GEOLOGICA CARPATHICA, FEBRUARY 2009, 60, 1, 3—14 doi: 10.2478/v10096-009-0003-6
www.geologicacarpathica.sk
Ti-in-biotite geothermometry in non-graphitic, peraluminous
metapelites from Crni vrh and Resavski humovi
(Central Serbia)
SUZANA ERIĆ
1
, MIHOVIL LOGAR
1
, DRAGAN MILOVANOVIĆ
1
, DANILO BABIČ
1
and BORIVOJ ADNAĐEVIĆ
2
1
Faculty of Mining and Geology, University of Belgrade, Đušina 7, Belgrade, Serbia; suzanaeric@yahoo.com
2
Faculty of Physical Chemistry, University of Belgrade, Studenski trg 12—16, Belgrade, Serbia
(Manuscript received August 16, 2007; accepted in revised form June 12, 2008)
Abstract: The study discusses the application of the Ti-in-biotite geothermometer of Henry et al. (2005) to the example of
biotites from non-graphitic peraluminous micaschists of Central Serbia. Three petrographically different micaschists were
distinguished on the basis of the following mineral assemblages: CV1 (St-Grt-Bt-Ms-Pg-Pl-Qtz), CV2 (Grt-St-Ky-Bt-
Ms-Pl-Qtz) and RH (Grt-St-Bt-Ms-Pl-Qtz). Applying different geothermobarometers it was estimated that the studied
micaschists were metamorphosed at average temperatures and pressures of 530 °C and 520 MPa (CV1
incl
),
580 °C and
670 MPa (CV1),
630 °C and 700 MPa (CV2) and 550 °C, 680 MPa (RH). The average temperatures obtained by the Ti-
in-biotite method revealed uniform values for CV1 and CV2 micaschists and these values are very similar to the tempera-
tures obtained by other methods. In contrast, the application of Ti-in-biotite geothermometer for RH micaschist yields the
temperature difference of 85—110 °C. The variability of temperature is interpreted as a result of a positive correlation of Ti
contents and X
Mg
values in RH biotite, which is in disagreement with the principles of the Ti-in-biotite method. The
positive Ti-X
Mg
correlation is a result of the compositional variability shown by RH biotites from different samples, which
can possibly be related to compositional inhomogeneities of the pelitic protolith. On the other hand, the Ti-in-biotite
geothermometer for CV2 biotite gave very uniform temperatures despite variable Ti contents (Ti = 0.260, sd = 0.018 apfu).
This is explained as result of the low sensitivity of Ti-in-biotite geothermometer for high Ti concentrations ( > 0.25 apfu).
Key words: Serbia, Ti-in-biotite geothermometer, micaschists, non-graphitic metapelite.
Introduction
Along with the commonly used geothermometer based on
Mg-Fe exchange between coexisting garnet and biotite (Fer-
ry & Spear 1978; Perchuk & Lavrenteva 1983; Bhattacharya
et al. 1992; Holldaway 2000) or the THERMOCALC pro-
gram (Powell & Holland 2001), the amount of Ti in biotite
could also be used to determine the temperature of metamor-
phism in metapelites (Henry et al. 2005). Experimental work
on phlogopites has shown that there is a positive correlation
between the amount of Ti in biotite and temperature and a
negative correlation with pressure (Forbes & Flower 1974;
Robert 1976).
Henry et al. (2005), suggested that geothermometric calcu-
lations based on the amount of Ti and Mg/(Mg + Fe) ratio in
biotite can be applied to graphitic peraluminous metapelites
that contain rutile and/or ilmenite and have equilibrated at pres-
sures and temperatures of about 400—600 MPa and 480—800 °C,
respectively. The authors suggest that this method can pro-
duce considerable disagreements with reference tempera-
tures when it is applied to non-graphitic peraluminous
metapelites containing Ti-saturated minerals. This paper
aims to shed more light on this problem by investigating bi-
otites from the micaschists in Serbia.
Micaschists from Crni vrh and Resavski humovi, associated
with other metamorphic rocks in Proterozoic to Upper Paleo-
zoic Crystalline Complexes in Central Serbia (Vujisić et al.
1981), correspond to non-graphitic peraluminous metapelites
containing Ti-saturated minerals.
The geothermometer of Henry et al. (2005) is most sensi-
tive for Ti contents in the range 0.0—0.3 apfu (atoms per for-
mula unit). Biotites from the micaschists of Crni vrh and
Resavski humovi, which have such a range of Ti content and
are characterized by different X
Mg
values 0.65), provide a
good basis for discussing the applicability and sensitivity of
the Ti-in-biotite geothermometer on non-graphitic peralumi-
nous metapelites.
Geological setting
Micaschists of Crni vrh are exposed about 10 km southwest
from Bagrdan, on the road between Strižilo and Donje Koma-
rice villages, as well as along the Pišljak and Kusača streams
(Fig. 1). Micaschists from Resavski humovi occur in the area
around Miljko’s monastery. The investigated micaschists are
exposed as 10 to 40 m long outcrops, where these rocks are
associated with amphibolites and pegmatites (Resavski hu-
movi), or alternate with quartzites and gneisses (Crni vrh).
Geotectonically, the metamorphic rocks of Crni vrh and Re-
savski humovi belong to the northern and northwestern parts of
the Serbo-Macedonian Composite Terrane (Karamata & Krstić
1996). The metamorphic rocks of the Serbo-Macedonian Mas-
sif were affected by polyphase metamorphism. According to
4
ERIĆ, LOGAR, MILOVANOVIĆ, BABIČ and ADNA
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EVIĆ
geochronological studies, these rocks were metamorphosed un-
der amphibolite-, locally eclogite-facies conditions during the
Caledonian and Hercynian events and underwent an Alpine
overprint in greenschist-facies conditions (Balogh et al. 1994;
Milovanović et al. 1998). They comprise wide variations of
ortho- and paragneisses, micaschists and rarely marbles, am-
phibolites and eclogites. Gneisses and micaschists are most
abundant. In places, gneisses and micaschists contain smaller
bodies of amphibole gneisses, amphibolites, eclogites, marbles,
or migmatitic patches and veins of quartz, feldspars and micas.
