Geophysical data and sparse subsurface samples suggest that the Spirit Lake trend (SLT) in the aeromagnetic fabric of northwest Iowa is the southern boundary of the Archean Superior craton. Several small-diameter, positive aeromagnetic anomalies south of the SLT have been interpreted as Middle Proterozoic anorogenic, magnetite-bearing granites that intruded juvenile Early Proterozoic crust. A scientific drill hole 609 m deep was drilled into a 1200 gamma aeromagnetic anomaly near Quimby, Iowa, to test these interpretations. Precambrian basement was reached at a depth of 510 m, and 99 m of undeformed, megacrystic, magnetite-bearing granite core was collected. Petrographic and chemical data are consistent with derivation of the granite magma by melting of preexisting tonalitic continental crust. A U-Pb zircon age of 1433 ±4 Ma shows that this granite is part of the older anorogenic granite-rhyolite suite of the United States. Sm-Nd data yield a depleted-mantle separation age of 1860 Ma, which confirms that the source for the pluton magma was Early Proterozoic crust. All data are consistent with the hypothesis that the aeromagnetic anomalies are caused by Middle Proterozoic anorogenic plutons and with interpretation of the SLT as an Archean/Early Proterozoic crustal boundary.