Magmatic layering

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Characteristics:

• Layering/banding in an igneous rock;

• The layered rocks are cumulates in most cases;

• Individual layers have a mineralogy and/or texture distinct from this of the nearby layer;

• Individual layers can have uniform or non-uniform (e.g., gradual upward variation) mineralogy or texture;

• Layering can be laterally continuous and vertically extensive / layering can also be intermittent and developed locally.

Type of layering in cumulate

• Graded layering – gradual variation in modal proportion and/or mineral size occurs within individual layers;

• Modal layering – the relative proportion of minerals varies;

• Rhythmic layering – layer that systematically repeats, with layers that can be centimetric (microrhythmic) or metric (macrorhythmic) in thickness;

• Cryptic layering – variation of the chemical composition of minerals.

Layering in volcanic rock

• Flow banding – similarly oriented phenocrysts in a glassy groundmass;

• Trachytic texture – idem, with feldspars being the main phenocrysts.

Layering in pegmatite

• Stockscheiders – laminated pegmatite. This is due to fluid accumulation along the margin of a magma intrusion;

• Brain rock, atoll texture – produced by fluid-saturated magma (forms quartz-rich and feldspar-rich irregular layers).

Origin:

• Layering produced by a magmatic process;

• Can be due to the appearance/disappearance of a phase during the crystallization sequence;

• Fractional crystallisation induces the progressive chemical evolution of the residual melt, which can produce cryptic layering;

• Can result from the mechanical sorting of minerals due to mineral mush slumping along the wall of a magma chamber;

• Can be due to a large-scale event in the magma chamber (magma replenishment, contamination), as has been proposed for the Bushveld Complex;

• Flow orientation, magma movement (flow banding).