THE MANCHESTER AND VINTON PLUTONS: KEY COMPONENTS OF A MIDCONTINENT RIFT-RELATED INTRUSIVE COMPLEX IN THE IOWA BASEMENT?

Large amplitude aeromagnetic and gravity anomalies over a ~16,000 km² area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC), a buried intrusive igneous complex composed of mafic/ultramafic rocks in the Yavapai Province (1.8-1.72 Ga). Long considered, but not proven, to be related to the ~1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent to the richly mineralized Duluth Complex and is similarly located near the margin of the MRS. Suspected faults mapped within the NEIIC commonly lie along a northwest trend, projecting into the MRS-related Belle Plaine fault zone, a possible lithospheric-scale structural discontinuity with a prolonged history of reactivation. Hundreds of meters of Paleozoic sedimentary cover and a paucity of basement drilling have prevented detailed studies of the NEIIC. Here we report on results from recently completed aeromagnetic and ground gravity surveys over the Manchester and Vinton mafic/ultramafic intrusions that are part of the southern portion of the NEIIC. Dikes similar in trend and petrologic character to MRS-related dikes in the region variably cut and are cut by the intrusions, suggesting an overall ~1.1 Ga age for the NEIIC. The Manchester and Vinton intrusions themselves are characterized by 25-40 km diameter ring-shaped aeromagnetic highs at their margins that surround central aeromagnetic lows, as well as gravity highs. These anomalies are interpreted to reflect dense rocks having mafic/ultramafic compositions that occur in lopolithic form and exhibit relatively weak total magnetization at their centers. Two types of intrusive systems are known to produce such geophysical anomalies, including alkaline ring complexes and other anorogenic intrusive complexes. Both types are known to exist in the broader North American craton; for example, the Coldwell Complex in Ontario and the Kiglapait intrusion in Labrador. Both examples have analogous geophysical expressions. Similarities with such examples imply notable potential for the Manchester and Vinton intrusions to host magmatic sulfide deposits and several other minerals recently deemed critical for national security.