Geological Characteristics, Genesis And Geological Significance Of Indosinian Granitic Pluton In Polostai Region,East Section Of East Kunlun Orogenic Belt

East Kunlun Orogenic Belt(EKOB)is a composite continental orogenic belt in the northern part of the Qinghai-Tibet Plateau.During its long-term tectonic evolution,due to complex and diverse tectonic magmatism,magmatic activity is intense and magmatic rocks are widely developed.The exposed area of granitic rocks occupies the dominant position in orogenic belts.The study of granitic rocks is an effective tool for further exploring the formation mechanism of orogenic belts.It is of great significance for inversing the deep dynamic process of orogeny and the mechanism of crust-mantle mixing.Previous studies have shown that the EKOB underwent two major evolutionary stages:the Original Tethyan Ocean and the Paleo-Tethyan Ocean,especially in the Late Paleozoic-Early Mesozoic.Therefore,this paper takes the Borostai granitic pluton in the eastern part of East Kunlun Mountains as the research object.By using the methods of field geology research and laboratory rock sample testing and analysis,the systematic petrology,isotope chronology,petrogeochemistry and mineralogy of the granitic pluton are studied.The characteristics,genetic relationship,crust-mantle mixing,geotectonic environment and geological significance of different intrusions in the rock mass are discussed in detail.Field geological characteristics show that the Borostai granitic pluton has a variety of rock types,including grayish-white medium-fine-grained massive granodiorite,grayish-white-light flesh red medium-coarse-grained massive monzonite and light flesh Red-Flesh red medium-coarse-grained massive syenite,in which intrusive contact relationship is dominant.Among them,there are a large number of mafic microgranular enclaves(MME)in granodiorite,which are elliptical,flame-shaped,irregular shape,etc.There are fewer enclaves in monzonite and syenite,mainly in elliptical shape.Isotopic chronology shows that the crystallization age of granodiorite is(254.8±1.5Ma),the age of monzonite is(249±2.4 Ma)and the age of MME in granodiorite is(253.9±1.1 Ma),indicating that the formation age of the Borostai granitic pluton is from the end of Late Permian to the early Early Triassic.Geochemical characteristics of the rocks show that the host rocks and dark microsome inclusions are quasi-aluminous-weak peraluminous medium-potassium-high-potassium calc-alkaline series rocks,which are rich in LREE and depleted in HREE,rich in large-ion lithophile elements such as Cs,Rb and Th,depleted in high-field strength elements such as Nb,Ta and Ti.Granodiorite-monzogranite-syenite show partial melting characteristics in different source areas in terms of(La/Yb)_N ratio,mafic index MF,Eu anomaly and REE content.According to the formula of temperature and pressure for different minerals,the crystallization temperature and pressure environment of different intrusions in rock mass are calculated as follows:the initial magma temperature of granodiorite crystallization is 785?,the total pressure value is 1.70-4.08kbar,and the depth is 6.42-18.14 km.The crystallization temperature of monzonitic granite intrusions ranges from 636 to 812?,and the initial magma temperature is 812?.The total pressure ranges from 1.05 to 1.11 kbar,and the depth is 3.97 to 4.19 km,which is similar to the pressure depth of its MME.The crystallization temperature of syenite intrusions ranges from 743 to 759?,the pressure ranges from 2.64 to 3.07 kbar,and the depth ranges from 9.98 to 11.58 km,which is larger than that of granodiorite and monzogranite.The crystallization temperature,pressure and depth of the MMEs are between 667 to 801?,1.02 to 5.05 kbar and 4.02 to 19.10 km,similar to host rock crystallization environment.Origin of the Borostai granitic pluton:the original magma of each host rock in the rock mass originated from the lower continental crust and was the product of partial melting of metabasic rocks and complex sandstones,while the mafic microgranular enclaves were the product of partial melting of mantle wedge metasomatized by subduction fluids.Comprehensive analysis shows that:during the Late Permian-Early Triassic,the Buqingshan-Animaqing Ocean subducted northward,causing the mantle-derived magma to rise and undermine the lower continental crust.The heat brought about by the subduction caused the partial melting of the lower continental crust metabasic source area and the meta-complex sandstone source area to form granitic magma of different properties.With the granitic magma subjected to ascending emplacement,separation and crystallization,etc.The process leads to the formation of the plutons with arc magmatic characteristics of different rock types.Combining with the characteristics of field geology,isotope chronology,mineral chemistry and petrogeochemistry,it is confirmed that there is incomplete mixing of magma in the Borostai granitic pluton.Mineral trapping crystals and amphibole and biotite with mixed crust-mantle characteristics in MMEs are all effective evidences for incomplete mixing.