At 12:44 02/11/01 -0500,
“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.