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1 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D–44780 Bochum, Germany
2 Planetary and Space Science Centre, Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick, E3B 5A3, Canada
E-mail address: claudia.trepmann{at}rub.de
The mineralogy and microstructure of Ca-rich metasomatic zones developed in crystalline target-rocks from the 54-km-diameter Charlevoix impact structure, in eastern Quebec, have been investigated by optical microscopy, analytical scanning electron microscopy and cathodoluminescence microscopy. The zones occur as discrete lenses (<0.5 m long) and as veins (<0.5 m wide) associated with regions of locally enhanced fracturing within Grenville Province charnockitic gneisses. The dominant metasomatic assemblage is prehnite + quartz 
calcite. Mineral fragments of the host gneiss also occur within the Ca-rich zones. Reaction of the fluid with the host gneiss and its entrained fragments is revealed by the partial replacement of shocked quartz by aggregates of authigenic quartz. Prehnite is considered to have supplanted the anorthite component of the host plagioclase. The predominance of prehnite indicates that precipitation mainly occurred at 250–380°C, probably following a higher-temperature phase. Mineralized cavities within the Ca-rich zones are themselves cross-cut by prehnite-filled fractures, and quartz partly shows a fibrous or spherulitic structure, indicating successive stages of precipitation during cooling. The Ca-metasomatic event occurred after the impact because it affected already shocked rocks and because the metasomatic minerals show no evidence of shock. The relatively high temperature of the fluid and the restriction of the Ca-rich zones to the impact structure suggest that the metasomatism is impact-related. Thermal energy to drive the hydrothermal system would have come from a combination of overlying impact-melt sheet and fallback, central uplift elevation and waste shock-induced heat. Given the Ca- and CO2-poor composition of the host gneiss, hydrothermal circulation through overlying carbonaceous Ordovician target-rocks may have provided an additional source for these components. Silicon, Al and Fe3+ were probably derived from the reaction of the fluid with the host gneiss. Ca-rich metasomatism was caused by the relatively short-lived (<1 Ma) thermal convection of aqueous fluids through shock-heated target-rocks that had acquired enhanced permeability due to impact-induced fracturing, with subsequent precipitation occurring in high-porosity zones.
Keywords: Ca metasomatism, prehnite, post-impact hydrothermal activity, planar deformation features, cathodoluminescence, shock features, Charlevoix impact structure, Quebec.
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