I can't tell if my last message went out or not. Bob's example I think
is instructive. If one picks up a single
kyanite-staurolite-muscovite-biotite-garnet-quartz schist, it is easy to
assign it to the middle amphibolite facies. However, if the rock happened
to be unusual in that the fluid was rather heavily diluted by some
otherwise inert fluid species, that single rock may have formed say in the
low amphibolite facies, which could show up if one compared the assemblage
to adjacent assemblages in other lithologies such as ordinary H2O dominated
metapelites and amphibolites. That is exactly the point of metamorphic
facies. And who knows whether the muscovite was not stabilized by Li, the
staurolite by Zn, and the garnet by Mn or even Fe3+? Of course, these
concerns are unlikely in most cases, but it serves to illustrate the
utility of the facies approach.
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.
>>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?
>>>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?
>>>School of Geological & Computer Sciences
>>>University of Natal
>>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
>Dr. Robert J. Tracy
>Professor of Geological Sciences
>Blacksburg VA 24061-0420
>[log in to unmask]
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