METAMORPHIC EVOLUTION OF THE NASHOBA FORMATION, EASTERN MASSACHUSETTS: A THERMODYNAMIC MODELING STUDY

The Nashoba terrane (NT) is a moderately northwest-dipping fault-bounded block within the New England Appalachians of eastern Massachusetts, located between the Avalon terrane to the southeast and terranes of Ganderian affinity to the northwest. The NT is a multiply-deformed and metamorphosed Cambrian-Ordovician arc-backarc complex composed of mafic to felsic metavolcanic rocks interlayered with metasedimentary rocks. This package was intruded by Silurian to earliest Carboniferous granitic and intermediate calc-alkaline composition plutons. This study focuses on the Nashoba Formation (NF), which forms the northwest portion of the NT and is the most extensive of the metasedimentary units.

The NF rocks are metamorphosed to sillimanite- and potassium feldspar-grade conditions, with localized migmatization. Areas of migmatitic NF are interlayered with non-migmatitic NF rocks and outcrop in linear northeast-trending belts that are parallel to the terrane boundaries. This study focused on three gneiss samples of the non-migmatitic NF. Petrographic analysis was carried out on standard polished thin sections. Peak metamorphic assemblages for all NF gneiss samples are biotite-garnet-sillimanite-potassium feldspar-magnetite-quartz. Garnets are zoned with cores rich with inclusions and inclusion free rims. Sillimanite occurs both as prismatic blades with biotite defining a foliation and as unoriented fibrolite masses.

In order to further define the pressure and temperature (P-T) conditions of metamorphism in the NF, P-T pseudosections were constructed using bulk rock compositions of homogeneous, non-migmatitic samples. Bulk rock compositions were determined by standard ICP-MS techniques. P-T pseudosections were prepared for each sample using Perple_X and subsequently THERMOCALC thermodynamic modeling software. All sections were modeled in the MnNCKFMASHTO system. The P-T pseudosections for all samples predict that peak assemblages equilibrated under conditions of ~600-700 degrees Celsius and ~4.5-6.0 kilobars, which is consistent with prior estimates from conventional thermobarometry. In many localities the NF was affected by retrograde greenschist-facies metamorphism.