Journal of African Earth Sciences 36 (2003) 73–87
www.elsevier.com/locate/jafrearsci
Petrography and geochemistry of the Singo granite,
Uganda, and implications for its origin
Betty Nagudi
a
a,1
, Christian Koeberl
a,*
, Gero Kurat
b
Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
b
Naturhistorisches Museum, Postfach 417, A-1014 Vienna, Austria
Received 31 July 2002; accepted 23 January 2003
Abstract
The Singo granite in western central Uganda intrudes metasedimentary rocks that have experienced low-grade regional and
contact metamorphism. Rocks of the main body are pink and coarse-grained with a porphyritic texture. Marginal gray mediumgrained biotite granite (BG) and several other varieties with intermediate composition occur with limited extent. The Singo granite is
generally massive, contains mainly plagioclase, K-feldspar, quartz, biotite, muscovite and opaques. The BG has higher Al2 O3 , MgO,
CaO, Fe2 O3 , TiO2 , Ba, Zr, and V, but lower SiO2 , Th, U, and rare earth elements (REE) and alkali totals than the pink porphyritic
granite (PPG). Spider diagrams (chondrite-normalized) show negative Eu, Sr, and Nb anomalies, and unfractionated heavy-REE
(HREE). The Eu and Sr anomalies and unfractionated HREE suggest the presence of plagioclase and absence of garnet in the
source, whereas the Nb anomaly implies a crustal component. Singo granite has both S- and I-type characteristics, and was emplaced in a syn- to post-collision tectonic setting in several magma pulses within relatively short time intervals. The pluton, in
general, shows zonation in texture, mineralogy, and geochemistry from the margin to the center. The BG is relatively old and less
felsic, whereas the PPG represents a younger and more felsic part of the pluton. There is continuity between the BG and the PPG
through the intermediate granite, suggesting a common origin. Field, petrographic, and geochemical characteristics support a
magmatic origin from a water-undersaturated, heterogeneous crustal source rock under low pressure conditions. Petrographic and
chemical variations were mainly the result of fractional crystallization and source heterogeneity. Late- and post-magmatic stages
were dominated by strong hydrothermal activity.
2003 Elsevier Science Ltd. All rights reserved.
Keywords: Singo granite; Buganda-Toro; Uganda; Fractional crystallization; Crustal melting
1. Introduction
The Singo granite in western central Uganda is similar
to the Mubende batholith (MacDonald, 1966). So far,
any information about the age and origin of granites in
the Buganda-Toro ‘‘System’’, to which the Singo granite belongs, remains limited. Previously, granites with
marked variations in mineral composition and a migmatitic boundary were considered to have originated
from the lower basement rocks by partial melting and
were believed to be older than the country rocks (King,
1947, and references therein). Those, which were more
*
Corresponding author. Tel.: +43-1-40103-2360; fax: +43-1-4039030.
E-mail address: [email protected] (C. Koeberl).
1
Present address: Department of Geology, Makerere University,
P.O. Box 7062, Kampala, Uganda.
uniform in composition and texture, were taken to be of
magmatic origin, younger than the country rocks, and
probably originated from remelting of portions of the
older granite. Recent petrographic and geochemical
work by Schumann et al. (1999) on granites and granite
gneisses elsewhere in the Buganda-Toro region (67 km
from the Singo granite) supports their magmatic origin
and emplacement in syn- to post-collision tectonic environment.
Details of the field relations of the Singo granite, as
well as some petrography, were given by King (1947),
Johnson (1960), and Johnson and Williams (1961).
Limited geochemical or age data exist, and, therefore,
the origin of the Singo granite is not yet well understood. Microtextural relationships studied by King
(1947) led him to suggest that the Singo granite formed
by a series of replacements (metasomatism) of the country rocks. The replacement stages varied from country
rock through contact rocks to the porphyritic granite
0899-5362/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0899-5362(03)00014-9
74
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
and were marked by varying degrees of quartz replacement by sericite and feldspars (King, 1947).
However, if the granite was formed by a replacement
process, some of the sedimentary structures should have
been preserved, the contact with the country rocks
should be gradational and the contact aureole should be
absent (Johnson and Williams, 1961). The parallel nature of bedding planes of the country rocks to the
granite contact in some places, suggests a contemporaneous emplacement of the Singo granite with the
country rocks. The quartz-sericite bodies, which were
considered to be intermediate stages in the formation of
the Singo granite from country rocks by King (1947),
were later found to be zones of alteration by hydrothermal fluids, and, therefore, postdate the pluton
(Johnson and Williams, 1961).
In this paper, we present petrographic, mineralogical
and geochemical data for the Singo granite and associated rocks. The objective is to describe and interpret the
petrographic and geochemical characteristics, as well as
field relationships of the Singo granite, and to conduct
inferences regarding its formation and possible source
rocks. The most probable tectonic setting and classification are also discussed.
2. Geological setting
The Singo granite in western central Uganda covers
an area of about 700 km2 (Fig. 1). It partly occupies a
syncline in the NE and generally cuts the country rocks,
although, locally, the granite contact seems to be parallel
to the bedding planes in the host rock. The country
rocks, of the Buganda-Toro ‘‘system’’, are believed to
have been folded prior to the emplacement of the granite
(King, 1947). Buganda-Toro rocks consist of sandstones, slates, phyllites, mica schists, basal quartzites,
amphibolites, and epidosites. The quartzites have resisted the emplacement in some localities. The granite
contact may contain different types of enclaves, but
breccias in the hornfelsed sandstones close to the contact
are also found (Johnson and Williams, 1961). The
country rocks have experienced both low grade regional
and contact metamorphism. The latter is only manifested in the form of indurations and silicification,
hornfelses, and enrichment of tourmaline and, in places,
hematite (Johnson and Williams, 1961). The granite is
associated with aplite dykes, quartz-sericite bodies,
pegmatites, greisen zones, quartz veins, hematite veins,
breccia and shear zones, steep joints and sinistral
faults. However, most parts of the batholith are covered
by swamps and dense vegetation, leaving poor and
weathered exposures. The granite boundary with the
country rocks is often only inferred (King, 1947). Alluvial gold, fluorite, wolframite, and beryl associated with
the Singo granite were economically mined in the past.
The emplacement of the Singo batholith cannot be
related to any orogeny in the immediate area. However,
Pinna et al. (2001, and references therein) suggested that
the Paleoproterozoic events affected this area on to a
limited extent and could have only led to the emplacement of an anorogenic granite, the Singo granite. The
Fig. 1. Simplified geological map of Uganda showing the location of Singo granite and some structural features (modified after MacDonald (1966)).
Table 1
Representative electron microprobe analyses (in wt%) for feldspars from the Singo granites, western central Uganda
K-feldspar
Plagioclase
Biotite granite
Pink porphyritic granite
Biotite granite
Pink porphyritic granite
36k1m
36k1c
36k2m
36k2c
56-km
56kc
46k3m
46k3c
88k16c
88k16m
36pm
36pc
36p2m
36p2c
56pm
56pc
88p15m
88p15c
SiO2
TiO2
Al2 O3
FeO
MnO
MgO
BaO
CaO
Na2 O
K2 O
64.0
0.04
18.8
0.01
b.d.
