Geochronology and geochemistry of the volcanics in the Zamuting, Inner Mongolia and its transfromation of Mesozoic tectonic system

As the origin, evolution and tectonic setting of late Mesozoic volcanic rocks in the Great Hinggan Range volcanic belt have always been controversial, our project carried out geochronology and geochemistry studies of the volcanics in the Zamuting area. LA-ICP-MS zircon U-Pb dating of the he Baiyingaolao Formation volcanic rocks in the Zhamuting area reveals that rhyolite and trachoma were formed at 131.6±1.2 Ma and 126.9±1.5 Ma respectively, which are consistent with the identification results of the animal and plant fossil in the Second Member volcanics of the Baiyingaolao Formation, indicating that the volcanic rocks were formed in the early Cretaceous. The rhyolites and trachyandesites of this region belong to alkali series volcanic rocks, with characteristics of high silicon, rich in alkali, low aluminum, low magnesium and calcium, as well as high FeO^T/(FeO^T+MgO) ratio; the rocks are relatively enriched in the large ion lithophile elements Rb, Th, U and Pb, and have a strong deficit in the high-field strong elements such as Nb, Ta, Ti, P, Ba and Sr; the rhyolitic rocks have the characteristics of A-type granite. The studies on trace elements and rare earth elements indicate that both rhyolite and trachyandesites are derived from the melting of the crust and have the same magmatic source. The Baiyingaolao volcanic rocks were formed in the post-orogenic extensional tectonic environment, which reflects that after the southward subduction of the Mongolia Okhotsk Ocean and the collision and integration of the Eerguna-Xingan landmass, the Da Hinggan Mountains volcanic region began to transform from compression to extensional tectonic system. The upwelling and underplating of asthenosphere mantle caused the partial melting of the lower crust, which led to the eruption of rhyolite. The further melting of residual phases led to large-scale eruption of trachyte.