The Ar-Ar age of diabase dykes of Youdong fault zone is 110.6 ± 2.0 Ma, which formed late Yanshaninan . Diabase is rich in SiO2（mean value 50.65%）、Al2O3（mean value 15.10%）、CaO（mean value 10.48%）、MgO（mean value 7.80%） and poor in TiO2（mean value 1.13%）、FeOt（mean value 9.90%）、Na2O（mean value 2.28%）、K2O （mean value 0.90%）、P2O5（mean value 0.11%.In rare earth elements, it is rich in large-ion lithophile Rb, Th, Ba, K, and poor in Nb、Ta、Zr 、Hf、P、Ti. Total rare earth is low, and bear the right-wing-type distribution patten eith obviously Eu deplation（δEu mean value 0.92）. Its characteristic is similar with which of active continental margin basalt, and this phenomenon reflects that primitive magma came from relative enrichment of the lithospheric mantle, didn’t come from original mantle.