Detrital Zircon U–Pb Geochronology Of

Home , Enticho Sandstone, Maturity (sedimentology)

Precambrian Research 154 (2007) 88–106

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 D. Avigad a,∗, R.J. Stern b,M.Beythc, N. Miller b, M.O. McWilliams d a Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel b Geosciences Department, University of Texas at Dallas, Richardson, TX 75083-0688, USA c Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel d Department of Geological and Environmental Sciences, Stanford University, CA 94305-2115, USA Received 1 May 2006; received in revised form 11 December 2006; accepted 14 December 2006

Abstract Detrital zircon geochronology of Neoproterozoic diamictites and Ordovician siliciclastics in northern Ethiopia reveals that the southern Arabian–Nubian Shield (ANS) formed in two major episodes. The earlier episode at 0.9–0.74 Ga represents island arc volcanism, whereas the later phase culminated at 0.62 Ga and comprised late to post orogenic granitoids related to crustal differentiation associated with thickening and orogeny accompanying Gondwana fusion. These magmatic episodes were separated by about ∼100 my of reduced igneous activity (a magmatic lull is detected at about 0.69 Ga), during which subsidence and deposition of marine carbonates and mudrocks displaying Snowball-type C-isotope excursions (Tambien Group) occurred. Cryogenian diamictite interpreted as glacigenic (Negash synclinoria, Tigrai) and polymict conglomerates and arkose of possible peri-glacial origin (Shiraro area, west Tigrai), deformed and metamorphosed within the Neoproterozoic orogenic ediﬁce, occur at the top of the Tambien Group. They were formed well after the shutdown of island arc igneous activity in this region and are pierced by the post-collision granitoids. Negash diamictite and Shiraro sequence contain detrital zircons derived from underlying ∼0.85–0.74 Ga volcanics, a small number of 1.1 Ga zircons (likely inherited within the underlying arc crust) were also detected. The youngest detrital zircons in these sequences are 0.75 and 0.74 Ga. A broadly Sturtian timing (i.e. ∼0.70 Ga) is plausible, but we note this is a lower time limit. Our investigation shows that clasts in the diamictite have a proximal provenance and are derived from underlying igneous rocks and metasediments (including Tambien carbonates). Diamictites were formed when subsidence and basin sedimentation ceased and the Tambien and its underlying igneous complex (Tsaliet Group) were uplifted and eroded (incision exceeded 1500 m). Thus, although bearing the hallmark of a Snowball Earth, the properties of Tambien diamictites indicate relief differentiation and vertical motions may have played a signiﬁcant role in shaping the glacial record of the southern ANS. © 2007 Elsevier B.V. All rights reserved.

Keywords: Arabian–Nubian Shield; Cryogenian; Neoproterozoic glaciation; Detrital zircon geochronology; Ethiopia

1. Introduction

∗ Corresponding author. Inasmuch as Rodinia rifted apart during the Neo- E-mail address: [email protected] (D. Avigad). proterozoic (e.g. Wang, 2003; Weil, 2004), great parts

0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2006.12.004 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 89 of that world were the site of plate convergence and were thus decorated by volcanic arcs, mountain belts and plateaus. Thus, Neoproterozoic Snowball Earth (e.g. Hoffman and Schrag, 2000) glaciers covered landscapes that were shaped under different geodynamic regimes. Convergence accommodating the dispersal of Rodinia gave birth to the Neoproterozoic Pan-African-Brasiliano orogeny which culminated in the assembly of Gond- wana (Unrug, 1997; Veevers, 2004). One major branch of the Pan-African-Brasiliano belt is the East African Orogen (EAO; Stern, 1994) which formed a 4000 km elongated mobile belt whose geological history spans almost all of Neoproterozoic time. Recently, glacigenic diamictites interleaved within the folded and weakly metamorphosed Neoproterozoic basement of the EAO in northern Ethiopia (Tigrai) have been reported (Beyth et al., 2003; Miller et al., 2003; Alene et al., 2006) and their potential significance for the Snowball Earth hypothesis (e.g. Hoffman and Schrag, 2000) has been considered. Stern et al. (2006) demonstrated that rocks of this type are found elsewhere in the Arabian–Nubian Shield (the northern segment of the EAO) and reviewed their mode of occurrence and possible timing. Because EAO diamictites are integrated in the Neoproterozoic orogenic edifice in Ethiopia, understanding the origin of the glacigenic sequence and its paleoclimatic signifi- Fig. 1. The Neoproterozoic East African Orogen and the cance requires understanding the orogenic processes that Arabian–Nubian Shield (after Stern, 2002). Approximate location of shaped this region. Here, we present U–Pb SHRIMP Fig. 2 is marked. dating of detrital zircons from Cryogenian diamictites and Lower Paleozoic sandstone to temporally constrain fragments in the east (e.g. the Afif-Abas terranes in Ara- crustal evolution and orogeny in north Ethiopia (south- bia and Azania in East Africa; Collins and Pisarevsky, ern Arabian Nubian Shield). We integrate these results 2005) soon after 630 Ma (Katz et al., 2004). Conver- with dating of igneous rocks and use these data to further gence related to the progressive amalgamation of East clarify the properties of Cryogenian diamictites in this Gondwana and its docking on the SE margins of the region, and attempt to place them within the history of EAO continued until 550 Ma (Meert, 2003; Collins and Neoproterozoic crust formation and orogeny. Pisarevsky, 2005)) leading to strike slip faulting, lateral displacements and northward extrusion (Bonavia and 2. Geological setting Chorowicz, 1993; Jacobs and Thomas, 2004). The ANS (Fig. 1) is dominated by supracrustal The East African Orogen (EAO; Fig. 1, Stern, 1994) metavolcanics including volcaniclastics and immature formed during Neoproterozoic time by closure of the sediments mostly metamorphosed in the greenschist Mozambique Ocean. It comprises two major segments: facies, variously deformed and intruded by granites, gab- the Arabian–Nubian Shield (ANS) in the north, and the bros, and dikes. Geochemical and isotopic signatures Mozambique Belt in the south. ANS is juvenile Neo- indicate that these rocks are dominantly mantle-derived proterozoic crust, its growth involved intra-oceanic arc juvenile crust (Stern, 2002; Stoeser and Frost, 2006). volcanism and perhaps accretion of oceanic plateaux In Ethiopia, the ANS merges with the Mozambique (Bentor, 1985; Stern, 1994; Stein and Goldstein, 1996; Belt which is the southern half of the EAO and which Stern, 2002; Johnson and Woldehaimanot, 2003). Arc accommodated the most intense collision between East terranes were welded together beginning about 780 Ma and West Gondwana fragments (e.g. Stern, 1994). and then tectonically thickened as a result of conver- The Mozambique Belt exposes higher temperature and gence between the East Sahara Metacraton in the west pressure suites with abundant amphibolite and granulite- (Abdelsalam et al., 2002), and a number of continental facies metamorphic rocks and gneiss terranes. 90 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 91

The lateral transition between greenschist-facies juvenile volcano-sedimentary sequences of the ANS and high-grade rocks of the Mozambique Belt occurs in Ethiopia. Northern Ethiopia (Tigrai, Fig. 1) and much of Eritrea and Ethiopia plateaus expose ANS-type greenschist facies volcano-sedimentary sequences, whereas high-grade rocks are abundant to the south, east and west. Earlier studies suggested that the high-grade rocks were Archean basement underlying the Neoproterozoic volcano-sedimentary sequence (Kazmin et al., 1978) but later geochronology showed that the high-grade sequence is similarly composed of Neoproteroic pro- tholiths (e.g. Ayalew et al., 1990; Teklay et al., 1998; Yibas et al., 2002; and review in Asrat et al., 2001). The diamictitic sequences studied in the present work occur in north Ethiopia (Tigrai), where they are preserved as greenschist facies metasediments above similarly metamorphosed arc volcanics, carbonates and mudrocks of the southern ANS.

