Abstract: The porphyritic granodiorite, located in the northern section of the Beishan area, is an important part of the Mingshui magmatic arc, but no reliable isotope ages have beenreported so far. On the basis of the detailed internal structures analysis of zircons from the porphyritic granodiorite using the technology of cathode luminescence, this study conducted isotope age analysis using LA-ICP-MS zircon U-Pbdating method. The result shows that single grain zircon U-Pb age is 277 Ma～283 Ma(averaging 279.5±1.6 Ma), indicating that the porphyritic granodiorite formed inlate Variscan. Geochemical study shows that porphyritic granodiorite has A/NCK>1.05 and Corundum molecular content is greater than 1%, with LREE/HREE ratio from 16.89 to 18.58 and weak negative Eu anomaly(δEu of 0.76～0.92). Enrichmentof Rb, Th and K and depletion of Nb, Ta, P and Ti indicates that porphyritic granodiorite has characteristics of high K calc-alkaline peraluminous granites. Combined with the regional geological background, it is considered that the porphyritic granodiorite formed in post collision tectonic environment, implying thatthe closing time of Hongshishan ocean was earlier than 279.5±1.6 Ma.