Abstract
Continental rifting at 1.1 Ga produced approximately 1.3 × 106 km3 of igneous rocks over a rift length of 2200 km (Midcontinent Rift System, MRS). These constitute one of the largest as well as oldest flood basalt provinces. Both geophysical (Hutchinson et al. 1990) and geochemical evidence (Nicholson and Shirey 1990) suggest that magma production resulted from decompression melting of a large, hot mantle plume centered beneath what is now Lake Superior. These rocks are now exposed only around Lake Superior in Ontario, Michigan, Wisconsin, and Minnesota, and in east-central Minnesota and northwestern Wisconsin; the rest of the igneous rift products are known from geophysics and scattered drill holes. The plateau lavas are geochemically bimodal, dominated by olivine tholeiites (many of them high-Al), transitional basalts, and basaltic andesites, with significant volumes of rhyolites, especially in Minnesota (Green 1982). Structural and stratigraphic relationships of these lavas indicate that they were erupted in several distinct plateaus, each of which subsided centrally as rifting intensity shifted in time and place along the MRS (Green 1977; 1983), but age determinations so far show no “unzipping” progression (or hot-spot progression) from Lake Superior to the extremities in Kansas and southeastern Michigan. The maximum known time span of MRS (Keweenawan) magmatism, from recent precise U/Pb zircon work by D.W. Davis and co-workers, is 1109 to 1086 Ma, but the greatest bulk of eruptive and intrusive activity (for exposed rocks) occurred during a magnetically reversed interval around 1108–1105 Ma and especially during a magnetically normal interval around 1097–1094 Ma.
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Green, J.C. (1995). Volcanic Rocks of the Midcontinent Rift System: A Review. In: Ojakangas, R.W., Dickas, A.B., Green, J.C. (eds) Basement Tectonics 10. Proceedings of the International Conferences on Basement Tectonics, vol 4. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-0831-9_13
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DOI: https://doi.org/10.1007/978-94-017-0831-9_13
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