Bruce
Thanks for pointing this out. I was trying to determine the origin of
said communications.
Yes, perhaps we could move on.. or perhaps this discussion is timeless.
cheers
On Thursday, Oct 16, 2003, at 03:52 US/Central, Bruce Yardley wrote:
> Should we be worried that our science is not really cutting edge and
> moving forwards with the times any more? Eric accidentally picked up
> an old email from a discussion of almost 2 years ago, rejoined the
> fray, and nobody seems to have noticed the hiatus ..........
>
> Bruce
>
> At 11:38 15/10/2003 -0400, you wrote:
>> Bob and all,
>> But there are amphibolites in the granulite facies, and also in
>> the
>> eclogite facies. One specimen does not a facies make!
>> I daresay that one could find a whiteschistin the eclogite
>> facies with
>> muscovite-biotite-garnet-staurolite-kyanite and we have seen just this
>> assemblage adjacent to obvious garnet granulites at 11-12 kbar and
>> 750-800 C
>> where the staurolite is zincian and the rock is starting to undergo
>> vapor
>> absent melting. QED -- one specimen does not a facies make!
>> eric
>>
>>
>> Quoting Robert Tracy <[log in to unmask]>:
>>
>> > As a follow-up to Eric's message, we also should keep in mind not
>> > only the historical context of the facies names as derived by Eskola
>> > and company, but also the fact that the original Eskola names are
>> > definitely composition-implicit, in a sense as a historical
>> > geographical/geological accident. The amphibolite facies, as
>> > displayed so well in the "Finnish Archipelago" of SW Finland where
>> > Eskola worked in the early 1900's, is mostly displayed in rocks of
>> > roughly andesitic or basaltic composition (or in some cases
>> > hydrothermally altered basalts, resulting in the classic
>> > orthoamphibole-cordierite rocks of that neck of the woods) which are
>> > areally abundant in outcrop there. Therefore, to Eskola the typical
>> > classic amphibolite-facies rock was, mirabile dictu, an amphibolite!
>> > A slightly lower-grade equivalent (a mafic schist?) was a
>> > greenschist. If George Barrow had named facies from the Glen
>> > Clova-Glen Esk areas 20 years earlier, we might have had
>> > "chlorite-schist facies" and "garnet-schist facies" instead of
>> > greenschist and amphibolite facies, and we'd be unhappy at
>> > facies-name assignments for rocks of mafic composition.
>> >
>> > I personally believe that one of the more likely reasons for the
>> > remarkable robustness over the last 75 years of the terms that
>> Eskola
>> > coined is that they are reasonably genetically neutral, i.e.,
>> > usefully descriptive, although compositionally derived.
>> Petrogenetic
>> > fads have come and gone through the twentieth century, but rock
>> > nomenclature (igneous or metamorphic) that avoids genetic
>> > implications and overly specific geographic references tends to
>> > persist, as Eric suggests.
>> >
>> > Finally, I disagree with Eric's rather absolutist point about never
>> > making a facies assignment based on one or a few samples. In some
>> > cases such caution might be justified, but I think most of us would
>> > be fairly confident in saying that a
>> muscovite-biotite-garnet-staurolite-kyanite schist reflected
>> > formation of the primary assemblage at amphibolite facies
>> conditions.
>> > I'd even be happy to stick my neck out for upper-middle amphibolite
>> > facies. Admittedly that type of potassic, aluminous lithology
>> > produces low-variance assemblages of quite limited P-T range,
>> > compared to a garden-variety "amphibolite" for example.
>> >
>> > Bob T.
>> >
>> >
>> > >Jürgen, Dugald and all,
>> > > No one should identify a metamorphic facies in hand specimen
>> at all.
>> > >Facies are distinguished by general associations in a variety of
>> rocks
>> > >subjected to the same P-T. Low pressure facies are also
>> identified by
>> > >assemblages, but not by their mechanism of formation. After all,
>> many
>> > >blueschist facies rocks are neither blue nor schists, yet no one
>> has a
>> > >problem with that term. If schists are not required for
>> blueschist or
>> > >greenschist facies rocks, why does anyone boggle at hornfels
>> facies rocks
>> > >without hornfelses? These are simply historical terms, well
>> established by
>> > >Eskola and subsequent workers. Hornfelses occur without contact
>> > >metamorphism and vice versa, so what?
>> > >eric
>> > >
>> > >
>> > >>I would fully support Dugald's statement. Can anybody tell me how
>> to
>> > >>differentiate between hornblende-hornfels facies and amphibolite
>> facies
>> > >>when looking at a hand specimen? What defines the upper pressure
>> limit of
>> > >>the "shallow contact metamorphic facies"? If we can use these
>> facies terms
>> > >>only in a field-related sense, where does "pure" contact
>> metamorphism end
>> > >>and where does low-pressure, regional-style thermal metamorphism
>> start?
>> > >>
>> > >>The idea that aureoles generally contain hornfelses is clearly
>> wrong. Do
>> > >>we then explain to students that a foliated
>> hornblende-plagioclase rock
>> > >>cannot be called a hornblende-hornfels, but rather an amphibolite
>> that
>> > >>originated in the hornblende-hornfels facies? What is lost if we
>> abandon
>> > >>these contact-metamorphic facies terms?
>> > >>
>> > >>Cheers,
>> > >>
>> > >>Jürgen
>> > >>
>> > >>J. Reinhardt
>> > >>School of Geological & Computer Sciences
>> > >>University of Natal
>> > >>Durban, 4041
>> > >>South Africa
>> > >
>> > >
>> > >Eric Essene
>> > >Professor of Geology
>> > >Department of Geological Sciences
>> > >2534 C.C. Little Bldg.
>> > >425 E. University Ave.
>> > >University of Michigan
>> > >Ann Arbor MI 48109-1063 USA
>> > >fx: 734-763-4690
>> > >ph: 734-764-8243
>> >
>> > --
>> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> > Dr. Robert J. Tracy
>> > Professor of Geological Sciences
>> > Virginia Tech
>> > Blacksburg VA 24061-0420
>> >
>> > 540-231-5980
>> > [log in to unmask]
>> > (FAX: 540-231-3386)
>> >
>
>
>
> Professor Bruce Yardley
> School of Earth Sciences
> University of Leeds
> Leeds LS2 9JT
> UK
>
> Tel: +44 (0)113 3435227
> Fax: +44 (0)113 3435259
>
>
Harold H. Stowell, Professor & Chair
Department of Geological Sciences
Box 870338
The University of Alabama
Tuscaloosa, AL 35487-0338
205 348 5098
205 348 0818 FAX
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