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 Ham pshire is fibrolitic and overprints F-2 folds (nappe stage) of earlier mica foliation. Regional sillimanite zone samples show no evidence fo r earlier staurolite parageneses, despite the fact that staurolite is abundant at lower grade, because sillimanite was produced directly fro m garnet + chlorite by the prograde (heating) reaction garnet + chlori te + muscovite + quartz = sillimanite + biotite + H2O. The pressure at which this reaction occurs is sensitive to the MnO and CaO contents o f garnet, and phase-equilibrium arguments reveal that at the regional pressures of west-central New Hampshire (2-4 kbar), staurolite paragen eses are only possible in rocks with low MnO + CaO. The inferred P-T p ath is counterclockwise with nearly isobaric initial heating at 2 kbar , followed by loading (+/- heating) to a peak metamorphic temperature of 600 +/- 25 degrees C at 4 kbar, followed by nearly isobaric cooling . Garnets were compositionally homogenized near the metamorphic peak, but subsequent cooling was rapid (> 100 degrees C/m.y.). These data ar gue against the folded isotherm model of Chamberlain (1986). Instead, the present distribution of metamorphic grades is interpreted to be th e result of regional stacking of high-grade thrust sheets on lower gra de rocks, followed by depression of high-grade rocks to lower structur al levels.