FRAGMENTATION PROCESSES AND XENOLITH TRANSPORT IN A PROTEROZOIC MINETTE DYKE, GRENVILLE PROVINCE, QUEBEC

The Mesoproterozoic Rivard minette dyke provides a case example of dyk e-parallel fracturing induced by propagation of low-viscosity melts at deep crustal levels. The dyke constitutes a xenolith-choked intrusion breccia and provides samples of an extensive section of the lithosphe re underlying the southwestern Grenville Province of Quebec at 1.08 Ga . The xenoliths occupy approximately 50% of the volume of the breccia. They are extremely diverse and include pyroxenites and mafic granulit es from the upper mantle and lower crust, and Grenvillian gabbroids, g neisses and quartzites. Wall-rock fragments are detached to various ex tents along dyke-parallel fractures and apophyses. The host lamprophyr e magma has a calculated viscosity of ca. 40 Pa . s. Rapid ascent of m agma and turbulence are suggested by the presence of large dense fragm ents and the brittle-to-ductile deformation at the margins of some xen oliths. A minimum ascent velocity of ca. 50 cm . s(-1) is estimated fr om the settling velocity of a 40 cm long pyroxenite xenolith. The prog ressive addition of xenoliths increased the effective viscosity of the liquid-solid mixture to 10(3) Pa . s during emplacement. This favoure d Bingham behaviour and laminar flow of the magma, and xenolith suspen sion. Field evidence suggests that xenoliths were formed through (1) e arly fracturing of wall rock by inelastic deformation during dyke prop agation, and (2) continuous delamination of wall rock by intrusion of magma along the dyke-parallel fractures. This led to removal of chille d margins, and to fluid infiltration, partial melting and microbreccia tion in the wall rock.