2.1. Ethiopia-Eritrea tectonostratigraphy and basement age relations

A simpliﬁed geological map of Tigrai is presented in Fig. 2. The ANS basement of northern Ethiopia (Tigrai) is divided (Fig. 3) into a lower Tsaliet Group and an Fig. 3. Schematic geologic columnar section of Tigrai region showing overlying Tambien Group (Beyth, 1972). The Tsaliet main rock units. Two major Neoproterozoic units are distinguished: Group is several kilometers thick and represents an arc Tsaliet Group (metamorphosed arc volcanics and syntectonic grani- volcano-sedimentary sequence (Teklay, 1997; Alene et toid intrusions) and overlying metasediments of the Tambien Group. al., 1998; Tadesse-Alemu, 1998; Tadesse et al., 1999) Diamictites comprise the top of the Tambien Group. The entire Neo- or an arc–back–arc system (Teklay, 2006), whereas the proterozoic section is pierced by post-tectonic Mereb-type granitoids. Ordovician (Enticho) sandstone and associated Endaga Arbi tillites overlying Tambien Group (Miller et al., 2003; Alene et overly the peneplained basement. al., 2006) is mainly a shallow marine sedimentary cover of carbonates and mudstones locally topped by a diamictite, preserved in limited outcrops as complex synclinoria at 820–740 Ma, but older (854 ± 3 Ma) arc volcanics surrounded by Tsaliet Group metavolcanics. The pres- occur in neighboring Eritrea (Teklay, 1997; Teklay et ence of marine carbonates low in the Tambien Group al., 2003; Anderson et al., 2006). Thus, Tsaliet Group sequence indicates that by the time the Tambien Group arc volcanism – and the most important episode of was deposited, large portions of the arc complex lay crust building – lasted in this region from ∼0.85 to below sea level. A complete shutdown of regional arc 0.74 Ga. volcanism is inferred from the absence of interbedded The Tambien Group section has been studied by lavas or tuffs from the Tambien carbonate and from the us at Negash and in Mai Kenetal synclinoria (Fig. 2). overlying clastics and diamictites. At Negash it contains (from base to top): ∼1.2 km A synthesis of available crystallization ages (mainly of shallow marine carbonate and mudrocks (now cal- zircon U–Pb and Pb–Pb) on Tsaliet arc crust in Ethiopia careous slate), overlain by 250 m of ﬁnely laminated (e.g. Tadesse et al., 1999; Alene et al., 2000; Tadesse et black limestone (deposited in a relatively deeper water al., 2000) shows that igneous activity occurred mainly under oxygen poor conditions) overlain by a ∼100 m

Fig. 2. Geological map of Tigrai region, North Ethiopia. Compiled and modified from Arkin et al. (1971), Kazmin (1973), Tadesse (1996a), Tadesse et al. (1999). Phanerozoic rock units simplified. Areas studied and sample locations are shown. Inset is an enlargement of the Negash synform including the diamictite. A and B are the locations of cross sections presented in Fig. 4. 92 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 transitional sequence of thinly bedded black limestone, volcanics, various phyllites, granites, quartz pegmatite calcareous slate, and non-calcareous slates, culminating and chert. Carbonate clasts were not observed. Cross with a diamictite. bedding indicates fluvial transport towards the SW. The diamictite, ca. 200 m thick, is pinched within Tadesse (1996b) divided northern Ethiopia into the Negash synclinoria (Fig. 4 transect B); it is weakly six blocks tectonically interleaved with two mafic- metamorphosed and displays vertical layering parallel ultramafic ophiolitic belts (Fig. 2). Tectonic boundaries to a pervasive N–S schistosity. It has been interpreted as and structures within the intervening blocks trend N to glacigenic because it contains a wide range of matrix- NNE, and E- or W-verging thrusts display significant supported clasts of many rock types that appear to be oblique slip (Tadesse et al., 1997). The age of the ophi- dropstones (see also Miller et al., 2003; Beyth et al., olites is not known but in Axum area (e.g. Tadesse et 2003). Yet, striated and/or faceted pebbles were not al., 2000) the ophiolitic belt is penetrated by syntec- observed by us and the interpretation of the diamic- tonic granitoid dated to 0.75–0.79 Ga (Tadesse et al., tites as glacigenic is therefore not unequivocal. Clasts, 2000) implying arc terranes were sutured by then. A sec- angular, subrounded or slightly elongated, include felsic ond orogeny took place after Tambien Group deposition, volcanics, fine-grained black limestone and dolomites, producing folding, thrusting and brittle–ductile lateral low-grade semipelitic sediments, rare volcanic conglom- displacements (Tadesse, 1996b) and probably reflects erates, pink granite and vein quartz, and are usually major squeezing and tightening of the EAO during final up to 10 cm in size. Some clasts appear to have been collision. The structure of the Neoproterozoic complex affected by low grade metamorphism prior to incor- is dominated by upright, NNE-trending, tight folds and poration into the diamictite, their presence reflects a layering that often dips steeply (Fig. 4). As indicated metamorphic phase precedent to the regionally observed by the orientation of boudin necks, stretching lineations collision-related metamorphism. Cap carbonates do not and microfractures, approximately E–W shortening was exist in the area as all carbonate units lie strati- accompanied by nearly N–S horizontal extension. The graphically below the Negash and Shiraro diamictites exposed Tsaliet-Tambien sequence was metamorphosed (e.g. Fig. 3). to greenschist facies. The Shiraro sequence also displays Miller et al. (2003) demonstrated that most Tam- low-grade metamorphism and faint ductile deformation. bien carbonates have 87Sr/86Sr between 0.7050 and Stretched pebbles are elongated SW–NE, and fold axes 0.7067, the maximum observed in the black limestone, measured at two localities trend 045◦, somewhat oblique which is also characterized by heavy δ13C (+3 to +6‰). to the boundary of adjacent blocks. At Negash (Fig. 2), the transition sequence below the Together with the Shiraro diamictite, the entire diamictite shows significantly lighter δ 13C(≤−2to metamorphic sequence at Tigrai was intruded by post- −9‰), similar to that observed elsewhere for Cryoge- orogenic 600–620 Ma Mereb granites (Miller et al., nian glacial transitions (e.g. Hoffman and Schrag, 2000). 2003; Asrat et al., 2004; Fig. 2), a somewhat older suit Miller et al. (2003) found that the C- and Sr-isotope (640–620 Ma) was recognized in Eritrea (Teklay et al., composition of the Tambien are most consistent with an 2001). Similar post-tectonic granitoids are characteris- age range of 720–750 Ma, broadly corresponding to the tic of the entire ANS, and their abundance increases Sturtian glaciation. Farther chemostratigraphic investi- northward (e.g. Bentor, 1985, and references therein). gations on Tambien carbonates were recently published by Alene et al. (2006) who generally accepted a Sturtian 2.2. Ordovician cover sequence: Enticho sandstone age for the overlying diamictite. The Shiraro diamictite (Beyth, 1972; Tadesse, 1996a) The earliest Phanerozoic sedimentary sequence cov- is exposed in the lowlands of westernmost Tigrai, strad- ering the Neoproterozoic basement in the region is dling the border with Eritrea (Figs. 2 and 4). Possibly the Ordovician Enticho sandstone (Garland, 1980; equivalent sediments in westernmost Eritrea are known Figs. 2 and 3). It comprises cross-stratified quartz sand- as the Gulgula Group (Teklay et al., 2003). The Shiraro stone that locally contains conglomerate layers, and may diamictite is widely distributed but is not well described. be partially glacial at its base (Saxena and Assefa, 1983; Tambien-like carbonates are locally exposed below it. Kumulainen et al., 2006). The unconformity at its base It contains polymictic conglomerates, arkosic sandstone is a remnant of the Afro-Arabian peneplain that records and siltstone and is weakly metamorphosed, locally the beveling of Neoproterozoic orogens across North folded and cleaved by strain slip, and pressure solution Africa and Arabia (Avigad et al., 2003). The Enticho is abundant. Clasts range up to 8 cm, mostly rounded and sandstone is a part of a widespread Cambro-Ordovician sorted, and usually comprise low-grade (Tsaliet-like) sandstone cover deposited on the eroded Neoproterozoic D. Avigad et al. / Precambrian Research 154 (2007) 88–106 93

Fig. 4. Schematic geological and structural sections of Tigrai Neoproterozoic domains investigated in the present study. Note deformation style is mainly via open to tight (usually upright) folding of the Tsaliet and Tambien Groups with local overturning. Shortening is considered the result of Gondwana fusion at around 630 Ma. Negash diamictite is pinched within the Negash synforms, whereas the structure of the Shiraro region is less tight. Locations of rock samples are marked by stars. The location of Transect A and B are marked on Fig. 2. basement of North Africa and Arabia in the aftermath of position of the southern Arabian–Nubian Shield and of the East African Orogeny (Avigad et al., 2003). Starting the northern Mozambique Belt. Farther south along the in Middle Cambrian time, the sedimentary cover spread East African Orogen, drainage must have been directed progressively from the north (in present coordinates) southward to yield the Table Mountain Group and other and blanketed the entire northern margin of Gondwana Early Paleozoic siliciclastics of South Africa and (then) (Garfunkel, 2002), ultimately reaching Yemen (e.g. the adjacent regions (Burke et al., 2003) so that the Enticho Wajid sandstone) and Ethiopia in Ordovician time. A sandstone must have a rather proximal provenance. In regional synthesis indicates that along the entire north view of the proximal provenance of Enticho sandstone in Gondwana margin these sands were transported from Ethiopia, its mineralogical maturity (quartz sandstone) a southern provenance (Avigad et al., 2005 and ref- probably reﬂects the intense chemical weathering that erences therein). Our reconnaissance study along the prevailed over north Gondwana at this time (Avigad et Adigrat-Axum road revealed transport directions trend- al., 2005). ing between NE and NW indicating an overall northward sense, but contrasting cross bedding directions were 2.3. Scope of the present work measured locally (Mekelle-Adigrat road). The Enticho sandstone is among the southernmost Early Paleozoic Here we use zircon U–Pb SHRIMP geochronology to sandstone units in the Middle East and North Africa, illustrate key events in the geological and environmen- and it was probably deposited near the headwaters of the tal history of Neoproterozoic Ethiopia, with a particular Early Paleozoic drainage system. The detrital zircon age effort to deﬁne the paleogeographic setting and age of spectrum of this unit should highlight the crustal com- the glaciogenic rocks. 94 D. Avigad et al. / Precambrian Research 154 (2007) 88–106

