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GEO-METAMORPHISM  2001

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Subject:

Re: metam facies confusion

From:

Robert Tracy <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Wed, 14 Nov 2001 14:03:09 -0500

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text/plain

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text/plain (101 lines)

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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