Abstract

The Keweenawan (1100 Ma) Mamainse Point volcanics, located along the eastern shore of Lake Superior in Ontario, formed in the Midcontinent Rift of North America. They are a 5250 m thick sequence of over 350 predominantly basaltic lava flows. The Mamainse Point section is the most continuous Keweenawan volcanic sequence and spans nearly the entire igneous history of the rift. The lower part of the section consists of high-MgO picrites and basalts, but the upper part of the section is composed of olivine tholeiites intercalated with numerous conglomerate layers. Major- and trace-element data reveal that the section consists of numerous stratigraphically constrained, geochemically distinct groups of lava flows. The comprehensive geochemical data on the entire sequence indicate that the section has no repetition due to faulting, as has been suggested by other workers on the basis of paleomagnetic studies. Evidently, the three paleomagnetic reversals previously found in the Mamainse Point section are real, and therefore there were multiple paleomagnetic reversals during Keweenawan time.

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