DETERMINING THE TIMESCALE AND AMOUNT OF SYN-METAMORPHIC FLUID FLOW DURING REGIONAL METAMORPHISM, TOWNSHEND DAM, VT, U.S.A

Fluids derived from progressive regional metamorphism have important implications for several characteristics of orogenic processes. These fluids, upon release, can migrate through the rock pervasively or through channelized flow, playing a role in the geochemical cycling of elements, transporting heat, and enhancing deformation in shear zones. An interpretation of the scale of fluid flow is fundamental in understanding the evolution of regional metamorphic environments.

Combining differential garnet geochronology and thermodynamic analysis permits calculation of the rate and amount of dehydration during garnet growth metamorphism of the pelitic schists of Townshend Dam, VT. High precision Sm-Nd geochronology was performed on several microsampled garnet segments from large (1-3 cm) porphyroblasts. Grouped core and rim ages reveal the timing of the initiation of garnet growth to be 380.3 ± 2.0 Ma, with the termination of garnet growth occurring at 376.5 ± 1.0 Ma, The duration of garnet growth is calculated to be 3.8 ± 2.2 My. Thermodynamic analysis, using both isochemical phase diagrams (pseudosections) and path dependent forward models (incorporating the fractionation of both garnet and fluid) predict that garnet growth initiated during nearly isothermal loading, at ~0.8 GPa and ~520°C, with continued burial to ~1.1 GPa and ~550°C. The rocks are subsequently heated isobarically, at which point garnet growth terminates at ~1.1 GPa and ~580°C. Decompressional heating (with no further garnet growth) is predicted until peak temperature is reached (~610°C at ~1.0 GPa). This falls in line with a broadly clockwise pressure-temperature evolution for regional metamorphism of these pelitic schists.

Further evaluation of the thermodynamic analysis allows for a prediction of the scale of dehydration. The predicted bulk rock dehydration during garnet growth is ~0.7 wt.% (or ~2 vol.%). Coupling the dehydration estimate with constraints on the duration of garnet growth, a minimum dehydration rate of 1.9 x 10^-10 moles H₂O per cm³ of rock per year is calculated. Implications for time-integrated fluid fluxes during metamorphism will be discussed.