Sillimanite from the regional sillimanite zone in west-central New Hampshire is fibrol-itic and overprints F2 folds (nappe stage) of earlier mica foliation. Regional sillimanite zone samples show no evidence for earlier staurolite parageneses, despite the fact that staurolite is abundant at lower grade, because sillimanite was produced directly from garnet + chlorite by the prograde (heating) reaction garnet + chlorite + muscovite + quartz = sillimanite + biotite + H2O. The pressure at which this reaction occurs is sensitive to the MnO and CaO contents of garnet, and phase-equilibrium arguments reveal that at the regional pressures of west-central New Hampshire (2–4 kbar), staurolite parageneses are only possible in rocks with low MnO + CaO.
The inferred P-T path is counterclockwise with nearly isobaric initial heating at 2 kbar, followed by loading (± heating) to a peak metamorphic temperature of 600 ± 25 °C at 4 kbar, followed by nearly isobaric cooling. Garnets were compositionally homogenized near the metamorphic peak, but subsequent cooling was rapid (> 100 °C/m.y.). These data argue against the folded isotherm model of Chamberlain (1986). Instead, the present distribution of metamorphic grades is interpreted to be the result of regional stacking of high-grade thrust sheets on lower grade rocks, followed by depression of high-grade rocks to lower structural levels.