Petrology of the regional sillimanite zone, west-central New Hampshire, USA, with implications for the development of inverted isograds

Sillimanite from the regional sillimanite zone in west-central New Hampshire is fibrolitic and overprints F₂ folds (nappe stage) of earlier mica foliation. Regional sillimanite zone samples show no evidence for earlier staurolite parageneses because sillimanite was produced directly from garnet - chlorite by a prograde (heating) reaction. 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. 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. -from Authors