At 08:31 AM 05/11/2001 +0100, prior to provoking JC's defense of "the noble
discipline", Christian wrote:
>... 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;...
Nomenclature aside, perhaps we can forge a link between the two noble
disciplines (:-)) by recognizing that the reaction in question is better
approximated by Hb + Pl + Qtz = Grt + Cpx + tonalitic liquid. In the
prograde direction this is a vapour-absent melting reaction (or
dehydration-melting reaction if you prefer). In the retrograde direction,
although Bowen did not recognize garnet as part of his Reaction Series,
this reaction is obviously capable of contributing to magmatic
differentiation if there is fractionation of the phases during
crystallization.
John Percival (1983 Am Mineral 68, 667-686) mapped a ~2400 km^2 area of
Archean migmatitic metabasites characterized by Grt+Cpx+Pl+Qtz
plus-or-minus Hb. Within this area are about 15 occurrences of Opx+Cpx+Pl.
The leucosomes have tonalite (sl) composition. His calculated curve for the
vapour-absent melting reaction has a steeply negative P-T slope, extending
from ~800C at 3 kbar to ~700C at 12 kbar.
Following Bas Hensen, Dirk deWaard and others, I would recognize
Grt+Cpx+Pl+Qtz in a metabasite as diagnostic of the high-P part of the
granulite facies. Otherwise, the P-T conditions of granulite facies and
eclogite facies do not conjoin.
Cheers, Dugald
Dugald M Carmichael Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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