mica granite, Wallagga area, western Ethiopia

Journal of African Earth Sciences, Vol. 32, No. 2, pp. 193-221, 2001
Pergamon
PlhS0899-5362(O0)O0035-5
© 2001 Elsev=er Sc=ence Ltd
All r=ghts reserved. Printed in Great Br=tam
0899-5362/01 $- see front matter
Magmatic evolution of the Suqii-Wagga garnet-bearing twomica granite, Wallagga area, western Ethiopia
T. KEBEDE 1, C. KOEBERL 1'* and F. KOLLER 2
~lnstitute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
2Institute of Petrology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
ABSTRACT--The Suqii-Wagga two-mica granite, situated in the western Ethiopian Precambrian,
is emplaced in a high-grade migmatitic terrane. It is composed of feldspars + quartz + muscovite
+ biotite ± garnet + zircon ± allanite ± apatite + Fe-Ti oxides + Fe sulphide. Textural studies and
microprobe analyses revealed t w o generations of almandine-spessartine-rich magmatic garnet.
The first is euhedral, fine-grained ( 3 0 0 - 3 5 0 pm), commonly occurs as inclusions in plagioclase
and alkali feldspars, and exhibits chemical zoning with almandine-rich cores and spessartine-rich
rims. In contrast, the second variety is medium- to coarse-grained (1-7 mm) and shows reverse
zoning with spessartine-rich cores and almandine-rich rims. Primary and secondary muscovites
were discriminated based on the concentrations of Ti, Fe, Mn and Na. Biotite is characterised by
a higher alumina saturation index than biotites of other granitoids in the area, suggesting considerable
alumina concentration in the source magma.
Garnet-biotite thermometry and phengite barometry were used to estimate the P-T conditions of
crystallisation for the Suqii-Wagga two-mica granite pluton at - 7 kbar and - 670°C. Mineral paragenesis,
the composition of aluminous minerals and the P-T conditions of crystallisation indicate that samples
containing fine-grained garnet crystallised earlier than those containing medium- to coarse-grained
garnet. Field and petrographic investigations, mineral chemistry, and whole rock major and trace element
studies suggest that the Suqii-Wagga two-mica granite has the characteristics of anatectic granite.
Highly variable normative Ab/Or ratios suggested melting under varying a,2o conditions and/or source
characteristics. The relatively high Rb/Sr, Rb/Ba and low CaO/Na20 ( < 0.3) ratios indicate the derivation
of the granitic magma from a plagioclase-poor politic source. Moreover, pronounced negative Eu anomalies
and large ion lithophile element modelling suggested crystal fractionation involving plagioclase. The
presence of the Suqii-Wagga Granite Pluton implies a significant contribution of older mature crustal
material to the magmatic evolution of the area. © 2001 Elsevier Science Limited. All rights reserved.
R¢SUME~--Le granite & deux micas de Suqii-Wagga, situ~ dans le socle prdcambrien de I'Ouest de
I'Ethiopie, se met en place darts un encaissant mdtamorphique de haut degrd et migmatitique. II est
composd de feldspaths + quartz + muscovite + biotite + grenat + zircon _ allanite ± apatite + oxydes FeTi + sulfures de Fe. Deux g6ndrations de grenat magmatique riches en almandin-spessartine sont mis en
dvidence sur la base d'arguments texturaux et d'analyses chimiques par microsonde.
Les premiers, automorphes et finement grenus ( 3 0 0 - 3 5 0 pm), apparaissent g6n6ralement en
inclusions dans les plagioclases et les feldspaths potassiques et montrent une zonation chimique
avec des coeurs et des bordures respectivement riches en almandin et en spessartine. Au contraire,
la seconde gdn6ration de grenat est de granulom6trie moyenne & grossi~re (1-7 mm) et poss~de
une zonation inverse, ceeurs riches en spessartine et bordures riches en almandin. Deux types de
muscovite sont discriminds sur leurs teneurs en Ti, Fe, Mn et Na. La biotite est caract6risde par un
indice de saturation en alumine supdrieur & celui des biotites des autres granitdides du secteur, ce
qui traduit le caract~re alumineux tr~s 61evd du magma parent.
Les gE~-thermom~es (biotite-grenat) et-barom~tres (phengites) ont ~d utilis6s pour esfimer les conditions
P-T de cristallisation du granite & deux micas de Suqii-Wagga ~ - 7 kbar et -670°C. La paragen6se
mindralogique, la composition des phases mindrales alumineuses et les conditions P-Tde cristallisation
*Corresponding author
[email protected]
Journal of African Earth Sciences 193
T. K E B E D E et al.
indiquent que les dchantillons contenant les grenats de petite taille ont cristallis6avant ceux qui renferment
les gros grenats. Les observations pdtrographiques et de terrain, la chimie des min(}raux, et la composition
en dldments majeurs et en traces des roches montrent que le granite & deux micas de Suqii-Wagga
poss~le les caractdres d'un granite d'anatexie. Des variations importantes du rapport Ab/Or normatives
sugg~rent des variations des conditions de all2o pendant la fusion et/ou des caractdristiques des matdriaux
sources. Des rapports Rb/Sr relativement dlevds et CaO/Na20 faibles ( < 0.3) supposent que le magma
granitique est issu d'une source pdlitique pauvre en plagioclase. D'autre part, un processus de
cristallisation fractionnde impliquant le plagioclase est indiqu6e par une anomalie ndgative marqude en
Eu et par une moddlisation bas6e sur les dl6ments lithophiles & fort rayon ionique. L'existence du pluton
granitique de Suqii-Wagga implique la forte contribution d'une croOtemature anciennedans la magmatisme
de cette rdgion. © 2001 Elsevier Science Limited. All rights reserved.
(Received 22/12/99: accepted 3/5/00)
INTRODUCTION
GEOLOGICAL SETTING
The Suqii-Wagga two-mica granite is located 2 0 - 3 5
km southeast of the town of Gimbi and underlies a
characteristically convex topography. The rock unit
is compact and massive (without any trace of mineral
alignment), medium- to coarse-grained and hypidiomorphic inequigranular. Generally the Suqii-Wagga
two-mica granite is leucocratic, containing subordinate mafic minerals, and pegmatitic and aplitic dykes
cut it at the top. The pegmatite is mineralogically simple
and devoid of any mineralogical zoning, whereas the
aplite is mainly composed of feldspars and quartz with
rare micas. The Suqii-Wagga two-mica granite is
emplaced parallel to the meridional to sub-meridional
structural trends in the high-grade migmatitic gneiss
terrane, which includes biotite gneiss, hornblendebiotite gneiss, amphibolite and granitic gneiss.
