Study On Magmatic Mixing And Their Dynamic Setting Of The Awengcuo Composite Plutons In The Western Section Of The Bangong Co-nujiang Metallogenic Belt,Tibet

Located at the western section of Bangong Co-Nujiang Metallogenic Belt,the Awengcuo composite plutons are the important part result in the magmatic response during the subduction and closing orogeny of Shiquanhe-Namiucou Tethys Ocean basin.It is featured by widely developed mafic microgranular enclaves and various discordant phenomena.They are ideal objects for the study of magma mixing,and their research study are of great significance to the tectonic evolution and the regional metallogenic dynamic mechanism of Bangong Co-Nujiang metallogenic zone.In order to explore the magmatic mixing and dynamic background of the Awengcuo composite plutons,based on the field geological survey,petrography,mineral chemistry,zircon U-Pb geochronology,zircon Lu-Hf isotope and whole-rock geochemistry of the host rock and dark microgranular enclave in the composite plutons were studied.This paper systematically summarizes various evidences of magmatic mixing in the composite plutons,and accurately determines the diagenetic age of the composite plutons.The magmatic mixing,tectonic setting and dynamic mechanism of the composite plutons are discussed.Through the research,the main achievements of this paper are as follows.(1)The Awengcuo composite plutons are the product of multiple magmatic intrusions.The intrusive sequence from early to late is quartz diorite(120 Ma)?granodiorite(115?114 Ma)? syenogranite(113?109 Ma)? monzogranite(104?103Ma).The rock are mainly calc alkaline high-k calc alkaline series quasi aluminous weakly peraluminous I-type granites,and a small amount of S-type granite exists in the late stage.(2)The mafic microgranular enclaves in the Awengcuo composite plutons have obvious plastic deformation characteristics,which are in direct or gradual contact with the host rock.The enclaves are interpenetrated and wrapped with the host rock.Occasionally,the veinlets are developed,and the enclaves have fine to medium coarse grain structure.A large number of plagioclase,orthoclase,quartz phenocrysts are captured from the host rock,and occasionally amphibole phenocrysts are found across the enclaves and the host rock,Special structures such as long columnar plagioclase,amphibole and acicular apatite are found in the enclaves and the host rock around the enclaves,indicating that the dark microgranular enclaves are the products of magma mixing.(3)A large number of dark microgranular enclaves are developed in the composite plutons.The An values of host rock plagioclase and enclave plagioclase show an oscillatory trend,which indicates that the magma mixing is mainly mixing with weak mingling,and the magma mixing is multi-stage.(4)The crystallization temperatures of host rock and enclaves are 632 ? 718 ?and 722 ? 768 ? respectively,and the formation depths are 8.15 ? 11.48 km and 10.55? 11.46 km,respectively.(5)Based on the study of the macroscopic geological characteristics,petrography,mineralogy,mineral chemistry,zircon U-Pb geochronology,zircon Lu-Hf isotopic composition and geochemical characteristics of the dark microgranular enclaves,it is shown that the Awengcuo composite plutons are the product of mixing of mantle derived mafic magma injected into felsic magma chamber for many times.(6)The results show that the Bangong-Nujiang Tethys ocean basin began to subduct southward in the early Middle Triassic.Affected by the southward subduction of the ocean basin,the central and Northern Lhasa block was torn along the line of Shiquanhe-Laguocuo-Asuo-Yongzhu-Namucuo-Jiali in the Late Triassic,forming the Shiquanhe-Namucuo back arc ocean basin,which began to subduct southward in the early Jurassic North subduction.The Awengcuo composite plutons is an important part of magmatic response during the northward subduction and closing orogeny of Shiquanhe-Namucuo Tethys ocean basin.