PALEOMAGNETISM OF PRECISELY-DATED MAFIC MAGMATIC EVENTS AND THE DRIFT OF SUPERIOR AND SLAVE CRATONS IN THE PALEOPROTEROZOIC

Only a handful of Paleoproterozoic paleomagnetic poles in the global database are sufficiently well-dated (ie., are obtained from precisely-dated rock units and are demonstrated primary) to be useful for rigorous tests of continental drift and paleocontinental reconstructions. Many of these “key” paleopoles are derived from gabbro dykes and sills in North America. Key poles of nine distinct Paleoproterozoic ages are now available from the Superior craton of east-central North America. They outline a rudimentary apparent polar wander path (APWP) that reflects the craton's drift through much of this period, especially between 2.22 and 1.88 Ga. The Superior “APWP” can be used as a reference track for comparison with key poles from other cratonic blocks in North America or elsewhere around the world. The Superior data are of sufficient quality that it is possible to compare results from either side of the Kapuskasing structural zone, a prominent zone of crustal uplift separating the eastern and western portions of the craton. Comparisons of such data at 2.45 Ga, 2.17 Ga and 2.08-2.07 Ga have recently demonstrated that a 10-20° relative vertical-axis rotation of the eastern and western Superior occurred some time after 2.07 Ga.

The first key Paleoproterozoic paleopole is reported for the Slave craton of northwestern North America. It was obtained from the 2.025 Ga Lac de Gras diabase dyke swarm. Comparison of the Lac de Gras pole with the Superior reference “APWP” demonstrates that the Slave and Superior cratons were not in their present relative positions at the time of Lac de Gras dyke emplacement. Paleopoles are also now available from four other precisely-dated (2.23-1.88 Ga) Paleoproterozoic dyke swarms in the Slave craton. However, they are not considered key poles, because either the remanences have not been demonstrated primary or the ages and poles were not determined from the same dykes. If the apparent ages of these poles are confirmed by further study, they, along with the Lac de Gras pole, would outline an APWP segment for the Slave craton between 2.23 and 1.88 Ga that differs in shape from that of the Superior craton in the same time interval. This would indicate that the two cratons were drifting separately during a portion of this period, rather than drifting together as part of a single supercontinent.