内蒙古巴彦敖包地区宝力高庙组火山岩地球化学特征、锆石U-Pb年龄及地质意义

Ages, origin and geological implications of the volcanic rocks in the Baoligaomiao Formation of East Ujimqin Banner,Inner Mongolia

内蒙古东乌旗巴彦敖包一带出露的宝力高庙组地层可划分为三段,可对应于查干敖包剖面二、三、四段。二段为一套杂色中酸性火山熔岩和火山碎屑岩类,火山喷发以喷溢相为主;三段为一套灰黄、灰白色火山-沉积碎屑岩类和陆源沉积岩,含Annularia植物化石,为火山间歇期沉积;四段为一套灰色、浅灰色酸性熔岩夹少量火山-沉积碎屑岩,火山喷发以溢流相为主。四段紫红色含晶屑流纹岩和二段灰黑色流纹岩锆石LA-ICP-MS U-Pb年龄分别为(305±4.1)Ma和(315.2±4.6)Ma,结合所含化石确定地层时代为晚石炭晚期—早二叠世早期。宝力高庙组中酸性火山岩为安山质、英安质、流纹质高钾钙碱性-钾玄岩系列火山岩组合,岩石呈现富硅、碱,贫钙、镁,A/NK值和Fe Ot/Mg O值高,富Rb、Nb,贫过渡族元素,并具有显著的铕负异常,为A2亚类花岗岩类,其可能是幔源岩浆底侵形成的晚古生代新生下地壳物质部分熔融而成,岩浆演化经历了钛铁矿物、铁镁矿物及斜长石分离作用。宝力高庙组中酸性火山岩形成于后碰撞构造背景环境,其特征揭示兴蒙造山带内蒙古东乌旗地区晚石炭世—早二叠世已处于同碰撞-碰撞后阶段,早二叠世已经进入碰撞后伸展阶段。

Baoligaomiao Formation is outcropped in Bayan Aobao Town of East Ujimqin Banner,Inner Mongolia.It can be divided into three segments corresponding to the second,the third and the fourth section of Chaganaobao.The second segment comprises a set of variegated intermediate-acidic volcanic lava and volcanic clasts.Volcanic eruption is mainly as extrusive facies.The third segment includes a set of Annularia fossil-bearing grayish yellow,grayish white volcanic sedimentary clasts and terrigenous sediments,being an intermittent volcanic deposit;the fourth segment has a set of gray,light-gray acidic lavas containing rare volcanic-clastic sedimentary rock.Volcanic eruption is mainly as extrusive facies.The fourth segment includes purplish red crystal rhyolite,and the second segment is grayish black rhyolite with granite age being(305±4.1)Ma and(315.2±4.6)Ma.Isotope age and the fossils suggest that the stratigraphic age is late Carboniferous to early Early Permian.Baoligaomiao Formation has intermediateacidic volcanic rock,with separate andesitic,dacite,high-K calc-alkaline to Shoshonis series volcanic association.Major elements are characterized by enrichment of SiO2and alkali and depletion of calcium and magnesium.Indexes of A/NK and FeOt/MgO are high,characterized by enrichment of Rb,Nb and depletion of transition group elements,with dramatic Eu negative anomaly,suggesting A2type sub-granite.It was probably the magma from the mantle and partly from conglomerate substance of the lower crust in late Paleozoic.Magmatic evolution experienced dissociation to produce titanium iron minerals,ferromagnesian minerals,and plagioclase.The intermediate-acidic volcanic rock in Baoligaomiao Formation was formed in a post-collisional setting.Its characteristics reveal that Xing'an-Mongolian orogenic belt of East Ujimqin Banner was in the syn-collision to post-collision stage during late Carboniferous to early Permian period.In Early Permian,the evolution entered a stage of stretching after collision.