Field relationships and detailed petrographic investigations, geochemical characteristics, petrogenesis, types of source magmas and emplacement age
of the Suqii-Wagga two-mica granite have not yet
been studied. As this granite is emplaced into the
high-grade gneissic terrane, which was interpreted
by Kazmin eta/. (1979) (based on structural data) as
pre-Pan-African basement, the results of this study,
together with other studies of western Ethiopian
Precarnbrian rocks (e.g. Ayalew and Peccerillo, 1998;
Kebede et al., 1999), can help to establish the
relationship between the granitoid rocks emplaced in
the low- and high-grade terranes.
Thus, in this paper the authors report new data on
petrography, mineral chemistry, bulk major and trace
element compositions of the Suqii-Wagga two-mica
granite, and the gneissic country rock samples. They
use the textural relationships and chemical composition of the aluminous minerals to constrain the
origin and evolution of this granite body. Tectonic
setting, crystallisation sequences, pressure and
temperature of crystallisation, subsolidus textural and
compositional modifications, and the origin of the
source magma are discussed.
The western Ethiopian Precambrian rocks constitute
high-grade gneissic terrane and low-grade volcanosedimentary sequences (Fig. 1 ). Shear zones mark
the lithological boundary between these two distinct
terranes (Abraham, 1989). The Homa granite, as it
intrudes between the contrasting low- and high-grade
terranes (Fig. 2), has recorded t h e e f f e c t s of shear
deformation (Kebede et aL, 2001). Plutonic rocks,
ranging in composition from gabbro to granite,
characteristically intruded the low-grade rock assemblage. The low-grade meta-volcano-sedimentary
rocks contain isolated ultrarnafic bodies, which may
be remnants of an ophiolitic sequence (e.g. Berhe,
1990). The low-grade terrane extends south into
Gore-Gambella, an area where it is known as the Birbir
194 Journal of African Earth Sciences
18ON
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Figure 1. Location map and tectonic terranes o f the southern
p a r t o f the Arabian Nubian Shield (after Vail, 1985, a n d
references therein). VST: volcano-sedimentary terrane; GT:
gneissic terrane; SZ: suture zones.
Magmatic evo/ution of the Suqi/-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
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Journal o f Afncan Earth Sciences 195
T. KEBEDE et al.
Domain (Moore et al., 1986; Ayalew et al., 1990;
Ayalew and Peccerillo, 1998). Ultimately, it is
truncated by the northwest-southeast running Surma
mega-shear zone (Davidson, 1983) in the southwestern verge of Ethiopia. The northern extension of
this low-grade belt is also exposed in northern Ethiopia
(Tadesse, 1996) and in Eritrea and Sudan (Drury and
Berhe, 1993; Drury and de Souza Filho, 1998). The
high-grade rocks are affected by emplacement of preto syn-kinematlc plutonic rocks of dioritic-granitic
composition and are generally exposed ~n river cuts
that are otherwise mostly covered by Tertiary volcanic
rocks. The Suqii-Wagga two-m~ca granite is one of
the granitic plutons in the migmatitic gneiss (Fig. 3a,
b) terrane.
PETROLOGY AND GEOCHEMISTRY
Analytical methods
Bulk major, minor and trace element analyses, and
petrographic investigations were conducted on 11
representative samples of Suqd-Wagga two-mica
granite and four samples of migmatised gneissic
country rocks. Microprobe mineral analyses were
done on three selected granite samples.
Whole-rock major and minor oxides and trace
elements, including Rb, Sr, Y, Zr, Nb, Co, Ni, Cu, Zn,
V, Cr and Ba abundances, were analysed at the
University of the Witwatersrand, Johannesburg,
South Africa, using X-ray fluorescence (XRF) spectrometry. The major- and minor-elements were done
on fused glass disks and the trace elements on
pressed pellets of powders. The precision and accuracy of the XRF analyses are detaded in Reimold et
aL (1994). The rare earth elements (REE) and other
trace elements were analysed by instrumental neutron
activation analysis (INAA) at the Institute of
Geochemistry, University of Vienna. For INAA,
several international rock standards, such as AC-E,
Allende, and G-2 (cf. Govlndaraju, 1989) were used.
The samples were irradiated for eight hours at a
neutron flux of 2 x 1012n cm 2s 1at the 250 kW TRIGA
Mark-II reactor of the Atomic Institute of the Austrian
Universities, Vienna. Analytical methods, correction
procedures (flux, geometric, interference and background), as well as data on precision and accuracy of
the INAA method, are described in Koeberl (1993).
Mineral analyses were conducted on a four
spectrometer Cameca SXlO0 microprobe at the
Institute of Petrology, University of Vienna. The
accelerating voltage was 15 kV, with a beam current
and beam diameter of 20 nA and 1 pm, respectively.
Well-characterised natural mineral standards were
used to calculate the concentrations of the unknowns.
Atomic number, absorption and fluorescence
196 Journal of African Earth Sciences
corrections, and data reduction was conducted
following the PAP procedure (Pouchou and Pichoir,
1991).
Petrography and mineral chemistry
The Suqii-Wagga two-mica granite is mainly composed
of plagioclase (PI), K-feldspar (Kfs) and quartz (Qtz)
with minor muscovite (Ms), biotite (Bt), + garnet (Grt),
zircon (Zrn), +allanite (AIn), _+apatite (Ap), +topaz
(Toz), _+cassiterite (Cst), + fluorite, + Fe-Ti oxides and
Fe sulphide. Quartz occurs in various forms, such as
large anhedral grains and graphic and myrmekitic
Jntergrowth with PI and Kfs, respectively. Other important textural features are shown in Fig. 4a-f. The
mineral abbreviations are those given by Kretz (1983).
Numbers of O atoms considered in mineral formula
recalculations are as recommended by Deer et al.
(1992).
Feldspars
Representative analyses of feldspars and recalculated
formulae are given in Table 1. The plagioclase composition ranges from Ab926An7Or04 to Ab85 4An13 3
Or12' Partial replacement of PI with microcline (Mc)
was rarely encountered. The rock generally appears
to be undeformed; however, bent albite twin lamellae,
suggestwe of mechanical deformation, were observed. Alkali-feldspar compositionally ranges from
Or98 9Abl 1 to Ors~ 5Abe85 and Ab947An48Oro s to
Ab989Ano 3Oro 9 Alkali-feldspars occur as large grains
of orthoclase (Or), crosshatched Mc, perthite and as
intergranular grains. Their occurrence as interstitial
grains suggests a late stage crystallisation from
residual melt. The rare occurrence of graphic ratergrowths further indicates simultaneous crystallisation
of Kfs and Qtz.
Muscovite
Muscovite from the Suqii-Wagga two-mica granite is
characterised by two texturally and chemically distinct
varieties. Bivariate plots of Fe, Mn and Na versus Ti,
and a Na-Ti-(Fe + Mg) ternary plot help to discriminate
the high- and Iow-Ti Ms end members (Fig. 5a-d). A
textural study revealed that the high-Ti Ms occurs as
well-developed large tabular crystals with irregular
termination often showing mica twinning. These large
Ms grafns are considered to be a primary magmatic
crystallisation product from peraluminous magma.