Detrital zircons from the Negash diamictite were analytical spot for 90 s before analysis to reduce common dated (from both, clasts and matrix) to constrain its depo- Pb, and the resulting analyses showed that 204Pb is gener- sitional age and the glaciation it may represent, to detect ally <0.01% of the total Pb. Isotope ratios were calibrated whether any far-traveled material is present and to define against AS57, with an assumed age of 1099 Ma (Paces the crustal composition of the provenance. A principal and Miller, 1993). Each spot analysis was the average goal was to constrain the age of the Negash diamictite so of five scans through nine mass-stations. Common lead that we could place it within the history of globally recog- was estimated using the method of Stacey and Kramers nized Neoproterozoic glacial intervals. Ideally, ash beds (1975) and was generally low. Data processing and plot- intercalated in the section would be targets for dating, ting were performed using Squid and Isoplot (Ludwig, but we have found none at Negash. Instead we treated 1994). Zircons yielding concordant ages (less than 10% the diamictite as a detrital sequence, separated zircons discordancy) were usually selected for presentation and from a melange´ of rock fragments and their matrix and 206Pb/238U ages were plotted in histograms. Analytical infer that deposition must postdate the youngest zircon data is presented in Tables 1a and 1b. found. Because the Tigrai volcano-sedimentary section is Neoproterozoic in age, this strategy allows us to detect 2.5. Detrital zircon geochronology of the Negash far-traveled exotic material because this would likely be diamictite much older. Similar targets were defined for the Shi- raro diamictite although a priori the sequence does not Two diamictite samples, ∼10 kg each, were taken unequivocally indicate that it formed by the action of from Negash syncline north of Mekelle (#T4-3-3; glaciers. 13.83541◦N; 39.61569◦E). Zircons were separated from Detrital zircon ages from the Enticho sandstone pro- both samples including matrix and clasts and were anal- vide a novel perspective on the evolution of Ethiopia’s ysed together. The detrital zircon ages are shown on a Neoproterozoic basement and on Cambro-Ordovician Concordia diagram and in Fig. 5a and as a histogram of paleogeography because these randomly sample sources spot ages in Fig. 6. We eliminated data that were more across a wide drainage system. The detrital zircon age than 10% discordant or that contained elevated levels of spectrum thus should reflect the age of the dominant common Pb. zircon-producing orogenic processes that shaped the Fig. 6A demonstrates that the sources for the region. Negash diamictite were principally Neoproterozoic Additionally, in order to augment the existing rocks (<1.0 Ga). The age spectrum is characterized by chronology of Tsaliet arc volcanism, a syn-tectonic a peak at ∼0.8 Ga with submodes at 0.75 and 0.86 Ga. micro-granite and a felsic metavolcanic sill from the The youngest concordant zircon is 0.75 Ga and seven top of the Tsaliet Group were dated to establish the age zircons lie in the 0.96–1.03 Ga bin. No 0.89–0.95 Ga of igneous activity and to search for inherited zircons. zircons were detected. A sample of post-tectonic Mereb granite that cuts the The 0.75—0.82 Ga interval represents igneous activ- folded and imbricate structure of the Tambien and Tsaliet ity and volcanic accretion in the southern ANS but groups in the Mai Kenatal region was also dated. 0.87 Ga rocks are not reported from Tigrai. These ages For the most part, our results from igneous rocks, are similar to slightly old zircon cores reported from diamictite, and sandstone yield a remarkably consistent the Ghedem high-grade gneisses in east Eritrea (e.g. history of crust formation in the region. Anderson et al., 2006) and from intrusive rocks in SE Sudan (Kroner¨ et al., 1991). Similar ages are also known 2.4. Analytical procedures in south Ethiopia where the oldest dated zircon yielded 0.87 Ga (Yibas et al., 2002). Our data suggest that the Zircons from 5 to 10 kg samples were isolated using Negash diamictite was derived mainly from proximal standard separation techniques. Representative zircon sources with no distinguishable outside-ANS compo- fractions of all samples, usually between 64 and 250 ␮m, nents, except for a few ∼1 Ga zircons, the source of were mounted in epoxy, polished, coated with gold and which is not straightforward identified (see discussion scanned by cathodoluminescence imaging. U–Th–Pb below). This is consistent with field and petrographic analyses were made with the SHRIMP RG (Reverse observations that show the diamictite is composed of Geometry) of the USGS-Stanford University facility, low-grade volcanic fragments and fine-grained carbon- using the calibrated SL 13 standard. Analytical spots ates with a few granitoid pebbles. A suitable source for − ∼30 ␮m in diameter were sputtered using ∼10-nA O2 most of these fragments is present in the underlying primary beam. The primary beam was rastered across the Tsaliet metavolcanics and related intrusive units. D. Avigad et al. / Precambrian Research 154 (2007) 88–106 95

Table 1a SHRIMP U–Pb–Th analytical data for detrital zircons from Tigre, North Ethiopia

Spot name Comm. U Th 232Th/ Rad 206Pb 206Pb/238U ±1σ Disc 238U/ Error 207Pb/ Error 206Pb (%) (ppm) (ppm) 238U (ppm) age (Ma) (%) 206Pb (%) 206Pb (%)

Negash diamictite #T4-3-3 DIAM1-13 0.89 48 14 0.30 5.1 743.5 14.1 −38 8.19 1.9 0.0633 4.2 DIAM1-3 0.82 101 89 0.91 11.7 811.4 10.4 −28 7.46 1.3 0.0659 2.8 DI AM 1-30 0.56 168 84 0.51 21.0 878.2 9.1 −27 6.88 1.1 0.0655 2.3 DIAM2-3 0.20 226 107 0.49 24.8 778.5 6.5 −26 7.83 0.8 0.0609 2.0 DIAM1-11 0.37 70 37 0.54 8.9 892.7 13.8 −26 6.77 1.6 0.0644 3.4 DI AM 1-47 0.27 79 35 0.47 10.1 906.8 13.6 −23 6.66 1.5 0.0646 3.2 DIAM1-19 0.20 233 89 0.39 26.5 803.6 7.1 −22 7.57 0.9 0.0623 2.0 DIAM2-2 0.83 181 101 0.57 17.4 681.5 7.7 −21 8.93 1.2 0.0655 2.3 DIAM2-18 0.42 77 70 0.94 8.5 781.6 13.1 −19 7.76 1.7 0.0644 3.2 DI AM 1-27 0.26 190 114 0.62 21.0 781.9 7.6 −18 7.78 1.0 0.0630 2.3 DI AM 1-25 0.38 102 38 0.38 12.4 857.3 11.5 −16 7.04 1.4 0.0661 3.1 DIAM2-15 0.22 260 142 0.56 29.3 796.3 6.2 −16 7.62 0.8 0.0637 1.8 DI AM 1-33 0.27 137 42 0.32 16.4 841.0 9.5 −16 7.20 1.2 0.0650 2.5 DI AM 1-46 0.52 277 132 0.49 31.8 808.8 6.2 −16 7.48 0.8 0.0663 1.8 DI AM 1-44 0.18 210 87 0.43 23.4 787.7 7.1 −13 7.71 0.9 0.0638 2.1 DIAM2-20 0.27 199 67 0.35 23.0 812.5 8.3 −12 7.45 1.0 0.0655 2.0 DI AM 1-37 0.37 208 159 0.79 25.7 868.3 8.0 −11 6.94 0.9 0.0678 2.0 DI AM 1-35 0.19 218 114 0.54 26.9 865.2 7.7 −10 6.97 0.9 0.0666 1.9 DIAM2-5 0.19 239 120 0.52 25.5 755.2 6.1 −10 8.05 0.8 0.0638 1.9 DIAM1-16 0.16 252 94 0.39 31.0 863.0 7.1 −10 6.99 0.8 0.0663 1.7 DIAM2-13 0.12 280 77 0.29 33.7 846.6 6.1 −10 7.14 0.7 0.0657 1.6 DIAM2-9 0.14 130 49 0.39 15.1 820.2 8.8 −10 7.38 1.1 0.0652 2.5 DIAM2-17 0.84 310 119 0.40 34.2 775.2 7.0 −8 7.78 0.9 0.0705 1.8 DIAM1-10 0.00 281 118 0.43 29.8 750.7 5.9 −8 8.12 0.8 0.0625 1.8 DI AM 1-39 0.15 159 91 0.59 18.1 800.7 7.2 −8 7.57 0.9 0.0651 2.1 DI AM 1-42 0.22 96 61 0.65 11.6 852.0 11.4 −7 7.08 1.4 0.0672 2.9 DIAM2-16b 0.66 116 37 0.33 16.9 1004.1 12.9 −7 5.91 1.3 0.0759 2.2 DIAM1-8 0.39 310 124 0.41 37.5 849.9 6.2 −7 7.09 0.7 0.0686 1.6 DI AM 1-40 0.17 224 120 0.55 25.4 801.2 6.9 −7 7.56 0.9 0.0655 2.0 DIAM1-21 0.00 226 93 0.42 27.7 860.2 7.6 −6 7.02 0.9 0.0661 1.9 DI AM 1-24 0.11 353 295 0.86 50.2 989.5 5.9 −5 6.03 0.6 0.0712 1.2 DIAM1-2 0.00 137 112 0.84 16.4 838.8 9.1 −4 7.21 1.1 0.0659 2.3 DI AM 1-32 0.27 125 25 0.20 18.6 1032.9 11.8 −4 5.75 1.2 0.0742 2.2 DIAM2-21 0.32 99 45 0.47 11.7 831.5 12.3 −4 7.25 1.5 0.0686 2.7 DIAM1-41 0.00 131 58 0.46 14.7 791.9 9.1 −4 7.66 1.2 0.0646 2.6 DIAM2-10 0.14 114 35 0.32 16.1 978.6 10.9 −4 6.10 1.1 0.0717 2.3 DI AM 1-20 0.13 586 184 0.32 62.3 752.0 4.2 −3 8.08 0.6 0.0646 1.3 DIAM1-4 0.09 300 101 0.35 32.4 761.9 5.9 −3 7.97 0.8 0.0647 1.8 DIAM2-11 0.20 184 72 0.41 21.2 808.5 7.2 −2 7.47 0.9 0.0672 2.0 DI AM 1-26 0.15 145 144 1.02 17.8 861.2 9.6 −2 6.99 1.1 0.0685 2.3 DI AM 1-38 0.15 402 103 0.26 45.2 791.5 4.3 −1 7.65 0.6 0.0665 1.2 DIAM2-7 0.13 142 114 0.83 16.6 820.4 8.6 −1 7.36 1.1 0.0673 2.3 DI AM 1-23 0.22 150 140 0.96 17.2 806.9 7.4 −1 7.48 0.9 0.0676 2.0 DI AM 1-34 0.33 56 14 0.25 8.0 983.0 16.0 1 6.05 1.6 0.0747 4.3 DIAM1-31 0.00 126 86 0.71 18.0 995.6 11.7 1 5.99 1.2 0.0726 2.3 DIAM2-8 0.12 327 197 0.62 37.3 803.0 5.3 1 7.53 0.7 0.0672 1.5 DIAM1-7 0.00 226 86 0.39 25.8 804.7 6.9 1 7.52 0.9 0.0662 1.9 DI AM 1-36 0.21 114 48 0.44 14.1 866.5 10.9 1 6.93 1.3 0.0700 2.8 DIAM1-9 0.31 71 36 0.52 8.9 876.4 13.1 3 6.88 1.5 0.0665 3.2 DIAM1-15 0.00 243 171 0.73 28.8 832.6 6.9 3 7.25 0.8 0.0677 1.7 DIAM1-5 0.00 84 41 0.51 10.1 844.8 11.9 4 7.13 1.4 0.0684 3.1 DIAM1-12 0.12 490 292 0.62 57.4 821.9 4.7 5 7.33 0.6 0.0688 1.2 DI AM 1-45 0.11 284 163 0.59 35.5 873.8 6.8 5 6.88 0.8 0.0688 1.7 DIAM1-6 0.00 425 312 0.76 47.4 785.8 5.0 5 7.70 0.6 0.0667 1.4 DIAM2-14 0.60 237 215 0.94 25.5 753.4 6.3 5 8.01 0.8 0.0709 1.9 DIAM2-6 0.00 216 88 0.42 25.6 831.4 8.5 7 7.25 1.1 0.0689 1.8 96 D. Avigad et al. / Precambrian Research 154 (2007) 88–106

