D. Morin et L. Corriveau, FRAGMENTATION PROCESSES AND XENOLITH TRANSPORT IN A PROTEROZOIC MINETTE DYKE, GRENVILLE PROVINCE, QUEBEC, Contributions to Mineralogy and Petrology, 125(4), 1996, pp. 319-331
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.