PALEOMAGNETISM, U-PB GEOCHRONOLOGY, AND GEOCHEMISTRY OF MARATHON DYKES, SUPERIOR PROVINCE, AND COMPARISON WITH THE FORT-FRANCES SWARM

We report the first detailed study of the paleomagnetism, U-Pb geochro nology and major element geochemistry of Paleoproterozoic north-trendi ng Marathon dykes north of Lake Superior. The paleomagnetic and geoche mical results demonstrate that Marathon dykes can be divided into two subsets, one of normal magnetic polarity, the other of reverse polarit y. Normal and reverse Marathon paleomagnetic poles, at 43 degrees N, 1 96 degrees E (dm = 9 degrees, dp = 7 degrees, number of dykes N = 16) and 51 degrees N, 175 degrees E (dm = 9 degrees, dp = 6 degrees, N = 1 2), respectively, are statistically distinct and may indicate differen t ages of normal and reverse dyke emplacement. A U-Pb baddeleyite age of 2121(-7)(+14) Ma has been obtained at a normally magnetized Maratho n paleomagnetic site. The reversely magnetized Marathon dykes are unda ted, but have a paleopole rather close to that of the reversely magnet ized 2076(-4)(+5) Ma Fort Frances dykes and major element geochemical signatures as portrayed on Jensen plots that are identical to those of the Fort Frances swarm. Therefore, reverse Marathon and Fort Frances dykes could define a giant radiating dyke swarm focused south of Lake Superior, supporting models that associate these dykes with Paleoprote rozoic rifting along the southern margin of the Superior Province. The Marathon and Fort Frances paleopoles continue a northwesterly trend i n southern Superior Province paleopoles, which has recently been defin ed by results for 2216(-4)(+8) Ma Senneterre dykes and 2167 +/- 2 Ma B iscotasing dykes. This trend contrasts with previous widely used polar wander paths for the same period that young in the opposite direction and illustrates the importance of collaborative studies of paleomagne tism and U-Pb geochronology.