There are large areas of late Mesozoic volcanic rocks in Mohe, north Da Hinggan Mountains. Among these volcanic rocks, the Tamulangou Formation erupted earlier than other formations. The Tamulangou Formation is a set of basic to intermediate volcanic rocks, including lavas and pyroclastic rocks. The study of petrogenesis of the Tamulangou Formation may provide a very important insight for the tectonic evolution of north Da Hinggan Mountains. A systematic geochemical analysis of major and trace elements was carried out with samples from the Tamulangou Formation. It is shown that most samples are alkaline basic to intermediate rocks with SiO₂ contents of 52.7%-57.8%. These rocks are basaltic trachy andesites and trachy andesites. Compatible transitional elements such as Cr, Co and Ni are low in contents. Trace element ratios such as Nb/La, Ba/Nb, Nb/U and Ce/Pb and the total rare earth elements contents of these samples vary in a large range, with a right-oblique rare earth element distribution mode. The petrogenesis study indicates that the partial melting of the enriched continental lithospheric mantle plus a slight crustal contamination and a certain degree of fractional crystallization are responsible for the formation of the Tamulangou Formation volcanic rocks.