Paleomagnetism of Keweenawan Intrusive Rocks, Minnesota

A paleomagnetic pole position calculated for 131 sites in normally (N) magnetized Keweenawan intrusive rock from Minnesota lies at 34.5°N, 168.5°W. Gabbro and diabase of reverse (R) polarity also are known but only from a limited area in northern Cook County, near the Canadian border. R-polarity sites yield a pole position of 42.5°N, 156.5°W and probably are older than the N-polarity sites. Magnetic intensities in these rocks vary by 4 orders of magnitude, in keeping with other strong differences in lithology, which together reflect active igneous differentiation, Dispersion of magnetic directions in the intrusive rocks indicates that the ratio of nondipole to dipole field-intensity has not changed markedly since Keweenawan time. It is suggested that, on the basis of current evidence, apparent polar wandering relative to North America is best described by a rather vaguely defined southwestwardly trending path several tens of degrees in width. Under favorable circumstances positions within the path of polar wandering can be used for geological correlation. Several episodes of Precambrian basaltic volcanism in North America are used to illustrate long-distance correlation by paleomagnetic techniques.