The timing, geometry, compositions and volumes of fluid fluxes associated with metamorphism of carbonates and pelites from east-central Vermont during the Acadian orogeny (380 Ma ago) are determined using petrological observations, calculation of petrogenetic grids, time-integrated flux calculations and construction of strontium isotope profiles across carbonate horizons. Qualitative petrological observations suggest flow to have been largely layer-parallel and focussed along permeability contrasts such as carbonate/pelite boundaries. Vein-deformation relationships constrain timing, implying that fluid flow occurred to some degree throughout the metamorphic event, but that the majority of mineral reaction driven by fluid flow occurred to the thermal peak of metamorphism. Textures also record both pre- and post- metamorphic events. Carbon distribution and isotope values may indicate that carbon deposition occurred in certain locations in response to the mixing of fluids of different compositions. Pseudosections based on the petrogenetic grid constructed in the system KCaNaFMASCH predict observed sequences of assemblages and reactions. Calculations studying the effect of alkali metal metasomatism on assemblages show that increasing potassium content stabilises biotite over amphibole at high temperatures, while addition of sodium stabilises plagioclase at the expense of all other phases, particularly chlorite and muscovite. Detailed temperature - X CO<SUB>2</SUB> work on four outcrops identifies gradients in fluid composition across carbonate bands on < 1m scales at biotite grade and on 5 -15 m scales at kyanite grade. Time-integrated fluid flux calculations, performed using an expression derived from the mass-continuity equation for this study, show that observed reaction progress could have been driven by fluid fluxes between 10<SUP>4</SUP> moles m<SUP>-2</SUP> if flow was layer-perpendicular and 10<SUP>8</SUP> moles m<SUP>-2</SUP> if flow was layer-parallel. Fluid release was continuous but varied in rate by up to two orders of magnitude, with the bulk of reaction and fluid release occurring over small (5-10°C) temperature intervals.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598864 |
| Date | January 1999 |
| Creators | Evans, K. A. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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