Paleomagnetism and Geochemistry of ~1144-Ma Lamprophyre Dikes, Northwestern Ontario: Implications for the North American Polar Wander and Plate Velocities

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©2018. American Geophysical Union. All Rights Reserved. We present new paleomagnetic and geochemical data from a suite of the ~1144-Ma ultramafic lamprophyre dikes that outcrop in the Canadian Shield northeast of Lake Superior (Ontario, Canada). Nineteen of 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (n = 5) and reversed (n = 14) polarity. The primary origin of characteristic remanent magnetization is bolstered by positive baked contact tests and a reversal test. The group mean direction (D = 306.4°, I = 72.1°, α95 = 5.5°, N = 19) obtained from the lamprophyre dikes is statistically indistinguishable from the group mean direction (D = 297.4°, I = 65.5°, α95 = 8.3°, N = 8) previously reported for the nearly coeval ~1142-Ma Abitibi dikes. The geochemistry of the lamprophyre dikes suggests strong affinity with magmas derived from ocean island basalt-type mantle sources, consistent with the mantle plume hypothesis for the formation of the ~1.1-Ga North American Midcontinent Rift. The similarity in age, trend, paleomagnetism, and geochemistry indicates that the lamprophyre and Abitibi dike suites represent the earliest magmatic event associated with the commencement of 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, 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 high-quality data from the ~1108-Ma Coldwell Complex, our data indicate an equatorward motion of Laurentia at 3.8 ± 1.4 cm/year, comparable with the present-day velocities of continental plates, before switching to extremely rapid motion between ~1108 and ~1099 Ma.

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Journal of Geophysical Research: Solid Earth