High- Mg andesites are mainly exposed in island arc environment along subduction zone of oceanic plate, which is a representative product of island arc magmatism. Detailed studies on the tectonic affinities of high- Mg andesites are of great significance for tracing the timing, tectonic setting, and evolution of lithospheric mantle during the oceanic plate subduction. The Changchun- Yanji suture lies between the Jiamusi- Khanka Block and the North China Craton, which provides crucial information on the closure of the Paleo- Asian Ocean and the commencement of the westward subduction of the Paleo- Pacific. However, the location of the suture between these two blocks and the subduction mode of the Paleo- Pacific are still controversial. In this study, we collected seven green schists from the Seluohe Group along the Changchun- Yanji suture and report the major and trace elements and zircon U- Pb age of these samples.The results show that they are a suite of high- Mg andesites with higher MgO, Cr and Ni, and lower Al2O3, TiO2, which are similar to the Sanukite in the Setouchi volcanic belt in Japan. They were formed by the interaction between the mantle wedge and the fluids from subducted slab (oceanic crust and overlying sediments), and were formed in a continental marginal arc on the North China Craton, which was subducted by the Jilin- Heilongjiang Ocean in the Middle Triassic, with zircon U- Pb age of 246±2 Ma. The evidence in this study suggests that the formation time of the Seluohe Group may be Early Mesozoic, and it is not an accretionary complex formed in the Proterozoic or the Late Paleozoic, but a suite of early Mesozoic continental marginal arc andesites.Regional tectonic analyses show that the Changchun- Yanji suture is not the eastern extension of the Tianshan- Solonker- Xar Moron- Changchun suture, but the southern margin of the Jilin- Heilongjiang high- pressure metamorphic belt, which was formed during the collision between the Jiamusi- Khanka Block and North China Craton caused by the westward subduction of the Paleo- Pacific plate.