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Subject:

Re: metam facies confusion

From:

Eric Essene <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Wed, 14 Nov 2001 16:55:50 -0500

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Bob,
    So what about that staurolite schist if the fluid was 90% CH4?
eric


>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)


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

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