U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior

Abstract

New single-crystal zircon U–Th–Pb ages for plutonic and rhyolitic Keweenawan Supergroup rocks from the south shore of Lake Superior provide geochronological constraints on magmatic evolution associated with the 1.1 Ga Midcontinent rift. Analyses of a granophyric phase of the Mineral Lake intrusion and the Mellen granite, both parts of the Mellen Intrusive Complex, and a laterally extensive rhyolite from the top of the Kallander Creek Volcanics have weighted average 207 Pb/ 206 Pb ages of 1102.0 ± 2.8 Ma ( N = 2), 1100.9 ± 1.4 Ma ( N = 5), and 1098.8 ± 1.9 Ma ( N = 4), respectively. Analyses of a pyroclastic rhyolite flow at the top of the Porcupine Volcanics result in variable 207 Pb/ 206 Pb ages that range from 1080 to 1137 Ma. This rhyolite exhibits a continuum between morphologically complex and simpler prismatic zircon crystals, the latter yielding concordant analyses having a weighted average 207 Pb/ 206 Pb age of 1093.6 ± 1.8 Ma ( N = 2). Four prismatic zircons from an aphyric rhyolite of the Chengwatana Volcanics in the Ashland syncline form a linear array intersecting concordia at 1094.6 ± 2.1 Ma (MSWD = 1.3). Another presumed Chengwatana rhyolite recovered from drill core intersecting the Hudson–Afton horst in southeast Minnesota yielded only ~20 morphologically indistinguishable zircons. Six analyses give 207 Pb/ 206 Pb ages ranging from 1112 to 1136 Ma, including one analysis with a virtually concordant age of 1130 Ma. This age, however, is considerably older than that obtained for the Chengwatana Volcanics in the Ashland syncline or any other precisely dated rock from the Midcontinent rift.