Based on petrochemical analyses and zircon U-Pb LA-ICP-MS dating combined with field investigation, the authors propose that these dykes represent a crustal extension event of the Bangong Lake arc zone after the closure of Bangong Lake middle-Tethys oceanic basin. The granite porphyries occurring only in NS-strike and the diorite porphyrites occurring both in NS- and EW-strike respectively yield a weighted mean age of (79.59±0.32) Ma (MSWD=1.08) with 13 zircons and (76.9±1.2) Ma (MSWD=2.8) with 6. The results indicate that the crustal extensional process of the Bangong Lake arc zone occurring in Late Cretaceous epoch was initialed only in EW-trending and slightly later also in NS-trending.