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At 12:44 02/11/01 -0500,
Dave wrote:
"I'd be careful about this. Grt+Cpx+Pl is the high-P equivalent of
  Opx+Cpx+Pl, the latter which everyone agrees is granulite. So Grt+Cpx+Pl is
  certainly characteristic of high-P granulites, but is it definitive? There
  is quite a division on this point, seemingly depending on perspective.
  There are a few unambiguous amphibolite-grade occurrences of Grt+Cpx+Pl,"

and Jim wrote:
         " I totally agree - I do not consider Grt+Cpx+Pl definitive of the
granulite facies; where I have that assemblage in amphibolitic rocks,
without nearby 2-px-bearing mafic rocks,"

I also agree and there is a good reason for that: Grt+Cpx+Pl+Q is a typical
paragenesis of HP granulite facies and the transition between HP granulite
facies and amphibolite facies is reached by the reaction Grt+Cpx+Q=Hb+Pl;
if Q is absent in a SiO2 undersaturated rock , the assemblage Grt+Cpx+Pl
(with or without Hb) may persist in the amphibolite facies. (if the rock is
very undersaturated like meta troctolites, then spinel appears)
It is exactly the same thing between the HP granulite facies and IP
granulite facies with the transition reaction Grt+Cpx+Q=Opx+Pl; the
paragenesis Grt+Cpx+Pl (with or witout Opx) can exist in SiO2
undersaturated IP metabasic granulites. I call this kind of paragenesis an
atypical paragenesis.

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