GEOCHRONOLOGY OF THE IGNEOUS ROCKS OF THE MIDCONTINENT RIFT SYSTEM: A REVIEW

Over 50 U/Pb isotopic analyses of zircon and baddeleyite, commonly on single grains, have provided an ever-expanding framework over the past 20 years for understanding the magmatic evolution of the Midcontinent Rift System. Many await formal publication and documentation; the following review includes data from abstracts.

A major paleomagnetic pole reversal (R to N), recorded throughout the Lake Superior basin, has been well constrained to the period from 1105 to 1102 Ma. This was a time of little or no recorded volcanic or subvolcanic activity, in contrast to the prolific magmatism both before and afterward. The latter, peaking from 1101 to 1094 Ma, produced the majority of both volcanic and plutonic rocks exposed (and thus sampled) around the Lake Superior district, including the bulk of the North Shore Volcanic Group and Duluth Complex, and the Beaver Bay Complex, the Mellen Complex, and the Portage Lake Volcanics. The “last gasps” of rift magmatism are represented by the Porcupine Volcanics (1093.6 Ma) and Lake Shore Traps (1087.2 Ma) in Michigan, a rhyolite on Michipicoten Island, ON (1086.5), and a rhyolite at the bottom of a drill hole near Munising, MI (1083 Ma).

The older, reversed-polarity rift magmatism is represented by many dates in a narrow range of 1108 +/-2 Ma, indicating an intense period of magma emplacement. These rock units include the Osler Group, Logan sills, Coldwell Complex, and part of the Mamainse Point Formation in Ontario, parts of the North Shore Volcanics and Duluth Complex in Minnesota, and most of the Powder Mill Group in Wisconsin and Michigan. There is still no date on the earliest volcanism in exposed Lake Superior sections. However, there is an increasing number of older dates ranging from 1110 to 1131 Ma on subvolcanic intrusions in rift-fringe areas (e.g. Nipigon sills, ON), and rhyolite from a drill hole near Minneapolis, MN (1131 Ma). Geophysical modeling shows that much if not most of the MRS igneous rocks, now buried under the lake and along the rift axis to the south, were emplaced during this early, reversed-polarity interval; these early, outlying dates may be representative of large volumes of unexposed rocks. The Lake Superior-radiating Abitibi dikes in Ontario, dated at 1140 Ma, may well be a symptom of the initial sublithospheric impact of the rift-generating plume.