Studies by Miller eta/. (1981 ) and Speer (1984) also
confirmed that primary magmatic Ms contains higher
Ti content than Ms of secondary origin. Zen (1988)
stressed the usefulness of Ti enrichment in Ms as an
indicator of magmatic origin, because it is not easily
affected by subsolidus reaction, as is Mg, Fe, Na and
K. Pnmary Ms (PMs), besides its higher TiO 2 ( > 0.4
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
Figure 3. Field appearance o f the high-grade migmatltic gneiss country rock. (a) Both
the interlayered leucosomes and melanosomes are folded. Such structures are c o m m o n
m the high-grade gneiss. (b) Pytlgmatic folding was found to occur in biotite gneiss
close to the Suqii-Wagga two-mica granite. The granitic leucosomes also s h o w pinch
and swell structures.
wt%) abundance, is characterised by rather homogeneous FeO (4-6 wt%), restricted MnO (< 0.2 wt%)
and variable Na20 (0.05-0.46 wt%) contents. The
PMs often show interlayering with stringers of Fe
oxide (Fig. 4a). These opaque inclusions generally run
parallel to the cleavage and also along the margins of
Ms. Early crystallising Ms can have a high Fe content,
which upon cooling exsolves as Fe oxides (Deer et
al., 1962). Ignoring the effect of Li and V, which were
not analysed, the AI w deficiency and the types of
cations that filled the octahedral site vacancies (Fig.
5e-f) also differentiate the PMs and SMs.
On the other hand, the Iow-Ti secondary Ms (SMs)
normally occurs dendritically intergrown with Qtz as
a replacement product along the cracks and resorbed
rims of Grt crystals (Fig. 4b, c) and as parallel intergrowth with Bt. The SMs is generally fine-grained
and is a product of subsolidus reactions of the primary
minerals and circulating fluids. Phillips et al. (1972)
suggested a retrogressive reaction involving Kfs and
a H20-rich fluid [3KAISi308+H20=KAI2(AISi3)
O10(OH)2+ 6SiO 2 + K20] to form Ms-Qtz symplectite.
The overall composition of the SMs seems to be influenced by the compositions of the parent minerals
Journal of African Earth Sciences 197
T. KEBEDE et al.
Figure 4. Back-scattered secondary electron images, showing textural relationships of the constituent minerals. (a) Large primary muscowte (PMs) (hght
grey) containing stringers of Fe oxtdes (white). It also reveals marginal resorption
and recrystalhsatlon of secondary muscowte (SMs) and quartz (dark grey)
symplectite (left centre). (b) Medium- to coarse*grained (MCG) garnet (Grt)
crystal (white), containing inclusions of Ms, K-feldspar (Kfs), quartz (Qtz),
chlorite (Chl) and corundum. The resorbed margtns and fractures are generally
lined with SMs. Note that the grain boundary with the adjacent Kfs (light grey,
left top) has no reaction rim.
and interacting fluids or the associated minerals, such
as Grt and Bt, which contain considerable amounts of
Fe and Mn. The SMs associated with Grt and Bt generally have relatively higher Fe and Mn than those coexisting with feldspars and Qtz. A systematic variation
198 Journal of A frlcan Earth Sciences
in concentrations of Fe and Mn occurred in SMs that
coexists with Grt. Contents of these elements in SMs
increase towards its grain boundary with Grt. Secondary Ms, containing relatively higher Mg and Ti
than the other SMs, was observed mantling chlorite
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
Figure 4. continued. Back-scattered secondary electron images, showing textural
relationships o f the constituent minerals. (c) Close up view o f marginal breakdown o f Grt shown above in (b) into SMs (centre). The SMs contains streaks
o f remnant Grt and is dendritically intergrown with Qtz. The dark grey mineral
(left bottom) is pyrophyllite. (d) Idlomorphic fine-grained (FG) Grt (centre, white)
embedded in plagioclase (PI).
grains. Both the Chl and Ms appear to be alteration
products after biotite. Despite these compositional
variations, the SMs is always distinct from the PMs.
Biotite
Biotite generally occurs in subhedral to anhedral
forms and is often altered to Chl, Ms, Ep and opaques
along cleavage traces and margins. Rare intergrowths of Mc and Bt were found in sample TK049.
Relatively high concentrations of AI203 ( 1 8 21 wt%), FeO (23-26 wt%) and TiO 2 (2-3 wt%,
with a few exceptions) characterise Bt grains of
the Suqii-Wagga two-mica granite. The biotite is
classified as annite with a significant proportion of
Journal of African Earth Sciences 199
T. KEBEDE et al.
Figure 4. continued. Back-scattered secondary electron images, showing textural
relationships of the constituent minerals. (e) Partially altered garnet (centre) in
Kfs (light grey). The alteration products include SMs, Chl and pyrophyllite. (f)
All the aluminous minerals, Ms (light grey), Bt (white elongated mineral from
lower right to centre) and FG Grt (centre) coexist with each other. The dark
grey mineral is Qtz.
siderophyllite. Biotite coexisting with Grt has
significantly lower TiO 2 (Table 2; analysis #090).
The Ti-poor analyses contain high AI w with a value
close to 2 and generally deviate from other Bt
analyses of the unit. This Bt is presumed to be an
alteration product of Grt, which normally has low
concentrations of Ti and is highly aluminous. The
200 Journal of African EarthSciences
Fe/(Fe + Mg) ratios are generally uniform and range
from 0.80 to 0.83 (Table 2; analysis #709 with
relatively higher MgO is an exception). The alumina
saturation index (ASI) of the Bt in the Suqii-Wagga
two-mica granite is significantly higher than that
from samples of other granitoids and associated
rocks in the region. Such high values of ASI in Bt
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
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204 Journal of Afncan Earth Sciences
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
coexisting with Ms and other peraluminous minerals
reflect increased alumina activity in the magma
(Zen, 1988).
Garnet
Garnet crystals were analysed from three garnetbearing samples (TK047, TK049, TK079) of the SuqiiWagga two-mica granite. Representative microprobe
analyses and formulae recalculated on the basis of
24 O are given in Table 3. Ferric Fe was calculated
following the procedures outlined by Droop (1987).
End member compositions were determined using the
method of Deer et aL (1992). Overall the garnets
from the three samples form a solid solution of
almandine-spessartine, which constitute 9 3 - 9 6
tool.% of the total. The other end members, such as
pyrope, andradite, grossular and very rare uvarovite,
together make up the remainder.
Detailed petrographic and microprobe studies revealed that the Grt in the garnet-bearing samples is
of two types. For simplicity, these two varieties are
referred to as fine-grained (FG) and medium- to coarsegrained (MCG).
