Fig. 6. Feldspar compositions in the samples of this study, plotted into the Or-An-Ab triangle. A alkalifeldspar; and B
plagioclase.
no compositional specialities apart from up to 0.3 wt. MgO and up to 0.8 wt. MnO. Hematite
component is below 0.03 in all analyses.
4. Thermobarometry and estimation of fluid parameters
4
.
1
. Methods Pressure-temperature calculations were per-
formed using conventional thermobarometers cali- brated — if possible — for mineral compositions
and under P-T conditions similar to the ones we found in our samples. For the first, higher pressure
granulite-facies event we used the following calibra- tions:
garnet-biotite thermometry after Patin˜o-Douce et al. 1993 because of the extremely high Ti
and Al content of the biotite in our samples;
garnet-ilmenite thermometry after Pownceby et al. 1987;
garnet-sillimanite-plagioclase-quartz barometry after Koziol 1989 with adjustments for the
sillimanite-kyanite transition after Holdaway 1971;
Fig. 7. Spinel compositions plotted in terms of Fe
2 +
-Fe
3 +
-Al A; and Fe
2 +
-Mg-Zn B. Note the extremely Zn-rich compo- sitions in the cordierite-free sample 163.
the absence of significant fluid species in the cordierite channels is inferred.
3
.
4
.
5
. Spinel Spinel composition is widely varying as a result
of the later reactions mentioned above. Spinel coexisting with magnetite and corundum or silli-
manite is the most Mg-rich in the respective sam- ples and reached an X
Mg
of about 0.45, while else it has X
Mg
values between 0.2 and 0.35 Fig. 7 and Table 4. Fe
3 +
contents are always below 0.1 in all samples. In sample 163, which has only a few spinel
grains, these are extremely rich in gahnite compo- nent with ZnO contents up to 13 wt., whereas
spinel in the other two samples is almost devoid of Zn.
3
.
4
.
6
. Other minerals Corundum, rutile and sillimanite were checked to
have almost perfect ideal composition. Ilmenite has
G .
Markl et
al .
Precambrian
Research
102 2000
47 –
68
57 Table 4
Selected cordierite and spinel microprobe analyses used for geothermobarometry 24-crd1
37-crd3 37-crd4
10-spl3 10-spl7
163-spl8 163-spl2
37-spi8 37-spi11
24-crd7 10-crd7
Mineral c 10-crd12
wt. 49.19
0.00 0.00
0.00 0.16
0.03 49.02
0.04 47.88
SiO
2
48.20 48.47
48.47 0.02
0.00 0.02
0.00 0.00
0.00 0.03
0.00 0.03
0.01 0.00
0.00 TiO
2
34.86 58.37
55.42 57.70
59.08 Al
2
O
3
60.84 32.93
57.18 33.00
34.23 34.15
34.59 5.26
31.41 38.59
27.52 30.30
30.98 5.83
35.48 7.14
6.58 FeO
7.68 7.64
10.29 8.57
10.46 9.11
4.80 3.59
5.75 8.46
7.38 8.86
9.40 9.34
MgO 0.07
0.11 0.05
0.08 0.03
0.31 MnO
0.24 0.00
0.04 0.12
0.06 0.06
0.00 0.59
0.62 9.83
4.55 0.02
0.00 0.13
0.00 ZnO
0.00 0.00
0.00 0.03
0.06 0.03
0.00 0.00
0.00 0.00
0.00 0.00
0.12 0.10
0.10 Na
2
O 99.93
99.76 100.03
99.49 100.00
100.95 Total
100.81 97.11
98.22 99.50
98.49 99.87
Formula based on ideal number of oxygens and, for spinel, cations 4.91
0.00 0.00
0.00 4.98
0.00 4.91
0.00 0.00
4.92 4.98
Si 4.91
4.11 4.09
4.10 1.89
1.85 1.96
1.95 1.94
1.86 4.00
Al 4.09
4.03 0.00
0.00 0.00
0.00 Ti
0.00 0.00
0.00 0.00
0.00 0.00
0.00 0.00
0.11 0.13
0.03 0.04
0.05 0.13
Fe
3+
0.49 0.67
0.44 0.61
0.78 0.64
0.67 0.65
0.69 0.66
0.60 0.56
Fe
2+
1.56 0.37
0.20 0.15
0.24 Mg
0.34 1.33
0.30 1.36
1.42 1.42
1.54 0.01
0.00 0.00
0.00 0.00
0.01 0.01
0.01 0.01
0.01 Mn
0.00 0.00
0.00 0.00
0.00 0.01
0.01 0.21
0.09 0.00
0.00 0.00
0.00 0.00
Zn 0.01
0.00 0.00
0.00 0.00
0.00 0.00
Na 0.01
0.02 0.02
0.02 0.01
11.03 3.00
3.00 3.00
3.00 3.00
11.04 3.00
11.06 Sum
11.01 11.03
11.04 X
Fe
0.22 0.33
0.62 0.79
0.80 0.74
0.66 0.69
0.33 0.30
0.28 0.24
Fig. 8. Results of the P-T-calculations for two representative samples. See text for discussion of the geothermobarometers used.
