A diabase dike of the Precambrian Mackenzie swarm from western Ontario has been sampled at two localities where it intrudes basaltic volcanics and granite respectively. The two profiles across the dike have rather similar contents of major and some trace elements (REE, U, Zr, Hf, Y, Nb, Sc, Co, Cr, and Sc) but differ in the abundances of K and related elements (Rb, Tl, Li, and Ba), which are significantly higher in the section of the dike intruding the granite. The differences can probably be attributed to selective contamination from granitic country rocks by diffusion.