The FG Grt ( 3 0 0 - 3 5 0 pm) is of uniform size,
generally euhedral (except where affected by resorption and subsolidus reaction), free of inclusions,
associated with Ms and Bt and commonly occurs
embedded in PI and Kfs (Fig. 4d, e). Plagioclase and
Grt are also found together as inclusions in Kfs.
Among others, the FG Grt forms a stable relationship
with PI, as demonstrated by distinct grain boundaries
without any reaction rims (Fig. 4d). On the other hand,
microprobe studies showed that FG Grt reacted with
Kfs to form aggregates of secondary minerals, such
as chlorite, pyrophyllite and secondary muscovite. The
FG Grt exhibits characteristic zoning, with a relatively
Fe-rich and Mn-poor core and a relatively Mn-rich and
Fe-poor rim (Table 3; Fig. 6a).
The MCG type ( 1 - 7 mm), on the other hand, is
subhedral (as it often shows resorbed margins),
contains inclusions of Ms, Kfs, Qtz, pyrophyllite,
carbonate and Fe-Ti oxides along margins, and microfractures. Biotite is not observed to coexist with this
type of garnet. The MCG variety also shows zoning
but in the opposite direction of the fine-grained type,
i.e. with Mn-rich cores and Fe-rich rims (Fig. 6b, c).
Leake (1967) recognised garnet zoning similar to the
latter in aplites within the Galway Granite, western
Ireland. He explained the zoning as a result of Rayleigh
fractionation process during crystal growth.
The absence of inclusions and perfect euhedral
crystals in the FG variety, according to Zen (1988),
suggests equilibrium with melt during crystallisation.
The FG and MCG varieties contain - 3 8 - 4 3 mol.%
and - 3 9 - 4 8 mol. % spessartine, respectively (Table
3). Magmatic Grt with characteristic almandinespessartine solid solutions has been reported from
highly peraluminous granites elsewhere (e.g. Clarke,
1981; Miller and Stoddard, 1981; Hogan, 1996).
Similarly, both types of garnets of the Suqii-Wagga
Granite with almandine-spessartine solid solutions
represent magmatic phases. Miller and Stoddard
(1981 ) explained the enrichment of Mn in garnets of
strongly peraluminous magma by the absence of
ferromagnesian minerals such as Hbl, which have a
higher affinity for Mn than Bt.
Accessories
Clusters of zircon crystals (8-11 grains) were found
to occur typically at mineral grain boundaries and as
inclusions in other minerals. Electron microprobe
analyses and back-scattered electron images have
shown that some zircon grains contain inclusions of
Chl and Ms. These types of zircons may represent
xenocrysts, which encompassed the other minerals,
presumably during metamorphic crystallisation and
growth. Allanite is generally replaced by a dark brown
metamict phase and is also marginally replaced by
carbonate. Fluorite often occurs lining the grain
boundaries and micro-fractures, particularly in sample
TK078.
Whole rock chemistry
Major, minor and trace elements
Major oxides, trace elements and normative mineral
compositions of the Suqii-Wagga two-mica granite
are given in Tables 4 and 5 and Fig. 7a-k. Restricted
ranges of SiO 2 concentrations ( 7 4 . 4 - 7 6 . 2 wt%)
characterise the garnet-bearing Suqii-Wagga two-mica
granite. The abundance of Fe203, CaO, Sr and Ba
tend to decrease with increasing SiO 2, whereas TiO 2,
AI203 and MgO show less variation. Na20 and K20
have an inverse relationship and show large variations
(Fig. 8a, b). Nevertheless, the Suqii-Wagga two-mica
granite pluton, unlike other granitoids in the region
(Kebede et aL, 1999; Kebede et aL, 2001), has a
rather homogeneous major element composition. Most
of the samples, with the exception of TK048, TK089
and TKO93a, are corundum-normative. In addition,
all samples that contain Grt have relatively high MnO
(0.03-0.04 wt%) (Fig. 7d).
Compared with the other granitoid bodies in the
region (Kebede et aL, 1999; Kebede et aL, 2001 ),
the Suqii-Wagga Granite is characterised by relatively low K/Rb; high Rb/Sr, Rb/Zr, Y/Zr and Nb/Zr;
and variable Rb/Ba, Ba/Sr, Ce/Nb, Tb/Ta and Y/Nb ratios
(see also Table 5). The relatively low concentrations
of high field strength elements, including Zr, suggest
the presence of zircon and other minor phases in
the precursor rocks (residual) that depleted the
Journal of African Earth Sciences 205
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Figure 6. Compositional prohles of representative garnet crystals from samples TK047, TK049 and TK079 o f the Suq#Wagga two-mica granite. (a) Fine-grained Grt with an almandine core and spessartine rim. (b) and (c) Medium- to coarsegrained (MCG) Grt, showing reverse zoning pattern. (d) Comparison of the zoning patterns between FG Grt (TK047) and MCG
Grt (TK079) are shown in (a) and (c), respectively. Increase in spessartine from core-rim o f the FG Grt, as well as to the core
of the MCG Grt, indicates crystallisation sequence of the different textural varieties of garnets. The reverse zoning in the
MCG Grt results from breakdown o f biotite that may have increased the almandine content progressively from core to rim.
elements from the source magma, or the precursor
rock may have originally been depleted in Zr. However,
the narrow range in the abundance of Zr suggests
that zircon was a crystallising phase (Hanson, 1989),
as shown by the presence of magmatic zircons in the
modal composition.
Rare earth elements
Chondrite-normalised REE plots of the Suqii-Wagga
two-mica granite and the high-grade gneiss country
rock are shown in Fig. 9a-c. Most of the Suqii-Wagga
two-mica granite samples (except TK070) show a
moderate to strong negative Eu anomaly, suggesting
fractionation of feldspars. Cullers and Graf (1984)
suggested that melting of igneous source rocks with
negative Eu anomalies may produce a melt with a
strong negative anomaly. Thus, the strong Eu negative
anomaly could be the combined effect of feldspar
fractionation and source characteristics. The samples
have vanable LREE enrichments, LREE/HREE ratios,
and absolute REE concentrations (Fig. 9a-c). The
208 Journal of African Earth Sciences
garnet-bearing samples have flat REE patterns with
slightly enriched HREE abundances (Fig. 9a). Garnet
has variable chondrite-normalised REE patterns depending on the composition of host rocks and/or grade
of metamorphism; however, Grt always has higher
HREE than LREE contents (e.g. Henderson, 1982;
Taylor and McLennan, 1985; Bea, 1996). Therefore,
garnet could be responsible for the relatively higher
concentrations of the HREE and variable absolute REE
concentrations.
