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The Jiazishan alkaline complex in the eastern Sulu ultrahigh pressure (UHP) metamorphic orogenic belt of eastern China is composed of potassic to ultrapotassic pyroxene syenite, quartz syenite and associated mafic dikes. A SHRIMP zircon ²⁰⁶Pb/²³⁸U age of 215±5 Ma was obtained for the quartz syenite and mineral ⁴⁰Ar/³⁹Ar dating gave emplacement ages of 214.4±0.3 and 214.6±0.6 Ma for the pyroxene syenite and 200.6±0.2 Ma for the mafic dike. These dates establish that the Jiazishan Complex was emplaced shortly after the UHP metamorphic event at 240 to 220 Ma due to the continental collision between the North China and Yangtze cratons. The ultrapotassic mafic dikes, with K₂O≈4.4–6.4 wt.% and K₂O/Na₂O≈3.5, have high MgO (8.06–12.44 wt.%), Ni (119–319 ppm) and Cr (477–873 ppm) and moderately low CaO/Al₂O₃ (∼0.76) and TiO₂ (∼1.12 wt.%). They also have high Sr (⁸⁷Sr/⁸⁶Sr∼0 …