40AR/39AR DATING OF A LOWER-GREENSCHIST FACIES SHEAR ZONE: PERMIAN DEFORMATION IN WESTERN NEW HAMPSHIRE

Optical and electron petrography across a retrograde shear zone in staurolite-grade rocks of the Devonian Littleton Formation of southwest New Hampshire demonstrate the presence of prograde (S1) and retrograde (S2) fabrics. ⁴⁰Ar/³⁹Ar step-heating experiments on white micas produce age spectra that constrain the age of cooling of S1 and crystallization of S2. Away from the shear zone, white mica and minor biotite define the dominant foliation (S1) which pre-dates staurolite growth, and open to tight F2 folds with NNE-trending axial surfaces (S2) deform S1. Within the shear zone, S1 is progressively overprinted by an S2 fabric defined by white mica and chlorite, and staurolite porphyroblasts are increasingly replaced by randomly oriented white mica locally undeformed by S2. These observations show that S1 micas experienced staurolite grade metamorphism and hence should record a cooling age, whereas S2 micas and staurolite-replacing micas grew at chlorite grade, below the closure temperature for argon, and should record growth ages.

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.