Samples TK089 and TKO93a, with lower absolute
REE content, have HREE/LREE ratios near unity. These
samples also contain very low amounts of Grt. Samples TK071 a, TK071 b and TKO76b, however, show
LREE enrichments, a flat HREE pattern and high LREE/
HREE ratios (LaN/YbN= 4.79 and 6.83, respectively).
Replacement of Bt by Ms+opaques and Kfs by
Ms + Qtz is ubiquitous in these samples, suggesting
significant fluid interaction. Petrographic studies
showed the presence of allanite in the modal composition. This mineral normally concentrates the LREEs
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
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Journal of African Earth Sciences 209
7-. K E B E D E
e t al.
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2 1 0 Journal o f African Earth Sciences
rr rr rr ,'~ >- Z
,,, ..j
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
0.40
'
I
'
I
0.05
'
m m
'
I
'
1
[a]
Cd)
0.35
m
0.04
~ 0.30
0.03
0¢-
i=-
0.25
0.02
I
0.20
74
I
,
75
I
0.01
76
77
74
I
'
I
75
SiO2 (wt%)
15
=
76
77
SiO2 (wt%)
0.3
I
I
(b)
'
I
•
14
(e)
0.2
o._m
<
O
13
m m
0.1
I
12
74
,
I
75
I
0.0
76
74
77
I
'
,
m
•
I
75
76
77
SiO2 (wt%)
SiO2 (wt%)
i
•
•
I
l.l
I
1.2
(C)
I
'
I
-
[f)
l.l
0.9
1.O
0 ~ 0.7
0 0.9
L)
u_
•
0.5
•
0.8
•
0.3
74
,m
75
mjm
76
77
0.7
I
74
,
75
SiO2 (wt%)
I
76
77
SiO2 (wt%)
Figure 7. Major and trace element variation diagrams o f the Suqfi-Wagga two-mica granite.
(Bea, 1996). The LREE enrichment presumably
resulted from assimilation of the gneissic country
rocks, which generally are characterised by enriched
LREEs and depleted HREEs, with the LaN/Yb N ratio
ranging from 8 to 24 (Fig. 9c; Table 5). Sample
TKO70 has low REE contents, a small positive Eu
anomaly (Eu/Eu* = 1.11 ), a relatively high LaN/Yb N
ratio (4.72) and a distinct chondrite-normalised REE
pattern. Very low REE content, as that observed in
TK070, is common in differentiated S i O j i c h rocks
of a particular suite of rocks (e.g. Cullers and Graf,
1984).
The hornblende-biotite gneiss sample (TKO86) of
the country rock has a relatively elevated absolute
REE content and, in particular, a Ce anomaly
indicating some alteration. Marsh (1991) and
Mongelli (1 993) suggested that a Ce anomaly is
related to the insolubility of C e O 2 (if Ce 4÷ occurs
Journal of African Earth Sciences 211
T. KEBEDE et am.
I
'
I
(g)
6
I
'
I
(J)
6OO
m
m
mm
4
~'400
3
I
=
75
74
I
77
76
•
=o
•
200
SiO 2 [W~%}
•
5
I
I
0
I
74
(h)
•
,
•
mm
I
75
76
77
SiO2 (wt%)
mm
4
o
200
3
'
I
'
I
[k)
2
I
74
,
75
I
76
77
SiO2 (wt%)
0,05
'
I
'
"~"
alO0
•
•
I
[i)
0.04
i
O
74
aiaii
m~ alp
75
76
77
SiO2 [wt%]
~. 0.03
0.02
0.01
74
i
m m m
__~,
I
=~ ,
75
Im
76""
77
SiO2 (wt%)
Figure 7. continued. Major and trace element variation diagrams o f the Suqii-Wagga two-mica granite.
instead of Ce 3+) that results in differential enrichment of Ce compared to the other REEs, which
are more soluble and leached by circulating ground
water. The overall REE patterns of the gneissic
country rocks are similar to the patterns of quartzintermediate greywackes (Taylor and McLennan,
1985). Such a similarity may indicate a sedimentary
precursor rock for the migmatitic gneissic rocks.
212 Journal of African Earth Sciences
DISCUSSION
Tectonic
discrimination
Trace element tectonic discriminations (e.g. Pearce
et aL, 1984) for the samples of the Suqii-Wagga twomica granite showed plots at the boundaries of WPG,
syn-COLG and r A G (Fig. 10). Thus, these trace
element discriminations do not sufficiently characterise the SuqiioWagga two-mica granite, either
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
100(
I
!
[a]
,,
\
2
"%.
Source
characteristics
"1
jl.
I
Suqii-Wagga two-mica granite
(garnet-bearing samples)
1 O(
l - - t - [ ] -
.*.
"--I
== " ~
,
74
I
-
== _ B I
-[]
La Ce Pr Nd
0
-.
,
75
I
S m Eu Gd T b D y Ho Er T m Yb Lu
,
76
77
1000
SiO2 (wt%)
! (b)
Suqli-Wagga two-mica granite
I--~-- TK(~7~hl
(contaminated >)
. . . . . . . . .
I . . . . . . . . .
I . . . . . . . . .
o
100
(b)
[]
10
5
o
z
I
4
La Ce Pr Nd
1000
3
,,,,,,,,=1,,,,,,,,,i
2
......
3
4
,,.
Figure 8. (a) Na/K ratios of the samples vary considerably
within the umt. The considerable variation m ratios may
result from different melting conditions. (b) The negative
correlation o f / ( 2 0 and Na20 indicates continuously varying
melting conditions (particularly aH2o).
because the concentration of Rb was significantly
decreased as a result of subsolidus fluid interaction,
which is apparent from textural modification and
formation of secondary minerals, or because the HFS
elements are enriched (variably in garnet-bearing
samples) and leads to scatter in the plots. However,
R 1 and R2 parameters, as defined by Batchelor and
Bowden (1985), constrained the Suqii-Wagga Granite
as anatectic two-mica leucogranite (Fig. 11 ). This is
also supported by the occurrence of aluminous
minerals (Ms, Bt and Grt), which are generally considered to be characteristic of peraluminous granites
(e.g. Anderson and Rowley, 1981 ; Clemens and Wall,
1981).
