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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