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GEO-METAMORPHISM  2001

GEO-METAMORPHISM 2001

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Subject:

Re: Hornfels facies: To be or not to be

From:

Bruce Bathurst <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Tue, 20 Nov 2001 04:21:01 -0800

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Jürgen Reinhardt wrote:
 
>> >>> [log in to unmask] 11/19/01 06:32PM >>>
>> Crossing vertically up-P from a hornfels to a regional facies does
>> not represent a change in grade, so nothing dramatic should happen.
>>
>> An ignomineous petrologist
 
> The change from amphibolite facies to eclogite facies is also not a change in grade, >but something does happen.

Yes, but because no change in the grade
of metamorphism has taken place, the
change must just be an artifact of
original differences in the protoliths.

>I hope you didn't mean crossing from contact to regional faies, because you might get >stoned to death as things stand right now.

Yes, perhaps temperature decreased when
passing from the facies of contact
metamorphism around a pluton to the
older, deeper facies of regional
metamorphism; if so, the metamorphism
decreased in grade with depth.

> Perhaps one way of keeping the distinction between different P-T gradients alive
> is to use a fourth facies series term as in Blatt & Tracy (I haven't seen this
> in any other book so far). I don't have hang-ups about "hornfels" or, alternatively, > maybe "very-low-pressure/high-temperature".
>
> A metaphoric petrologist

The great value metamorphic petrologists
place on nomenclature is most
gratifying, and speaks well of all.

The GEO-LEXICOGRAPHY list!

I'm sure all will agree with Prof
Yardley that classification &
definition, being the sole of petrology,
are worthy of their own
geo-lexicographical list. Unfortunately,
natural classifications cannot be
separated from theory, and operational
definitions cannot be separated from
technique. So, there is no pure
nomenclature to separate.

Natural Classification

'The quartz-feldspathoid and
quartz-olivine boundaries are natural
dividing-lines, and as such they have a
scientific importance which artificial
boundaries can never have.' -S.J. Shand,
Eruptive Rocks, p228. Cf. Yoder &
Tilley.

Operational Definition

'Thus, Eskola (1939) attempted to give
an excessively positivistic definition
which was based only on directly
observable chemical and mineral
compositions. This attempt stemmed
probably from the positivistic spirit he
had aquired in his youth under the
influence of F.W. Ostward, Ernst Mach,
and others.' -A.Miyashiro, Metamorphism
& Metamorphic Belts, p294. (For
'excessively positivistic', today read
'operational'.)

Lexicography (with etymology!)

Facies, taken directly from the Latin
for form or shape (especially of the
face), share a conceptual meaning in
several sciences. (1) It's more general
use in Victorian England, as today, was
generally synonymous with 'appearance'.
(2) It's more restricted use was a more
abstract 'appearance' that attempted to
capture a phenomenon by ignoring
distracting features. That property used
to identify the phenomenon is usually
prefixed.

Examples of definition 2

Hence mineral facies capture
metamorphism by ignoring compositional
changes of the protolith. Lithofacies
capture sedimentary environments by
ignoring chronostratigraphic changes of
the protolith. Faunal facies capture
chronostratigraphic environments by
ignoring lithostratigraphic changes of
the protolith. (This was one of
geology's earliest 'natural
classifications', sought by Edward
Forbes, a contemporary of Sedgwick &
Murchison.)

Metamorphic Facies

Goldschmidt studied the hornfels facies
of his contact metamorphic aureole (dfn
1). His classes neatly defined a mineral
facies for Eskola (dfn 2), whose
holotypic rock was a hornfels! (Eskola's
contact facies near Orijarvi were used
to define the amphibolite facies.)

Some objected to this restricted use of
facies, though it had old precedents.

'In petrology the expression "facies"
had, until now, designated areas within
a single geological region that varied
either in "Struktur" (Strukturfacies) or
in the rock's mineral assemblage &
chemical composition
(Konstitutionsfacies).' -F.Becke, 1921,
Zur Facies-Klassifikation der
metamorphen Gesteine, T.M.P.M., v35,
p218. (My translation.
Facies also brought to Becke's mind the
'tektonische Facies' of B.Sander.)

'The term "facies" as applied by Eskola
must be considered as somewhat
unfortunate, as Becke has already
pointed out. Facies is a term long since
established in geology, but its use is
now bewilderingly lax. It has been
adopted in paleontology, but is also
used i tectonics and stratigraphy.
Petrology has not escaped, the term
being used with reference to a variant
of a rock type, as in "marginal facies",
&c.' -C.E.Tilley, 1924, Facies of
metamorphic rocks, Geol Mag, v.61,
p.168.

Very low temperature & high temperature
contact metamorphic facies

So, if the context of discussion is not
the mineral-facies classification of
rocks, one should be able to use
'aphanitic facies' or 'schistose
facies'. 'Hornfels facies' seems
ambiguous, at best. If the appearance of
the rock makes its petrogenesis very
clear, one might use
'contact-metamorphic facies' or
'regional-metamorphic facies'. Or, one
can bravely extend this definition of
facies even further. Personally, I see
no ambiguity and wouldn't object.


Bruce Bathurst

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