Download PDF

Precambrian Research 206–207 (2012) 159–167
Contents lists available at SciVerse ScienceDirect
Precambrian Research
journal homepage: www.elsevier.com/locate/precamres
U–Pb zircon geochronology of the eastern part of the Southern Ethiopian Shield
R.J. Stern a,∗ , Kamal A. Ali b , Mohamed G. Abdelsalam c , Simon A. Wilde d , Qin Zhou e
a
Geosciences Dept., U Texas at Dallas, Box 830688, Richardson, TX 75083-0688, USA
Department of Mineral Resources and Rocks, Faculty of Earth Sciences, King Abdulaziz University, P.O. Box 80206, Jeddah 21589, Saudi Arabia
c
Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA
d
Department of Applied Geology, Curtin University, Perth, 6845 WA, Australia
e
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10029, China
b
a r t i c l e
i n f o
Article history:
Received 5 October 2011
Received in revised form 2 February 2012
Accepted 12 February 2012
Available online xxx
Keywords:
East African Orogen
Neoproterozoic
Ethiopia
U–Pb zircon geochronology
Pan-African
a b s t r a c t
The Southern Ethiopian Shield (SES) in the central East African Orogen lies at the junction of Neoproterozoic (880–550 Ma) largely greenschist-facies juvenile crust of the Arabian-Nubian Shield in the
north and more metamorphosed and remobilized older crust of the Mozambique Belt to the south.
The SES exposes a polycyclic sialic gneissic basement (represented by the Alghe Terrane in this study)
with interleaved ophiolitic–volcano–sedimentary (Kenticha, Megado, and Bulbul) terranes. U–Pb zircon
SHRIMP and LA-ICP-MS dating of 8 igneous and metamorphic intermediate and felsic bodies in the eastern SES yield Neoproterozoic crystallization ages: 847 ± 11, 855 ± 14, 732 ± 5, 665 ± 8, 657 ± 6, 560 ± 8,
and 548 ± 8 Ma. From these and previously published ages, we infer 4 principal magmatic episodes:
840–890 Ma (Late–Tonian–Early Cryogenian), 790–700 Ma (Cryogenian), ∼660 Ma (Moyale Event), and
Pan-African (630–500 Ma; Ediacaro-Cambrian). Neoproterozoic zircon xenocrysts (941, 884, 880, 863,
762, 716 and 712 Ma) confirm the dominance of Neoproterozoic crust in the study area. One sample
of undeformed granite from the Alghae Terrane has abundant Archean zircons and may be a ∼550 Ma
melt of ∼2.5 Ga crust, demonstrating for the first time that Archean crust or sediments with abundant
Archean zircons exists in the SES. In spite of ∼300 million years of Neoproterozoic igneous activity, we
see no evidence of systematic compositional evolution in SES igneous rocks from early low-K suites to
late high-K suites. Ediacaran deformation and magmatism of the SES reflects Late Tonian and Cryogenian
formation of mostly juvenile crust that was subsequently deformed and chemically modified as a result
of collision between large fragments of East and West Gondwana. Terminal collision began at ∼630 Ma
and caused crustal thickening, melting, uplift, erosion, orogenic collapse, and tectonic escape over a broad
region of the East African Orogen, including up to ∼25 km of erosion of SES crust. Plate convergence was
likely continuous from ∼630 Ma, forming major N-S structures. Deformation stopped at ∼550 Ma and
was followed by exhumation between ∼ 530 and 500 Ma.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
The East African Orogen (EAO) is one of Earth’s great deformation belts, stretching ∼6000 km N-S along the eastern flank of
Africa (Fig. 1a). Evolution of the EAO reflects a NeoproterozoicCambrian Wilson Cycle that began with the breakup of Rodinia
(870–800 Ma: Kusky et al., 2003; Li et al., 2008) and led to the final
amalgamation of Gondwana in Cambrian time. The EAO was even
longer before rifting truncated it in the north and opening of the
Southern Ocean removed ∼2000 km of the orogen in Antarctica
(thus reconstructed, the orogen is sometimes called the East
∗ Corresponding author.
E-mail addresses: [email protected] (R.J. Stern), [email protected] (K.A.
Ali), [email protected] (M.G. Abdelsalam), [email protected] (S.A. Wilde),
[email protected] (Q. Zhou).
0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.precamres.2012.02.008
African-Antarctic Orogen or EAAO; Jacobs and Thomas, 2004). The
northern EAO consists of the Arabian-Nubian Shield (ANS), composed largely of juvenile Neoproterozoic crust flanking the Red
Sea (Fig. 1b), whereas the southern part comprises the Mozambique Belt (MB) with predominantly reworked older crust. The
ANS contains abundant ophiolite fragments, in contrast to the MB,
where ophiolites are not found. The MB is where Ediacaran collision between large continental blocks of East and West Gondwana
was most intense, where deep erosion has exposed medium- to
high-grade gneisses and granulites, and where great mountains
rose and were eroded in Late Ediacaran and Early Paleozoic time
(Squire et al., 2006). Crust formation in the EAO encompassed
nearly all of Neoproterozoic (∼900–540 Ma) and much of Early
Paleozoic time. The final stage of EAO evolution witnessed collision of large continental blocks: the Congo and Kalahari cratons
and Saharan metacraton in the west, the Indian and Australian
cratons and Cryogenian continental lithosphere of E Arabia in
160
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
Eritrea
Tertiary & Younger Sediments
Miocene - Quaternary Volcanics
Oligocene - Miocene Volcanics
Mesozoic Sediments
12°N
Sudan
Precambrian Basement
East African Rift
Somalia
8°
South
Sudan
Somalia
4°
Kenya
Fig. 3
36°E
40°
South Ethiopian Shield
44°
48°
Fig. 2. Geologic map of Ethiopia, showing the location of the Southern Ethiopian
Shield. White lines and irregular shapes are rivers and lakes, respectively. Rectangle
shows location of Fig. 3.
Map is modified from Tefera et al. (1996).
erupted from the Afar mantle plume ∼30 Ma ago (Hofmann et al.,
1997). However, there is a ∼200 km × 300 km region in southern
Ethiopia where EAO basement is exposed, which we call the Southern Ethiopian Shield (SES; Figs. 1b and 2). The SES is situated at the
junction of the ANS and MB and has affinities to both. Studying SES
rocks provides an opportunity to better understand the nature of
the ANS-MB transition and thus the evolution of the EAO in space
and time.
2. The Southern Ethiopian Shield
Fig. 1. (a) Location of the East African Orogen at the juncture between East and
West Gondwana, reconstructed prior to the Jurassic and younger rifting. Note the
location of Southern Ethiopian Shield at the transition between the Arabian-Nubian
Shield and Mozambique Belt. (b) Location of the Southern Ethiopian Shield near the
junction of the Arabian-Nubian Shield and the Mozambique Belt.
Modified after Tsige and Abdelsalam (2005).
the east (Stern, 1994; Squire et al., 2006). This collision was key
for forming the Neoproterozoic/Cambrian supercontinent Greater
Gondwana. In addition to thickening the EAO crust, this collision caused significant but unquantified along-strike transport of
crust via tectonic escape (Stern, 1994). Both EAO segments were
intruded by countless Neoproterozoic and Early Paleozoic plutons
from ∼880 to ∼500 Ma. These igneous bodies and their metamorphosed equivalents provide opportunities to obtain radiometric
ages and chemical compositions that can be used to infer tectonic
evolution of the EAO, which is the strategy we use in this contribution.
