This paper is the result of the geological survey engineering
[Objetive]The Yidun island arc, as a key part of Sanjiang Tethys orogenic belt, has been hottly debated on granitic intrusions and the related mineralization in recent years. But few studies are concerned with the rhyolites such as the Tumugou Formation. [Methods] Based on detailed field mapping, this paper has conducted research on petrology, geochemistry and chronology of the Tumugou Formation’ s rhyolite. [Results] LA-ICP-MS zircon U-Pb dating on one sample yield an age of 216.5 Ma, and proved that the rhyolites were formed in the Late Triassic. The rhyolites have contents of SiO₂ from 73.24% to 74.72%, alkali (K₂O+Na₂O) of 5.26% to 6.27% as calc-alkaline rock series. All the samples are enriched in large ion lithophile elements such as Rb, Th, U, K and light rare earth elements, but depleted in the high strength filed elements such as Nb, Ta, Ti, P and heavy rare earth elements, indicative of the characteristics of island- arc volcanic rocks. [Conclusions] According to the comprehensive study, it is concluded that the rhyolites were formed by partial melting of the lower continental crust during the westward subduction of the Ganzi-Litang oceanic crust in the Indo-Chinese epoch.