Dear Javier,
This case seems to be not so rare. I observed sectorially zoned
zoisite blasts (up to 1 cm across elongation) in the Qtz and
Ms-Qtz veins cutting eclogites of Marun-Keu (the Polar Urals,
Russia). I can send you their micropotograph. These blasts are not
pseudomorphs and really were formed at disequilibrium
crystallisation. I met sector and oscillating zoning in
clinozoisite crystals from Hbl-Czo metasomatites after amphibolic
schists at the Kostamuksha greenstone stucture (Karelia, the
Baltic shield). Thise clinozoisite corroding primary hornblende
reveal disequilibrium at this case. Oscillating zoning can be
found in many other zoisites and clinozoisites.
Honeycomb (mesh-like) pattern in Zo-Grt-Omph eclogites (also with
Ky and carinthine) from Marun-Keu was described by N.G.Udovkina in
her monographs (1971,1985) - unfortunately, in Russian. There one
can see all stage of conversion from troctolites through coronites
to eclogites. Plagioclase pseudomorphed by fine-grained Zo-Ky
aggregate; olivine and bronzite was transformed into omphacite;
and garnet formed in coronas between plagiocalse and clinopyroxene
zone. However zoisite was recrystallized and therefore does not
reflect the primary inhomogeneity of plagiocalse grains.
Zoisite in Marun-Keu Qtz veins is more late. All the more its
zoning is primary-growth.
Disequilibrium origin of sector zoning was considered in monograph
"Modern Crystallography", v.III. Crystal Growth. / Springer Series
in Solid-State Science, 36. Springer Verlag, 1984. Many examples
of sector zoning are described in book of D.P.Grigoriev, Ontogeny
of minerals (I don't know exact reference; the English translation
was published in Jerusalem).
Pavel.
13-Dec-2001 15:17 Javier Rodrøguez Aller wrote:
> Dear all,
> I've read about chemical aspects of sector zoning in zoisite
> (variations in
> Fe3/Fe3+Al ratios: Deer, Howie & Zussman, 1986)... Going further,
> does
> anybody have any opinion about the origin(s) of this rare feature?
> I've found sector-zoned zoisite porphyroclasts in poorly deformed
> eclogite.
> Zoisite coexists with omphacite porphyroclasts and garnet, which is
> arranged in honeycomb patterns (surrounding omphacite and zoisite
> clasts/domains).
> It seems therefore that eclogitization took place in this rock at
> nearly
> static conditions, with low-range chemical diffusion... Is then
> zoisite
> reflecting some chemical inhomogeneity in a pristine (now
> diappeared)
> phase? ... I'm thinking about possible An-rich, (zoned?) igneous
> plagioclase.
> PS:... Trace amounts of trivalent cations in the former phase can
> also
> account for tiny rutile inclusions, arranged in preferred
> orientations
> within zoisite clasts.
> Thanks,
> -------------------ooo---------------------
> Javier Rodrøguez
> Dpto. Mineralogøa y Petrologøa
> Facultad de Ciencias, Univ. del Paøs Vasco
> Apdo. Correos 644
> E-48080 Bilbao, Spain
> Tfn: + 34 94 601 5456
> Fax: + 34 94 601 3078
> e-mail: [log in to unmask]
> ===========================================
--
Pavel Azimov, PhD
Institute of Precambrian Geology and Geochronology
Russian Academy of Sciences
2, Makarov Embankment
St.Petersburg, 199034, Russia
Phone: +7(812)328-03-62
Fax: +7(812)328-48-01
E-Mail: [log in to unmask]
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