Litho-structural layering within the Archean Lac du Bonnet Batholith, at AECL’s Underground Research Laboratory, Southeastern Manitoba

Abstract

The Lac du Bonnet Batholith (LDBB) is one of many late to post-tectonic granites in the western Superior Province which appear to be compositionally homogeneous and structurally simple on the surface. However, mapping in the 443 m deep access shaft at the Underground Research Laboratory (URL), borehole logs, and detailed surface mapping clearly demonstrate that decametre-scale layering is widespread in the LDBB. Individual layers or litho-structural domains are distinguished by the type and abundance of xenoliths and late residual or metasomatic segregations, by auto-intrusive contacts, and by variations in style and orientation of mesoscopic fabric. The litho-structural domains are inconspicuous at surface due to their low dip and the flat topography; however, detailed mapping in the central part of the URL has identified their surface expression as a series of broad and open domes and intervening saddles. The litho-structural domains host three systems of auto-intrusive dykes, sills, and recrystallized zones whose abundance and mode of occurrence change with depth. Late magmatic granodioritic dykes are the predominant rock type below 300 m depth at the URL, but the swarm narrows rapidly up-dip, and at surface is represented only by narrow zones of alteration and ductile deformation. Late pegmatite–aplite dykes are pervasive across all rock types at surface, but are limited to the larger granodiorite dykes at 420 m depth at the URL. The distribution of inclusions, alteration and fractures in the LDBB suggest the present topographic surface is close to the original roof zone of the batholith.