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GEO-METAMORPHISM  2001

GEO-METAMORPHISM 2001

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Subject:

Re: Amphibolite

From:

Bruce Bathurst <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Tue, 6 Nov 2001 14:57:33 -0800

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text/plain

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Christian Nicollet wrote:

> At 10:17 05/11/01 +0000, vous [Msr Watt] avez écrit:
> >And I wondered why students were being put off metamorphic geology...
>
> is it so different in magmatic petrology ?

As Prof Banno remarked, 'It is too
covetous to hope single name can
represent protolith, mineral assemblage
and facies it belongs to.'

1. In a good nomenclature, several names
can represent the same object. It fails
if one name represents several objects.
'Granulite' fails, and 'amphibolite'
probably fails.

2. In a good petrological nomenclature,
a name is expressed using terms that
create a picture in the mind, so the
rock can preferably be recognized in the
field. It must not denote a
petrogenesis, though it may connote one
(which changes with research).
'Granulite' fails, and 'amphibolite' may
fail.

As Nigel Kelly wrote: 'As an undergrad,
I was taught that the simplest method
for naming
metamorphic rocks was to use the main
(or important) minerals along with the
dominant texture (schist, gneiss,
hornfels...).'

Fortunately, the undergrad may choose to
reserve the Germanic term 'amphibolite'
for use with 'facies', and use a base
name such as 'schist' for a rock. One
may also reserve 'granulite' for use
with 'facies', and use a rock name such
as 'leptite' for the granulose example
chosen by Eskola. This still leaves the
general problem of what base name to use
for a granular rock with granulose
fabric or no visible fabric at all. Are
there any?

Having separated rock name from facies,
the undergrad can prepend mineral names.
This identifies the mineral assemblage
in the rock, for those interested in
intensive thermodynamic states.

If the minerals are listed in increasing
abundance, one has included a coarse
measure of the composition of the rock,
for those interested in protoliths and
extensive thermodynamic states, as
igneous petrologists are. Until
metamorphic petrology focuses less on
intensive states, it might be good to
avoid using relative amounts of minerals
to name rocks. In the future one may
wish to use modal distinctions that
connote a genetic distinction.

Shouldn't the metamorphic literature
currently distinguish rock, assemblage,
composition, and facies?


Bruce Bathurst







   






As an undergrad, I was taught that the
simplest method for naming
metamorphic rocks was to use the main
(or imortant) minerals along
with the dominant texture (schist,
gneiss, hornfels...). Eg. a
clinopyroxene and amphibole bearing rock
that has a hornfels texture
would be a cpx-amph hornfels (as
originally refered to by Anthi). This
sort of classification, to me at least,
conveys the most information about
the rock. By adding garnet to a
pyroxene amphibolite you may get a
granulite, but the term 'granulite' is
so commoly used to cover any rock
that formed/recrystallised in the
granulite facies, that it really conveys
little information apart from just
that. Just a thought, anyway ...

Cheers,

Nigel.


At 15:49 05/11/01 +0200, vous avez
écrit:
> I am not so sure whether we can restrict the term granulite to basic
> rocks after it had been used for some 100 years in a different sense.

Perhaps …. But should the things never
change? In Europe, (french and
German ?), we gave up the old term
leptynite (leucocratic gneiss).


> In fact, the original term referred to a felsic rock.

But who restricts the granulite word to
the only felsic rocks? On the
contrary, this word took a too general
sense! Why not restrict it to the
metabasites?
If not, I'd use : 2prx metabasic
granulite, etc.

Christian
http://christian.nicollet.free.fr/

 We have our blue
book! The same could not be said for the
world of metamorphic
nomenclature.
With amusement,
JC

The name of the various metamorphic
facies comes from the basic rocks
metamorphized under
these conditions: a metabasic rock is
(generally) an amphibolite in the
amphibolite facies (but a
metapelite is not an amphibolite) , an
eclogite in the eclogite facies (a
metapelite is an eclogitic
micaschist), a 2prx hornfels in the 2prx
hornfels facies. But in the granulite
facies, unfortunately,
ALL the rocks are named granulite! We
must reserve the term granulite for
metabasites, since it is
the case for the other facies!.

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