THE IGNEOUS PETROLOGY AND MAGMATIC EVOLUTION OF THE MIDCONTINENT RIFT SYSTEM

The 1100 Ma Midcontinent rift system of North America is one of the world's great igneous provinces, containing over one million km3 of mafic volcanic and plutonic rocks. Volcanic rocks range from picrite to rhyolite, although tholeiitic basalt predominates. Plutonic rocks range from peridotite to granite, with plagioclase-rich gabbros and troctolites being most abundant. The igneous history of the rift can be divided into early and late stages; flows of the early stage form a trend having low-alumina, high-magnesia picrite as the primitive endmember, whereas flows from the late stage form a trend having high-alumina olivine tholeiite as the primitive endmember. The early basalts have enriched Nd isotopic compositions and were derived largely from old, heterogeneous subcontinental lithosphere. The late basalts have chondritic to depleted Nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle. Apparently, melting began initially in the subcontinental lithosphere as it was stretched. Subsequently, a lower mantle plume, which had entrained asthenospheric mantle during its ascent, rose beneath the rift and melted at large degrees to yield the voluminous later basalts.