Abstract: The volcanic rocks of the Abaga Formation in Sonid Left Banner area of central Inner Mongolia are mainly composed of andesites. These andesites have high alkali, potassium and aluminum, but low magnesium, and are obviously enriched in LILEs such as Rb, Ba, U, K and LREE, and depleted in HFSEs such as Nb, Ta, Ti with obvious negative anomalies. They have high ∑REE values, and show intense REE fractionation with LREE enrichment and weak negative Eu anomalies. Their geochemical characteristics suggest that they belong to potassic volcanic rocks, and were likely generated from a small degree of partial melting of phlogopite-bearing garnet lherzolite in an enriched sub-lithospheric mantle. The enriched sub-lithospheric mantle was metasomatized by fluids released from the stagnant Pacific slab in the mantle transition zone. Geochemical features also reveal that fractional crystallization with the removal of plagioclase and ferromagnesian minerals played an important role in the evolution, and the potassic magma was not affected by crustal contamination as it passed through the thick continental crust. These potassic rocks were formed in an intraplate extensional tectonic setting resulting from the rollback of the stagnant Pacific slab in the mantle transition zone.