PALEOMAGNETISM AND GEOCHEMISTRY OF ~1144-MA LAMPROPHYRE DIKES, NORTHWESTERN ONTARIO: IMPLICATIONS FOR THE NORTH AMERICAN POLAR WANDER PATH AND PLATE VELOCITIES

The ~1.1 Ga North American Midcontinent Rift (MCR) represents the best-preserved example of a failed intra-continental rift. Although paleomagnetic, geochronological, and geochemical studies of the MCR rocks from the main and final stages of the MCR are abundant, data from the precursor stage remain limited. Here we present new paleomagnetic and geochemical data from a suite of the ~1144 Ma ultramafic lamprophyre dikes that outcrop around northeastern Lake Superior (Ontario, Canada). Nineteen out of the 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (N=5) and reversed (N=14) polarity. The primary origin of this direction is confirmed by positive baked contact tests and a reversal test. The group mean directions of the lamprophyre dykes (D=306.4°, I=72.1°, α₉₅=5.5°, N=19) and the previously reported nearly coeval ~1142 Ma Abitibi dikes (D=297.4°, I=65.5°, α₉₅=8.3°, N=8) are statistically indistinguishable. The geochemistry of the lamprophyres suggests an affinity with magmas derived from Ocean Island Basalt‐ type mantle sources, consistent with the mantle plume hypothesis for the MCR. The similarity in age, trend, paleomagnetism, and geochemistry indicate that the lamprophyre and Abitibi dikes represent the earliest magmatic event associated with the rifting. The combined mean direction (D=303.1°, I=70.2°, α₉₅=4.5°, N=27) corresponds to a paleomagnetic pole at Plat=55.8°N and Plong=220.0°E (A₉₅=7.3°). The new pole merits the highest classification on the Q‐scale of paleomagnetic reliability and represents a key pole defining the North American apparent polar wander path during the late Mesoproterozoic. Combined with data from the ~1108 Ma Coldwell Complex, our data indicate an equator‐ward motion of Laurentia at 3.8±1.4 cm/yr, comparable with the present‐day velocities of continental plates, before switching to very rapid motion between ~1108 Ma and ~1099 Ma.