Fig. 1. Geological map of the Crni vrh and Resavski humovi in Central Serbia (Vujisić et al. 1979).
Petrography
Two different varieties of micaschists from Crni vrh (CV1
and CV2) and one of Resavski humovi (RH) are petrographi-
cally distinguished, mostly according to the amount and size
of staurolite and garnet porphyroblasts and on the basis of
the presence and absence of kyanite (Table 1). Micaschists
of Crni vrh are composed of quartz, muscovite, biotite, sodic
plagioclase, garnet, staurolite, kyanite and rarely of alkali
feldspar. Tourmaline, rutile and ilmenite occur as accessory
Table 1: Mineral associations of micaschists from Crni vrh and Resavski humovi.
Micaschists from Crni vrh
Micaschists from Resavski humovi
St + Grt + Bt + Ms + Pg + Pl + Qtz (CV1)
Grt + St + Ky + Bt + Ms + Pl + Qtz (CV2)
Grt + St + Bt + Ms + Pl + Qtz (RH)
Grt — garnet, St — staurolite, Ky — kyanite, Bt — biotite, Ms — muscovite, Pg — paragonite, Pl — plagioclase, Qtz — quartz
5
Ti-IN-BIOTITE GEOTHERMOMETRY IN NON-GRAPHITIC, PERALUMINOUS METAPELITES (CENTRAL SERBIA)
Fig. 2. First variety of micaschist from Crni vrh (CV1).
Fig. 3. Garnet porphyroblasts from CV1, N ll. Grt – garnet, Bt –
biotite, Ilm – ilmenite.
Fig. 4. Second variety of micaschist from Crni vrh (CV2).
Fig. 5. Staurolite and garnet from CV2, N+. St – staurolite, Qtz –
quartz, Grt – garnet, Bt – biotite.
Fig. 6. Garnet with micas from RH, N ll. Bt – biotite, Ms – mus-
covite, Grt – garnet.
Fig. 7. Garnet and biotite from RH, N ll. Bt – biotite, Grt – gar-
net, Ilm – ilmenite.
6
ERIĆ, LOGAR, MILOVANOVIĆ, BABIČ and ADNA
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EVIĆ
minerals. CV1 micaschist contains very large staurolite por-
phyroblasts as idiomorphic (Fig. 2), rarely hypidiomorphic
grains up to 10 cm in size, which compose nearly 25 vol. %
of the rock. These porphyroblasts contain frequent inclu-
sions of garnet, biotite, muscovite, plagioclase and quartz.
Garnet occurs either as inclusions or porphyroblasts up to
0.4 cm in diameter (Fig. 3). Paragonite has been observed in
the groundmass along with muscovite and biotite. The domi-
nant mineral of CV2 micaschist is garnet appearing as large,
idiomorphic porphyroblasts up to 1 cm in diameter (Fig. 4),
while staurolite (Fig. 5) and kyanite porphyroblasts, ranging
in size from 0.5 mm to 0.5 cm, are less abundant.
Micaschists from Resavski humovi (RH) consist of quartz,
micas (biotite and muscovite, Fig. 6), sodic plagioclase, gar-
net and staurolite. Garnet appears as large (up to 0.7 cm in
diameter), idiomorphic porphyroblasts usually containing in-
clusions of muscovite and quartz.
In all the micaschist varieties biotite is texturally associat-
ed with muscovite. It occurs as orientated flakes and sheet-
like aggregates defining the main rock fabric characterized
by a pronounced schistosity. In some samples the micas
build patches or bands up to a few mm in thickness. These
mica-rich bands sometimes alternate with quartz aggregates
producing a banded structure. Flakes of micas are up to
2 mm, commonly from 0.3 to 1 mm in length (Fig. 7). Be-
sides the matrix biotite, tiny biotite flakes (from 0.05 to
0.3 mm in size) included in staurolite porphyroblasts were
also observed in the CV1 micaschist variety.
Analytical methods
The mineral chemistry of micaschists from both localities
was obtained using an EMPA, CAMECA, SX 100 electron
microprobe at the Institute of Mineralogy and Petrography,
University of Hamburg, Germany. Operating conditions
were 15 kV accelerating voltage and 20 nA sample current.
Four representative samples were taken from each mica-
schist variety. More than 100 microprobe analyses of biotite
and a similar number of garnet analyses was carried out,
while plagioclase and other major minerals were analysed to
a lesser extent.
Robust regression analysis for defining predominant ex-
change vectors in biotites were performed using the statisti-
cal program NCSS.
Chemistry of major minerals
All analysed garnets exhibit almandine compositions con-
taining significant amounts of pyrope and spessartine and
subordinate grossular component (Table 2). The amounts of
almandine and pyrope components increase, while the
amounts of spessartine and grossular component decrease
from core to rim in garnet grains of all investigated mica-
schists. All the studied micaschists contain typical ferrugi-
nous staurolite characterized by low MgO (from 0.30 to
maximally 2.00 % – Erić 2005). The plagioclase of all the
investigated micaschists corresponds to oligoclase (Table 3)
containing 26—30 mol % anorthite. Paragonite was identified
only in the CV1 variety and it is worth noting that this mica-
schist contains sodium-rich muscovite that is muscovite with
the highest content of paragonite component (Table 3).
