Skip to Main Content
Book Chapter

8: Geochronology of Keweenawan rocks of the Lake Superior region: A summary

By
W. R. Van Schmus
W. R. Van Schmus
Search for other works by this author on:
J. C. Green
J. C. Green
Search for other works by this author on:
H. C. Halls
H. C. Halls
Search for other works by this author on:
Published:
January 01, 1982

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.

You do not currently have access to this article.

Figures & Tables

Contents

GSA Memoirs

Geology and Tectonics of the Lake Superior Basin

Richard J. Wold
Richard J. Wold
Search for other works by this author on:
William J. Hinze
William J. Hinze
Search for other works by this author on:
Geological Society of America
Volume
156
ISBN print:
9780813711560
Publication date:
January 01, 1982

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal