INTERPRETATION OF PLAGIOCLASE ZONATION IN CALCIC PELITIC SCHIST, SOUTH STRAFFORD, VERMONT, AND THE EFFECTS ON THERMOBAROMETRY

Metamorphic plagioclase in calcic pelitic schists from South Strafford , Vermont, grew during heating of the assemblage garnet + biotite +/- chlorite + plagioclase + epidote +/- calcite + quartz + muscovite + gr aphite + fluid during namer growth with chlorite in the assemblage, or during garnet consumption after chlorite was removed from the assembl age. These grains have a variety of patterns of compositional zoning. The simplest pattern is continuous concentric zoning, which records th e sequence of plagioclase compositions produced in the rock during pro gressive metamorphism. The most strongly zoned single grain found vari es from An(20) to An(70) from core to rim. Sodic plagioclase was consu med during growth of calcic plagioclase, and the locations of growth a nd 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 pa tterns in plagioclase can also reflect the character of the matrix ove rgrown: smoothly zoned plagioclase overgrew relatively homogeneous mat rix, whereas plagioclase with complex zoning patterns overgrew crenula ted muscovite or other inhomogeneities. The peristerite gap is display ed as alternating lamellae of two plagioclase compositions, as a simpl e gap in the zoning of concentrically zoned grains, and as portions of grains that are nearly unzoned with compositions near An(18). This va riety of zoning features and the complexity of possible zoning pattern s make it imperative to document carefully any correlation of mineral compositions. Petrologically unreasonable interpretations can potentia lly lead to wild errors in thermobarometric estimates.