Representative biotite analyses of micaschist varieties
from different samples are shown in Tables 4—6. All chemi-
cal analyses are normalized on the basis of 22 O assuming all
Fe as Fe
2 +
. Biotites from investigated micaschists differ in
their X
Mg
values (CV1–0.57—0.62, CV2–0.47—0.50 and
RH–0.52—0.57) (Fig. 8). Additionally, biotites from CV1
and RH micaschists have almost identical Ti contents rang-
ing 0.15—0.20 and 0.13—0.22 atoms per formula unit (apfu),
respectively, while biotite of the CV2 variety shows higher
Ti contents (0.20—0.30 apfu). Significant within-grain (Ta-
ble 6, analyses 2a—2b) and especially between grains (indi-
vidual grains from different samples: analyses 3—6, 4—7)
compositional variations are found only for biotites from the
RH variety. Finally, biotites occurring as inclusions in CV1
variety (Table 7) display similar X
Mg
values and significant-
ly lower Ti contents (0.10—0.13 apfu) in comparison to ma-
trix biotite (Table 4). The observed variability in chemical
composition, particularly in Ti content and X
Mg
values in bi-
otite from the investigated micaschist varieties as well as in
different biotite generations (CV1), can be a good opportuni-
ty for applying the Ti-in-biotite geothermometer (Henry et
al. 2005) whereas the frequency of isotherms suggests that all
biotites developed in the most sensitive part of the Ti—X
Mg
di-
agram (Fig. 9).
Biotite crystallochemistry
Numerous studies have confirmed that the amounts of Ti
incorporated in biotite depend not only on temperature, but
also on the mineral assemblage, pressure and biotite crystal
chemistry (Guidotti et al. 1977; Dymek 1983; Lobotka 1983;
Guidotti 1984; Henry & Guidotti 2002; Henry et al. 2005).
The presence of titanium in biotite is related to substitutions
in octahedral and/or tetrahedral and octahedral sites (Dymek
1983; Henry & Guidotti 2002). The substitution:
Ti
4 +
+ 2O
2—
+ H
2
R
2+
+ 2OH
—
(where R
2 +
is a divalent octahedral cation), which repre-
sents deprotonation (dehydrogenation) through the loss of H
(Waters & Charnley 2002; Cesare et al. 2003), often takes
place in high-temperature metapelitic biotites. As possible
substitution mechanisms to account for Ti incorporation in
the investigated biotites we used the exchange vectors Ti R
—2
(2
VI
R
2+
=
VI
Ti +
V I
; where R
2 +
is a divalent octahedral cation
and is vacancy) and TiRAl
—2
(2
VI
Al = Ti +
VI
R
2+
) for octahe-
dral sites and TiAl
2
R
—1
Si
—2
(2
IV
Si +
VI
R
2+
=
VI
Ti + 2
IV
Al) and
Al
2
R
—1
Si
—2
(
VI
R
2+
+
IV
Si = 2
IV
Al + 2
VI
Al) for substitutions be-
tween octahedral and tetrahedral sites, respectively. The di-
rection of these exchange vectors for biotites from particular
micaschist varieties was determined using the robust regres-
sion method. The obtained results are presented in Table 8
and Fig. 10a—d and all of them display statistical signifi-
cance (p < 0.001) without taking the value R
2
into account.
7
Ti-IN-BIOTITE GEOTHERMOMETRY IN NON-GRAPHITIC, PERALUMINOUS METAPELITES (CENTRAL SERBIA)
Fig. 8. Varieties of investigated micaschists with respect to Ti con-
tent and X
Mg
values in biotites.
Fig. 9. Isotherms according to the Ti-in-biotite geothermometer in
metapelites (Henry et al. 2005).
Fig. 10. Regression lines of substitutions in investigated biotites. a – Ti R
—2
, b – TiAl
2
R
—1
Si
—2
, c – TiRAl
—2
, d – Al
2
R
—1
Si
—1
.
8
ERIĆ, LOGAR, MILOVANOVIĆ, BABIČ and ADNA
Đ
EVIĆ
Garnet Biotite
CV1-r CV2-r RH-r CV1-incl.
CV1-r CV2-r RH-r CV1-incl.
SiO
2
37.03 37.02 36.52
37.25
SiO
2
36.34 36.96 37.04 35.97
TiO
2
0.02
0.05
0.02
0.00
TiO
2
1.49
2.25
1.48
1.21
Al
2
O
3
21.17 21.43 21.11
21.14
Al
2
O
3
18.61 19.00 19.80 19.64
FeO
31.58 31.78 34.62
33.66
FeO
16.52 18.57 16.66 15.71
MnO
3.32
3.45
1.58
3.05
MnO
0.04
0.00
0.00
0.08
MgO
3.98
3.68
3.05
3.10
MgO
13.17 10.02 11.76 12.83
CaO
2.70
2.45
2.95
1.96
CaO
0.02
0.00
0.00
0.02
Na
2
O
0.01
0.00
0.02
0.00
Na
2
O
0.38
0.31
0.51
0.32
K
2
O
0.00
0.00
0.01
0.00
K
2
O
8.73
9.10
7.86
8.92
Sum
99.81 99.86 99.88 100.16
Sum
95.30 96.21 95.11 94.70
Si
2.961
2.964
2.937
2.991
Si
5.429
5.514
5.495
5.388
Al
IV
0.039
0.036
0.063
0.009
Al
IV
2.571
2.486
2.505
2.612
Al
VI
1.955
1.986
1.938
1.992
Al
VI
0.706
0.855
0.958
0.855
Ti
0.001
0.003
0.001
0.000
Ti
0.167
0.252
0.165
0.136
Fe
3+
0.083
0.044
0.126
0.016
Mg
2.933
2.228
2.601
2.865
Mg
0.474
0.439
0.366
0.371
Fe
2+
2.064
2.317
2.067
1.968
Fe
2+
2.028
2.084
2.203
2.244
Mn
0.005
0.000
0.004
0.010
Mn
0.225
0.234
0.108
0.207
Ca
0.003
0.000
0.000
0.003
Ca
0.231
0.210
0.254
0.169
Na
0.110
0.090
0.147
0.093
Na
0.002
0.000
0.003
0.000
K
1.664
1.732
1.488
1.705
K
0.000
0.000
0.001
0.000
cations
15.562 15.474 15.430 15.635
cations
7.999
8.000
8.000
7.999
X
Mg
0.587
0.490
0.557
0.593
Alm
68.5
70.2
75.1
75.0
Py
16.0
14.8
12.5
12.4
Sp
7.6
7.9
2.5
6.9
Grs
3.7
4.8
3.7
4.8
r — rim of grain,
Alm — almandine, Py — pyrope, Sp — spessartine, Grs — grossular
X
Mg
= Mg/(Fe+Mg)
Table 2: Representative microprobe analyses of garnet and biotite in the investigated micaschists.
