A PROPOSED GIANT RADIATING DYKE SWARM FRAGMENTED BY THE SEPARATION OF LAURENTIA AND AUSTRALIA BASED ON PALEOMAGNETISM OF CA.780 MA MAFIC INTRUSIONS IN WESTERN NORTH-AMERICA

We propose that mafic dykes and sheets, recently dated at 780 Ma, in three widely separated areas of western North America (the Canadian Shield, the Mackenzie Mountains in the northern Cordillera, and the Wyoming Province) may represent subswarms of a giant radiating dyke swarm. Paleomagnetic data from these intrusions demonstrate the general tectonic integrity of the whole region, which further suggests that relative strikes of the subswarms are primary. The roughly radial pattern of the subswarms is focused off the western coast of North America and may define the ancient location, relative to North America, of a mantle plume responsible for the giant radiating dyke swarm. We suggest that the recently proposed ca. 800 Ma plume centre defined by the Willouran volcanic rocks in central-southern Australia and the associated Gairdner dyke swarm could represent, respectively, the missing plume centre and a missing subswarm of our proposed giant radiating dyke swarm. If so, the giant radiating dyke swarm was fragmented sometime after 780 Ma by the breakup of the supercontinent which included Laurentia and Australia. Indeed, the 780 Ma magmatism may have been a precursor to the breakup.