Abstract: A set of volcanic rocks were recently found in Palaeozoic strata in the Bayinsukhtuore district of South Mongolia. The RbSr isotopic age of the rocks is 409±19Ma, which is consistent with the chronological age (Silurian-Devonian) obtained from previous work. The SiO₂ content of dacite is 63.23%~73.36%, while the Al₂O₃ content is 10.25%~15.56%. The dacite is LREE enrichment, HREE loss, and has strong Eu negative anomaly. Based on petrology and geochemistry, it is held that this set of volcanic rocks evolved in the same period and came from the same source. The lithologic character of the rocks is mainly of dellenite-dacite type, with little change. According to the Kalium analysis, the volcanic rocks were derived from the potassium calc-alkaline series. It is suggested that the melted mantle source area was reformed by the subducting plate. During this process the original magma was generated from the liquid mantle of the subducting zone. At the same time crustal contamination happened. The magmatic evolution would be the main reason for generating the combined set of the volcanic rocks. It also implies that this set of volcanic rocks was formed in the island arc during the closing of pal-Asia, which indicates that the closing of pal-Asia was later than the formation of this set of volcanic rocks.