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Precambrian Research 206– 207 (2012) 159– 167

Contents lists available at SciVerse ScienceDirect

Precambrian Research

journa l h omepa g e: www.elsev ier .com/ locate /precamres

–Pb zircon geochronology of the eastern part of the Southern Ethiopian Shield

.J. Sterna,∗, Kamal A. Alib, Mohamed G. Abdelsalamc, Simon A. Wilded, Qin Zhoue

Geosciences Dept., U Texas at Dallas, Box 830688, Richardson, TX 75083-0688, USADepartment of Mineral Resources and Rocks, Faculty of Earth Sciences, King Abdulaziz University, P.O. Box 80206, Jeddah 21589, Saudi ArabiaGeological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USADepartment of Applied Geology, Curtin University, Perth, 6845 WA, AustraliaInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10029, China

r t i c l e i n f o

rticle history:eceived 5 October 2011eceived in revised form 2 February 2012ccepted 12 February 2012vailable online xxx

eywords:ast African Orogeneoproterozoicthiopia–Pb zircon geochronologyan-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 Neopro-terozoic (880–550 Ma) largely greenschist-facies juvenile crust of the Arabian-Nubian Shield in thenorth 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 zirconSHRIMP and LA-ICP-MS dating of 8 igneous and metamorphic intermediate and felsic bodies in the east-ern 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), andPan-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 sampleof undeformed granite from the Alghae Terrane has abundant Archean zircons and may be a ∼550 Mamelt of ∼2.5 Ga crust, demonstrating for the first time that Archean crust or sediments with abundantArchean zircons exists in the SES. In spite of ∼300 million years of Neoproterozoic igneous activity, wesee no evidence of systematic compositional evolution in SES igneous rocks from early low-K suites tolate 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 resultof collision between large fragments of East and West Gondwana. Terminal collision began at ∼630 Maand caused crustal thickening, melting, uplift, erosion, orogenic collapse, and tectonic escape over a broadregion of the East African Orogen, including up to ∼25 km of erosion of SES crust. Plate convergence waslikely continuous from ∼630 Ma, forming major N-S structures. Deformation stopped at ∼550 Ma and

tion b

was followed by exhuma

. Introduction

The East African Orogen (EAO) is one of Earth’s great defor-ation belts, stretching ∼6000 km N-S along the eastern flank offrica (Fig. 1a). Evolution of the EAO reflects a Neoproterozoic-ambrian Wilson Cycle that began with the breakup of Rodinia870–800 Ma: Kusky et al., 2003; Li et al., 2008) and led to the finalmalgamation of Gondwana in Cambrian time. The EAO was even

onger before rifting truncated it in the north and opening of theouthern Ocean removed ∼2000 km of the orogen in Antarcticathus reconstructed, the orogen is sometimes called the East

∗ Corresponding author.E-mail addresses: rjstern@utdallas.edu (R.J. Stern), alik6588@yahoo.com (K.A.

li), abdelsam@mst.edu (M.G. Abdelsalam), S.Wilde@curtin.edu.au (S.A. Wilde),houqin@mail.iggcas.ac.cn (Q. Zhou).

301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2012.02.008

etween ∼ 530 and 500 Ma.© 2012 Elsevier B.V. All rights reserved.

African-Antarctic Orogen or EAAO; Jacobs and Thomas, 2004). Thenorthern EAO consists of the Arabian-Nubian Shield (ANS), com-posed largely of juvenile Neoproterozoic crust flanking the RedSea (Fig. 1b), whereas the southern part comprises the Mozam-bique Belt (MB) with predominantly reworked older crust. TheANS contains abundant ophiolite fragments, in contrast to the MB,where ophiolites are not found. The MB is where Ediacaran colli-sion between large continental blocks of East and West Gondwanawas most intense, where deep erosion has exposed medium- tohigh-grade gneisses and granulites, and where great mountainsrose and were eroded in Late Ediacaran and Early Paleozoic time(Squire et al., 2006). Crust formation in the EAO encompassednearly all of Neoproterozoic (∼900–540 Ma) and much of EarlyPaleozoic time. The final stage of EAO evolution witnessed colli-

sion of large continental blocks: the Congo and Kalahari cratonsand Saharan metacraton in the west, the Indian and Australiancratons and Cryogenian continental lithosphere of E Arabia in

160 R.J. Stern et al. / Precambrian Research 206– 207 (2012) 159– 167

Fig. 1. (a) Location of the East African Orogen at the juncture between East andWest Gondwana, reconstructed prior to the Jurassic and younger rifting. Note thelocation of Southern Ethiopian Shield at the transition between the Arabian-NubianShield and Mozambique Belt. (b) Location of the Southern Ethiopian Shield near thej

M

tfGscifpaet

ephmP

Precambrian Basement

Mesozoic SedimentsOligocene - Miocene VolcanicsMiocene - Quaternary Volcanics

Tertiary & Younger Sediments

12°N

8°

4°

36°E 40° 44° 48°

Sudan

Erit rea

East African Rift

SouthSudan

Kenya

Somalia

Somalia

South Ethiopian ShieldFig. 3

Fig. 2. Geologic map of Ethiopia, showing the location of the Southern EthiopianShield. White lines and irregular shapes are rivers and lakes, respectively. Rectangle

