内蒙古乌拉特后旗乌兰敖包地区二叠纪中-基性火山岩地球化学、年代学、Hf同位素特征及其地质意义

内蒙古乌拉特后旗乌兰敖包地区中-基性火山岩位于华北陆块北缘西段,野外出露岩性主要为灰黑色-深灰绿色玄武岩、深灰绿色玄武安山岩和灰绿色安山岩等。岩石样品中SiO_2含量49.33~65.84%,Na_2O含量0.53~5.49%,K_2O平均值为0.25~6.40%,属于钙碱性系列;由基性到中性岩石中稀土总量呈增加趋势,轻稀土元素富集程度增加,δEu负异常逐渐明显;微量元素显示火山岩普遍具有不同程度的Ta、Nb亏损,U、Ba、K等大离子亲石元素富集特征,具有俯冲带弧岩浆岩的特征。火山岩中岩浆锆石U-Pb测年结果为285.0±2.3 Ma和265.0±1.5Ma,表明其形成时代为早-中二叠世;火山岩与下伏地层接触关系及捕获锆石的年龄特征指示,乌兰敖包地区火山岩形成在华北陆块北缘之上。锆石Hf同位素信息指示岩浆起源于受俯冲板块影响的亏损地幔,岩浆上升过程中加入了壳源物质。

GEOCHEMISTRY, CHRONOLOGY AND HF ISOTOPE FEATURES OF THE PERMAIN INTERMEDIATE-BASIC VOLCANIC ROCKS IN WULANAOBAO AREA, URAD HOUQI, INNER MONGOLIA AND ITS GEOLOGICAL SIGNIFICANCE

The Wulanaobao intermediate-basic volcanic rocks, distributed in Urad Houqi, Inner Mongolia, are located in the western north margin of the north China block. The outcropped lithologies mainly are grey black-spruce basalt, spruce basaltic andesite and greyish-green andesite. All samples show chemical signatures of calc-alkaline series which are characterized by contents of SiO₂ (49.33~65.84%), Na₂O (0.53~5.49%) and K₂O (0.25~6.40%). From basic to intermediate, the amount of rare earth elements, light rare earth elements enrichment degree and Eu negative anomaly degree show a trend of increase. The trace elements show subduction zone arc magmatite features which are characterized by wide depletion of Ta and Nb, and enrichment of the large ion lithophile elements (U, Ba, K). The zircon U-Pb dating results of the magma from volcanic rocks are 285.0±2.3Ma and 265.0±1.5Ma, which indicates that the volcanic rocks were formed at the early-middle Permian period. The age characteristics of captured zircons and the contact relations between volcanic rocks and underlying strata suggest that volcanic rocks from Wulanaobao were formed on the crust of the north China block. Zircon Hf isotopic information indicates that the magma was derived from the depleted mantle which was affected by the subducting plate and partly joined with crust-derived material during the rising stage.