P- T of crystallisation
Garnet-biotite thermometry
The garnet-biotite thermometry, based on the
exchange of Fe and Mg, as first experimentally
,c)
TKO94a
5
K20 (wt%}
S m Eu Gd Tb D y Ho Er T m Yb Lu
Gneisslc country rocks
.~
1 O0
g~
lO
La Ce Pr Nd
S m Eu Gd Tb Dy Ho Er T m Yb Lu
Figure 9. Chondrite-normalised rare earth element plots. (a)
and (b) The Suqii-Wagga two-mica granite and (c) the gneissic
country rock samples (normalisation factors are after Taylor
and McLennan, 1985).
calibrated by Ferry and Spear (1978), can be used
for garnet containing Xca[Ca/(Ca + Fe + Mg + Mn)]
+ X M , [ M n / ( M n + F e + M g + C a ) ] up to - 0 . 2 and
biotite Al~V+ T i up to - 0 . 1 5 . As XM, in the SuqiiWagga two-mica granite is on the order of 0 . 3 8 0.48, the Ferry and Spear (1978) thermometer was
not suitable. The thermometers of Ganguly and
Saxena (1984) and Williams and Grambling (1990)
yielded unreasonably low temperatures, probably
due to the effect of high XM, on Fe-Mg mixing in
garnet. Nevertheless, the thermometric calibration
of Indares and Martignole (1 985) recorded three
Journal of African Earth Sciences 213
T. KEBEDE et al.
of the body as indicated in Fig. 12. Relatively low
temperatures of - 4 1 0 ° C were estimated from a
two-feldspar thermometer (Whitney and Stormer,
1977). This result may also have been affected by
subsolidus perthitic exsolution.
10 g
Phengite barometry
1 I0
Garnet in the Suqii-Wagga two-mica granite generally
occurs in equilibrium either with PI or Ms + Bt, but
never with PI + Ms + Bt. Therefore, it was not possible
to determine the pressure of crystallisation using
barometric calibrations (e.g. Ghent and Stout, 1981 )
that are based on the equilibrium assemblage
Grt + Bt + PI + Ms. However, the muscovite in the
Suqii-Wagga two-mica granite contains a considerable
celadonitic component, which makes it suitable for
phengite barometry. Velde (1965, 1967) calibrated
the phengite thermobarometer based on the content
of celadonite, which raises with increasing pressure.
This barometer is best suitable for low P-Tconditions.
Later Massonne and Schreyer (1987) formulated a
new geobarometric calibration that shows a strong
linear increase of Si per formula unit (pfu) with pressure
and a moderate decrease of Si with temperature. In
this study, the authors used the graphic solution of
this calibration as given by Anderson (1996) to
estimate the P o t crystallisation of the Suqii-Wagga
two-mica granite at - 7 kbar corresponding to the
highest T (670°C). Pressures of - 3 - 5 kbar were recorded for lower cooling temperature ranges ( - 4 0 0 500°C). Pressures were calculated for extensions
0
1
1
10
100
Yb+Ta (ppm]
Figure 10. Discrimination of the Suqii-Wagga two-mica granite
using plot o f Rb versus Yb + Ta (field boundaries are after
Pearce et al., 1984). ORG: Ocean ridge granite; Syn-COLG:
syn-collislon granite; VAG: volcanic-arc granite; WPG: withinplate gramte.
significantly different temperatures at - 4 4 0 ° C ,
500°C and - 670°C. The - 670°C temperature
is interpreted as a reasonable approximation of the
crystallisation temperature of the Suqii-Wagga twomica granite (see also Fig. 12). The other t w o
temperatures ( - 450°C and - 500°C), on the other
hand, are considered to represent the cooling path
2500
'
'
'
I
. . . .
I
'
'
'
I
1 Mantle fractionates
2 Pre-plate collision
3 Post-collision uplift
4 Late-orogenic
5 Anorogenic
'
'
I
J
J
7 Post-orogenic
-
'
'
'
'
'
I
'
'
'
,
,
'
/
6 Syn-collision
] 500
'
\
1
\
J
\
-
2
500
,
,
,
I
,
500
,
,
,
I
,
1000
,
,
,
I
,
,
1500
a
~
I
. . . .
2000
I
,
2500
3000
R1
Figure 11. R~ versus R2 diagram classifwng the Suqii-Wagga two-mica granite as
anatectic granite (field boundaries are after 8atchelor and Bowden, 1985). R 7=4Si11(Na+K)-2(Fe+ Ti); R 2 = 6 C a + 2 M g + A I .
214 Journal of African Earth Sciences
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
40
E
o
.Q
v
J¢:
20 ~.
400
500
600
700
800
900
Temperature [°C]
Figure 12. P-T conditions o f melt generation, crystallisation and cooling path. Water-saturated granite solidus,
muscovite and biotite stability curves and their approximate ranges o f dehydration are from Hyndman (1981). The
stability curve o f Grt is from Clarke (1981). The overlapping stability fields o f Ms + B t + G r t are drawn based on
the stability field of muscovite as indicated by the horizontally hatched field. The patterned area in the overlapping
fields o f the aluminous minerals represents crysta//isation conditions o f the Suqii-Wagga two-mica granite. The
broken line with arrow indicates the cooling path. Two geothermal gradients, at 20°C km -r and 30°C km ~, are
shown.
below and above crystallisation temperatures to
monitor the variability of the barometer. Accordingly,
the barometer shows a significant uncertainty of + 1
kbar for a + 74°C error in temperatures. Anderson
(1996) estimated that a + 50°C variation, and an
analytical uncertainty of _+0.05 atoms Si pfu,
results in an overall error of _+2 kbar. Nevertheless,
the relatively high pressure ( - 7 kbar) obtained is
geologically reasonable as this granite body is
emplaced in a migmatitic high-grade rock. To exclude
the effects of subsolidus reactions on the determinations, only analyses from coarse-grained Tirich PMs crystals were used. Ultimately, the P-T
conditions of crystallisation at - 7 kbar and 670°C
fall within the overlapping stability fields of Ms +
Bt + Grt (Fig. 12), indicating that they are reasonably
well constrained.
Crystallisation sequence
The crystallisation sequence in the Suqii-Wagga
two-mica granite was outlined using the AFM phase
diagram (Fig. 13) of Miller and Stoddard (1981 ).
Muscovite generally occurs together with other
aluminous minerals, specifically Bt and Grt. There
are no cases where Bt and Grt coexisted w i t h o u t
muscovite. The fine-grained Grt with Ms and Bt
(Fig. 4f) suggests crystallisation at the tributary
reaction point (Fig. 13). In contrast, the MCG variety
is only found in association with Ms, implying
crystallisation along the Ms-Grt phase boundary
(Fig. 13). Textural relationships and P-Tof crystallisation suggested that the FG Grt variety crystallised at relatively higher temperatures than the MCG
one, indicating temperature decrease in the order
Ms + B t - , M s + Bt + FG Grt--~Ms + MCG Grt during
Journal of African Earth Sciences 2 1 5
T. KEBEDE et al.
A
o
"+"°
y /
\
Garnet
F
50
~kk
M
Figure 13. A F M phase diagram (after Miller and Stoddarcl, 1981) relating
the crystallisation order o f the aluminous minerals. Shaded areas indicate
the respective mineral analyses. The arrows ( 1 - 4 ) indicate possible
crystallising phase assemblages. A r r o w s on the cotectic lines indicate
directions o f decreasing temperatures. A = AI-(2Ca + Na + K); F = Fe + Mg;
M = Mn.
crystallisation. Crystallisation sequences involving
different parageneses of the aluminous minerals
(Ms + Bt, Ms + Bt + Grt, Ms + Grt) from peraluminous
granitic magmas were discussed by Abbott (1981 ),
Clemens and Wall (1981 ), Miller and Stoddard (1981,
1982), Speer and Becker (1992) and Hogan (1996).