0.02
0.06
0.04
1.2
14.9
63.5
b.d.
19.1
b.d.
0.02
b.d.
0.06
0.04
0.43
15.8
64.1
0.01
19.1
b.d.
b.d.
0.02
0.07
0.03
1.29
14.8
63.9
b.d.
19.2
0.05
b.d.
0.02
b.d.
0.01
0.9
15.1
64.8
b.d.
19.0
0.02
b.d.
0.01
0.02
b.d.
0.33
15.7
64.1
b.d.
19.0
b.d.
b.d.
b.d.
b.d.
0.02
0.28
15.9
64.3
b.d.
18.8
0.01
b.d.
0.02
b.d.
0.07
0.25
15.5
63.7
0.01
18.9
0.08
b.d.
0.01
0.04
0.01
0.27
15.4
63.8
0.01
18.7
0.04
0.01
b.d.
b.d.
b.d.
0.23
16.2
63.3
b.d.
18.6
0.02
b.d.
b.d.
b.d.
0.02
0.22
16.3
66.1
0.03
22.4
0.02
0.02
b.d.
b.d.
1.77
10.0
0.2
65.7
0.01
22.1
0.02
b.d.
b.d.
b.d.
2.1
9.9
0.1
68.0
0.01
21.3
0.04
0.02
b.d.
b.d.
0.57
10.3
0.2
63.0
b.d.
25.0
0.09
b.d.
0.02
0.05
2.22
8.9
1.6
67.4
0.03
20.3
0.02
b.d.
b.d.
b.d.
0.4
11.3
0.2
68.1
0.02
20.8
0.06
0.01
b.d.
b.d.
0.26
10.8
0.1
66.2
0.01
21.2
0.11
0.01
0.01
0.01
1.57
10.3
0.3
65.3
b.d.
21.5
0.04
0.02
0.01
b.d.
1.88
10.2
0.2
Total
99.19
98.83
99.46
99.17
99.88
99.22
98.96
98.37
99
98.43
100.59
99.95
100.36
100.83
99.59
100.2
99.83
99.19
Rb2 O
Si
Al
Ti
Fe2
Mn
Mg
Ba
Ca
Na
K
Cations
X
Z
Ab
An
Or
b.d.
5.94
2.06
0.003
0.001
n.c.
0.003
0.002
0.004
0.22
1.77
10.0
b.d.
5.91
2.09
n.c.
n.c.
0.002
n.c.
0.002
0.004
0.08
1.87
9.96
b.d.
5.92
2.08
0.001
n.c.
n.c.
0.003
0.003
0.003
0.23
1.75
9.99
b.d.
5.91
2.09
n.c.
0.004
n.c.
0.003
n.c.
0.001
0.16
1.79
9.96
b.d.
5.95
2.05
n.c.
0.002
n.c.
0.001
0.001
n.c.
0.06
1.84
9.91
b.d.
5.93
2.07
n.c.
n.c.
n.c.
n.c.
n.c.
0.002
0.05
1.87
9.92
0.05
5.96
2.05
n.c.
0.001
n.c.
0.003
n.c.
0.007
0.05
1.84
9.89
b.d.
5.93
2.08
0.001
0.006
n.c.
0.001
0.001
0.001
0.05
1.83
9.89
b.d.
5.94
2.06
0.001
0.003
0.001
n.c.
n.c.
n.c.
0.04
1.93
9.98
b.d.
5.95
2.05
n.c.
0.002
n.c.
n.c.
n.c.
0.002
0.04
1.95
10.0
0.03
5.72
2.29
0.002
0.001
0.001
n.c.
n.c.
0.164
1.67
0.03
9.86
0.01
5.73
2.27
0.001
0.001
n.c.
n.c.
n.c.
0.196
1.68
0.01
9.88
0.02
5.85
2.15
0.001
0.003
0.001
n.c.
n.c.
0.052
1.71
0.02
9.78
0
5.45
2.55
n.c.
0.007
n.c.
0.003
0.002
0.206
1.49
0.17
9.88
0.04
5.91
2.09
0.002
0.001
n.c.
n.c.
n.c.
0.038
1.92
0.02
9.98
0.02
5.88
2.12
0.001
0.004
0.001
n.c.
n.c.
0.024
1.81
0.02
9.86
0.06
5.81
2.19
0.001
0.008
0.001
0.001
n.c.
0.148
1.76
0.04
9.95
0.03
5.76
2.24
n.c.
0.003
0.001
0.001
n.c.
0.178
1.75
0.03
9.96
8
1.99
10.9
0.2
88.9
8
1.96
4
0.2
95.8
8
1.99
11.7
0.2
88.2
8
1.95
8.3
0.1
91.7
8
1.91
3.1
n.c.
96.9
8
1.92
2.6
0.1
97.3
8
1.89
2.4
0.4
97.2
8
1.89
2.6
0.1
97.3
8
1.98
2.1
n.c.
97.9
8
2
2
0.1
97.9
8
1.86
89.8
8.8
1.3
8
1.88
89
10.4
0.5
8
1.78
96
2.9
1.1
8
1.88
79.7
11.1
9.3
8
1.98
97.1
1.9
1
8
1.86
97.9
1.3
0.8
8
1.95
90.4
7.6
2
8
1.96
89.5
9.1
1.3
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
Sample
All samples are NB; K1m: potassium feldspar margin of grain 1; K1c: potassium feldspar center of grain 1; p2m: plagioclase margin of grain 2; p2c: plagioclase center of grain 2; b.d.: below detection
limit; n.c.: not calculated (calculations were based on 16 oxygens).
75
76
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
Paleoproterozoic event, together with the Kampala
orogenic event, is believed to represent the remnants of
an outer-zone related to the late Archaean Neovictorian
orogeny. Recent age data (zircon evaporation) indicate
emplacement of the Singo granite during the Paleoproterozoic (1847 6 Ma, Pinna et al., 2001; 1615 19 Ma,
Nagudi et al., 2001).
This granite has similar characteristics to the Mubende granite and both are believed to have been
emplaced at the same time, during the Palaeoproterozoic (MacDonald, 1966; Johnson and Williams, 1961).
However, the Mubende granite has contradictory emplacement ages of 1.8 Ga (Rb/Sr ages on biotites) and
<1.3 Ma (stratigraphic age relative to the Singo ‘‘Series’’
rocks; Johnson, 1960; King, 1947).
3. Sampling and analytical techniques
A total of about one hundred rock samples from the
granite body, quartz-diorite and aplite dykes, and en-
claves were collected. Eighty one thin sections of these
samples were prepared and studied by optical microscopy and 10 others were selected for electron microscopy. Mineral compositions, both quantitative (Tables
1 and 2) and qualitative, were determined using a
Cameca SX-100 electron microprobe at the University
of Vienna, Austria. A JOEL scanning electron microscope (SEM), equipped with a Kevex energy dispersive spectrometer, at the Natural History Museum in
Vienna, was also used for qualitative and semi-quantitative mineral identification.
