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/