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 ..........
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!
>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
> > >>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
Tel: +44 (0)113 3435227
Fax: +44 (0)113 3435259