Abstract

A primary magnetization about 2.1 Ga old is well preserved in the Kenora–Kabetogama dyke swarm in Minnesota and Ontario. Its direction, the mean for 12 dykes, incorporating results from 30 paleomagnetic sites, is 115.9°, −55.6°, yielding a paleomagnetic pole position of latitude 42.7°S, longitude 4.2°E (dm = 7.3°, dp = 5.2°).The primary nature of the magnetization can be demonstrated using geochemistry to correlate individual dykes over distances that can exceed 300 km. Whereas the remanence direction varies between dykes, it remains constant along a dyke, sufficient proof that it was formed during initial cooling of the intrusion.A weak, longitudinal increase in hydrous alteration, MgO content, and degree of magnetic overprinting is observed as the swarm is followed to the south and may reflect increasing proximity to a Precambrian continental margin that underwent rifting, sedimentation, and deformation during the time interval 2.2–1.6 Ga BP. Although the cause of the longitudinal changes in terms of margin evolution can only be surmised, the results illustrate the potential of dyke swarms as sensitive indicators of how shield terrains may respond to events occurring around their margins.

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