Fig. 9. Results of the P-T calculations for all samples of this study. The solid lines labelled ‘dry’, ‘0.1’, ‘0.2’, and ‘0.3’ are
solidus curves for granitic melts at different water activities after Johannes and Holtz 1996.
Further calculations concerning the H
2
O activ- ity, K
+
H
+
ratio, oxygen fugacity or silica activity in the metamorphic fluids were performed using the
GEOCALC software of Lieberman and Petrakakis 1991 with the thermodynamic database of
Berman 1988.
4
.
2
. Results of P-T calculations P-T calculations using the various geothermo-
barometers show two distinctly different sets of metamorphic conditions which are in agreement
with additional constraints on minimum and max- imum pressures and with petrogenetic grids for the
observed assemblages e.g. Bucher and Frey, 1994; p. 209. Fig. 8 shows the representative calculations
for the two samples 24 and 37 and the relatively narrow overlap of the various geothermobarome-
ters.
The earlier assemblage — in the following re- ferred to as high-temperaturehigh-pressure assem-
blage HTHP — equilibrated in the range 820 – 950°C at 6.5 – 9.5 kbar. This is the range
spanned by all samples, every single sample, how- ever, shows a much smaller range of equilibration
conditions see Fig. 9. The very high temperatures of these granulites are indicated by all geother-
mometers and by the combination of all geother- mobarometers
used. The
pressures are
independently constrained by the appearance of rutile Itaya et al., 1985 to be higher than about
6 kbar and by the absence of kyanite to be lower than about 10 – 12 kbar. The various samples ap-
pear to represent slightly different depths where metamorphism occurred, but obviously, they all
have been subjected to the same geotherm during equilibration. The geotherm spanned by the five
samples corresponds to very roughly 100°Ckbar see Fig. 9.
The later, cordierite-bearing assemblage — the medium-temperaturemedium-pressure assemblage
MTMP — equilibrated between 650 and 730°C and between 3 and 5 kbar Fig. 9. The range span-
ned by all samples is significantly smaller than the one indicated by the earlier HTHP assemblage, but
only three samples show the later overprint at all.
garnet-rutile-ilmenite-plagioclase-quartz barometry after Bohlen and Liotta 1986;
garnet-rutile-ilmenite-sillimanite barometry af- ter Bohlen and Liotta 1986;
garnet-spinel-sillimanite-quartz thermobarome- try after Nichols et al. 1992 with garnet activ-
ities calculated after Berman 1990.
The conditions for the lower pressure, cordierite- forming event were estimated based on the follow-
ing cordierite-involving calibrations:
garnet-cordierite thermometry after Nichols et al. 1992;
garnet-cordierite-sillimanite-quartz thermo-
barometry after Nichols et al. 1992. For these calculations, garnet and biotite core
compositions were used to constrain the higher pressure event, while intermediate garnet composi-
tions not rim, these were thought to have possibly equilibrated later as a result of lower temperature
diffusion were used with average cordierite com- positions in the calculations for the lower pressure
event. Garnet-biotite thermometry was not used to constrain the cordierite-event as a result of the
difficulties in deciding whether biotite was still stable with cordierite or not. Preliminary tests
revealed, however, temperatures in the same range as those recorded by the garnet-cordierite assem-
blage.
4
.
3
. Constraints on H
2
O acti6ity and the composi- tion of the fluid
4
.
3
.
