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-centr al Vermont records two nappe-style events, D1 and D2, followed by domi ng. D1 formed a muscovite + biotite +/- ilmenite schistosity subparall el to compositional layering, SO, and was followed by heating to garne t grade. The temperature and pressure at the end of D1 are estimated t o be c. 450 degrees C and 6-8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1-2 kbar, calculate d from compositions of garnet, which have inclusions trails with progr essive crenulation and rotation of the S1 fabric. Similar P-T paths ar e 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 Straffo rd-Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasing X(An) overgrew S2 in calcic pelites, refl ecting post-D2 heating to a maximum of 550-600 degrees 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 t he thermal peak of metamorphism decrease by more than 4 kbar east of t he dome. The observed pattern of isotherms and isobars is mainly the r esult of post-metamorphic, differential uplift and unroofing. Finally, a minor, retrograde metamorphism produced the assemblage albite + epi dote + K-feldspar + muscovite + chlorite, with grade increasing east t oward the Connecticut River.