Northwest trending diabase dikes having alkali olivine basalt chemistry occur abundantly in the Southern Structural Province of the Canadian Shield of Ontario. In several subregions these dikes can be traced across the Grenville Front for a short distance into the Grenville Province. Within the Southern Province the dikes possess a westerly directed primary magnetization yielding a paleomagnelie pole of high precision and accuracy (168° W. 21/2°S, A95 = 21/2°). The geological age of these dikes is probably about 1250 Ma. At distances ranging from 2–8 km (northwest) of the Grenville Front these dikes are re magnetized and are characterized by anomalously high apparent K–Ar ages. The acquisition of this later magnetization having a direction of 111°, +261/2° with a corresponding pole at 161/2° W, 4° S. A95 = 61/2° takes place without any general mineralogical expression of metamorphism within the dikes. A few dikes sharing the above direction of magnetization were intruded during this time of reheating at about 1000 Ma. A short dike was located in the Grenville Front region southwest of Sudbury which possesses tholeiitic chemistry, early Paleozoic apparent K–Ar ages, and a cleaned remanence direction of 125°, +59° with a corresponding pole at 42° W, 10° N. These early Paleozoic dikes may be more extensive within the Grenville Province.