Formation Age and Geochemical Characteristics of Dongcaogou Pluton in the Western Central Qilian Belt and Its Implications for Arc-Granitic Magmatism

Active continental margin is the main place for the formation of granitic magma, which is of great significance for our understanding of crust-mantle interaction of subduction zone. In this study, detailed petrological, geochemical and zircon U-Pb geochronological analyses have been carried out on diorite and biotite monzogranite in the Dongcaogou granitic pluton in the western Central Qilian Belt (CQB). LA-ICP-MS zircon dating shows that fine-grained diorite and biotite monzogranite of the Dongcaogou pluton formed about 461±1 Ma and 445±1 Ma, respectively, representing two stages of magmatic activity in the Early Paleozoic of the CQB. Geochemical characteristics show that the Dongcaogou pluton is a series of shoshonite-calc-alkaline, metaluminous-weakly peraluminous rocks. Fine-grained diorites are characterized by low silicon, alkali-rich and rare earth element-rich, which may be the products of physical mixing of dehydration melting of subducted sediments and mantle wedge peridotite. Biotite monzogranites has the charaterisits of high Si and Sr, accompanied by high Sr/Y and La/Yb values, which may be the production formed by partial melting of subducted oceanic crust under eclogite facies. Combined with regional magmatism studies indicate there are two magmatic events in the Dongcaogou pluton, which were formed in a continental margin island arc environment with significance crust-mantle interaction.