Representative samples were then selected for whole
rock geochemistry. Sample weights were 1–1.5 kg before
crushing and powdering. Major, minor, and trace
element abundances were determined by X-ray fluorescence (XRF), using a Philips PW 1400 XRF spectrometer at the University of the Witwatersrand in South
Africa. The elements include SiO2 , TiO2 , Al2 O3 , Fe2 O3T ,
MnO, MgO, CaO, Na2 O, K2 O, P2 O5 , V, Cr, Co, Ni, Cu,
Zn, Rb, Sr, Zr, Y, Nb and Ba. Accuracy and precision
for the data are given in Reimold et al. (1994). Other
Table 2
Representative electron microprobe analyses (in wt%) for biotites from the Singo granite
Sample
Biotite granite
Pink porphyritic granite
36b3m
36b3c
36b5m
36b5c
56bc
88b1c1
88-b1c2
46b5c
465m
SiO2
TiO2
Al2 O3
FeO
MnO
MgO
BaO
CaO
Na2 O
K2 O
F
H2 O
35.8
3.4
17.0
20.1
0.4
8.6
b.d.
0.04
0.16
9.4
b.d.
1.86
35.7
3.9
16.6
19.8
0.5
8.5
0.23
0.02
0.17
9.4
b.d.
1.86
33.0
3.6
17.2
22.0
0.5
9.3
b.d.
1.40
0.08
7.0
b.d.
1.83
36.0
3.7
17.0
19.8
0.4
8.6
0.32
b.d.
0.16
9.5
b.d.
1.87
39.2
2.0
18.0
16.9
0.9
8.3
0.01
0.02
0.22
9.4
0.07
1.88
40.0
1.5
14.3
14.6
1.0
12.9
b.d.
b.d.
0.18
9.9
3.48
0.26
40.1
1.4
14.3
14.6
1.0
12.8
b.d.
b.d.
0.18
9.8
3.44
0.28
39.4
2.0
14.8
15.0
0.7
11.6
b.d.
b.d.
0.13
9.7
1.76
1.06
39.0
2.1
15.0
15.4
0.7
11.4
0.01
b.d.
0.13
9.7
1.45
1.2
Total
96.71
96.66
95.82
97.33
96.86
98.057
97.86
96.13
96.03
Rb2 O
Si
AlIV
AlVI
Ti
Fe2
Mn
Mg
Ba
Ca
Na
K
Cations
CF
OH
O
Mg/Fe + Mg
b.d.
5.76
2.24
0.98
0.41
2.71
0.05
2.07
n.c.
0.006
0.05
1.9
16.2
n.c.
2
24
0.43
b.d.
5.76
2.24
0.92
0.47
2.67
0.07
2.05
0.02
0.004
0.05
1.9
16.2
n.c.
2
24
0.43
b.d.
5.40
2.60
0.72
0.44
3.01
0.07
2.26
n.c.
0.245
0.03
1.5
16.2
n.c.
2
24
0.43
b.d.
5.76
2.24
0.96
0.45
2.66
0.06
2.05
0.02
n.c.
0.05
1.9
16.2
n.c.
2
24
0.44
0.25
6.14
1.86
1.46
0.23
2.22
0.12
1.93
n.c.
0.004
0.07
1.9
15.9
0.07
1.964
24
0.47
0.10
6.30
1.71
0.95
0.18
1.92
0.13
3.01
n.c.
n.c.
0.05
2.0
16.2
3.46
0.27
24
0.61
0.12
6.31
1.69
0.96
0.17
1.90
0.13
3.01
n.c.
n.c.
0.05
2.0
16.2
3.42
0.29
24
0.61
0.30
6.26
1.74
1.03
0.24
2.0
0.09
2.74
n.c.
n.c.
0.04
2.0
16.1
1.76
1.119
24
0.58
0.07
6.22
1.78
1.02
0.25
2.06
0.09
2.70
n.c.
n.c.
0.03
2.0
16.1
1.46
1.271
24
0.57
All samples are NB; b: biotite; m: margin; c: center; b.d.: below detection limit; n.c.: not calculated (calculations were based on 22 oxygens).
Table 3
Bulk chemical compositions of the Singo granite and associated rocks, western central Uganda
Sample
Biotite granite
MG
Intermediate granite
NB9
NB15
NB36
NB37
NB38
NB43
NB52b
NB25
NB8
NB13
NB14
NB23
NB41
NB56
NB67
NB79
NB85
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
68.15
0.77
14.27
5.43
0.14
1.12
1.80
3.19
4.32
0.23
1.05
69.99
0.64
14.04
4.60
0.10
0.95
1.85
3.17
4.30
0.19
0.80
67.66
0.75
14.01
5.23
0.12
1.07
1.87
3.20
4.33
0.22
0.98
69.43
0.68
13.84
4.70
0.11
1.00
1.78
3.23
4.32
0.21
1.11
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
70.70
0.41
14.28
3.04
0.12
0.56
1.26
3.87
4.47
0.20
1.24
69.09
0.68
14.07
4.70
0.13
0.95
1.72
3.47
4.36
0.20
0.93
n.d
n.d
n.d
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
76.75
0.13
12.78
0.63
0.05
0.00
0.48
3.88
5.00
0.13
0.47
72.89
0.43
13.23
2.35
0.07
0.71
1.24
3.49
4.77
0.17
1.03
72.99
0.37
13.63
1.85
0.06
0.60
0.87
3.62
5.34
0.16
0.98
72.26
0.43
13.96
2.16
0.07
0.60
0.71
3.47
5.53
0.16
1.08
73.49
0.43
13.12
2.64
0.08
0.79
0.84
2.98
5.02
0.12
0.95
71.83
0.40
13.88
2.19
0.08
0.62
1.24
3.70
5.27
0.17
0.92
76.21
0.20
12.28
1.03
0.06
0.27
0.55
3.71
5.28
0.04
0.78
71.23
0.45
14.00
3.01
0.10
0.70
1.26
3.32
5.24
0.12
0.79
70.74
0.51
13.72
2.85
0.07
0.87
1.88
3.54
4.73
0.16
0.96
72.82
0.47
12.49
2.49
0.07
0.71
1.21
3.63
4.67
0.12
0.88
Total
100.45
100.64
99.44
100.42
n.d.
100.16
100.29
n.d.
100.32
100.36
100.46
100.43
100.46
100.28
100.42
100.22
100.03
99.57
Sc
V
Cr
Co
Ni
Cu
Zn
Rb
Sr
Y
Zr
Sb
Nb
Cs
Ba
La
Ce
Nd
Sm
Eu
Gd
Tb
Tm
Yb
Lu
Hf
Ta
Th
U
12.1
56
34.1
12.6
18
9
104
216
167
47
259
0.62
19
8.29
943
54.5
113
49.5
9.26
1.83
7.39
1.46
0.49
4.49
0.7
7.88
1.6
18.8
4.61
11.9
50
35.9
10.2
12
<2
75
227
151
37
206
0.39
17
11.3
877
52.2
104
45.9
8.66
1.78
6.86
1.18
0.73
4.39
0.59
7.01
1.56
19.8
4.48
13.1
52
62
12.6
24
6
84
216
155
38
246
0.08
18
7.07
933
59.5
121
52.2
9.7
1.84
7.69
1.33
<0.18
4.19
0.56
7.71
1.37
20.3
4.74
10.8
49
27.3
9.44
16
6
61
199
154
40
227
1.13
17
6.46
925
48.8
98.6
42.5
7.76
1.8
8.06
1.13
0.69
3.88
0.61
6.12
1.4
16.3
3.53
7.21
n.d.
