内蒙古西乌旗大石寨组火山岩年代学和地球化学特征及地质意义

中亚造山带南缘二叠纪火山岩的成因及形成环境一直存在争议。本文以内蒙古西乌旗罕乌拉地区发育的大石寨组火山岩为研究对象,对其开展了野外地质、岩石学、锆石U-Pb同位素年代学、地球化学研究。大石寨组火山岩为一套中酸性火山熔岩-碎屑岩组合,岩性以流纹岩为主。锆石LA-ICP-MS U-Pb同位素定年结果显示,2件流纹岩样品的~(206)Pb/~(238)U年龄加权平均值分别为276±0.81Ma(MSWD=1.3)和280±0.76Ma(MSWD=0.69),说明大石寨组流纹岩喷发于早二叠世,反映了早二叠世的构造岩浆作用事件。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、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,具一致的右倾"海鸥式"配分型式。在微量元素原始地幔标准化蛛网图上显示明显的Ba、Sr、P、Eu和Ti的负异常。10000Ga/Al平均值3.6。地球化学特征显示该套火山岩具有岛弧火山岩的属性,类似A型花岗岩的地球化学特征,为高温低压下长英质地壳部分熔融的产物。结合地球化学特征及区域地质资料,大石寨组火山岩最可能形成于弧后扩张环境,是早二叠世古亚洲洋闭合前洋壳俯冲消减作用的产物。

Geochronology and geochemistry for volcanic rocks of Dashizhai Formation and its geological significance in Xi U jimqin Banner, Inner Mongolia

The petrogenesis and tectonic environments of the Permian volcanic rocks in southern the Central Asia Orogenic Belt (CAOB) are controversial for a long time. The authors choose the volcanic rocks of Dashizhai Formation, which located in Hanwula of Xi U jimqin Banner, Inner Mongolia, as the studied object, and study detailed on filed geology, petrology, zircon U-Pb isotopic geochronology and geochemistry. The volcanic rocks occur petrologically as a set of intermediate-acid volcanic lava-clastic rocks, and are mainly composed of rhyolite. LA-ICP-MS zircon U-Pb ages of 276±0.81Ma (MSWD=1.3) and 280±0.76Ma (MSWD=0.69) from two rhyolite samples uniformly indicate that these rhyolites of the Dashizhai Formation erupted in the Early Permian, representing a violent tectono-magmatism event. The petrological and geochemical analysis reveal that the intermediate-acid rocks are characterized by enrichment of Si and alkaline, depletion of Ti, Mg, Fe and Ca, and abundance in trace elements and REEs, exhibiting a right-inclined seagull-type distribution pattern. The LILE, e.g. Rb, Th, U, K and LREE are richer than HFSE like Nb, Ta and HREE. The rocks are also featured remarkably by negative anomalies of Ba, Sr, P, Eu and Ti on the primitive mantle-normalized trace element diagram. The average value of 10000Ga/Al is 3.6. All these characteristics suggest that the volcanic rocks were geochemically formed in an island-arc setting, and are similar to geological features of A-type granites originated from the partial melting of felsic crust under high temperature and low pressure conditions. Combined with the geochemistry characteristics and the regional geology data, the rocks were most probably formed in a back-arc spreading environment, and are products of subduction of Paleoasian Oceanic crust before its closure in Early Permian.