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 Le bel Stock which is bound to the north by a major late-Archean shear zo ne (Larder Lake-Cadillac deformation zone; LCDZ). The AMS defines a ma gnetic foliation which is parallel to a strong, planar preferred miner al orientation fabric considered to have been acquired during flow of a crystal-laden magma during emplacement. The geometry of the foliatio n indicates that the stock has an horizontal dish shape at depth. The syenites also contain a well defined magnetic lineation which consiste ntly points toward the LCDZ apart from localities in the south where i t trends E-W, parallel to the stock's margin. AMS ellipsoids are domin antly of oblate type except in samples showing solid-state strain and alteration overprints, which often have plane to prolate ellipsoids. T he intensity of the magnetic fabric shows a weak spatial variation, be ing 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 invol ving southward migration of a relatively thin sheet of syenitic magma from a linear source corresponding to the present location of the LCDZ .