The nature of the transition between the MB and ANS – which
encompasses the region now occupied by Ethiopia and Kenya – is
poorly defined. This is largely because the basement rocks of Kenya
have not been studied with modern isotopic and geochronologic
methods and because Ethiopian basement is mostly buried beneath
Phanerozoic sediments and flood basalts of the Ethiopian Plateau,
The SES consists of granite-gneiss basement interleaved
with three belts (Megado, Kentisha and Bulbul) of low-grade
greenschist-facies ophiolitic–volcano–sedimentary rocks (Fig. 3;
Yibas et al., 2002). The granite-gneiss terrane consists of granitoids and quartzofeldspathic para- and orthogneiss, intercalated
with amphibolite and sillimanite–kyanite-bearing schist. Granitoids vary from undeformed to strongly foliated plutons and
range compositionally from diorite to granite (Worku and Yifa,
1992; Worku and Schandelmeier, 1996; Yibas et al., 2002;
Yihunie et al., 2004; Tsige, 2006; Abdelsalam et al., 2008). The
ophiolitic–volcano–sedimentary belts are composed of mafic,
ultramafic and metasedimentary rocks thought to mark Cryogenian
arc–arc sutures overprinted by Ediacaran deformation (Abdelsalam
and Stern, 1996). The gneisses in the Alghe terrane near Kenticha
(∼5◦ 10 N, 39◦ E) were metamorphosed to upper amphibolite facies.
Garnet–biotite and amphibole–plagioclase geothermometry indicate peak metamorphism at 630–650 ◦ C and 7 kbar, corresponding
to a depth of ∼25 km (Tsige, 2006). Yihunie et al. (2004) documented three episodes of metamorphic mineral growth in the
Alghe terrane – up to 590–640 ◦ C and 7 kbar – and inferred that
these accompanied deformation. In contrast, the ophiolitic belts
were subjected to lower T and P metamorphism (480–520 ◦ C and
4–5 kbar) indicating 13–17 km depth in the Kenticha terrane. On
this basis, Yihunie et al. (2004) concluded that Kenticha and Alghe
terrane protoliths were metamorphosed and brought up from the
mid-crust before juxtaposition with the Kenticha sequence along a
N-S trending thrust, during or after terminal EAO collision.
Several models are proposed to explain the relationship between the granite-gneiss basement and the
ophiolite–volcano–sedimentary belts: (1) Worku and Yifa (1992),
and Worku and Schandelmeier (1996) suggested that the Megado,
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
2
Adola Foldand-thrust
Belt
4,5
6°N
9,10,11
DidesaAdola SZ
A
B E
1
3
G
7
15
8
12
7
Fig. 4
F
5°
D
7
Thrust
1
Antiform
4°
Eth
iop
ia
Synform
7
C
Strike-Slip
13
Kenya
100 km
38°E
Granitoids
Normal
Fault
6,14
39°
40°
Low-grade volcano-sedimentary and mafic-ultramafic rocks
High-grade gneisses and migmatites
Fig. 3. Geology and geochronology of the Southern Ethiopian Shield. Principal tectonic elements (after Tsige and Abdelsalam, 2005): A = Megado belt; B = Kenticha
belt; C = Moyale belt; D = Bulbul belt; E = Wadera shear zone; F = Bulbul shear zone;
G = Alghe Terrane/Melka Guba Domain. Previous zircon and Ar geochronological
results: 1 = Metoarbasebat granite magmatic age U–Pb zircon SHRIMP 526 ± 5 Ma;
biotite 40 Ar/39 Ar 506 ± 4 Ma; hornblende 40 Ar/39 Ar 511 ± 4 Ma cooling ages (Yibas
et al., 2002); 2 = Meleka granodiorite gives 3 groups of U–Pb zircon SHRIMP ages:
610 ± 9 Ma; 560 ± 8 Ma; 489–515 Ma. Biotite 40 Ar/39 Ar 534.5 ± 8 Ma, 512 ± 4 Ma;
muscovite 40 Ar/39 Ar 515 ± 3 Ma (Yibas et al., 2002); 3 = Digati diorite gneiss emplacement age 570 ± 7 Ma U–Pb zircon SHRIMP; biotite 40 Ar/39 Ar 502 ± 4 Ma (Yibas et al.,
2002); 4 = Wadera diorite gneiss magmatic age 579 ± 5 Ma U–Pb zircon SHRIMP
(Yibas et al., 2002); 5 = Wadera deformed granitic dyke magmatic age 576 ± 7 Ma
(Yibas et al., 2002); 6 = Moyale granodiorite 666 ± 5 Ma U–Pb zircon SHRIMP
(Yibas et al., 2002); 7 = Melka dioritic gneiss magmatic age 778 ± 23 Ma U–Pb zircon SHRIMP and metamorphic age 552 ± 13 Ma U–Pb zircon (Yibas et al., 2002);
8 = Bulbul mylonitic diorite gneiss magmatic age 876 ± 7 Ma U–Pb zircon SHRIMP
(Yibas et al., 2002); 9 = Granite gneiss 752.3 ± 0.6 Ma Pb–Pb zircon evaporation
(Teklay et al., 1998); 10 = Metarhyolite crystallization age 605 ± 7 Ma; 1125 ± 2.5 Ma,
1657 ± 2 Ma xenocrysts Pb–Pb zircon evaporation (Teklay et al., 1998); 11 = Alghae
augengneiss magmatic age 559.9 ± 0.9 Ma Pb–Pb zircon evaporation (Teklay et al.,
1998); 12 = Tonalitic gneiss magmatic age 884.2 ± 0.8 Ma Pb–Pb zircon evaporation (Teklay et al., 1998); 13 = Amphibolitic ophiolitic metagabbro magmatic age
700.5 ± 1.3 Ma Pb–Pb zircon evaporation (Teklay et al., 1998); 14 = Moyale trondhjemite magmatic age Pb–Pb zircon evaporation 657.9 ± 0.8 Ma (Teklay et al., 1998);
15 = Yavello granitic gneiss magmatic age Pb–Pb zircon evaporation 716.2 ± 1.2 Ma
(Teklay et al., 1998). Not shown (lies west of area) is charnockitic Konso gneiss,
magmatic age 721 ± 12 Ma U–Pb zircon SHRIMP; 728.6 ± 0.6 Ma Pb–Pb zircon evaporation (Teklay et al., 1998).