Several models are proposed to explain the rela-

unction of the Arabian-Nubian Shield and the Mozambique Belt.

odified after Tsige and Abdelsalam (2005).

he east (Stern, 1994; Squire et al., 2006). This collision was keyor forming the Neoproterozoic/Cambrian supercontinent Greaterondwana. In addition to thickening the EAO crust, this colli-ion caused significant but unquantified along-strike transport ofrust via tectonic escape (Stern, 1994). Both EAO segments werentruded by countless Neoproterozoic and Early Paleozoic plutonsrom ∼880 to ∼500 Ma. These igneous bodies and their metamor-hosed equivalents provide opportunities to obtain radiometricges and chemical compositions that can be used to infer tectonicvolution of the EAO, which is the strategy we use in this contribu-ion.

The nature of the transition between the MB and ANS – whichncompasses the region now occupied by Ethiopia and Kenya – isoorly defined. This is largely because the basement rocks of Kenya

ave not been studied with modern isotopic and geochronologicethods and because Ethiopian basement is mostly buried beneath

hanerozoic sediments and flood basalts of the Ethiopian Plateau,

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 southernEthiopia where EAO basement is exposed, which we call the South-ern Ethiopian Shield (SES; Figs. 1b and 2). The SES is situated at thejunction of the ANS and MB and has affinities to both. Studying SESrocks provides an opportunity to better understand the nature ofthe ANS-MB transition and thus the evolution of the EAO in spaceand time.

2. The Southern Ethiopian Shield

The SES consists of granite-gneiss basement interleavedwith three belts (Megado, Kentisha and Bulbul) of low-gradegreenschist-facies ophiolitic–volcano–sedimentary rocks (Fig. 3;Yibas et al., 2002). The granite-gneiss terrane consists of grani-toids and quartzofeldspathic para- and orthogneiss, intercalatedwith amphibolite and sillimanite–kyanite-bearing schist. Gran-itoids vary from undeformed to strongly foliated plutons andrange 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). Theophiolitic–volcano–sedimentary belts are composed of mafic,ultramafic and metasedimentary rocks thought to mark Cryogenianarc–arc sutures overprinted by Ediacaran deformation (Abdelsalamand 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 indi-cate peak metamorphism at 630–650 ◦C and 7 kbar, correspondingto a depth of ∼25 km (Tsige, 2006). Yihunie et al. (2004) doc-umented three episodes of metamorphic mineral growth in theAlghe terrane – up to 590–640 ◦C and 7 kbar – and inferred thatthese accompanied deformation. In contrast, the ophiolitic beltswere subjected to lower T and P metamorphism (480–520 ◦C and4–5 kbar) indicating 13–17 km depth in the Kenticha terrane. Onthis basis, Yihunie et al. (2004) concluded that Kenticha and Algheterrane protoliths were metamorphosed and brought up from themid-crust before juxtaposition with the Kenticha sequence along aN-S trending thrust, during or after terminal EAO collision.

tionship between the granite-gneiss basement and theophiolite–volcano–sedimentary belts: (1) Worku and Yifa (1992),and Worku and Schandelmeier (1996) suggested that the Megado,

R.J. Stern et al. / Precambrian Resear

40°38°E 39°

4°

5°

6°N

High-grade gneisses and migmatitesLow-grade volcano-sedimentary and mafic-ultramafic rocks

Granitoids

Didesa-Adola SZ

Fig. 4

7

1

1

7

7

NormalFault

Strike-Slip

Synform

Antiform

Thrust

100 km

2

3

4,5

6,14Kenya

Ethiopia

7

8

9,10,11

12

13

Adola Fold-and-thrustBelt

EA B

C

DF

G

1515

Fig. 3. Geology and geochronology of the Southern Ethiopian Shield. Principal tec-tonic elements (after Tsige and Abdelsalam, 2005): A = Megado belt; B = Kentichabelt; C = Moyale belt; D = Bulbul belt; E = Wadera shear zone; F = Bulbul shear zone;G = Alghe Terrane/Melka Guba Domain. Previous zircon and Ar geochronologicalresults: 1 = Metoarbasebat granite magmatic age U–Pb zircon SHRIMP 526 ± 5 Ma;biotite 40Ar/39Ar 506 ± 4 Ma; hornblende 40Ar/39Ar 511 ± 4 Ma cooling ages (Yibaset al., 2002); 2 = Meleka granodiorite gives 3 groups of U–Pb zircon SHRIMP ages:610 ± 9 Ma; 560 ± 8 Ma; 489–515 Ma. Biotite 40Ar/39Ar 534.5 ± 8 Ma, 512 ± 4 Ma;muscovite 40Ar/39Ar 515 ± 3 Ma (Yibas et al., 2002); 3 = Digati diorite gneiss emplace-ment age 570 ± 7 Ma U–Pb zircon SHRIMP; biotite 40Ar/39Ar 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 zir-con 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 = Alghaeaugengneiss 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 evapora-tion (Teklay et al., 1998); 13 = Amphibolitic ophiolitic metagabbro magmatic age700.5 ± 1.3 Ma Pb–Pb zircon evaporation (Teklay et al., 1998); 14 = Moyale trond-hjemite 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,mo

