Petrology and genetic mechanism of volcanic magma can be studied thro-ugh low pressure experiments.The Ol-Di-Ne tertiary phase diagram shows that the Ol-CPx-Pl saturated cotectic lines of alkaline basalts formed in different geotectonic environments do not coincide with each otber,and Ol-CPx-Pl saturated cotectic lines of alkaline basalts formed in the same tectonic environment simply move toward the Ol vertex with the increasing oxygen fugacity.The silica-alkali evolution trend of low pressure crystallization fractionation of volcanic magma exhibits positive slope and is approximately parallel to the boundary of volcanic series in the TAS diagram(Le Bas,1986).The 1-atom experimental liquid lines of descent are different from the evolution trend of the natural rocks,implying that low-pressure crystallization fractionation could not be the major mechanism for the magmatic evolution of the Datong volcanic cluster.It seems that high pressure crystallization fractionation was the major factor controlling the evolution of the volcanic magma from the early stage to the late stage of Middle Pleistocene.