Dear All,
Jürgen wrote:
"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."
Then there is a standard nomenclature for metamorphic rock. Is it published
somewhere? As John Clemens already said, the IUGS subcommission on
metamorphic rocks nomenclature has not given yet its recommendations. In the
field, when dealing with metamorphic rocks and several geologists, the first
day generally consists in discussions about nomenclature to be able to
communicate. Even for words as "gneiss" or "micaschist" this is no agreement
(is gneiss indeed a purely textural term? what about the mylonitic
texture?). Working mainly in magmatic rocks where nomenclature problem is
mostly (not entirely) solved, I can see the softening in exchanges that this
gives. To answer to the magmatic nomenclature of Christian Nicollet, it is
indeed easy to give him the reference of the IUGS recommendation. Example:
an andesite is an intermediate volcanic rock usually porphyritic defined
first by mode (pl+px+hb +/-bi) and if not available by its SiO2 (57-63%) and
Na2O+K2O (max 6-7%) contents; this is a good example of the unambiguity of
the recommended classification. There is no relation made to the C-A or Alk
series (can be debated) but only to alkali content (could be debated if the
rock is altered, but at a lower level).
I would then be grateful to know what this standard nomenclature for
metamorphic rock consists in.
Have a nice day,
Jean-Paul Liegeois
Head of Section, Isotope Geology
Africa Museum
B-3080 Tervuren, Belgium
Tel & Fax: +32 (0)2 6502252
e-mail: [log in to unmask]
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