And I wondered why students were being put off metamorphic geology...

-----Original Message-----
From:   Jurgen Reinhardt [SMTP:[log in to unmask]]
Sent:   Monday, November 05, 2001 7:30 AM
To:     [log in to unmask]
Subject:        Re: Amphibolite


There are some hang-ups about rock nomenclature that I don't quite understand.
An amphibolite, as any reasonable textbook would state, is a rock consisting
mainly of hornblende and plagioclase (not any amphibole plus plagioclase!).
Additional phases can/should be added as qualifyers (epidote amphibolite,
garnet amphibolite, Cpx (diopside- or whatever) amphibolite, etc.).
"Metabasite" is a term referring to a bulk compositional category, with no
information about mineral assemblage, metamorphic grade, nor structure. Hence,
it is used in the same sense as metapelite, but is not acceptable as a specific
rock name.

The term "pyribole" as defined by J.B. Thompson is firmly entrenched in the
literature. For that reason alone, I would not accept "pyribolite" as a rock
term for any type of pyroxene-amphibole rock. As S. Banno says: no more new
rock names, if it can be avoided.

I agree with Jim Eckert that the term amphibolite should be used based on
mineral assemblage only and not necessarily imply a facies. Keeping in mind,
however, that any middle-of-the-road amphibolite would commonly have formed
under amphibolite facies conditions, the minerals that (may) indicate otherwise
should be included in the rock term, even if it becomes lengthy. I cannot see
any problem in using "two-pyroxene amphibolite" (just like "two-mica granite").
It's certainly more informative than " amphibolite". There is, of course, only
so much a rock name can tell you, and if the hormblende is prograde or
retrograde remains a different matter.

There are always borderline cases where standard nomenclature may be
unsatisfactory. However, metamorphic rock nomenclature has the advantage that
it can cater for a lot of variety, and if one wants to be precise, one can use
as many qualifyers as necessary. It really depends on what one intends to
communicate to others.

The only real mishap in rock nomenclature is "granulite", a non-descriptive
term that refers to conditions of formation only and can be applied to all
sorts of rock compositions. Hence is not particularly useful as a stand-alone
rock name. However, before I open another can of worms, I stop here.



J. Reinhardt
School of Geological & Computer Sciences
University of Natal
Durban, 4041
South Africa