NEW ADVANCES ON THE DEFORMATION AND ANATECTIC HISTORY OF THE NASHOBA TERRANE, NEW ENGLAND APPALACHIANS

The Nashoba terrane (NT) in eastern Massachusetts is a moderately NW-dipping shear zone-bounded, Cambro-Ordovician, backarc complex intruded by Silurian to earliest Carboniferous plutons. It is bounded the by Merrimack belt to the NW and the Avalon terrane to the SE. The NT is multiply deformed and metamorphosed. This study focused on the locally migmatitic metasedimentary units of the deformed Nashoba Formation (NF). The NF is dominated by isoclinal folds, overprinted by NW-side-down asymmetric folds. The gneissic fabrics in the NW are cut by subvertical, NW-side-down ~0.5 m wide shear zones. Previously, three discrete metamorphic events at ~423, ~395 and ~378 Ma have been interpreted based on monazite ages, with ~395 Ma representing migmatization.

To elucidate the timing of deformation we carried out U-Pb Secondary Ion Mass Spectrometry analyses on complex zircons from variably folded anatectic melts. The maximum age of the isoclinal folding event is constrained by a ~413 Ma granitic leucosome that has been folded by both the isoclinal and the asymmetric folds. Zircons from a granitic leucosome that is also folded by both fold sets preserves ~412 Ma cores indicating the intrusive age. Two sets of overgrowths are ~404 Ma and ~393 Ma.

U-Pb Chemical Abrasion Thermal Ionization Mass Spectrometry was used to date zircons from intrusive bodies that cut the migmatitic fabric of the NF. A crosscutting ~369 Ma tonalitic intrusive is cut by younger NW-side-down shear zones. An asymmetrically-folded fine grained granitic dike recorded ~367 Ma old zircons, placing a maximum age on folding, and lastly, the weakly foliated Acton Granite cuts the NF migmatitic fabric and shows a ~361 Ma crystallization date, indicating that folding had ceased by this time.

Our new data suggest, in contrast to the previous interpretation, a continuum of zircon ages from ~413 to ~393 Ma, suggesting partial melting conditions lasted for at least ~20 million years. The ~367 Ma dike was ductilely deformed, suggesting that elevated temperatures persisted until that time. The NT cooled much earlier than the adjacent terranes, indicating different tectonic histories. We propose that the NT was ductilely extruded between the cooler and more rigid adjacent terranes into the upper crust due to increased buoyancy from partial melting.