Characteristics and tectonic setting of volcanic rocks of Baiyingaolao Formation in Hanwula of Xi Ujimqin Banner, Inner Mongolia

There exist different opinions concerning the petrogenesis and tectonic background of Mesozoic volcanic rocks developed in the Da Hinggan Mountains. The Late Mesozoic volcanic rocks in the Xi Ujimqin Banner of Inner Mongoliais a very important part of the huge volcanic rock belt in eastern China.The authors studied the volcanic rocks of Baiyingaolao Formation in Hanwula of Xi Ujimqin Banner in such aspects as field occurrence,petrology,zircon U-Pb isotopic geochronology and geochemisty in order to constrain their petrogenesis and tectonic background. The volcanic rocks of Baiyingaolao Formation are composed of rhyolite and volcanic clastic,which are a set of felsic volcanic rocks. The cathodoluminescence (CL) images of analyzed zircons of the pyromeride and dacite porphyry from Baiyingaolao Formation and their Th/U ratios(0.34-1.25) imply the igneous origin. LA-ICP-MS U-Pb dating shows that their ages are about(140±0.8) Ma and(133±0.7) Ma respectively,suggesting the early period of Early Cretaceous. Petrological and geochemical data reveal that the rocks belong to the high potassium calc-alkaline rock series characterized by rich Si and alkali,poor magnesium and calcium,high FeO^T/MgO ratio and low Mg^#,Nb/Ta ratio. LREE are richer than HREE.The trace element geochemistry is characterized evidently by enrichment of LILE,depletion of Ba,Sr and HFSE. All these geochemical characteristics of rocks show an affinity with the A-type granites,which were most probably formed in an extensional setting and originated from the partial melting of the crust. Combined with spacial distribution of the Mesozoic volcanic rocks,the authors hold that they were probably related to the post-orogenic extension following the closure of the Mongol-Okhotsk orogen,and were also affected by the subduction of the Paleo-Pacific plate.