The ultrabasic lamprophyre dikes from the vicinity of McKellar Harbour, northwestern Ontario, are composed essentially of a mixture of phlogopite, olivine, primary carbonate, spinel, and apatite with associated perovskite and (?) melilite. Petrologically and chemically they have strong affinities with alnöites and magmas derived from upper mantle sources in stable continental regimes.Rb–Sr isotopic studies give an isochron age of 1.65 ± 0.12 Ga (2σ) with an initial 87Sr/86Sr ratio of 0.70167 ± 0.00036 (2σ). This age is considerably older than the Neohelikian Keweenawan magmatic event associated with the development of the Lake Superior Basin (e. g., 950–1170 Ma). K–Ar data indicate a mean whole-rock age for the dikes of 1144 Ma and a pseudoisochron age of 1120 ± 34 Ma (2σ). These ages are thought to reflect partial degassing of phlogopite and incomplete updating of the rocks under thermal influences induced by the intrusion of the Coldwell alkaline complex at 1044 Ma.Ages of carbonatitic and alkaline rock intrusions in the general vicinity of the McKellar Harbour – Marathon region closely approximate those associated with the intrusion of similar magmas in the Kapuskasing structural zone, i.e., 1650–1700 and 1020–1050 Ma. By analogy with the Kapuskasing zone, it is suggested that periodic activation of a deep-seated, long-lived structural zone in the Marathon region has lead to the tapping of upper mantle magmas. The McKellar Harbour ultrabasic lamprophyres are the product of one such event.

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