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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




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Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK

Tel. 0113 233 5227        Fax  0113 233 5259
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