Close study of the progress of growth toward higher-Ca compositions finds that the compositional change was stepwise and not smooth. The steps cross several pixel values each, and are therefore not an artifact of image enhancement but are real.
It is suggested that the stepwise increases in grossular component in the garnet result from deformational pulses during a prograde metamorphism involving steady pressure increase. Garnet growth would have the effect of armoring plagioclase grain boundaries with increasingly albitic compositions . Further growth of garnet would require access to the more anorthitic cores of matrix plagioclase grains. This access cannot be provided by diffusion. Deformational pulses affecting matrix plagioclase grains would have the desired effect - rehomogenization of most matrix plagioclase and consequent pulsed increases in the activity of anorthite in the intergranular medium.
For more information about this image contact: Michael L. Williams