Dave,
Yes, I understand the difference but thought some readers might be
interested in that old reference. In a sense, many carbonates are
ultramafic at high T because they are not stable with quartz. That
allows reaction of igneous rocks with carbonate rocks to yield
desilicated assemblages. Indeed, producing nepheline gneisses and even
nepheline syenites in the Bancroft area by metasomatic reactions of
syenite with marble is an old idea, dating back 50 years to Tilley &
Gittins.
eric
On Dec 9, 2004, at 12:27 PM, David Dolejs wrote:
> Eric and others,
>
> Corundum along serpentinite-felsic rock or even magnesite-gneiss
> contacts is, indeed, relatively more common. One of the many Russian
> papers:
>
> Rossovskii, L. N.; Konovalenko, S. I. Corundum plagioclasites of the
> southwestern Pamirs. Doklady Akademii Nauk SSSR (1977), 235(3),
> 663-6 [Petrogr.].
>
> I fear that its setting and explanation are quite different from the
> previous examples though. Here, progressive reaction zones (in the
> latter state) or melt contamination (analogous to blackwall alteration,
> in the former) seem the most obvious. The Grenville examples lack any
> ultramafic rocks and further grade into bio-corundum-nepheline
> gneisses.
>
> David
>
> Eric Essene wrote:
>> Dave and all,
>> Lawson (1901, Geol. Soc. Am. Bull.) described a corundum pegmatite
>> cross-cutting a serpentinite in the foothills of the Sierra Nevada,
>> CA.
>> He called the corundum-albite rock a plumasite. The term has since
>> been
>> used on occasion.
>> eric
>>
>>
>> On Dec 9, 2004, at 9:47 AM, David Dolejs wrote:
>>
>> 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
>>
>> --
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> David Dolejs (postdoc/geochemistry)
>> Bayerisches Geoinstitut
>> Universitaet Bayreuth
>> D-95440 Bayreuth Tel: +49 921 553717
>> Germany Fax: +49 921 553769
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>>
>
> --
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> David Dolejs (postdoc/geochemistry)
> Bayerisches Geoinstitut
> Universitaet Bayreuth
> D-95440 Bayreuth Tel: +49 921 553717
> Germany Fax: +49 921 553769
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
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