Systematic retrograde metamorphism of sillimanite-staurolite schists, New Salem area, Massachusetts

Abstract

The New Salem retrograde metamorphic zone occupies an approximately triangular area covering ∼26 km 2 in sillimanite-staurolite-grade rocks, largely pelitic schists of the Devonian Littleton Formation. Acadian prograde metamorphism at ∼6 kbar and ∼600 °C produced the common assemblage quartz-muscovite-biotite-garnet-staurolite-iunenite-albite-graphite±sillimanite±pyrrhotite. Systematic hydration of the prograde assemblages by an influx of H 2 O produced the retrograde assemblages chlorite–K-feldspar–celadonitic musco-vite–sphene–anatase±pyrite (common) and chlorite-chloritoid-musco-vite-anatase-sphene (rare). Retrograde sillimanite-out, staurolite-out, and garnet-out isograds have been mapped, representing the completion of important retrograde hydration reactions. These isograds plus the thicknesses of chlorite rims on garnets (0 and 0.2 mm) can be used to subdivide the area into six concentric retrograde metamorphic zones, designated R1 to R6. During retrograde metamorphism, the exchange reactions K ⇌ Na, Mn ⇌ Fe+Mg, and Fe ⇌ Mg proceeded while more or less consistent tie lines between the sheet silicates and ilmenite were maintained. Biotite and chlorite became more Fe rich owing to staurolite, garnet, and ilmenite breakdown. Biotite, chlorite, and ilmenite became more Mn rich owing to garnet breakdown. Coexisting biotite and muscovite became more K rich during biotite breakdown. Muscovite also became progressively more celadonite rich. Chlorite, biotite, and ilmenite are homogeneous in single thin sections. Muscovite is zoned with respect to M +2 /(M +2 +Al IV ) and K/(K+Na) ratios, which are higher in muscovite rims. There is no evidence for re-equilibration of staurolite, garnet, or albite during retrograde metamorphism. Temperature and pressure estimates for retrograde metamorphism are poorly constrained to ∼3.5 kbar and ∼280 °C. Each kilometre of vertical extent of the retrograde zone required an influx of ∼0.26 km 3 of H 2 O (at STP). Water may have been derived from a large volume of surrounding rock and concentrated in the study area by a pressure shadow effect around the north end of the Prescott intrusive complex.