relatively high total of Rara Earth Elements （REE） content with LREE riched patterns ／[（La/Yb）N=2.86~6.14／] of chondrite normalized, LREE are differentiated intenser than HREE, Eu show obviously negative anomaies（δEu=0.46~1.02,average=0.69）; The rocks enriched in large ion lithophile elements （LILE） Rb, Th and K, while strongly depleted in high field strength elements （HFSE） Nb,Ta, Ti and P; these show that the Nalati quartz monzonite has the characteristics of A2type granite.The high Th values （8.68~14.62μg/g） and low Nb/Ta ratios （12.23~29.90,average18.13） imply that the mainl source of magma was continental crust, but the magma experienced a significant contamination by mantle. In the tectonic setting discrimination diagrams （Nb-Y-Ce, Y/Nb-Ce/Nb, Rb-（Nb＋Y） et al.）, the most data project into the area of post orogenic or late orogenic granite. Writers reach a consensus that the rock was the product formed during the period syn orogen transformed to post orogen or late orogen, it was formed by post orogenic or late orogenic magmatism. Combining with regional geological setting, this study suggests that the Ili tiny Plate and Junggar tiny Plate had collided before the time of 307.0±2.5 Ma, the northern Tianshan oceanic basin had closed before Late Carboniferous.These new data provide new constrains for tectonic evolution of the Western Tianshan Mountains.