GEOCHEMICAL EVOLUTION OF 1.1 GA MIDCONTINENT RIFT MAGMATISM

Magmatism accompanying the development of the 1.1 Ga Midcontinent rift system (MCR) is a result of upwelling and decompression melting of a mantle plume beneath North America. Rift-related intrusions and flows were emplaced mainly during two episodes between about 1115-1105 Ma and 1102-1094 Ma. During the first magmatic episode, three geochemically distinctive mafic/ultramafic compositions are recognized in both intrusions and basalt flows, allowing for regional correlation of successive volcanic units around the Lake Superior basin. The earliest magmas ( high-MgO basalts, picrites, and intrusions) are characterized by clinopyroxene phenocrysts, steep REE patterns, and no Nb-Ta anomaly. Overlying these magmas are regionally extensive basalts, characterized by olivine phenocrysts, prominent negative Nb-Ta anomalies, and flatter REE patterns. Overlying the second magma type is a more evolved basaltic composition with an overall trace element composition similar to the earliest picrites. After a magmatic hiatus, the second magmatic episode is characterized by a fourth basaltic composition, occurring as voluminous flood basalts and large complex igneous intrusions, characterized by less enriched REE patterns and mild Nb-Ta anomalies. As rifting waned after 1094 Ma, a MORB-like basaltic composition appears at the top of the section.

The succession of magma compositions observed in the 1.1 Ga Midcontinent rift system has a younger analogue in Tertiary East Greenland flood basalts, suggesting similar processes occurred through time. In the MCR, this succession is attributed to the interaction of three mantle sources, augmented locally by crustal melts. Early picritic magmas are derived by small degrees of partial melting deep within an enriched, ocean-island-type mantle source (ε_{Nd(1100 Ma)} ~ 0), followed by variable interaction of those melts at a somewhat higher level with another mantle source, most likely continental lithospheric mantle (CLM; ε_Nd(1100Ma) <0). Over time the relative contribution of CLM diminished, as relatively shallow partial melts of the plume became the dominant source for basalts (ε_Nd(1100Ma) ~ 0) erupted throughout the Lake Superior basin. Late basalts (ε_Nd(1100Ma) ~ 0 to +3) were derived from mixtures of melts from the plume and a depleted asthenospheric mantle source.