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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.

The Nuvvuagittuq Supracrustal belt in northern Que ´bec is a rare ϳ9 km 2 fragment of the Earth's early crust. The belt contains a metamorphosed volcanosedimentary sequence that is at least Eoarchean in age, including amphibolites of the Ujaraaluk Unit that may have protoliths as ancient as 4400 million years old (Ma; O'Neil and others, 2012). Upper amphibolite facies metamorphism and high-strain deformation have obscured many primary field relationships and disturbed whole-rock isotope systematics, leading to debate over the interpretation of previous geochronological data. We report new SHRIMP U-Pb isotopic analyses of zircons from key meta-igneous and newly identified metasedimentary units of the belt. The analyzed samples fall into four categories: (a) felsic gneisses, including sheets interlayered with the supracrustal assemblage and a tonalitic orthogneiss at the margin of the belt; (b) a newly identified metasedimentary unit; (c) fuchsite-bearing quartz-rich layers; (d) mafic gneisses, including a garnet-biotite amphibolite from the Ujaraaluk Unit and a meta-gabbro. The felsic lithologies place constraints on the minimum age of the supracrustal assemblage; oscillatory-zoned zircons separated from a felsic orthogneiss sheet form a discordant array with an upper intercept age of 3774 ؎ 32 Ma, and the tonalitic orthogneiss yielded oscillatory zoned zircons with an upper intercept age of 3781 ؎ 11 Ma. A metasedimentary unit, dominated by quartz, albite and clinozoisite, has been identified within a lower strain domain, interfolded with amphibolites of the Ujaraaluk Unit. A sample of this unit yielded zircons with highly variable CL structures and near concordant ages of between 3390 and 3780 Ma. Zircons from a fuchsite-bearing quartz layer have oscillatory zonation similar to the orthogneiss grains, and define an upper intercept age of 3794 ؎ 16 Ma. Petrologic observations and geochemical data suggest that this lithology is a metasomatically altered felsic orthogneiss band, rather than a meta-sedimentary rock belonging to the supracrustal package. Electron microscopy of polished thin sections was used to locate zircons within 6 samples of the Ujaraaluk Unit amphibolites and two samples of meta-gabbro. Large (<500 m) zircons separated from a garnet-biotite amphibolite, together with grains from a meta-gabbro, are sector zoned and yield ages of 2700 to 2640 Ma, recording the timing of pervasive hightemperature metamorphism in the region. The new zircon U-Pb data add important new detail to the geological history of the Nuvvuagittuq belt. The volcano-sedimentary sequence and mafic-ultramafic intrusive rocks were formed prior to 3800 million years ago. Subsequent geological evolution included extensive and protracted early Archean TTG magmatism and crustal remelting, Paleo-to Mesoarchean uplift and sedimentation and Neoarchean high-grade metamorphism and regional magmatism during consolidation of the Superior craton.