In order to enhance the understanding of the stage of magmatism and the relationship between Weishan alkaline complex and REE mineralization, the zircon U-Pb age and geochemistry of Weishan alkaline complex are conducted. The geochemical result shows that syenite series has the same geochemical characteristics, proving that they are from the same resource, and the syenite belong to metaluminous and alkaline series. They have high ∑REE, and distinct fractionation between HREE and LREE, exhibiting depleted in HREE, Nb, Ta, Zr, but enriched in LREE, Rb, Ba, Sr, and have slight Eu abnormality, which is consistent with the characteristics of mantle-derived magmatic rocks. The element-geochemical features reflect that the magma comes from enriched mantle. LA-ICP-MS zircon U-Pb ages of the ore-bearing quartz syenite and the aegirine-augite syenite are separately 122.4±2.0 Ma (MSWD=5.2) and 130.1±1.4 Ma (MSWD=9), and the presence of 2536±6.1 Ma (MSWD=1.6) inherited zircon is also an evidence that crustal materials have been involved in the magmatism process. It is concluded that Weishan syenites had come into being in an extensional setting after the transformation of the tectonic regime in the NCC in the Mesozoic area, when a large-scale enriched mantle melted to cause the generation of alkaline magma and a massive upwelling of the mantle materials from the asthenosphere to form finally the REE deposit after the formation of the alkaline rock complex.