Kenticha and Bulbul belts were preserved between the granitegneiss rocks as N-trending synforms resulting from folding of
a once coherent E-verging nappe about N-trending axes. They
further inferred that the these synforms and complementary
gneissic antiforms represent N-trending “shortening zones”
developed due to terminal collision between fragments of East
Gondwana and the Saharan metacraton (Abdelsalam and Stern,
1996; Abdelsalam et al., 2002; Avigad et al., 2007). Additionally, this model implies that the MB-ANS in southern Ethiopia
161
represents a “basement–cover” relationship in which reworked
continental crust of the MB extends northward beneath the
juvenile crust of the ANS. (2) Yihunie and Tesfaye (2002), Yihunie
(2003) and Yihunie et al. (2004) examined the northern part of
the Bulbul belt and proposed that it evolved from a SW-verging
fold and thrust belt to N-trending dextral strike-slip shear zone
due to NE-SW directed oblique collision between the Alghe and
the Bulbul terranes. (3) Different from the previous constructional
deformation style model, Burke and Sengor (1986), Bonavia and
Chorowicz (1992) and Stern (1994) interpreted the Megado,
Kenticha and Bulbul belts as strike-slip shear zones representing
escape routes of northward expulsion of the ANS from the MB
because of continental collision between East and West Gondwana
along the MB. (4) Tsige and Abdelsalam (2005) used the presence
of SE-plunging stretching lineations and kinematic indicators
showing top-to-the-southeast tectonic transport to propose that
the southern Bulbul belt developed by regional gravitation tectonic
collapse where the Bulbul terrane slipped east relative to the Alghe
terrane. (5) To reconcile the along-strike variation in structural
style within the Bulbul belt (SW-verging fold and thrust belt in the
north, dextral strike-slip shear zone in the center, and low-angle
top-to-the-southeast normal fault in the south), Abdelsalam et al.
(2008) proposed an anti-clockwise terrane rotation during NW-SE
oblique collision between the Alghe and Bulbul terranes.
There are two modern geochronological studies of SES basement published in the peer-reviewed literature. Teklay et al. (1998)
used Pb–Pb zircon evaporation techniques to date 8 samples of
gneissic and granitic rocks (Fig. 3). They identified 3 important
magmatic periods in the SES: ∼850, ∼750–700, and ∼650–550 Ma.
Teklay et al. (1998) found pre-Neoproterozoic zircons of ∼1125 Ma
and ∼1660 Ma in 605 Ma meta-rhyolite. In the second study, Yibas
et al. (2002) identified 5 major tectonic episodes (1160–1030 Ma,
875 Ma, 770–720 Ma, 700–550 Ma, and 550–500 Ma, accompanied by 7 intrusive episodes Gt1 (>880 Ma), Gt2 (800–770 Ma),
Gt3 (770–720 Ma), Gt4 (720–700 Ma) Gt5 (700–600 Ma), Gt6
(580–550 Ma), Gt7 (550–500 Ma). According to Yibas et al. (2002),
intrusive rocks of all but Gt7 are deformed.
The east-central SES is the focus of this study (Fig. 3). The structure of this region is dominated by the N-trending Bulbul Shear
Zone, which separates the Bulbul terrane in the east from the Alghe
terrane to the west (Fig. 4). The Alghe terrane (Yihunie and Tesfaye,
2002) represents the southern continuation of the Alghe gneisses
of Kazmin (1975) and Kazmin et al. (1978) and is the equivalent
of the Zambaba domain of Worku and Schandelmeier (1996). This
terrane is dominated by hornblende and quartzofeldspathic gneiss
and granitic migmatite. These rocks show well-developed NNW- to
NNE-striking gneissic layering and are intruded by deformed and
undeformed granitoids. The gneissic layering is disrupted by early
rootless intrafolial folds and late map-scale NNE-trending open
antiforms (Abdelsalam et al., 2008).
The Bulbul terrane (Yihunie and Tesfaye, 2002) is dominated by
amphibolite schists, ultramafic slivers, and pre- and syn-tectonic
granitoids, all showing N- to NNW-trending, moderately E- and
ENE-dipping foliation. This foliation is cut by N- to NNW-trending
cleavage associated with W- and WSW-verging folds and thrusts.
The intersection of foliation and cleavage produces strong subhorizontal pencil structure.
The Bulbul Shear Zone is a 100 km long, N-S trending belt that is
5–7 km wide. The northern extension of the shear zone is intruded
by a granitic body whereas its southern part terminates against
a bow-shaped dextral strike slip shear zone (Fig. 3). This zone
is dominated by E-dipping mylonitic fabric with stretching lineations that plunge NE in the north and SE in the south. Kinematic
indicators show that the northern Bulbul Shear Zone represents
a SW-verging fold-and-thrust belt; its central part is dominated
by dextral strike–strike shearing, and top-to-the-southeast normal
162
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
Fig. 5. K2 O vs. SiO2 plot of analyzed samples, series after Peccerillo and Taylor
(1976). Colors denote ages in 100 m.y. intervals, the same color code used in Fig. 8.
(For interpretation of the references to color in the figure caption, the reader is
referred to the web version of the article.)
Fig. 4. Simplified geological map of study area showing sample localities. Location
of this map is shown by dashed rectangle in Fig. 3.
faulting characterizes the southern part (Abdelsalam et al., 2008).
In addition, many parts of the shear zone also show co-axial but
non-co-planar complex folds reflecting strong E-W shortening.
Samples were collected in the course of detailed structural studies by Abdelsalam et al. (2008) in a ∼55 km × 30 km region bounded
by 5◦ 15 –4◦ 45 N, 39◦ 15 –39◦ 30 E (Fig. 4). The study area is a relatively small part of the SES but the 8 analyzed samples are from
lithologic units that are broadly representative of the SES, including gneiss of the Alghe terrane in the west and greenschist-facies
metavolcanic and metasedimentary rocks of the Bulbul Terrane in
the east. Field photographs of typical lithologies and structures in
the Alghe and Bulbul terranes, and the Bulbul Shear Zone can be
found in Abdelsalam et al. (2008).
3. Analytical methods
Eight samples for age determination were collected at locations (determined by GPS) provided in Section 4. Samples were
crushed at UT Dallas. Powdered samples were analyzed for major
oxides and some trace elements (Ba, Sr, Y, Zr, Sc, and V) at Actlabs
(http://www.actlabs.com/) using routine ICP fusion techniques.
Zircons were isolated at UT Dallas by crushing, magnetic separation, and heavy liquids. Grains from the non-magnetic fractions
were hand-picked, mounted on double-sided adhesive tape, and
set in EpirezTM resin along with several grains of Sri Lanka zircon
standard (CZ3) which has a conventionally measured 206 Pb/238 Pb
age of 563 Ma (Nelson, 1997). The mounted zircons were ground
and polished to effectively cut them in half and then gold-coated.
U–Th–Pb analyses were performed on 1 sample (BSZ-04-1-4) using
a SHRIMP II ion microprobe at Curtin University in Australia following techniques described by Nelson (1997) and Williams (1998)
utilizing five-cycle runs through the mass stations. Another 7 samples were analyzed for U–Th–Pb concentrations and Pb isotopic
compositions using an Agilent 7500a Q-ICP-MS connected to a
193 nm Excimer laser ablation system at the Institute of Geology
and Geophysics, Chinese Academy of Sciences, Beijing, following
techniques described by Xie et al. (2008). For the SHRIMP session, a
total of 12 spots were analyzed on 11 zircons of sample BSZ-04-1-4,
along with 17 analyses of the CZ3 standard, which yielded a 1 sigma
variation in Pb/U of 0.42% for those sites located in the same mount.
A total of 129 spots on 129 zircons were analyzed by LA-ICP-MS,
along with 43 analyses of the 91500 standard and 21 analyses of the
GJ-1 standard. The relative standard deviations of reference values
for 91500 were set at 2%. Data were processed using SQUID (Ludwig,
2001a) and Isoplot (Ludwig, 2001b) software. Ages >1000 Ma are
calculated from 207 Pb/206 Pb and ages <1000 Ma are calculated from
206 Pb/238 U because young zircons provide low count rates on the
207 Pb peak, thus increasing uncertainties (Black et al., 2004). Both
data sets were corrected using the measured 204 Pb (Compston et al.,
1992).
