Zircon SHRIMP U-Pb age and geochemical characteristics of the Mesozoic volcanic rocks in Xi Ujimqin Banner, Inner Mongolia

The Xi Ujimqin Banner area in southern Da Hinggan Mountains is characterized by extensively-distributed Mesozoic volcanic rocks. Two groups of different age volcanic rocks in the Late Mesozoic were chosen for SHRIMP zircon U-Pb dating. The SHRIMP zircon U-Pb age of the intermediate rock is about (163±2)Ma, and the dating of the Chagannuoer Formation rhyolite yields 144.2Ma±1.4Ma, indicating that these volcanic rocks were formed from Late Jurassic to Early Cretaceous, which is incompatible with the stratigraphic column. The intermediate rocks fall into trachytes belonging to the alkaline series in the TAS diagram, with enrichment of LILE and LREE and depletion of HREE and some HFSE. The rhyolites with characteristics of A2-type granite are of the high-K calc-alkaline series, with enrichment of LILE and LREE and depletion of HREE and, especially, Ba, Sr, HFSE. The trachytes originated from the fractional crystallization of the alkaline or peralkalic depleted mantle source and the rhyolites came from the middle-upper crustal magma that had suffered partial melting. Combined with the distinct geochemical features of Xi Ujimqin Banner volcanic samples and many previous researches in the study area, it is held that the Late Jurassic (163Ma) volcanism occurred in an intense lithopheric extension tectonic setting which resulted from closure of the Mongol-Okhotsk Ocean and subsequent orogenic collapse.