petrographyandgeochemistryofthesingogranite, uganda,andimplicationsforitsorigin · 2003-05-19 ·...

Petrography and geochemistry of the Singo granite,Uganda, and implications for its origin

Betty Nagudi a,1, Christian Koeberl a,*, Gero Kurat b

a Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austriab 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 medium-

grained 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 Al2O3, MgO,

CaO, Fe2O3, 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 em-

placed 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 similarto 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 gran-

ite belongs, remains limited. Previously, granites with

marked variations in mineral composition and a mig-

matitic 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

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 theolder 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 en-

vironment.

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 under-

stood. Microtextural relationships studied by King

(1947) led him to suggest that the Singo granite formed

by a series of replacements (metasomatism) of the coun-

try rocks. The replacement stages varied from country

rock through contact rocks to the porphyritic granite

Journal of African Earth Sciences 36 (2003) 73–87

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* Corresponding author. Tel.: +43-1-40103-2360; fax: +43-1-403-

9030.

E-mail address: [email protected] (C. Koeberl).1 Present address: Department of Geology, Makerere University,

P.O. Box 7062, Kampala, Uganda.

0899-5362/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0899-5362(03)00014-9

mail to: [email protected]

and were marked by varying degrees of quartz replace-ment 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 na-

ture of bedding planes of the country rocks to the

granite contact in some places, suggests a contempora-neous 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 hydro-

thermal fluids, and, therefore, postdate the pluton

(Johnson and Williams, 1961).

In this paper, we present petrographic, mineralogicaland geochemical data for the Singo granite and associ-

ated 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 classifi-

cation 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 parallelto 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 sand-

stones, slates, phyllites, mica schists, basal quartzites,

amphibolites, and epidosites. The quartzites have re-

sisted the emplacement in some localities. The granite

contact may contain different types of enclaves, butbreccias 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 mani-

fested 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). Allu-

vial 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 emplace-

ment 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)).

74 B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87

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

Sample 36k1m 36k1c 36k2m 36k2c 56-km 56kc 46k3m 46k3c 88k16c 88k16m 36pm 36pc 36p2m 36p2c 56pm 56pc 88p15m 88p15c

SiO2 64.0 63.5 64.1 63.9 64.8 64.1 64.3 63.7 63.8 63.3 66.1 65.7 68.0 63.0 67.4 68.1 66.2 65.3

TiO2 0.04 b.d. 0.01 b.d. b.d. b.d. b.d. 0.01 0.01 b.d. 0.03 0.01 0.01 b.d. 0.03 0.02 0.01 b.d.

Al2O3 18.8 19.1 19.1 19.2 19.0 19.0 18.8 18.9 18.7 18.6 22.4 22.1 21.3 25.0 20.3 20.8 21.2 21.5

FeO 0.01 b.d. b.d. 0.05 0.02 b.d. 0.01 0.08 0.04 0.02 0.02 0.02 0.04 0.09 0.02 0.06 0.11 0.04

MnO b.d. 0.02 b.d. b.d. b.d. b.d. b.d. b.d. 0.01 b.d. 0.02 b.d. 0.02 b.d. b.d. 0.01 0.01 0.02

MgO 0.02 b.d. 0.02 0.02 0.01 b.d. 0.02 0.01 b.d. b.d. b.d. b.d. b.d. 0.02 b.d. b.d. 0.01 0.01

BaO 0.06 0.06 0.07 b.d. 0.02 b.d. b.d. 0.04 b.d. b.d. b.d. b.d. b.d. 0.05 b.d. b.d. 0.01 b.d.

CaO 0.04 0.04 0.03 0.01 b.d. 0.02 0.07 0.01 b.d. 0.02 1.77 2.1 0.57 2.22 0.4 0.26 1.57 1.88

Na2O 1.2 0.43 1.29 0.9 0.33 0.28 0.25 0.27 0.23 0.22 10.0 9.9 10.3 8.9 11.3 10.8 10.3 10.2

K2O 14.9 15.8 14.8 15.1 15.7 15.9 15.5 15.4 16.2 16.3 0.2 0.1 0.2 1.6 0.2 0.1 0.3 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

Rb2O b.d. b.d. b.d. b.d. b.d. b.d. 0.05 b.d. b.d. b.d. 0.03 0.01 0.02 0 0.04 0.02 0.06 0.03

Si 5.94 5.91 5.92 5.91 5.95 5.93 5.96 5.93 5.94 5.95 5.72 5.73 5.85 5.45 5.91 5.88 5.81 5.76

Al 2.06 2.09 2.08 2.09 2.05 2.07 2.05 2.08 2.06 2.05 2.29 2.27 2.15 2.55 2.09 2.12 2.19 2.24

Ti 0.003 n.c. 0.001 n.c. n.c. n.c. n.c. 0.001 0.001 n.c. 0.002 0.001 0.001 n.c. 0.002 0.001 0.001 n.c.

