40AR/39AR CONSTRAINTS ON THE PALEOZOIC COOLING HISTORY OF SOUTHWEST NEW HAMPSHIRE AND ADJACENT VERMONT

New ⁴⁰Ar/³⁹Ar data from southwest NH and adjacent VT provide constraints on the cooling history of the Bronson Hill anticlinorium, and indicate late Paleozoic to early Mesozoic deformation in these classically Acadian rocks. Near Whites River Junction new ages reproduce the published cooling age discontinuity at the VT-NH border, with amphibole and muscovite (Ms) ages of ~380 and ~330 in VT, and 330 and 270 in NH. An amphibole age discontinuity is also found across the Northey Hill fault (NHf), with an age of 279 ± 2 Ma in the footwall near the Meriden antiform and ages of 310 ± 2, 324 ± 3, and 326 ± 2 Ma in the hanging wall near the Sugar River dome. The young age near the NHf is consistent with published ages from monazite and amphibole to the north in the Salmon Hole Brook Syncline and suggests that the NHf is a structure with major Paleozoic offset. Ms age spectra from the Littleton Formation in the hanging wall are ~250 Ma near the NHf and are older and more complex eastward. Detailed sample characterization by EMP and XRD shows that these age spectra reflect mixing of cooling ages of ~300 Ma and growth ages of ~250 Ma associated with ductile deformation along the Bald Mountain shear zone near Claremont. The Ms age gradient observed in earlier studies may also reflect a mixture of cooling and growth ages rather than a cooling age gradient as previously suggested. Within limits of error the age of ductile deformation overlaps the age of pseudomorphic replacement of staurolite by fine-grained Ms (243-247 Ma, n = 4).

Ms separates from the Littleton Formation 20 km along strike to the south near Hackett Swamp exhibit similarly complex age spectra and intrasample variation in Ms composition. The data are consistent with mixing of Ms cooling and growth ages of ~320 and ~250 Ma, respectively. Pseudomorphic Ms after staurolite records an age of ~250 Ma. Similar pseudomorphs and compositional variation in matrix Ms occur as far south as northern Massachusetts and we speculate that retrograde metamorphism may be regionally contemporaneous. The ⁴⁰Ar/³⁹Ar data indicate that retrograde deformation in the Connecticut River Valley continued into the latest Paleozoic and earliest Mesozoic. This is consistent with the timing of well-documented deformation in CT, and highlights the importance of inherited structures in controlling the locus of later deformation.