A counterclockwise P–T path for anatectic pelites, south-central Massachusetts

Pelitic gneisses, cordierite ± garnet-bearing pegmatites and leucocratic garnetiferous melt segregations (garnet leucosomes) from within the Acadian (360 Ma) granulite-facies of south-central Massachusetts were investigated to further understand the counterclockwise P–T evolution of the Merrimack synclinorium. The rocks were investigated with specific reference to phase equilibria in the system Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O (NaKFMASH) in the context of the petrogenetic grid proposed by Spear et al. (1999). An early low pressure history is suggested by the widespread occurrence of sillimanite pseudomorphs after andalusite as well as abundant matrix K-feldspar in both gneisses and partial melts. Biotite-consuming, fluid-absent melting of pelitic gneisses produced cordierite ± garnet-bearing pegmatites and garnet ± cordierite leucosomes at conditions of 700–750 °C and 4–5 kbar. The major reaction is Qtz + Sil + Bt = Kfs + Crd + Grt + Melt (KFMASH). Magnesian gneisses melted to form cordierite ± garnet-bearing pegmatites and iron-richer gneisses melted to form garnet ± cordierite-bearing leucosomes. Previous studies of the quartz ± plagioclase–K-feldspar–sillimanite–biotite ± garnet ± cordierite assemblages in the gneisses suggest peak metamorphism at 700–750 °C and 6–7 kbar. Retrograde reactions are essentially the reverse of prograde reactions and produced intergrowths of pale green biotite + sillimanite + quartz on cordierite and/or garnet grain boundaries within the melt products as well as late muscovite. Water dissolved in the melt during prograde metamorphism was available for retrogression as the melts cooled. The rocks investigated record part of a counterclockwise P–T path.