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

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

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。

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.