To whom interested:
I never thought the Turner's system of defining hornfels series
is useful, But I will use the pyroxene hornfels facies in honour of
Goldscmidt and Eskola. We need Eskola(1939) 's faceis and modified
Coombs facies, which are zeolite, prehnite-actinolite and
pumpellyite-actinolite facies. I think prhenite-pumpelllyite facies is
confusing and better not to use.
One can subdivide a facies by well-defined mineral assemblages
depending the areas they are working, It is something like period in
Geological time scale. If one has good isotopic ages one may use it
instead of Jurassic etc
Shohei.
1-23-16-104
Takano Higashi-Hiraki
Sakyo-ku, Kyoto
606-8107, Japan
T&F (+) 81-(0)75-722-7175
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-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]]
On Behalf Of Ed Ghent
Sent: Thursday, October 16, 2003 1:47 AM
To: [log in to unmask]
Subject: Re: metam facies confusion
Worse yet, how do you tell albite-epidote hornfels from greenschist
facies?
I agree with Bob, that there are some individual mineral assemblages
that
do characterize a facies. What bothers me most is naming a facies on
inadequate evidence. In some cases this is because the term used is
thought to have more impact, e.g.,
K-feldspar-sillimanite-quartz-almandine
as granulite facies without supporting evidence.
Ed Ghent
At 11:38 AM 10/15/2003 -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)
> >
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