Fe2 0.001 n.c. n.c. 0.004 0.002 n.c. 0.001 0.006 0.003 0.002 0.001 0.001 0.003 0.007 0.001 0.004 0.008 0.003

Mn n.c. 0.002 n.c. n.c. n.c. n.c. n.c. n.c. 0.001 n.c. 0.001 n.c. 0.001 n.c. n.c. 0.001 0.001 0.001

Mg 0.003 n.c. 0.003 0.003 0.001 n.c. 0.003 0.001 n.c. n.c. n.c. n.c. n.c. 0.003 n.c. n.c. 0.001 0.001

Ba 0.002 0.002 0.003 n.c. 0.001 n.c. n.c. 0.001 n.c. n.c. n.c. n.c. n.c. 0.002 n.c. n.c. n.c. n.c.

Ca 0.004 0.004 0.003 0.001 n.c. 0.002 0.007 0.001 n.c. 0.002 0.164 0.196 0.052 0.206 0.038 0.024 0.148 0.178

Na 0.22 0.08 0.23 0.16 0.06 0.05 0.05 0.05 0.04 0.04 1.67 1.68 1.71 1.49 1.92 1.81 1.76 1.75

K 1.77 1.87 1.75 1.79 1.84 1.87 1.84 1.83 1.93 1.95 0.03 0.01 0.02 0.17 0.02 0.02 0.04 0.03

Ca-

tions

10.0 9.96 9.99 9.96 9.91 9.92 9.89 9.89 9.98 10.0 9.86 9.88 9.78 9.88 9.98 9.86 9.95 9.96

X 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

Z 1.99 1.96 1.99 1.95 1.91 1.92 1.89 1.89 1.98 2 1.86 1.88 1.78 1.88 1.98 1.86 1.95 1.96

Ab 10.9 4 11.7 8.3 3.1 2.6 2.4 2.6 2.1 2 89.8 89 96 79.7 97.1 97.9 90.4 89.5

An 0.2 0.2 0.2 0.1 n.c. 0.1 0.4 0.1 n.c. 0.1 8.8 10.4 2.9 11.1 1.9 1.3 7.6 9.1

Or 88.9 95.8 88.2 91.7 96.9 97.3 97.2 97.3 97.9 97.9 1.3 0.5 1.1 9.3 1 0.8 2 1.3

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).

B.Nagudietal./JournalofAfrica

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75

Paleoproterozoic event, together with the Kampalaorogenic 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 Paleopro-

terozoic (1847� 6 Ma, Pinna et al., 2001; 1615� 19 Ma,

Nagudi et al., 2001).

This granite has similar characteristics to the Mu-

bende granite and both are believed to have beenemplaced at the same time, during the Palaeoprotero-

zoic (MacDonald, 1966; Johnson and Williams, 1961).

However, the Mubende granite has contradictory em-

placement 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 thesesamples were prepared and studied by optical micro-

scopy and 10 others were selected for electron micro-

scopy. 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 micro-

scope (SEM), equipped with a Kevex energy disper-

sive spectrometer, at the Natural History Museum inVienna, was also used for qualitative and semi-quanti-

tative 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 fluores-

cence (XRF), using a Philips PW 1400 XRF spectro-

meter at the University of the Witwatersrand in SouthAfrica. The elements include SiO2, TiO2, Al2O3, Fe2O3T ,

MnO, MgO, CaO, Na2O, K2O, P2O5, 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 35.8 35.7 33.0 36.0 39.2 40.0 40.1 39.4 39.0

TiO2 3.4 3.9 3.6 3.7 2.0 1.5 1.4 2.0 2.1

Al2O3 17.0 16.6 17.2 17.0 18.0 14.3 14.3 14.8 15.0

FeO 20.1 19.8 22.0 19.8 16.9 14.6 14.6 15.0 15.4

MnO 0.4 0.5 0.5 0.4 0.9 1.0 1.0 0.7 0.7

MgO 8.6 8.5 9.3 8.6 8.3 12.9 12.8 11.6 11.4

BaO b.d. 0.23 b.d. 0.32 0.01 b.d. b.d. b.d. 0.01

CaO 0.04 0.02 1.40 b.d. 0.02 b.d. b.d. b.d. b.d.

