Dear Pavel and others,
I think this is a rather common texture in high-grade metapelites, widely
reported in the literature to form from generalized reactions like:
Bt + Als + Qtz = Crd + Spl + Kfs + H2O/melt, where locally the quartz may
be totally consumed, leaving 'silica-understaturated spots' with cordierite
containing inclusions of Als+Bt and newly formed Spl. These are often
surrounded by Kfs. If the Spl is Zn-rich it is stable to lower
temperatures, even in the presence of Qtz (e.g. Montel et al. 1986)
Montel, J.-M., Weber, C. & Pichavant, M. 1986. Biotite-sillimanite-spinel
assemblages in high-grade metamorphic rocks: Occurrences, chemographic
analysis and thermobarometric interest. Bull. Minéral. 109: 555-573.
Ulf B.
>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 1000=9CC. 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 1000=9CC.
>
>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]
|