NEW 40AR/AR AGE CONSTRAINTS ON THE TIMING OF METAMORPHISM AND DEFORMATION IN THE WESTERN NASHOBA TERRANE, EASTERN MASSACHUSETTS

New single-grain total fusion ⁴⁰Ar/³⁹Ar ages provide temporal constraints on metamorphic events in the western portion of the Nashoba terrane in eastern MA. This terrane is metamorphosed to the upper amphibolite facies and lies between the Avalon terrane to the east and the Merrimack terrane to the west. The northwestern portion of the Nashoba terrane is underlain by biotite gneisses and schists of the Nashoba Fm. The Tadmuck Brook Schist (TBS), a rusty-weathering pelitic unit, occurs along the boundary between the Nashoba and Merrimack terranes, northwest of the Nashoba Fm. The polymetamorphic TBS varies in grade from lower greenschist facies in the northwest, adjacent to the Merrimack terrane, to the sillimanite zone in the southeast. An amphibolite in the Nashoba Fm., near the contact of the TBS, yielded ages of 391 ± 14 Ma (hornblende) and 330 ± 7 Ma (biotite). Randomly-oriented late “fish-scale” muscovite porphyroblasts from the higher grade southeastern TBS yielded an age of 316 ± 6 Ma. Samples from the Ball Hill mylonite zone, an intraterrane shear zone separating members of the Nashoba Fm., yielded ages of 291 ± 6 Ma (muscovite) and 267 ± 5 Ma (biotite).

The 391 Ma hornblende age from the Nashoba terrane amphibolite is consistent with a period of widespread migmatization in the terrane as previously interpreted from 400-385 Ma electron microprobe monazite ages (Stroud et al. 2009). The 330 Ma biotite age from the same amphibolite is interpreted as a cooling age from this same event, giving a cooling rate for the terrane of ~3 ºC/ Ma over this time interval, similar to that calculated for the Putnam terrane farther south (Wintsch et al., 1992). The hornblende age of 391 Ma from the western Nashoba terrane is considerably older than Ar isotope correlation ages previously found in hornblendes from the eastern portion of the terrane (351-325 Ma; Hepburn et al., 1987). The 316 Ma cross-foliation large muscovites in the TBS sample suggest the occurrence of a late hydrothermal event, while the 291 and 267 mica ages from the Ball Hill mylonite zone likely represent Alleghanian fault reactivation and recrystallization.