MAGMATIC PLUMBING EVOLUTION DURING THE LATENT PHASE OF THE MIDCONTINENT RIFT

The Mamainse Point Formation (MPF) is a 5250m thick flood basalt pile with thick interbedded conglomerate layers that represents an uninterrupted record of almost all of the igneous activity within the Midcontinent Rift. The MPF contains an extensive dike network which allows for the probing of relationships between the magmatic plumbing system and rift progression. 26 basaltic dikes collected throughout the MPF and the surrounding Archean basement were analyzed for major- and trace-element chemistry. These dikes were chemically correlated with 4 of the 8 chemostratigraphic units of the basalt flows of the MPF – groups 3, 5, 6, and 8. ~60% of the dikes sampled are related to group 5, while less than 20% of the flood basalt sequence belongs to group 5. All of the group 5 dikes were <1m in thickness while the thickness of all the other dikes ranged from 2-20m. The group 5 flows occur following a brief hiatus in magmatic activity that is evidenced by a ~200m thick conglomerate, and the end of group 5 magmatic activity is marked by another hiatus in which a ~500m thick conglomerate was deposited. The decrease in magmatic flux during group 5, the presence of silicic rocks within group 5, and the increased frequency and relative thinness of the group 5 dikes provides compelling evidence for a shallowing of magmatic activity during this latent period of rift activity. These group 5 dikes provide a window into the evolution of the magmatic plumbing system between two distinct periods of magma flux within the Midcontinent Rift System.