THE MIDCONTINENT RIFT IN ONTARIO: A SHORT, BUT COMPLICATED TALE OF CRUSTAL SAGGING, SOFTENING, AND SPLITTING

William Logan first described the copper-bearing rocks of the Midcontinent Rift (MCR) in Ontario in 1863. Subsequent studies by a litany of researchers has lead to our current understanding of the MCR in Ontario, which comprises 4 main tectonic elements: 1) the volcanic and sedimentary rocks in and along the margin of Lake Superior, 2) the associated alkalic complexes, 3), the mafic and ultramafic intrusions centred on Lake Nipigon, and whether they represent a "failed arm" of the main rift, and 4) the interplay between the MCR and the Grenville orogen. Work in Ontario has tended to focus on the Lake Nipigon area, because of accessibility and mineral potential. Pre-conditioning of the Lake Nipigon area as the result of A-type granitic magmatism between 1535 and 1590 Ma, and deposition of the Sibley Group sediments, likely after 1450 Ma, may constitute an important fifth element. Study of the on-land portions of the MCR indicate the presence of an earlier period of magnetically normal, localized alkalic and mafic magmatism between 1150 and 1135 Ma, including the possibility of localized volcanism and sedimentation south of Armstrong. The bulk of the mafic and ultramafic rocks in the Lake Nipigon and Superior areas were emplaced in a short, magnetically reversed, internal between 1115 and 1100 Ma. Emplacement of alkalic intrusions, such as the Coldwell complex, as well as filling of much of the submerged part of the rift in Lake Superior, also occurred then. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore. Several pieces of evidence call into question the viability of the "failed arm" model. These include the fact that most north-trending dikes east and west of Lake Nipigon belong to the 2121 to 2100 Ma Marathon dike swarm, the lack of any thickening of the sills toward the centre of the basin or spectrum of ages across the basin, and the apparent continuity of major Archean structures across Lake Nipigon. MCR magmatism occurred during a hiatus in Grenville orogenesis from 1150 to 1090 Ma. Although the 2 events may not be directly related, heating of the lower crust in the Great Lakes region during MCR magmatism resulted in the ductile character of Grenvillian deformation between 1080 and 1040 Ma.