Daba Mountain tectonic belt is located in the transitional position of the Qinling orogenic belt and the Yangtze plate,formed in Indosinian Yanshanian, taken the shape in Himalayan period.Previous studies focused on geochronology of North Daba Mountain, less reserach work was reported in high precision age in South Daba Mountain. The dating of LA ICP MS zircon U Pb fordiabase porphyry dike sample from the Donghe area in Fangxian, South Daba Mountain shows the that the Donghe diabase porphyry was formed at 4406～4422Ma,equivalent to the early Silurian System.It is almost consistent with that of diabase in the south area of North Daba Mountain,(Zhenping area), and with that of the basic dike and volcanic rochks in the middle northern area of North Daba Moutain,(Ziyang Langao area).Combined with previous studies, the authors suggest that South Daba Moutain was a rift during the middle late stage of Early Silurian,and the diabase porphyry in the Donghe area was the product of continental rift.The age of 4393Ma represents that the north margin of Yangtze plate was generally in the extensional tectonic environment, suggesting the extension of this rift to its peak in Early Silurian. The age date obtained in this study determines the surrounding rock Zhushan Formation formed no later than early Silurian, defining the strata are Cambrian Ordovician in age.