KgafgdG1a

eations that plunge NE in the north and SE in the south. Kinematic

agmatic age 721 ± 12 Ma U–Pb zircon SHRIMP; 728.6 ± 0.6 Ma Pb–Pb zircon evap-ration (Teklay et al., 1998).

enticha and Bulbul belts were preserved between the granite-neiss rocks as N-trending synforms resulting from folding of

once coherent E-verging nappe about N-trending axes. Theyurther inferred that the these synforms and complementaryneissic antiforms represent N-trending “shortening zones”eveloped due to terminal collision between fragments of East

ondwana and the Saharan metacraton (Abdelsalam and Stern,996; Abdelsalam et al., 2002; Avigad et al., 2007). Addition-lly, this model implies that the MB-ANS in southern Ethiopia

ch 206– 207 (2012) 159– 167 161

represents a “basement–cover” relationship in which reworkedcontinental crust of the MB extends northward beneath thejuvenile crust of the ANS. (2) Yihunie and Tesfaye (2002), Yihunie(2003) and Yihunie et al. (2004) examined the northern part ofthe Bulbul belt and proposed that it evolved from a SW-vergingfold and thrust belt to N-trending dextral strike-slip shear zonedue to NE-SW directed oblique collision between the Alghe andthe Bulbul terranes. (3) Different from the previous constructionaldeformation style model, Burke and Sengor (1986), Bonavia andChorowicz (1992) and Stern (1994) interpreted the Megado,Kenticha and Bulbul belts as strike-slip shear zones representingescape routes of northward expulsion of the ANS from the MBbecause of continental collision between East and West Gondwanaalong the MB. (4) Tsige and Abdelsalam (2005) used the presenceof SE-plunging stretching lineations and kinematic indicatorsshowing top-to-the-southeast tectonic transport to propose thatthe southern Bulbul belt developed by regional gravitation tectoniccollapse where the Bulbul terrane slipped east relative to the Algheterrane. (5) To reconcile the along-strike variation in structuralstyle within the Bulbul belt (SW-verging fold and thrust belt in thenorth, dextral strike-slip shear zone in the center, and low-angletop-to-the-southeast normal fault in the south), Abdelsalam et al.(2008) proposed an anti-clockwise terrane rotation during NW-SEoblique collision between the Alghe and Bulbul terranes.

There are two modern geochronological studies of SES base-ment published in the peer-reviewed literature. Teklay et al. (1998)used Pb–Pb zircon evaporation techniques to date 8 samples ofgneissic and granitic rocks (Fig. 3). They identified 3 importantmagmatic periods in the SES: ∼850, ∼750–700, and ∼650–550 Ma.Teklay et al. (1998) found pre-Neoproterozoic zircons of ∼1125 Maand ∼1660 Ma in 605 Ma meta-rhyolite. In the second study, Yibaset al. (2002) identified 5 major tectonic episodes (1160–1030 Ma,875 Ma, 770–720 Ma, 700–550 Ma, and 550–500 Ma, accompa-nied 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 struc-ture of this region is dominated by the N-trending Bulbul ShearZone, which separates the Bulbul terrane in the east from the Algheterrane to the west (Fig. 4). The Alghe terrane (Yihunie and Tesfaye,2002) represents the southern continuation of the Alghe gneissesof Kazmin (1975) and Kazmin et al. (1978) and is the equivalentof the Zambaba domain of Worku and Schandelmeier (1996). Thisterrane is dominated by hornblende and quartzofeldspathic gneissand granitic migmatite. These rocks show well-developed NNW- toNNE-striking gneissic layering and are intruded by deformed andundeformed granitoids. The gneissic layering is disrupted by earlyrootless intrafolial folds and late map-scale NNE-trending openantiforms (Abdelsalam et al., 2008).

The Bulbul terrane (Yihunie and Tesfaye, 2002) is dominated byamphibolite schists, ultramafic slivers, and pre- and syn-tectonicgranitoids, all showing N- to NNW-trending, moderately E- andENE-dipping foliation. This foliation is cut by N- to NNW-trendingcleavage associated with W- and WSW-verging folds and thrusts.The intersection of foliation and cleavage produces strong sub-horizontal pencil structure.

The Bulbul Shear Zone is a 100 km long, N-S trending belt that is5–7 km wide. The northern extension of the shear zone is intrudedby a granitic body whereas its southern part terminates againsta bow-shaped dextral strike slip shear zone (Fig. 3). This zoneis dominated by E-dipping mylonitic fabric with stretching lin-

indicators show that the northern Bulbul Shear Zone representsa SW-verging fold-and-thrust belt; its central part is dominatedby dextral strike–strike shearing, and top-to-the-southeast normal

162 R.J. Stern et al. / Precambrian Research 206– 207 (2012) 159– 167

Fo

fIn

ibtlimttf

3

tco(ZtwssaaUaiupc1

Fig. 5. K2O vs. SiO2 plot of analyzed samples, series after Peccerillo and Taylor

ig. 4. Simplified geological map of study area showing sample localities. Locationf this map is shown by dashed rectangle in Fig. 3.

aulting characterizes the southern part (Abdelsalam et al., 2008).n addition, many parts of the shear zone also show co-axial buton-co-planar complex folds reflecting strong E-W shortening.

