Geochronology and geochemistry for volcanic rocks of Dashizhai Formation and its geological significance in Xi U jimqin Banner, Inner Mongolia

The petrogenesis and tectonic environments of the Permian volcanic rocks in southern the Central Asia Orogenic Belt (CAOB) are controversial for a long time. The authors choose the volcanic rocks of Dashizhai Formation, which located in Hanwula of Xi U jimqin Banner, Inner Mongolia, as the studied object, and study detailed on filed geology, petrology, zircon U-Pb isotopic geochronology and geochemistry. The volcanic rocks occur petrologically as a set of intermediate-acid volcanic lava-clastic rocks, and are mainly composed of rhyolite. LA-ICP-MS zircon U-Pb ages of 276 +/- 0. 81Ma (MSWD = 1. 3) and 280 +/- 0. 76Ma (MSWD = 0.69) from two rhyolite samples uniformly indicate that these rhyolites of the Dashizhai Formation erupted in the Early Permian, representing a violent tectono-magmatism event. The petrological and geochemical analysis reveal that the intermediate-acid rocks are characterized by enrichment of Si and alkaline, depletion of Ti, Mg, Fe and Ca, and abundance in trace elements and REEs, exhibiting a right-inclined seagull-type distribution pattern. The LILE, e. g. Rb, Th, U, K and LREE are richer than HFSE like Nb, Ta and HREE. The rocks are also featured remarkably by negative anomalies of Ba, Sr, P, Eu and Ti on the primitive mantle-normalized trace element diagram. The average value of 10000Ga/Al is 3.6. All these characteristics suggest that the volcanic rocks were geochemically formed in an island-arc setting, and are similar to geological features of A-type granites originated from the partial melting of felsic crust under high temperature and low pressure conditions. Combined with the geochemistry characteristics and the regional geology data, the rocks were most probably formed in a back-arc spreading environment, and are products of subduction of Paleoasian Oceanic crust before its closure in Early Permian.