Temporal and directional constraints on a major shear zone in eastern Massachusetts: Monazite dating of the Assabet River fault zone

Electron microprobe dating of monazite grains found within the major intra-terrane Assabet River shear zone, coupled with shear sense indicators, is 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 and below 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.