Dear Colleagues,
I appreciate you for your responses. They are very helpful for me
(not only for mentioned case; some of them are similar to other
textures, kept "in my eyes and mind").
Summarizing: spinel (hercynite) in cordierite is pseudomorph after
an alumina-rich phase. In my case this phase seems to be
staurolite (so, my case seems to remind of the Dave Waters'
description). Actually, cordierite porphyroblasts contains
corroded inclusions of staurolite; we can surmise reaction 2St +
+ 15Qtz = 10And + 4Crd + 4H2O. The isolated relics of staurolite
can be disintegrated as 2St = 6Hc + Crd + 10And + 4H2O. Spinel
cannot arise from reaction And + Chl since cordierite
porphyroblasts in this case does not contain Al2SiO5 and
chlorite inclusions (andalusite is only as porphyroblasts in
matrix).
However, such an item is unclear: if a mass transfer during
the reaction was sufficient to remove Al2SiO5 from cordierite why
silica was not transported toward a reaction area? Obviously the
PT conditions did not allow to coexist Qtz and Hc (the lower
temperature for this assemblage is 750øC at 3 kbar; it is
determined by the reactions 5Qtz + 2Hc = fCrd and 2Sil + Alm = 3Hc
+ 5Qtz (calculated from TWQ 2.02b dataset). This temperature was
not achieved (presence And and Chl in rock matrix; migmatites are
quite absent). So, some inscrutability remains.
More sure explanation can be make for Spl (both hercynite and
magnesia spinel) + fibrolite ingrowths in metapelitic cordierites.
They can originate just from the reaction 2St = 6Hc + Crd + 10Sil
+ 4H2O after mantling of St relics with Crd. This reaction thought
to be more probable than Bt + Als + Qtz = Crd + Spl + Kfs +
H2O/melt since fibrolite grew (not dissolved).
Spinel-cordierite and sapphirine-cordierite symplectites as I
suppose have other origin. Most probable, that they formed as
pseudomorphs after sillimanite in the reactions like Opx + Sil =
Crd + Spr (as described by Harley's works). Now we (me and Dima
Dolivo-Dobrovolsky) are studying also such textures in
sapphirine-bearing rocks and preparing a paper on their genesis.
Many of these textures were often formed not isochemically (e.g.,
we observed increase of Mg content in Spr and Crd relatively Opx,
and so on). It is not amazing since such textures forms not
frequently at fluid flow and metasomatism (this follows from
geological data).
Warm metamorphic regards,
Pavel.
--
Pavel Azimov, PhD
Institute of Precambrian Geology and Geochronology
Russian Academy of Sciences
2, Makarov Embankment
St.Petersburg, 199034, Russia
Phone: +7(812)328-03-62
Fax: +7(812)328-48-01
E-Mail: [log in to unmask]
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