Abstract: There has been increasing interest in the study of dyke swarms which are related to rifting and continental break-up. Ku1uktag dyke swarms are located in the northeast part of Tarim Basin, Xinjiang, China. The dykes, parallel to each other, intruded into the Archaean and early Proterozoic rocks. Four samples collected from the dykes are dated with K-Ar method. The apparent ages are discordant, ranging from 455 to 673 Ma. Using the Ludwing’s ISOPLOT, an isochron age of 287± 13 Ma was obtained with a correlation cotfficicent of 0.9850, This age is concordant with the ages of volcanic rocks along the Tianshan Mt. structure zone dated around 280Ma. The K-Ar isochrom method has some advantage for dating mafic dykes：(1)These rocks are often fresh and slightly affected by alteration; (2) The closure temperature of mafic rocks for argon is high; (3) The measurement precision of mass spectrometer is much higher than it was before, so that low K samples can be well measured; The potassium abundance of mafic dykes is about 0.5-1.5％ and suitable for K-Ar dating; (4) The whole rock method is used, so that the tough work of selecting monomineral fraction is not necessary. Moreover, the initial argon can be eliminated by this method, but the atmospheric argon must be removed in the experimental process. Igneous rocks originated from different sources have different initial argon ratios. The initial argon ratio for Kuluktag dykes is 508, belonging to mantle plume-type according to Kaneoka and Takaoka(1985). We infer that the magma of the dykes originated from “enriched mantle”. The REE and trace elements features of the dykes am also typical of “enriched mantle” and are in good agreement with those of basalts related to plume.