Dear Kare & others,
The descriptions remind me of several regions in the southwestern
Grenville province, Ontario (Bancroft-Wilberforce area) which contain
nepheline- and scapolite-bearing gneisses with corundum. This belt also
contains locally developed corundum-feldspar pegmatites with largest
corundum crystals exceeding 30 cm. The classical description is:
Carlson, H. D.. Origin of the corundum deposits of Renfrew County,
Ontario, Canada. Geological Society of America Bulletin (1957), 68
1605-36.
See also:
Anderson, Gregor M.; Cermignani, Claudio. Mineralogical and
thermodynamic constraints on the metasomatic origin of the York River
nepheline gneisses, Bancroft, Ontario. Canadian Mineralogist (1991),
29(4), 965-80.
One can observe continous transitions from calc-silicate
parametamorphics (amphibole-plagioclase) through biotite-scapolite-plg
towards the corundum-plagioclase-scapolite-bio+/-magnetite assemblages.
I believe that these compositional trends could be tight to
brine-induced metasomatism.
More below:
> Most of the partly pseudomorphed corundum crystals occur within the
> quartz-free leucosomes of the migmatite. Generally, the leucosomes are
> composed of plagioclase, while the mesosomes are composed of amphibole +
> plagioclase. The present interpretation is that corundum crystallized in
> situ from the partial melt.
Have you checked equilibria of the type:
amphibole = anorthite + mtt + cor + H2O
amphibole + cor = spinel + plg + H2O
to get some constraints on PT, a(H2O) etc?
Best wishes,
David
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David Dolejs (postdoc/geochemistry)
Bayerisches Geoinstitut
Universitaet Bayreuth
D-95440 Bayreuth Tel: +49 921 553717
Germany Fax: +49 921 553769
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