Na2O 0.16 0.17 0.08 0.16 0.22 0.18 0.18 0.13 0.13

K2O 9.4 9.4 7.0 9.5 9.4 9.9 9.8 9.7 9.7

F b.d. b.d. b.d. b.d. 0.07 3.48 3.44 1.76 1.45

H2O 1.86 1.86 1.83 1.87 1.88 0.26 0.28 1.06 1.2

Total 96.71 96.66 95.82 97.33 96.86 98.057 97.86 96.13 96.03

Rb2O b.d. b.d. b.d. b.d. 0.25 0.10 0.12 0.30 0.07

Si 5.76 5.76 5.40 5.76 6.14 6.30 6.31 6.26 6.22

AlIV 2.24 2.24 2.60 2.24 1.86 1.71 1.69 1.74 1.78

AlVI 0.98 0.92 0.72 0.96 1.46 0.95 0.96 1.03 1.02

Ti 0.41 0.47 0.44 0.45 0.23 0.18 0.17 0.24 0.25

Fe2 2.71 2.67 3.01 2.66 2.22 1.92 1.90 2.0 2.06

Mn 0.05 0.07 0.07 0.06 0.12 0.13 0.13 0.09 0.09

Mg 2.07 2.05 2.26 2.05 1.93 3.01 3.01 2.74 2.70

Ba n.c. 0.02 n.c. 0.02 n.c. n.c. n.c. n.c. n.c.

Ca 0.006 0.004 0.245 n.c. 0.004 n.c. n.c. n.c. n.c.

Na 0.05 0.05 0.03 0.05 0.07 0.05 0.05 0.04 0.03

K 1.9 1.9 1.5 1.9 1.9 2.0 2.0 2.0 2.0

Cations 16.2 16.2 16.2 16.2 15.9 16.2 16.2 16.1 16.1

CF n.c. n.c. n.c. n.c. 0.07 3.46 3.42 1.76 1.46

OH 2 2 2 2 1.964 0.27 0.29 1.119 1.271

O 24 24 24 24 24 24 24 24 24

Mg/Fe+Mg 0.43 0.43 0.43 0.44 0.47 0.61 0.61 0.58 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

NB1 NB9 NB15 NB36 NB37 NB38 NB43 NB52b NB25 NB8 NB13 NB14 NB23 NB41 NB56 NB67 NB79 NB85

SiO2 68.15 69.99 67.66 69.43 n.d. 70.70 69.09 n.d 76.75 72.89 72.99 72.26 73.49 71.83 76.21 71.23 70.74 72.82

TiO2 0.77 0.64 0.75 0.68 n.d. 0.41 0.68 n.d 0.13 0.43 0.37 0.43 0.43 0.40 0.20 0.45 0.51 0.47

Al2O3 14.27 14.04 14.01 13.84 n.d. 14.28 14.07 n.d 12.78 13.23 13.63 13.96 13.12 13.88 12.28 14.00 13.72 12.49

Fe2O3 5.43 4.60 5.23 4.70 n.d. 3.04 4.70 n.d. 0.63 2.35 1.85 2.16 2.64 2.19 1.03 3.01 2.85 2.49

MnO 0.14 0.10 0.12 0.11 n.d. 0.12 0.13 n.d. 0.05 0.07 0.06 0.07 0.08 0.08 0.06 0.10 0.07 0.07

MgO 1.12 0.95 1.07 1.00 n.d. 0.56 0.95 n.d. 0.00 0.71 0.60 0.60 0.79 0.62 0.27 0.70 0.87 0.71

CaO 1.80 1.85 1.87 1.78 n.d. 1.26 1.72 n.d. 0.48 1.24 0.87 0.71 0.84 1.24 0.55 1.26 1.88 1.21

Na2O 3.19 3.17 3.20 3.23 n.d. 3.87 3.47 n.d. 3.88 3.49 3.62 3.47 2.98 3.70 3.71 3.32 3.54 3.63

K2O 4.32 4.30 4.33 4.32 n.d. 4.47 4.36 n.d. 5.00 4.77 5.34 5.53 5.02 5.27 5.28 5.24 4.73 4.67

P2O5 0.23 0.19 0.22 0.21 n.d. 0.20 0.20 n.d. 0.13 0.17 0.16 0.16 0.12 0.17 0.04 0.12 0.16 0.12

LOI 1.05 0.80 0.98 1.11 n.d. 1.24 0.93 n.d. 0.47 1.03 0.98 1.08 0.95 0.92 0.78 0.79 0.96 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 12.1 11.9 13.1 10.8 7.21 9.85 12.7 25.6 2.53 8.57 7.55 6.74 7.01 8.33 5.01 8.78 7.84 8.29

V 56 50 52 49 n.d. 31 45 n.d. <15 33 31 37 26 33 <15 38 42 37

Cr 34.1 35.9 62 27.3 18.9 34.5 38.4 15.5 20.3 22.2 27.5 20.5 33.7 24.8 23.6 51.5 24.2 30.3

