ALLEGHANIAN DEFORMATION AND RECRYSTALLIZATION IN THE CONNECTICUT VALLEY OF VT AND NH

New ⁴⁰Ar/³⁹Ar data collected in support of 1:24,000-scale bedrock mapping in SW NH and adjacent VT indicate that retrograde deformation and recrystallization are widespread and that the region was transpressive in the Early Permian.

Staurolite in the Littleton Fm. in NH is locally replaced by coarse or fine-grained muscovite (Ms). ⁴⁰Ar/³⁹Ar ages from coarse-grained Ms pseudomorphs (n=2) and coexisting foliation-forming Ms agree (~320 Ma), consistent with both populations recording cooling ages. In contrast, fine-grained pseudomorphic Ms (n=2) yields ages of ~250 Ma, while coexisting foliation-forming Ms yields complex spectra and older ages. These data document the replacement of staurolite by Ms at both high and low grade, the latter at 250 Ma. In the Bald Mountain shear zone (BMsz), some 250 Ma pseudomorphs are deformed indicating Triassic ductile deformation along a reactivated regional S₂ foliation.

Near Alstead, NH, Ms separated from F₃-crenulated schist in the Partridge Fm. yields a ⁴⁰Ar/³⁹Ar total gas age of ~310 Ma. In the directly overlying calc-silicate layer, randomly oriented Ms truncates the dominant foliation and yields an age of ~280 Ma—30 Myr younger than the schist centimeters away. We suggest recrystallization in the schist during F₃ folding led to neocrystallization of Ms in the adjacent calc-silicate and that 280 Ma is the age of F₃ folding at this locality. To the NW, new and published 280-300 Ma amphibole and monazite ages around the Northey Hill Line (NHL) suggest deformation in a narrow zone along the fault at about the same time as F₃.

In the Connecticut Valley trough the sinistral Westminster West shear zone (WWsz) was active at ~300 Ma. Mapping documents several similar shear zones to the north. Ms separated from one of these zones yields an age spectrum consistent with an Alleghanian overprint of an older schistosity. Electron microscopy of Ms in a foliated granite in this belt documents narrow (~10 μm) high Mg rims. Preliminary ⁴⁰Ar/³⁹Ar data indicate that the Mg-rich rims record an Alleghanian growth age perhaps related to sinistral motion on the nearby WWsz.

The similar ages of deformation along the WWsz, rapid cooling around the NHL, and F₃ folding are consistent with region wide transfer of strain inboard of a colliding Gondwana resulting in transpression at the NH-VT border in the Permian.