Abstract: There is a group of nearly E–W trending mafic dyke of Late Carboniferous–Early Permian along the Longmu Co–Shuanghu suture zone in Qiangtang area, Qinghai–Tibet plateau. The surrounding rocks are mainly slightly-metamorphosed rocks which belong to rift sedimentary type formations of Late Carboniferous–Early Permian with basic volcanic rocks intercepted. From north to south, amount and thickness of these interceptions decreased gradually. The main rock type of the mafic dyke is diabase while for basic volcanic rock is mainly basalt, secondly basalt andesite. The mafic dyke swarm and basalt formed nearly in the same time and both of them experienced greenschist facies metamorphism. There are no significant differences in the concentrations of the major elements, REE elements and the trace elements between the mafic dyke swarm and the basalt. The elements concentrations of the mafic dyke swarm and the basalt have the same feature as the within–plate basalt .On that basis, combined with the regional geological data, the authors think that the mafic dyke and the basalt come from same source magma. They are the products of the breakup of the northern margin of Gondwana during Late Carboniferous to Early Permian.