1
. H
2
O acti6ity and K
+
H
+
ratios As the samples were subjected to partial melt-
ing at temperatures in the range 850 – 900°C, the H
2
O activity must have been at or above 0.3 according to Johannes and Holtz 1996. Apart
from this estimate, the H
2
O activity was calcu- lated independently from equilibria like
Ann + Sill + 2Qtz = Kfsp + Alm + H
2
O 6
for both assemblages or, if it is inferred that cordierite in the spinel-bearing samples was still in
equilibrium with biotite, from the reaction
2Ann + 6Sill + 9Qtz = 2Kfsp + 3Fe-Crd + 2H
2
O 7
for the MTMP assemblage. Calculations used the same garnet, cordierite and biotite compositions
as for the respective P-T estimations with a slightly more Fe-rich biotite for reaction 7. Al-
mandine activities were calculated after Berman 1990, biotite, K-feldspar and cordierite activities
with an ideal site mixing formulation.
These equilibria provide, as a result of the difficulty in discerning the accurate biotite compo-
sition with which the garnet under high-pressure and the cordierite under low-pressure conditions
were in equilibrium, and as a result of uncertain- ties in the mixing properties of biotite, only rough
estimates. As shown on Fig. 10A, H
2
O activities during both the HTHP and the MTMP event
are constrained between 0.6 and 1, if the whole range of possible pressures, temperatures, biotite
and garnet compositions are taken into account. In detail, sample 24, 99 and 163 indicate slightly
higher H
2
O activities than sample 10 and 37 in the HTHP calculations, but all samples with cordier-
ite indicate H
2
O activities of more than about 0.75 and possibly close to 1 Fig. 10B. These
estimates are in accordance with the relatively low halogen and especially Cl contents of the biotite
which indicate a fluid of low salinity.
The formation of the cordierite-quartz symplec- tite by reaction 2 is dependent on pressure,
temperature, H
2
O activity and the K
+
H
+
activ- ity ratio. As pressure and temperature are rela-
tively well constrained, the K
+
H
+
activity ratio can be readily calculated for a fixed H
2
O activity. In addition to Eq. 2, the following equilibria
also allow the calculation of K
+
H
+
activity ra- tios in various samples during the MTMP
equilibration:
Fig. 10. Diagrams relating the activity of annite component in biotite to the activity of water at 7 – 9 kbar, 850 – 950°C A; and at 4 kbar, 650°C B. The range of a
H2O
relevant for the different samples is constrained by the intersection of the sample box with the respective reaction curve. See text for discussion.
Fig. 11. Diagrams relating oxygen fugacity to temperature A; and silica activity B. These diagrams were constructed to
explain the corundum- and sillimanite-magnetite-spinel tex- tures found in sample 10 and 37. Corundum-bearing assem-
blages are evidently stable under low silica activities only, irrespective of the temperature. See text for discussion.
the rock during cordierite formation could pro- mote redox reactions. Hence, spinel breakdown
was investigated as a function of temperature, oxygen fugacity and silica activity, as these are the
parameters of most importance for equilibria 4 and 5. In Fig. 11A, equilibria among corundum,
hercynite, sillimanite and quartz are calculated at 5 kbar and with an ideal site mixing model for the
average spinel analysis from samples 10 and 37. For comparison, also the fayalite-magnetite-
quartz buffer is shown. In Fig. 11B, these equi- libria are calculated at 5 kbar, 650°C and with the
same hercynite activity. Silica activity is defined such that pure quartz at standard conditions has
an activity of 1. Interestingly, although the rocks are all quartz saturated, differences in silica activ-
ity explain the different textures more readily Fig. 11B than temperature differences. This indi-
cates that the coexisting fluid was not in equi- librium with quartz in the corundum-producing
samples, but had silica activities below 0.8. Differ- ences in temperature would always favour the
formation of sillimanite in the presence of a quartz-saturated fluid Fig. 11A. Involvement of
non-equilibrium growth of corundum is another possibility, but in the presence of a fluid at 650°C,
this is considered unlikely. Furthermore, the pres- ence of the sillimanite in one sample argues
against this possibility. Oxygen fugacities in the fluid as indicated by these reaction textures are at
least 2 – 3 orders of magnitude above the FMQ buffer at the same temperature see Fig. 11A —
independent on the actual temperature.
Oxygen fugacity estimates of the high-tempera- ture assemblage which involves ilmenite and rutile
can be performed using the oxygen barometer of Zhao et al. 1999. Estimates based on this cali-
bration using ilmenite compositions as reported in Ba¨uerle 1999 indicate oxygen fugacities of
QFM 9 1.
5. Discussion and conclusions
