Metamorphic plagioclase in calcic pelitic schists from South Strafford, Vermont, grew during heating of the assemblage garnet + biotite + or - chlorite + plagioclase + epidote + or - calcite + quartz + muscovite + graphite + fluid during garnet growth with chlorite in the assemblage, or during garnet consumption after chlorite was removed from the assemblage. These grains have a variety of patterns of compositional zoning. The simplest pattern is continuous concentric zoning, which records the sequence of plagioclase compositions produced in the rock during progressive metamorphism. The most strongly zoned single grain found varies from An 20 to An 70 from core to rim. Sodic plagioclase was consumed during growth of calcic plagioclase, and the locations of growth and consumption can be affected by partitioning of deformational strain in a rock. As a result of discontinuous growth of a particular grain, plagioclase can have discontinuous zoning or patchy zoning. Zoning patterns in plagioclase can also reflect the character of the matrix overgrown: smoothly zoned plagioclase overgrew relatively homogeneous matrix, whereas plagioclase with complex zoning patterns overgrew crenulated muscovite or other inhomogeneities. The peristerite gap is displayed as alternating lamellae of two plagioclase compositions, as a simple gap in the zoning of concentrically zoned grains, and as portions of grains that are nearly unzoned with compositions near An 18 . This variety of zoning features and the complexity of possible zoning patterns make it imperative to document carefully any correlation of mineral compositions. Petrologically unreasonable interpretations can potentially; lead to wild errors in thermobarometric estimates.

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