Dear Alexander and others,
The orthoamphibole-bearing assemblages I mentionned are muscovite and
k-feldspar absent, and they are obviously FMASH. THERMOCALC P-T
results for these assemblages indicate c. 640 °C / 7.9 kbar, and
Qtz-Ky O-isotope thermometry indicate T ‰ 660 - 680 °C. It appears
thus that the stability of these assemblages would fit quite nicely
the equilibria located between three FMASH invariants located around
640 °C and between 6 and 8 kbar in the THERMOCALC FMASH grid of Xu et
al. (1994) (invariant nb. [13] orthoamphibole + staurolite +
cordierite + chlorite + aluminosilicate, [14] orthoamphibole +
staurolite + cordierite + garnet + aluminosilicate, and [17]
orthoamphibole + staurolite + chlorite + garnet + kyanite in fig 3 of
Xu et al.). Yet, it is not clear how these invariants have been
calculated as Xu et al. (1994) does not give informations about
non-ideality in orthoamphibole, and ideal activities do not seem to
do the job. Xu et al. (1994) announced that orthoamphibole-bearing
assemblages will be dicussed in a separate paper but I can't find
this publication. Does someone know about it ?
The above-mentionned P-T results also appear consistent with the
clinoamphibole (cummingtonite - grunerite) involving invariant at
648°C, 8.2 kbar you mention. Yet, in order to be as "internally
consistent" as possible, I would prefer to be able to calculate
equilibria involving the anthophyllite - Fe-anthophyllite - gedrite
Fe-Mg and Tschermak solution, rather than to use the cummingtonite -
grunerite Fe-Mg solution as a proxy.
It is also funny that you are mentionning the garnet + staurolite +
chlorite + chloritoid + biotite + muscovite + quartz invariant
problem, as I found one such assemblage in the same Himalayan section
I am currently studying, and this is also another problem I have. In
the KFMASH grid I calculated, this invariant is stable at 539 °C /
4.18 kbar. In the new "unpublished" Spear & Cheney grid found in
Spear (1999), this invariant is comparably at about 570 °C / 4.7
kbar. I was not able to determine a pressure for the
chloritoid-bearing assemblage because it does not contain
plagioclase. Yet, other nearby samples indicate P-T conditions in the
range 550-650 °C / 6.5-8.5 kbar, i.e. significantly higher pressures
compared to the predictions of both THERMOCALC and Spear (1999)
KFMASH grids.
Bonjour chez vous !
Vannay Jean-Claude
Spear F. (1999): "Real-time AFM diagrams on your Macintosh".
Geological Materials Research 1/3, 1-9, http://gmr.minsocam.org/
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