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All,
    Once again a rock term (hornfels) is confused with a facies name.  The
term regional facies is also unnecessarily restrictive, as contact
metamorphism can also occur at high pressure.
eric


>Overall, I think that Dugald is right to question the validity of the
>Hornfels facies, which have worried me for a while. However there is one
>important exception at high-T. At the low-T end of the low-P spectrum there
>is indeed little clear distinction between geothermal field alteration and
>regional assemblages in buried volcaniclastic sequences, although
>metamorphic assemblages may form at very shallow depths, constrained only
>by the boiling curve of water. Greenschist facies assemblages with epidote
>and amphibole are known from very shallow levels by drilling, and do not
>obviously need a separate facies name. The rocks are certainly NOT
>hornfelses, usually they are fractured and veined volcanics. Shallow
>aureole sequences rich in wet sediments probably stay cool and don't
>develop classical contact metamorphism, while shallow crystalline rocks
>above c.350C are not permeable enough to show hydrothermal contact
>metamorphism, and don't show many other changes (see Fournier's analysis).
>As a result, the really high temperature effects caused by melting close to
>shallow igneous contacts are actually very distinctive, not the end of a
>continuous spectrum, and I would argue that they do merit their own facies
>name, if only to emphasise that they cannot fit into any other facies.
>However whether you can realistically justify both pyroxene hornfels and
>sanidinite I very much doubt. If I were to redraw the facies diagram now I
>might actually put a zone of "not found in nature" over part of Turner's
>hornfels facies at very low pressures and medium temperatures, and merge
>the higher-P parts of them into the regional facies, as Jurgen and Dugald
>advocate.
>
>Bruce Yardley
>
>
>
>
>--------------------------------------------
>Professor Bruce Yardley
>School of Earth Sciences
>University of Leeds
>Leeds LS2 9JT
>UK
>
>Tel. 0113 233 5227        Fax  0113 233 5259
>---------------------------------------------
>
>GEOFLUIDS now exists!    http://www.blackwell-science.com/gfl


Eric Essene
Professor of Geology
Department of Geological Sciences
2534 C.C. Little Bldg.
425 E. University Ave.
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