Samples were collected in the course of detailed structural stud-es by Abdelsalam et al. (2008) in a ∼55 km × 30 km region boundedy 5◦15′–4◦45′N, 39◦15′–39◦30′E (Fig. 4). The study area is a rela-ively small part of the SES but the 8 analyzed samples are fromithologic units that are broadly representative of the SES, includ-ng gneiss of the Alghe terrane in the west and greenschist-facies

etavolcanic and metasedimentary rocks of the Bulbul Terrane inhe east. Field photographs of typical lithologies and structures inhe Alghe and Bulbul terranes, and the Bulbul Shear Zone can beound in Abdelsalam et al. (2008).

. Analytical methods

Eight samples for age determination were collected at loca-ions (determined by GPS) provided in Section 4. Samples wererushed at UT Dallas. Powdered samples were analyzed for majorxides and some trace elements (Ba, Sr, Y, Zr, Sc, and V) at Actlabshttp://www.actlabs.com/) using routine ICP fusion techniques.ircons were isolated at UT Dallas by crushing, magnetic separa-ion, and heavy liquids. Grains from the non-magnetic fractionsere hand-picked, mounted on double-sided adhesive tape, and

et in EpirezTM resin along with several grains of Sri Lanka zircontandard (CZ3) which has a conventionally measured 206Pb/238Pbge of 563 Ma (Nelson, 1997). The mounted zircons were groundnd polished to effectively cut them in half and then gold-coated.–Th–Pb analyses were performed on 1 sample (BSZ-04-1-4) using

SHRIMP II ion microprobe at Curtin University in Australia follow-ng techniques described by Nelson (1997) and Williams (1998)

tilizing five-cycle runs through the mass stations. Another 7 sam-les were analyzed for U–Th–Pb concentrations and Pb isotopicompositions using an Agilent 7500a Q-ICP-MS connected to a93 nm Excimer laser ablation system at the Institute of Geology

(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 isreferred to the web version of the article.)

and Geophysics, Chinese Academy of Sciences, Beijing, followingtechniques described by Xie et al. (2008). For the SHRIMP session, atotal 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 sigmavariation 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 theGJ-1 standard. The relative standard deviations of reference valuesfor 91500 were set at 2%. Data were processed using SQUID (Ludwig,2001a) and Isoplot (Ludwig, 2001b) software. Ages >1000 Ma arecalculated from 207Pb/206Pb and ages <1000 Ma are calculated from206Pb/238U because young zircons provide low count rates on the207Pb peak, thus increasing uncertainties (Black et al., 2004). Bothdata sets were corrected using the measured 204Pb (Compston et al.,1992).

4. Results

Geochemical results are listed in Table 1. Fig. 5, modified afterPeccerillo and Taylor (1976), enables identification of the mafic,intermediate, and felsic components. Analytical results for zirconsare listed in Tables A1 (ion probe results) and A2 (LA-ICP-MS)(supplementary material). Whole rock abundances of SiO2 and K2Oare also summarized below for each sample. SiO2 contents indi-cate whether igneous (sometimes metamorphic) rocks are mafic(derived by melting of the mantle) or felsic (generated by crustalmelting or by fractionation of mafic melts). In contrast, K2O con-tents reflect if the source – whether mantle or crust – was enrichedor depleted. Considered together, these two elements approximatethe most important aspects of igneous rock compositions. Compo-sitional 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-Kgranodiorite (∼59% SiO2, ∼1.3% K2O; Fig. 5) from the north-ern Bulbul Shear Zone (Fig. 4). This pluton is deformed, with awell-developed mylonitic fabric striking 010◦ and dipping 60◦E,along with a NE-trending stretching lineation. Three data pointsare discordant and were not used in age calculations (Fig. 6a).Nine data points are concordant and cluster together yieldinga weighted mean 206Pb/238U 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 amphibo-lite schist from the Bulbul Terrane (Fig. 4) with a well-developedschistosity that strikes 020◦ and dips 70◦E. It contains ∼55%

R.J. Stern et al. / Precambrian Research 206– 207 (2012) 159– 167 163

Table 1Geochemistry of Southern Ethiopian Shield basement rocks.

Sample 1–4 1–5 2–1A 2–1B 2–1C 3–2 3–5 4–3 5–1

Lithology Granodiorite Amphibolite Gneiss Granite Felsic dike Granodiorite Gneiss Granite GranodioriteAge (Ma) 851 659 560 548 665 731 ∼2500 840Major elements (wt. %)

SiO2 58.99 55.03 68.4 73.9 76.76 73.77 71.73 65.07 76.36TiO2 0.79 0.18 0.44 0.17 0.06 0.18 0.53 0.83 0.12Al2O3 18.43 15.46 15.16 13.91 13.42 13.11 13.79 15.5 13.01Fe2O3T 6.22 9.1 4.47 2.19 1.26 3.56 3.84 4.29 1.16MnO 0.09 0.15 0.06 0.03 0.02 0.04 0.06 0.05 0.03MgO 2.72 5.87 1.81 0.46 0.09 1.3 0.65 1.32 0.22CaO 6.12 7.45 3.3 1.61 1.21 2.43 1.86 2.37 1.05Na2O 4.41 3.68 4.23 3.14 3.97 3.67 2.98 4.47 3.66K2O 1.26 1.78 1.79 4.63 3.99 1.53 4.18 4.07 4.4P2O5 0.2 0.12 0.18 0.11 0.02 0.04 0.21 0.42 0.04LOI 0.71 1.15 0.46 0.42 0.11 0.21 0.45 0.58 0.83

