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Petrology of high-grade schists, gneisses, and cordierite pegmatites in south-central Massachusetts
Abstract
Pelitic gneisses and pegmatites from within the Acadian granulite-facies of south-central Massachusetts were investigated to understand the P-T evolution of the Merrimack synclinorium. Qtz$\pm$Pl-Kfs-Sil-Bt$\pm$Grt$\pm$Crd (X$\sb{\rm Fe}$ = 0.227-0.294) assemblages in 28 schists suggest peak metamorphism at 710-740$\sp\circ$C and 6-7 kbar. One mylonite records similar conditions, suggesting it did not re-equilibrate during mylonitization. Some schists record temperatures of retrograde ion exchange. Several pristine pyrrhotite-rich schists containing Bt(X$\sb{\rm Fe}$ to 0.284)-Crd(X$\sb{\rm Fe}$ to 0.165)-Sil-Qtz-Pl-Kfs-Po-Rt$\pm$Ilm suggest an early metamorphic fluid was dominated by H$\sb2$O and CH$\sb4.$ Biotite-consuming, fluid-absent melting of pelitic gneisses produced cordierite$\pm$garnet-bearing pegmatites and leucocratic gametiferous melt segregations. The major reaction is Qtz+Sil+Bt=Kfs+Crd+Grt+Melt (KFMASH). Magnesian gneisses melted to form cordierite (X$\sb{\rm Fe}$ = 0.274-0.347)$\pm$garnet pegmatites at 710-800$\sp\circ$C, 4-5 kbar. Iron-richer gneisses melted to form garnet$\pm$cordierite (X$\sb{\rm Fe}$ = 0.237-0.284) melt segregations at 700-720$\sp\circ$C, 6 kbar. Retrograde reactions produced intergrowths of pale green Bt$\pm$Sil$\pm$Qtz on cordierite and/or garnet grain boundaries within the melt products. Fe-rich pegmatitie cordierites contain aggregates of Grt+Sil+Qtz around which cordierite is strongly zoned down to 0.16 adjacent to aggregate garnet. Aggregate garnets, observed only in pegmatites, show complicated composition zoning and are the result of cordierite breakdown via: 3Crd=2Grt+4Sil+5Qtz. This began at 760-810$\sp\circ$C and 5-5.5 kbar and continued through 570-600$\sp\circ$C and 6-7 kbar. Thus, the pegmatites record part of a counterclockwise P-T path. A mineralogically and chemically zoned coticule pod places constraints on granulite-facies diffusional processes. Compositional gradients are preserved over a distance of 7 cm between the manganiferous core and the surrounding host pelitic schist. Homogeneous volume diffusion allowed the preservation of side-to-side chemical gradients within individual garnet grains. Solution of simple diffusion equations suggest that, even at 700$\sp\circ$C, temperatures were too low to homogenize the pod over the available time for metamorphism. Evidence suggests a lack of metamorphic fluid during the later stages of metamorphism.
Subject Area
Geology
Recommended Citation
Thomson, Jennifer A, "Petrology of high-grade schists, gneisses, and cordierite pegmatites in south-central Massachusetts" (1992). Doctoral Dissertations Available from Proquest. AAI9305909.
https://scholarworks.umass.edu/dissertations/AAI9305909