Abstract: The volcanic rocks from the Upper Triassic Nadigangri Formation in the Mugxung region,Zhidoi,southern Qinghai consist of intermediate to basic rocks assembled by alkaline basalt and basaltic andesite.Petrochemically,the volcanic rocks are characterized by low Ti,high Al,enrichment of LREE,high fractionation of LREE and HREE,slightly negative Eu anomaly,enrichment of macro-ion lithophile elements,and depletion of high field strength elements.The(⁸⁷Sr/⁸⁶Sr) i values between 0.70774 and 0.70801 and the εNd(t) values between-3.3 and-4.5 suggest an island-arc environment.Tectonically,the volcanic rocks from the Late Triassic Nadigangri Formation constitute a magmatic arc originated during the southward subduction of the Lazhuglung-Jinshajiang(or Garze-Litang) oceanic crust.