Structure, magnetic fabric and emplacement of the Archean Lebel Stock, SW Abitibi Greenstone Belt

The anisotropy of magnetic susceptibility (AMS) has been determined in a suite of samples from the ca 2680 Ma old, sub-circular, syenitic Lebel Stock which is bound to the north by a major late-Archean shear zone (Larder Lake-Cadillac deformation zone; LCDZ). The AMS defines a magnetic foliation which is parallel to a strong, planar preferred mineral orientation fabric considered to have been acquired during flow of a crystalladen magma during emplacement. The geometry of the foliation indicates that the stock has an horizontal dish shape at depth. The syenites also contain a well defined magnetic lineation which consistently points toward the LCDZ apart from localities in the south where it trends E-W, parallel to the stock's margin. AMS ellipsoids are dominantly of oblate type except in samples showing solid-state strain and alteration overprints, which often have plane to prolate ellipsoids. The intensity of the magnetic fabric shows a weak spatial variation, being strongest at the southern margin. The magnetic and magmatic fabric pattern, AMS parameter variation and the sense of displacement of the western wall-rocks are used to propose an emplacement mechanism involving southward migration of a relatively thin sheet of syenitic magma from a linear source corresponding to the present location of the LCDZ.