Dear Pavel,
I have observed hercynite inclusions in cordierite in
cordierite-biotite-quartz gneisses. These gneisses are metapelitic rocks
occurring in a wide contact metamorphic aureole (northern Coast Belt, North
American Cordillera), close to the intrusive contact. Hercynite inclusions
are located in the center of cordierite porphyroblasts. Around the
hercynite, separated by an inclusion-free zone, is fibrolitic sillimanite.
It is also observed that hercynite forms tabular (or needle-like)
pseudomorphs after sillimanite. In a few lower grade samples garnet is also
present as an inclusion in the cordierite (along with sil and hc). Quartz
is abundant, but never in contact with hercynite. K-feldspar is also
present, in the matrix. We infer that cordierite production results from
the breakdown of quartz, biotite and sillimanite (which also produces
k-feldspar and liquid), and to a lesser degree to the reaction Grt+Sil+Qtz.
Migmatites have been observed as well. The hercynite then is thought to be
the result of breakdown of garnet and sillimanite (also to form cordierite).
For a temperature estimate we calculated 700°C at 4 kbar for Grt+Sil=Crd+Hc
using TWQ 2.02 (this worked only for Fe-Crd and Alm). Grt-Bt-Pl-Crd
geothermometry of the same sample provided P-T of 4.7 kbar and 719°C (also
TWQ 2.02).
This can be found in Mezger, Chacko & Erdmer, 2001 (Journal of Metamorphic
Geology 19, 121-137). Fig. 3d in that paper shows the inclusions of grt, hc
and sil in cordierite.
Ciao,
Jochen
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Dr. Jochen Mezger
Postdoctoral Fellow
Johannes Gutenberg-University Mainz Tel.: 49-(0)6131-3924767
Institut fuer Geowissenschaften Fax.: 49-(0)6131-3923863
Tectonophysics Group
Becherweg 21
55099 Mainz Email: [log in to unmask]
GERMANY http://www.uni-mainz.de/~mezger/
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