Abstract: There is a suite of mantle xenoliths bearing olivine basalt discovered recently in the south of Siziwangqi area，Inner Mongolia．The rock is grey-black，with compact block structure and is characterized by alkaline．rich(K2O+Na20=5．91％～6．47％)，high K(K2O=2．82％ ～3．12％)and high Ti(Ti02=2．22％ ～2．36％)compositions．The whole rock K-Ar isotopic age is 21．89 4±1．65 Ma． The rock types of xenoliths are lherzolites，spinel lherzolites and anorthositic gabbro．Geothermometer and geobarometer calculations manifest that the temperature and pressure ranges for the xenoliths are 1 100-1 250 oC and 1.2～1.7 GPa，respectively，which indicates a lithospheric mantle source．Considering that the high Ti character of the olivine basalt is similar to Hannuoba basalt，the basalt magma might come from the asthenosphere．Deep mantle plume possibly began to work since the Mesozoic under the northern margin of North China Platform，which resulted in the accumulation of vast magma remaining tinder the lithosphere，followed by underplating and ascending of some magma carrying the mantle xenoliths upwards．