Abstract: The formation mechanism of Eogene igneous rocks in Dongbu sag in Liaohe depression is analyzed in terms of forming environment and structural evolution. It is suggested that the various igneous rocks in this area belong to cogenetic magma without hybridism, and the crystal fractionation period of related trachyte is longer than those of basalt and diabase. The diabase is intrusive mass of Late Oligocene. The volcanic rock of the middle section of Sha-3 member features underwater eruption, while the upper section of Sha-3 member to Dongying formation is dominated by above-water eruption. The magma activity is mainly controlled by such faults as Jiexi, Jiazhangsi and Erjiegou, etc., and its center in Eocene is located in the middle section of Dongbu sag, the active center of magma in Oligocene is located in two sides of south and north Dongbu sag. The trachyte is mainly found in the midsection of Sha-3 member in south-central Dongbu sag.

Key words: Liaohe depression, Dongbu sag, igneous rock, volcanic rock, forming environment, structural evolution