Stepwise outgassing experiments have been carried out on nine whole rock samples of material from these diabase dikes. None of the samples is entirely free of alteration and all of the age spectra exhibit at least some degree of discordance. Over the intermediate temperature interval (775–1025 °C, arbitrarily defined), apparent age plateaus (or plateau-like features) appear to be the rule. The mean apparent age of gas released at these temperatures for six of the samples is ~900–910 Ma; the average apparent Ca/K ratio is ~3. These samples have a mean total gas age of ~910 Ma and total gas apparent Ca/K ratios ranging from ~5 to ~7. The above value is interpreted as a probable minimum age for K-rich phases (biotite and sericite).The dikes were intruded into the host gneisses probably while they were still above the temperature at which argon is quantitatively retained in biotites. Although the magnetic blocking temperatures of the dikes are not known, the Frontenac paleopole is probably in the vicinity of ~900 Ma old. This value is consistent with other recent estimates of the age of this middle portion of the Grenville apparent polar wander track.