EVIDENCE FOR A CONCEALED MIDCONTINENT RIFT-RELATED NORTHEAST IOWA INTRUSIVE COMPLEX

Large amplitude aeromagnetic and gravity anomalies over a ~9500 km² area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC). This buried intrusive complex is composed of mafic/ultramafic rocks intruding the Yavapai Province (1.8-1.7 Ga). Hundreds of meters thick Paleozoic sedimentary cover and a paucity of drilling into Proterozoic basement rocks have prevented detailed studies of the NEIIC. Long considered, but not proven, to be related to the ~1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent and rift-marginal position to the richly mineralized Duluth Complex. The NEIIC is largely characterized by multiple zoned intrusions, many of which contain large volumes of mafic-ultramafic rocks and have strong geophysical similarities to alkaline intrusive complexes elsewhere, including the MRS-related Coldwell Complex of Ontario. The largest of the zoned intrusions are ~40 km wide. Suspected faults, orientations of intrusions, and intrusive margins tend to be aligned along and crudely co-located with northwest and northeast trends that match the projected trends of the Belle Plaine fault zone and Fayette structural zone. Both are previously interpreted as pre-MRS, possibly lithospheric-scale discontinuities that may have controlled NEIIC emplacement. New geochronological and geophysical data together support an MRS affinity for the NEIIC. A borehole-intercepted and unmetamorphosed dike swarm imaged in aeromagnetic data is cut by intrusions of the NEIIC, and a new apatite U-Pb date of ~1170 Ma on one of the dikes thus represents a maximum age for the NEIIC. A minimum age constraint is suggested by (1) large-volume mafic/ultramafic magmatism associated with the MRS that was the last such event to affect the region; and (2) the presence of reversely magnetized dikes, similar in geophysical character to MRS-related dikes elsewhere, that cut several intrusions of the NEIIC. The MRS affinity, comparison to the setting of the Duluth Complex, apparent emplacement along major preexisting structures, and geophysical evidence for a large volume of mafic/ultramafic and alkaline rocks collectively imply notable potential for the NEIIC to host several different types of undiscovered base metal and critical mineral deposits.