Geology of the Shrewsbury Quadrangle, East-Central Massachusetts

Abstract

The Shrewsbury quadrangle was geologically mapped at a scale of 1:24,000. The quadrangle spans the entire Nashoba terrane, a belt of amphibolite-grade rocks related to an early Paleozoic peri-Gondwanan arc. Petrofabric studies of fault-rocks indicated that the final motion on several major shear zones - Ball Hill fault, Sulfur Hill shear zone, and Assabet River fault - was sinistral strike-slip with an oblique NW over SE thrust component. Monazites from these shear zones were dated using an electron microprobe. Regional metamorphism (M1) took place around 420 Ma. A second regional metamorphism (M2) produced anatectic conditions around 394Ma. A group of dates in the range 360-385 Ma indicates that the Nashoba terrane was also affected by Neoacadian metamorphism and/or deformation. Major shear zones were active throughout the Devonian and may have persisted into the Carboniferous.