Sintering as a key process in the textural evolution of chromitite seams in layered mafic-ultramafic intrusions

ABSTRACT Nearly monomineralic stratiform chromitite seams of variable thickness (millimeters to meters) occur in many of the world's layered mafic-ultramafic intrusions. These seams are often associated with economically significant quantities of platinum group metals, yet the petrogenesis of these societally important materials remains enigmatic. Here we evaluate processes associated with late-magmatic (postcumulus) textural maturation of chromitite seams from four layered mafic-ultramafic intrusions of different ages and sizes. From largest to smallest, these intrusions are the ∼2060 Ma Bushveld Complex (South Africa), the ∼2710 Ma Stillwater Complex (USA), the ∼1270 Ma Muskox Intrusion (Canada), and the ∼60 Ma Rum Eastern Layered Intrusion (Scotland). Three endmember chromitite textures are described, based on chromite grain size and degree of textural equilibration: (1) coarse-grained chromite crystals (>0.40 mm) that occur in the central portions of seams and exhibit high degrees of solid-state textural equilibration; (2) fine-grained chromite crystals (0.11–0.44 mm) at the margins of seams in contact with and disseminated throughout host anorthosite or pyroxenite; and (3) fine-grained chromite crystals (0.005–0.28 mm) hosted within intra-seam orthopyroxene, clinopyroxene, and olivine oikocrysts. Crystal size distribution and spatial distribution pattern analyses are consistent with coarsening occurring through processes of textural maturation, including the sintering of grains by coalescence. We propose that textural maturation initially occurred in the supra-solidus state followed by an important stage of solid-state textural maturation and that these equilibration processes played a major role in the eventual microstructural and compositional homogeneity of the chromitite seams.

The role of transiently fine-grained reaction products in syntectonic metamorphism: natural and experimental examples

It is proposed that one of the most important mechanisms of interrelationship between deformation and metamorphism is the facilitation of one of several grain-size-sensitive deformation mechanisms resulting from the formation of fine-grained products of a metamorphic reaction. During prograde metamorphism, such effects are likely to be transient, because grain coarsening and textural equilibration are likely in response to rising temperature conditions. Thus deformation mechanisms are often difficult to infer from such naturally deformed rocks.In localized shear zones exhibiting retrogressive metamorphism, evidence of enhanced deformability by such mechanisms is most likely to be preserved, because cooling conditions inhibit grain growth and both deformed and undeformed rocks are likely to be preserved.An experimental study has been made of the effects of deformation on serpentinite under conditions of progressive dehydration but with controlled pore pressure. A marked weakening (near-linear viscous rheology) at low strain rates was observed in association with the onset of dehydration to olivine. The enhancement of deformability is interpreted as due to the formation of thin, planar zones of ultrafine-grained but equiaxed (0.25 μm) olivine, which deform by diffusion-accommodated grain-boundary sliding. The experimental data therefore support the idea that a great deal of natural deformation during prograde metamorphism may occur in association with the transient existence of fine-grained reaction products, followed by grain growth and textural equilibration under essentially static conditions of relaxed stress.