8: Geochronology of Keweenawan rocks of the Lake Superior region: A summary
W. R. Van Schmus, J. C. Green, H. C. Halls, 1982. "8: Geochronology of Keweenawan rocks of the Lake Superior region: A summary", Geology and Tectonics of the Lake Superior Basin, Richard J. Wold, William J. Hinze
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There have been many U-Pb, Rb-Sr., and K-Ar geochronologic studies of Keweenawan rocks. U-Pb results on zircons by Silver and Green show that most of the igneous activity (by volume) occurred 1,110 ± 10 m.y. ago in the Lake Superior region. This includes rocks of the upper normal magnetic polarity sequence as well as upper units of the underlying reversed magnetic polarity sequence, thus dating that reversed-to-normal change at 1,110 ± 10 m.y. ago.
Many of the Rb-Sr and K-Ar results on the 1,110 m.y. old units are concordant although in many other instances there is clear discordance with the Rb-Sr and K-Ar ages being too young. Other units, not yet dated by U-Pb methods, also give young ages, potentially suggesting that upper Keweenawan igneous activity may have extended to as young as 900 m.y. ago. However, review of paleomagnetic pole positions for such units shows no evidence for such young crystallization ages; the paleomagnetic data are consistent with all younger units being about 1,110 m.y. old.
K-Ar results suggest ages of 1,150-1,250 m.y. for stratigraphically older units (for example, Logan Sills) of the reversed sequence and, along with Rb-Sr results, for normal polarity dikes of the Sudbury dike swarm. Paleomagnetic pole positions are also consistent with early Keweenawan igneous activity occurring about 1,200 m.y. ago. Thus, we conclude that Keweenawan rifting and associated igneous activity began 1,200-1,225 m.y. ago, peaked at 1,110 m.y. ago, and ceased shortly thereafter.