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The Midcontinent Rift in the Lake Superior region of North America is one of the best preserved examples of an aborted Precambrian intercontinental rift, one that hosts a diverse suite of rock types in addition to the well-studied and voluminous rift-fill flood basalts. Although there is a growing database of high-precision age information for the main volcanic packages and the largest mafic intrusions, there is relatively little information available on the absolute timing of mafic-ultramafic intrusions, dyke swarms, and alkaline complexes, especially in the Ontario portion of the rift. We report new high-precision U–Pb ages for 29 samples, primarily collected in the Lake Nipigon area, Ontario. From these new age results, it is now possible to expand the known distribution of Geon 15 magmatism in the region, confirm an early stage of Midcontinent Rift mafic magmatism between 1150 and 1130 Ma, provide evidence that significant mafic–ultramafic magmatism occurred in the Lake Nipigon region slightly earlier (~1115–1110 Ma) than the main stage of rift magmatism (1108–1094 Ma), and further document synchronous ~1110–1100 Ma tholeiitic and alkaline magmatism.

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The Nipigon Embayment is underlain by Archean rocks of the English River, Wabigoon, and Quetico subprovinces, and intruded along the west side by late- to post-tectonic mafic to ultramafic intrusions. The early Mesoproterozoic ultramafic to felsic Badwater intrusion and felsic English Bay Complex are located in the northwest corner of the Nipigon Embayment. Three mafic to ultramafic intrusions, the Disraeli, Seagull, and Hele intrusions, are located south of Lake Nipigon, and the Kitto intrusion is located east of the lake. A number of mafic to ultramafic bodies (Jackfish (Island), Shillabeer, Kama Hill, Nipigon Bay) have only limited outcrops. The gabbroic Nipigon diabase sills intrude all other rocks in the Nipigon Embayment and generally have a consistent mineralogy and geochemistry, except for the Inspiration sill(s) and the McIntyre Sill. Geological and geophysical data suggest emplacement of the ultramafic intrusions by mechanisms similar to those controlling emplacement of the saucer-shaped diabase sills. These mechanisms are partially dependent on a series of pre-existing north-, northwest-, and northeast-trending faults formed prior to Keweenawan magmatism. The presence of sills, rather than dykes, indicates that the Nipigon Embayment was not extensional during the Keweenawan Midcontinent Rift, suggesting that the Nipigon Embayment is not a classic failed arm.