The Mogou alkalic pluton, located in the southern margin of the North China Block, is mainly composed of aegirine-augite syenite, with minor syenite and porphyritic (aegirine-augite) syenite as well as mafic magma enclaves. LA-ICP-MS U-Pb dating of the aegirine-augite syenite yielded two ages of 237.6±2.9 Ma (MSWD=0.95) and 210.4±2.0 Ma (MSWD=0.39), suggesting the middle and late Triassic epoch, as also shown by previous studies. The aegirine-augite syenite is of peralkaline series and belongs to potassic alkaline rock with σ being 8.93～11.08, K₂O 10.70%～13.76%, and K₂O/Na₂O 3.87～10.35. The REE and trace elements are enriched in LREE and large ion lithophile elements, and depleted in high-field strength elements, with slightly negative to no obvious anomalies of Eu. The aegirine-augite syenite shows high (⁸⁷Sr/⁸⁶Sr)_i values with a large variation and significantly negative εNd(t) values. The zircon Hf isotopic compositions are characterized by εHf(t) values of -21.6 to -4.5, and the two-stage model ages of 1 555 to 2 629 Ma. The formation of the Mogou pluton could be interpreted as mixing/mingling of crustal-melted magma and mantle-derived magma which originated from phlogopite-bearing enriched mantle by low degree partial melting. The early stage magmatic activity of the Mogou pluton occurred in a local extension setting during a great regional collision period, while the late stage magmatism took place in the post-collision environment.