40AR/39AR DATING OF A LOWER-GREENSCHIST FACIES SHEAR ZONE: PERMIAN DEFORMATION IN WESTERN NEW HAMPSHIRE
40Ar/39Ar age spectra on white mica separates are complex, consistent with the preservation of these two age populations. Total gas ages systematically decrease from 274 Ma to 258 Ma as the shear zone is approached, consistent with the increased mode of S2 mica. Steps in individual spectra range in age by ~20 m.y., but age maxima cluster at ~280 Ma and age minima reach ~255 Ma. These ages are interpreted to provide a minimum estimate for the time of cooling through S1 white mica closure, and a maximum estimate for the time of S2 growth respectively. 40Ar/39Ar age spectra from undeformed white mica in pseudomorphic rims around staurolite are flat and reproduce at 242-247 Ma (n=4) regardless of proximity to the shear zone and matrix age. The pseudomorph and matrix ages constrain the age of S2 development to 242-255 Ma. Regionally, fabric growth at this time is consistent with mounting evidence for Permo-Triassic reactivation of the geologic structures near the VT-NH border, and highlights the importance of the Alleghanian orogeny in controlling the post-peak metamorphic juxtaposition of these rocks. This study also demonstrates the power of linking argon isotopic data with textural and petrologic observations to constrain the timing of deformation at lower-greenschist facies conditions.