Fragmentation processes and xenolith transport in a Proterozoic minette dyke,Grenville Province,Québec |
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| Authors: | D Morin Louise Corriveau |
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| Institution: | (1) Centre géoscientifique de Québec, INRS-Géoressources, C.P. 7500, Québec, QC, Canada, G1V 4C7, CA;(2) Centre géoscientifique de Québec, Commission géologique du Canada, C.P. 7500, Québec, QC, Canada, G1V 4C7, CA |
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| Abstract: | The Mesoproterozoic Rivard minette dyke provides a case example of dyke-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 lithosphere underlying the southwestern Grenville Province of Québec at 1.08 Ga. The xenoliths occupy approximately
50% of the volume of the breccia. They are extremely diverse and include pyroxenites and mafic granulites from the upper mantle
and lower crust, and Grenvillian gabbroids, gneisses and quartzites. Wall-rock fragments are detached to various extents along
dyke-parallel fractures and apophyses. The host lamprophyre magma has a calculated viscosity of ca. 40 Pa·s. Rapid ascent
of magma and turbulence are suggested by the presence of large dense fragments and the brittle-to-ductile deformation at the
margins of some xenoliths. A minimum ascent velocity of ca. 50 cm·s–1 is estimated from the settling velocity of a 40 cm long pyroxenite xenolith. The progressive addition of xenoliths increased
the effective viscosity of the liquid-solid mixture to 103 Pa·s during emplacement. This favoured Bingham behaviour and laminar flow of the magma, and xenolith suspension. Field evidence
suggests that xenoliths were formed through (1) early fracturing of wall rock by inelastic deformation during dyke propagation,
and (2) continuous delamination of wall rock by intrusion of magma along the dyke-parallel fractures. This led to removal
of chilled margins, and to fluid infiltration, partial melting and microbrecciation in the wall rock. Pre-existing discontinuities
played a minor role in the fracturing process. Xenoliths may thus be abundant in alkaline magmas not only because these magmas
ascend rapidly and can transport xenoliths, but possibly also, because their low viscosities promote intense fracturing of
wall rock.
Received: 10 June 1995 / Accepted: 10 February 1996 |
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