Paleomagnetism of the Beaver Bay Complex, Minnesota

The Beaver Bay complex is a series of intrusions of gabbroic rocks related to the Middle Keweenawan Duluth gabbro complex. The Beaver Bay ferrogabbro, one of three units into which the complex has been divided, is an iron-rich gabbro with a very prominent igneous lamination. Magnetic anisotropy caused by preferential alignment of inequant magnetite grains within the lamination appears to have deflected the direction of remanent magnetization of the ferrogabbro by about 8.5°. When samples from the ferrogabbro are eliminated, the resulting paleomagnetic pole for the Beaver Bay complex falls at 27.5°N, 170.5°W, where it forms part of a distinct cluster of paleomagnetic poles for Keweenawan igneous rock units of negative (normal) polarity. Positively magnetized Keweenawan igneous rocks, which appear to be older than the negatively magnetized units, give paleomagnetic pole positions that are scattered over a considerable area to the northeast of the cluster of negative poles. Magnetic intensities vary markedly throughout the complex, probably in response to the ordinary processes of magmatic differentiation.