CONSTRAINING THE TIMING OF DEFORMATION IN THE NASHOBA FORMATION, EASTERN MASSACHUSETTS: A STRUCTURAL AND GEOCHRONOLOGICAL STUDY

The Nashoba terrane (NT) is a moderately NW-dipping fault-bounded block within the New England Appalachians. The NT is a multiply deformed Cambrian-Ordovician arc-backarc complex metamorphosed up to Sil+K-Spar conditions, resulting in localized migmatization. This package was intruded by Silurian to earliest Carboniferous plutons. The Nashoba Fm (NF) is the best exposed of the metasedimentary units and occurs in the NW part of the NT.

Deformation in NF gneiss is dominated by isoclinal folds, overprinted by top-down-to-the-NW asymmetric folds. The gneissic fabrics in the NW are cut by subvertical, top-down-to-the-NW ~0.5 m wide shear zones. Steep top-down-to-the-NW ultra-cataclasites cut these shear zones. To understand the temporal relationship of the folding events we carried out high-precision U-Pb geochronology on zircons from folded and crosscutting dikes. The minimum age of the isoclinal folding event is fixed by a crosscutting pegmatitic dike, containing Plag+Qtz+Musc+Bt and intruded into isoclinally folded fabrics. Zircons were ~418 Ma, indicating isoclinal folding predates the Late Silurian. A migmatitic K-Spar+Musc+Bt+minor Qtz dike is folded by the asymmetric folding. The zircons show a variety of dates, but the youngest zircon is ~365 Ma and suggests that part of the asymmetric folding was younger than that. Zircons from a planar Plag+Qtz+Bt+Amph dike that crosscuts the asymmetric folds is ~363 Ma. The dike is foliated with an orientation oblique to that in the country rock, indicating folding had stopped by the time of dike emplacement, but some deformation persisted.

The ~418 Ma pegmatite may be related to the Late Silurian Andover granite to the northeast that intrudes the NF and is interpreted to be a product of the amalgamation of the Avalon terrane to the southeast. Previously, only sparse Middle Devonian monazite dates have been reported and interpreted as resulting from metamorphism and anatexis within the NF. Late Devonian monazite ages from shear zones within the NF have been interpreted to result from recrystallization during fluid influx. On the contrary, the Late Devonian zircons in the migmatitic dike from our study are the youngest ones found to date in the NF; they indicate that melting conditions continued much later than originally thought and persisted until the Late Devonian.