Objective The northern Alxa block is the key area for studying the closing of the Paleo−Asian Ocean. Since the Permian volcanics have been rarely investigated in the Yagan area, the tectonic evolution of the Yagan Fault in the Paleozoic remains uncertaintu; and more importantly, the closing timing of the Paleo−Asian Ocean in northern Alxa block is restricted.

Methods In this paper, we carried out a systematic study of the petrography, geochemistry, zircon U−Pb dating for the intermediate−acid volcanics, which are located in the south of the Yagan Fault.

Results The intermediate−acid volcanics in the Yagan region are mainly composed of andesite, rhyolite, dacite, rhyolitic crystal tuff and dacitic crystal tuff. The zircon LA−MC−ICP−MS U−Pb dating results show that the rhyolite and dacite were formed during the Early Permian at 278±2 Ma, 279±2 Ma, and 283±1 Ma. The Yagan intermediate−acid volcanics are characterized by high K calc−alkaline or calc−alkaline feature, relative enrichment in large ion lithophile elements, such as Rb,U and Th, and depleted in high field strength elements, such as Nb,Ta,P and Ti. They show right−leaning chondrite−normalized patterns,which indicate enrichment in LREE and depletion in HREE, and exhibit relatively moderate−strong negative Eu anomalies (average value of δEu is 0.42~0.67).

Conclusions Based on the formation age and geochemical characteristics, it is concluded that the Yagan intermediate−acid volcanics were likely formed in a continental margin arc setting as products of the subduction of the Paleo−Asian ocean during Early Permian.Combined with regional geological background, it is inferred that the closure of the Paleo−Asian ocean in the northern margin of the Alxa block should postdate the late period of the Early Permian.