TIME RESOLUTION OF GEOLOGIC EVENTS ON THE KEWEENAW PENINSULA AND IMPLICATIONS FOR DEVELOPMENT OF THE MIDCONTINENT RIFT SYSTEM

Three flood-type lava flows from the Keweenaw Peninsula in upper Michigan have been precisely dated, using UPb analyses of zircon and baddeleyite. Zircons were extracted from pegmatoid segregations within two basaltic flows. Ages from the Copper City Flow, near the base of the Portage Lake Volcanics, and the Greenstone Flow, near the top, are 1096.2 ± 1.8 Ma and 1094.0 ± 1.5 Ma, respectively (95% confidence errors). UPb analyses of baddeleyite from these samples are in general agreement with the zircon results although some fractions appear slightly older, possibly due to 222Rn loss. The age of an andesite flow from the Lake Shore Traps, 1000 m above the Portage Lake Volcanics and within the Copper Harbour Conglomerate, is 1087.2 ± 1.6 Ma from analyses of zircon. This gives an age for sedimentation within the Copper Harbor Conglomerate. These ages, along with stratigraphic data, indicate a subsidence rate of 1.3-0.6+1.8 mm/y during eruption of the Portage Lake Volcanics and 0.2 ± 0.1 mm/y (1σ errors) for the transitional period from flood basalt eruption to clastic sedimentation. Magma eruption rates were at least 0.02-0.05 km3/y during emplacement of the Portage Lake Volcanics. Rates as high as 0.2-0.6 km3/y are inferred from regional stratigraphic data and currently available precise dates from throughout the Lake Superior basin. These values are comparable to the high rates of magma production estimated for Phanerozoic continental flood basalt provinces. Analyses of three detrital zircon grains from a sandstone at the base of the Copper Harbor Conglomerate give ages from 1101 to 1106 Ma. A fraction consisting of 29 detrital zircon grains has an age of 1104 ± 2 Ma indicating that sediments within the Copper Harbor Conglomerate were probably derived almost entirely from lower Keweenawan rocks. Since the Portage Lake Volcanics are paleomagnetically normal, the age of the Keweenawan paleomagnetic polarity reversal is now bracketed between 1096.2 ± 1.8 Ma and 1097.6 ± 3.7 Ma, the age of a reversed flow within the Osler Group. From this and other studies it appears that apparent polar wander rates are different for reversed and normal poles and that normal and reversed paleomagnetic poles of comparable age have different geographic positions. This suggests that the asymmetry between reversed and normal Keweenawan poles for igneous rocks may be due to a non-geocentric dipole. © 1990.