Mantle temperature and lithospheric thinning beneath the Midcontinent rift system: evidence from magmatism and subsidence

The tectono-magmatic history of the Midcontinent rift system can be explained by the presence of a mantle plume bringing elevated-temperature mantle beneath the rift system at about 1110 Ma. Huge volumes of extrusive and intrusive igneous rocks were generated as abnormally hot mantle decompressed beneath the lithospheric rift. Geochemical and isotopic data from the Keweenawan volcanics show that the earliest melts were derived from small-degree melting of primitive plume mantle, coupled with enriched metasomatic melts derived from the continental lithosphere. As rifting progressed, the main bulk of the volcanics was generated primarily from the plume mantle, with the melting starting at depths of about 120 km and extending to as shallow as the base of the stretched lithosphère at 45 km depth. Elevated mantle temperatures of 1500–1560 °C, approximately 150–200 °C above normal, are inferred from the rare earth element concentrations in the volcanic rocks. Further constraints on the mantle temperature come from combined subsidence and melt-generation modelling. I assume that rifting occurred in two main periods, during 1110–1105 and 1100–1094 Ma, with a reduced rate of stretching and greatly decreased melt production during the intervening period, 1105–1100 Ma. At the centre of the rift, production of more than 15 km of volcanic rocks close to, or above, sea level was followed by the accumulation of up to 8 km of mainly coarse terrigenous sediments in the postrift subsidence phase. This can be explained by lithospheric thinning by a factor of approximately 6 above mantle with a potential temperature of about 1550 °C. Subsequently, the mantle cooled to a normal potential temperature of 1350 °C as the plume thermal anomaly died away.