GEOCHEMICAL EVOLUTION OF THE VOLCANIC ROCKS OF THE 1.1 GA MIDCONTINENT RIFT SYSTEM

During the past two decades there has been an explosion of geochemical and isotope determinations for igneous rocks of the 1.1 Ga Midcontinent rift (MCR) exposed in the Lake Superior region. Detailed stratigraphic and geochemical studies of MCR basalts around Lake Superior have identified five distinctive flood basalt compositions, which can be correlated across the region. The compositional groups are caused by varying contributions of distinct mantle sources to flood basalt magmatism during rift development. The earliest flood basalts were derived by small degrees of partial melting of an enriched, ocean-island type plume mantle source (ε_{Nd(1.1 Ga)} about 0). These were followed by basalts representing hybrid melts derived from the plume source and interaction with another mantle source, perhaps continental lithospheric mantle (ε_{Nd(1.1 Ga)} <0). During the main stage of rift development, larger degree partial melts of the plume became the dominant source for the voluminous younger basalts (ε_{Nd(1.1 Ga)} about 0). The youngest phase of basaltic magmatism is best explained by a mixture of melts from the plume and a depleted asthenospheric mantle source (ε_{Nd(1.1 Ga)} about 0 to +3).

Like many continental rifts, MCR-related volcanic rocks are dominantly basaltic, with felsic rocks making up <<5% of the total volcanic section. However, in some locations, such as the North Shore Volcanic Group, rhyolites comprise as much as 25% of the volcanic pile. Rhyolites occur both within localized volcanic complexes (e.g., Porcupine Mountains, Michipicoten Island) and in very large flows, some of which are estimated to be as much as 600 km³ in volume. Geochemical and isotopic data for many MCR rhyolites suggest derivation, at least in large part, from melting of crust, with Nd isotopic values reflecting the age of the crust involved. Rhyolites formed in the interior of the central MCR graben (e.g., Portage Lake Volcanics) are more likely to have been formed by melting of early MCR igneous rocks (ε_{Nd(1.1 Ga)} about 0), whereas rhyolites erupted near the margin of the rift are more likely to be generated by melting of older crust, with examples including the Porcupine Volcanics (ε_{Nd(1.1 Ga)} -9 to -21) and the major rhyolite flows on the North Shore of Minnesota (ε_{Nd(1.1 Ga)} -2 to -15).