Regional Metamorphism of the Waits River Formation, Eastern Vermont: Delineation of a New Type of Giant Metamorphic Hydrothermal System

Abstract

Metamorphic isograds and time-integrated fluid fluxes were mapped over the ∼ 1500 km2 exposure of the Waits River Formation, eastern Vermont, south of latitude 44°30'N. Isograds based on the appearance of oligoclase, biotite, and amphibole in metacarbonate rocks define elongated metamorphic highs centered on the axes of two large antiforms. The highest-grade isograd based on the appearance of diopside is closely associated spatially with synmetamorphic granitic plutons. Pressure, calculated from mineral equilibria, was fairly uniform in the area, 7± 1.5 kb; calculated temperature increases from ˜ 480°C at the lowest grades in the area to ˜ 575°C in the diopside zone. Calculated Xco2f equilibrium metamorphic fluid increases from <0-03 at the lowest grades to ∼ 0.2 in the amphibole zone and decreases to ∼ 0.07 in the diopside zone. Time-integrated fluid fluxes increase with increasing metamorphic grade, with the following mean values for each metamorphic zone (in cm3/cm2): ankerite-oligoclase zone, 1 x 104; biotite zone, 7 x 104; amphibole zone, 2 x 105; diopside zone, 7 x 105. The mapped pattern of time-integrated fluxes delineates two large deep-seated (˜ 25-km depth) regional metamorphic hydrothermal systems, each centered on one of the major antiforms. Fluid flowed subhorizontally perpendicular to the axis of the antiforms from their low-temperature flanks to their hot axial regions and drove prograde decarbonation reactions as they went. Along the axes of the antiforms fluid flow was further focused around synmetamorphic granitic intrusions. In the hot axial region fluid changed direction and flowed subvertically out of the metamorphic terrane, precipitating quartz veins. Estimates of the total recharge, based on progress of prograde decarbonation reactions, nearly match estimates of the total discharge, based on measured quartz vein abundance, (2-10) x 1012 cm3 fluid per cm system measured parallel to the axes of the antiforms. Within the axial regions fluids had lower XCO2 and rocks record greater time-integrated fluxes close to the intrusions than at positions more than ˜ 5 km from them. The differences in both fluid composition and time-integrated flux can be explained by mixing close to the intrusions of regional metamorphic fluids of XCO2/ with fluids from another source with XCO2˜ 0 in the approximate volume ratio of 1:2.