Bruce
Bob noticed, as did you, as did I almost
immediately after sending the message.
eric
>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 ..........
>
>Bruce
>
>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!
>>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)
>>>
>
>
>
>Professor Bruce Yardley
>School of Earth Sciences
>University of Leeds
>Leeds LS2 9JT
>UK
>
>Tel: +44 (0)113 3435227
>Fax: +44 (0)113 3435259
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