Genesis Of The Late Ordovician Granodiorite In Zhiyu Area,The Eastern Segment Of East Kunlun Orogen

The origin of early Paleozoic adakitic granitoids in the eastern part of the East Kunlun Mountains has two main viewpoints:the melting origin of the subduction oceanic crust and the partial melting origin of the thickened lower crust.Through detailed field geological observation of the granite batholith in Zhiyu area,it is found that the partially exposed diorite with mafic microgranular enclave?MME?intrudes into the granodiorite in reverse vein.EMPA data show that amphibole in granodiorite,diorite and MME is mainly magnesio-hornblende,showing mixed source characteristics of crust and mantle.The biotite in granodiorite is ferrous-magnesian biotite,which shows the characteristics of crust-mantle mixed source-crust source.Plagioclase in granodiorite is Ordovician-Mesofeldspar with the Oscillatory zones.The An number is 21-33,indicating a turbulent crystalline environment.Zircon U-Pb dating by LA-ICP-MS results show that the granodiorite basement in Zhiyu area was formed at 445-440 Ma,belonging to Late Ordovician.The main and trace data of the whole rock show that the granodiorite and diorite are rich in Na₂O?3.0-4.8wt.%?,low K₂O?1.4-3.4 wt.%? and MgO?0.9-1.5 wt.%and 4.3-4.8 wt.%?,and A/CNK is about0.87-1.05.They are quasi-aluminous and calc-alkaline to high potassium calc-alkaline rocks.Strong enrichment of light rare earth elements?LREE? and light rare earth elements?LILE:eg.Rb,Ba,K,Sr?,loss of high field strength elements HFSEs?eg.Nb,Ta,Ti?,and heavy rare earth elements?HREE?,?Eu=0.51-1.03.Granodiorites are adakitic rocks with high Sr and low Y and Yb,and diorites are normal arc magmatic rocks.The granodiorite and diorite have similar REE and trace element characteristics,showing that they are homologous magma.The MME are hornblend-gabbro,with the shape of lenticular and elliptical,.It's rich in MgO?11.33 wt.%??Cr?553?g/g?and Ni?176?g/g?,rich in light rare earth elements,and flat in heavy rare earth elements,?Eu=1.1,rich in Rb,Ba,K,Sr,and depleted HREE?Nb,Ta,Ti,Zr,Hf?,named calc-alkaline basalt.Sm-Nd isotope results show that the granodiorite?_Nd?t?is between-4.3-1.0,corresponding to the value of T_DM of 1120-1358 Ma.The?_Nd?t?of diorite and amphibole gabbro xenoliths are-3.7-3.5 and 2.4,respectively.The corresponding T_DM values of diorite are 1490-1519 Ma.In summary,it is considered that the Late Ordovician granodiorites and diorites in Zhiyu area were formed by partial melting of the thickened lower crust,and the MME in the diorites were formed by melting of metasomatic mantle wedge by subduction fluid,which was an active continental margin environment.During the Late Ordovician?445-440 Ma?,the south of East Kunnan terrane was still in the stage of ocean-continent subduction,and collision orogeny began in the early Silurian when the oceanic closed.