Timing and Style of Deformation in the Nashoba Formation of the Nashoba Terrane, Eastern Massachusetts

Wesley Buchanan, Colorado School of Mines
Yvette Kuiper, Colorado School of Mines
Robet Buchwaldt, Massachusetts Institute of Technology

The Nashoba terrane (NT) is a moderately northwest-dipping fault-bounded block within the New England Appalachians and is located between the Avalon terrane to the southeast and terranes of Ganderian affinity to the northwest. The NT is a multiply deformed Cambrian-Ordovician arc-backarc complex metamorphosed up to sillimanite + potassium feldspar conditions, resulting in migmatization. This package was intruded by Silurian to earliest Carboniferous granitic and calc-alkaline intermediate-composition plutons. The Nashoba Formation (NF) in the NW portion of the NT is the best exposed metasedimentary unit in the NT and preserves a complex deformational history. The NF is comprised of biotite-feldspar-quartz ± garnet ± sillimanite ± muscovite ± amphibole ± magnetite gneiss with interlayered calc-silicate, impure quartzite, marble and sillimanite-bearing pelitic schist.

Deformation in the Nashoba Formation is dominated by isoclinal folds, overprinted by top-down-to-the-NW asymmetric folds. These folds and gneissosity in the northwest portion of the formation is cut by subvertical NW-side-down ~0.5 m wide shear zones, and later local steeply NW-dipping top-down-to-the-NW ultra-cataclasites. U-Pb zircon chemical abrasion – thermal ionization mass spectrometry (CA-TIMS) geochronology was used to bracket the ages of folding within the Nashoba Formation. A ~418 Ma pegmatitic dike cross-cuts isoclinal folds. Three migmatitic dike samples were dated and contain metamorphic, igneous, and inherited zircon. All ages reported below are interpreted as crystallization ages based on zircon morphology and high Th/U ratios. A migmatitic dike folded only by top-down-to-the-NW asymmetric folds is 364.6 ± 0.6 Ma. A planar migmatitic dike that crosscuts the asymmetric folds yielded ~364-361 Ma zircon. These two samples appear to constrain the age of asymmetric folding between ~365 and ~361 Ma. However, an isoclinally folded dike, refolded by top-down-to-the-NW asymmetric folds, is 361.4 ± 0.3 Ma, suggesting that both generations of folding are younger than that. Perhaps, folding and anatexis are diachronous across the field area, or there is possible mixing of age domains within zircon grains.

The leucosome crystallization ages presented above indicate crustal melting conditions existed between ~365 and ~361 Ma. Such Late Devonian, post-Acadian migmatitic melting has not been recognized previously in this portion of the Nashoba Formation. Partial melting may have resulted from either Neo-Acadian accretion of the Meguma terrane to the composite Laurentian margin, or from post-Acadian exhumation of the Nashoba terrane.