Abstract: Based on the studies on apatites from the Altay No.3 granite pegmatite，Xinjiang，China．together with those peraluminous granites and pegmatites from various areas in the world，this paper discusses the fractionation of the coherent elemental pairs that have similar ionic radii and same valences ，and also the origin of the lanthanide tetrad effect． It is suggested that the fractionation of the coherent elemental pairs and occurrence of the lanthanide tetrad effect are genetically related to formation of volatile-rich fluid during magmatic evolution，and that the fractionation of the elemental pairs an d the intensity of the lanthanide tetrad effect ar1e dependent on the extent of fluid-melt interactions．