The Petrological Characteristics Of The Lower Segment Of Baoligaomiao Volcanic Rocks In Sonidzuoqi, Inner Mongolia

This paper based on "Inner Mongolia1:50000Zhunheremuyin Sumu six pieces ofregional geological survey" project, carry out research on Baoligaomiao Formationinvolcano rock of Sonidzuoqi Area Inner Mongolia. The study area is located in the InnerMongolia based north western border areas, tectonic location belonging to the CentralAsian orogenic belt Xinmeng orogenic belt. It belonged to the paleo Asian tectonic domainof the orogenic belt when in Paleozoic. while in Mesozoic, it belonged to West Pacifictectonic domain. Study division of tectonic units belongs to the northern orogenic belt ofXingmeng orogenic belt. Its south boundary is Erlianhot-Hegenshan fault zone and theNorth belongs to the Siberia active continental margin. The Paleozoic tectonic actsfrequently, magmatic acts strongly.In Xingmeng orogenic belt, Baoligaomiao formation volcano rocks widely exposed, isone of the most widely distributed, exposed largest Paleozoic volcano rock area of northernorogenic belt. The volcano rock strata are mainly exposed on Chaganaobao area,where isthe north of Erlianhot-Hegenshan fault zone,and in zhe north east it exposed on to theDongwuqi-Gala city etc. The lower segment of Baoligaomiao volcanic rocks mainlycombined volcano rock, it contains the section of andesite, dacite, andesitic tuff, coarserhyolitic tuff, rhyolite breccia tuff. And there are thin rhyolite and shallow metamorphicclastic rock.This paper use LA-ICP-MS to analyze zircon U-Pb year of the lower segment ofBaoligaomiao volcanic rocks. Isotopic analysis of andesite samples of16zircon U-Pbshows that the weighted average age of zircon206Pb/238U is310.9±8.1Ma (MSWD=6.76). The age of310.9Ma represents the magma crystallization age, belonging to the latecarboniferous. With Xin Houtian in Dongwuqi of Carboniferous Baoligaomiao Formationvolcano rock age (303.4±6.7Ma~320±7.2Ma) and James et al. in the southeast ofMongolia in the Carboniferous arc volcano rock age (323±0.7Ma), the author thinks thethree things belonging to the same magmatic events in the same tectonic domain during thesame period.Geochemical data shows the content of SiO2of volcano rock changes between65.94~80.31wt%, and that the Litman index δ is1.38~3.13, belonging to the calcalkalineseries, ratio range between0.92~1.07A/CNK, A/NK0.99~1.53, belonging to themetaluminous to weakly peraluminous rocks. REE diagram shows the volcano rock LREEenrichment, HREE loss, presenting rightist trend. The trace element spider diagrams show that the volcano rocks are enriched in large ion dear stone elements (K, Rb, U) and Th, lossof high field strength elements (Nb, Ta, Ti). In the Zr/Yb-Nb/Yb diagram, volcano rockmostly distributes in enriched mantle range, showing the magma source region is enrichedmantle wedge possibly fluid transformation by subduction zones. The petrologicalcharacteristics of the lower segment of Baoligaomiao volcanic rocks are similar withAndean type continental margin volcano rock of the arc tectonic environment. It hasevolution characteristics of continental marginal arc volcanic rocks. And in theTa/Yb-Th/Yb figure, the whole rocks fall into the active continental margin regionalenvironment, and this shows that the lower segment of Baoligaomiao volcanic rocks inSonidzuoqi formed in continental arc environment.Combined with the age and tectonic environment of the lower segment of Baoligaomiaovolcanic rocks, it shows that Late Carboniferous in Inner Mongolia eastern area is still inthe stage of subduction, and then we can the conclusion that the closing of the paleo AsianOcean should occur after the late carboniferous.