Carla,
These structures are uncommon but not rare, are typically in uppermost amphibolite facies or granulite facies rocks, and almost certainly represent oxidation/dehydration haloes; they are commonly called "magnetite eyes". In your illustration the "missing" phase is biotite, but in other cases (as from granulite gneisses in the Poudre River Canyon in Colorado) the mineral uniformly present in the matrix but missing from the haloes is hornblende. The oxidation/dehydration event appears to have produced magnetite as a product mineral, along with feldspar (Kfs in the case of biotite, plagioclase in the case of hornblende). An isochemical model would not predict quartz in the haloes, since both biotite and hornblende are silica-deficient minerals, but it is commonly present in the haloes and therefore some local metasomatism must clearly be going on. Where the magnesium goes in the reaction is an interesting question, as yet unsolved as far as I know, and I know of no publications that attempt an explanation of the haloes. An interesting test would be to see if there is a subtle change in biotite or hornblende chemistry (increase in Mg) for some distance surrounding the haloes.
The photos you sent show relatively equant haloes, but they are commonly ovoid or elongated where I have seen them, and the elongation typically defines a lineation. In one rock from New England USA, there is even an example of magnetite eyes where the magnetite cores are elongated and all have a common alignment forming a lineation, and the haloes are also aligned but at about a 30-40 degree angle to the magnetites and defining a different lineation. Rather difficult to explain that one...
Bob T.
Dr. Robert Tracy
Professor of Geosciences
Associate Department Head
Director, Museum of Geosciences
Virginia Tech
Blacksburg VA 24061-0420
540-231-5980
540-231-3386 (F)
On Feb 25, 2013, at 1:17 PM, Carla Cristine Porcher <[log in to unmask]> wrote:
> Dear All,
>
> I am trying to find publications that describe reactions and metamorphic conditions to the development of the "Bright-eyed" gneiss from Death Canyon. The texture composed of dark magnetite core, about 1/4 inch in diameter surrounded with a rim composed of quartz, feldspar in a gneiss matrix with quartz, feldspar, and biotite (see attached image from
> http://www.nps.gov/history/history/online_books/grte/grte_geology/sec5.htm)
>
>
> Many thanks
>
> Carla
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> --
> Profa. Carla Porcher
> Departamento de Geologia -IG/UFRGS
> <fig20[1].jpg>
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