This probably is not the case for your samples, but it is possible to
get intergrowths of spinel in cordierite via the pseudomorphous
replacement of aluminosilicate porphyroblasts via a prograde (c.
550°C) reaction of the type: andalusite + Mg-rich chlorite = spinel +
cordierite + fluid. In the Reynolds Range, Northern Territory,
Australia, this happened. I think it is a result of a complex
history, involving later prograde metamorphism, to low-P granulite
facies, affecting areas where I think earlier "diagenetic
hydrothermal" alteration had produced Mg-rich chlorite. In the most
Mg-rich areas, the pseudomorphs have aluminosilicate cores partly
pseudomorphed at the rims by a cordierite + corundum intergrowth; in
slightly more iron-rich areas, the blocky pseudomorphs consist of
intergrown cordierite + green spinel, inside a clear cordierite rim.
The pseudomorphs occur in rocks that contain quartz.
>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
> Phone: +7(812)328-03-62
> Fax: +7(812)328-48-01
> E-Mail: [log in to unmask]
Dr. Julie Vry
Senior Lecturer - Geology
School of Earth Sciences
PO Box 600
Wellington, New Zealand
phone, office: 64 4 463 6432
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