The Paleoproterozoic Marathon Large Igneous Province: New evidence for a 2.1 Ga long-lived mantle plume event along the southern margin of the North American Superior Province

Three Paleoproterozoic diabase dyke swarms, the Marathon, Kapuskasing and Fort Frances, which outcrop around the northern margin of Lake Superior, radiate from a focal region south of the lake. U–Pb dating shows that the Marathon and Kapuskasing swarms range in age between 2126 and 2101 Ma, and the Fort Frances between 2076 and 2067 Ma. The convergence of the dyke swarms to a common focal area and the lack of apparent polar wander during the time interval 2125–2067 Ma, suggest that a single, periodically active, plume was responsible. This 60 m.y. period of magmatism, manifest over an area of at least 400,000 km2, is designated as the Marathon Large Igneous Province. Only a single magnetic field reversal has been recognized throughout the 60 m.y. of plume activity. This observation, together with U–Pb geochronology, has helped to place dykes within each swarm in a chronological order based on their magnetic polarity. For the dyke population as a whole, older dykes tend to show more enriched and more fractionated incompatible trace element patterns compared to younger ones, a result which suggests that the Marathon, Kapuskasing and Fort Frances swarms are genetically related and underwent a secular reduction both in the degree of crustal contamination and/or depth of mantle melting over the course of plume activity. A similar geochemical relationship has also been found for the 2171–2166 Ma Biscotasing swarm but determining whether it is a common feature of plume development must await further study.