Temporal evolution of mantle temperature and lithospheric thickness beneath the ∼1.1 Ga Midcontinent Rift, North America: Implications for rapid motion of Laurentia

Abstract

The production of the magma volumes necessary for the formation of continental flood basalts is thought to result from extension-related decompression melting of thermo-chemically anomalous mantle. The relative importance of elevated mantle temperature or decompression associated with lithospheric thinning remains difficult to constrain because the temporal relationship between the initiation of extension and continental flood basalt is often ambiguous. The ∼1.1 Ga Midcontinent Rift (MCR) in North America provides an opportunity to probe these magma generation factors because the mantle thermo-chemical anomaly (Keweenaw plume) thought responsible for the continental flood basalts of the Keweenaw Large Igneous Province (LIP) intersected an existing rift. The role of elevated mantle temperatures in Keweenaw LIP magmatism remains ambiguous because a temporal decrease in MgO content in primitive lavas may reflect decreasing melting depths with time due to progressive lithospheric thinning, a temporal decrease in mantle temperature, or both. Furthermore, paleomagnetic studies show that Laurentia was moving rapidly during Keweenaw LIP magmatism, raising the question of how the plume remained connected to the MCR during 25 Myr of volcanism. Using samples from the most complete volcanic section preserved at Mamainse Point, Ontario, Canada, we constrain mantle potential temperature and lithospheric thickness by combining three major element thermobarometry models with forward modeling of rare earth element compositions. We find that the earliest stage of Keweenaw LIP magmatism (Early phase; 1109-1104 Ma) was governed by initially deeper melting (>85-100 km thick lithosphere) and high mantle potential temperatures (>1480-1630°C) related to the plume. Potential temperatures decreased to near-ambient conditions (∼1400-1445°C) during the latter portion of the Early phase, with these lower temperatures persisting throughout the Latent (1104-1098 Ma) and Main (1098-1090 Ma) phases. Major element-derived pressures of melt equilibration place broad constraints on lithospheric thickness, which decreased from ∼60-110 km at the onset of magmatism to 45-65 km by the Main phase. The decrease in primitive MgO contents thus reflects thinning lithosphere and decreasing mantle potential temperatures. We argue that the temporal decrease in mantle potential temperature reflects the severing of the connection between the MCR and Keweenaw plume after the Early phase of magmatism by the rapid plate motion. Our constraints on mantle potential temperature resolve the relationship between the MCR, the plume, Keweenaw LIP magmatism, and rapid plate motion, and indicate that an alternative mechanism, such as enhanced small-scale convection due to rapid plate motion and plume-lithosphere interaction, was required to generate the voluminous Main magmatic phase.