Abstract

Normally magnetized dikes and reversely magnetized sills of Neohelikian age near the northwest shore of Lake Superior form two distinct paleomagnetic groups with mean pole positions of 179° W, 35° N, and 140° W, 47° N respectively. Thermal and alternating field demagnetization and the study of magnetic properties and opaque minerals indicate that directions of magnetization of these rocks were acquired at the time of their intrusion. Field evidence suggests that the sills are older than the dikes and radiogenic age determinations indicate that the intrusion of both occurred between 1000 and 1100 m.y. ago.These pole positions, together with those for the Franklin intrusions pole at 167° E, 08° N (675 m.y.), the Abitibi dikes, at 134° W, 27° N (1150 m.y.), and the Mackenzie igneous events, at 171° W, 4° N (1200 m.y.) are used to define the Logan Loop, the path that the pole took in Neohelikian time relative to the Canadian Shield. Other poles well defined magnetically, but less well dated, from rocks of this era, fit the curve quite well.Analysis of available data supports the hypothesis that the relative polar movement that gave rise to the Logan Loop was preceded and followed by polar stability vis à vis North America, whereas polar movement may have been quite rapid during the formation of the loop, which coincided with the time of the Grenvillian orogeny. Comparison of the depositional environment of Neohelikian rocks of the Canadian Shield with their probable paleolatitude as indicated by the 5 key points on Logan Loop should provide a test for the validity of this movement.

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