Total 99.94 99.97 100.3 100.57 100.91 99.84 100.28 98.97 100.88Trace elements (ppm)

Ba 531 172 262 922 99 478 956 1849 886Sr 536 251 404 271 55 158 160 854 184Y 10 29 9 9 5 9 33 8 3Sc 11 27 5 2 2 3 7 5 2Zr 147 157 154 244 12 109 295 337 61V 109 183 62 28 <5 33 32 61 9

206 Pb/238 U

207Pb/235U

1000

900

800

700

600

500

0.05

0.07

0.09

0.11

0.13

0.15

0.17

0.5 0.7 0.9 1.1 1.3 1.5

(d)BSZ-04-2-1C

206Pb/238U weighted mean age = 548 ± 8 Ma (N = 4)(MSWD = 0.32) data-point error ellipses are 2σ

(b)BSZ-04-1-5820

780

740

700

660

620

0.095

0.105

0.115

0.125

0.135

0.7 0.8 0.9 1.0 1.1 1.2 1.3

206Pb/238U weighted mean age = 657 ± 6 Ma (N = 12)(MSWD = 0.76) data-point error ellipses are 2σ

206Pb/238U weighted mean age = 762 ± 8 Ma (N = 6)(MSWD = 0.39) data-point error ellipses are 2σ

920

880

800

0.128

0.132

0.136

0.140

0.144

0.148

0.152

0.156

1.15 1.25 1.35 1.45 1.55

BSZ-04-1-4(a)

840

206Pb/238U weighted mean age = 847 ± 11 Ma (N = 9)(MSWD = 0.55) data-point error ellipses are 2σ

BSZ-04-2-1B(c)

250

350

450

550

650

750

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60.02

0.04

0.06

0.08

1.00

0.12

0.14

206Pb/238U weighted mean age = 560 ± 8 Ma (N = 3)(MSWD = 0.91) data-point error ellipses are 2σ

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-04-2-1Bundeformed 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.

1 Resear

3

4

5

6

64 R.J. Stern et al. / Precambrian

SiO2 and ∼1.8% K2O and plots on the boundary betweenmafic-intermediate and medium- to high-K calc-alkaline suites(Fig. 5). We interpret this as having formed from a meltprior to being metamorphosed to amphibolite facies (ortho-amphibolite). Twenty zircons were analyzed. Two data pointsare discordant and were not used in age calculations (Fig. 6b).Twelve data points yield a weighted mean 206Pb/238U age of657 ± 6 Ma (MSWD = 0.76), which we interpret as the time ofcrystallization. The remaining six analyses are concordant witha weighted mean 206Pb/238U age of 762 ± 8 Ma (MSWD = 0.39),which we interpret as the age of xenocrysts (Fig. 6b). Alter-natively, the igneous rock could have formed at ∼760 Ma andthen been metamorphosed ∼660 Ma, as suggested by the factthat Th/U for ∼660 Ma zircons = 0.60 ± 0.21 (1�), significantlylower than Th/U = 1.02 ± 0.35 for ∼760 Ma zircons. Further workis 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 intrudesthe Alghe Terrane. It is a felsic, high-K calc-alkaline igneousrock (∼74% SiO2 and ∼4.6% K2O; Fig. 5). The body has a well-developed foliation that is concordant with layering in thesurrounding gneiss. One analysis was conducted for each of 18zircon grains. Twelve data points are discordant and were notused in the calculations, whereas 3 concordant analyses yieldeda weighted mean 206Pb/238U age of 560 ± 8 Ma (MSWD = 0.91),which we interpret as the best estimate of the age of crystalliza-tion (Fig. 6c). The remaining three analyses are concordant andyield 206Pb/238U ages of 613 ± 8 Ma and 716 ± 9 Ma, also inter-preted as xenocrysts. Spot 18 yields a younger 206Pb/238U age(540 ± 8) than the other 18 analyses, which may record Pb-lossor 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. Thisdike is a felsic, high-K calc-alkaline igneous rock (∼77% SiO2 and∼4% K2O; Fig. 5). One analysis was conducted for each of 9 zircongrains. Three young data points are discordant and were omittedfrom the age calculation (Fig. 6d), whereas 4 data points are con-cordant and yield a weighted mean 206Pb/238U age of 548 ± 8 Ma(MSWD = 0.32), which we interpret as the crystallization age ofthe dike. The remaining two analyses are concordant and yield206Pb/238U ages of 880 ± 13 Ma and 884 ± 13 Ma, which we inter-pret as xenocrysts.