The occurrence of FG Grt inclusions in PI, according
to Flood and Vernon (1988), suggests that crystallisation of FG Grt ceased while PI continued to
crystallise. On the other hand, the spessartine
contents of the late-crystallising garnets decrease
towards the rim (Hogan, 1996), which is also the
case in the MCG Grt of the unit. Medium- to coarsegrained Grt is never found as inclusion in PI or any
other mineral.
Figure 6d shows a decrease of almandine from the
core of the FG Grt in TK047 to the core of the
MCG Grt in TK079. Assuming crystal fractionation,
in which Mn is enriched in the evolved magma (Hall,
1965; Hsu, 1968; Miller and Stoddard, 1981 ), the
FG Grt ceased to grow before the beginning of
crystallisation of the MCG Grt, as indicated by a
decrease of the abundance of almandine and an
increase in the abundance of spessartine from FG
to the core of MCG garnets, but with a significant
compositional gap between the two (Fig. 6d). Biotite,
which crystallised along Bt-Ms cotectics or at the
tributary reaction point, may break down as the
216 Journal of African Earth Sciences
melt cools along the Ms-Grt cotectic line (cf. Ehlers,
1972). Breakdown of Bt by reaction with the melt
results in Grt + M s (Cawthorn and Brown, 1976;
Miller and Stoddard, 1981), and the absence of
other crystallising ferromagnesian mineral phases
that take up Fe, can cause reverse zoning in the
MCG Grt by increasing the concentration of Fe in
the melt as crystallisation proceeds. This is supported by the annite-rich end member composition
and extremely rare occurrence of Bt in samples
containing MCG Grt, which presumably crystallised
along the Ms-Grt cotectic at relatively lower temperatures than the FG garnet.
The mineralogical compositions and textures of
the Suqii-Wagga two-mica granite were also
significantly affected by subsolidus interactions of
the constituent minerals with H20-rich fluids, as
suggested from the hydrous nature of the secondary minerals. Occurrence of carbonate as an alteration product is also suggestive of CO 2 in the
fluid phase. Both Grt varieties are partially or wholly
altered to secondary mineral phases, such as
pyrophyllite, secondary Ms, carbonate, Fe-Ti oxides
and quartz. The textural modification resulting from
the intergrowth of quartz and secondary Ms can
be ascribed to subsolidus reactions of primary
minerals (e.g. feldspars and PMs) with residual H20rich fluid.
Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
Origin and evolution of the magma
The mineralogical composition (particularly muscovite,
biotite and garnet), emplacement in a migmatitic
terrane and the absence of intermediate and/or basic
rock associations favours the origin of the SuqiiWagga two-mica granite by the partial melting of
crustal rocks. In a normative Ab-Qtz-0r ternary
diagram (Fig. 14), all samples cluster fairly close to
the granite minimum melting region, further suggesting
an anatectic derivation of the source magma (cf.
Barbarin, 1996). The data show tendencies to migrate
towards the Ab-Qtz sideline, indicating a relatively
high all2o melting condition. This is consistent with
the study by Barbarin (1996) who showed that twomica granites are largely produced by 'wet' anatexis
of crustal materials in orogenic areas which experienced crustal thickening accompanied by shearing and
thrusting. Thompson (1996) showed that the Ab and
Or contents vary significantly when all2o changes from
0 to 1 (Fig. 14). The Suqii-Wagga two-mica granite
generally has high Na/K ratios (reaching up to 2.32),
suggesting also melt generation at high all2o and P~t,~
(e.g. Huang and Wyllie, 1975; Hall, 1996). According
to Philpotts (1990), the portions containing high
normative Ab may have crystallised from a magma
generated at the H20-saturated solidus. Further
melting at H20-undersaturated conditions progressively enriched the melt with an Or-rich component (Fig. 8a, b). Therefore, similar continuously
changing melting conditions (P-T-all2o) and assimilation of the gneissic country rock could be the
contributing factors for the major oxides and trace
element compositional variations observed within
the unit.
The characteristic pronounced negative Eu anomalies and very low concentrations of Sr in the
Suqii-Wagga two-mica granite samples indicate the
generation of the magma from a plagioclase-poor
precursor rock (cf. Sylvester, 1998). The garnetbearing samples of the Suqii-Wagga Granite compared to the other samples have higher Rb/Sr, Rb/Ba
and Ba/Sr ratios. According to Sylvester (1998),
magmas of peraluminous granites with such chemical characteristics are derived by partial melting of
clay-rich and plagioclase-poor pelitic sediments.
Furthermore, the relatively low K/Rb ratios in the
Suqii-Wagga two-mica granite (113-1 97), compared to other granitoids in the region (e.g. Kebede
etaL, 2001 ), are suggestive of biotite in the residue
of the partial melt.
Qtz
Ab
50
Or
Figure 14. Normative Ab-Qtz-Or ternary plot. The Suqii-Wagga two-mica
granite samples cluster at, and close to, the pseudotemary minimum granite
melt. M i n i m u m melting compositions and dependence on water activity
are m o d i f i e d from Thompson (1996) a n d references therein. Total P
increases towards the feldspar sideline, and temperature increases with
decreasing A b / O r ratio (Thompson, 1996). The solid arrow indicates the
direction o f increase o f all2o in the source magma o f the Suqii-Wagga twomica granite.
Journal of African Earth Sciences 217
7". KEBEDE et al.
Crystallisation of the Suqii-Wagga two-mica granite
at P-Tconditions of - 7 kbar and - 6 7 0 ° C suggests
emplacement at a crustal depth of - 2 0 - 2 5 km. The
country rocks are not extensively migmatised, ruling
out an in situ crystallisation of the body. Therefore, it
is not unreasonable to infer that the melt was
generated in the lower crust, from where it later
segregated and was emplaced.
The Suqii-Wagga two-mica granite is characterised
by low AI203/TiO2 ( < 60) and CaO/Na20 ( < 0 . 3 ) (Fig.