4. Results
Geochemical results are listed in Table 1. Fig. 5, modified after
Peccerillo and Taylor (1976), enables identification of the mafic,
intermediate, and felsic components. Analytical results for zircons
are listed in Tables A1 (ion probe results) and A2 (LA-ICP-MS)
(supplementary material). Whole rock abundances of SiO2 and K2 O
are also summarized below for each sample. SiO2 contents indicate whether igneous (sometimes metamorphic) rocks are mafic
(derived by melting of the mantle) or felsic (generated by crustal
melting or by fractionation of mafic melts). In contrast, K2 O contents reflect if the source – whether mantle or crust – was enriched
or depleted. Considered together, these two elements approximate
the most important aspects of igneous rock compositions. Compositional and age results are summarized below for each sample.
1. BSZ-04-1-4 (05◦ 15 17.0 N, 39◦ 30 44.0 E): This is a medium-K
granodiorite (∼59% SiO2 , ∼1.3% K2 O; Fig. 5) from the northern Bulbul Shear Zone (Fig. 4). This pluton is deformed, with a
well-developed mylonitic fabric striking 010◦ and dipping 60◦ E,
along with a NE-trending stretching lineation. Three data points
are discordant and were not used in age calculations (Fig. 6a).
Nine data points are concordant and cluster together yielding
a weighted mean 206 Pb/238 U age of 847 ± 11 Ma (MSWD = 0.55),
which we interpret as the age of intrusion.
2. BSZ-04-1-5 (05◦ 08 57.5 N; 39◦ 28 50.7 E): This is an amphibolite schist from the Bulbul Terrane (Fig. 4) with a well-developed
schistosity that strikes 020◦ and dips 70◦ E. It contains ∼55%
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
163
Table 1
Geochemistry of Southern Ethiopian Shield basement rocks.
Sample
1–4
Lithology
Granodiorite
851
Age (Ma)
Major elements (wt. %)
58.99
SiO2
0.79
TiO2
18.43
Al2 O3
6.22
Fe2 O3 T
MnO
0.09
MgO
2.72
CaO
6.12
Na2 O
4.41
1.26
K2 O
0.2
P2 O5
LOI
0.71
Total
99.94
Trace elements (ppm)
531
Ba
Sr
536
10
Y
Sc
11
Zr
147
109
V
0.156
1–5
2–1A
2–1B
2–1C
3–2
3–5
4–3
5–1
Amphibolite
659
Gneiss
Granite
560
Felsic dike
548
Granodiorite
665
Gneiss
731
Granite
∼2500
Granodiorite
840
55.03
0.18
15.46
9.1
0.15
5.87
7.45
3.68
1.78
0.12
1.15
99.97
68.4
0.44
15.16
4.47
0.06
1.81
3.3
4.23
1.79
0.18
0.46
100.3
73.9
0.17
13.91
2.19
0.03
0.46
1.61
3.14
4.63
0.11
0.42
100.57
76.76
0.06
13.42
1.26
0.02
0.09
1.21
3.97
3.99
0.02
0.11
100.91
73.77
0.18
13.11
3.56
0.04
1.3
2.43
3.67
1.53
0.04
0.21
99.84
71.73
0.53
13.79
3.84
0.06
0.65
1.86
2.98
4.18
0.21
0.45
100.28
65.07
0.83
15.5
4.29
0.05
1.32
2.37
4.47
4.07
0.42
0.58
98.97
76.36
0.12
13.01
1.16
0.03
0.22
1.05
3.66
4.4
0.04
0.83
100.88
172
251
29
27
157
183
262
404
9
5
154
62
922
271
9
2
244
28
99
55
5
2
12
<5
478
158
9
3
109
33
956
160
33
7
295
32
1849
854
8
5
337
61
886
184
3
2
61
9
(b) BSZ-04-1-5
(a) BSZ-04-1-4
820
0.135
920
206
Pb/ U weighted mean age =
762 ± 8 Ma (N = 6)
(MSWD = 0.39)
data-point error ellipses are 2σ
0.152
0.148
238
0.125
880
780
740
0.144
840
660
0.136
206
U
238
Pb/
238
Pb/ U weighted mean age =
847 ± 11 Ma (N = 9)
(MSWD = 0.55)
data-point error ellipses are 2 σ
800
0.132
0.128
1.15
0.14
1.25
1.35
1.45
0.105
206
0.095
0.7
1.55
0.17
750
0.12
0.8
0.9
1.0
(d) BSZ-04-2-1C
800
550
700
0.08
206
238
0.11
Pb/ U weighted mean age =
560 ± 8 Ma (N = 3)
(MSWD = 0.91)
data-point error ellipses are 2 σ
450
600
0.09
350
500
250
206
0.05
0.02
0.6
0.8
1.0
1.2
1.4
1.6
207
Pb/
238
Pb/ U weighted mean age =
548 ± 8 Ma (N = 4)
(MSWD = 0.32)
data-point error ellipses are 2σ
0.07
0.4
1.3
900
0.13
0.2
1.2
1000
650
0.04
1.1
0.15
1.00
0.06
238
Pb/ U weighted mean age =
657 ± 6 Ma (N = 12)
(MSWD = 0.76)
data-point error ellipses are 2σ
620
(c) BSZ-04-2-1B
206
700
0.115
0.140
0.5
0.7
0.9
1.1
1.3
1.5
235
U
Fig. 6. U–Pb concordia plots. (A) BSZ-04-1-4 granodiorite from the northern Bulbul Shear Zone; (B) BSZ-04-1-5 amphibolite schist from the Bulbul Terrane; (C) BSZ-04-2-1B
undeformed granite; (D) BSZ-04-2-1C undeformed felsic dike. All error ellipses are 2␴. Dotted symbols are excluded from age calculations, as discussed in text.
164
3.
4.
5.
6.
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
SiO2 and ∼1.8% K2 O and plots on the boundary between
mafic-intermediate and medium- to high-K calc-alkaline suites
(Fig. 5). We interpret this as having formed from a melt
prior to being metamorphosed to amphibolite facies (orthoamphibolite). Twenty zircons were analyzed. Two data points
are discordant and were not used in age calculations (Fig. 6b).
Twelve data points yield a weighted mean 206 Pb/238 U age of
657 ± 6 Ma (MSWD = 0.76), which we interpret as the time of
crystallization. The remaining six analyses are concordant with
a weighted mean 206 Pb/238 U age of 762 ± 8 Ma (MSWD = 0.39),
which we interpret as the age of xenocrysts (Fig. 6b). Alternatively, the igneous rock could have formed at ∼760 Ma and
then been metamorphosed ∼660 Ma, as suggested by the fact
that Th/U for ∼660 Ma zircons = 0.60 ± 0.21 (1␴), significantly
lower than Th/U = 1.02 ± 0.35 for ∼760 Ma zircons. Further work
is required to resolve this uncertainty.
