Dear Pulak,
Biotite may well begin to appear early, but continue to grow
progressively, as a product of muscovite-consuming reactions that also
produce staurolite and andalusite. Because staurolite and andalusite
tend to produce much larger porphyroblasts than biotite, quite a bit of
biotite can grow before the nearest large porphyroblast impinges on it,
making it look as if biotite always predates the large porphyroblasts.
Have you tried looking to see if the sizes and spatial distributions of
biotites included in andalusite or staurolite are similar or dissimilar
to those of biotite in the matrix?
For a recently-published account of this sort of reaction and growth
sequence (albeit complicated by disequilibrium effects), see pp. 138-142
of Waters and Lovegrove (2002) Assessing the extent of disequilibrium
and overstepping of prograde metamorphic reactions in metapelites from
the Bushveld Complex aureole, South Africa. Journal of Metamorphic
Geology, 20, 135-149.
Regards,
Dave Waters
PULAK SENGUPTA 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
--
Dave Waters - Lecturer in Metamorphic Petrology, Oxford University
Dept of Earth Sciences, Parks Rd, Oxford OX1 3PR, UK
Tel: +44 1865 272000
Direct: +44 1865 272058 Email: [log in to unmask]
Fax: +44 1865 272072 http://www.earth.ox.ac.uk/~davewa/
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