The timing and direction of metamorphic fluid flow in Vermont |
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| Authors: | Alasdair D L Skelton |
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| Institution: | (1) Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh, Scotland EH9 3JW, GB |
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| Abstract: | A suite of metabasite dykes, emplaced within the Albee Formation, east-central Vermont, preserves evidence of interaction with a CO2-bearing hydrous fluid during Acadian metamorphism. Coupled advective and diffusive cross-layer fluxes of CO2 are recorded by forwards progress of the hydration-carbonation reaction: 3 amphibole + 2 epidote + 8 H2O + 10 CO2 = 3 chlorite + 10 calcite + 21 quartz Advection, diffusion and mineral reaction rates are modelled from reaction progress data through application of an analytical solution to the 1-D mass conservation equation for linear reaction kinetics (Lichtner 1988; Lasaga and Rye 1993; Skelton et al. in press). Dimensionless Peclet (Pe) and Damköhler (ND) numbers which describe the ratios of advection to diffusion and reaction rate to advection are thus resolved, from which time-integrated fluid fluxes are calculated. Small Pe (<10), large ND (>10) and small time-integrated fluid fluxes (<5 m3/m2) are obtained, predicting that cross-layer fluxes of CO2 are largely diffusive. It follows that, within the Albee schists, fluid flow must have been layer parallel. Where fluid flow is layer parallel, it is both interesting and informative to consider the geometry of fluid flow at a fold hinge. For layer-parallel fluid flow to be maintained, the flow direction must reverse which is difficult to justify with regards to the driving force. It is perhaps more reasonable that fluid should “escape” through the fold hinge (cf. Skelton et al. 1995), either via close-spaced microveins (cf. Cole and Graham 1994) or macro-scale axial-planar quartz veins (Ferry 1992, 1994). At present, the Albee schists are ～ vertical and strike ～ N–S. This reflects: (1) E–W recumbent folding of strata (early Acadian “nappe-stage” deformation; D1); (2) N–S arching of strata (late Acadian “dome-stage” deformation; D2). In the Strafford Quadrangle, east-central Vermont, curious “Coarse Garnet Schists” occur in the axial regions of D1 fold closures. On the basis of this spatial association, crystal morphologies, reaction textures and chemical zoning profiles, it is postulated that the Coarse Garnet Schists developed in response to structurally focused fluid escape. That fluid flow was syn-D1 is implicit to this argument. As such, layer-parallel fluid flow must have been ～ horizontal. Although this conclusion is in close agreement with that of Ferry (1994), up-temperature fluid flow is not implicated. |
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