Structure, petrology, and tectonic implications of highly strained rocks along the west margin of the Acadian granulite-facies high, south-central Massachusetts

The Conant Brook shear zone, identified along the Bronson Hill anticlinorium - Merrimack synclinorium boundary in south-central Massachusetts, coincides with the west margin of the granulite-facies metamorphic high. This kilometer-wide shear zone deforms pelitic schists of the Silurian Rangeley Formation, is bounded on the west by the Ordovician Monson Gneiss, and truncates major Acadian structural features and intrusive bodies along its margins. Sillimanite, quartz, and feldspar lineations define two directions of transport during shearing in this zone, with a progressive change from earlier backfold-stage, reverse dip-slip motion to later dome-stage, dextral strike-slip motion, parallel to the orogen. Fabric relations observed in this zone have led to a model for regional development of orogen-parallel lineations. Minerals in the peak metamorphic assemblage, garnet-sillimanite-orthoclase-biotite, are deformed during both shearing events, suggesting that both took place after the peak of metamorphism. The apparent activity of prism slip in quartz during both phases of shearing indicate relatively high temperature deformation, $\u3e$600$\sp\circ$C. The eastward transition to granulite facies, characterized by the addition of cordierite to the above peak assemblage, coincides with the eastern margin of the shear zone. Samples from the shear zone and from the granulite-facies region show some differences in garnet and biotite chemistry, but no consistent difference in estimated temperatures of peak metamorphism. Diffusion modelling and mineral textures indicate that peak conditions, estimated at 750$\sp\circ$C at 6.0 kbar, are relatively robust. Mineral zoning textures from granulite-facies rocks and those from the shear zone are very similar and indicate relatively dry conditions after the metamorphic peak. The chemical potential of water ($\mu$H$\sb2$O) appears to be higher in assemblages within the shear zone than in cordierite-bearing assemblages to the east. This implies that a low $\mu$H$\sb2$O helped to stabilize cordierite in the granulite-facies assemblages and that high $\mu$H$\sb2$O either contributed to or resulted from focused high strain within the shear zone. The Quabbin Park fault was identified by a discontinuity in isograds, west of the shear zone, and may be the result of late- or post-metamorphic faulting