Table 1a (Continued )

Spot name Comm. U Th 232Th/ Rad 206Pb 206Pb/238U ±1σ Disc 238U/ Error 207Pb/ Error 206Pb (%) (ppm) (ppm) 238U (ppm) age (Ma) (%) 206Pb (%) 206Pb (%)

DI AM 1-22 0.42 76 41 0.56 9.6 886.5 13.3 9 6.79 1.5 0.0682 3.2 DIAM2-16 0.09 90 50 0.58 12.5 958.7 14.6 10 6.20 1.6 0.0756 2.6 DIAM1-17 0.07 516 120 0.24 52.6 721.0 4.2 11 8.43 0.6 0.0654 1.3 DI AM 1-28 0.00 97 62 0.67 11.1 805.2 10.7 12 7.49 1.3 0.0691 2.9 DI AM 1-43 0.00 235 121 0.53 24.4 734.1 6.4 15 8.25 0.9 0.0673 2.0 DIAM2-4 1.48 163 54 0.34 14.5 621.3 6.2 24 9.69 1.0 0.0780 2.2 DIAM2-19 1.27 693 733 1.09 65.4 659.9 4.2 30 9.09 0.6 0.0790 1.1 Shiraro arkose #T4-15-1 SHIRA1-49 0.85 43 16 0.40 5.4 888.3 16.7 −47 6.82 1.9 0.0631 4.2 SHIRA2-18 0.83 71 21 0.31 8.7 850.6 15.0 −31 7.10 1.8 0.0667 3.3 SHIRA1-38 0.49 149 86 0.59 18.7 884.2 9.3 −31 6.84 1.1 0.0640 2.3 SHIRA1-16 0.51 78 51 0.67 9.9 893.5 13.0 −29 6.76 1.5 0.0647 3.2 SHIRA2-23 0.43 82 44 0.55 10.1 868.0 14.0 −23 6.96 1.7 0.0654 3.0 SHIRA1-24 0.36 53 26 0.50 7.1 925.4 16.2 −22 6.51 1.8 0.0660 4.9 SHIRA1-28 1.10 69 36 0.53 8.7 879.3 13.3 −22 6.82 1.5 0.0711 3.2 SHIRA2-11 0.71 87 76 0.90 10.4 834.4 13.3 −21 7.23 1.6 0.0677 3.0 SHIRA1-39 0.44 90 51 0.59 11.2 874.3 11.7 −18 6.90 1.4 0.0666 2.9 SHIRA2-25 0.38 213 235 1.14 26.4 867.4 8.7 −16 6.95 1.0 0.0668 1.9 SHIRA1-36 0.46 170 132 0.80 21.8 896.4 8.6 −16 6.71 1.0 0.0679 2.1 SHIRA2-14 0.45 95 37 0.40 11.0 819.3 12.4 −16 7.38 1.5 0.0664 2.9 SHIRA1-3 0.42 90 30 0.34 11.3 881.1 11.6 −15 6.84 1.3 0.0674 2.8 SHIRA1-30 0.11 184 234 1.32 23.1 883.5 8.4 −15 6.84 1.0 0.0651 2.1 SHIRA2-4 0.30 58 37 0.65 7.2 868.0 13.8 −14 6.95 1.6 0.0666 3.6 SHIRA2-26 0.47 77 59 0.80 8.7 799.7 13.2 −14 7.57 1.7 0.0665 3.2 SHIRA1-25 0.61 74 44 0.62 9.3 870.3 12.7 −13 6.91 1.5 0.0692 3.4 SHIRA1-23 0.16 136 37 0.28 15.7 810.0 8.9 −13 7.49 1.1 0.0641 2.5 SHIRA2-19 0.31 61 24 0.41 7.3 837.8 15.8 −13 7.21 1.9 0.0662 3.6 SHIRA1-4 0.14 294 310 1.09 35.3 844.9 6.3 −13 7.16 0.8 0.0649 1.6 SHIRA1-55 0.25 82 31 0.40 9.9 850.1 11.9 −12 7.11 1.4 0.0660 3.4 SHIRA1-19 0.28 268 141 0.54 33.5 877.7 6.7 −11 6.87 0.8 0.0673 1.6 SHIRA2-3 0.30 123 64 0.54 14.2 817.3 9.0 −11 7.40 1.1 0.0661 2.5 SHIRA1-45 0.23 175 87 0.51 21.9 877.9 8.6 −11 6.87 1.0 0.0670 2.1 SHIRA1-33 0.68 68 39 0.59 8.5 870.2 13.9 −11 6.90 1.6 0.0704 4.8 SHIRA1-8 0.00 36 18 0.51 4.6 899.6 18.9 −10 6.70 2.1 0.0660 4.6 SHIRA1-7 0.13 303 157 0.54 36.7 851.4 6.2 −9 7.09 0.7 0.0661 1.6 SHIRA1-14 0.09 240 227 0.98 29.3 858.1 7.3 −9 7.04 0.9 0.0660 1.9 SHIRA1-47 0.07 321 227 0.73 36.4 801.5 5.8 −8 7.57 0.7 0.0644 1.6 SHIRA1-18 0.56 71 34 0.50 9.1 893.9 13.5 −7 6.70 1.5 0.0711 3.2 SHIRA1-12 0.25 133 82 0.64 16.1 848.8 9.4 −7 7.11 1.1 0.0673 2.4 SHIRA2-24 0.31 325 206 0.66 35.9 779.1 6.5 −7 7.78 0.9 0.0661 1.6 SHIRA1-60 0.13 293 189 0.67 35.3 847.4 6.2 −7 7.13 0.7 0.0664 1.6 SHIRA1-5 0.14 141 96 0.71 16.9 844.7 9.0 −7 7.15 1.1 0.0664 2.3 SHIRA1-6 0.00 264 3 0.01 30.4 811.3 6.5 −7 7.47 0.8 0.0644 1.8 SHIRA1-15 0.00 86 48 0.57 10.7 880.3 12.4 −7 6.85 1.4 0.0664 3.0 SHIRA1-40 0.32 102 71 0.73 12.7 877.9 11.0 −6 6.85 1.3 0.0690 2.6 SHIRA1-9 0.00 163 104 0.66 20.4 878.0 8.8 −6 6.87 1.0 0.0665 2.2 SHIRA1-32 0.23 82 37 0.47 10.8 921.5 13.1 −5 6.51 1.5 0.0699 2.9 SHIRA1-34 0.00 191 126 0.68 24.2 887.6 8.2 −5 6.79 0.9 0.0670 2.0 SHIRA1-29 0.00 233 128 0.57 29.9 901.1 7.5 −5 6.68 0.9 0.0675 1.8 SHIRA1-48 0.33 204 156 0.79 25.5 874.7 7.9 −4 6.87 0.9 0.0696 1.9 SHIRA2-17 0.40 141 60 0.44 17.3 857.8 10.8 −4 7.01 1.3 0.0701 2.3 SHIRA1-20 0.40 53 27 0.52 6.4 848.0 15.0 −4 7.09 1.8 0.0695 3.8 SHIRA1-17 0.00 85 43 0.53 9.9 820.1 11.0 −3 7.38 1.4 0.0656 3.1 SHIRA1-21 0.00 65 27 0.43 7.9 862.2 13.5 −3 7.00 1.6 0.0670 3.5 SHIRA2-4 0.15 161 82 0.53 19.3 841.2 8.2 −3 7.17 1.0 0.0677 2.3 SHIRA1-22 0.06 145 116 0.83 18.3 883.3 9.3 −2 6.82 1.1 0.0674 2.2 SHIRA2-7 0.00 143 91 0.66 16.9 832.8 8.7 −2 7.26 1.1 0.0663 2.3 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 97