18.9
0.77
b.d.
b.d.
4.9
145
b.d.
b.d.
b.d.
3.05
b.d.
1.36
b.d.
29.4
44.4
20.9
3.01
0.56
4.7
0.33
0.4
1.02
0.17
4.42
1.94
19.8
1.19
9.85
31
34.5
9.05
17
9
55.8
294
108
35
135
0.1
16
11
571
31.4
65.6
31.7
5.52
1.17
<3.8
0.93
0.43
3.34
0.49
4.25
1.72
10.8
4.34
12.7
45
38.4
10.9
15
8
72.8
239
138
41
203
0.13
19
13.5
794
50
103
46.9
8.39
1.64
6.06
1.32
<0.14
4.71
0.67
6.48
1.75
17.7
3.17
25.6
n.d.
15.5
34.8
b.d.
b.d.
117
80.8
b.d.
b.d.
b.d.
0.75
b.d.
8.33
b.d.
34.2
68
37
6.92
1.95
7.05
1.03
0.64
4.22
0.61
5.38
0.78
8.64
2.46
2.53
<15
20.3
1.47
<9
<2
13
407
36
26
27
0.79
11
18.8
162
7.13
18.4
9.13
2.24
0.28
2.77
0.44
0.37
2.63
0.35
1.3
1.4
6.24
10.3
8.57
33
22.2
5.78
14
<2
19.8
343
132
36
156
<0.53
19
7.26
529
57.6
106
43
6.85
1.17
5.66
0.75
<0.17
3.4
0.61
5.21
2.09
25.8
7.8
7.55
31
27.5
2.57
15
<2
22
430
115
35
135
0.13
17
8.35
512
39.1
73.8
24.7
5.41
0.6
2.89
0.66
0.85
2.9
0.64
4.24
1.71
20
6.83
6.74
37
20.5
3.59
11
5
20
378
122
74
153
0.26
19
7.89
708
76.7
155
61.1
12.2
2.02
12
1.54
0.76
6.63
0.98
4.15
1.72
22.5
22
7.01
26
33.7
5.05
10
<2
27
298
101
42
150
0.2
19
8.46
619
52.2
105
43
8.3
1.29
6.87
1.12
0.93
4.26
0.64
5.97
1.82
29.1
12.4
8.33
33
24.8
7.72
15
<2
30
345
137
59
147
0.4
19
9.21
659
65
110
50.3
9.76
1.93
10.13
1.74
<0.16
6.89
1
5.38
2.05
26.9
6.77
5.01
<15
23.6
1.28
9
<2
15
530
46
30
133
0.27
24
4.16
220
80
117
37.3
7.04
0.51
4.81
0.75
0.4
2.93
0.49
5.54
1.86
50.8
32.1
8.78
38
51.5
7.4
12
18
40
259
142
28
144
0.31
18
11.5
817
40.5
78.9
36
5.82
1.19
5.07
0.92
<0.11
3.51
0.42
4.72
1.34
17.4
5.12
7.84
42
24.2
6.75
12
<2
39
293
173
29
180
0.27
20
4.82
744
74.1
132
51.9
8.12
1.41
5.59
0.92
<0.14
3.17
0.47
6.15
2.21
36.3
11.8
8.29
37
30.3
6.41
20
<2
66
245
136
576
206
0.13
20
3.67
569
83.8
149
75.9
22.9
5.84
75.7
12
6.85
54.7
8.61
5.42
1.93
40.9
11.4
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
NB1
77
78
Table 3 (continued)
Sample
K2 O/Na2 O
ASI
Mg#
Ti/P
Sr/Ba
Rb/Sr
Eu/Eu*
(La/Yb)cn
Biotite granite
MG
Intermediate granite
NB9
NB15
NB36
NB37
NB38
NB43
NB52b
NB25
NB8
NB13
NB14
NB23
NB41
NB56
NB67
NB79
NB85
1.35
1.08
29.01
4.62
0.177
1.29
0.93
8.2
1.36
1.06
29.03
4.61
0.72
1.5
0.96
8.04
1.35
1.05
28.84
4.68
0.166
1.39
0.92
9.59
1.34
1.05
29.65
4.42
0.17
1.29
0.89
8.5
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.65
19.49
1.16
1.06
26.73
2.83
0.189
2.72
n.d.
6.35
1.26
1.04
28.59
4.68
0.174
1.73
0.98
7.17
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.99
5.48
1.29
1.01
0.00
1.36
0.22
11.3
0.6
1.83
1.37
1.01
37.44
3.49
0.25
2.6
0.85
11.45
1.48
1.02
39.12
3.17
0.225
3.74
0.86
9.11
1.59
1.08
35.49
3.69
0.172
3.09
0.77
4.08
1.69
1.11
37.22
4.96
0.163
2.95
0.81
8.28
1.42
0.99
35.93
3.24
0.208
2.5
0.82
6.38
1.42
0.96
34.18
7.06
0.209
11.52
0.61
16.82
1.58
1.04
31.54
5.19
0.174
1.82
0.91
7.8
1.34
0.96
37.68
4.37
0.233
1.69
0.94
15.8
1.29
0.94
36.10
5.42
0.239
1.8
0.52
1.04
QD
ME
Pink porphyritic granite
NB76A
NB76B
NB4
NB6A
NB45A
NB57
NB84
NB88
NB87
NB61
NB86
NB46
NB48
NB89
NB74B
NB45B
NB58A
NB1X
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
75.04
0.26
13.53
1.26
0.04
0.63
0.13
0.06
7.04
0.14
1.97
76.25
0.18
12.39
0.75
0.04
0.28
0.32
4.64
4.37
0.12
0.96
76.51
0.24
12.49
0.81
0.02
0.23
0.28
4.07
4.95
0.04
0.75
76.37
0.21
12.11
1.04
0.04
0.35
0.14
3.67
5.35
0.04
0.95
74.10
0.28
13.51
1.35
0.06
0.35
0.69
3.81
4.93
0.12
0.92
75.88
0.19
12.30
1.04
0.04
0.13
0.63
4.05
4.83
0.04
0.83
76.50
0.17
12.39
0.43
0.04
0.02
0.48
3.33
6.37
0.05
0.51
76.36
0.13
12.17
0.74
0.03
0.07
0.59
4.35
4.78
0.02
0.79
76.26
0.14
12.67
0.76
0.04
0.01
0.50
4.09
5.22
0.02
0.72
74.84
0.21
12.74
1.28
0.06
0.24
0.51
3.93
5.32
0.05
0.94
77.23
0.17
12.33
0.42
0.04
0.16
0.59
4.16
4.65
0.02
0.84
75.93
0.19
12.51
0.87
0.05
0.09
0.62
3.90
5.33
0.04
0.75
76.22
0.19
12.37
0.62
0.05
0.19
0.36
4.11
4.99
0.03
0.81
75.53
0.18
12.70
1.06
0.06
0.13
0.54
4.15
5.20
0.02
0.89
76.99
0.23
12.26
1.08
0.06
0.18
0.54
3.53
4.89
0.06
0.59
77.07
0.17
12.34
0.69
0.04
0.06
0.47
4.31
4.66
0.02
0.67
52.06
1.27
15.06
12.12
0.18
5.36
8.97
2.71
1.09
0.11
1.34
65.80
0.75
15.79
5.87
0.13
1.21
2.07
3.49
4.03
0.22
1.17
Total
100.11
100.31
100.40
100.28
100.12
99.97
100.29
100.04
100.46