Table 3: Representative microprobe analyses of plagioclase, muscovite and paragonite in the investigated micaschists.
Plagioclase Muscovite
Paragonite
CV1-r CV2-r RH-r CV1-incl.
CV1-r CV2-r RH-r CV1-incl. CV1-r
SiO
2
61.28
61.25
60.83
61.20
SiO
2
45.34
45.28
47.16
47.00
45.09
TiO
2
0.00
0.00
0.00
0.00
TiO
2
0.35
0.47
0.74
0.55
0.15
Al
2
O
3
24.25
24.38
24.51
24.31
Al
2
O
3
35.54
35.64
34.47
36.32
39.00
FeO
0.00
0.00
0.03
0.00
FeO
2.29
1.31
3.11
0.61
0.98
MnO
0.00
0.00
0.00
0.00
MnO
0.03
0.00
0.03
0.14
0.00
MgO
0.00
0.00
0.00
0.00
MgO
0.52
0.73
1.21
1.12
0.09
CaO
6.29
6.26
5.56
6.07
CaO
0.02
0.01
0.00
0.00
0.31
Na
2
O
8.23
8.25
8.52
8.29
Na
2
O
2.62
0.94
1.87
1.60
6.55
K
2
O
0.06
0.05
0.07
0.05
K
2
O
7.43
9.72
8.82
8.20
1.74
Sum
100.11
100.19
99.52
99.92
Sum
94.14
94.10
97.41
95.54
93.91
Si
2.721
2.717
2.714
2.721
Si
6.079
6.089
6.617
6.151
5.900
Al
1.269
1.275
1.289
1.274
Al
IV
1.921
1.911
1.833
1.849
2.100
Ti
0.000
0.000
0.000
0.000
Al
VI
3.696
3.737
3.479
3.752
3.915
Fe
0.000
0.000
0.001
0.000
Ti
0.035
0.048
0.073
0.054
0.015
Mg
0.000
0.000
0.000
0.000
Mg
0.104
0.146
0.236
0.218
0.018
Mn
0.000
0.000
0.000
0.000
Fe
2+
0.257
0.147
0.340
0.067
0.107
Ca
0.299
0.298
0.266
0.289
Mn
0.003
0.000
0.003
0.016
0.000
Na
0.708
0.710
0.737
0.715
Ca
0.003
0.001
0.000
0.000
0.043
K
0.003
0.003
0.004
0.003
Na
0.681
0.245
0.474
0.406
1.662
cations
5.000
5.003
5.011
5.002
K
1.271
1.667
1.471
1.369
0.290
Ab
70.1
70.3
73.2
71.0
cations
14.050
13.991
14.526
13.882
13.557
An
29.6
29.5
26.4
28.7
X
Ms
0.651
0.872
0.756
0.736
0.148
Or
0.3
0.3
0.4
0.3
X
Pg
0.349
0.128
0.244
0.264
0.852
Ab — albite, An — anorthite, Or — orthoclase
X
Ms
=X
K
/(X
K
+X
Na
); X
K
=K/(K+Na+Ca); X
Na
=Na/(K+Na+Ca); X
Pg
=1–X
Ms
r — rim of grain
The directions of exchange vectors (regression lines,
Fig. 10a—d) for biotites from RH micaschist crosscut the
mutually sub-parallel directions of exchange vectors shown
by biotites from the CV1 and CV2 varieties. These dis-
crepancies are obvious in all cases, especially for
VI
R
2+
+
IV
Si = 2
IV
Al + 2
VI
Al substitution (Fig. 10d). The rea-
son for these discrepancies is most probably related to the
observed compositional variations shown by RH biotites
(note that the individual biotite analyses have different
weights in the regression model).
9
Ti-IN-BIOTITE GEOTHERMOMETRY IN NON-GRAPHITIC, PERALUMINOUS METAPELITES (CENTRAL SERBIA)
Table 4: Representative microprobe analyses of biotites in CV1 micaschists.
Bt-1a
Bt-1b Bt-2a Bt-2b Bt-3 Bt-4 Bt-5 Bt-6 Bt-7
SiO
2
36.13
36.38
36.08
36.22
36.51
37.91
38.35
35.74
37.13
TiO
2
1.43
1.47
1.57
1.52
1.40
1.37
1.40
1.56
1.46
Al
2
O
3
18.79
19.13
19.15
18.95
18.64
18.36
19.53
18.55
18.73
FeO
16.62
16.43
15.73
15.94
16.45
17.06
15.66
15.83
15.90
MnO
0.07
0.04
0.12
0.07
0.04
0.05
0.09
0.07
0.12
MgO
12.67
12.56
12.79
12.78
13.24
12.67
12.77
12.51
13.02
CaO
0.02
0.01
0.03
0.00
0.02
0.00
0.00
0.06
0.00
Na
2
O
0.40
0.53
0.45
0.34
0.37
0.74
0.91
0.14
0.62
K
2
O
8.44
8.98
9.01
9.19
8.80
8.22
8.23
8.83
8.35
Sum
94.57
95.53
94.93
95.01
95.47
96.38
96.94
93.31
95.33
Si
5.434
5.422
5.401
5.424
5.441
5.579
5.563
5.442
5.508
Al
IV
2.566
2.578
2.599
2.576
2.559
2.421
2.437
2.558
2.492
Al
VI
0.764
0.782
0.779
0.769
0.716
0.763
0.901
0.771
0.783
Ti
0.162
0.165
0.177
0.171
0.157
0.152
0.153
0.181
0.163
Mg
2.841
2.791
2.854
2.853
2.942
2.780
2.761
2.840
2.879
Fe
2+
2.090
2.048
1.969
1.996
2.050
2.100
1.900
2.016
1.973
Mn
0.009
0.005
0.015
0.009
0.005
0.006
0.011
0.009
0.015
Ca
0.003
0.002
0.005
0.000
0.003
0.000
0.000
0.010
0.000
Na
0.117
0.153
0.131
0.099
0.107
0.211
0.256
0.041
0.178
K
1.619
1.707
1.721
1.756
1.673
1.543
1.523
1.715
1.580
cations
15.605
15.653
15.651
15.653
15.653
15.555
15.505
15.583
15.571
X
Mg
0.576
0.577
0.592
0.588
0.589
0.570
0.592
0.585
0.593
Henry
et
al.