15). Low CaO/Na20 ratios ( < 0 . 3 ) characterise
peraluminous granite magmas derived from clay-rich,
plagioclase-poor ( < 5 % ) pelitic rocks (Sylvester,
1998). He also suggested that high temperature
(_>875°C) melting induced by basaltic magma underplating in collisional orogens produces peraluminous
granites with low AI203/TiO 2 ratios. In strongly
peraluminous granites derived from pelitic sediments,
the Rb/Ba and Rb/Sr ratios are expected to be high
(Sylvester, 1998). However, these ratios in the
samples of the Suqii-Wagga two-mica granite are
scattered, with very low values for the ratios for some
samples (Table 5). The low Rb/Ba and Rb/Sr ratios in
samples (TK070, TK071 a, TK071 b, TKO76b) with
relatively higher CaO/Na20 (0.26-O.31) are possibly
caused by mixing (assimilation?) with the gneissic
country rocks, which are characterised by higher
concentrations of Sr and Ba, as well as significantly
higher CaO/Na20 (0.55-1.98), than the Suqii-Wagga
Granite samples. Assuming the samples with low
CaO/Na20 ratios were uncontaminated, mixing of 5 25% of the gneissic rock samples (depending on their
ratios) is required to produce the average CaO/Na20
ratios of 0.27 shown by the contaminated granite
samples. In Fig. 16, it is also shown that the contaminated samples, with higher K20 than Na20 (Table 4),
appear to be affected by K-feldspathisation. However,
the trend of the other samples, as indicated by a pronounced negative Eu anomaly (Fig. 9a), is controlled
by fractional crystallisation of plagioclase (Fig. 16).
Regional implication
The emplacement of the Suqii-Wagga two-mica granite
in the migmatised biotite gneiss in the eastern highgrade gneissic block (Fig. 2), its content of aluminous
minerals such as Ms, Bt and Grt, and its geochemical
characteristics indicate that it is an anatectic granite.
Generally, anatectic granites are believed to be
associated with crustal thickening as a result of continent-continent collision tectonics (e.g. Pitcher,
1987). Hence, the occurrence of the Suqii-Wagga twomica granite body implies that continent-continent or
arc-continent collision processes may have contributed to the crust formation in the region. Furthermore,
a significant involvement of continent-derived clayrich plagioclase-poor mature s e d i m e n t s (e.g.
Sylvester, 1998) in the source magma is indicated.
This observation supports the idea that microcontinents might have been involved in the accretionary processes that are believed to have formed
the Arabian Nubian Shield (ANS) (e.g. Gass, 1977).
The gneissic country rocks and underlying precursor
rocks are probably the northern extension of the
Kenyan Mozambique Mobile Belt (Key et al., 1989;
10.C
o
1.C
i
0.1
10
i
i
~
=
= i
= i
100
,
1000
A1203/TiO2
Figure 15. AI203/TiO2 versus CaO/Na20 plot of the Suqii-Wagga two-mica
granite. The trapezoid represent compositional range of the strongly peraluminous
granites (after Sylvester, 1998).
218 Journal of African EarthSciences
Magmatic evolution of the Suqfi-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia
10000
Kfs
1000
~
1 O0
n
8
taminated
10
. . . . . . . .
0
~
1 O0
.
.
.
.
===ll
1000
,
=
*
= , , , ,
10000
Ba(ppm)
Figure 16. Barium and Sr modelling of the Suqii-Wagga granite.
The arrows a and b indicate the effects of K-feldspathisation and
fractional crystallisatlon o f plagioclase, respectively. The corresponding arrows to plagloclase (PI), K-feldspar (Kfs) and biotite
(Bt) s h o w which minerals control the variations (partition coefficients are from Rollinson, 1993, and references therein).
Mosley, 1993), which is believed to form the highgrade basement rocks in Ethiopia (e.g. Kazmin et al.,
1978, 1979). The arc-arc accretion (e.g. Abdelsalam
and Stern, 1996) is considered a major crust forming
process in the ANS. However, as the ANS narrows
southward (Fig. 1), the involvement of the northern
extension of the relatively older Mozambique Belt
rocks becomes important. The occurrence of inherited zircons of Mesoproterozoic age in members
of the granitoid populations emplaced in the highgrade g n e i s s i c and the l o w - g r a d e v o l c a n o sedimentary terranes (Kebede et aL, 2000, 2001 )
supports the involvement of older pre-Pan-African
crusts in the origin and magmatic history of the
Precambrian rocks of western Ethiopia. Accordingly, the geochemical features of the Suqii-Wagga
t w o - m i c a granite may have been largely influenced by the involvement of the older crustal rocks.
Kebede et aL (2000, 2001) report that the SuqiiWagga two-mica granite was emplaced at - 700
Ma during the arc-continent collision of east and
west Gondwana with the ANS at 7 5 0 - 6 5 0 Ma
(Abdelsalam and Stern, 1996, and references
therein).
CONCLUSIONS
All available data, including field and petrographic
relationships, mineral chemistry, and bulk major and
trace element compositions indicate that the SuqiiWagga two-mica granite is anatectic in origin. The
granitic melt was presumably generated by partial
melting of a meta-pelitic source at lower crustal
depths ( > 25 km) at - 700 Ma during arc-continent
collision, which may have thickened the crust. The
negative Eu anomalies, low CaO/Na20 (<0.3), as well
as low concentrations of Sr in the Suqii-Wagga
Granite indicate the generation of the source magma
from a plagioclase-poor metapelitic rock. The low
K/Rb ratios in the unit suggest the presence of biotite
in the unmelted residue. Large ion lithophile element
(Sr and Ba) modelling and principal minerals (PI, Kfs,
Bt) fractionation trends indicate modification of the
magma by plagioclase crystal fractionation and
possibly K-feldspathisation. The significant variation
in Na20 concentration within the Suqii-Wagga twomica granite is related to varying all2o during partial
melting and presumed compositional variations within
the precursor rock. The aluminous minerals recorded
P-T conditions of crystallisation at - 7 kbar and
670°C, which in turn suggests that the Suqii-Wagga
two-mica granite was emplaced deep in the migmatitic
gneissic terrane. The textures and mineralogical compositions of the granite were modified by interaction
with H20- and CO2-rich circulating fluids, as indicated
by ubiquitous hydrous and carbonate secondary
minerals. Overall, the occurrence of the Suqii-Wagga
two-mica granite indicates crustal thickening as a
result of arc-continent collision, which is an observation that is consistent with the idea that microcontinents might have had a significant role in the
evolution of the ANS.
Journal of African Earth Sciences 219
T. KEBEDE et al.
ACKNOWLEDGEMENTS
T.K. is grateful to T. Alemu for logistic assistance and
important discussions in the field. The authors very much
appreciate T. Ntaflos (Institute of Petrology, University
of Vienna) for access to, and assistance with, the
electron microprobe. S. Farrell (University of the
Witwatersrand, Johannesburg) is thanked for help with
the XRF analyses. The authors are also grateful to Profs
A. Peccerillo and A. Mogessie for their helpful reviews
of the manuscript. In addition, T.K. would like to
acknowledge the (3sterreichischer Akademischer
Austauschdienst (0AD) for a Ph.D. scholarship.
Laboratory work was supported by the Austrian FWF,
grant Y58-GEO (to C.K.).
Editorial handling - P. Bo w d e n
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