Zircon SHRIMP U-Pb dating of the Fanshan potassic alkaline ultramafite-syenite complex in Hebei Province, China.

As one of alkaline intrusives in northern margin of the North China block, the Fanshan potassic ultramafite-syenite complex in Hebei Province contains the largest endogenous magnetic-apatite deposite of China. However, the pre-existing K-Ar, Rb-Sr and Sm-Nd ages are so different that more precise dating is very important to constrain the timing of the Fanshan complex. This study reports a SHRIMP zircon U-Pb age 218 +/- 2Ma from a pyroxene syenite sample. Because the dated sample was collected from the main part of the complex, which was produced by the last (third-stage) pulse of magmatism, the zircon U-Pb age provides a strong constaint on the timing of the Fanshan complex. This is consistents with the zircon U-Pb ages from the literature for other alkaline intrusives in northern, eastern, and southern margins of the North China block, indicating that the alkaline magmatism at around 220Ma is one of the most important tectonomagmatic events in eastern China.