Calcium depletion halos and Fe-Mn-Mg zoning around faceted plagioclase inclusions in garnet from a high-grade pelitic gneiss

Faceted, reversely zoned plagioclase inclusions surrounded by Ca depletion halos and Fe-Mg-Mn zoning in garnet are present in sillimanite-bearing rocks from the Skagit Gneiss in the North Cascade Range of Washington. Garnets are homogeneous except near the rim and near plagioclase inclusions. In the vicinity of plagioclase inclusions, the grossular component of garnet decreases by 2-8 mol%, almandine increases by 4-10 mol%, pyrope decreases by 1-13 mol%, and spessartine increases by <1-4 mol% from the average core composition. Chemical communication between plagioclase inclusions and matrix phases is proposed. Observations demonstrate that postentrapment changes in plagioclase inclusions can occur during evolving P-T conditions. -from Author