Granulation of weak rock as a precursor to peperite formation: coal peperite, Coombs Hills, Antarctica

Peperite formed by mingling of magma with coal, and with fragmented coal plus other country rock, is exposed at Coombs Hills, Antarctica, in rocks of the Mawson Formation, where Ferrar Supergroup basalt encountered the Beacon Supergroup continental sedimentary succession. An internally laminated, 0.5-m-thick coal bed passes gradationally through a coal-fragment-dominated peperite into a glass-rich, basalt-dominated coal fragment–matrix peperite, and then into coherent basalt. Initial interaction of magma with water-saturated coal and host sediments locally brecciated the coal. Subsequent mingling of basalt with a slurry of coal fragments and water, driven by viscosity and density contrasts between the basalt and the slurry and by flow inhomogeneities in intruding magma, led to increasingly complex mingling of the two fluids via bifurcation of fingering viscous flows. This mingling was complicated by the participation of phases with time- and space-dependent thermal properties and viscosity (coal fragments, coal–water slurry, and basaltic magma), and the generation of multiple (?) gas phases through devolatilisation of heated coal. The initial elastic response of the coal fragments to stress resulted in: (a) fluidal mingling of coal and basalt facilitated by softening of coal during heating associated with intrusion; and (b) localised formation of blocky coal clasts during high stress events associated with passage of a liquid, and/or a gas phase(s), through the coal. The presence of fluidal- and blocky-shaped coal fragments in the coal peperite, with both curved and planar surfaces bounding single coal clasts, suggests that clast morphology in the magma–sediment dispersion was controlled at very small scales by the time-, temperature-, and stress-dependent qualities of the coal. Fluidal and blocky clast shapes in the coal peperite, coupled with vesicles in the coal clasts, suggest that transient properties of either the host or intrusion during non-explosive magma–sediment interaction may exert a strong control on clast morphology and mingling characteristics.