100.13
100.62
100.29
99.95
100.47
100.42
100.51
100.28
100.54
Sc
V
Cr
Co
Ni
Cu
Zn
Rb
Sr
Y
Zr
Sb
Nb
Cs
Ba
La
Ce
6.87
32
8.9
2.07
<9
<2
8.53
382
24
35
102
0.39
15
7.71
425
49.2
89.5
5.4
15
16.3
1.26
9
<2
19.7
470
59
33
63
0.41
15
9.9
226
13.8
33.6
6.77
<15
16.4
1.36
<9
<2
17
617
53
24
116
0.16
26
2.79
213
100
134
5.41
15
31
1.6
12
<2
13
424
29
26
122
0.24
23
3.04
241
38
77.2
6.79
27
30.1
3.76
13
<2
13
444
96
50
98
0.33
16
10
423
35
69
6.86
<15
20
0.73
<9
<2
8.5
514
33
21
120
<0.5
24
3.6
170
76
102
2.62
<15
12.7
1.09
<9
<2
27
291
61
25
62
0.18
15
2.84
191
35.7
51.9
5.08
<15
13.9
0.79
<9
<2
12
442
27
34
106
<0.52
19
2.05
100
82.8
108
5.97
<15
5.7
0.87
<9
<2
14.1
516
15
23
140
<0.54
21
2.75
65
50.5
69
5.19
<15
16.1
1.09
<9
<2
14
467
36
27
129
0.37
27
4.25
260
74.6
117
5.65
<15
15.1
1.25
<9
<2
5
451
30
22
110
0.19
26
1.68
167
67.6
101
7.39
<15
6.88
2.2
<9
<2
12
632
43
31
125
1.24
25
3.1
231
69.7
110
5.52
<15
1.91
0.94
<9
<2
8.5
402
27
15
127
0.38
21
1.71
196
70.2
100
5.95
<15
31
1.48
14
<2
16
529
32
22
118
0.16
21
2.96
147
70.4
87.9
5.46
<15
15.1
2.23
<9
<2
27
384
69
65
79
0.13
17
9.22
327
29.6
60.7
5.83
<15
9.28
0.63
<9
<2
11
498
26
24
99
31
307
44.1
39.5
76
131
104
89.4
234
24
91
<0.31
8
1.57
200
10.1
20.7
13
60
45.5
13.9
20
5
104
260
178
43
223
0.8
17
10.3
861
58.1
113
38
2.99
97
48.8
75
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
NB1
Major elementspinffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
wt %; trace and minor elements in ppm; MG: muscovite granite; QD: quartz-diorite; b.d.: below detection limit; ME: mafic enclave; n.d.: not determined; all Fe as Fe2 O3 .
Eu/Eu ¼ EuN/ ðSmN GdNÞ; (Mg# ¼ 100[MgO/(MgO + Fe2 O3T (0.8998))]).
1.16
1.14
28.99
4.68
0.207
1.46
0.86
8.03
0.4
0.69
46.70
15.84
1.17
0.38
1.14
2.47
1.08
0.95
14.69
10.2
0.268
19.15
0.44
12.54
1.39
1.01
24.82
5.31
0.211
5.57
0.54
2.56
117.33
1.70
49.76
2.56
0.054
15.91
0.58
7.98
K2 O/Na2 O
ASI
Mg#
Ti/P
Sr/Ba
Rb/Sr
Eu/Eu*
(La/Yb)cn
0.94
0.96
42.51
2.077
0.261
7.97
0.64
2.37
1.22
0.99
36.00
8.47
0.249
11.64
0.61
34.31
1.46
1.00
40.00
7.41
0.12
14.6
0.5
9.84
1.29
1.05
33.93
3.23
0.227
4.63
0.81
5.26
1.19
0.94
19.85
6.7
0.194
15.58
0.54
23.14
1.91
0.94
8.44
4.64
0.319
4.77
0.49
10.01
1.1
0.91
15.78
7.8
0.27
16.37
0.5
18.34
1.28
0.95
2.54
8.4
0.231
34.4
0.31
14.97
1.35
0.97
27.08
5.73
0.138
12.97
0.53
20.57
1.12
0.95
43.01
10.2
0.18
15.03
0.46
18.95
1.37
0.94
17.01
6.33
0.86
14.7
0.53
13.93
1.21
0.97
37.77
8.76
0.138
14.89
0.49
25.37
1.25
0.95
19.55
10.8
0.218
16.53
0.74
23.08
19.3
3.01
0.17
3.02
0.67
0.56
2.63
0.57
4.55
4.04
39.4
16.8
38.4
6.07
0.93
9.88
1.46
<0.14
4.17
0.48
3.14
1.94
15.3
3.12
Nd
Sm
Eu
Gd
Tb
Tm
Yb
Lu
Hf
Ta
Th
U
14
3.01
0.45
3.87
0.47
<0.14
3.93
0.54
2.11
2.45
9.8
10.2
31.4
4.97
0.54
5.12
0.69
<13
1.97
0.36
6.14
2.34
60.9
14.6
20.8
3.57
0.36
5.06
0.8
<0.22
2.61
0.36
4.92
1.89
52.2
15.6
33.1
5.3
1
5.37
0.86
<0.15
4.5
0.7
313
1.78
14.4
5.75
24.2
3.83
0.35
4.18
0.54
<0.17
2.22
0.34
5.36
1.76
51.6
23
17.6
3.21
0.36
5.22
1
<0.12
2.41
0.35
2.41
1.68
27.8
11.1
38.7
7.24
0.42
5.88
1.13
<0.11
3.05
0.61
4.64
1.5
48.2
77.8
22.3
3.45
0.13
4.61
0.44
<0.15
2.28
0.49
7.54
1.89
47.6
30.6
39.2
4.93
0.54
6.88
1.36
<0.14
2.45
0.59
5.3
2.32
49.7
10.1
30.3
3.75
0.38
6.31
0.74
<0.14
2.41
0.42
4.78
2.52
51
8.52
34.8
6.11
0.45
5.5
0.72
0.7
3.38
0.71
4.81
3.3
43.4
22.5
19.3
2.9
0.32
4.72
0.49
0.52
1.87
0.4
5.92
1.7
52.2
13.3
19.4
3.46
0.33
2.08
0.34
0.29
2.06
0.47
5.33
1.64
47.7
16.7
29.1
7
0.7
8.41
1.44
1
7.83
1.3
2.93
2.01
16.9
13.7
11.7
2.93
1.19
3.24
0.49
0.16
2.76
0.34
2.6
0.35
1.45
0.55
47.3
9.38
1.9
9
1.3
0.7
4.89
0.69
7.34
1.82
19.2
4.11
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
79
trace elements abundances were obtained by instrumental neutron activation analysis at the University of
Vienna. These include Fe, Na, Sc, Cr, Co, Zn, As, Rb,
Sr, Zr, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb,
Lu, Hf, Ta, Th and U. Measurements were done following procedures described by Koeberl (1993); this
reference also gives information on instrumentation,
standards, data reduction, accuracy, and precision. The
results of these analyses are reported in Table 3.
