Paleomagnetism of the Abitibi dyke swarm, southern Superior Province, and implications for the Logan Loop

Abstract

The trace of the Middle Proterozoic Logan Loop of the North American apparent polar wander path is controversial. The older 1270–1110 Ma limb of the loop is poorly constrained, while the depth of the loop, based on the 1110–1080 Ma Keweenawan data of the younger limb, is thought by some to be largely an artifact of reversal asymmetry in the Earth's magnetic field. Paleomagnetism of the 1141 Ma Abitibi mafic dyke swarm is one of the keys to constraining the geometry of the Logan Loop.Unfortunately, previous paleomagnetic studies failed to distinguish dykes of the northeast-trending Middle Proterozoic olivine-bearing Abitibi swarm from subparallel Early Proterozoic olivine-free Biscotasing (formerly Preissac) dykes, and hence paleomagnetic poles determined in these studies should no longer be used. In the present study, sampling of eight Abitibi dykes has identified 23 normally magnetized sites in four dykes and, for the first time, five reversely magnetized sites in three dykes. One of the normally magnetized sites corresponds to the locality for which a high-precision U–Pb age was previously reported. A baked contact test establishes that the characteristic remanence of one normally magnetized dyke is primary. In addition, sites along individual dykes exhibit much smaller secular variation than is observed between dykes, indicating that the remanences of the other dykes are also primary. One of the normally magnetized dykes, the 700 km long Great Abitibi dyke, exhibits two primary directions that correspond to two geochemically distinct magma pulses. The five normally magnetized units, which consist of four separate dykes plus the second pulse of the Great Abitibi dyke, yield a well-defined mean paleomagnetic pole at 42.8°N, 151.5°W, dm = 16.3°, dp = 12.5°. It falls close to the reversely magnetized poles from the Keweenawan Track and establishes a minimum depth for the Logan Loop of about 40°. The reversed-polarity data from three other dykes are more scattered and may not average out secular variation. Hence, the present study is inconclusive regarding asymmetry of the Earth's magnetic field at 1141 Ma, even though a mean pole based on combined normal-and reversed-polarity dykes is indistinguishable from that based on normal-polarity dykes alone.