Co 12.6 10.2 12.6 9.44 0.77 9.05 10.9 34.8 1.47 5.78 2.57 3.59 5.05 7.72 1.28 7.4 6.75 6.41

Ni 18 12 24 16 b.d. 17 15 b.d. <9 14 15 11 10 15 9 12 12 20

Cu 9 <2 6 6 b.d. 9 8 b.d. <2 <2 <2 5 <2 <2 <2 18 <2 <2

Zn 104 75 84 61 4.9 55.8 72.8 117 13 19.8 22 20 27 30 15 40 39 66

Rb 216 227 216 199 145 294 239 80.8 407 343 430 378 298 345 530 259 293 245

Sr 167 151 155 154 b.d. 108 138 b.d. 36 132 115 122 101 137 46 142 173 136

Y 47 37 38 40 b.d. 35 41 b.d. 26 36 35 74 42 59 30 28 29 576

Zr 259 206 246 227 b.d. 135 203 b.d. 27 156 135 153 150 147 133 144 180 206

Sb 0.62 0.39 0.08 1.13 3.05 0.1 0.13 0.75 0.79 <0.53 0.13 0.26 0.2 0.4 0.27 0.31 0.27 0.13

Nb 19 17 18 17 b.d. 16 19 b.d. 11 19 17 19 19 19 24 18 20 20

Cs 8.29 11.3 7.07 6.46 1.36 11 13.5 8.33 18.8 7.26 8.35 7.89 8.46 9.21 4.16 11.5 4.82 3.67

Ba 943 877 933 925 b.d. 571 794 b.d. 162 529 512 708 619 659 220 817 744 569

La 54.5 52.2 59.5 48.8 29.4 31.4 50 34.2 7.13 57.6 39.1 76.7 52.2 65 80 40.5 74.1 83.8

Ce 113 104 121 98.6 44.4 65.6 103 68 18.4 106 73.8 155 105 110 117 78.9 132 149

Nd 49.5 45.9 52.2 42.5 20.9 31.7 46.9 37 9.13 43 24.7 61.1 43 50.3 37.3 36 51.9 75.9

Sm 9.26 8.66 9.7 7.76 3.01 5.52 8.39 6.92 2.24 6.85 5.41 12.2 8.3 9.76 7.04 5.82 8.12 22.9

Eu 1.83 1.78 1.84 1.8 0.56 1.17 1.64 1.95 0.28 1.17 0.6 2.02 1.29 1.93 0.51 1.19 1.41 5.84

Gd 7.39 6.86 7.69 8.06 4.7 <3.8 6.06 7.05 2.77 5.66 2.89 12 6.87 10.13 4.81 5.07 5.59 75.7

Tb 1.46 1.18 1.33 1.13 0.33 0.93 1.32 1.03 0.44 0.75 0.66 1.54 1.12 1.74 0.75 0.92 0.92 12

Tm 0.49 0.73 <0.18 0.69 0.4 0.43 <0.14 0.64 0.37 <0.17 0.85 0.76 0.93 <0.16 0.4 <0.11 <0.14 6.85

Yb 4.49 4.39 4.19 3.88 1.02 3.34 4.71 4.22 2.63 3.4 2.9 6.63 4.26 6.89 2.93 3.51 3.17 54.7

Lu 0.7 0.59 0.56 0.61 0.17 0.49 0.67 0.61 0.35 0.61 0.64 0.98 0.64 1 0.49 0.42 0.47 8.61

Hf 7.88 7.01 7.71 6.12 4.42 4.25 6.48 5.38 1.3 5.21 4.24 4.15 5.97 5.38 5.54 4.72 6.15 5.42

Ta 1.6 1.56 1.37 1.4 1.94 1.72 1.75 0.78 1.4 2.09 1.71 1.72 1.82 2.05 1.86 1.34 2.21 1.93

Th 18.8 19.8 20.3 16.3 19.8 10.8 17.7 8.64 6.24 25.8 20 22.5 29.1 26.9 50.8 17.4 36.3 40.9

U 4.61 4.48 4.74 3.53 1.19 4.34 3.17 2.46 10.3 7.8 6.83 22 12.4 6.77 32.1 5.12 11.8 11.4


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Table 3 (continued)

Sample Biotite granite MG Intermediate granite

NB1 NB9 NB15 NB36 NB37 NB38 NB43 NB52b NB25 NB8 NB13 NB14 NB23 NB41 NB56 NB67 NB79 NB85

K2O/Na2O 1.35 1.36 1.35 1.34 n.d. 1.16 1.26 n.d. 1.29 1.37 1.48 1.59 1.69 1.42 1.42 1.58 1.34 1.29

