Biotite and plagioclase samples from the quartz monzodiorite of Xinwuli granitoids yield 40Ar-39Ar plateau ages of (152.1±1.7) and (135±1) Ma. Samples from the granodiorite give 40Ar-39Ar plateau ages of (143.8±0.9) Ma and (140.4±0.9) Ma. The similar ages suggest an important stage of Yanshanian magmatic activity in the Middle and