西天山玉希莫勒盖达坂石英闪长岩的微量元素地球化学及同位素年代学研究

Geochronological and geochemical studies on quartz diorite in Yuximolegai Daban,West Tianshan and its tectonic implication

玉希莫勒盖达坂石英闪长岩的主量元素组成相对稳定,SiO2、TiO2、Al2O3、Fe2O3T、MgO、CaO、Na2O和K2O的含量分别为57.08%～59.95%、0.70%～0.85%、16.34%～16.81%、6.73%～8.51%、2.82%～3.55%、5.72%～6.91%、3.22%～3.90%和1.44%～1.96%。该类岩的CA/TH指数在1.2左右变化,应属钙碱性系列岩石。主量元素和微量元素地球化学特征显示,玉希莫勒盖达坂石英闪长岩与区域内的大哈拉军山组钙碱性玄武安山岩为同源岩浆岩,它们是富集的尖晶石二辉橄榄岩部分熔融的产物,原始地幔楔的富集受俯冲板块脱水形成流体控制,这种源区富集交代过程导致玉希莫勒盖石英闪长岩具有明显的陆壳混染特征。玉希莫勒盖石英闪长岩明显亏损Nb-Ta、Zr-Hf、Ti等高场强元素,同时富集Pb,显示出与板块俯冲作用有关造山带岩浆岩的特征,形成于活动陆缘岛弧。与大哈拉军山组高钾系列火山熔岩形成于板片撕裂诱发的伸展构造环境不同,石英闪长岩形成于挤压的构造环境。单颗粒锆石U-Pb定年结果显示,石英闪长岩的形成年龄为309.9±2.7Ma,暗示阿吾拉勒东段大哈拉军山组火山岩的形成时代为晚石炭世。

The relatively less variable major element contents of the quartz diorite from Yuximolegai Daban and their CA/TH index about 1.2 indicated that they should belong to the calc-alkaline series rocks. The quartz diorite might be the co-magmatic rocks with the associated calc-alkaline basaltic andesite of the Dahalajunshan Formation. Both of them are the products of partial melting of the enriched spinel-lherzolite,and their formation was controlled by the fluid derived from the dehydration of the subducting plate. This process of enrichment and metasomatism in primary mantle wedge illustrated the significant characteristics of continental contamination in the quartz diorite of Yuximolegai Daban. The obvious depletion of HFSE,including Nb,Ta,Zr,Hf,Ti and the Pb enrichment demonstrate that the characteristics of igneous rocks in orogenic belt related to the plate subduction,which formed in the island arc of the active continental margin. So,this quartz diorite occurred in a compressive tectonic setting while the high-K series of volcanic lava in the Dahalajunshan Formation was formed in an extensional tectonic setting induced by the plate-slit. The concordia age of 309.9±2.7Ma obtained from zircon grains in the quartz diorite,implicated that the igneous rocks of the Dahalajunshan Formation in eastern Awulale Mountains was formed in Late Carboniferous.