4. Petrography and mineral chemistry
The Singo granite occurs in a number of texturally
and compositionally different subtypes: (a) a pink, very
coarse-grained, and porphyritic granite (PPG); (b) a
pinkish gray, very coarse-grained granite; (c) a pinkish
gray medium-grained granite; (d) a pinkish fine/aplitic to
medium-grained muscovite granite (MG); and (e) a fineto medium-grained biotite granite (BG).
The PPG forms the largest part of the Singo batholith
and the phenocrysts occasionally show magmatic flow
alignment. Other varieties occur in a few places, and
have fewer phenocrysts compared to the PPG. The BG
is richer in biotite, calcic plagioclase, and zircon but
poorer in K-feldspar and quartz relative to other varieties. The BG forms most of the pluton margin, but is
occasionally found within the main body itself. Enclaves
are very common in the BG and are dark gray and fine
grained, ranging from a few mm to cm in size, and from
ovoid or rounded to narrow streaks in shape. Some have
a very sharp contact with the host granite, whereas
others show gradation.
The Singo granite, therefore, shows zonation in texture, color, and mineral composition. Modally, most
samples of the Singo granite belong to monzogranite
and a few others are syenogranites. However, the mineralogy is generally uniform: quartz, plagioclase (An1–11 ),
K-feldspar, biotite, muscovite and opaques. K-feldspars, plagioclase and quartz constitute both the phenocrysts and the groundmass. Minor phases include zircon,
apatite, sphene, monazite, xenotime, opaques and thorite
most of which are inclusions in biotite. Chlorite, epidote
and fluorite are secondary minerals.
Potassium feldspar is anhedral and may enclose
quartz and plagioclase or occupy interstices in between
the two. K-feldspar forms the majority of the phenocrysts up to 6 cm in length and includes microclineperthites, orthoclase, and microcline. The K-feldspars
are usually cloudy or altered to white mica and are in
some cases rimmed by albite and recrystallized quartz.
Inclusions in K-feldspar are altered plagioclase, quartz,
biotite, opaques, muscovite, and zircon. Microclineperthites show a zonation in Ba contents. The microcline-perthites of the BG have higher Na contents than
those of the PPG (Table 1).
80
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
Plagioclase forms rectangular to subhedral plates of 4
cm or less with variable degrees of sericitisation. Plagioclase is commonly zoned with Ca-rich cores and Capoor margins, but oscillatory zoning is manifested in a
few grains. They are mostly oligoclase, which may have
twin-free margins or are not twinned at all.
Quartz grains up to 2 cm occur as anhedral isolated
grains or aggregates and may be recrystallized. Quartz
intergrowths with K-feldspar and plagioclase form
micrographic and/or granophyric texture and myrmekites, respectively. Large phenocrysts almost exclusively
show wavy extinction, whereas intergrowths and some
smaller grains may have uniform extinction.
Biotite occurs as dark brown or greenish yellowish
flakes in the BG and the PPG, respectively. Most biotite
has altered to chlorite, or broken down to sphene, epidote, muscovite, opaques, and quartz. The PPG biotites
are enriched in F, Rb2 O, MgO, MnO, Al2 O3 , and SiO2 ,
but depleted in FeO, Na2 O, BaO, and TiO2 compared to
the BG ones (Table 2). Zircon, apatite, monazite, and
opaques are common inclusions. The BG biotites are
usually kinked.
Muscovite occurs almost exclusively as a secondary
mineral in feldspars. In NB25 (Fig. 2), however, large
plates of primary muscovite without inclusions and with
distinct boundaries are found. In some other areas
muscovite may be deformed.
Magnetite is the most common opaque phase
and occasionally contains ilmenite lamellae. It occurs
within, or close to, biotite or chlorite as euhedral crystals. Prismatic to anhedral ilmenite crystals are also
common. Most ilmenite is intergrown with titanite and
rutile and is rich in Mn.
Apatite is the most common accessory mineral, occurring in various sizes from large anhedral to small
hexagonal crystals as inclusions in biotite, chlorite, and
Fig. 2. Generalised geological map of the Singo granite with some sample locations and the surrounding areas, western central Uganda (modified
from the geological map of Kampala, sheet N.A. 36-14, published by the Geological Survey of Uganda, 1962).
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
magnetite. Similarly, prismatic to rounded zircon crystals also occur and may show oscillatory zoning. Monazites are anhedral to euhedral and are rich in Th.
Epidote (secondary) is associated with magnetite, fluorite, ilmenite and titanite and usually rich in rare earth
elements (REEs). Epidote forms inclusions in biotite
and is most common in NB85. Xenotime occurs as small
anhedral and rarely as large subhedral grains close to
monazite. Thorite is common and forms anhedral grains
in rutile with ilmenite. Some euhedral thorite crystals
were also observed in quartz. Chalcopyrite is rare and
occurs as inclusions in K-feldspar.
81
5. Whole rock geochemistry
The Singo granite has a variable composition, with
SiO2 contents of 67.7–77.2 wt% and K2 O > Na2 O (Table
3). On variation diagrams, the BG samples plot at one
end, whereas the PPG ones plot at the other end of an
evolution line populated by the rest of the samples (Fig.
3). The BG samples represent a relatively old and less
felsic unit and the PPGs are younger and more felsic.
The BGs also have lower total alkalis (Na2 O + K2 O) and
REE contents, and higher TiO2 , Al2 O3 , MgO, CaO, Ba
and Sr than the PPGs. In general, the contents of MgO,
Fig. 3. Harker variation diagrams for the Singo granite. All the diagrams show general fractional crystallization trends in which the granite evolved
from the primitive BG through intermediate granite to the pink porphyritic granite (see (g) and (f)).
82
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
Fig. 3 (continued)
CaO, Fe2 O3T , Al2 O3 , TiO2 , P2 O5 , Ba, Sr, Eu, Zr, and V
decrease with increasing SiO2 contents defining nearly
linear trends (Fig. 3). Element ratios of highly incompatible and immobile elements vary, except the K2 O/
Na2 O ratio, which is relatively constant (Table 3).
Chondrite-normalized REE patterns show that
the BGs are enriched in light REEs (LREEs) relative
to heavy REEs (HREEs), have a weak negative Eu
anomaly (Eu=Eu P 0:8), a flat HREE pattern, and are
less fractionated ððLa=YbÞcn ¼ 5–8Þ. The PPGs are enriched in the LREEs, but show a stronger and variable
negative Eu anomaly (Eu/Eu up to 0.31), with most
samples having steep REE patterns ððLa=YbÞcn ¼
18–34Þ. In general, the Singo granites (both BG
and PPG) show unfractionated HREE patterns, except
sample NB85, which is enriched in the HREEs and Y
relative to the LREEs (Fig. 4). Spider diagrams (chondrite-normalized) also show negative Nb and Sr anomalies (Fig. 5).