. BSZ-04-3-2 (05◦02′26.3′′N; 39◦28′01.8′′E): This sample is from ahighly sheared granodiorite body in the Bulbul Terrane (Fig. 4). Itis a medium-K felsic igneous rock (∼74% SiO2, ∼1.5% K2O; Fig. 5).This pluton has a mylonitic fabric that strikes 010◦ and dips50◦E. One analysis was conducted for each of 8 zircons. One datapoint is discordant and was not used in age calculations (Fig. 7a).Six concordant analyses yield a weighted mean 206Pb/238U ageof 665 ± 8 Ma (MSWD = 0.096), which we interpret as when therock crystallized. The remaining analysis gives a 206Pb/238U ageof 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 froma quartzo-feldspathic gneiss at the contact between the Bul-bul Shear Zone and the Bulbul terrane. The dated sample is afelsic, high-K calc-alkaline metamorphic rock (∼72% SiO2 and∼4.2% K2O; Fig. 5) that probably formed from an igneous pro-tolith (orthogneiss). It has a well-developed mylonitic fabric thatstrikes 020◦ and dips 50◦E and also a well-developed stretchinglineation (azimuth 066◦, plunge 28◦). One analysis was con-ducted on each of 36 zircon grains. Seven analyses are discordant

and were excluded from age calculations (Fig. 7b). Twenty-fivedata points cluster around a weighted mean 206Pb/238U age of732 ± 5 Ma (MSWD = 0.75). We interpret these zircons to haveformed by crystallization from a melt, as indicated by mean

ch 206– 207 (2012) 159– 167

Th/U = 0.60 ± 0.24 (1�), so this age is taken to reflect when theprotolith crystallized. The four remaining analyses are concor-dant but younger; one (spot 36) yields a 206Pb/238U age of699 ± 12 Ma and three others yield a weighted mean 206Pb/238Uage of 605 ± 11 Ma (MSWD = 0.80), perhaps reflecting metamor-phic recrystallization, an interpretation supported by the lowTh/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 froman undeformed granite intruding the Alghe Terrane. It has∼65% SiO2 and ∼4% K2O, and plots near the boundary betweenintermediate and felsic igneous rocks with high-K calc-alkalineaffinities (Fig. 5). Twenty-four zircons were analyzed and con-cordant ages range from 2505 ± 36 Ma to 568 ± 9 Ma. Three ofthese yield a weighted mean 207Pb/206Pb age of 2514 ± 18 Ma(MSWD = 0.18). The youngest ages are identical to the weightedmean age of sample BSZ-04-2-1B; a granite that also intrudes theAlghe terrane. It is not clear if this is a ∼2.5 Ga igneous rock meta-morphosed in Late Neoproterozoic time or a Late Neoproterozoicmelt of Archean crust, but the fact that the body is undeformedsuggests that it is an Ediacaran intrusion that was generated bymelting of ∼2.5 Ga crust or sediments with a concentration ofsuch zircons.

8. BSZ-04-5-1 (04◦55′51.3′′N; 39◦26′59.4′′E): This sample is from agranodiorite at the contact between the Bulbul Shear Zone andthe Alghe Terrane. It is a felsic, high-K calc-alkaline igneous rock(∼76% SiO2 and ∼4% K2O; Fig. 5). It has both a well-developedmylonitic fabric (strike 020◦, dip 50◦E and stretching lineation(azimuth 075◦, plunge 45◦). One analysis was conducted for eachof 14 grains. Six were discordant and were omitted from agecalculations (Fig. 7d). Four data points yield a weighted mean206Pb/238U age of 855 ± 14 Ma (MSWD = 0.28) which we inter-pret as the time of intrusion; three points are discordant aroundthis age and may reflect Pb-loss. The remaining four concor-dant ages gave scattered 206Pb/238U ages of 941 ± 16, 800 ± 14,710 ± 14 and 648 ± 12 Ma. The oldest of these ages may be axenocryst, whereas the three younger ones may reflect youngerthermal disturbance.

5. Discussion

SES exposures provide a chance to examine the middle EAOcrust, 13–25 km deep, as it existed at ∼550 Ma when deformationstopped. Because it juxtaposed metamorphic rocks of very differentgrade, it is likely that prior to terminal collision, the present steepcontact between lower grade ophiolitic–volcano–sedimentarybasins and higher grade granite-gneiss middle crust was subhor-izontal, so that gneiss and low grade sequences were separatedby a shear zone that facilitated lateral movements of the uppercrustal “superstructure” relative to the mid- and lower crust“infrastructure” (Abdelsalam et al., 2008). Such shear zones wouldhave likely concentrated fluids, including those responsible forgold mineralization in the area (Tsige, 2006). Thus the resultspresented above should be viewed in the context of a mid-crustal environment, with the possible exception of the youngestintrusions.

These results largely confirm conclusions from earliergeochronologic studies that the SES is largely a juvenile Neo-proterozoic crustal addition. With the exception of ∼2.5 Ga zirconsin BSZ-04-4-3, the inferred crystallization ages are 851, 840,731, 665, 659, 560, and 548 Ma. The dominantly Neoproterozoicnature of the crust in the study area is also confirmed (again

with the exception of ∼2.5 Ga material) by the fact that zirconxenocrysts, 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– 167 165

207Pb/235U

206 Pb/238 U

(a) BSZ-04-3-2900

860

820

780

740

700

660

620

5800.09

0.11

0.13

0.15

0.7 0.9 1.1 1.3 1.5

206Pb/238U weighted mean age = 665 ± 8 Ma (N = 6)(MSWD = 0.096) data-point error ellipses are 2s

BSZ-04-4-3(c)

