Contact metamorphism of roof pendants at Hope Valley,Alpine County,California, USA |
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Authors: | John M Ferry |
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Institution: | (1) Department of Earth and Planetary Sciences, The Johns Hopkins University, 21218 Baltimore, MD, USA |
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Abstract: | Calcareous hornfelses and marbles all contain calcite+K-feldspar+quartz+sphene±diopside±plagioclase ±scapolite±clinozoisite. In addition, rocks on one side of a fault contain combinations of biotite, amphibole, and muscovite while those on the other side contain combinations of grossular, wollastonite, and axinite. At
bars, mineral-fluid equilibria in biotite and amphibole-bearing rocks record T= 440° C and
garnet-bearing rocks record T= 540° C and
Conventional volumetric fluid-rock ratios were calculated using measured progress of prograde decarbonation reactions and the
conditions of metamorphism: marbles, 0–0.4; amphibole-bearing hornfelses, 1.0–1.4; garnet-bearing hornfelses, 2.8–6.7. Decarbonation reactions were driven by pervasive infiltration of rock by reactive aqueous fluids. Differences in fluid-rock ratio between interbedded marble and hornfels and lack of correlation between fluid-rock ratio and whole-rock Cl-content, however, argue for channelized fluid flow along lithologic layers. A new analysis of reaction progress allows estimation of time-integrated fluxes for a specified temperature gradient along the direction of flow. Results are: marbles, 0–0.1×105 cm3/cm2; amphibole-bearing hornfelses, 0.8–1.3×105 cm3/cm2; garnet-bearing hornfelses, 1.2–2.5 × 105 cm3/cm2. Fluid flowed from regions of low to regions of high temperature. Using a simple thermal model for the area, the duration of contact metamorphism was estimated as 105 years. Assuming the time of fluid flow was the same as the duration of the thermal event, the first measurements of average metamorphic fluxes (q) and permeabilities (k) are: average marbles, q=0–0.3×10–8 cm/s and k = 2×10–6 darcy; hornfels, q=3–8×10–8 cm/s and k =20–53×10–6 darcy. Estimated premeabilities are within the range of values measured for metamorphic rocks in the laboratory. Fluxes, permeabilities, and whole-system fluidrock ratios are similar to those estimated for the Skaergaard hydrothermal system by Norton and Taylor (1979). |
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