Geochemistry of Nanba complex rocks in Yunchang, Gansu Province, and its tectonic significance

Abstract：There are a lot of microgranular enclaves hosted in the Naba complex, a part of the Shiyangdong batholith. Based on field geological, petrographic and geochemical characteristics, the authors reveal that the microgranular enclaves have igneous texture. In spite of the geochemical difference between the microgranular enclaves and the host granite, their SiO2 oxides (such as MgO, CaO) and bi-element scatter diagrams assume hyperbolic or linear relationship, whereas their K2O/CaO-A2O3/CaO，Na2O/CaO-A2O3/CaO，Zr/Sr-La/Sr，Y/Sr-Zr/Sr，FeOT/TiO2-K2O/CaO，Rb/Sr-Al2O3/SiO2 covariant diagrams show linear variation, suggesting a crust-mantle mingling granite. Tectonically, geochmical features of granites of Ayila complex in R1-R2 diagram and Nb－Y and Rb－Y+Nb diagram all indicate they are syn-collision granites. It is thus held that the Ayila complex and the mingling granites are products of the collision between the Alxa terrane and the Qilian terrane, and the related underplating is an important factor responsible for the growth of the transitional zone crust.