Abstract: The petrogenesis and tectonic background of Mesozoic volcanic rocks from the middle-south part of the Da Hinggan Mountains are closely related to the late stage evolution of the Southem Mongolia-Da Hinggan Mountains orogenic belt and to the genesis of copper-polymetallic deposits．Based on detailed fieldwork，petrological and geochemical studies on volcanic rocks have been carried on．The results show that the volcanic rocks，including basaltic trachyte，trachyandesite，trachyte and dacite，belong to cale-alkaline series．In major elements they are enriched in Si02，Na20 and K20，and depleted in Fe203，FeO，MgO and CaO；and in trace elements they are enriched in lithophile elements，and depleted in siderophile elements and Nb，Ta，Ti etc．In REE patterns they are generally LREE enriched，(87Sr／86Sr)t ratios are 0．7045～0．7077，mostly 0．7045～0．7055，and most of εNd(t)values are positive．The results suggest that partial melting of upper mantle in different degrees Was the major mechanism for generation of the volcanic rocks in the middle-south part of the Da Hingan Mountains。and the volcanic rocks were developed in the tensional tectonic setting．The Mesozoic volcanic activity was a response to the extension caused by upwelling of mantle plume of asthenosphere during the post-orogenic period of the Tianshan-Da Hingan Mountains belt，to the post-orogenic extension background of the Inner Mongolia-Jihei orogenic belt，and to compression background caused by closure of Mongolia-Okhotsk residual ocean in a scissors form．