TEMPORAL AND DIRECTIONAL CONSTRAINTS ON A MAJOR SHEAR ZONE IN EASTERN MASSACHUSETTS: MONAZITE DATING OF THE ASSABET RIVER FAULT ZONE

There is much debate over the sequence of accretionary events along the eastern margin of the Appalachian Orogen in southeastern New England; however, few studies use direct methods to determine the direction and timing of terrane accretion. The Nashoba terrane is an Early Paleozoic arc located between the Merrimack terrane to the west and the Avalon terrane to the east. It is comprised of metamorphosed mafic volcanic and volcanogenic sedimentary rocks now metamorphosed at sillimanite and sillimanite + K-feldspar zone conditions to amphibolites, biotite-feldspar gneisses, schists, calc-silicate gneisses, and feldspathic gneisses. Plutonic rocks of the Nashoba terrane range in age from Late Ordovician to Early Carboniferous and are a temporally overlapping series of calc-alkaline intermediate composition diorites and peraluminous granitoids. The oldest rocks found within the Nashoba terrane to date are from the 499 Ma, largely orthogneissic, Fish Brook Gneiss.

Electron microprobe dating of monazite grains found within the major intra-terrane Assabet River fault zone, coupled with shear sense indicators, was used to reconstruct the tectonic history of the Nashoba terrane in eastern Massachusetts. Microstructural evidence indicates that the principal ductile shearing is sinistral with the eastern side moving to the north-northwest beneath the western side. Three distinct metamorphic events were discerned in the Nashoba terrane. The first metamorphic event (M1) occurred from ~420-407 Ma. A second metamorphic event (M2) occurred at ~390 Ma and is associated with widespread migmatization. A third metamorphic event (M3) occurred during the ~378-371 Ma time interval and is likely associated with the Neoacadian Orogeny.