Abstract

Some areas of the Canadian Shield are profusely intruded by swarms of subparallel basaltic dykes. These typically have a diabasic texture and formerly were considered to be strictly the intrusive equivalents of tholeiitic flood basalt.About 650 oriented samples were collected from about 25 dyke swarms and preliminary palaeomagnetic data from six of them are presented here. The Mackenzie swarm has a K–Ar age of 1 295 million years, occurs throughout the western Canadian Shield, and is the most extensive swarm of basic dykes known anywhere in the world. The other swarms are the Molson dykes (1 445 m.y.) in northeastern Manitoba, the Marathon dykes (1 810 m.y.) just north of Lake Superior, the Sudbury dykes (1 285 m.y.) of southeastern Ontario, the Matachewan dykes (2 485 m.y.) of western Quebec and northeastern Ontario, and the Abitibi dykes (1 230 m.y.), which occur from Sault Ste. Marie, Ontario, to Lake Mistassini, Quebec.The palaeomagnetic virtual pole positions of six swarms were derived from the mean of their measured remanent magnetization directions. These directions of magnetization were determined from the samples after they were magnetically washed in an a-c. field of 80 or 130 oersteds. The reasons for the dispersion within the individual swarms are discussed. The pole positions of the Sudbury and Mackenzie dykes are almost identical and the two swarms are the same age within the limits of analytical uncertainty. However, basalt of the Sudbury swarm is alkalic and more undersaturated than basalt of the Mackenzie dykes.No evidence was found to contradict the usual assumption that unaltered basic igneous rocks of this type acquire and generally retain a stable magnetization which was parallel with the earth's magnetic field at the time of the dyke intrusion.

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