Evolution of Texture and Layering in Layered Intrusions - A Review

It is becoming increasingly apparent form a number of studies that igneous textures in layered intrusions evolve over time. Mechanisms include crystal aging, the effects of which are to produce crystal size distributions to develop characteristic profiles that, unfortunately, mimic those produced by hydraulic sorting. However, crystal aging does allow crystal in the center of large intrusions (where heat loss is slow) to continue to grow at the expense of energetically unfavored crystals. Furthermore, field observations, laboratory experiments and numerical modeling demonstrate that many small scale features (e.g., inch-scale doublets of the Stillwater Complex) can be formed by such processes. In this regard, igneous layering has many parallels with metamorphic banding. Compaction tends to affect both texture and bulk composition and is well documented in many intrusions, but recognizing its development on sloping crystal piles where it may have a shear component is less clear. Future work should more fully understand the transition from ophitic to cumulus textures. Modeling too often focuses either on nucleation and crystal growth leading to an overall increase in the crystal number density, or models of crystal aging that assume an existing crystal assemblage as a starting point. Future modeling should include a fusion of these two approaches to develop a full model of textural development from liquid to solid rock, and encompass textural development in high temperature metamorphic rocks as well.