Abstract:

SHRIMP U-Pb ages and in situ Hf isotope compositions have been measured to constrain the timing of the volcanic tuff at
the bottom of the Pinghu Formation from a core drilled at the Wuyunting structure belt, western of the Xihu depression. Zircons U-Pb ages showed the age dispersed in 32~2636 Ma. Three youngest zircon ages yield a mean weighted age of (32.6±1.3) Ma (MSWD=0.29, n= 3), which constrains the oldest age of the Pinghu Formation. The motion direction of the Pacific plate changed from NNW to NWW during 42~44 Ma, and then the Pinghu Formation became under compressive stress control in E-W direction, with the regional stress field characteristic of the depression deposition. Furthermore, the εHf(t) values of the inherited zircons are much similar to those of the coeval zircons from the Cathsysia Block, suggesting that they might be originated from a common source.

Key words: Xihu Depression, Pinghu Formation, SHRIMP, U-Pb Age, Hf isotopes