THE NORTHEAST IOWA INTRUSIVE COMPLEX: A MAJOR UNWRITTEN CHAPTER IN THE STORY OF THE MIDCONTINENT RIFT SYSTEM?

Large amplitude gravity and aeromagnetic anomalies over a ~16,000 km² area of northeast Iowa and southeast Minnesota have been interpreted to reflect a buried intrusive complex composed of mafic/ultramafic rocks, the northeast Iowa Intrusive Complex (NEIIC), intruding Yavapai Province (1.8-1.72 Ga) rocks. Because only four boreholes reach the complex, which is covered by 200-700 m of Paleozoic sedimentary rocks, geophysical methods are critical to developing a better understanding of the fundamental nature of the NEIIC. Among the most significant geophysical anomalies present are 15-40 km diameter ring- and horseshoe-shaped aeromagnetic highs that correspond with gravity highs. These anomalies have previously been interpreted to reflect dense mafic/ultramafic lopolithic intrusions with relatively weak central magnetizations. The age of these intrusions is unproven due to a lack of dateable samples from the limited drilling. Two types of intrusive systems are known to produce such geophysical anomalies. The first is alkaline ring complexes, with examples including the Keweenawan (i.e., related to the ~1.1 Ga Midcontinent Rift System) Coldwell and Killala Lake Complexes in Ontario. The second is 1.3-1.6 Ga anorogenic intrusive complexes, known to exist in a swath from the southwest U.S. through eastern Canada and into the Baltic Shield. Possible geophysical analogues include the Kiglapait and Hettasch intrusions in Labrador and the Ahvenisto Complex in Finland. However, the NEIIC is considered to mostly be Keweenawan, based on the large volume of mafic/ultramafic rocks indicated by the gravity highs, its position at the eastern margin of the recognized Midcontinent Rift, and northeast-trending linear aeromagnetic highs consistent with Keweenawan dikes that both cut and are cut by other interpreted NEIIC intrusions. Taken together, the available evidence suggests the NEIIC is a major intrusive part of the Midcontinent Rift System, and in terms of areal extent and position at the margin of the rift is comparable to the Duluth Complex.