RIFT-WIDE CORRELATION OF 1.1 GA MIDCONTINENT RIFT SYSTEM BASALTS - IMPLICATIONS FOR MULTIPLE MANTLE SOURCES DURING RIFT DEVELOPMENT

Magmatism that accompanied the 1.1 Ga Midcontinent rift system (MRS) i s attributed to the upwelling and decompression melting of a mantle pl ume beneath North America. Five distinctive flood-basalt compositions are recognized in the rift-related basalt succession along the south s hore of western Lake Superior, based on stratigraphically correlated m ajor element, trace element, and Nd isotopic analyses. These distincti ve compositions can be correlated with equivalent basalt types in comp arable stratigraphic positions in other MRS localities around western Lake Superior. Four of these compositions are also recognized at Mamai nse Point more than 200 km away in eastern Lake Superior. These region ally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood-basalt magmatism during rift development, extending a model originally devel oped for eastern Lake Superior. In this refined model, the earliest ba salts were derived from small degrees of partial melting at great dept h of an enriched, ocean-island-type plume mantle source (epsilon(Nd(11 00)) value of about 0), followed by magmas representing melts from thi s plume source and interaction with another mantle source, most likely continental lithospheric mantle (epsilon(Nd(1100)) < 0). The relative contribution of this second mantle source diminished with time as lar ger degree partial melts of the plume became the dominant source for t he voluminous younger basalts (epsilon(Nd(1100)) value of about 0). To wards the end of magmatism,mixtures of melts from the plume and a depl eted asthenospheric mantle source became dominant (epsilon(Nd(1100)) = 0 to +3).