ASI 1.08 1.06 1.05 1.05 n.d. 1.06 1.04 n.d. 1.01 1.01 1.02 1.08 1.11 0.99 0.96 1.04 0.96 0.94

Mg# 29.01 29.03 28.84 29.65 n.d. 26.73 28.59 n.d. 0.00 37.44 39.12 35.49 37.22 35.93 34.18 31.54 37.68 36.10

Ti/P 4.62 4.61 4.68 4.42 n.d. 2.83 4.68 n.d. 1.36 3.49 3.17 3.69 4.96 3.24 7.06 5.19 4.37 5.42

Sr/Ba 0.177 0.72 0.166 0.17 n.d. 0.189 0.174 n.d. 0.22 0.25 0.225 0.172 0.163 0.208 0.209 0.174 0.233 0.239

Rb/Sr 1.29 1.5 1.39 1.29 n.d. 2.72 1.73 n.d. 11.3 2.6 3.74 3.09 2.95 2.5 11.52 1.82 1.69 1.8

Eu/Eu* 0.93 0.96 0.92 0.89 0.65 n.d. 0.98 0.99 0.6 0.85 0.86 0.77 0.81 0.82 0.61 0.91 0.94 0.52

(La/Yb)cn 8.2 8.04 9.59 8.5 19.49 6.35 7.17 5.48 1.83 11.45 9.11 4.08 8.28 6.38 16.82 7.8 15.8 1.04

Pink porphyritic granite QD ME

NB76-

A

NB76B NB4 NB6A NB45-

A

NB57 NB84 NB88 NB87 NB61 NB86 NB46 NB48 NB89 NB74B NB45B NB58-

A

NB1X

SiO2 75.04 76.25 76.51 76.37 74.10 75.88 76.50 76.36 76.26 74.84 77.23 75.93 76.22 75.53 76.99 77.07 52.06 65.80

TiO2 0.26 0.18 0.24 0.21 0.28 0.19 0.17 0.13 0.14 0.21 0.17 0.19 0.19 0.18 0.23 0.17 1.27 0.75

Al2O3 13.53 12.39 12.49 12.11 13.51 12.30 12.39 12.17 12.67 12.74 12.33 12.51 12.37 12.70 12.26 12.34 15.06 15.79

Fe2O3 1.26 0.75 0.81 1.04 1.35 1.04 0.43 0.74 0.76 1.28 0.42 0.87 0.62 1.06 1.08 0.69 12.12 5.87

MnO 0.04 0.04 0.02 0.04 0.06 0.04 0.04 0.03 0.04 0.06 0.04 0.05 0.05 0.06 0.06 0.04 0.18 0.13

MgO 0.63 0.28 0.23 0.35 0.35 0.13 0.02 0.07 0.01 0.24 0.16 0.09 0.19 0.13 0.18 0.06 5.36 1.21

CaO 0.13 0.32 0.28 0.14 0.69 0.63 0.48 0.59 0.50 0.51 0.59 0.62 0.36 0.54 0.54 0.47 8.97 2.07

Na2O 0.06 4.64 4.07 3.67 3.81 4.05 3.33 4.35 4.09 3.93 4.16 3.90 4.11 4.15 3.53 4.31 2.71 3.49

K2O 7.04 4.37 4.95 5.35 4.93 4.83 6.37 4.78 5.22 5.32 4.65 5.33 4.99 5.20 4.89 4.66 1.09 4.03

P2O5 0.14 0.12 0.04 0.04 0.12 0.04 0.05 0.02 0.02 0.05 0.02 0.04 0.03 0.02 0.06 0.02 0.11 0.22

LOI 1.97 0.96 0.75 0.95 0.92 0.83 0.51 0.79 0.72 0.94 0.84 0.75 0.81 0.89 0.59 0.67 1.34 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 6.87 5.4 6.77 5.41 6.79 6.86 2.62 5.08 5.97 5.19 5.65 7.39 5.52 5.95 5.46 5.83 31 13

V 32 15 <15 15 27 <15 <15 <15 <15 <15 <15 <15 <15 <15 <15 <15 307 60

Cr 8.9 16.3 16.4 31 30.1 20 12.7 13.9 5.7 16.1 15.1 6.88 1.91 31 15.1 9.28 44.1 45.5

Co 2.07 1.26 1.36 1.6 3.76 0.73 1.09 0.79 0.87 1.09 1.25 2.2 0.94 1.48 2.23 0.63 39.5 13.9