2600

2200

1800

1400

1000

600

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14

Youngest concordant age = 568 ± 9 Ma

207Pb/206Pb weighted mean age = 2514 ± 18 Ma (N = 3)(MSWD = 0.18) data-point error ellipses are 2s

(c)BSZ-04-3-5 800

720

680

640

600

560

0.085

0.095

0.105

0.115

0.125

0.135

0.6 0.8 1.0 1.2 1.4

760

206Pb/238U weighted mean age = 732 ± 5 Ma (N = 25)(MSWD = 0.75) data-point error ellipses are 2s

206Pb/238U weighted mean age = 605 ± 11 Ma (N = 3)(MSWD = 0.80) data-point error ellipses are 2s

950

850

750

650

550

0.08

0.10

0.12

0.14

0.16

0.18

0.6 0.8 1.0 1.2 1.4 1.6

BSZ-04-5-1(d)

206Pb/238U weighted mean age = 855 ± 14 Ma (N = 4)(MSWD = 0.28) data-point error ellipses are 2s

F 4-3-5B mbol

SectOeTTw8gEctta∼(se

as(

ig. 7. U–Pb concordia plots. (A) BSZ-04-3-2 Alghe sheared granodiorite; (B) BSZ-0SZ-04-5-1 Bulbul-Alghe mylonitic granodiorite. All error ellipses are 2�. Dotted sy

ES mostly comprises juvenile Neoproterozoic crust. We see novidence, either in crystallization or xenocryst ages, to support theonclusion that significant tracts of 1160–1030 Ma crust (Adolaectonothermal event of Yibas et al., 2002) are present in the SES.ur zircon crystallization ages mostly agree with the 3 magmaticpisodes (∼850, ∼750–700, and ∼650–550 Ma) identified byeklay et al. (1998) for the SES. However, considering the data ofeklay et al. (1998) and Yibas et al. (2002) along with our own,e infer 4 principal magmatic episodes in the region (Fig. 8):

40–890 Ma (Late Tonian-Early Cryogenian); 790–700 Ma (Cryo-enian), ∼660 Ma (Moyale Event), and Pan-African (630–500 Ma;diacaro-Cambrian). The first episode marks the beginning ofrust formation and encompasses the 876 ± 5 Ma Bulbul–Awataectonothermal event, which Yibas et al. (2002) correlated withhe Samburian-Sabachian episode in Kenya (Rb–Sr whole-rockges of Key et al., 1989). The first two episodes overlap with0.85–0.74 Ga Tsaliet arc magmatism in NE Ethiopia and Eritrea

Avigad et al., 2007). The Cryogenian episode is essentially theame as the 770–720 Ma Megado tectonothermal event of Yibast al. (2002).

There is no evidence for the distinctive ∼720–750 Ma carbon-tes and other sediments of the Tambien Group, which defineynformal keels in the Neoproterozoic basement of NE EthiopiaMiller et al., 2009). It is unknown whether the Tambien was

Bulbul quartzo-feldspathic gneiss; (C) BSZ-04-4-3 undeformed Alghe granite; (D)s are excluded from age calculations, as discussed in text.

removed by erosion (more extensive in the SES than in NEEthiopia) or was never deposited on SES crust.

The ∼660 Ma “Moyale” event seems to have been impor-tant for SES evolution, including both granitic intrusions (likethat of Moyale granodiorite) and formation of amphibolite schist(BSZ-04-1-5) of this study. There may be a range of ages for amphi-bolite protoliths; Teklay et al. (1998) report ages of 700.5 ± 1.3 Mafor amphibolite from the Moyale area, and Yibas et al. (2002)note that amphibolite xenoliths are common in the ∼650 MaMoyale granodiorite. Worku (1996) reported a Sm–Nd whole- rockage 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 bythis event, and to identify the full age range of amphibolites andophiolitic rocks.

SES rocks studied here also provide insights into magma-tism and deformation accompanying continental collision betweenfragments of East Gondwana and the Congo-Tanzanian craton –Saharan metacraton, beginning at ∼630 Ma. These events formedthe EAO (Fig. 1B) and other Ediacaran orogens referred to as Pan-

African (Kennedy, 1964; Kröner and Stern, 2004). High mountainswere created by these collisions, and the continental assemblyand 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

450 500 550 600 650 700 750 800 850 900 950 1000 2500

Crystallization age, this study

Xenocryst age, this study

Crystallization age, Yibas et al (2002)

Ar/ Ar age (Yibas et al., 2002)40 39

B = Biotite; M = Muscovite; H = Hornblende

B

H

B

B

M

B

Crystallization age, Teklay et al (1998)

1125

Xenocryst age, Teklay et al (1998)T

T

1650

T

Ma

Moy

ale

even

t

Pan-African Early Cryogenian

LateCryogenian U-Pb & Pb-Pb Zircon Ages

Collision Denudation

F s et ala et al.

d ion, th

2twa∼esitir(t5bb

oendS1EntasdtallolS

N∼f2∼m

who worked extensively in the SES, and helped MGA collect the

ig. 8. Age summary diagram comparing results of this study with those of Yibas zircon Pb–Pb evaporation and one SHRIMP zircon age and xenocrysts of Teklayiscussion of timing. (For interpretation of the references to color in the figure capt