(2005)
[T(°C)]
568 573 593 584 566 552 562 595 576
for all analyses: Ti
apfu
sd = 0.011, X
Mg
sd = 0.009; sd — standard deviation
Table 5: Representative microprobe analyses of biotites in CV2 micaschists.
Bt-1a Bt-1b Bt-2a Bt-2b Bt-3 Bt-4 Bt-5 Bt-6 Bt-7
SiO
2
36.58
37.24
35.57
35.48
35.88
35.47
35.08
35.42
35.29
TiO
2
2.11
2.36
2.23
2.45
2.36
2.39
2.13
2.24
2.09
Al
2
O
3
18.56
18.92
18.52
18.78
18.74
18.60
18.62
18.61
18.42
FeO
18.84
19.01
19.89
19.70
19.40
19.53
19.34
20.08
19.84
MnO
0.00
0.00
0.04
0.04
0.04
0.07
0.04
0.13
0.01
MgO
10.39
10.19
10.27
10.12
10.14
10.22
10.37
10.20
10.36
CaO
0.00
0.00
0.00
0.00
0.02
0.01
0.00
0.02
0.00
Na
2
O
0.49
0.44
0.05
0.28
0.08
0.24
0.07
0.18
0.11
K
2
O
8.82
8.59
9.45
9.39
9.76
9.03
9.18
9.44
9.33
Sum
95.79
96.75
96.02
96.24
96.42
95.56
94.83
96.32
95.45
Si
5.494
5.518
5.384
5.355
5.401
5.378
5.363
5.357
5.375
Al
IV
2.506
2.482
2.616
2.645
2.599
2.622
2.637
2.643
2.625
Al
VI
0.779
0.822
0.687
0.696
0.726
0.702
0.718
0.673
0.681
Ti
0.238
0.263
0.254
0.278
0.267
0.273
0.245
0.255
0.239
Mg
2.326
2.251
2.317
2.277
2.275
2.310
2.363
2.300
2.352
Fe
2+
2.366
2.356
2.518
2.487
2.442
2.477
2.473
2.540
2.527
Mn
0.000
0.000
0.005
0.005
0.005
0.009
0.005
0.017
0.001
Ca
0.000
0.000
0.000
0.000
0.003
0.002
0.000
0.003
0.000
Na
0.143
0.126
0.015
0.082
0.023
0.071
0.021
0.053
0.032
K
1.690
1.624
1.825
1.808
1.874
1.747
1.790
1.821
1.813
cations
15.542
15.442
15.621
15.633
15.615
15.591
15.615
15.662
15.645
X
Mg
0.496
0.489
0.479
0.478
0.490
0.483
0.489
0.475
0.482
Henry
et
al.
(2005)
[T(°C)]
621 637 629 644 630 642 625 629 618
for all analysis: Ti
apfu
sd = 0.018, X
Mg
sd = 0.007; sd — standard deviation
Geothermobarometry
The temperature of metamorphism for the investigated
micaschists was calculated using garnet-biotite (GB) Fe-Mg
exchange (Bhattacharya et al. 1992; Holdaway 2000), garnet-
muscovite (GM) (Wu et al. 2002), muscovite-paragonite (MP)
(Blencoe et al. 1994) and Ti-in-biotite geothermometer (Henry
et al. 2005). Pressures were determined using the geobarome-
ters based on the net-transfer reactions: anorthite-grossular-kya-
nite-quartz (GASP) (Powell & Holland 1988), garnet-biotite-
plagioclase-quartz (GBPQ) (Wu et al. 2004a) and garnet-mus-
covite-plagioclase-quartz (GMPQ) (Wu et al. 2004b).
The average values of temperature and pressure based on
the all above mentioned calibrations and those obtained from
the THERMOCALC 3.21 program are given in Table 9.
Calculated temperatures using the Henry et al. (2005) for-
malism for all studied biotites are listed in Tables 4—7 and
plotted on the Ti versus X
Mg
diagram (Fig. 11a). Biotites
10
ERIĆ, LOGAR, MILOVANOVIĆ, BABIČ and ADNA
Đ
EVIĆ
Table 6: Representative microprobe analyses of biotites in RH micaschists.
