Dear Pulak
I just noticed your note about the absence of muscovite in your pelites.
This is a typical feature of the pelites of Connemara at this grade, (which
also sometimes contain your exact assemblage) which I described in J.Pet
1980, and arises as a result of the bulk composition. Once Al-silicate
develops from a reaction involving muscovite and staurolite etc., we find
in these rocks that it is muscovite that gets consumed first, because they
don't have much muscovite left in the staurolite zone to begin with. Its
actually quite common worldwide to find pelites like this, just not in the
favoured areas of the old New England pelite enthusiasts (Hi Guys!). I
would be very wary of proposing open system behaviour for rocks with such
low variance as these assemblages - the Connemara examples are clearly
isochemical (Lithos 1977). You would need to find strong evidence of
external buffering to be able to support an open system model (eg biotite
always having exactly the same Fe:Mg:Mn), and since Fe is a more minor
component of the fluid than alkalis and Ca, I would expect to see extensive
mica-feldspar exchange before obsrvable muscovite-biotite exchange. We know
too much solution chemistry these days to use open systems as a panacea for
all our woes!
Good wishes
Bruce Yardley
At 20:38 18/01/02 +0530, you wrote:
>Dear friends,
>Thanks a lot for the references on the staurolite-andalucite-garnet-biotite
>bearing rocks. In all the occurrences, muscovite is present. Interestingly,
>in my rock muscovite is practically absent and the textural features(bt
>occurs as inclusions in all other porphyroblasts) show early stability of
>biotite. I am therefore, thinking some kind of hydrolysis reaction(s) in an
>open system to convert biotite to the other non potassic phases! Your
>suggestion in this regard will be highly appreciated.
>Thanks to you all!
>Pulak
>
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Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK
Tel. 0113 233 5227 Fax 0113 233 5259
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GEOFLUIDS now exists! http://www.blackwell-science.com/gfl
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