BSZ-04-2-1B (2-1A, B, and C are all from 04◦ 44 24.3 N;
39◦ 15 52.5 E): This sample is from a granite body that intrudes
the Alghe Terrane. It is a felsic, high-K calc-alkaline igneous
rock (∼74% SiO2 and ∼4.6% K2 O; Fig. 5). The body has a welldeveloped foliation that is concordant with layering in the
surrounding gneiss. One analysis was conducted for each of 18
zircon grains. Twelve data points are discordant and were not
used in the calculations, whereas 3 concordant analyses yielded
a weighted mean 206 Pb/238 U age of 560 ± 8 Ma (MSWD = 0.91),
which we interpret as the best estimate of the age of crystallization (Fig. 6c). The remaining three analyses are concordant and
yield 206 Pb/238 U ages of 613 ± 8 Ma and 716 ± 9 Ma, also interpreted as xenocrysts. Spot 18 yields a younger 206 Pb/238 U age
(540 ± 8) than the other 18 analyses, which may record Pb-loss
or perhaps the effects of a slightly younger thermal event.
BSZ-04-2-1C: Is an undeformed felsic dyke intruded into gneiss
(BSZ-04-2-1A; undated but chemical analysis listed in Table 1)
and ∼560 Ma granite (BSZ-04-2-1B) of the Alghe terrane. This
dike is a felsic, high-K calc-alkaline igneous rock (∼77% SiO2 and
∼4% K2 O; Fig. 5). One analysis was conducted for each of 9 zircon
grains. Three young data points are discordant and were omitted
from the age calculation (Fig. 6d), whereas 4 data points are concordant and yield a weighted mean 206 Pb/238 U age of 548 ± 8 Ma
(MSWD = 0.32), which we interpret as the crystallization age of
the dike. The remaining two analyses are concordant and yield
206 Pb/238 U ages of 880 ± 13 Ma and 884 ± 13 Ma, which we interpret as xenocrysts.
BSZ-04-3-2 (05◦ 02 26.3 N; 39◦ 28 01.8 E): This sample is from a
highly sheared granodiorite body in the Bulbul Terrane (Fig. 4). It
is a medium-K felsic igneous rock (∼74% SiO2 , ∼1.5% K2 O; Fig. 5).
This pluton has a mylonitic fabric that strikes 010◦ and dips
50◦ E. One analysis was conducted for each of 8 zircons. One data
point is discordant and was not used in age calculations (Fig. 7a).
Six concordant analyses yield a weighted mean 206 Pb/238 U age
of 665 ± 8 Ma (MSWD = 0.096), which we interpret as when the
rock crystallized. The remaining analysis gives a 206 Pb/238 U age
of 863 ± 14 Ma, which we interpret as a xenocryst.
BSZ-04-3-5 (05◦ 02 42.2 N; 39◦ 27 14.6 E): This sample is from
a quartzo-feldspathic gneiss at the contact between the Bulbul Shear Zone and the Bulbul terrane. The dated sample is a
felsic, high-K calc-alkaline metamorphic rock (∼72% SiO2 and
∼4.2% K2 O; Fig. 5) that probably formed from an igneous protolith (orthogneiss). It has a well-developed mylonitic fabric that
strikes 020◦ and dips 50◦ E and also a well-developed stretching
lineation (azimuth 066◦ , plunge 28◦ ). One analysis was conducted on each of 36 zircon grains. Seven analyses are discordant
and were excluded from age calculations (Fig. 7b). Twenty-five
data points cluster around a weighted mean 206 Pb/238 U age of
732 ± 5 Ma (MSWD = 0.75). We interpret these zircons to have
formed by crystallization from a melt, as indicated by mean
Th/U = 0.60 ± 0.24 (1␴), so this age is taken to reflect when the
protolith crystallized. The four remaining analyses are concordant but younger; one (spot 36) yields a 206 Pb/238 U age of
699 ± 12 Ma and three others yield a weighted mean 206 Pb/238 U
age of 605 ± 11 Ma (MSWD = 0.80), perhaps reflecting metamorphic recrystallization, an interpretation supported by the low
Th/U for some of these zircons of ∼0.10.
7. BSZ-04-4-3 (05◦ 13 06.8 N; 39◦ 29 44.6 E): This sample is from
an undeformed granite intruding the Alghe Terrane. It has
∼65% SiO2 and ∼4% K2 O, and plots near the boundary between
intermediate and felsic igneous rocks with high-K calc-alkaline
affinities (Fig. 5). Twenty-four zircons were analyzed and concordant ages range from 2505 ± 36 Ma to 568 ± 9 Ma. Three of
these yield a weighted mean 207 Pb/206 Pb age of 2514 ± 18 Ma
(MSWD = 0.18). The youngest ages are identical to the weighted
mean age of sample BSZ-04-2-1B; a granite that also intrudes the
Alghe terrane. It is not clear if this is a ∼2.5 Ga igneous rock metamorphosed in Late Neoproterozoic time or a Late Neoproterozoic
melt of Archean crust, but the fact that the body is undeformed
suggests that it is an Ediacaran intrusion that was generated by
melting of ∼2.5 Ga crust or sediments with a concentration of
such zircons.
8. BSZ-04-5-1 (04◦ 55 51.3 N; 39◦ 26 59.4 E): This sample is from a
granodiorite at the contact between the Bulbul Shear Zone and
the Alghe Terrane. It is a felsic, high-K calc-alkaline igneous rock
(∼76% SiO2 and ∼4% K2 O; Fig. 5). It has both a well-developed
mylonitic fabric (strike 020◦ , dip 50◦ E and stretching lineation
(azimuth 075◦ , plunge 45◦ ). One analysis was conducted for each
of 14 grains. Six were discordant and were omitted from age
calculations (Fig. 7d). Four data points yield a weighted mean
206 Pb/238 U age of 855 ± 14 Ma (MSWD = 0.28) which we interpret as the time of intrusion; three points are discordant around
this age and may reflect Pb-loss. The remaining four concordant ages gave scattered 206 Pb/238 U ages of 941 ± 16, 800 ± 14,
710 ± 14 and 648 ± 12 Ma. The oldest of these ages may be a
xenocryst, whereas the three younger ones may reflect younger
thermal disturbance.
5. Discussion
SES exposures provide a chance to examine the middle EAO
crust, 13–25 km deep, as it existed at ∼550 Ma when deformation
stopped. Because it juxtaposed metamorphic rocks of very different
grade, it is likely that prior to terminal collision, the present steep
contact between lower grade ophiolitic–volcano–sedimentary
basins and higher grade granite-gneiss middle crust was subhorizontal, so that gneiss and low grade sequences were separated
by a shear zone that facilitated lateral movements of the upper
crustal “superstructure” relative to the mid- and lower crust
“infrastructure” (Abdelsalam et al., 2008). Such shear zones would
have likely concentrated fluids, including those responsible for
gold mineralization in the area (Tsige, 2006). Thus the results
presented above should be viewed in the context of a midcrustal environment, with the possible exception of the youngest
intrusions.
