THE RICH PALEOMAGNETIC RECORD OF PROTEROZOIC MIDCONTINENT RIFT INTRUSIVES: AN UPDATED SYNTHESIS WITH A NEW POLE FROM THE BEAVER RIVER DIABASE

Paleomagnetic data are essential for studying the paleogeography of Laurentia. A recent compilation of the Laurentia apparent polar wander path (APWP) based on Midcontinent Rift volcanics coupled with high-precision geochronology data has shown a very rapid motion of Laurentia from high to low latitude. Studies from previous decades have also developed a rich paleomagnetic record of many Keweenawan intrusive complexes coeval with the volcanics. We compile existing intrusive paleomagnetic data and contextualize them in a modern geological and geochronological framework. In addition, we develop new paleomagnetic data from the intrusive Beaver Bay Complex. In particular, we have focused on adding new data from the extensive mafic dike and sill network called the Beaver River diabase as well as from anorthosite xenoliths within the diabase. These xenoliths range in size from centimeters to over a hundred meters.

The Beaver River diabase intrudes the Palisade rhyolite (1093.94 ± 0.28 Ma) and is intruded by the Silver Bay aplite (1091.61 ± 0.14 Ma). These high-precision U-Pb dates bracket the age of the diabase, making it an excellent target for investigating the geomagnetic pole position during the latter part of the main stage of Keweenawan magmatism. In addition, a cooling model suggests that the anorthosite xenoliths would have acquired a full thermal remanent magnetization during emplacement and cooling of the diabase sills. New paleodirectional data derived from diabase and anorthosite sites show good agreement between sites and across lithologies. Tilt-corrected paleomagnetic pole positions from both lithologies fall within the Keweenawan APWP based on the volcanics between ca. 1090 Ma and ca. 1095 Ma, which is consistent with the geochronology constraints.

Once integrated with the records from Midcontinent Rift volcanics, paleomagnetic pole positions from the Beaver Bay Complex together with our updated intrusive paleomagnetic data compilation further constrain the plate motion of Laurentia. We also better constrain the age of intrusive units with paleomagnetic data but poor or no radiometric geochronology by comparison with the improved Laurentia APWP given the significant plate motion that occurred through Midcontinent Rift development.