PALEOMAGNETISM OF THE MIDCONTINENT RIFT SYSTEM INTRUSIVES IN THE THUNDER BAY AREA (ONTARIO, CANADA): A REVIEW

Numerous mafic sills and dykes found in the Thunder Bay area (Ontario, Canada) arguably represent the northern extension of the ~1.1 Ga Midcontinent Rift System (MRS). Recent geochemical and geochronological studies have significantly improved our understanding of the MRS sequences in this area. We will present the results of our paleomagnetic and rock magnetic investigation of a suite of MRS intrusives exposed in the vicinity of Thunder Bay. Paleomagnetic directions have been obtained from 490 independently oriented samples from 53 sites (37 dykes and two gabbroic units) using detailed thermal and alternating field demagnetizations preceded by low-temperature magnetic cleaning. After removal of a soft viscous overprint, most samples reveal a single-component characteristic remanent magnetization (ChRM). Rock magnetic analyses, including magnetic hysteresis and thermomagnetic behavior, suggest pseudo-single-domain low-Ti titanomagnetite as the principal magnetic carrier in the studied rocks. Most of the paleomagnetic directions show the typical Keweenawan asymmetry of the normal and reverse polarity directions. A positive dyke-to-dyke baked contact test from an E-W trending Pigeon River dyke cutting through a NNW trending Cloud River dyke confirms the primary nature of ChRM. We re-evaluate the previously published paleomagnetic data for the MRS intrusives in this area (90 dykes and 50 sills) and combine them with our data. We analyze the updated Thunder Bay paleomagnetic database together with the high quality geochronological and petrographical observations, and detailed geochemical and isotope data, and discuss the implications of these results for the magnetostratigraphy of MRS and the tectonic history of the rifting.