内蒙古锡林浩特乌拉苏太大石寨组火山岩年代学、地球化学特征及其地质意义

通过1∶5万区域地质调查,对中亚造山带南缘内蒙古锡林浩特乌拉苏太地区发育的大石寨组酸性火山岩进行了野外地质、岩石学、锆石U-Pb同位素年代学、地球化学等研究。LA-ICP-MS锆石U-Pb同位素定年结果显示,该火山岩年龄为287.5±1.4Ma(MSWD=3.1),形成于早二叠世早期。岩石地球化学研究表明,大石寨组火山岩为一套中酸性火山岩,以高硅、富碱、高铝为特征,Ti、Mg、Fe、Ca等元素含量较低。微量元素总体含量较高,具有一致的配分曲线,Rb、Ba、Th、U、K、LREE等大离子亲石元素相对于Nb、Ta、HREE等高场强元素明显富集。稀土元素总量偏高,具有一致的右倾海鸥式配分型式。地球化学特征显示,该套火山岩具有岛弧火山岩的属性。结合大石寨组岩石学及地质学特征,大石寨组火山岩最可能形成于弧后扩张(或弧间)盆地,是早二叠世早期古亚洲洋闭合前洋壳俯冲消减作用的产物。

Geochronological and geochemical features of volcanic rocks of Dashizhai Formation in Ural Sutai of Xilin Hot, Inner Mongolia, and their geological significance

Based on 1:50000 regional geological survey, the authors studied the volcanic rocks of Dashizhai Formation developed in southern Central Asia Orogenic Belt (CAOB) within Ural Sutai of Xilin Hot, Inner Mongolia in such aspects as filed occurrence, petrology, zircon U-Pb isotopic geochronology and geochemistry. LA-ICP-MS zircon U-Pb dating results show that the rocks were formed at about 287.5±1.4Ma (MSWD=3.1) in the early period of Early Permian. Petrological and geochemical data reveal that the rocks are a suite of mid-acid volcanic rocks characterized by rich Si, alkali and Al, poor Ti, Mg, Fe and Ca, and abundant trace elements and REEs with coincident distribution pattern exhibiting a right-inclined seagull-type distribution pattern. The LILE, such as Rb, Ba, Th, U, K, and LREE are richer than HFSE like Nb, Ta and HREE, suggesting that the volcanic rocks were formed in an island-arc setting geochemically. Together with their petrological and geological features, the rocks were most probably formed in a back-arc or intra-arc spreading basin triggered by oceanic crust subduction before closure of the Paleoasian Ocean in the early period of Early Permian.