Study On The Petrogenesis And Tectonic Geological Significance Of The Yusupuleke Tagh Granitic Composite Intrusion,Southern Altun

The Yusupuleke Tagh granitic composite intrusion is located in the southern margin of the Altun orogenic belt,and is one of the largest granite bodies in the area of south Aultun.According to the field geological investigation,the granitic composite intrusion is formed by multi-period magmatic staging.The texture,structure and the material compositions of the intrusion were obviously different and contained a large number of enclaves.This provide us a good condition to explore the relationship between magma evolution and crust-mantle interaction,it also provide us a chance to discuss the relationship of Paleozoic magmatism and tectonic evolution of the South Altyn Tagh.However the detailed results of the study on Yusupuleke Tagh granitic composite intrusion have not been published so far.So the granitic composite intrusion is selected as the research object.Through the comparative study of the field geology,mineralogy,petrology,geochemistry and other characteristics to explore the causes of the composite intrusion and the formation of the kinetic process,and combined with zircon U-Pb chronology and Lu-Hf isotopic composition to determine the age of the complex rock formation,material sources and genetic types.Finally,based on the previous studies,the classification of the magmatic activity period and the tectonic significance of the rock formation are discussed.The main part of the Yusupuleke tagh granitic composite intrusion is composed with medium-coarse grained porphyroid biotite monzonitic granite and is the result of the first magmatic emplacement.The edge is mainly composed with medium-fine grained biotite monzonitic granite with porphyritic,and potash feldspar porphyroblasts and dark enclaves also occurred in the edge.It is the result of the second magmatic emplacement.Major element characteristics showing the composite intrusion belongs to high-K calc alkaline I-type granites and the first stage granite has a higher Iron,Magnesium content and lower silicon content.Trace elements showing the first stage granite has a relative enrichment of Large iron lithophile and depletement of high field strength elements.The rare earth element distribution curve shows the right-inclined type,and has characteristics of obvious enrichment of light rare earth and depletion heavy rare earth elements and moderate negative Eu anomalities.Through zircon LA-MC-ICP-MS U-Pb isotopes dating,obtained the age of the first stage granite is?458.4±4?Ma and the?418.2±3.6?Ma of the second stage.The results show that the formation age of the composite intrusion is Early Paleozoic and the formation of the first stage granite is relatively early,which is the result of the two phases of magmatic staging.Most of the ?_Hf?t?values of the composite intrusion is positive but very few are negative,which indicates that the composite intrusion is partially melted from the new crust.The ?_Hf?t?value of the first stage granite is relatively higher,indicating that the magmatic mixture of the composite intrusion in its middle part is more thorough.Based on the above findings,we believe that the southern Altyn Tagh region began to enter the late orogenic stage from 460 Ma,and the the collision of the southern Altun ocean began to weaken until the 420 Ma entered the equilibrium adjustment and extension phase after plate collision.In this process,the subducted slabs of the southern Algin oceanic crust is dehydrated,the mantle wedge were substituted by the fluid,melted and finally formed basaltic magma.The fluid and heat brought by the basaltic magma made new crust partially melted and formed a granitic magma chamber in the deep part of the crust.The magma chamber undergoes magma mixing and crystal fractionation and then formed Yusupuleke tagh granite complex.