The enclave (NB1X) shows higher Al2 O3 , MgO,
Fe2 O3T , TiO2 , CaO, Na2 O, Yb, Zn, Ni, Eu, and lower
SiO2 , K2 O and Ba values than all the granite samples. It
has REE and trace element patterns that are similar to
those of the host BG samples (Figs. 4 and 5). The
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
Fig. 4. Chondrite-normalized rare earth element patterns of the Singo
granite rocks. NB1X: mafic enclave, NB58A: quartz-diorite. Normalisation factors after Taylor and McLennan (1985).
quartz-diorite dyke (NB58A) is characterized by greater
contents of TiO2 , Fe2 O3T and MgO with lower Th, Zr,
Y and REEs compared to all granite samples. It shows
a near-parallel REE pattern to that of the BG, but
with lower abundances and a positive Eu anomaly
(Eu=Eu ¼ 1:14). Also, the quartz-diorite dyke is
slightly fractionated, with ðLa=YbÞcn ¼ 2:47, and has a
high Mg# (46.7).
6. Classification and tectonic setting
Based on the alumina saturation index (ASI) of
Shand (1947), Singo granite is metaluminous to peraluminous, except NB88 (peralkaline) (Table 3). The BG
has S-type tendencies (i.e. two-mica granite, has mo-
83
Fig. 5. Chondrite-normalized plot of incompatible trace elements for
the Singo granite rocks. NB1X: mafic enclave, NB58A: quartz-diorite.
Normalisation factors after Taylor and McLennan (1985).
nazites and normative corundum >1 wt%) although the
ASI < 1.1 is typical of I-type granites (Vetter and Tessensohn, 1987). On the other hand, most PPG samples
have normative diopside, ASI < 1.1, normative corundum <1 wt%, and sphene, which characterize I-type
granites. Discrimination in the Pearce et al. (1984) diagram (Rb ) (Y + Nb)) shows that the Singo granite
(especially the PPGs) have some geochemical characteristics similar to those of recent syn-collision granite
(high-Rb, low-Zr, -Hf, -Sr contents) (Fig. 6a). On an
Hf–Rb–Ta diagram, all the BGs plot at the boundary
between volcanic arc granite field, whereas most PPGs
plot in the syn-collisional granite field area (Fig. 6b).
The Al2 O3 –SiO2 diagram shows that all the PPG
and intermediate samples plot in post orogenic granite
(POG) region, whereas most BGs plot outside this
region (Fig. 6c).
84
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
Fig. 6. Discrimination diagrams for the Singo granite rocks. Syn-COLG: syn-collision granite; VAG: volcanic arc granite; WPG: within-plate
granite; ORG: ocean ridge granite; IAG: island arc granitoids; CAG: continental arc granitoids; CCG: continental collision granitoids; RRG: riftrelated granitoids; CEUG: continental epeirogenic uplift granitoids; POG: post orogenic granite. (a) Rb versus Y + Nb plot, showing that most of the
samples belong to Syn-COLG. Field boundaries after Pearce et al. (1984). (b) Hf–Rb–Ta ternary diagram for rocks of the Singo granite. Field
boundaries after Harris et al. (1986). (c) Al2 O3 versus SiO2 , showing that all the intermediate samples and the pink porphyritic granite plot in the
POG region. Field boundaries after Maniar and Piccoli (1989).
7. Discussion
Secondary minerals, such as epidote, sphene, and
fluorite, sericitisation of feldspars, and chloritisation of
biotite, are a result of late hydrothermal activity. The
scatter in the SiO2 versus K2 O may also be due to hydrothermal alteration, although low grade regional
metamorphism or weathering could also be responsible
(Opiyo-Akech et al., 1999).
The shape of the Singo granite was probably controlled by the structure of the country rocks and is
possibly laccolith-like. This has been a common observation from recent geophysical and structural data
elsewhere (Clarke, 1992; Holness, 1997; Middlemost,
1997). The alignment of the feldspars is not related to
the shape of the pluton and probably reflects the variability in viscosity of the magma, whereas a general lack
of foliation in the BG implies a short time interval between the emplacement of BG and PPG.
The zonation in the Singo granite is consistent with
crystal differentiation, with high temperature minerals
crystallizing first on the walls and concentration of the
lower temperature constituents towards the center. Although marginal contamination and heterogeneity of
the source rock is also possible (Hall, 1987; Barbarin,
1996, 1999), the observed major and trace element plots
(Fig. 3) are in agreement with general trends for fractional crystallization described elsewhere (Hall, 1987;
Opiyo-Akech et al., 1999). Also, variations in Ba contents is probably due to biotite and K-feldspar fractionation (Kebede et al., 1999) (Table 3). The general
decrease of the Zr, P2 O5 , TiO2 , and V contents from BG
to PPG, and the complex behavior of the immobile trace
elements with differentiation, was likely due to crystallization/removal of accessory minerals, such as rutile,
apatite, and zircon from the melt.
The reduction in grain size at the Singo granite margin
could possibly mark chilling. However, the coarsegrained variety reaches the margin in some places, e.g.,
NB4 (Fig. 1 and Table 3).This could represent a ‘‘phase’’
which was emplaced in a preheated country rock (Cox
et al., 1979). Continued pulsing of the magma probably
detached the chilled skin such that the magma came into
contact with preheated country rock and, therefore,
suffered no accelerated cooling. The BG that is enclosed
in the main body may represent small detached fragments of the chilled margin. Also, in the BG, K-feldspars
reveal finer microperthites and higher Na content than
those of the PPG, because the latter cooled more slowly
and had enough time to exsolve most Na as albite.
The general porphyritic texture represents an early
growth of large feldspar crystals during slight cooling of
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
magma followed by finer grains resulting from more
rapid cooling due to emplacement at shallow levels
(Best, 1982). Micrographic and granophyric textures
indicate rapid and simultaneous crystallization of quartz
and K-feldspars from an undercooled liquid also at
shallow depth (Barker, 1983; Clarke, 1992). Finegrained textures possibly resulted from rapid cooling as
the magma was emplaced at shallow depth or cool part
of the pluton. Myrmekite formed by metasomatism,
exsolution (Cox et al., 1979; Pitcher, 1993) or direct
crystallization during deformation (Pitcher, 1993). Deformation is supported by the wavy extinction and recrystallization of quartz, kinked biotite and muscovite,
joints, faults, breccias, and shear zones, which are a
likely response to post-emplacement regional stresses,
imposed on the Singo granite.
High contents of Mg in the PPG biotites (Table 2) are
probably due to early removal of magnetite during fractional crystallization, leading to enrichment of Mg in the
melt with time. Biotites are always in equilibrium with
the melt (Barbarin, 1999; Usta€
omer, 1999), hence later
biotites are Mg-rich. The inconsistent variation of Mg#
in PPG (Table 3) may be due to variability in oxidation
conditions and the amount of magnetite that crystallized
from the melt (Deer et al., 1969; Cox et al., 1979).
