PALEOMAGNETISM OF THE ABITIBI DYKE SWARM, SOUTHERN SUPERIOR-PROVINCE,AND IMPLICATIONS FOR THE LOGAN LOOP

The trace of the Middle Proterozoic Logan Loop of the North American a pparent polar wander path is controversial. The older 1270 - 1110 Ma l imb of the loop is poorly constrained, while the depth of the loop, ba sed on the 1110 - 1080 Ma Keweenawan data of the younger limb, is thou ght by some to be largely an artifact of reversal asymmetry in the Ear th's magnetic field. Paleomagnetism of the 1141 Ma Abitibi mafic dyke swarm is one of the keys to constraining the geometry of the Logan Loo p. Unfortunately, previous paleomagnetic studies failed to distinguish dykes of the northeast-trending Middle Proterozoic olivine-bearing Ab itibi swarm from subparallel Early Proterozoic olivine-free Biscotasin g (formerly Preissac) dykes, and hence paleomagnetic poles determined in these studies should no longer be used. In the present study, sampl ing of eight Abitibi dykes has identified 23 normally magnetized sites in four dykes and, for the first time, five reversely magnetized site s in three dykes. One of the normally magnetized sites corresponds to the locality for which a high-precision U - Pb age was previously repo rted. A baked contact test establishes that the characteristic remanen ce of one normally magnetized dyke is primary. In addition, sites alon g individual dykes exhibit much smaller secular variation than is obse rved between dykes, indicating that the remanences of the other dykes are also primary. One of the normally magnetized dykes, the 700 km lon g Great Abitibi dyke, exhibits two primary directions that correspond to two geochemically distinct magma pulses. The five normally magnetiz ed units, which consist of four separate dykes plus the second pulse o f the Great Abitibi dyke, yield a well-defined mean paleomagnetic pole at 42.8-degrees-N, 151.5-degrees-W, dm = 16.3-degrees, dp = 12.5-degr ees. It falls close to the reversely magnetized poles from the Keweena wan Track and establishes a minimum depth for the Logan Loop of about 40-degrees. The reversed-polarity data from three other dykes are more scattered and may not average out secular variation. Hence, the prese nt study is inconclusive, regarding asymmetry of the Earth's magnetic field at 1141 Ma, even though a mean pole based on combined normal- an d reversed-polarity dykes is indistinguishable from that based on norm al-polarity dykes alone.