THE END OF MIDCONTINENT RIFT MAGMATISM AND THE PALEOGEOGRAPHY OF LAURENTIA

Paleomagnetism of the North American Midcontinent Rift provides a robust record of the paleogeography of Laurentia (cratonic North America) from ca. 1110 to 1070 Ma, revealing rapid equatorward motion of the continent throughout the main rifting episode. Existing age and paleomagnetic constraints on the youngest rift volcanic and sedimentary rocks have been interpreted to signal a slowdown of this motion as rifting waned. We present new high-resolution paleomagnetic and U-Pb geochronologic data from the 1090–1084 Ma “late-stage” volcanics exposed as the Lake Shore Traps (Michigan), the Schroeder-Lutsen basalts (Minnesota), and the Michipicoten Island Formation (Ontario). Paleomagnetic results from 40 and 23 additional cooling units lead to robust paleomagnetic poles for the Schroeder-Lutsen basalts and Michipicoten Island Formation, respectively. New temporal constraints are provided by high-precision, CA-ID-TIMS ²⁰⁶Pb/²³⁸U zircon dates from a Lake Shore Traps andesite, a Michipicoten Island tuff and a rhyolite, and a Beaver Bay Complex aplitic dike overlain by the Schroeder-Lutsen flows. Successful mitigation of Pb loss effects in zircon allows weighted mean ²⁰⁶Pb/²³⁸U dates to be calculated with internal uncertainties typically on the order of ±200 kyr to ±300 kyr (2σ), nearly an order of magnitude improvement in precision over the previous geochronology. These dates are the youngest yet obtained from Midcontinent Rift volcanics and indicate that rift magmatism was active for at least 25 million years. The addition of our late-stage paleomagnetic poles (Schroeder-Lutsen basalts and Michipicoten Island Formation) to the Laurentian apparent polar wonder path suggests that Laurentia’s rapid rate of motion was maintained throughout the entirety of rift volcanism.