Ni <9 9 <9 12 13 <9 <9 <9 <9 <9 <9 <9 <9 14 <9 <9 76 20

Cu <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 131 5

Zn 8.53 19.7 17 13 13 8.5 27 12 14.1 14 5 12 8.5 16 27 11 104 104

Rb 382 470 617 424 444 514 291 442 516 467 451 632 402 529 384 498 89.4 260

Sr 24 59 53 29 96 33 61 27 15 36 30 43 27 32 69 26 234 178

Y 35 33 24 26 50 21 25 34 23 27 22 31 15 22 65 24 24 43

Zr 102 63 116 122 98 120 62 106 140 129 110 125 127 118 79 99 91 223

Sb 0.39 0.41 0.16 0.24 0.33 <0.5 0.18 <0.52 <0.54 0.37 0.19 1.24 0.38 0.16 0.13 <0.31 0.8

Nb 15 15 26 23 16 24 15 19 21 27 26 25 21 21 17 38 8 17

Cs 7.71 9.9 2.79 3.04 10 3.6 2.84 2.05 2.75 4.25 1.68 3.1 1.71 2.96 9.22 2.99 1.57 10.3

Ba 425 226 213 241 423 170 191 100 65 260 167 231 196 147 327 97 200 861

La 49.2 13.8 100 38 35 76 35.7 82.8 50.5 74.6 67.6 69.7 70.2 70.4 29.6 48.8 10.1 58.1

Ce 89.5 33.6 134 77.2 69 102 51.9 108 69 117 101 110 100 87.9 60.7 75 20.7 113

78


nEarth

Scien

ces36(2003)73–87

trace elements abundances were obtained by instru-mental 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 fol-

lowing 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 fine-

to medium-grained biotite granite (BG).

The PPG forms the largest part of the Singo batholithand 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 varie-

ties. 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 finegrained, 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 tex-

ture, color, and mineral composition. Modally, most

samples of the Singo granite belong to monzogranite

and a few others are syenogranites. However, the mine-ralogy is generally uniform: quartz, plagioclase (An1–11),

K-feldspar, biotite, muscovite and opaques. K-feld-

spars, plagioclase and quartz constitute both the pheno-

crysts 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 enclosequartz and plagioclase or occupy interstices in between

the two. K-feldspar forms the majority of the pheno-

crysts up to 6 cm in length and includes microcline-

perthites, 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. Microcline-perthites show a zonation in Ba contents. The micro-

cline-perthites of the BG have higher Na contents than

those of the PPG (Table 1).

