Abstract

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 < l–4 mol% from the average core composition. The observations (1) that garnet is zoned around plagioclase inclusions, (2) that zoning is most pronounced around inclusions in pervasively fractured garnets, and (3) that the extent of garnet zoning around each inclusion is related to the location of fractures are evidence for chemical communication between plagioclase inclusions and matrix phases.

The facets of the plagioclase inclusions are garnet crystal faces. All inclusions are reversely zoned; anorthite content increases by 12–24 mol% from core to rim. Zoning parallels the plagioclase-garnet interface of both negative crystals and irregularly shaped inclusions, suggesting that zoning is not a preentrapment feature.

These observations demonstrate that postentrapment changes in plagioclase inclusions can occur during evolving P-T conditions. This possibility must be considered when using the composition of plagioclase inclusions in garnet for geobarometric calculations and quantitative P-T path construction.

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