内蒙古东乌珠穆沁旗宝力高庙组流纹岩年代学、地球化学特征及地质意义

Geochronology,Geochemistry Characteristics and Geological Significance of the Rhyolite of the Baoligaomiao Formation in the East Wuzhumuqin County,Inner Mongolia

兴安-蒙古造山带是古亚洲洋东段洋盆俯冲、闭合形成的增生造山带,但对该地区古亚洲洋俯冲形式、闭合时限及兴蒙造山带构造背景仍存在较大争议。东乌珠穆沁旗巴彦呼热地区位于兴蒙造山带中部,区内出露宝力高庙组为一套陆相中酸性火山碎屑岩-正常沉积碎屑岩建造。通过对该组流纹岩主、微量元素地球化学分析和LA-ICP-MS锆石U-Pb定年,精确厘定宝力高庙组火山岩形成时代,分析岩石地球化学特征。宝力高庙组流纹岩富集Rb,Th,Sr,亏损Nb,Ta,Ti元素。符合典型高分异“A”型花岗岩特点,显示后碰撞花岗岩特征,形成于造山后地壳减薄阶段。宝力高庙组流纹岩锆石U-Pb定年显示^(206)Pb/^(238)U加权平均年龄为(299.6±2.5)Ma(MSWD=1.5),时代为晚石炭—早二叠世。流纹岩SiO2含量70.80%~79.81%,MgO含量0.05%~0.50%,TiO2含量0.05%~0.23%,K2O含量3.25%~9.33%;微量元素Rb,La,Nd,Zr,Sm富集,相对亏损Sr,Ba,强烈亏损P,Ti;明显负铕异常及高锆石饱和温度(708.5℃~899.3℃),表明宝力高庙组流纹岩为形成于造山后地壳减薄阶段地壳部分熔融的产物。综合岩石地球化学、年代学及构造背景,认为兴蒙造山带中段东乌珠穆沁旗巴彦呼热地区在晚石炭—早二叠世处于后碰撞伸展环境,这为兴蒙造山带构造-岩浆演化提供新的证据。

Xing'an-Mongolia orogenic belt is a accretive orogenic belt formed by subduction and closure of the oceanic basin in the eastern part of the ancient Asian Ocean.However,there are still great disputes about the subduction form,closure time and the tectonic setting of the Xing'an-Mongolia orogenic belt in this area.The Bayanhure area of East Wuzhumuqin County is located in the central part of Xing’an-Inner Mongolia Orogenic Belt.Baoligaomiao Formation consist of intermediate-acidic and terrigenous sediments.Based on the geochemical analysis of the main and trace elements of the whole rhyolite and LA-ICP-MS zircon U-Pb dating,the formation age of the volcanic rocks of the Baoligaomiao Formation in the study area was accurately determined,and the geochemical characteristics of the rocks are analyzed.These rocks are enriched in Rb、Th、Sr,and depleted in Nb,Sr,P and Ti,resembling typical characteristics of highly fractionated A type grantites.It shows the characteristics of post-collision granite,which formed in the post-orogenic crustal thinning stage.The rhyolite of Baoligaomiao Formation has SiO2 contents of 70.80%~79.81%,MgO contents of 0.05%~0.50%,TiO2 contents of 0.05%~0.23%,K2O contents of 3.25%~9.33%.Meanwhile,these rocks are enriched in Rb,La,Nd,Zr,Sm,depleted in Sr,Ba,P,Ti,with slightly negative Eu anomaly.and highly zircon saturation temperatures.The geochemical characteristics of rhyolite show A2-type granites,which should be formed under post-collision settings and partly from conglomerate substance of the lower crust.Zircons from Baoligaomiao Formation rhyolite yielded weighted mean ^(206)Pb/^(238)U ages of 299.6±2.5Ma(MSWD=1.5).According to all the above,It is concluded that the Bayanhure area of East Wuzhumuqin County in the central Xing-Meng Orogenic belt was under a post-collision extensional setting in the Late Paleozoic.This provides new evidence for the tectonic-magmatic evolution of the Xing-Meng Orogenic belt.