Paleomagnetism of the ~1.1 GA Portage Lake Volcanics (Michigan, USA)
Thursday, 18 December 2014
The ~1094 Ma Portage Lake Volcanics (PLV) consist of more than 200 lava flows and represent the most voluminous phase of the North American Midcontinent Rift (MCR) extrusive magmatism. Paleomagnetism of the PLV basalts has been investigated since the 1960s, but most of the paleomagnetic datasets are affected by remagnetization due to an extensive mineralization event that affected the PLV flows in the central part of the Keweenaw Peninsula. We report new paleomagnetic data from 74 and 35 PLV lava flows exposed at the eastern tip of the Keweenaw Peninsula and on Isle Royale, respectively. These lava flows represent the uppermost part of the PLV sequence and are much less affected by mineralization and metamorphism than their counterparts in the center of the Keweenaw Peninsula and the older PLV flows. Detailed thermal demagnetization reveals multiple natural remanent magnetization (NRM) components. In addition to a soft, low-temperature NRM component, removed by heating to 350-375°C, two components carried by magnetite were observed. A secondary magnetite component was isolated between ~375 and 525°C. The characteristic component of NRM was isolated between ~525°C and 585°C. The primary origin of this component is supported by positive paleomagnetic field tests. While the magnetite components have the same declination, the primary remanence is characterized by systematically steeper inclinations. The PLV lava flows also reveal a high-temperature NRM component carried by hematite. The direction of the hematite remanence is indistinguishable from that of the primary magnetite component. The new high-quality paleomagnetic pole for the PLV is obtained. We will discuss implications of the new paleomagnetic data for the MCR evolution as well as for the North American apparent polar wander path and plate motion rate at ~ 1.1 Ga.