PASSIVE RIFTING – TIME TO RETHINK THE PLUME PARADIGM FOR THE MIDCONTINENT RIFT

Although widely accepted, the mantle plume origin for the Midcontinent Rift (MCR) is not fully consistent with recent geochronological, geochemical and mineralogical data making it necessary to evaluate alternate models. Evidence against a simple plume model includes new geochronological data which shows that MCR magmatism spans at least 20 million years compared to the majority of plume-related Large Igneous Provinces (LIPs) which are characterized by a short-duration magmatic pulse or pulses (less than 1–5 m.y.). Similarly, unlike plume-related LIPS, which are typically associated with, or even recognized by, the presence of giant, radiating dike swarms up to 3000 km long, no radiating dike swarm has been associated with the MCR. Rather, the majority of MCR-related dikes occupy extensional, rift arm-parallel structures, which is not consistent with a central piercing point, like a plume. An argument often put forward in favour of a plume origin for the MCR is the presence of ultramafic rocks. Although there are, indeed, ultramafic rocks in the MCR, all of which are hosted in intrusions, mineral chemistry analyses from these intrusions shows that they have maximum olivine forsterite compositions not in equilibrium with the mantle but rather are consistent with a parental magma with 8-10 wt% MgO.

The long duration of MCR magmatism, absence of primary ultramafic magmas and lack of a radiating dike swarm all suggest that a passive rifting model may be more appropriate for the rift with upwelling of material underplated by earlier plume events thought to have been centered in the vicinity of the present-day Lake Superior (e.g. the Marathon LIP) accounting for the OIB-like geochemistry of the MCR. Alternatively, the presence of recently recognized ~1100 Ma magmatism in the Kalahari Craton, the Congo, the Amazon and India, suggests that there may have been a widespread perturbation of the Earth’s mantle that manifested as one or more mantle plume events. The presence of multiple, broadly contemporaneous LIP events suggests that the Mesoproterozoic may have been a period of significant mantle and atypically increased magmatic activity.