U-TH-PH ZIRCON AGES OF SOME KEWEENAWAN SUPERGROUP ROCKS FROM THE SOUTH SHORE OF LAKE SUPERIOR

New single-crystal zircon U-Th-Pb ages for plutonic and rhyolitic Kewe enawan Supergroup rocks from the south shore of Lake Superior provide geochronological constraints on magmatic evolution associated with the 1.1 Ca Midcontinent rift. Analyses of a granophyric phase of the Mine ral Lake intrusion and the Mellen granite, both parts of the Mellen In trusive Complex, and a laterally extensive rhyolite from the top of th e Kallander Creek Volcanics have weighted average Pb-207/Pb-206 ages o f 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 tile Porcupine Volcanics result in Variable Pb-207/Pb-2 06 ages that range from 1080 to 1137 Ma. This rhyolite exhibits a cont inuum between morphologically complex and simpler prismatic zircon cry stals, the latter yielding concordant analyses having a weighted avera ge Pb-207/Pb-206 age of 1093.6 +/- 1.8 Ma (N = 2). Four prismatic zirc ons from an aphyric rhyolite of the Chengwatana Volcanics in the Ashla nd syncline form a linear array intersecting concordia at 1094.6 +/- 2 .1 Ma (MSWD = 1.3). Another presumed Chengwatana rhyolite recovered fr om drill core intersecting the Hudson-Afton horst in southeast Minneso ta yielded only similar to 20 morphologically indistinguishable zircon s. Six analyses give Pb-207/Pb-206 ages ranging from 1112 to 1136 Ma, including one analysis with a virtually concordant age of 1130 Ma. Thi s age, however, is considerably older than that obtained for the Cheng watana Volcanics in the Ashland syncline or any other precisely dated rock from the Midcontinent rift.