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 km 2 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 X CC)2 of equilibrium metamorphic fluid increases from <0O3 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 cm 3 /cm 2): ankerite-oligoclase zone, 1 x 10 4 ; biotite zone, 7 x 10*; amphibole zone, 2 x 10 5 ; diopside zone, 7 x 10'. 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 10 12 cm 3 fluid per cm system measured parallel to the axes of the antiforms. Within the axial regions fluids had lower X COl 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 XQQ^O-I with fluids from another source with X COl~0 in the approximate volume ratio of 1:2.

Geologic controls of deep natural gas resources in the United States

“…These twelve are only the systems documented in the literature and are biased by the fact that they are now exhumed and exposed at the surface where they can be easily studied. Fluid fluxes of similar magnitude have been documented in recent work by Ferry (1992) in giant vein systems of more common dynamothermal metamorphic terranes.…”

“…An additional source of information on fluids in crustal rocks is provided by fluid inclusions in quartz veins associated with ore mineralization. Reviews of fluid inclusions in ore deposits have been recently prepared by Landis and Hofstra (1991) and Kerrich and Feng ( 1992), and related observations from a nonmineralized setting are given by Ferry (1992). All of this information is important because it records the flux of fluids from deep to shallower levels of the crust and provides a basis for quantitative estimates of the flux of gases to shallow crustal levels as discussed in the following section.…”

The effects of metamorphism on iron mineralogy and the iron speciation redox proxy

“…The Waits River and Gile Mountain Formations were folded, intruded by granitic plutons, and then regionally metamorphosed during the Devonian Acadian orogeny (McWilliams et al, 2013;Osberg et al, 1989;Spear and Harrison, 1989;Thompson and Norton, 1968;. Mapped metamorphism of pelitic schist units spanned the chlorite through biotite, garnet, staurolite, and kyanite zones (Doll et al, 1961;Ferry, 1994); within the metamorphosed carbonate rocks of the Waits River Formation, isograds and zones were mapped based on the presence of ankerite, oligoclase, biotite, amphibole, and diopside (Ferry, 1992).…”

New evidence for syntectonic fluid migration across the hinterland‐foreland transition of the Canadian Cordillera

“…In section 8.1.1 we refuted the notion that age variation is responsible for increasing wall rock depletion westward toward the Purcell Thrust, and we must therefore propose a secondary process capable of altering and homogenizing rock compositions. Dehydration reactions, which occur during the formation of chlorite and muscovite from clay minerals, are capable of generating a substantial volume of fluid during regional metamorphism and cleavage development [e.g., Etheridge et al, 1984;Valley, 1986;Ferry, 1992]. For example, Ferry [1992] estimated that Appalachian metacarbonates of similar grade to those in the Western Ranges experienced a time-integrated fluid flux of 4.0 Â 10 2 mol H 2 O cm À2 during regional metamorphism.…”

“…Dehydration reactions, which occur during the formation of chlorite and muscovite from clay minerals, are capable of generating a substantial volume of fluid during regional metamorphism and cleavage development [e.g., Etheridge et al, 1984;Valley, 1986;Ferry, 1992]. For example, Ferry [1992] estimated that Appalachian metacarbonates of similar grade to those in the Western Ranges experienced a time-integrated fluid flux of 4.0 Â 10 2 mol H 2 O cm À2 during regional metamorphism. Pervasive circulation of locally derived metamorphic fluid can produce systematic carbonate depletions by (1) promoting isotopic exchange between carbonates and coexisting, lower-d 18 O siliceous phases (i.e., scenario 3 above) or (2) promoting isotopic exchange between carbonates and the metamorphic fluid itself (scenario 2).…”