This is an article in the field of earth sciences.Magmatic rocks of the Gondwana Belt are one of the most popular problems in basic geological research on the Xizang-Qinghai Plateau.In this paper,we analyze the physical origin and regional tectonic properties of the Gachatai granite using LA-ICP-MS zircon U-Pb dating method,combined with the geochemical characteristics of the rocks and various graphical methods.The rocks are characterized by high alkali(average Na2O+K2O content of 6.26％),potassium(K2O/Na2O=1.03),low TiO2(average 0.50％)and quasi-aluminous(aluminum saturation index A/CNK=0.933～1.033,average 0.991)high potassium-calcium-alkaline rock system.Rare earth elements show negative anomalies at Eu,which is the Eu-deficient type,and Ce has weak negative anomalies;trace elements show enrichment of large ion-parental elements Rb,Th,Nd,La and K;high field strength elements Nb,Ba,U,Ta,Ce,Sm and Ti are deficient;reflecting the mixed crust-mantle type of magma origin and the presence of subduction oceanic crustal melting.The zircon LA-ICP-MS U-Pb isotopic age is(345.3±1.8)Ma,and its formation age is Early Carboniferous.The analysis of the regional geological data suggests that this magmatic event is a product of the formation of the northern part of the eastern Gondwana continent after the breakup of the Rodinia supercontinent.The project points go to I-type granite,the discriminant diagrams indicate it's formed from continental island arc,going through time before plate collision to co-collisional orogenic movement,which indicates a long period time of magmatic evolution;the rock geochemical characteristics show the environment of magmatic evolution changed from the start of island arc(primary island arc)-development(early stage island arc)-maturement(full-grown island arc).
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