内蒙古西乌珠穆沁旗地区中生代中酸性火山岩SHRIMP锆石U-Pb年龄和地球化学特征

Zircon SHRIMP U-Pb age and geochemical characteristics of the Mesozoic volcanic rocks in Xi Ujimqin Banner,Inner Mongolia

西乌旗地区处于大兴安岭南部,广泛分布中生代火山岩。中生代火山岩中性岩组SHRIMP锆石U-Pb年龄为163Ma±2Ma,为岩浆上侵结晶的年龄;研究区南区下营子地区查干诺尔组流纹岩SHRIMP锆石U-Pb年龄为144.2Ma±1.4Ma,表明西乌旗地区晚中生代火山岩形成时代为晚侏罗世—早白垩世。在TAS图中,中性岩样品数据点落入粗面岩区,属碱性岩系列,REE分馏明显,LILE富集,Nb、Ta等元素亏损,Eu异常不明显,具有正高εNd(t)值,推断粗面岩是由地幔的碱性、过碱性岩浆经过分离结晶过程形成的;流纹岩属高钾钙碱性岩系列,REE分馏明显,部分LILE富集,Ba、Sr和HFSE强烈亏损,推测该岩石由相对较浅的中下地壳物质部分熔融形成并具有A2型花岗岩的特征。综合研究并结合前人的资料,认为从晚侏罗世(163Ma)开始,西乌旗所在大兴安岭地区已经处于伸展构造环境。

The Xi Ujimqin Banner area in southern Da Hinggan Mountains is characterized by extensively-distributed Mesozoic volcanic rocks.Two groups of different age volcanic rocks in the Late Mesozoic were chosen for SHRIMP zircon U-Pb dating.The SHRIMP zircon U-Pb age of the intermediate rock is about（163±2）Ma,and the dating of the Chagannuoer Formation rhyolite yields 144.2Ma±1.4Ma,indicating that these volcanic rocks were formed from Late Jurassic to Early Cretaceous,which is incompatible with the stratigraphic column.The intermediate rocks fall into trachytes belonging to the alkaline series in the TAS diagram,with enrichment of LILE and LREE and depletion of HREE and some HFSE.The rhyolites with characteristics of A2-type granite are of the high-K calc-alkaline series,with enrichment of LILE and LREE and depletion of HREE and,especially,Ba,Sr,HFSE.The trachytes originated from the fractional crystallization of the alkaline or peralkalic depleted mantle source and the rhyolites came from the middle-upper crustal magma that had suffered partial melting.Combined with the distinct geochemical features of Xi Ujimqin Banner volcanic samples and many previous researches in the study area,it is held that the Late Jurassic（163Ma） volcanism occurred in an intense lithopheric extension tectonic setting which resulted from closure of the Mongol-Okhotsk Ocean and subsequent orogenic collapse.