These results largely confirm conclusions from earlier
geochronologic studies that the SES is largely a juvenile Neoproterozoic crustal addition. With the exception of ∼2.5 Ga zircons
in BSZ-04-4-3, the inferred crystallization ages are 851, 840,
731, 665, 659, 560, and 548 Ma. The dominantly Neoproterozoic
nature of the crust in the study area is also confirmed (again
with the exception of ∼2.5 Ga material) by the fact that zircon
xenocrysts, although common, are entirely Neoproterozoic (941,
863, 880, 884, 762, 716 and 712 Ma). Sr–Nd isotopic studies
(Teklay et al., 1998; Yihunie et al., 2006) also indicate that the
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
0.135
(a) BSZ-04-3-2
900
0.15
206
238
Pb/ U weighted mean age =
665 ± 8 Ma (N = 6)
(MSWD = 0.096)
data-point error ellipses are 2s
0.13
(c) BSZ-04-3-5
206
0.125
860
165
800
238
Pb/ U weighted mean age =
732 ± 5 Ma (N = 25)
(MSWD = 0.75)
data-point error ellipses are 2s
760
720
820
0.115
780
680
740
0.105
700
0.11
660
600
580
0.09
0.7
0.6
0.9
1.1
1.3
1.5
0.085
0.6
0.18
(c) BSZ-04-4-3
0.5
2600
0.8
207
0.3
1400
1.2
Pb/ U weighted mean age =
855 ± 14 Ma (N = 4)
(MSWD = 0.28)
data-point error ellipses are 2s
950
206
0.16
1.4
238
850
0.14
206
Pb/ Pb weighted mean age =
2514 ± 18 Ma (N = 3)
(MSWD = 0.18)
data-point error ellipses are 2s
1800
1.0
(d) BSZ-04-5-1
2200
0.4
238
Pb/ U weighted mean age =
605 ± 11 Ma (N = 3)
(MSWD = 0.80)
data-point error ellipses are 2s
560
U
238
Pb/
206
0.095
620
206
640
750
0.12
0.2
1000
650
0.10
0.1 600 Youngest concordant age = 568 ± 9 Ma
550
0.0
0
2
4
6
8
10
12
14
207
0.08
0.6
Pb/
0.8
1.0
1.2
1.4
1.6
235
U
Fig. 7. U–Pb concordia plots. (A) BSZ-04-3-2 Alghe sheared granodiorite; (B) BSZ-04-3-5 Bulbul quartzo-feldspathic gneiss; (C) BSZ-04-4-3 undeformed Alghe granite; (D)
BSZ-04-5-1 Bulbul-Alghe mylonitic granodiorite. All error ellipses are 2␴. Dotted symbols are excluded from age calculations, as discussed in text.
SES mostly comprises juvenile Neoproterozoic crust. We see no
evidence, either in crystallization or xenocryst ages, to support the
conclusion that significant tracts of 1160–1030 Ma crust (Adola
tectonothermal event of Yibas et al., 2002) are present in the SES.
Our zircon crystallization ages mostly agree with the 3 magmatic
episodes (∼850, ∼750–700, and ∼650–550 Ma) identified by
Teklay et al. (1998) for the SES. However, considering the data of
Teklay et al. (1998) and Yibas et al. (2002) along with our own,
we infer 4 principal magmatic episodes in the region (Fig. 8):
840–890 Ma (Late Tonian-Early Cryogenian); 790–700 Ma (Cryogenian), ∼660 Ma (Moyale Event), and Pan-African (630–500 Ma;
Ediacaro-Cambrian). The first episode marks the beginning of
crust formation and encompasses the 876 ± 5 Ma Bulbul–Awata
tectonothermal event, which Yibas et al. (2002) correlated with
the Samburian-Sabachian episode in Kenya (Rb–Sr whole-rock
ages of Key et al., 1989). The first two episodes overlap with
∼0.85–0.74 Ga Tsaliet arc magmatism in NE Ethiopia and Eritrea
(Avigad et al., 2007). The Cryogenian episode is essentially the
same as the 770–720 Ma Megado tectonothermal event of Yibas
et al. (2002).
There is no evidence for the distinctive ∼720–750 Ma carbonates and other sediments of the Tambien Group, which define
synformal keels in the Neoproterozoic basement of NE Ethiopia
(Miller et al., 2009). It is unknown whether the Tambien was
removed by erosion (more extensive in the SES than in NE
Ethiopia) or was never deposited on SES crust.
The ∼660 Ma “Moyale” event seems to have been important for SES evolution, including both granitic intrusions (like
that of Moyale granodiorite) and formation of amphibolite schist
(BSZ-04-1-5) of this study. There may be a range of ages for amphibolite protoliths; Teklay et al. (1998) report ages of 700.5 ± 1.3 Ma
for amphibolite from the Moyale area, and Yibas et al. (2002)
note that amphibolite xenoliths are common in the ∼650 Ma
Moyale granodiorite. Worku (1996) reported a Sm–Nd whole- rock
age of 789 ± 36 Ma for meta-mafic igneous rocks near Megado.
Further research is needed to resolve the importance of the
∼660 Ma episode, to map the extent of the region affected by
this event, and to identify the full age range of amphibolites and
ophiolitic rocks.
SES rocks studied here also provide insights into magmatism and deformation accompanying continental collision between
fragments of East Gondwana and the Congo-Tanzanian craton –
Saharan metacraton, beginning at ∼630 Ma. These events formed
the EAO (Fig. 1B) and other Ediacaran orogens referred to as PanAfrican (Kennedy, 1964; Kröner and Stern, 2004). High mountains
were created by these collisions, and the continental assembly
and relief affected climate and other aspects of the Earth System,
including the rapidly evolving biosphere (Meert and Lieberman,
166
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
Denudation
Collision
40
Pan-African
M
B
B
B
B = Biotite; M = Muscovite; H = Hornblende
Early
Cryogenian
Late
Cryogenian
Moyale event
B
Ar/ 39Ar age (Yibas et al., 2002)
U-Pb & Pb-Pb Zircon Ages
Crystallization age, Teklay et al (1998)
Crystallization age, Yibas et al (2002)
H
T
Crystallization age, this study
Xenocryst age, Teklay et al (1998)
Xenocryst age, this study
T
450
500
550
600
650
700
750
800
850
900
950
1000
1125
T
1650
2500
Ma
Fig. 8. Age summary diagram comparing results of this study with those of Yibas et al. (2002), including zircon crystallization ages and mineral 40 Ar/39 Ar ages, as well
as zircon Pb–Pb evaporation and one SHRIMP zircon age and xenocrysts of Teklay et al. (1998). Color-coded 100 million year intervals as used in Fig. 5. See text for more
discussion of timing. (For interpretation of the references to color in the figure caption, the reader is referred to the web version of the article.)
2008). This collision triggered crustal thickening in the EAO south of
the Ariab-Nakasib-Tharwah-Bir Umq suture (Johnson et al., 2003),
which in turn engendered crustal melting, uplift, and erosion over
a broad region. Deformation is likely to have been continuous from
∼630 Ma, and includes the ∼580 Ma Wadera shearing event (Yibas
et al., 2002); other SES shear zones need to be dated. Post-630 Ma
shortening probably formed the major N-S structures, overprinting older structures. Deformation stopped in at least part of Alghe
terrane at ∼550 Ma (constrained by 560 ± 8 Ma deformed granite
intruded by 548 ± 8 Ma undeformed felsic dyke), and this probably
reflects localization and diminishing of strain as shortening stopped
(after 70 million years) and the orogen relaxed. This approximates
the start of the post-orogenic Berguda tectonothermal event at
550–500 Ma (Yibas et al., 2002) and was followed by exhumation
between ∼500 and 530 Ma, as indicated by 40 Ar/39 Ar hornblende,
biotite and muscovite ages (Yibas et al., 2002).
