Geochemical Characteristics of Carboniferous Bimodal Volcanic Rocks in Shibanquan, Beishan and Its Tectonic Significance

Located in the southern margin of Beishan orogenic belt, the carboniferous volcanic rocks(Hongliuyuan Group) have constituted a unique bimodal volcanic rock combination in time and space, which are mainly characterized byshorter Daly discontinuous and a large number of acidic volcanic rock, with a small amount of intermediate and basic volcanic rocks. In which, the andesites are rich in sodium and poor in potassium, with high Al values and low Mg contents, belonging to the peraluminous high potassium calc-alkaline series, containing substantial amount of depleted mantle materials, and indicating that these andesites may be derived from depleted mantle, with geochemical characteristics of intraplate andesite. The rhyolites belong to the low Ti type, indicating that they may be formed by partial melting of lower crust materials under theinfluence from the underplating of basic magma, with some plagioclaseresidued inmagmatic source.And, these rhyolites were formed in a continental rift environment, its dynamics system may be associated with intracontinental stretching tension effect, suggesting that the late Paleozoic rift were developed to the peak stage in the late period of Early Carboniferous and/or the early period of Late Carboniferous. Thus, the Late Carboniferous serves as the major turning point for the the geodynamic environment gradually changing from the extension and tension to the collision and compression, and this studying area was steped into the ollision orogenic evolution stage at the end of Permian.