The primary muscovites in NB25 are magmatic in
origin. However, the limited development of the muscovite granite, the rare occurrence of pegmatites and aplites in the Singo granite and the textural features
support emplacement at shallow depth from a primary
magma that was water-undersaturated. In contrast, wet
magmas are very viscous, mostly form plutons close to
their source, and are usually deep-seated (Barker, 1983).
The calcic cores in plagioclase are likely to indicate
an earlier phase of crystallization (e.g., Hibbard, 1995).
The rimming of K-feldspars by plagioclase is usually a
result of magma mixing (Silva et al., 2000) and is typical of rapakivi granites. Rapakivi textures also imply
decrease of lithostatic pressure, hence rapid emplacement. Oscillatory zoning indicates variations in the
conditions local to the crystal (Holten et al., 1999). In
microcline-perthites, Ba-rich areas represent narrow
zones that have exsolved on a fine scale (Lee and
Parsons, 1997) as a result of intracrystalline differences
in rates of interdiffusion during exsolution in slowly
cooled felsic rocks. Untwinned feldspars represent a
different generation of development from the twinned
ones (Liren et al., 1985).
Most magnetite grains are euhedral, suggesting their
primary nature (Clarke, 1992). Together with other
early forming minerals, such as apatite and zircon, they
form the majority of inclusions in other minerals.
Euhedral zircons may be considered as magmatic
zircons as opposed to anhedral ones which can be partially melted restitic crystals (Pupin, 1980; Pitcher, 1993).
From the semi-quantitative data (SEM), the Singo
85
granite zircons are relatively poor in Th, U, and Y, implying that they crystallized from water-poor magmas
(Pupin, 1980). The high Rb/Sr (>2.6) and low Sr/Ba
(<0.4) ratios (Table 3) for most of the PPG samples are
consistent with plagioclase fractionation. The relatively
constant K2 O/Na2 O ratios and K2 O > Na2 O indicate
differentiation of different magma pulses from similar
sources (Jung et al., 1999) whereas Ti/P ratio of up to
10.8 suggest a deep-seated crustal source (Opiyo-Akech
et al., 1999). The scarcity of mafic rocks, the peraluminosity of the BG, the negative Nb anomaly and a granitic
gneiss basement support a crustal source for the Singo
granite. The quartz-diorite dyke cross-cuts the granite
and has the most primitive composition in this analytical
set (Table 3). It is younger than the granite.
The negative Eu and Sr anomalies are either the result
of early crystallization of plagioclase from the melt by
fractional crystallization, or retention of these elements
in feldspars at the source during partial melting (Rollinson, 1993). The unfractionated HREE (and Y) patterns suggest that the magmas were produced outside
the garnet stability field whereas the negative Eu and Sr
anomalies could indicate that plagioclase was stable in
the source. All these features are consistent with rather
low pressures (<8 kbar) (Arth, 1979; Barker, 1979;
Mark, 1999 and references therein). The occurrence of
primary muscovite also places pressure limits of the
crystallization of the granitic rocks at 4–2.6 kbar (Green
and Pearson, 1986 and references therein). As these
granites, in particular the PPG, are rich in K and incompatible trace elements, melting of a metaluminous
crustal source is a possible model for their origin.
In NB85, the enrichment of the HREE and Y is attributed to the presence of fluorine. Fluorine complexes
are important for REE transport in hydrothermal fluids, causing enrichments of the HREEs over the LREEs.
The fluorine source may be the late magmatic fluids, or
altered biotite (e.g. Hecht et al., 1999). The variable
REE contents in PPG are due to variable proportions of
minor mineral phases, such as apatite, monazite, epidote, and zircon (e.g. Kebede et al., 1999, and references
therein; Opiyo-Akech et al., 1999).
The complex behavior of immobile trace elements
(high field strength elements, HFSE) may result from
mobilization, especially in magmatic and hydrothermal
environments with strong complexing agents such as
fluorine and sulphides (Hecht et al., 1999), or heterogeneity of the source rock (Rollinson, 1993). Such
variations could also result from the influence of accessory minerals that can settle out if the magma viscosity is low (Opiyo-Akech et al., 1999). However, the
broad correlation between Th and La suggests that the
abundance of these elements was controlled by monazite
and (+ thorite?) fractionation.
The enclave NB1X shows the same geochemical
characteristics as the BG samples (Figs. 4 and 5). This
86
B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87
enclave, more mafic than the rest of the granite, witnesses a more primitive magma reinjected within the BG
magma at some stage of its differentiation.
Peraluminosity of BG suggests a dominantly crustal
origin whereas sample NB88 with mild peralkalinity
((Na + K)/Al ¼ 1.01), could be the result of local enrichment in Na and/or K.
In general, although the Singo batholith consists of
granite types with different geochemical characteristics,
the BG and the PPG seem to both belong to I-type
granite (ASI < 1.1). The variation in geochemical characteristics suggests the mixing of mafic and felsic magmas during the genesis of the Singo batholith and this is
in line with its association with mafic rocks (quartzdiorite and mafic enclaves) and heterogeneity of the
country rocks.
8. Summary and conclusions
We have studied about one hundred samples from the
Singo granite and associated rocks. The research involved fieldwork, petrographic studies, mineral chemistry and whole rock geochemistry. The results enabled us
to make some conclusions regarding the origin of the
Singo granite.
• The Singo granite petrographic and geochemical
characteristics suggest that fractional crystallization
was the main differentiation process during its formation.
• The felsic magmas probably formed from a heterogeneous crustal source which was water-undersaturated. Geochemical and field data (e.g. mafic
enclaves) suggest that this melting process was linked
to the emplacement of the mafic magmas in the crust.
• Unfractionated and high HREE contents and low Sr
and Eu contents suggest that melting and differentiation took place under low-P conditions (plagioclase
stable without garnet). Textural and field features
also indicate a shallow pluton in which the magma
was emplaced in batches/several magma pulses by dyking as shown by the small contact aureole as compared to the batholithic size of the Singo granite.
• Late- and post-magmatic stages (late-magmatic pneumatolic phase) were dominated by hydrothermal activity, which led to leaching of quartz, and formation
and concentration of minerals, such as fluorite, wolframite, beryl, and gold. Hydrothermal activity also
led to the formation of quartz-sericite bodies and secondary minerals such as hematite, chlorite, and sericite.
• There is a coexistence of two main types of granites in
the Singo massif; the subordinate BG of mildly felsic
peraluminous composition and the dominant PPG of
highly felsic and metaluminous composition, with lo-
cally rapakivi texture. Both BG and PPG belong to Itype granites.
• The continuous geochemical trends between these
two granites argue for a synplutonic emplacement,
with a clear crustal signature for the BG.
• The Singo granite shows similarities with recent
POG.
• The overall time for the emplacement of the Singo
batholith was rather short.
Acknowledgements
Field work was funded by the Austrian Academic
€ AD). The authors are grateful to
Exchange Service (O
W.U. Reimold and S. Farrell (Johannesburg) for help
with the XRF analyses. Laboratory work was supported
by the Austrian FWF, grant Y58––GEO (to C. Koeberl). We would also like to thank F. Brandst€
atter
(Natural History Museum, Vienna) for help with SEM
analyses, and T. Ntaflos (Univ. Vienna) for help with
electron microprobe analyses.
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