准噶尔盆地西北缘中拐凸起石炭纪安山岩年代学、地球化学特征及其构造意义

为加深对中亚造山带西段石炭纪构造背景的认识,对准噶尔盆地西北缘中拐凸起金龙10井区石炭纪安山岩岩心样品开展岩相学、年代学和地球化学研究。安山岩的LA-ICP-MS锆石U-Pb定年结果显示,其结晶年龄为(322.4±1.1) Ma,属中石炭世。地球化学分析结果表明,研究区安山岩属于低钾-中钾钙碱性岩石系列,其稀土元素配分曲线均略向右倾,具微弱的Eu负异常;微量元素原始地幔标准化蛛网图显示整体相对富集Pb和大离子亲石元素Rb、Ba等,亏损高场强元素Nb、Ta、Zr等,具有消减带弧火山岩的特征。安山岩的Th/La、Th/Ce和Lu/Yb值均低于大陆地壳的平均值,而接近幔源岩浆的比值。研究区安山岩属于正常的岛弧火山岩,源于富水地幔楔,是由俯冲板片脱水产生的流体交代地幔楔发生部分熔融所形成的。结合区域地质背景并对比相近时期岩浆岩,综合认为研究区安山岩形成于大陆弧环境,为西准噶尔残余洋盆俯冲消减作用的产物,且残余洋盆在(322.4±1.1) Ma时还未完全关闭。

Geochronology,Geochemistry,and Tectonic Significance of Carboniferous Andesite in the Zhongguai Uplift,Northwestern Margin of the Junggar Basin

To deepen our understanding in the Carboniferous tectonic background in the western Central Asian Orogenic Belt, drill-core samples of andesite in Jinlong 10 well area of Zhongguai uplift (northwestern margin of Junggar basin) were studied via petrography, chronology and geochemistry. The results show that our andesite samples are low-K to medium-K calc-alkaline, with slightly right-inclining REE diagram and weakly negative Eu anomaly. In the primitive mantle-normalized spider diagram, the rocks have Pb and LILE (Rb, Ba) enrichments and HFSE (Nb, Ta, Zr) depletions, resembling those of typical subduction-related arc volcanic rocks. The Th/La, Th/Ce and Lu/Yb ratios of andesite are below those of the average continental crust, but similar to those of typical mantle-derived magmas. The andesite samples belong to the normal island arc volcanic rocks, which are derived from the water-rich mantle wedge and by partial melting of the fluid-metasomatized mantle wedge (produced by subducting-slab dehydration). LA-ICP-MS zircon U-Pb dating of the andesite yielded Middle Carboniferous age of (322.4±1.1) Ma. Combined with regional geological background and compared with coeval magmatism, we considered that the andesite in the Zhongguai uplift was formed in a continental arc setting, and was the subduction product of residual oceanic basin in West Junggar, which was not completely closed at (322.4±1.1) Ma.