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 In the new "unpublished" Spear & Cheney grid found in Spear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/