Bt-1a Bt-1b Bt-2a Bt-2b Bt-3 Bt-4 Bt-5 Bt-6 Bt-7
SiO
2
37.70 38.13 37.58 37.56 35.62 35.47 35.87 37.72 38.91
TiO
2
1.58
1.65
1.49
1.72
1.26
1.55
1.78
1.92
1.93
Al
2
O
3
20.02 19.53 19.51 19.70 19.94 19.41 19.39 19.52 19.93
FeO
17.57 17.33 17.35 16.67 18.30 18.05 16.55 16.59 16.86
MnO
0.00
0.11
0.08
0.18
0.00
0.02
0.08
0.00
0.09
MgO
11.64 11.72 11.38 11.37 10.94 11.55 11.96 11.82 11.73
CaO
0.00
0.00
0.00
0.00
0.04
0.00
0.04
0.00
0.14
Na
2
O
0.55
0.44
0.72
0.60
0.25
0.30
0.32
0.37
0.21
K
2
O
8.20
8.50
7.86
8.09
8.64
8.84
8.82
8.40
7.12
Sum
97.26 97.41 95.97 95.89 94.99 95.19 94.81 96.34 96.92
Si
5.493
5.545
5.543
5.534
5.375
5.346
5.387
5.531
5.613
Al
IV
2.507
2.455
2.457
2.466
2.625
2.654
2.613
2.490
2.387
Al
VI
0.932
0.892
0.935
0.955
0.922
0.794
0.819
0.904
1.001
Ti
0.173
0.180
0.165
0.191
0.143
0.176
0.201
0.212
0.209
Mg
2.529
2.541
2.502
2.497
2.460
2.595
2.678
2.584
2.522
Fe
2+
2.141
2.108
2.140
2.054
2.309
2.275
2.079
2.034
2.034
Mn
0.000
0.014
0.000
0.022
0.000
0.003
0.010
0.000
0.011
Ca
0.000
0.000
0.000
0.000
0.006
0.000
0.006
0.000
0.022
Na
0.155
0.155
0.206
0.171
0.073
0.088
0.093
0.105
0.059
K
1.524
1.577
1.479
1.521
1.663
1.700
1.690
1.571
1.310
cations
15.454 15.467 15.427 15.411 15.577 15.631 15.576 15.473 15.168
X
Mg
0.541
0.546
0.539
0.549
0.516
0.533
0.563
0.559
0.554
Henry
et
al.
(2005)
[T(°C)]
570 581 559 593 516 571 608 616 612
for all analysis: Ti
apfu
sd = 0.022, X
Mg
sd = 0.017; sd — standard deviation
Table 7: Representative microprobe analyses of biotite inclusions in staurolite grains (CV1 micaschists).
Bt-1 Bt-2 Bt-3 Bt-4 Bt-5 Bt-6 Bt-7 Bt-8 Bt-9
SiO
2
35.97
36.04
37.93
38.66
36.00
38.29
37.31
37.15
36.65
TiO
2
1.21
1.18
1.20
1.00
1.19
1.10
1.10
1.15
1.20
Al
2
O
3
19.64
19.07
19.18
18.82
19.35
19.00
19.23
19.18
19.30
FeO
15.71
15.91
14.92
14.78
15.81
14.85
15.24
15.33
15.51
MnO
0.08
0.03
0.13
0.10
0.05
0.11
0.09
0.08
0.08
MgO
12.83
12.79
12.84
12.61
12.81
12.72
12.72
12.77
12.82
CaO
0.02
0.03
0.00
0.00
0.02
0.00
0.01
0.01
0.02
Na
2
O
0.32
0.37
0.22
0.68
0.34
0.45
0.50
0.40
0.30
K
2
O
8.92
8.96
8.35
8.18
8.94
8.27
8.55
8.60
8.74
Sum
94.70
94.38
94.77
94.83
94.51
94.79
94.75
94.67
94.62
Si
5.388
5.426
5.606
5.700
5.332
5.653
5.545
5.532
5.475
Al
IV
2.612
2.574
2.394
2.300
2.668
2.347
2.455
2.468
2.525
Al
VI
0.855
0.810
0.948
0.970
0.807
0.960
0.914
0.899
0.874
Ti
0.136
0.134
0.133
0.111
0.136
0.122
0.124
0.129
0.135
Mg
2.865
2.871
2.829
2.772
2.908
2.799
2.817
2.834
2.854
Fe
2+
1.968
2.003
1.844
1.822
2.014
1.834
1.894
1.909
1.938
Mn
0.010
0.004
0.016
0.012
0.006
0.014
0.011
0.010
0.010
Ca
0.003
0.005
0.000
0.000
0.003
0.000
0.002
0.002
0.003
Na
0.093
0.108
0.063
0.194
0.100
0.129
0.144
0.115
0.087
K
1.705
1.705
1.575
1.539
1.737
1.556
1.621
1.634
1.666
cations
15.635
15.640
15.408
15.420
15.711
15.414
15.527
15.532
15.567
X
Mg
0.593
0.589
0.605
0.603
0.591
0.604
0.598
0.597
0.596
Henry
et
al.
(2005)
[T(°C)]
535 529 535 485 534 512 514 522 534
for all analysis: Ti
apfu
sd = 0.008, X
Mg
sd = 0.005; sd — standard deviation
from CV1 micaschist and most biotites from RH micaschist
conform to the 580 °C geotherm (550—600 °C). Biotites oc-
curring as inclusions in staurolite in CV1 micaschist exhibit
lower Ti contents suggesting equilibration at lower tempera-
tures (Fig. 11a). Temperatures calculated for biotites from
CV2 micaschist range from 600° to 640 °C.
A comparison of calculated temperatures for individual
micaschist varieties obtained by Ti-in-biotite and garnet-bi-
otite geothermometers (Bhattacharya et al. 1992; Holdaway
2000) is graphically shown in Fig. 11b—d.
Discussion
The investigated micaschists from Crni vrh and Resavski
humovi represent non-graphitic peraluminous metapelites
with Ti-saturated minerals. Three micaschist varieties,
CV1, CV2 and RH have been petrographically distin-
guished. The average values of different geothermobaro-
metric methods suggest that the RH micaschist variety
(Grt + St + Bt + Ms + Pl + Qtz) reached a temperature of about
550 °C and pressure of 680 MPa, while CV2 micaschists
11
Ti-IN-BIOTITE GEOTHERMOMETRY IN NON-GRAPHITIC, PERALUMINOUS METAPELITES (CENTRAL SERBIA)
Table 8: Results of robust regression for possible substitutions in biotites of the investigated micaschists.
