ORIGIN AND EVOLUTION OF THE NASHOBA AND MERRIMACK TERRANES IN SOUTHEASTERN NEW ENGLAND: WHAT HAVE WE LEARNED IN THE PAST DECADE?

The Nashoba terrane in eastern Massachusetts is located between the Avalon terrane to the SE and Ganderian terranes to the NW. It consists of Cambrian-Ordovician mafic to felsic metavolcanic and interlayered metasedimentary rocks, intruded by a series of Silurian to earliest Carboniferous dioritic to granitic plutons. Trace element geochemistry and Sm/Nd results of the oldest volcanic rocks indicate an early Paleozoic arc/back-arc origin of the terrane. The mafic volcanic rocks had a primitive source. The oldest intermediate and felsic rocks incorporated older, isotopically evolved crustal material, indicative of a Ganderian basement below the terrane. Based on negative εNd values, model ages and detrital zircon ages, metasedimentary rocks were derived from mixed, generally evolved, sources with Archean though Ordovician input, indicating a Ganderian source.

We interpret the Nashoba terrane as a Ganderian arc-backarc complex similar to the Penobscot and/or Popelogan-Victoria arc systems and the Tetagouche-Exploits backarc basin on the leading (NW) margin of Ganderia in the northern Appalachians. The Nashoba terrane, however, is now located on the trailing (SE) margin of Ganderia and must have been there since ~430 Ma, when subduction-related plutons intruded the terrane and deposition of the ~430-407 Ma Merrimack terrane sediments to the NW took place.

Major and trace element geochemical and Sm/Nd data of ~430-350 Ma dioritic and granitic plutons in the Nashoba terrane indicate subduction-related convergence. Significantly younger model ages in the youngest granitic rocks may reflect Avalonian input after 350 Ma. At least three periods of metamorphism occurred between ~425 Ma and ~370 Ma, related to convergence between the Nashoba, Merrimack and Avalon terranes. Monazite growth between ~360 and ~305 Ma and biotite and muscovite ⁴⁰Ar/³⁹Ar age populations of ~300 Ma and ~267 Ma may indicate post-Neoacadian normal movement or Alleghanian metamorphism.