Late Pan-African exhumation engendered tremendous erosion
of the SES and other parts of the central and southern EAO (Johnson
et al., 2003). Vast volumes of clastic detritus were transported
northwards and an estimated (15–90) × 106 km3 of sandstone was
deposited on the northern margin of Gondwana (Avigad et al., 2005;
Squire et al., 2006). The higher estimate is equivalent to eroding
12.5 km of crust from the 8000-km-long and at least 1000-km-wide
East African–Antarctic Orogen. Of course, uplift and erosion were
not distributed evenly across the orogen, mostly being greater in
the south. One difference between the ANS and MB is that uplift
and erosion was much greater in the MB than the ANS. In that
sense, the SES is more like the MB in showing erosion to ∼25 km
depth (Tsige, 2006). We suspect that much of this rapid erosion was
the product of Alpine-style glaciation. The SES lay at ∼30◦ S (Meert
and Lieberman, 2008) but glacial deposits are inferred from lowlatitude deposits as late as ∼580 Ma (e.g., Bingen et al., 2005). It is
likely that high-altitude regions such as the Ediacaran mountains
of the EAO were extensively glaciated during this time and perhaps
later, even though no clear signs of glaciation are known from the
SES.
In this study, we document the presence of reworked
Neoarchean/Paleoproterozoic crust (or sediment with abundant
∼2.5 Ga zircons) in the undeformed granite sample BSZ-04-4-3
from the Alghe Terrane. Zircons with similar ages (2489.3 ± 0.3 Ma;
2947 ± 2 Ma) were reported by Teklay et al. (1998) from E Ethiopia,
∼600 km NE of the SES, near 9◦ N, 42◦ E (Fig. 2). Such old ages are
much less common than Neoproterozoic ages from the Ethiopian
basement, but indicate that scattered remnants of older crust may
be present in the Ethiopian basement. Further geochronological
and isotopic work is needed to understand how important preNeoproterozoic crust has been for the evolution of the SES and
Ethiopian basement in general.
Finally, we note that there is no strong evidence of geochemical
evolution of igneous rocks with time shown in the potassium–silica
diagram using both our data and those of Teklay et al. (1998) (Fig. 5).
The youngest igneous rocks are enriched in incompatible elements,
as shown by the fact that all have >4% K2 O, but older rocks may
have high or low contents of incompatible elements. This further
indicates the complex magmatic history that the middle crust of
this region experienced during Proterozoic time.
6. Conclusions
New and existing geochronologic data from the Southern
Ethiopian Shield indicate that the rocks mostly formed in the middle crust and represent a Neoproterozoic addition to the crust,
although further studies are needed to assess the extent and role
of Archean components. Neoproterozoic crustal growth spanned
∼300 million years, from ∼850 Ma to ∼550 Ma. This age range is
indistinguishable from that seen for crustal growth of the ANS to the
north. This, together with the fact that SES basement includes a significant proportion of ophiolitic and volcano-sedimentary terranes,
indicates that the generation of juvenile Neoproterozoic crust in
the EAO affected regions well to the south of the ANS. On the other
hand, the abundance of high grade metamorphic rocks, along with
the recognition of Archean protoliths in the Alghae Terrane, indicate similarities to crust of the MB, and uplift and deep erosion
of the SES is readily ascribed to terminal collision between large
fragments of what became East and West Gondwana, with erosion aided by glaciation. Deformation in the SES related to terminal
collision along the EAO continued until 548 ± 8 Ma.
Acknowledgments
We dedicate this study to the memory of Dr. Lulu Tsige. Lulu
who worked extensively in the SES, and helped MGA collect the
samples in 2005. He passed away on September 5, 2011 of cancer
in Addis Ababa at the age of 45. This work was supported by NSFOISE grant #0804749 and a Blaustein-Cox fellowship at Stanford
R.J. Stern et al. / Precambrian Research 206–207 (2012) 159–167
University to RJS and a post-doctoral fellowship from Curtin University in Perth, Australia to KAA. The SHRIMP II facility in Perth
is operated jointly by Curtin University, the University of Western Australia and the Geological Survey of Western Australia, with
support from the Australian Research Council. The manuscript was
greatly improved following the constructive criticism of J. Jacobs,
editor P. Cawood, and two anonymous reviewers. This is UTD
Geosciences contribution number #1226, TIGeR publication #400,
and Missouri S&T Geology and Geophysics Program contribution
#41.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.precamres.2012.02.008.
References
Abdelsalam, M.G., Liégeois, J.P., Stern, R.J., 2002. The Saharan Metacraton. J. Afr. Earth
Sci. 34, 119–136.
Abdelsalam, M.G., Stern, R.J., 1996. Sutures and shear zones in the Arabian-Nubian
Shield. J. Afr. Earth Sci. 23, 289–310.
Abdelsalam, M.G., Tsige, L., Yihunie, T., Hussien, B., 2008. Terrane rotation during the
East African Orogeny: evidence from the Bulbul shear zone, southern Ethiopia.
Gondwana Res. 14, 497–508.
Avigad, D., Sandler, A., Kolodner, K., Stern, R.J., McWilliams, M., Miller, N., Beyth,
M., 2005. Mass-production of Cambro-Ordovician quartz-rich sandstone as a
consequence of chemical weathering of Pan-African terranes: environmental
implications. Earth Planet. Sci. Lett. 240, 818–826.
Avigad, D., Stern, R.J., Beyth, M., Miller, N., McWilliams, M.O., 2007. Detrital zircon
U–Pb geochronology of Cryogenian diamictites and Lower Paleozoic sandstone
in Ethiopia (Tigrai): age constraints on Neoproterozoic glaciation and crustal
evolution of the southern Arabian-Nubian Shield. Precambrian Res. 154, 88–106.
Bingen, B., Griffin, W.L., Torsvik, T.H., Saeed, A., 2005. Timing of Late Neoproterozoic glaciation on Baltica constrained by detrital zircon geochronology in the
Hedmark Group, south-east Norway. Terra Nova 17, 250–258.
Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil,
R., Campbell, I.H., Korsch, R.J., Williams, I.S., Foudoulis, C., 2004. Improved
206
Pb/238 U microprobe geochronology by the monitoring of a trace-elementrelated matrix effect; SHRIMP, ID-TIMS. ELA-ICP-MS and oxygen isotope
documentation for a series of zircon standards. Chem. Geol. 205 (1–2), 115–140.
Bonavia, F.F., Chorowicz, J., 1992. Northward expulsion of the Pan-African of northeast Africa guided by a reentrant zone of the Tanzanian Craton. Geology 20,
1023–1026.
Burke, K., Sengor, A.M.C., 1986. Tectonic escape in the evolution of the continental
crust. Am. Geophys. Union Geodyn. Ser. 14, 41–53.
Compston, W., Williams, I.S., Kirschvink, J.L., Zhang, M.G., 1992. Zircon U–Pb ages
for the early Cambrian time-scale. J. Geol. Soc. Lond. 149, 171–184.
Hofmann, C., Courtillot, V., Feraud, G., Rochette, P., Yirgu, G., Ketefo, E., Pik, R., 1997.
Timing of the Ethiopian flood basalt event and implications for plume birth and
global change. Nature 389, 838–841, doi:10.1038/39853.