Table 9: Results of geothermobarometric analyses of micaschist from Crni vrh and Resavski humovi.
Temperature (average values) (T°C)
References CV1
CV2
RH
CV1
incl.
garnet-biotite geothermometry (GB)
Bhattacharya et al. (1992)
574 sd = 9
631 sd = 6
545 sd = 10
516 sd = 11
Holdaway (2000)
589 sd = 10
636 sd = 6
557 sd = 8
530 sd = 15
garnet-muscovite geothermometry (GM)
Wu et al. (2002)
590 sd = 9
640 sd = 8
548 sd = 19
550 sd = 18
muscovite-paragonite geothermobarometry (MP)
Blencoe et al. (1994)
600 sd = 20
–
–
–
Ti-in-biotite geothermometry
Henry et al. (2005)
581 sd = 14
633 sd = 11
571 sd = 32
522 sd = 16
Pressure (average values) (MPa)
CV1 CV2
RH
CV1
incl.
garnet-plagioclase-kyanite-quartz geobarometry (GASP)
Powell & Holland (1988)
600 sd = 50
630 sd = 60
590 sd = 60
410 sd = 60
garnet-biotite-plagioclase-quartz geobarometry (GBPQ)
Wu et al. (2004a) (exp. 2)
690 sd = 60
720 sd = 50
700 sd = 50
560 sd = 60
garnet-muscovite-plagioclase-quartz geobarometry (GMPQ)
Wu et al. (2004b) (exp. 2)
720 sd = 60
750 sd = 50
740 sd = 50
590 sd = 60
TERMOCALC 3.21
Powell & Holland (2001)
T = 582
°C (sd = 19)
T = 622
°C (sd = 17)
T = 548 °C (sd = 20)
P = 590 MPa (sd=120)
P = 620 MPa (sd = 140)
P = 600 MPa (sd = 160)
(St + Grt + Bt + Ms + Pg + Pl + Qtz) were metamorphosed at
630 °C and 700 MPa (Fig. 12). The CV1 variety
(St + Grt + Bt + Ms + Pg + Pl + Qtz) is characterized by the pres-
ence of two biotite and garnet generations. The first genera-
tion of garnet and biotite (which occurs as an inclusion in
staurolite) revealed an average temperature of 530 °C and
pressure of 520 MPa. The second generation of matrix gar-
net and biotite records temperatures of around 580 °C and
pressures of 670 MPa. The matrix garnet and biotite formed
along with continuously growing staurolite porphyroblasts.
Note that for those varieties not containing kyanite the re-
sults obtained by GASP geobarometry are not included in
the final calculations of pressure.
The average temperatures obtained by Ti-in-biotite ther-
mometer of Henry et al. (2005) for biotites from CV1 and
CV2 micaschists revealed uniform temperatures of 581 °C
(sd = 14 °C) and 633 °C (sd = 11 °C), respectively. In addi-
tion, the calculated temperatures show very small differences
in comparison to the temperatures obtained from other geo-
thermometers (Table 9), in spite of the fact that the pressure
values are higher ( ~ 700 MPa) than those (P = 400—600 MPa)
used by Henry et al. (2005) in their calibration.
For biotite of the RH variety application of the Ti-in-bi-
otite geothermometer gave an average temperature of 571 °C
(sd = 32 °C). This value differs by 21 °C from the average
temperature obtained by other methods, which is close to the
error of the Ti-in-biotite geothermometer (Henry et al.
2005). However, a rather high standard deviation of 32 °C is
related to the fact that more than one third of the analysed
RH biotites (14 out of 37) displays a temperature difference
in the range 85—110 °C. The largest differences were ob-
served for calculated temperatures below 520 °C (n = 6, ex-
cluding T = 502 °C as an extreme value) and above 605 °C
(n = 8) (Fig. 12). The pronounced variability of calculated
Exchange vectors
x,y
CV1
CV2
RH
Ti R
–2
x = Ti +
y = 2R
2+
y = 11.216 – 4.207·x
RMSE = 0.048
n = 26 R
2
= 0.97
y = 11.556 – 3.972·x
RMSE = 0.080
n = 29 R
2
= 0.84
y = 10.572 – 3.048·x
RMSE = 0.127
n = 37 R
2
= 0.71
TiAl
2
R
–1
Si
–2
x = Ti + 2Al
IV
y = 2Si + R
2+
y = 17.540 – 0.338·x
RMSE = 0.072
n = 27 R
2
= 0.38
y = 17.116 – 0.279·x
RMSE = 0.031
n = 28 R
2
= 0.51
y = 18.237 – 0.497·x
RMSE = 0.042
n = 36 R
2
= 0.79
TiRAl
–2
x = Ti + R
2+
y = 2Al
VI
y = 7.274 – 1.134·x
RMSE = 0.028
n = 27 R
2
= 0.97
y = 7.413 – 1.186·x
RMSE = 0.017
n = 28 R
2
= 0.98
y = 8.103 – 1.286·x
RMSE = 0.056
n = 37 R
2
= 0.88
Al
2
R
–1
Si
–1
x = 2Al
VI
+ 2Al
IV
y = R
2+
+ Si
y = 15.675 – 0.803·x
RMSE = 0.022
n = 25 R
2
= 0.97
y = 14.621 – 0.670·x
RMSE = 0.043
n = 29 R
2
= 0.49
y = 11.280 – 0.161·x
RMSE = 0.025
n = 34 R
2
= 0.40
n — number of analyses; y = b
o
–b
1
x; R
2
— coeficient of determination; RMSE — Root Mean Square Error
12
ERIĆ, LOGAR, MILOVANOVIĆ, BABIČ and ADNA
Đ
EVIĆ
Fig. 11. a – Obtained temperatures according to Ti-in-biotite geothermometer. Calculated temperature values according to different geo-
thermometers: b – CV1 without biotite inclusions, c – CV2, d – RH.
