Metamorphic P‐T paths from calcic pelitic schists from the Strafford Dome, Vermont, USA

Metamorphism of the Gile Mountain Formation and Waits River Formation in the Strafford Dome and Townshend‐Brownington Syncline in east‐central Vermont records two nappe‐style events, D1 and D2, followed by doming. D1 formed a muscovite + biotite ± ilmenite schistosity subparallel to compositional layering, SO, and was followed by heating to garnet grade. The temperature and pressure at the end of D1 are estimated to be c . 450d̀ C and 6‐8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1‐2 kbar, calculated from compositions of garnet, which have inclusions trails with progressive crenulation and rotation of the S1 fabric. Similar P‐T paths are computed for most of the area, suggesting that the later schistosity developed during emplacement of a regional nappe 3‐6 km thick. There is a general lack of D3 (dome‐stage) microstructures. Near the Strafford‐Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasing X An overgrew S2 in calcic pelites, reflecting post‐D2 heating to a maximum of 550‐600d̀ C. Metamorphic pressures at the end of D2 are fairly constant on the west side of the dome, indicating minor dome‐stage uplift. In contrast, pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome. The observed pattern of isotherms and isobars is mainly the result of post‐metamorphic, differential uplift and unroofing. Finally, a minor, retrograde metamorphism produced the assemblage albite + epidote + K‐feldspar + muscovite + chlorite, with grade increasing east toward the Connecticut River.