Dear Pavel
Another attractive possibility in addition to those you have received is
that these represent prograde pseudomorphs after a rather refractory
aluminous phase, quite likely staurolite. The initial rimming of the
staurolite by reaction products (cordierite) may isolate remaining
staurolite from the quartz-bearing matrix. Subsequent staurolite
breakdown on composition could give the assemblage cordierite + spinel +
sillimanite. More likely, though, there is partial communication between
the decomposing staurolite and the matrix. In my experience many such
intergrowth and nodular textures approximately conserve Al and Si but
allow exchange of Fe and Mg. Maintaining Al/Si should generate a Crd-Spl
intergrowth in about 60:40 proportions.
I referred to various nodular textures like this as prograde
pseudomorphs in a study of sapphiring-bearing rocks (J Petrology 27,
541-565, 1986). A similar example with relic staurolite had been
described by Stoddard, 1979, Am Min 64. Spinel-bearing pseudomorphic
intergrowths after Al-silicates are well known from contact metamorphic
settings, particularly in xenoliths and enclaves (Didier 1973 "Granites
and their enclaves"; Van Bergen, 1983, Geol Rundschau 72).
Subsequently one of my graduate students (Huw Humphreys) studied low-P
metapelites from eastern Namaqualand, South Africa, in which
Crd-Spl+/-Sil nodules preserve the characteristic crystallographic
outlines of staurolite porphyroblasts, in rocks known to carry such
staurolites at lower grade. These get a passing mention in Humphreys and
Van Bever Donker 1990, J Metamorphic Geol 8, 159-170.
Dave
Pavel Azimov wrote:
>
> Dear geo-metamorphists,
>
> I would like read your opinion on rather usual texture: inclusions
> of Mg-spinel or hercynite in the core of cordierite porphyroblasts
> in quartziferous rocks (generally, gneisses). Sometimes these
> porphyroblasts contain also fibrolite needles. The direct contact
> between spine/hercynite and quartz is invariably absent.
>
> Spinel-bearing cordierites are known mostly in granulitic gneisses
> however without evidences for temperatures above 1000œC. I have
> met also hercynite inclusions in cordierite from an assemblage
> Qtz+Pl+Bt+Chl+Crd+St+And. The mineral composition and geological
> setting (central part of greenstone belt) of this rock indicates
> that it did not heat up to 1000œC.
>
> So, spinel or hercynite in silica-saturated rock below this
> temperature must be metastable. What can you say about origin of
> such inclusions? Why they forms? Why spinel is located in
> cordierite, not in other minerals?
>
> Pavel.
> --
>
> 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]
--
Dave Waters - Lecturer in Metamorphic Petrology, Oxford University
Dept of Earth Sciences, Parks Rd, Oxford OX1 3PR, UK
Tel: +44 1865 272000
Direct: +44 1865 272058 Email: [log in to unmask]
Fax: +44 1865 272072 http://www.earth.ox.ac.uk/~davewa/
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