Jacobs, J., Thomas, R.J., 2004. Himalayan-type indenter-escape tectonics model
for the southern part of the late Neoproterozoic-early Paleozoic East AfricanAntarctic orogen. Geology 32, 721–724.
Johnson, P.J., Abdelsalam, M.G., Stern, R.J., 2003. The Bir Umq-Nakasib Suture Zone
in the Arabian-Nubian Shield: a key to understanding crustal growth in the East
African Orogen. Gondwana Res. 6, 523–530.
Kazmin, V., 1975. The Precambrian of Ethiopia and some aspects of the geology of
the Mozambique Belt. Geophysical Observatory Bulletin, vol. 1. Addis Ababa,
Ethiopia, 14 pp.
Kazmin, V., Shifferaw, A., Balcha, T., 1978. The Ethiopian basement: stratigraphy and
possible manner of evolution. Geologische Rundshau 67, 531–546.
Kennedy, W.Q., 1964. The structural differentiation of Africa in the Pan-African
(±500 m.y.) tectonic episode. Annual Report of the Research Institute of African
Geology, vol. 8. University of Leeds, pp. 48–49.
167
Key, R.M., Charsley, T.J., Hackman, B.D., Wilkinson, A.F., Rundle, C.C., 1989. Superimposed upper Proterozoic collision-controlled origins in the Mozambique Belt of
Kenya. Precambrian Res. 44, 197–225.
Kröner, A., Stern, R.J., 2004. Pan-African Orogeny. Encyclopedia of Geology, vol. 1.
Elsevier.
Kusky, T.M., Abdelsalam, M., Stern, R.J., Tucker, R.D., 2003. Preface: evolution of the
East African and related orogens, and the assembly of Gondwana. Precambrian
Res. 123, 81–85.
Li, Z.X., Bogdanova, S.V., Collins, A.S., Davidson, A., De Waele, B., Ernst, R.E., Fitzsimons, I.C.W., Fuck, R.A., Gladkochub, D.P., Jacobs, J., Karlstrom, K.E., Lu, S.,
Natapov, L.M., Pease, V., Pisarevsky, S.A., Thrane, K., Vernikovsky, V., 2008.
Assembly, configuration, and break-up history of Rodinia: a synthesis. Precambrian Res. 160, 179–210.
Ludwig, K.R., 2001a. SQUID 1.02: A User’s Manual. Berkeley Geochronology Center,
Berkeley, CA, Special Publication No. 2, 19 pp.
Ludwig, K.R., 2001b. Users Manual for Isoplot/Ex Version 2.05. Berkeley Geochronology Center, Berkeley, CA, Special Publication No. 1a, 48 pp.
Meert, J.G., Lieberman, B.S., 2008. The Neoproterozoic assembly of Gondwana and
its relationship to the Ediacaran-Cambrian radiation. Gondwana Res. 14, 5–21.
Miller, N., Stern, R.J., Avigad, D., Beyth, M., Schilman, B., 2009. Cryogenian carbonateslate sequences of the Tambien Group. Northern Ethiopia (I) – pre-“Sturtian”
chemostratigraphy and regional correlations. Precambrian Res. 170, 129–156.
Nelson, D.R., 1997. Compilation of SHRIMP U–Pb Zircon Geochronology Data, 1996.
Geological Survey of Western Australia, Record 1997/2, 189 pp.
Peccerillo, A., Taylor, S.R., 1976. Geochemistry of Eocene Calc-alkaline volcanic rocks
from the Kastamonu area, northern Turkey. Contrib. Mineral. Petrol. 58, 63–81.
Squire, R.J., Campbell, I.H., Allen, C.M., Wilson, C.L., 2006. Did the Transgondwanan
Supermountain trigger the explosive radiation of animals on Earth? Earth Planet.
Sci. Lett. 250, 116–133.
Stern, R.J., 1994. Arc assembly and continental collision in the Neoproterozoic East
African Orogen: implications for the consolidation of Gondwanaland. Annu. Rev.
Earth Planet. Sci. Lett. 22, 319–351.
Tefera, M., Chernet, T., Haro, W., 1996. Geological Map of Ethiopia, second edition.
Regional Mapping Department of the Ethiopian Geological Survey.
Teklay, M., Kröner, A., Mezger, K., Oberhänsli, R., 1998. Geochemistry, Pb–Pb single
zircon ages and Nd–Sr isotope composition of Precambrian rocks from southern
and eastern Ethiopia: implications for crustal evolution in East Africa. J. Afr. Earth
Sci. 26, 207–227.
Tsige, L., 2006. Metamorphism and gold mineralization of the Kenticha-Katawicha
area: Adola belt, southern Ethiopia. J. Afr. Earth Sci. 45, 16–32.
Tsige, L., Abdelsalam, M.G., 2005. Neoproterozoic–Early Paleozoic gravitational tectonic collapse in the southern part of the Arabian-Nubian Shield: the Bulbul Belt
of southern Ethiopia. Precambrian Res. 138, 297–318.
Worku, H., Yifa, K., 1992. The tectonic evolution of the Precambrian metamorphic
rocks of the Adola belt, southern Ethiopia. J. Afr. Earth Sci. 14, 37–55.
Worku, H., Schandelmeier, H., 1996. Tectonic evolution of the Neoproterozoic Adola
Belt of southern Ethiopia: evidence for a Wilson Cycle process and implications
for oblique plate collision. Precambrian Res. 77, 179–210.
Williams, I.S., 1998. U–Th–Pb geochronology by ion microprobe. Rev. Econ. Geol. 7,
1–35.
Worku, H., 1996. Geodynamic development of the Adola Belt (southern Ethiopia) in
the Neoproterozoic and its control on gold mineralisation. Ph.D. thesis. Berlin
Technical University, 156 pp.
Xie, L.W., Zhan, Y., Zhnag, H., Sun, J., Wu, F., 2008. In situ simultaneous determination
of trace elements. U–Pb and Lu–Hf isotopes in zircon and beddeleyite. Chin. Sci.
Bull. 53, 1565–1573.
Yibas, B., Reimold, W.U., Armstrong, R., Koeberl, C., Anhaesseur, C.R., Phillips, D.,
2002. The tectonostratigraphy, granitoid geochronology and geological evolution of the Precambrian of southern Ethiopia. J. Afr. Earth Sci. 34, 57–84.
Yihunie, T., Tesfaye, M., 2002. Structural evidence for the allochthonous nature of the
Bulbul terrane in southern Ethiopia: a west-verging thrust nappe. J. Afr. Earth
Sci. 34, 85–93.
Yihunie, T., Adachi, M., Takeuchi, M., 2004. P–T conditions of Metamorphism in the
Neoproterozoic rocks of the Negele Area, southern Ethiopia. Gondwana Res. 7,
489–500.
Yihunie, T., Adachi, M., Yamamoto, K., 2006. Geochemistry of the Neoproterozoic
metabasic rocks from the Negele area, southern Ethiopia: tectonomagmatic
implications. J. Afr. Earth Sci. 44, 255–269.
Yihunie, T., 2003. Chemical Th–U-total Pb isochron ages of zircon and monazite from
granitic rocks of the Negelle area, southern Ethiopia. J. Earth Planet. Sci. Nagoya
Univ. 50, 1–12.