Dear all:
My name is Guillermo, and I am a PhD student from
Spain.
I am looking for references dealing with anticlockwise
PT paths in medium pressure pelitic schists.
I am particularly interested in the secuence of
mineral assemblages and the description of reactional
textures.
As a hint, I send you the secuence of mineral
assemblages and some reactions that I have found
during the textural analysis.
First of all is noticeable the widespread presence of
big andalusite crystal (up to 20 cm) which are now
absolutely replaced by kyanite.
These andalusite polymorphs are found either as
Qtz+And veins or in the pelitic schists matrix.
Inside of the last kind of crystals I have found an
association composed of staurolite and garnet. So the
first paragenesis was And + St + Grt I.
This paragenesis is replaced by other composed mainly
of Grt + Ky but there are evidences of the existence
of other minerals such as sillimanite and cordierite
(now replaced by an aggregate of kyanite and biotite).
Finally, during the retrogression, there is a reaction
between kyanite and garnet which forms a brown
simplectitic reaction rim, probably composed of
staurolite and biotite.
Using the Powell and Holland petrogenetic grid for
pelitic schists (and taking into account other
considerations, as the mineral chemistry) it is
clearly seen that the first part of the PT path is an
isobaric heating at low pressures (2-4 kbar).
Then, at 650 ºC the schists are pressurized up to 9.5
kbar (calculated with the function average PT included
in THERMOCALC).
Finally, the absence of another heating after the
metamorphic peak and the presence of the reaction Grt
+ Ky = St + Bt justifies the anticlockwise geometry of
the PT path.
Anybody knows some similar example?
Thank you for your patience and sorry for the
longitude of this message.
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