A schematic crustal cross-section is presented for the southwestern Grenville Province based on reprocessed Lithoprobe near-vertical incidence seismic reflection data and compiled seismic refraction - wide-angle velocity models interpreted with geological constraints. The schematic crustal architecture of the southwest Grenville Province from southeast to northwest comprises allochthonous crustal elements (Frontenac-Adirondack Belt and Composite Arc Belt) that were assembled prior to ca. 1160 Ma, and then deformed and transported northwest over reworked rocks of pre-Grenvillian Laurentia and the Laurentian margin primarily between 1120 and 980 Ma. Reworked pre-Grenvillian Laurentia and Laurentian margin rocks are interpreted to extend at least 350 km southeast of the Grenville Front beneath all of the Composite Arc Belt. Three major structural boundary zones (the Grenville Front and adjacent Grenville Front Tectonic Zone, the Central Metasedimentary Belt boundary thrust zone, and the Elzevir-Frontenac boundary zone) have been identified across the region of the cross-section based on their prominent geophysical signatures comprising broad zones of southeast-dipping reflections and shallowing of mid-crustal velocity contours by 12-15 km. The structural boundary zones accommodated southeast over northwest crustal stacking at successively earlier times during orogeny (ca. 1010-980 Ma, 1080-1060 Ma, and 1170-1160 Ma, respectively). These shear zones root within an interpreted gently southeast-dipping regional décollement at a depth of 25-30 km corresponding to the top of a high-velocity lower crustal layer.

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40Ar–39Ar ages have been determined on single grains of biotite and amphibole from rocks on a 40 km traverse across the Grenville Front (GF) in the Temagami area, Ontario. Minerals in the vicinity of the GF contain excess Ar in varying amounts characterized by both anomalously high integrated and plateau ages. The excess Ar in biotite varies with distance from the GF, such that the overall age distribution along the GF forms an asymmetric wave-like pattern (an argonami) with two age maxima, one on either side of the GF. The integrated age maxima are 2.34 Ga for biotite and 3.21 Ga for amphibole located 6 and 0.2 km north of the GF, respectively, whereas maxima are 1.33 Ga for biotite and 1.50 Ga for amphibole 7 km south of the GF. This wave-like pattern of apparent ages shows a remarkable similarity with that seen in an earlier K–Ar study across the GF at Chibougamau, Quebec, and also with a small-scale pattern of excess Ar associated with the emplacement of a dyke. Variations in the magnitude of excess Ar in minerals can be interpreted in terms of the interaction of variable mineral blocking trajectories with the ambient partial pressure of Ar, and their patterns may also provide clues to underlying tectonic structures associated with the anomalies.The time–temperature relationships for minerals geographically located beyond the effects of significant excess Ar suggest that the crust in the Temagami area was subjected to more rapid cooling relative to gneisses of the interior of the Grenville Province.

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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