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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? Clark 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 > > Hi, > > 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. > > Cheers, > > Jurgen > > 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 UK email [log in to unmask] direct dial/voice mail: 01865 483610 international: +44 1865 483610 FAX 01865 483242