Fig. 12. PT path for investigated micaschists with temperature varia-
tions according to Ti-in-biotite geothermometer (Henry et al. 2005) and
KFMASH petrogenetic grids according to Holland & Powell (1998).
temperatures for RH biotites obtained by the Ti-in-biotite
method as well as differences in comparison to temperatures
obtained by garnet-biotite geothermometers (Bhattacharya et
al. 1992 and Holdaway 2000) are illustrated in Fig. 11d.
A greater variability of temperature estimates for RH bi-
otites in comparison to calculated temperatures of biotites
from CV1 and CV2 micaschists can be related to the prob-
lem of correlation of Ti contents and X
Mg
values. The appli-
cation of the Ti-in-biotite geothermometer generally requires
low variability of Ti contents and X
Mg
values. Biotites of
CV1 micaschist (excluding the biotite inclusions in garnet)
do not show correlations between Ti concentrations and X
Mg
values, biotite of CV2 shows a negative correlation (r = —0.67;
p < 0.01), whereas RH biotite exhibits a significant positive
correlation (r = 0.56; p < 0.01) of these parameters. Hence, the
observed positive correlation between Ti contents and X
Mg
values in RH biotites is in disagreement with the principles
of the Ti-in-biotite geothermometric method. Moreover, the
method suggests that an increase in titanium concentrations
should be accompanied by a decrease of Mg/(Mg + Fe) ratio
(Henry et al. 2005). The positive Ti-X
Mg
correlation is the re-
13
Ti-IN-BIOTITE GEOTHERMOMETRY IN NON-GRAPHITIC, PERALUMINOUS METAPELITES (CENTRAL SERBIA)
sult of the compositional variability shown by RH biotites
from different samples (Table 6). Such compositional vari-
ability can be produced by the effects of chloritization or
other types of alteration (Veblen & Ferry 1983) but such
characteristics were not detected by optical investigations.
The observed compositional variability might have been pro-
duced by temperature fluctuations during metamorphism in a
relatively small area (representative samples were taken in
intervals of 2—5 m). However, this possibility is not likely
given the uniform results of garnet-biotite geothermometry.
It is more probable that the observed compositional differ-
ences are related to compositional inhomogeneities of the
pelitic protolith, with respect to Ti contents. Thus, all the
analysed biotites within a single RH sample, for which tem-
peratures above 600 °C were calculated (e.g. Table 6, analy-
ses 5, 6 and 7), contain less iron and more titanium in
octahedral position than do biotites from other RH samples.
Biotites from the CV2 variety also show variable Ti con-
tents (Ti = 0.260, sd = 0.018 apfu) but they did not produce
variations in calculated temperatures. On the contrary, the
temperature estimates using the Ti-in-biotite geothermome-
ter for CV2 biotite display a very small standard deviation of
sd = 11 °C. A possible explanation can be that, as already
mentioned, the composition of CV2 biotite is characterized
by a negative correlation between Ti contents and X
Mg
val-
ues. The second explanation can be that the small variations
of temperature estimates are due to different sensitivity of
the Ti-in-biotite geothermometer with respect to different Ti
contents and X
Mg
values. The configuration of isotherms on
Henry’s diagram (Fig. 2) shows that titanium concentrations
above 0.25 apfu and low values of X
Mg
produce smaller vari-
ations in calculated temperatures and this was the case of
CV2 biotite. On the other hand, to obtain uniform tempera-
ture estimates for biotites that compositionally correspond to
the high sensitivity field (e.g. Ti < 0.25 apfu as observed in
CV1 and RH biotite) very uniform Ti contents are needed
(sd < 0.02). Simultaneously, the standard deviation of X
Mg
values should not be higher than 0.05 because it would also
produce variable temperature estimates.
The high sensitivity of Ti-in-biotite with respect to Ti con-
tents enables the calculation of temperature for two genera-
tions of biotite within individual samples of non-graphitic
peraluminous metapelites. For instance, the average Ti
apfu
value for biotite inclusions from the CV1 variety is consider-
ably lower (0.129, sd = 0.008 apfu) than the Ti
apfu
value for
matrix biotite (Ti = 0.169, sd = 0.011 apfu) produces substan-
tial differences in calculated temperatures of around 60 °C. It
is noteworthy that similar differences are found in other geo-
thermometric calibrations.
Conclusions
The application of the Ti-in-biotite geothermometer (Hen-
ry et al. 2005) to biotites (Ti
apfu
< 0.30 and X
Mg
< 0.65) from
three varieties of micaschists, metamorphosed in a tempera-
ture range of 550—650 °C and at pressures of 600—800 MPa,
revealed important constraints on the applicability of the
method for non-graphitic peraluminous micaschists. Notable
variations in calculated temperature (85—110 °C) for biotites
from the RH micaschist variety are the result of a significant
positive correlation between the Ti amounts and X
Mg
values
in these biotites. It can be concluded that such positive corre-
lation should not exist within the same biotite generation and
that the standard deviation for Ti contents should be less
than 0.02. The biotites of two other micaschist varieties CV1
and CV2 do not show positive correlations between Ti con-
tents and X
Mg
, therefore, the temperature estimates based on
their composition are very uniform and show close agree-
ments with temperatures calculated by other geothermome-
ters. The method shows high sensitivity for biotites with Ti
amounts below 0.25 apfu. This enables the determination of
temperature for different biotite generations which is con-
firmed by results from two biotite generations in CV1 mic-
aschist revealing a temperature difference of 60 °C.
Consequently, the Ti-in-biotite geothermometry can be suc-
cessfully applied to similar mineral associations in non-gra-
phitic peraluminous micaschists.
Acknowledgments: This study has been supported by the
Ministry of Science of the Republic of Serbia.
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