Table 1a (Continued )

Spot name Comm. U Th 232Th/ Rad 206Pb 206Pb/238U ±1σ Disc 238U/ Error 207Pb/ Error 206Pb (%) (ppm) (ppm) 238U (ppm) age (Ma) (%) 206Pb (%) 206Pb (%)

SHIRA1-1 0.00 50 27 0.56 5.9 834.5 15.1 −2 7.24 1.8 0.0665 4.1 SHIRA2-6 0.00 130 104 0.82 21.0 1107.6 11.4 −1 5.34 1.1 0.0759 2.1 SHIRA2-20 0.15 387 334 0.89 43.0 783.3 6.0 −1 7.73 0.8 0.0663 1.5 SHIRA1-52 0.00 167 116 0.72 20.8 871.4 8.6 −1 6.91 1.0 0.0678 2.1 SHIRA1-53 0.14 142 126 0.92 17.5 867.3 9.4 −1 6.94 1.1 0.0688 2.2 SHIRA2-8 0.13 132 111 0.87 14.9 795.6 8.4 0 7.60 1.1 0.0667 2.3 SHIRA2-9 0.06 321 335 1.08 38.6 844.6 5.9 0 7.14 0.7 0.0677 1.5 SHIRA1-56 0.13 143 34 0.25 17.7 866.0 9.2 0 6.95 1.1 0.0690 2.2 SHIRA1-50 0.09 215 149 0.71 26.5 861.9 7.5 1 6.98 0.9 0.0688 1.9 SHIRA1-11 0.13 161 79 0.51 19.7 857.2 8.9 1 7.02 1.1 0.0690 2.2 SHIRA1-44 0.34 186 160 0.89 22.2 838.3 7.8 2 7.17 0.9 0.0701 2.1 SHIRA1-27 0.00 132 94 0.73 16.1 855.0 9.3 2 7.05 1.1 0.0680 2.3 SHIRA1-37 0.06 174 150 0.89 22.1 890.8 8.4 2 6.75 1.0 0.0688 2.0 SHIRA1-35 0.00 227 144 0.65 34.0 1032.6 8.4 2 5.75 0.8 0.0744 1.5 SHIRA1-10 0.12 184 143 0.80 21.9 833.9 8.1 3 7.23 1.0 0.0686 2.0 SHIRA1-51 0.14 219 156 0.73 27.0 862.5 7.3 3 6.99 0.9 0.0675 1.9 SHIRA1-59 0.21 139 68 0.50 16.2 822.4 8.8 3 7.36 1.1 0.0655 2.9 SHIRA2-15 0.21 99 77 0.80 11.2 792.0 11.6 3 7.63 1.5 0.0682 2.8 SHIRA1-42 0.46 140 50 0.37 15.9 800.6 8.7 4 7.59 1.1 0.0632 2.5 SHIRA1-54 0.00 165 143 0.89 19.7 836.6 8.2 5 7.20 1.0 0.0684 2.1 SHIRA1-26 0.37 224 204 0.94 23.5 739.2 6.3 5 8.19 0.9 0.0681 1.9 SHIRA1-57 0.19 156 89 0.59 18.7 842.8 8.5 6 7.16 1.0 0.0672 2.2 SHIRA1-13 0.00 96 28 0.30 12.2 884.1 11.6 7 6.78 1.3 0.0707 2.8 SHIRA1-2 0.00 37 22 0.61 4.3 812.5 17.3 8 7.42 2.2 0.0683 4.7 SHIRA1-58 0.00 86 37 0.44 9.3 762.6 11.0 8 7.94 1.5 0.0666 3.7 SHIRA1-46 0.18 153 112 0.76 17.5 800.4 8.1 8 7.53 1.0 0.0694 2.2 SHIRA2-16 0.00 98 73 0.77 11.8 837.0 12.4 11 7.19 1.5 0.0703 2.7 SHIRA1-43 0.00 250 281 1.16 27.2 766.7 6.2 12 7.89 0.8 0.0677 1.8 SHIRA1-31 0.75 75 32 0.44 9.2 865.0 12.2 14 6.98 1.4 0.0662 3.2 SHIRA2-10 0.00 25 13 0.56 2.9 821.2 20.0 17 7.32 2.5 0.0712 5.3 SHIRA2-13 0.63 64 48 0.77 7.0 764.4 14.5 39 7.80 1.9 0.0807 3.3 Enticho sandstone (Ordovician) #T3-21-1 Ordo1-59* 0.00 42 67 1.63 17.3 2526.1 23.8 0 2.09 1.7 0.1668 1.4 Ordo1-27* 0.00 36 38 1.08 13.0 2572.5 27.2 14 2.39 1.9 0.1715 1.6 Ordo1-55* 0.08 164 124 0.78 55.6 2096.6 14.3 −2 2.53 0.8 0.1305 0.8 Ordo1-24* 0.16 48 43 0.92 14.8 2059.6 39.8 5 2.80 1.7 0.1284 2.2 Ordo1-54* 0.00 128 122 0.98 38.4 1851.2 21.1 −4 2.86 1.0 0.1132 1.2 Ordo1-13 0.00 49 45 0.96 7.6 1108.7 64.8 3 5.52 1.7 0.0765 3.2 Ordo1-58 0.10 149 36 0.25 22.9 979.3 40.6 −8 5.59 1.0 0.0726 1.9 Ordo1-7 0.32 212 165 0.80 29.4 958.3 8.4 1 6.22 0.9 0.0738 1.7 Ordo1-37 0.00 54 45 0.86 7.2 926.0 15.3 17 6.42 1.7 0.0759 3.2 Ordo1-29 0.00 73 59 0.83 9.6 912.4 13.0 −5 6.59 1.4 0.0679 3.0 Ordo1-33 0.55 67 42 0.65 8.5 884.4 13.2 −3 6.77 1.5 0.0719 3.1 Ordo1-44 0.00 209 78 0.39 24.0 808.9 7.0 −7 7.50 0.9 0.0642 1.9 Ordo1-39 0.34 86 75 0.90 9.8 800.3 10.5 12 7.56 1.3 0.0663 3.0 Ordo1-35 0.18 218 139 0.66 24.9 802.2 6.7 3 7.52 0.8 0.0682 1.8 Ordo1-28 0.00 116 66 0.59 13.1 798.4 9.2 −9 7.61 1.2 0.0635 2.6 Ordo1-1 0.00 56 49 0.92 6.3 796.9 13.1 −10 7.62 1.7 0.0631 3.8 Ordo1-52 0.00 130 107 0.85 14.5 789.0 8.6 −13 7.71 1.1 0.0623 2.5 Ordo1-41 0.24 99 75 0.78 11.0 781.8 8.3 7 7.72 1.1 0.0690 2.3 Ordo1-6 0.31 111 106 0.99 12.3 784.3 11.6 −11 7.73 1.5 0.0652 2.8 Ordo1-38 0.10 83 73 0.91 9.2 782.5 10.7 −6 7.75 1.4 0.0647 3.1 Ordo1-21 0.81 75 28 0.38 8.2 770.2 9.6 42 7.83 1.3 0.0701 2.7 Ordo1-32 0.11 184 212 1.19 20.4 783.1 7.2 −14 7.77 0.9 0.0627 2.1 Ordo1-15 0.07 129 83 0.67 14.2 773.1 8.2 7 7.84 1.1 0.0663 2.4 Ordo1-20 1.11 36 31 0.88 4.0 775.4 16.4 −9 7.76 2.1 0.0718 4.7 Ordo1-53 0.53 58 41 0.73 6.3 765.7 12.6 30 7.89 1.7 0.0684 3.6 98 D. Avigad et al. / Precambrian Research 154 (2007) 88–106

