Compositionally-zoned microporphyroblasts of plagioclase are found in phyllites from southwestern Massachusetts. Rocks from the lowest metamorphic grade yield unzoned albite.
At slightly higher grade, albite constitutes the core and a thin outer rim with an intervening narrow zone of An 13–17. At higher grades, plagioclase is complexly zoned in one of two patterns: either the composition drops stepwise from an oligoclase core to an albitic rim, or the composition of the core gradually increases outward from An 19 to An 24, then abruptly drops to an albite rim. All of these patterns are present in rocks at metamorphic grades below the first appearance of chloritoid and garnet. Above the garnet isograd, the zoning pattern is simple: a core of An 13 and a gradual outward increase to the maximum value of An 25.
Transmission-electron-microscopy studies of an oligoclase core, an An 9–16 intermediate zone, and an An 1–5 outer albite rim show that two sets of lamellae are present in the composition range An 2–25. In the range An 9–16, the (1, 18, 1) set of lamellae is strong, has a periodicity of about 122A, and gives rise to satellites; the other set, (201), is weak. In the range An 22–25 both sets are very weak, and diffuse streaking is parallel to *. All areas have only a single lattice. The strong microstructures are interpreted as products of spinodal decomposition, and the weak lamellar microstructures as fluctuations in the nucleation-only zone of the peristerite solvus.
The rocks were metamorphosed probably at about 350°C and under at most a few kilometers overburden, which represents conditions considerably below the peak of the peristerite solvus. Metastable crystallization is indicated. Available data, however, do not permit an unequivocal conclusion on whether the zoned plagioclase grew during a single episode of metamorphism, or whether the albitic rim represents a second Acadian metamorphism superposed on preexisting Taconian mineral assemblages.