Geochronology And Geochemistry Researches Of Early Carboniferous Volcanic Rocks In Sonid Zuoqi,Inner Mongolia

The study area is located in the Shaertala area,northern Sonid Zuoqi,Inner Mongolia.The tectonic location is in thesouthwestern Xing'an-Airgin Sum Block,the western of the Xing-Meng Orogenic Belt(XMOB).Previous research mainly focused on the southern part of Sonid Zuoqi.There is a lack of systematic research on the temporal and spatial distribution,geochemical features and tectonic settingsof the early Carboniferous magmatismin northern Sonid Zuoqi.Therefore,We conducted systematic researches on petrography,zircon U-Pb dating and Hf isotopes,whole-rock geochemistry works for the volcanic rocks from previously assigned Early Permian Dashizhai Formation in the Shaertala area,northern Sonid Zuoqi.This work ascertains the temporal and spatial,magmatc sources and tectonic settings of the early Carboniferous volcanic rocks in different stages of the Sonid Zuoqi area.On the basis of zircon Hf isotopes of felsic igneous rocks,we reveal the crustal nature and evolution of the Xing'an-Airgin Sum Block in Carboniferous.The main conclusionsare as follows:Firstly,zircon U-Pb dating results suggest that the formation ages of these volcanic rocks can be divided into two stages:?346.5Ma and 335.1Ma.Their rock associations are basaltic andesite,andesite,dacite and rhyolite,and dacite,rhyolite,respectively.The ages of basaltic andesite and rhyolite are 349.0±5.2Ma and 335.1±2.0Ma respectively,it indicates that their formation ages should belong to early Carboniferous rather than Early Permian.Secondly,the early early Carboniferousbasaltic andesites have relatively high Al₂O₃(>17%),low Mg O(2.43%?2.70%)and Cr,Co,Ni,suggesting that the primary magma could undergo fractional crystallization of olivine and pyroxene(early stage)and plagioclase(late stage),at the hydrous condition with high temperature and low pressure.They are relatively enriched in light rare earth elements(LREEs),large ion lithophilic elements(LILEs,such as Rb,Ba,Sr),Pb,Zr and Hf,depleted in heavy rare earth elements(HREEs)and Nb,Ta and Ti,have high positive zircon?_Hf(t)values(8.92?13.79),and a large number of early Pelaozoic captured zircons.These features imply that the primary magma was mostlikelygenerated by the partial melting of the depleted lithospheric mantle which was previously modified by subduction-related fluids under the intracontinental extension mechanism,and experienced a certain degree of crustal contamination.The rhyolites and dacites are characterized by high Si O₂ and Al₂O₃,low Fe₂O₃^T and Mg O,enrichment in LREEs and LILEs,depeletion in HREEs and HSFEs.Moreover,the 335Ma dacites and rhyolites show high temperature(average value of 876°C)and A-type granite signatures,suggesting an extensional environment.The depleted zircon Hf isotopic compositions of these felsic rocks(?_Hf(t)values of 7.97?12.14 and 8.35?14.71,t _DM2 of 838Ma?577Ma and 736Ma?407Ma),suggest that their primary magma was formed by the partial melting of the late Neoproterozoic and Early Paleozoic juvenile crust.The geochemical features of the andesite and less occurence ofbasaltic rocks imply that the early early Carboniferous andesite magma is probably the product of a mixture of the above intermediate to basic and felsic magma.Additionally,the Carboniferous crustal evolution was dominated by partial melting of juvenile crust,and the zircon Hf t _DM2 for the felsic volcanic rocks gradually decrease with their formation ages.The temporal and spatial variation of Zircon Hf isotopic composition of the Carboniferous felsicigneous rocks reveals the lateral heterogeneity of the deep crust beneath the Xing'an-Airgin Sum Block.In summary,considering the magma mixing origin of the andesites,the basaltic andesites with within-plate origin and the coeval dacites and rhyolites constitute a bimodal volcanic rock association,indicating that the early early Carboniferous was most likely under an extensional setting.The formation of middle early Carboniferous A-type felsic volcanic rocks further implied that the regionalextension continuously occurred.Together with the previous researches on sedimentology and metamorphism,we therefore propose that the Carboniferous extension could be associated with the post-collisional regime after the closure of Paleo-Asian Ocean.