I have been subject to the same suite of correspondence. Awful eh? Yes, it is no
wonder people get put off. Coincidentally, I satrted on metamorphic petrology to
the "nd yeas this morning!
Any progress on the detritals?
Gordon Watt wrote:
> 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
Dr Clark R.L. Friend
Dept of Geology
Oxford Brookes University
Oxford OX3 0BP
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