Compositional zoning in individual garnets is principally the result of depletion of the constituent elements of the garnet from a homogeneous matrix as the garnet grows. Minerals such as garnet, which by growth can change the composition of the matrix due to the fact that they do not internally equilibrate in response to changing external conditions, are termed “refractory” phases. It is shown that staurolite and probably ilmenite are “refractory” phases. During resorbtion of the refractory phase ilmenite, iron is added to the matrix, which gives rise to decreasing Mg/Fe in the concurrently growing garnet and staurolite. Biotite, chlorite, and presumably muscovite are homogeneous minerals that continuously internally equilibrate in response to changing external conditions, including retrograde metamorphism; therefore, they are considered to form the matrix from which the refractory minerals grow.
Differences between measured electron micro-probe profiles of garnet grains and those calculated for a depletion model are due mainly to the effect of Mn on the Mg/Fe ratio in the garnet and partly to changing relative proportions of chlorite, muscovite, and biotite during the growth of the garnet.
Higher grade assemblages appear to be the result of bypassing of lower grade assemblages. This leads to a model of overstepping of reactions that is tested by an analysis of garnet zoning profiles from assemblages that contain a limiting garnet composition (that is, the garnet composition projects at a corner of an assemblage polyhedron). The composition of these garnets is a function of the temperature at which they grew, which was nearly constant for each garnet.
Magnesian chlorite persists stably with sillimanite, in the absence of muscovite; staurolite with a moderate zinc content persists with quartz in the highest grade rocks of the Kwoiek area.