The Studies Of Geological Characteristics And Igneous Rocks From Northern Abaga Banner Of Inner Mongolia,China

The northern Abaga Banner of Inner Mongolia is located in the northwest of the plate suture zone from Erlian to Hegenshan. In the Late Paleozoic, this region was in an accretion zone, which is a part of the southeast continental margin of the Siberian plate. In the Mesozoic, this region was located at west margin of the magmatic rock band of the Greater Khingan Mountains, which is a part of the Pacific Rim tectonic zone. The strata, intrusive rocks and volcanic rocks in the area are detailedly studied in this paper according to the regionally geological mapping of1:50000and measured geological section of1:5000, as well as the analyses of samples.The stratigraphical units in this region are well emerged based on the regional geological mapping of1:50000. These strata are the Ordovician, the Devonian, the Carboniferous-Permian, the Jurassic, the Paleogene and the Neogene from old to new. The magmatic activities were mainly occurred in three periods, i.e. the Carboniferous-Permian, the Jurassic and the Pleistocene. Volcanic rocks are widely distributed and intrusive rocks are quite developed. The overall direction of structure line in this region is north-east-east. Faults are main geological structures, and folds are next.According to the lithologic features and chemical composition, the intrusive rocks of northern Abaga Banner can be recognized to three types, i.e. syenite granite, alkali feldspar granite and granite porphyry. All of them are separate intrusive bodies. The diagenesis age of syenite granite is the Early Permian, while both of latter are the Late Jurassic. Furthermore, by analyzing contents of major elements and trace elements, syenite granite and alkali feldspar granite are characterized by A-type granite, while the granite porphyry is characterized by both A-and S-type granites. All of them were formed under the post-orogenic tectonic environment. The magmatic origin of these intrusive bodies is considered to be related with the partial melting of the continental crust metagreywacke.In this paper, the volcanic rock types of the second member of the Baoligaomiao Formation (C7P1bl2) and the Baiyingaolao Formation (J3b) were studied, and stratigraphic profiles also were analysed. Moreover, the volcanic eruption cycle features were discovered during the Late Carboniferous-Early Permian and the Late Jurassic in this region. Volcanic lithofacies of C2P\bl2are mainly eruptive facies, secondarily effusive facies, and occasionally with some eruption discontinue. Based on volcanic lithofacies, five eruption rhythms can be recognized. From the lower part to upper part, every eruption rhythm is with a change which is from effusive facies to eruptive facies. These rocks are along with a gradual rising of the burst index, and a transforming of the composition of magma, i.e., basic to acidic. Volcanic lithofacies of J3b also are mainly eruptive facies and secondarily effusive facies, without discontinue. J3b also has five eruption rhythms according to the lithofacies. The composition of magma at the bottom of every eruption rhythm generally is dacitic or andesitic, and rhyolitic upwardly. As a whole, J3b reflects effusive-burst facies and burst facies cycle, and is with the abnormal rhythms, i.e. weak to strong. Besides, analyses of geochemical characteristics suggest that both C2P1bl2and J3b were formed under the orogenic tectonic environment.The magmatic activities in this region were summarized based on the geologic features of intrusive rocks, volcanic rocks and geological tectonics. Furthermore, the tectonic evolution history in this region was explained on the basis of the combination of the geologic history of adjacent areas. In the Paleozoic, there were two ocean trough deposits in this region. One was formed by terrigenous detrital minerals during the Middle-Late Ordovician, and the other was formed by terrigenous material with a little tephra during the Devonian. During the Carboniferous-Permian, the closing of the Paleo-Asian Ocean and the building of tectonic zone of Variscan caused a series of magmatic events, and the Baoligaomiao Formation and the syenogranite intrusive body were come forth. In the Late Jurassic, frequent magmatic activities occurred in the stretch zones due to the subducation of the Pacific Plate, and the formation of J3b volcanic rocks and alkali feldspar granite were formed.