Geochronological and geochemical features of volcanic rocks of Dashizhai Formation in Ural Sutai of Xilin Hot, Inner Mongolia, and their geological significance

Based on 1:50000 regional geological survey, the authors studied the volcanic rocks of Dashizhai Formation developed in southern Central Asia Orogenic Belt (CAOB) within Ural Sutai of Xilin Hot, Inner Mongolia in such aspects as filed occurrence, petrology, zircon U-Pb isotopic geochronology and geochemistry. LA-ICP-MS zircon U-Pb dating results show that the rocks were formed at about 287.5±1.4Ma (MSWD=3.1) in the early period of Early Permian. Petrological and geochemical data reveal that the rocks are a suite of mid-acid volcanic rocks characterized by rich Si, alkali and Al, poor Ti, Mg, Fe and Ca, and abundant trace elements and REEs with coincident distribution pattern exhibiting a right-inclined seagull-type distribution pattern. The LILE, such as Rb, Ba, Th, U, K, and LREE are richer than HFSE like Nb, Ta and HREE, suggesting that the volcanic rocks were formed in an island-arc setting geochemically. Together with their petrological and geological features, the rocks were most probably formed in a back-arc or intra-arc spreading basin triggered by oceanic crust subduction before closure of the Paleoasian Ocean in the early period of Early Permian.