Hercynite is stable with quartz down to 750°C. Its reaction to
form garnet or cordierite of course depends on P as well. On the
other hand Mg-rich spinel is not stable with quartz. I wonder if
your matrix assemblage is a migmatite that is introduced later, or
whether it is a quartzose metasediment.
>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°C. 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°C.
>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 Azimov, PhD
> Institute of Precambrian Geology and Geochronology
> Russian Academy of Sciences
> 2, Makarov Embankment
> St.Petersburg, 199034, Russia
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