Table 1a (Continued )

Spot name Comm. U Th 232Th/ Rad 206Pb 206Pb/238U ±1σ Disc 238U/ Error 207Pb/ Error 206Pb (%) (ppm) (ppm) 238U (ppm) age (Ma) (%) 206Pb (%) 206Pb (%)

Ordo1-18 0.35 108 39 0.37 11.8 767.6 9.6 −3 7.89 1.3 0.0669 2.8 Ordo1-31 0.14 306 49 0.17 32.7 755.8 5.5 −9 8.05 0.7 0.0635 1.7 Ordo1-36 0.15 144 103 0.74 15.4 752.5 7.9 6 8.05 1.1 0.0670 2.4 Ordo1-42 0.35 182 76 0.43 19.4 753.0 7.0 −12 8.07 0.9 0.0644 2.2 Ordo1-4 0.00 91 48 0.55 9.6 747.4 10.0 5 8.12 1.4 0.0652 3.0 Ordo1-12 0.34 127 120 0.97 13.3 739.8 8.2 −13 8.22 1.1 0.0638 2.6 Ordo1-25 0.59 493 361 0.76 50.3 719.5 4.2 −3 8.43 0.6 0.0674 1.3 Ordo1-26 0.53 67 46 0.70 6.8 716.5 10.8 −41 8.55 1.5 0.0595 3.7 Ordo1-19 0.40 87 50 0.59 8.1 657.8 8.9 23 9.29 1.4 0.0630 3.3 Ordo1-43 0.36 165 84 0.53 15.3 659.1 6.7 −13 9.29 1.0 0.0620 2.5 Ordo1-45 0.00 102 95 0.97 9.2 650.2 8.5 −13 9.45 1.3 0.0589 3.4 Ordo1-9 0.44 132 34 0.27 11.9 638.3 7.2 −7 9.58 1.1 0.0632 2.7 Ordo1-60 0.23 177 145 0.85 15.8 634.2 6.4 20 9.65 1.0 0.0630 2.5 Ordo1-50 0.04 118 53 0.46 10.5 631.7 7.3 5 9.71 1.2 0.0614 2.9 Ordo1-22 0.67 76 41 0.56 6.8 626.4 9.2 23 9.68 1.5 0.0704 3.5 Ordo1-49 0.33 102 120 1.21 9.0 630.5 7.9 −9 9.72 1.3 0.0617 3.2 Ordo1-17 0.49 62 64 1.06 5.5 626.3 10.7 −4 9.76 1.7 0.0639 4.7 Ordo1-16 0.00 87 91 1.07 7.7 626.1 7.5 −4 9.81 1.2 0.0598 3.1 Ordo1-8 0.51 79 193 2.52 6.9 623.3 9.3 −18 9.84 1.5 0.0616 3.8 Ordo1-30 0.27 159 226 1.47 13.8 620.0 6.4 −2 9.88 1.0 0.0623 2.6 Ordo1-48 0.00 49 1 0.02 4.2 607.4 11.2 2 10.12 1.9 0.0605 4.8 Ordo1-47 0.27 194 112 0.60 16.4 604.8 5.6 −11 10.16 0.9 0.0603 2.4 Ordo1-5 0.52 101 85 0.87 8.5 604.8 7.8 −36 10.18 1.3 0.0586 3.4 Ordo1-56 0.50 119 104 0.90 10.0 600.0 6.9 −20 10.24 1.2 0.0606 3.0 Ordo1-51 0.00 46 52 1.17 3.8 592.2 11.8 20 10.35 2.0 0.0632 5.0 Ordo1-40 0.52 108 93 0.89 8.8 584.1 7.2 −50 10.58 1.2 0.0563 3.3 Ordo1-3 0.36 186 360 2.00 13.6 529.0 5.1 −39 11.73 1.0 0.0557 2.7 Ordo1-57 0.64 253 158 0.65 16.5 462.5 4.0 80 13.18 0.8 0.0723 2.0 Ordo1-10 0.47 406 236 0.60 25.3 445.8 3.2 60 13.77 0.7 0.0671 1.9

The provenance of the 0.95–1.1 Ga age zircons is also within Neoproterozoic rocks of southern Ethiopia (a less obvious. These zircons crystallized during Kibaran metarhyolite from Waderagroup; Teklay et al., 1998) and (Grenvillian) magmatism that predated the onset of it is thus most likely that the Kibaran zircons detected in Arabian–Nubian Shield activity at ∼0.87 Ga (Stern, the diamictites have their provenance in similar rocks, in 1994). Kibaran rocks are generally not known north of the vicinity of Tigrai. Therefore, the Kibaran-age detrital Tanzania and the presence of these zircons may a priori zircons in the diamictites do not represent long-distance indicate a distal provenance, but if this is correct, other transport. pre-Neoproterozoic zircons would be expected also. Yet, Based on the age of the youngest concordant zircon, pre-Kibaran zircons are not detected in the diamictites. the Negash diamictite is younger than 0.75 Ga, consistent Recent studies of the Cambro-Ordovician sandstone with the constraints for the underlying Tsaliet volcanism. blanketing the northern Arabian–Nubian Shield show that 1.1 Ga zircons are a prominent component of the 2.6. Detrital zircon geochronology of the Shiraro detrital zircon age spectra of this terrane (Avigad et sequence al., 2003; Kolodner et al., 2006). Although the possibility that they represent far traveled ice-rafted material We separated zircons from a pebble-free Shi- cannot be refuted, Avigad et al. (2003) also raised the raro arkosic sandstone (#T4-15-1; 14.404840◦N; possibility that Kibaran age rocks and/or zircons reside 37.80902◦E) to determine the source of the basin detritus within ANS, and Hargrove et al. (2006) recently indi- and to constrain its depositional age and possible rela- cated some ANS crustal segments were contaminated tionship to the Negash diamictite. The detrital zircon by pre-Neoproterozoic (including Kibaran) material. ages are presented on a Concordia diagram (Fig. 5b) and Approximately 1.1 Ga xenocrystic zircons were reported plotted on a histogram in Fig. 6B. The age distribution is D. Avigad et al. / Precambrian Research 154 (2007) 88–106 99

Table 1b SHRIMP U–Pb–Th analytical data for zircons in igneous rocks from Tigre, North Ethiopia

Spot name Comm. U Th 232Th/ Rad 206Pb 206Pb/238U 1S.E. Disc 238U/ Error 207Pb/ Error 206Pb (%) (ppm) (ppm) 238U (ppm) age (Ma) (%) 206Pb (%) 206Pb (%)

Sill-top Tsaliet#T3-11-10 (Concordia age = 775.9 ± 6.5 (95%-conf.)) S-1.1 0.27 76 29 0.39 8.8 812.2 7.6 −1 7.43 1.0 0.0684 1.5 S-2.1 0.32 157 64 0.42 17.5 784.6 5.0 −4 7.70 0.6 0.0680 1.1 S-4.1 0.37 185 88 0.49 20.6 779.3 7.3 3 7.75 1.0 0.0682 1.2 S-5.1 0.58 146 56 0.39 15.9 766.8 5.2 13 7.87 0.7 0.0696 1.1 S-8.1 0.27 129 67 0.54 14.1 768.4 5.8 −1 7.88 0.8 0.0670 1.3 S-10-2 0.29 170 64 0.39 18.8 779.9 7.6 −7 7.75 1.0 0.0676 1.2 S-10-4 0.46 130 64 0.51 13.9 756.1 7.8 0 8.00 1.1 0.0683 1.3 S-10-6 0.19 83 40 0.50 9.3 788.5 9.1 −11 7.70 1.2 0.0638 2.2 Mai Kenetal granite post-tectonic #T3-17-01 (Concordia age = 612.3 ± 5.7 (95%-conf.)) T317-01-3 0.18 365 126 0.36 31.4 615.2 5.3 8 9.99 0.9 0.0618 1.0 T317-01-4 0.13 147 48 0.33 12.4 601.1 6.0 −9 10.20 1.0 0.0610 2.8 T317-01-6 0.02 560 502 0.93 48.8 622.7 5.1 −3 9.86 0.9 0.0603 0.8 T317-01-7 0.05 384 159 0.43 33.4 622.0 5.2 −5 9.87 0.9 0.0601 0.9 T317-01-8 0.09 360 124 0.36 31.2 617.8 5.4 −8 9.92 0.9 0.0612 1.0 T317-01-10 0.06 323 199 0.64 27.1 601.3 5.4 3 10.23 0.9 0.0604 1.6 T317-01-11 0.24 463 175 0.39 39.0 603.2 5.1 6 10.18 0.9 0.0619 1.0 T317-01-14 0.19 964 583 0.62 82.3 609.8 4.9 −1 10.06 0.8 0.0618 0.6 T317-01-1 0.21 253 79 0.32 20.7 587.3 5.3 −2 10.46 0.9 0.0613 1.1 T317-01-9 0.19 303 119 0.40 25.1 593.4 5.3 6 10.36 0.9 0.0613 1.1 T317-01-2 1.48 455 178 0.40 35.6 552.6 4.8 −12 10.98 0.9 0.0707 1.6 T317-01-5 2.66 299 73 0.25 18.0 429.1 4.1 48 14.24 0.9 0.0768 1.1 T317-01-12 1.23 385 197 0.53 25.9 480.8 4.4 14 12.78 0.9 0.0667 1.1 T317-01-13 3.07 327 168 0.53 23.2 495.1 4.9 −16 12.11 0.9 0.0821 1.0 T317-01-15 0.54 449 416 0.96 34.5 550.4 4.7 16 11.19 0.9 0.0629 1.0 Aplite granite syn-tectonic #T4-7-1 (Concordia age = 784.2 ± 14.1 (95%-conf.)) AG-1 0.75 87 27 0.33 9.5 771.8 12.8 −26 7.86 1.7 0.0651 2.7 AG-2 0.24 106 35 0.34 11.9 789.4 12.4 −4 7.67 1.6 0.0663 2.4 AG-3 0.32 85 24 0.29 9.5 780.9 12.9 1 7.74 1.7 0.0680 2.7 AG-4 0.25 150 54 0.37 16.6 782.6 11.4 −6 7.74 1.5 0.0657 2.1 AG-5 0.00 74 19 0.27 8.1 776.6 13.4 3 7.80 1.8 0.0657 2.9 AG-6 0.65 161 64 0.41 17.6 769.3 11.2 −6 7.85 1.5 0.0686 2.0 AG-7 0.13 111 38 0.35 12.6 797.7 12.3 1 7.58 1.6 0.0671 2.3 AG-8 0.12 163 93 0.59 19.6 843.1 12.1 −4 7.16 1.5 0.0671 1.9 AG-9 0.00 127 43 0.35 13.6 754.9 11.5 8 8.03 1.6 0.0663 2.2 AG-10 0.00 93 26 0.29 10.4 789.1 12.8 1 7.68 1.7 0.0656 2.5 AG-11 0.48 133 45 0.35 15.4 817.2 12.4 −21 7.41 1.6 0.0650 2.2 AG-12 0.00 190 78 0.43 19.7 735.3 10.4 4 8.27 1.4 0.0647 1.8

