Bruce
I don't understand this comment. Personally I think it is important to
distinguish rock names from facies names, even though we all speak rather
casually about one meaning the other. You should know it's not
terminology itself that I personally care about. However, I do not see why
we should throw out the higher T hornfels facies just because some seem to
be formed during regional metamorphism. The facies concept does not
involve identification of the thermal cause, only the assemblages that are
found. Yet the argument that has appeared on the net is as follows:
(1) hornblende and pyroxene hornfels facies rocks are not always
hornfelses, and
(2) rocks of these facies are also found in regional metamorphic terranes,
therefore
(3) these facies names should be dropped in favor of the amphibolite and
granulite facies.
I am also puzzled by the statement "there are some extreme conditions
of high-T, low-P contact metamorphism with no regional equivalents, in
which some very distinctive assemblages are produced. The science behind
why this should be the case is clearly of interest.". I think it is very
evident why this is the case. The only way that regional metamophism could
get to 900°C and 0.5 kbar is if the entire crust were molten, so it simply
doesn't occur (except very early on in earth history?). I think it is
likely that apparent regional metamorphism of 4 kbar and 700-800°C is
occult contact metamorphism unless isothermal decompression during
synmetamorphic unroofing is involved.
eric
>It is a sad comment on where Metamorphic Petrology has got to, that some of
>us do not appear to read the scientific content of each other's emails any
>more, we just worry about terminology. I attempted to make the scientific
>point, which Jorge Julian Restrepo has also picked up on, that, although we
>all agree that there are a wide range of P-T conditions in which either
>classic hornfelses or apparently normal regionally metamorphosed rocks may
>be produced, there are some extreme conditions of high-T, low-P contact
>metamorphism with no regional equivalents, in which some very distinctive
>assemblages are produced. The science behind why this should be the case is
>clearly of interest. The fact that I referred in passing to Turner's
>regional/honfels facies terminology, even though it was only to point out
>that it was not appropriate, may perhaps be of interest to lexicographers
>but it is not of scientific interest. I would like to propose that someone
>sets up a separate geo-lexicography mailing list for people who want to
>discuss matters of pure terminology.
>
>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.
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