PALEOMAGNETISM OF THE ALLARD LAKE ANORTHOSITE SUITE

Abstract

Analysis of paleomagnetic data on 112 samples of Precambrian anorthosite, norite, pyroxene–syenite and hemo-ilmenite ore from four main localities in the Allard Lake area reveals: (1) A relatively soft magnetic component resides in magnetite, which is of variable abundance in all rock types. The magnetite remanent magnetic vector is of normal polarity, and is more or less directly opposed to a hard component residing in hemo-ilmenite, the predominant oxide. Self-reversal of the permanent magnetism in the hemo-ilmenite is the favored explanation. (2) Disregarding polarity, the stable remanent magnetic vector has similar orientation in all rock types from all four localities (N = 4, α₉₅ = 18.0). This precludes significant differential rotation of individual structural blocks since the rocks became magnetized. Furthermore, either no polar wander occurred during the emplacement and cooling of the entire igneous rock suite, or all these rock types acquired their magnetism simultaneously during subsequent metamorphism. (3) On the centered-axial-dipole model for the earth's magnetic field, the remanent vector in the Allard Lake rocks gives a pole southeast of New-foundland, at coordinates 38°36′ N., 39°36′ W. (α₉₅ = 18.0).