内蒙古林西地区满克头鄂博组火山岩形成时代及构造环境

Formation age and tectonic environment of volcanic rocks from Manketouebo Formation in Linxi area,Inner Mongolia

为确定内蒙古林西地区五十家子盆地火山岩地层归属及形成环境,对火山岩岩石组合及盆地内流纹岩进行了研究。流纹岩LA-ICP-MS锆石U-Pb年龄为151.4±1.4 Ma,表明其喷发于晚侏罗世晚期。流纹岩具有高SiO_2含量和全碱含量,稀土元素配分图呈右倾趋势,具Eu负异常,富集Rb、K和Th,亏损Ba、Sr、Ti和Nb。归纳近年来大兴安岭南段的火山岩年代学数据表明,160~150 Ma为满克头鄂博组火山活动峰期,综合地层层序及测年结果将五十家子盆地火山岩划归为满克头鄂博组。流纹岩具有与A型花岗岩相同的化学组成,指示林西地区在晚侏罗世晚期处于伸展环境,可能与蒙古—鄂霍茨克洋闭合后的岩石圈伸展有关。

Wushijiazi Basin is located in northern Linxi area,Inner Mongolia. In order to determine the stratigraphic attribution and formation environment of volcanic rocks in the Wushijiazi Basin,we analyze the volcanic rock association and the rhyolites of the basin. The LA-ICP-MS Zircon U-Pb dating of the rhyolites is 151. 4 ± 1. 4Ma,which indicates its eruption in the late Late Jurassic. Geochemical analysis indicates that the samples high in SiO_2 and alkaline content. The REE distribution patterns show right deviated and Eu negative. The elements are enriched in Rb,K and Th,while depleted in Ba,Sr,Ti and Nb. Summarizing the isotopic dating data of volcanic rocks in the south of Da Hinggan Mountains published for the recent years,which suggests 160 ~ 150 Ma as the peak period of the Manketouebo Formation volcanic activity,and combining the stratigraphic sequence with the chronological characteristics,we consider the volcanic rocks belong to the Manketouebo Formation. The geochemical features of rhyolites are consistent with A- type granite,indicating the Linxi area was in an extensional environment during the late Late Jurassic,which was possibly related to the lithosphere extension after the closure of the Mongol-Okhotsk Ocean.