On the northern margin of the Tarim basin, the Cenozoic alkaline basalt s formed the latest during the formation of the alkaline rock belt. The alkaline basalts are composed mainly of alkaliolivine basalts and alkaliolivine gabrros, the former are characterized by continental central eruption in the Tertiary basin and the latter occurred as pipes or dikes and are related to the same rocks in the underlying Palaeozoic strata. The two kinds of rocks were formed as a result of comagmatic crystallization from enrichment mantle in diffe rent space.