西准噶尔由后碰撞向板内体制转变的地质记录--来自克西克A型花岗岩的证据

The Geological Record of the Post-collision Shift to Intraplate System in West Junggar-Evidence from Kexike A-type Granite

西准噶尔地区广泛发育晚古生代A。型和A2型花岗岩类。克西克岩体侵位于谢米斯台断裂北侧的志留纪一石炭纪火山沉积地层中，锆石LA-ICP-MSU-Pb年龄为（293．3±3．4）Ma，形成于早二叠世，属于西准噶尔地区后碰撞岩浆活动的产物。岩石地球化学研究表明，克西克花岗岩具有高硅（SiO2〉68．6oA）、富碱（Na2O＋K2O〉8．63％）、低钛（TiO2〈0．48％）、贫钙（CaO〈1．48％）、相对高铝（Al2O3〉13．56％）的特征，属于准铝质-过铝质（A／CNK=0．95～1．11）钾玄岩系列。岩石相对富集稀土元素（除Eu）和大离子亲石元素（Rb）及高场强元素（Th、Hf、Zr），相对亏损Ba、Sr、P和Ti等元素，为A1-A2怠过渡型花岗岩。这种延续性的岩浆演化很好的记录了前后相继的两个构造演化阶段，为标定西准噶尔地区由后碰撞向板内体制转变时限提供了重要佐证。

Late Paleozoic Al-type and A2-type granitoids are widespread in West Junggar. The Kexike granite intruded into the Silurian-Carboniferous volcanic strata in the north of the Xiemisitai fault. Zircon LA-ICP-MS U-Pb dating on the Kexike granite yields weighted mean 206 pb/238U ages of 293.3± 3.4Ma, corresponding to the Early Permian, implying the granite is the product of post-collisional magma activity around the Junggar area. Petrogeochemistry analyses suggest the Kexike pluton is characterized by high silica （SiO2〉68.6%）, high alkaline （NazO＋K2O〉8.63%）, low titanium （TiO2〈0.48%）, low calcium （CaO% 1.48%）, and relatively high alumina （A12 03 〉 13. 560/oo）, indicating shoshonite series of metaluminous or peraluminous （ACNK = 0.95 ~ 1.11 ）. The granites belong to A1-Az granite of transition type, with relative high REE content （except for Eu）, large ion lithophile elements such as Rb, high field strength elements such as Th, Hf and Zr, relative depletion in Ba, Sr, P and Ti. The sustainable magmatic evolution well recorded the two continuous tectonic evolutions, providing important evidence for demarcating the timing of the post-collision shift to intrap｜ate system in West Junggar.