Discussion On The Formation Age And Tectonic Environment Of The Baoerhantu Group In The North Of Bayan Obo

The northern part of Bayan Obo lies across the accretion zone on the northern margin of the North China Plate and the North China Plate.It is an ideal area for studying the accretion of the Paleo-Asian Ocean and the North China Plate.There are a large number of early Paleozoic volcanic rocks and intrusive rocks in the area.It is of great significance to study the tectonic evolution of the Xingmeng orogenic belt in the early Paleozoic.The volcanic rocks and intrusive rocks of the Baoerhantu Group in northern Bayan Obo are studied in detail by lithofacies,geochemistry and zircon U-Pb dating in order to discuss the early Paleozoic magmatic activity and tectonic evolution in northern Bayan Obo.Zircon U-Pb dating results show that the magmatic activity of the volcanic rocks and the corresponding intrusive rocks of the Baoerhantu Group was mainly in the Late Ordovician.The LA-ICP-MS zircon U-Pb isotope analysis of rhyolite(BLS-7),dacite tuff(BLS-8)and Ordovician granite(BH-9)near Bulongshan in the study area shows that there are obvious oscillating growth zones in zircon,and the Th/U ratio is high(0.27-0.70,0.33-0.71,0.22-1.01).It shows that zircon is a typical magmatic zircon,which can represent the formation age of the rocks.The zircon U-Pb ages of rhyolite(BLS-7),dacite tuff(BLS-8)and Ordovician granite(BH-9)samples are 483.0±2.9 Ma,483.4 ±2.4 Ma and 481.9 ±2.3 Ma,respectively,indicating that the volcanic rocks and corresponding intrusive rocks of the Baoerhantu Group were formed in the Late Ordovician.The results of petrogeochemical analysis show that the volcanic rocks of the Baoerhantu Group are mainly divided into two types.One type of volcanic rocks is a set of quasi-aluminous alkaline medium-basic rocks with the characteristics of rich aluminium,rich iron,rich sodium and poor potassium.Compared with the first type of volcanic rocks,the second type of volcanic rocks are more silicon-rich and potassium-rich,and the content of other major elements has been reduced,which is a set of quasi-aluminous rocks Aluminous acid rocks.The trace elements of these two types of volcanic rocks are characterized by enrichment of Th,U and La,depletion of Rb,Nb,Ta,P and Ti,and enrichment of light rare earth elements and depletion of heavy rare earth elements.The magma may originate from the mantle source area affected by subduction and be affected by terrigenous material and crystallization differentiation.Ordovician granites in intrusive rocks have similar geochemical characteristics with typical adakites.They are rich in aluminium,alkali,poor in potassium,magnesium and calcium.Trace elements are characterized by enrichment of Th,U,La,depletion of Rb,Nb,Ta,P and Ti.Rare earth elements are characterized by enrichment of light rare earths and depletion of heavy rare earths.The magma of the Ordovician granite originates from the melting of the thickened lower crustal basement under the mechanism of subduction and subsidence.The geochemical characteristics of these rocks indicate that the volcanic rocks and related intrusive rocks of the Baoerhantu Group were formed in the environment of plate subduction.Combined with previous studies,it can be further explained that the collision between the southern subduction of the Paleo-Asian Ocean and the North China Plate in the Early Paleozoic formed the accretion zone on the northern margin of the North China Plate.