Analysis Of Southern Qilian Hala Lake Nanshan Rock Magma Sources And Tectonic Environments

Southern Qilian Hala Lake Nanshan exposed two types of intermediate acid intrusive rocks for granodiorite and monzogranite,they were all located in the early Paleozoic strata.Due to the south of the rock mass is the ultrahigh-pressure?UHP?metamorphic belts of the north margin of Qaidam Basin,it has special geological significances.This paper is based on the field survey in the area of intrusive rock,through the study of the rock petrography?thin section identification??LA-ICP-MS zircon U-Pb geochronology?whole-rock geochemical and Hf isotope,in order to investigate the origins,magma sources and tectonic environments of Hala Lake Nanshan rocks.LA-ICP-MS zircon U-Pb dating of measured granodiorite,age 439.8±1.7Ma and441.6±2.5Ma;The age of monzogranite was 440.2±3.2Ma and 443.9±2.4Ma.They are all formed in the Early Silurian period.Geochemical characteristic shows that Granodiorite belong to Metaluminous of subalkaline series that is a weakly peraluminous calc-alkaline granite.It has a significant REE fractionation?LREE/HREE=10.31¹6.83?with a medium-weak negative euanomaly??Eu=0.58⁰.87?.The spider diagram of trace element shows a significant Nb-Ta slot.Overall,it own the chemical characteristics of I type granite;Monzogranite is a high potassium calcium alkaline granite of strongly peraluminous with strongly negative Eu anomaly??Eu=0.19⁰.42?.Trace elements enriched in?LILE?Rb,Th,U,K,and loss incompatible elements Nb,Sr,P,Ti for the characteristics which is a typical S type granite.The Zircon Hf isotope studies of the two types of intrusions revealed that granodiorite's?_Hf?t?distribute in-12.3^-5.3,T^C_DM?Ma?distribute in 1.85².35Ga;monzogranite's?_Hf?t?distribute in-12.1^-5.8 and T^C_DM?Ma?distribute in 1.95².33Ga.The two have very stable and similar Hf isotopic compositions.Comprehensive analysis of the two kinds of intrusive rocks they all came from partial melting of the ancient earth's crust.Different is the granodiorite from crustal mafic rocks and monzogranite from terrigenous clastic rocks.Through above research of the rock mass of Hala Lake Nanshan,and combined with regional geological characteristics,we think the rock mass has tight connections with the early Paleozoic UHP belt of the north margin of Qadaim Basin.The magma derived from partial melting of the basement of the early Proterozoic crust,under the tectonic setting of deep subduction of Qaidam crust and the collision between the Qaidam crust and the southern Qilian crust.Therefore,these two types of granitic magma are in different layers within the same subduction zone,near simultaneous formation.,Meanwhile reveals that the South Qilian ocean may have closed before 440Ma,at this time is active period of continental deep subduction and collision.