Effect of anisotropy on the paleomagnetic contact test for a Grenville Dike

Abstract

We report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the Precambrian Grenville Province of Canada. Directions of natural remanent magnetization (NRM) in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric, using the measured anisotropy of laboratory‐induced thermoremanent magnetization (TRM). The TRM ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility (AMS) ellipsoids, but the anisotropy of remanence is more pronounced than the AMS. Anisotropy correction moves NRM directions in the unbaked zone almost 20°, bringing them close to typical Grenvillian paleofield directions. The anisotropy seems to be controlled by multidomain rather than single‐domain magnetic minerals. NRM directions from the baked gneiss near the dike contact after anisotropy correction agree well with the NRM directions from the chilled margin of the early Cambrian (570 ± 3 Ma) dike. This positive baked contact test is augmented by the presence of a “hybrid zone”, in which country rock NRMs are increasingly overprinted by the dike NRM as one moves from the unbaked toward the baked zone. The dike NRM is therefore primary. However, the virtual geomagnetic pole (VGP) for the dike NRM: 51°N, 131°W (dp=10°, dm=12°, N=24 samples) or 41°N, 153°W (dp=13°, dm=18°, N=4 chilled margin samples) does not fall near most established Cambrian paleopoles. Post‐emplacement tectonic movement of the dike is unlikely; it may be that the dike cooled and acquired TRM during a geomagnetic field excursion because of its geologically short cooling (blocking) period of a few tens of years.