TECTONIC HISTORY OF THE AVALON AND NASHOBA TERRANES ALONG THE WESTERN FLANK OF THE MILFORD ANTIFORM, MASSACHUSETTS

The Bloody Bluff fault zone (BBfz) juxtaposes the peri-Gondwanan Nashoba and Avalon terranes in Massachusetts. Cambrian to Silurian rocks in the Nashoba terrane include intrusive rocks of the Grafton Gneiss, metavolcanic rocks of the Marlboro Formation, and metasedimentary and metavolcanic rocks of the Nashoba Formation. A SHRIMP U-Pb zircon age of 515 ± 4 Ma from the cross-cutting Grafton Gneiss constrains the Marlboro Formation to Early Cambrian or older. Neoproterozoic rocks in the Avalon terrane include metasedimentary rocks intruded by arc-related plutonic rocks. New SHRIMP U-Pb zircon ages exist for the Ponagansett Gneiss, a megacrystic biotite granite gneiss (612 ± 5 Ma), Northbridge Granite Gneiss, a coarse grained biotite granite gneiss (607 ± 5 Ma), and the Hope Valley Alaskite Gneiss (606 ± 5 Ma). These newly dated rocks in the Nashoba and Avalon terranes are calc-alkaline continental arc granites with age-corrected Epsilon Nd and Pb isotope values that overlap the fields for Gander and Avalon basement.

In the Nashoba terrane, dominant deformation is associated with upper amphibolite facies metamorphism. Metamorphic zircons from amphibolite in the Marlboro Formation are 355 ± 4 Ma, and record no evidence of an earlier Silurian event reported elsewhere by others. Major structural features are not continuous across the BBfz. In the Avalon terrane, the age of relict D1 deformation in the metasedimentary rocks may be Neoproterozoic. Late Paleozoic Alleghanian deformation (D2) produced a regional dominant foliation with associated mylonites that increase in intensity towards the BBfz. Subsequent Alleghanian D3 folding produced the NE-trending Milford antiform and NW-trending Oxford anticline. Thin, steeply dipping, NE-trending D3 mylonitic shear zones increase towards the southwestern part of the Milford antiform. The shear zones may be splays of the Hope Valley shear zone (HVsz), and may represent the structural termination of the HVsz northward into the Milford antiform within the Avalon terrane. Here, the HVsz is not a terrane boundary. Extensional veins, shear bands, and thin mylonitic fault zones indicate west-side-down normal displacement and show that the Milford antiform is truncated by late movement along the BBfz.