Reported ages are based on 207 corrected data. Ages older than 1.0 Ga are reported as 207Pb/206Pb ages based on 204 corrected data and are marked by asterisk. Uncertainties are reported at the 1σ level. Rad stands for Radiogenic and Disc for Discordancy. quite similar to that of the Negash diamictite, indicating a 2003), and the Gulgula Group contains clasts of this age provenance that is principally ANS crust with practically (Teklay et al., 2003). no contribution from pre-Neoproterozoic sources (two In terms of its zircon age distribution, the Shiraro zircons yielded Kibaran ages; Fig. 6B). The youngest sequence resembles the Negash diamictite in that it has concordant grain is 739 ± 6 Ma but the majority of the no zircons younger than 0.74 Ga, but the Shiraro con- detrital zircon ages range between 0.8 and 0.9 Ga, with tains a greater proportion of detrital zircons older than a peak at 0.87 Ga. This time interval corresponds to the 0.85 Ga. Just a small proportion of the detrital zircon early stages of ANS crustal growth, but – as noted for inventory of Shiraro potentially represents rocks dated Negash diamictite – rocks of this age have not been in the Tsaliet of Tigrai (∼0.82–0.75 Ga). Thus, although reported from Tigrai. On the other hand, 0.85 Ga igneous Negash and Shiraro sedimentary sections were derived activity was reported from western Eritrea (Teklay et al., from ANS crust and may have been deposited about the 100 D. Avigad et al. / Precambrian Research 154 (2007) 88–106

Fig. 6. (A) U–Pb SHRIMP detrital zircon ages (206Pb/238U) from Negash diamictites. Zircons younger than 750 Ma are either discordant or contain elevated 204Pb. (B) U–Pb SHRIMP (206Pb/238U) ages Fig. 5. Concordia plots for detrital zircons from the Negash diamictite, of detrital zircons from the Shiraro sequence. (C) U–Pb SHRIMP ages σ Shiraro arkose and Enticho sandstone. 2 error ellipses. (206Pb/238U, ages older than 1.0 Ga are 207Pb/206Pb ages based on 204 corrected data) of detrital zircons from the Enticho Ordovician same time, Shiraro may have been preferentially derived sandstone. Note change in time scale with respect to A and B. from western Eritrea basement sources. Concordia diagram (Fig. 5c), a cumulative histogram in 2.7. Detrital zircon geochronology of the Enticho Fig. 6C and a probability plot on Fig. 7. sandstone Figs. 6C and 7 show that the detritus in Enticho sandstone is dominated by Neoproterozic sources, with We collected samples of white Enticho sandstone subordinate contributions from older Kibaran crust and on the Adwa-Adigrat road (#T3-21-1; 14.27648◦N; minor contributions (∼10%) from pre-Kibaran sources. 39.11582◦E) from a stratigraphic position several tens Within the Neoproterozoic two modes are distinguished: of meters above the peneplain surface. The detrital zir- the older mode is at ∼0. 8 Ga with a major concentra- con ages from the Enticho sandstone are presented on a tion between 0.82 and 0.76 Ga, and the younger mode D. Avigad et al. / Precambrian Research 154 (2007) 88–106 101

oproterozoic zircons distinguishes Enticho as having been derived from erosion of a broader drainage basin. The Kibaran–Grenvillian zircons have counterparts in the Negash diamictite and in Shiraro and therefore proximal (as discussed above). The Ordovician-age zircons may have been derived from igneous rocks in southern Ethiopia (Yibas et al., 2002) but these young zircons are 60–80% discordant (Table 1a) and do not reﬂect a reliable age.

2.8. Basement geochronology of Tsaliet Group and Mereb granites Fig. 7. Cumulative probability diagram of 53 detrital zircon U–Pb ages from the Ordovician Enticho sandstone. Several older grains at In order to augment the existing data on the timing 1.8, 2.0, 2.2 and 2.6 Ga are not shown. The zircon age distribution is of igneous activity in Tigrai we dated three rock units. interpreted as generally reflecting the crustal evolution of the south- The results of U–Pb zircon geochronology and analytical ern ANS in Ethiopia. We suggest that onset of Tambien deposition data are presented in Table 1b. best fits the lull defined at around 0.7 Ga because Tambien deposition marks the complete shutdown of Tsaliet igneous activity and predated 2.9. Post-tectonic Mai Kenetal granite post-tectonic magmatism. Note that the timing of Neoproterozoic East African Orogeny and crustal thickening (pertaining to the onset of We sampled the post-tectonic granite (#T3-17-01) Gondwana collision) is very similar to the timing of Marinoan Snow- ◦ ball Earth glaciation. S—the timing of Sturtian glaciation, M—the from the Mai Kenetal area (Fig. 2) (14.04972 N; ◦ timing of Marinon glaciation (e.g. Hoffman et al., 2004). Note that 38.96864 E). Our analyses yield a mean age of Marinoan glaciation slightly predated the peak of post-orogenic mag- 612.3 ± 5.7 Ma, similar to ages of the Mereb granite to matism so that both Sturtian and Marinoan diamictites would share the east (606 ± 0.9; 613.4 ± 0.9; 608 ± 7 Ma, Miller et similar field relation to the major igneous phases that shaped the EAO in Ethiopia. al., 2003; Asrat et al., 2004). This age reflects regional granitic melt generation in the lower crust of the thickened Ethiopian basement (see also Teklay et al., 2001). has a peak at 0.62 Ga with a concentration between By this time, contractional deformation in Tigrai had 0.66 and 0.58 Ga. The two major igneous phases were ceased, but the currently exposed metamorphic rocks lay ∼ separated by a 100 m.y. interval of reduced igneous at a depth of 6–8 km (e.g. Asrat et al., 2004). activity with a magmatic lull about 0.69 Ga. We interpret the older mode to indicate contributions from Tsaliet- 2.10. Syntectonic aplite microgranite from the like crust, whereas the younger mode is a contribution Tsaliet Group from abundant late- to post-tectonic ‘Mereb’ granitoids. While the younger granites make up a smaller propor- A deformed aplite micro-granite body within the tion of Neoproterozoic crust in the region (∼20% in volcano-sedimentary section of Tigrai (“Tsaliet Gran- Tigrai, e.g. Tadesse-Alemu,1998; and our Fig. 2), the late itoid” in Fig. 2; #T4-7-1; 13.89018◦N; 39.41531◦E) was Neoproterozoic peak is almost as large as the early Neo- dated to establish the intrusion age of deformed syn- proterozoic peak because the Mereb granites are very tectonic intrusives in this area. SHRIMP analysis of 12 rich in zircon. The histogram contains also a small num- zircons detected no inherited zircons and yielded a con- ber of 0.89–0.93 and 0.9–1.1 Ga (Kibaran–Grenvillian) cordia age of 784 ± 14 Ma (Table 1b). This age is similar zircons as well as a few at 1.8, 2.0 and 2.5 Ga. These Pale- to the range of ∼740–780 Ma obtained for Tsaliet arc- oproterozoic and Late Archean zircons may come from related igneous activity in Tigrai by previous works. The old basement remobilized during the Neoproterozoic age and lack of inherited zircons in this sample support such as detected in eastern Ethiopia (Teklay et al., 1998) the notion that crust in this region formed in Neoprotero- and western Ethiopia (e.g. Kebede et al., 2001), or from a zoic time. more distal source such as in Yemen (Whitehouse et al., 1998) where Late Archean–Early Proterozoic crust has 2.11. Metavolcanics/Sill from the top of the Tsaliet been reworked during the Neoproterozoic and intruded Group by abundant 0.76 Ga granitoids. Unlike the local provenance reflected by zircons from the Negash diamictite In the Madhane-Alem region, the section is over- and Shiraro sequence, the presence of Archean and Pale- turned and the Tsaliet metavolcanics overly the Tambien 102 D. Avigad et al. / Precambrian Research 154 (2007) 88–106

