东天山石灰窑钾长花岗岩体LA-ICP-MS锆石U-Pb年龄与地球化学特征

LA-ICP-MS Zircon U-Pb Dating and Lithogeochemistry of Shihuiyao Potassium Granite Pluton,Eastern Tianshan Mountain

石灰窑花岗岩位于卡拉麦里蛇绿岩带的最东段,岩性为中粗粒钾长花岗岩。岩体具有高硅（74.13%～75.76%）、高钾（4.26%～5.59%）、低镁铁[w（MgO＋Fe2O3＋TiO2）介于1.55%～3.37%之间]、高A/CNK（0.99～1.12）特征,刚玉标准分子略高（0.003%～1.46%）,属高钾钙碱性过铝质S型花岗岩。岩体富钾（K2O/Na2O值介于1.12～2.06之间）,高Rb、Cs,低CaO/Na2O值（0.18～0.34之间）,显示源区主体为富云母的变泥质岩区。岩石明显富集Rb、Th、U、K和Pb等大离子亲石元素,强烈亏损Ba、Sr、P、Ti、Eu等元素,并具有弱的Nb亏损,具有俯冲相关的火山弧和后碰撞伸展花岗岩类的地球化学特征。岩体侵位于下石炭统山梁砾石组地层,南界被蛇绿混杂岩的边界断裂切割,岩体LA-ICP-MS锆石U-Pb年龄（282.9±3.6）Ma应代表了卡拉麦里蛇绿混杂岩带东段的闭合年龄。结合区域构造背景分析,石灰窑花岗岩形成于卡拉麦里俯冲碰撞向碰撞后伸展的过渡阶段,是局部减压和地幔热流异常导致上地壳物质发生部分熔融的产物。

The Shihuiyao granite in the eastern end of Karamaili ophiolite belt,is composed of coarse grain potassium granite.Chemical analyses show that the granite has high SiO2（74.13%-75.76%）,high potassium（4.26%-5.59%）,and low magnesium-iron（MgO＋Fe2O3＋TiO2 between 1.55%-3.37%）,slightly peraluminous with A/CNK values ranging from 0.99 to 1.12,and moderate to high normalized corundum molecules（0.003%-1.46%）,reflecting the high-potassium calc-alkaline peraluminous S-type granite affinity.The rock,enriched in potassium（K2O/Na2 O within 1.12-2.06）,Rb,Cs,and low CaO/Na2O（0.18-0.34）,indicates a prodominant metapelitic source region with abundant mica.The rock displays obvious enrichment of LILE such as Rb,Th,U,K,Pb,noticeable depletion of Ba,Sr,P,Ti,Eu,etc,and slightly depletion of Nb.The trace element characteristics of Shihuiyao granite reflect that it is formed under volcanic arc setting and post-collisional extension setting related with subduction.The pluton intruded into the Lower Carboniferous Shanlianglishi Formation and had been cut off by the boundary faults of ophiolite belt to the south margin.Therefore,the LA-ICP-MS zircon U-Pb dating of 282.9±3.6Ma should represent the closure time of Karamaili ophiolite belt.Combined with the tectonic analyses,we suggest that the Shihuiyao granite probably has been generated in the transitonal phase from collision stage to post-collision extentional stage by increasing heat from decreasing pressure in local extentional,ascending anomalous mantle and heat flow under compressional environment.