Geochronology And Geochemistry Researches Of Permian Volcanic Rocks In The North Linxi,Inner Mongolia

The study area is located in the northern Linxi,Inner Mongolia,the tectonic location is in the central Xing’an-Mongolia Orogenic Belt and the western Songliao-Hunshandake Block.There are a large volume of Permian intermediate-felsic rocks developed in this area,which is of great significance for revealing the evolution of the Paleo-Asian Oceanic tectonic system.In this paper,the volcanic rocks from the Dashizhai Formation originally scheduled for the Early Permian in the study area were selected as the research objects,and detailed researches on petrography,zircon U-Pb geochronology,whole-rock major and trace elements and zircon Hf isotopes were conducted to determine their petrological characteristics and formation ages,and petrogenesis.The in-situ zircon Hf isotopes,crustal thickness estimated by zircon Eu/Eu*,and zircon Ti temperatures revealed the Early-Middle Permian crustal evolution process in northern Linxi.The main insights gained are as follows:The zircon U-Pb dating results show that the formation ages of andesite-dacite and dacite-rhyolite in the northern Linxi are 275.1 Ma～278.4 Ma and 263.4 Ma～264.7 Ma,respectively,indicating that the formation ages of this Permian volcanic rocks are Early and Middle Permian,showing multiple stages of magmatic activities.Early Permian Andesites have moderate SiO₂（60.44 wt.%～61.09 wt.%）,Al₂O₃（16.76 wt.%～17.13 wt.%）contents,low MgO（1.66 wt.%～2.02 wt.%）and P₂O₅（0.19 wt.%～0.20 wt.%）contents and high ^TFe₂O₃（7.42 wt.%～7.77 wt.%）belongs to the tholeiitic series.These geochemical features,together with the large volumes of coeval felsic volcanic rocks,suggests their mixing origin between mafic and felsic magma.In addition,the andesites have high positiveεHf（t）value with an average value of 11.02,suggesting that its magma source is depleted,which may be the product of the mixing of depleted lithospheric mantle-derived magma and newly formed crust-derived magma.The Early Permian dacites and middle Permian dacite-rhyolite are enriched in silica and Alumina,but depleted in iron oxide and magnesium,indicating the crustal origin.TheεHf（t）values??of magmatic zircon in Permian dacite（19LX4）and dacite-rhyolite（BLS2）are 5.55～12.19 and 8.27～11.27,respectively,and the corresponding Hf isotopic two-stage model ages(T_DM2)are between 636 Ma～947 Ma and 828 Ma～968 Ma,indicating that the magma of the Early and Middle Permian（278 Ma and 263 Ma）felsic rocks came from the partial melting of the Neoproterozoic juvenile lower crust.In addition,they all show stronger negative anomalies of Sr,Eu and Ti,indicating the residual plagioclase in the source area or fractional crystallization of plagioclase,and the fractional crystallization of Fe-Ti oxides.The Early-Middle Permian felsic rocks in the study area all show post-collisional signatures,with extension to the intraplate magmatic rock area,suggesting that they may have formed in the post-orogenic extensional environment,which is consistent with the intraplate origin of the Early Permian basaltic andesite rocks.The crystallization temperatures of the felsic magma in the northern Linxi are relatively high,tend to increase with the change of age,revealing the continuous extension process from Early to the Middle Permian in the study area.The zircon Eu/Eu*in magmatic rocks and the calculated median crustal thickness show that there is a significant crustal thinning process in the study area at 282 Ma～276Ma and 271 Ma～263 Ma,and the zircon Ti temperatures shows an overall increasing trend,which further confirmed that there is a dynamic process of crustal thinning in 290 Ma～260Ma.Based on the above researches in this study and the results of previous studies,we speculate that the lithospheric extension occurred in Permian within the central Xing’an-Mongolia Orogenic Belt.