Ages, origin and geological implications of the volcanic rocks in the Baoligaomiao Formation of East Ujimqin Banner, Inner Mongolia

Baoligaomiao Formation is outcropped in Bayan Aobao Town of East Ujimqin Banner, Inner Mongolia. It can be divided into three segments corresponding to the second, the third and the fourth section of Chaganaobao. The second segment comprises a set of variegated intermediate-acidic volcanic lava and volcanic clasts. Volcanic eruption is mainly as extrusive facies. The third segment includes a set of Annularia fossil-bearing grayish yellow, grayish white volcanic sedimentary clasts and terrigenous sediments, being an intermittent volcanic deposit; the fourth segment has a set of gray, light-gray acidic lavas containing rare volcanic-clastic sedimentary rock. Volcanic eruption is mainly as extrusive facies. The fourth segment includes purplish red crystal rhyolite, and the second segment is grayish black rhyolite with granite age being (305±4.1) Ma and (315.2±4.6) Ma. Isotope age and the fossils suggest that the stratigraphic age is late Carboniferous to early Early Permian. Baoligaomiao Formation has intermediateacidic volcanic rock, with separate andesitic, dacite, high-K calc-alkaline to Shoshonis series volcanic association. Major elements are characterized by enrichment of SiO₂ and alkali and depletion of calcium and magnesium. Indexes of A/NK and FeO^t/MgO are high, characterized by enrichment of Rb, Nb and depletion of transition group elements, with dramatic Eu negative anomaly, suggesting A2 type sub-granite. It was probably the magma from the mantle and partly from conglomerate substance of the lower crust in late Paleozoic. Magmatic evolution experienced dissociation to produce titanium iron minerals, ferromagnesian minerals, and plagioclase. The intermediate-acidic volcanic rock in Baoligaomiao Formation was formed in a post-collisional setting. Its characteristics reveal that Xing'an-Mongolian orogenic belt of East Ujimqin Banner was in the syn-collision to post-collision stage during late Carboniferous to early Permian period. In Early Permian, the evolution entered a stage of stretching after collision.