The Upper Precambrian plateau lavas of the Lake Superior region were produced in response to tensional rifting of North America roughly 12001120 x 106 years ago (White, 1972a). Geological and geophysical evidence shows that they consist of a group of about eight separate volcanic accumulations which partly overlap in time and space as the locus and activity of rifting changed along what is now the Midcontinent Gravity High (Green, 1977). Nearly all of these lava accumulations ("plateaus") contain preserved sections which range in thickness from 2.5 to 7 km and are made of hundreds of individual flows. They have subsided centrally during and/or after eruption. Basalt of various compositions predominates; the most common type is nonporphyritic olivine tholeiite with unusually high Al content and ophitic texture. Transitional to weakly alkaline basalt is also common. Quartz tholeiite, basaltic andesite and rhyolite are moderately abundant in most of the plateaus, and icelandite (intermediate quartz latite) is found in some.
Prepared for the Annual Meeting of THE GEOLOGICAL SOCIETY OF AMERICA, NORTH-CENTRAL SECTION and THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Duluth, Minnesota, 1979, 22 p.
Green, John C..
Guidebook 11. Field Trip Guidebook for the Keweenawan (Upper Precambrian) North Shore Volcanic Group, Minnesota.
Minnesota Geological Survey.
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