Group (e.g. Fig. 4). A sample of concordant, green- logical data and geological consideration would favor ish, fine grained volcanic rock (#T3-11-10; 13.94560◦N; the formation of the diamictites starting 0.72–0.70 Ga 39.61816◦E), possibly a sill injected into volcaniclas- (broadly Sturtian or later) but the exact timing cannot be tics ∼20 m below the first massive Tambien carbonate constrained further. (Didikama Fm) beds, was dated to establish the age of Additional insights into the origin of the Neopro- igneous activity at the top (and presumably the youngest) terozoic glacial record (assuming Tambien diamictites part of the Tsaliet. A mean age of 774.7 ± 5.7 Ma was are indeed glacigenic) may be obtained by considering derived from a cluster of 8 zircons measured (Table 1b). the content of the diamictites. Unlike typical tillites in We interpret this result as the eruption/intrusion age which far-traveled clasts are usually observed, Tambien of this body. The observation that a syntectonic aplite diamictites appear to have been derived from a proxi- and this sill/metavolcanic from different localities yield mal source. Neither the clast inventory nor the detrital similar ages provides further support for the conclusion zircon spectra contain contributions from extra-ANS that igneous activity was widespread in the region at pre-Neoproterozoic sources so that an origin from a 0.77–0.78 Ga, and that the Tambien Group is younger. continental-scale ice sheet is difficult to envisage. The diamictite clast lithologies contain Tambien carbonate 2.12. On the timing and nature of diamictite and Tsaliet volcanics, indicating that Tambien sedimen- formation tation must have been interrupted to deposit the Negash and Shiraro diamictites. The source area comprising Neither Shiraro nor Negash diamictites contain clasts both Tambien and Tsaliet rocks must have been locally or zircons from the widespread and distinctive late uplifted and exposed, eroded and re-deposited at the Neoproterozoic post-tectonic ∼610 Ma Mereb granites. top of the Tambien sequence to yield the diamictite. This is consistent with both diamictites being older The presence of Tsaliet Group clasts in the diamictite than the post-tectonic granitoid intrusions in the region may farther indicate that the entire Tambien (∼1500 m) (Tadesse, 1996a). They thus significantly differ from was locally removed by erosion. This implies significant late Neoproterozoic molasse basins which are abun- vertical movements which cannot be accommodated by dant in the northern ANS and which contain abundant sea level drop due to Neoproterozoic glaciation alone. detritus derived from post-orogenic granitoids (Jarrar et One of the carbonate clasts identified in the diamictite al., 1991; Garfunkel, 1999; Johnson, 2003; Wilde and yielded (e.g. Miller et al., 2006) negative δ13C vlaue of Youssef, 2002; Weissbrod and Sneh, 2002). the type identified by Miller et al. (2006) and by Alene The youngest concordant zircons in the Negash et al. (2006) at the very bottom of the Tambien sequence diamictite and Shiraro sequence are ∼745 Ma and these (Assem limestone; Mai Kenetal synclinoria). This fea- sequences must therefore be younger. Thus, as far as the ture implies reworking of the deepest part of the Tambien zircon data tell the diamictites may have been deposited sequence and is consistent with the Tambien basin being at any time between 0.745 and ∼0.62 Ga. locally uplifted and tectonically inverted to generate the Miller et al. (2003) suggested an age of ca. diamictite. 0.75–0.72 Ga for the Negash diamictite on the basis of The current incomplete understanding of the geo- strontium isotope ratios measured in the underlying Tam- tectonic evolution of the southern ANS as well as the bien carbonates. Deposition of the diamictites must have absence of a definite age for the diamictite hinder the been preceded by thermal contraction and subsidence of recognition of the exact link between ANS tectonics and the Tsaliet arc complex (starting ∼0.74 Ga), and by the the formation of Tigrai diamictites. Two significant tec- deposition of carbonate and clays. The Tsaliet arc may tonic phases are usually reported to have affected the have been inundated for quite some time to accumu- ANS. The older phase corresponds to the closure of var- late a 1 km thick Tambien carbonate section. While the ious ocean basins and to the accretion of island arcs duration is unknown, a few millions of years to several (Abdelsalam and Stern, 1996), and went on in different tens of million years is not an unreasonable estimate. ANS segments from ∼0.78 to ∼0.63 Ga. In Ethiopia, The time interval defined in Fig. 7 as reduced igneous the exact timing of arc suturing and ophiolite emplace- activity (starting ∼0.74 Ga) may thus be appropriate for ment is not well constrained but it appears to predate the Tambien carbonate sedimentation. Formation of Negash deposition of the Tambien Group. In the Axum area (e.g. diamictite and Shiraro sequences should have taken Tadesse et al., 2000; and Fig. 2) the ophiolitic belt is pen- place afterwards, but prior to ∼0.62 Ga. Allowing few etrated by syntectonic granitoid dated to 0.75–0.79 Ga tens of million years for Tsaliet thermal subsidence and (Tadesse et al., 2000) implying arc terranes were sutured Tambien carbonate deposition, the available geochrono- by then. In western Ethiopia, metamorphism possibly D. Avigad et al. / Precambrian Research 154 (2007) 88–106 103 pertaining to arc suturing was reported by Ayalew et cambrian basement and is mineralogically and texturally al. (1990) at 0.76 Ga suggesting orogenic movements more evolved. The sandstones originated from a wider related to arc suturing predated diamictite deposition. drainage area so that they tap an important area of the Moreover, sutured arcs would be a plausible source for underlying ANS basement, and for this reason we think the low-grade metamorphic clasts observed by us in that the detrital zircon age inventory reflects the surface Ethiopian diamictites and by Teklay et al. (2003) in the age-composition of the Ethiopian zircon-bearing base- equivalent Gulgula section of western Eritrea. ment in Ordovician times. This sample largely excludes A subsequent major phase of crustal shortening and mafic and ultramafic rocks that usually have low zircon orogeny affected ANS as well as other parts of the content and will be biased towards felsic rocks that are EAO in relation to the protracted fusion of Gondwana rich in zircon. starting at about 0.63 Ga. Direct evidence from western Unlike previous studies that (tacitly perhaps, but not Ethiopia include a major basement isotopic rehomoge- explicitly) assume continuous crustal growth for the nization of the Rb–Sr system at 632 ± 8 Ma, (e.g. Ayalew ANS from 870 Ma until the end of Neoproterozoic time, et al., 1990) and the development of zircon growth rim the detrital zircon ages reveal that in Ethiopia, ANS at 629 Ma (Kebede et al., 2001). This is considered to zircon-producing igneous activity took place in two be the time of collision over the length of the EAO from major phases. Island arc volcanism lasted until ∼0.75 Ga central Tanzania northward (Sommer et al., 2005), but followed by a period of ca. 100 m.y. during which subsi- the ages quoted above probably represent the culmina- dence and deposition of shallow marine carbonates and tion of crustal thickening and thermal re-equilibration of mudrocks of the Tambien Group ocurred; this was later the thickened orogen, so contractional tectonics prob- followed by the major phase of collision orogeny and ably commenced earlier. Remarkably, EAO collision crustal thickening (culminating at ca. 0.63 Ga) related to culminated about the same time as Marinoan glacia- Gondwana fusion. In Tigrai, terminal collision is mani- tion at 635 ± 2Ma (Hoffman et al., 2004). Although fested by upright folding (such as Negash synclinoria), a Marinoan age for the Negash diamictite and Shiraro thrusting and lateral displacements. This was followed sequence is 100 Ma younger than the youngest detri- by a phase of late- to post-orogenic igneous activity that tal zircon detected by us in the diamictites, we note peaked at around 0.62 Ga. The first phase of island arc that it is not inconsistent with the detrital zircon data volcanism represents crustal growth via the subduction (see Fig. 7). Marinoan glacial diamictites in Ethiopia, if factory, whereas the later phase of post-tectonic igneous there were any, would not be expected to collect zir- activity was related to crustal thickening and collision cons younger than 0.75 Ga anyway, because igneous and as such likely involved melting of previous lower activity in the region was reduced significantly starting crust rocks (see also Teklay et al., 2001; Bentor, 1985; 0.75 Ga and peaked again at 0.63–0.62 Ga, as demon- Stein, 2003). strated by the Entichio detrital zircon ages. