The study area extends for 80 km through regionally metamorphosed rocks in the meta-morphic high of southern New England, in which Acadian (Devonian) metamorphism ranged from sillimanite-muscovite grade through sillimanite-garnet-cordierite-orthoclase grade. Over this metamorphic interval, amphibolites containing the assemblages plagioclase + hornblende ± quartz ± cummingtonite ± orthoamphibqls ± garnet were transformed by a series of reactions into pyroxene granulites with assemblages of plagioclase + orthopyroxene + augite ± quartz ± hornblende ± garnet. Three particularly important reactions were examined in detail. These reactions, along with others, define an assemblage series in quartz-bearing rocks in the transition from amphibolite facies to granulite facies. This assemblage series, listed below, is based on the coexistence of diagnostic minerals in common mafic rocks with increasing grade.

hornblende + orthoamphibole assemblage (Mg-rich rocks only)

orthoamphibole + hornblende + quartz = cummingtonite + plagioclase + H2O

hornblende + cummingtonite assemblage

cummingtonite + plagioclase = hornblende + orthopyroxene + quartz + H2O

hornblende + orthopyroxene assemblage

hornblende + quartz = augite + orthopyroxene + plagioclase + H2O

orthopyroxene + augite assemblage.

This transition is estimated to have occurred over a temperature interval of 580–730 °C.

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