008). This collision triggered crustal thickening in the EAO south ofhe Ariab-Nakasib-Tharwah-Bir Umq suture (Johnson et al., 2003),hich in turn engendered crustal melting, uplift, and erosion over

broad region. Deformation is likely to have been continuous from630 Ma, and includes the ∼580 Ma Wadera shearing event (Yibast al., 2002); other SES shear zones need to be dated. Post-630 Mahortening probably formed the major N-S structures, overprint-ng older structures. Deformation stopped in at least part of Algheerrane at ∼550 Ma (constrained by 560 ± 8 Ma deformed granitentruded by 548 ± 8 Ma undeformed felsic dyke), and this probablyeflects localization and diminishing of strain as shortening stoppedafter 70 million years) and the orogen relaxed. This approximateshe start of the post-orogenic Berguda tectonothermal event at50–500 Ma (Yibas et al., 2002) and was followed by exhumationetween ∼500 and 530 Ma, as indicated by 40Ar/39Ar hornblende,iotite and muscovite ages (Yibas et al., 2002).

Late Pan-African exhumation engendered tremendous erosionf the SES and other parts of the central and southern EAO (Johnsont al., 2003). Vast volumes of clastic detritus were transportedorthwards and an estimated (15–90) × 106 km3 of sandstone waseposited on the northern margin of Gondwana (Avigad et al., 2005;quire et al., 2006). The higher estimate is equivalent to eroding2.5 km of crust from the 8000-km-long and at least 1000-km-wideast African–Antarctic Orogen. Of course, uplift and erosion wereot distributed evenly across the orogen, mostly being greater inhe south. One difference between the ANS and MB is that upliftnd erosion was much greater in the MB than the ANS. In thatense, the SES is more like the MB in showing erosion to ∼25 kmepth (Tsige, 2006). We suspect that much of this rapid erosion washe product of Alpine-style glaciation. The SES lay at ∼30◦S (Meertnd Lieberman, 2008) but glacial deposits are inferred from low-atitude deposits as late as ∼580 Ma (e.g., Bingen et al., 2005). It isikely that high-altitude regions such as the Ediacaran mountainsf the EAO were extensively glaciated during this time and perhapsater, even though no clear signs of glaciation are known from theES.

In this study, we document the presence of reworkedeoarchean/Paleoproterozoic crust (or sediment with abundant2.5 Ga zircons) in the undeformed granite sample BSZ-04-4-3

rom the Alghe Terrane. Zircons with similar ages (2489.3 ± 0.3 Ma;

947 ± 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 areuch less common than Neoproterozoic ages from the Ethiopian

. (2002), including zircon crystallization ages and mineral 40Ar/39Ar ages, as well(1998). Color-coded 100 million year intervals as used in Fig. 5. See text for moree reader is referred to the web version of the article.)

basement, but indicate that scattered remnants of older crust maybe present in the Ethiopian basement. Further geochronologicaland isotopic work is needed to understand how important pre-Neoproterozoic crust has been for the evolution of the SES andEthiopian basement in general.

Finally, we note that there is no strong evidence of geochemicalevolution of igneous rocks with time shown in the potassium–silicadiagram 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% K2O, but older rocks mayhave high or low contents of incompatible elements. This furtherindicates the complex magmatic history that the middle crust ofthis region experienced during Proterozoic time.

6. Conclusions

New and existing geochronologic data from the SouthernEthiopian Shield indicate that the rocks mostly formed in the mid-dle crust and represent a Neoproterozoic addition to the crust,although further studies are needed to assess the extent and roleof Archean components. Neoproterozoic crustal growth spanned∼300 million years, from ∼850 Ma to ∼550 Ma. This age range isindistinguishable from that seen for crustal growth of the ANS to thenorth. This, together with the fact that SES basement includes a sig-nificant proportion of ophiolitic and volcano-sedimentary terranes,indicates that the generation of juvenile Neoproterozoic crust inthe EAO affected regions well to the south of the ANS. On the otherhand, the abundance of high grade metamorphic rocks, along withthe recognition of Archean protoliths in the Alghae Terrane, indi-cate similarities to crust of the MB, and uplift and deep erosionof the SES is readily ascribed to terminal collision between largefragments of what became East and West Gondwana, with ero-sion aided by glaciation. Deformation in the SES related to terminalcollision along the EAO continued until 548 ± 8 Ma.

Acknowledgments

We dedicate this study to the memory of Dr. Lulu Tsige. Lulu

samples in 2005. He passed away on September 5, 2011 of cancerin Addis Ababa at the age of 45. This work was supported by NSF-OISE grant #0804749 and a Blaustein-Cox fellowship at Stanford

Resear

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R.J. Stern et al. / Precambrian

niversity to RJS and a post-doctoral fellowship from Curtin Uni-ersity in Perth, Australia to KAA. The SHRIMP II facility in Perths operated jointly by Curtin University, the University of West-rn Australia and the Geological Survey of Western Australia, withupport from the Australian Research Council. The manuscript wasreatly improved following the constructive criticism of J. Jacobs,ditor P. Cawood, and two anonymous reviewers. This is UTDeosciences contribution number #1226, TIGeR publication #400,nd Missouri S&T Geology and Geophysics Program contribution41.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.precamres.2012.02.008.

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