Abstract：The Qiangtang area is a key area to studying the Palaeo-Tethys，and the emplacement timing and tectonic setting of mafic dike swarms in southern Qiangtang have great significance in determining the breakup age of Pangaea and tectonic evolution of the Qinghai-Tibet Plateau. Mafic dikes were dated by using the single-grain zircon U-Pb and whole-rock Sm-Nd methods and the zircon U-Pb concordia age of 312±4 Ma and Sm-Nd isochron ages of 299±13 and 314±5 Ma were obtained.On that basis, combined with the regional geological data, the authors think that the mafic dikes are the product of the breakup of the Qiangtang block and that the isotopic ages represent their emplacement age. This constrains the age of breakup of the Qiangtang block and provides important information for the reconstruction of the formation and evolution of the Lungmu Co-Shuanghu Lake Palaeo-Tethys ocean basin.