The evolution of texture and layering in layered intrusions

Abstract

Mineral segregation in igneous systems, and igneous layering in particular, can occur during ageing of crystal assemblages because areas composed of nominally larger grains or texturally favoured regions grow at the expense of adjacent unfavoured regions owing to the concentration gradients in the liquid that arise from surface energy effects. Numerical modelling demonstrates that this simple process can give rise to a number of observable phenomena, including ‘inch-scale doublets’, sharp modal contacts, and size-graded layers, among which the modelled size-graded layering mimics that produced by crystal settling. For a variety of physical situations, the characteristic times and lengths are scaled by the ratio of the mass of minerals initially present to the liquid compositional gradients driving the mineral segregation. On scales where both are appropriate, igneous layering shows many similarities to metamorphic banding that is unrelated to any pre-existing bedding, in that both can develop spontaneously and become more sharply defined with time.

Keywords:

• igneous layering
• igneous texture
• crystal ageing
• thermodynamics
• mafic intrusions

Acknowledgements

I thank Tony Philpotts and Peter Nabelek for their constructive reviews of this article. This work was supported by National Science Foundation grant EAR 04-07928.