Nd

38.4

14

31.4

20.8

33.1

24.2

17.6

38.7

22.3

39.2

30.3

34.8

19.3

19.4

29.1

19.3

11.7

47.3

Sm

6.07

3.01

4.97

3.57

5.3

3.83

3.21

7.24

3.45

4.93

3.75

6.11

2.9

3.46

73.01

2.93

9.38

Eu

0.93

0.45

0.54

0.36

10.35

0.36

0.42

0.13

0.54

0.38

0.45

0.32

0.33

0.7

0.17

1.19

1.9

Gd

9.88

3.87

5.12

5.06

5.37

4.18

5.22

5.88

4.61

6.88

6.31

5.5

4.72

2.08

8.41

3.02

3.24

9

Tb

1.46

0.47

0.69

0.8

0.86

0.54

11.13

0.44

1.36

0.74

0.72

0.49

0.34

1.44

0.67

0.49

1.3

Tm

<0.14

<0.14

<13

<0.22

<0.15

<0.17

<0.12

<0.11

<0.15

<0.14

<0.14

0.7

0.52

0.29

10.56

0.16

0.7

Yb

4.17

3.93

1.97

2.61

4.5

2.22

2.41

3.05

2.28

2.45

2.41

3.38

1.87

2.06

7.83

2.63

2.76

4.89

Lu

0.48

0.54

0.36

0.36

0.7

0.34

0.35

0.61

0.49

0.59

0.42

0.71

0.4

0.47

1.3

0.57

0.34

0.69

Hf

3.14

2.11

6.14

4.92

313

5.36

2.41

4.64

7.54

5.3

4.78

4.81

5.92

5.33

2.93

4.55

2.6

7.34

Ta

1.94

2.45

2.34

1.89

1.78

1.76

1.68

1.5

1.89

2.32

2.52

3.3

1.7

1.64

2.01

4.04

0.35

1.82

Th

15.3

9.8

60.9

52.2

14.4

51.6

27.8

48.2

47.6

49.7

51

43.4

52.2

47.7

16.9

39.4

1.45

19.2

U3.12

10.2

14.6

15.6

5.75

23

11.1

77.8

30.6

10.1

8.52

22.5

13.3

16.7

13.7

16.8

0.55

4.11

K2O/N

a2O

117.33

0.94

1.22

1.46

1.29

1.19

1.91

1.1

1.28

1.35

1.12

1.37

1.21

1.25

1.39

1.08

0.4

1.16

ASI

1.70

0.96

0.99

1.00

1.05

0.94

0.94

0.91

0.95

0.97

0.95

0.94

0.97

0.95

1.01

0.95

0.69

1.14

Mg#

49.76

42.51

36.00

40.00

33.93

19.85

8.44

15.78

2.54

27.08

43.01

17.01

37.77

19.55

24.82

14.69

46.70

28.99

Ti/P

2.56

2.077

8.47

7.41

3.23

6.7

4.64

7.8

8.4

5.73

10.2

6.33

8.76

10.8

5.31

10.2

15.84

4.68

Sr/Ba

0.054

0.261

0.249

0.12

0.227

0.194

0.319

0.27

0.231

0.138

0.18

0.86

0.138

0.218

0.211

0.268

1.17

0.207

Rb/Sr

15.91

7.97

11.64

14.6

4.63

15.58

4.77

16.37

34.4

12.97

15.03

14.7

14.89

16.53

5.57

19.15

0.38

1.46

Eu/Eu*

0.58

0.64

0.61

0.5

0.81

0.54

0.49

0.5

0.31

0.53

0.46

0.53

0.49

0.74

0.54

0.44

1.14

0.86

(La/Y

b)cn

7.98

2.37

34.31

9.84

5.26

23.14

10.01

18.34

14.97

20.57

18.95

13.93

25.37

23.08

2.56

12.54

2.47

8.03

Majorelem

ents

inwt%;trace

andminorelem

ents

inppm;MG:muscovitegranite;

QD:quartz-diorite;b.d.:below

detectionlimit;ME:maficenclave;

n.d.:notdetermined;allFeasFe 2O

3.

Eu/Eu�¼EuN/

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðSmN

�GdNÞ

p;(M

g#¼100[M

gO/(MgO+Fe 2O

3T(0.8998))]).

B. Nagudi et al. / Journal of African Earth Sciences 36 (2003) 73–87 79

Plagioclase forms rectangular to subhedral plates of 4cm or less with variable degrees of sericitisation. Pla-

gioclase is commonly zoned with Ca-rich cores and Ca-

poor 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 formmicrographic and/or granophyric texture and myrme-

kites, 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, epi-

dote, muscovite, opaques, and quartz. The PPG biotitesare enriched in F, Rb2O, MgO, MnO, Al2O3, and SiO2,

but depleted in FeO, Na2O, BaO, and TiO2 compared tothe 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 crys-

tals. 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, oc-

curring in various sizes from large anhedral to smallhexagonal 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).


magnetite. Similarly, prismatic to rounded zircon crys-tals also occur and may show oscillatory zoning. Mo-

nazites are anhedral to euhedral and are rich in Th.

Epidote (secondary) is associated with magnetite, fluo-

rite, 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 grainsin rutile with ilmenite. Some euhedral thorite crystals

were also observed in quartz. Chalcopyrite is rare and

occurs as inclusions in K-feldspar.

5. Whole rock geochemistry

The Singo granite has a variable composition, with

SiO2 contents of 67.7–77.2 wt% and K2O>Na2O (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 (Na2O+K2O) and

REE contents, and higher TiO2, Al2O3, 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)).


CaO, Fe2O3T , Al2O3, TiO2, P2O5, Ba, Sr, Eu, Zr, and V

decrease with increasing SiO2 contents defining nearly

linear trends (Fig. 3). Element ratios of highly incom-

patible and immobile elements vary, except the K2O/

Na2O 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 Euanomaly (Eu=Eu� P 0:8), a flat HREE pattern, and are

less fractionated ððLa=YbÞcn ¼ 5–8Þ. The PPGs are en-

riched 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 ¼18–34Þ. 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 (chon-

drite-normalized) also show negative Nb and Sr ano-

malies (Fig. 5).

The enclave (NB1X) shows higher Al2O3, MgO,Fe2O3T , TiO2, CaO, Na2O, 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

Fig. 3 (continued)


quartz-diorite dyke (NB58A) is characterized by greater

contents of TiO2, Fe2O3T and MgO with lower Th, Zr,

Y and REEs compared to all granite samples. It showsa 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) ofShand (1947), Singo granite is metaluminous to pera-

luminous, except NB88 (peralkaline) (Table 3). The BG

has S-type tendencies (i.e. two-mica granite, has mo-

nazites and normative corundum >1 wt%) although the

ASI< 1.1 is typical of I-type granites (Vetter and Tes-

sensohn, 1987). On the other hand, most PPG samples

have normative diopside, ASI < 1.1, normative corun-

dum <1 wt%, and sphene, which characterize I-type

granites. Discrimination in the Pearce et al. (1984) dia-

gram (Rb) (Y+Nb)) shows that the Singo granite

(especially the PPGs) have some geochemical charac-teristics 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 Al2O3–SiO2 diagram shows that all the PPG

and intermediate samples plot in post orogenic granite

(POG) region, whereas most BGs plot outside thisregion (Fig. 6c).

