Studies On Stratum Affiliation And Tectonic Setting Of Volcanic Rocks In The Western Lucao Well Of Beishan Mountain,Inner Mongolia

The Beishan area in the Inner Mongolia,which is located in the southern margin of the Central Asian orogenic belt,is a key junction between the Siberia Plate,the Tarim Plate and the North China Plate.With the special structural location,strong magmatic activities happened in this area.Palaeozoic is an important period for the formation and evolution of different oceanic basins and landmasses in the Beishan area.Oceanic opening and closing,plate collages and collisions lead to widespread distribution of various magmatic rocks.Among them,Variscan magmatic rocks are the most common.The Lucao well area is located in the northern part of the Beishan orogenic belt,with volcanic rocks quite developed.However,the study of this area is relatively low.Rock associations,lithologyand petro-geochemistry of the volcanic rocks are not very clear,and precise isotope dating data are specially lacking.This study focuses on the volcanic rocks in the western Lucao well with a comprehensive study including petrology,petrography,isotopicgeochronology and geochemistry etc.,to provide chronological evidence for the magmatic rocks and strata in the northern Beishan area,and new datum for regional tectonic evolution of this area as well.Based on rock associations,petrography and geochronoglogy of volcanic rocks,combined with comparison to regional strata in the western Lucao Well,their affiliation is re-determined.Meanwhile,according to geochemical characteristics of the volcanic rocks,their magmatic source and tectonic setting are discussed.Combined with regional geological datum,the following conclusions are obtained.(1)The volcanic rocks in the western Lucao well are composed of dacite,andesite,dacite(breccia-bearing)ignimbrite and a small amount of terrigenous clastic rocks,belonging to marine-continental alternating volcanic rocks.In terms of minera composition and petrography,it is closer to the Baishan Formation of the Upper Carboniferous in the region than to the Queershan Group of the Devonian.(2)Two samples of the volcanic rocks in the western Lucao Well yield zircon LA-ICP-MS U-Pb weighted mean ages of 304.0±1.4 Ma and 311.98±0.86 Ma,respectviely,constraining their formation age to be late Carboniferous.The dated ages provide indirect basis for determining the volcanic rocks into the Baishan formation of the Upper Carboniferous.(3)The volcanic rock in the western Lucao well displaycalc-alkaline affinity.They are enriched in strongly incompatible element(e.g.,Th,Hf)and large ion lithophile elements(e.g.,Rb),while strongly deplete high field strength elements(HFSE)(e.g.,Nb?Ta?P and Ti).The LREE and HREE of rocks are obviously differentiated with weak negative Euanomaies.(4)Magma source of the volcanic rocks in the western Lucao well is proposed to be a crust-mantle mixed source,but mainly composed of crust-sourced materials.It is suggested that they formed in a continental margin arc setting.