Fig. 4. Chondrite-normalized rare earth element patterns of the Singo

granite rocks. NB1X: mafic enclave, NB58A: quartz-diorite. Normali-

sation factors after Taylor and McLennan (1985).

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).


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 K2O may also be due to hy-

drothermal 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 con-

trolled by the structure of the country rocks and is

possibly laccolith-like. This has been a common obser-

vation 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 vari-ability in viscosity of the magma, whereas a general lack

of foliation in the BG implies a short time interval be-

tween 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. Al-

though marginal contamination and heterogeneity ofthe 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 frac-

tional crystallization described elsewhere (Hall, 1987;

Opiyo-Akech et al., 1999). Also, variations in Ba con-

tents is probably due to biotite and K-feldspar frac-

tionation (Kebede et al., 1999) (Table 3). The general

decrease of the Zr, P2O5, TiO2, and V contents from BG

to PPG, and the complex behavior of the immobile trace

elements with differentiation, was likely due to crystal-lization/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 coarse-

grained 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 probablydetached 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 frag-

ments 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

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: rift-

related 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) Al2O3 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).


magma followed by finer grains resulting from morerapid 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). Fine-

grained 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). De-

formation is supported by the wavy extinction and re-

crystallization 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) areprobably due to early removal of magnetite during frac-

tional crystallization, leading to enrichment of Mg in the

melt with time. Biotites are always in equilibrium with

the melt (Barbarin, 1999; Usta€oomer, 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 musco-

vite granite, the rare occurrence of pegmatites and ap-

lites 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 typ-

ical of rapakivi granites. Rapakivi textures also imply

decrease of lithostatic pressure, hence rapid emplace-

ment. Oscillatory zoning indicates variations in the

conditions local to the crystal (Holten et al., 1999). Inmicrocline-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 theirprimary 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 par-

tially melted restitic crystals (Pupin, 1980; Pitcher, 1993).

From the semi-quantitative data (SEM), the Singo

granite zircons are relatively poor in Th, U, and Y, im-plying 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 K2O/Na2O ratios and K2O>Na2O 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-Akechet al., 1999). The scarcity of mafic rocks, the peralumi-

nosity 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 byfractional crystallization, or retention of these elements

in feldspars at the source during partial melting (Roll-

inson, 1993). The unfractionated HREE (and Y) pat-

terns 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 in-

compatible 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 at-tributed to the presence of fluorine. Fluorine complexes

are important for REE transport in hydrothermal flu-

ids, 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, epi-

dote, and zircon (e.g. Kebede et al., 1999, and referencestherein; 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 hetero-

geneity of the source rock (Rollinson, 1993). Such

variations could also result from the influence of ac-cessory minerals that can settle out if the magma visco-

sity 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


enclave, more mafic than the rest of the granite, wit-nesses 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)/Al¼ 1.01), could be the result of local en-

richment 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 char-

acteristics suggests the mixing of mafic and felsic mag-

mas during the genesis of the Singo batholith and this is

in line with its association with mafic rocks (quartz-

diorite 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 in-

volved fieldwork, petrographic studies, mineral chemis-

try and whole rock geochemistry. The results enabled us

to make some conclusions regarding the origin of theSingo granite.

• The Singo granite petrographic and geochemical

characteristics suggest that fractional crystallization

was the main differentiation process during its forma-

tion.

• The felsic magmas probably formed from a hetero-

geneous crustal source which was water-undersatu-rated. 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 differentia-

tion took place under low-P conditions (plagioclase

stable without garnet). Textural and field features

also indicate a shallow pluton in which the magmawas emplaced in batches/several magma pulses by dy-

king as shown by the small contact aureole as com-

pared to the batholithic size of the Singo granite.

• Late- and post-magmatic stages (late-magmatic pneu-

matolic phase) were dominated by hydrothermal ac-

tivity, which led to leaching of quartz, and formation

and concentration of minerals, such as fluorite, wol-

framite, beryl, and gold. Hydrothermal activity alsoled to the formation of quartz-sericite bodies and sec-

ondary minerals such as hematite, chlorite, and seri-

cite.

• 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 I-type 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

Exchange Service (€OOAD). The authors are grateful to

W.U. Reimold and S. Farrell (Johannesburg) for help

with the XRF analyses. Laboratory work was supportedby the Austrian FWF, grant Y58––GEO (to C. Koe-

berl). We would also like to thank F. Brandst€aatter(Natural History Museum, Vienna) for help with SEM

analyses, and T. Ntaflos (Univ. Vienna) for help with

electron microprobe analyses.

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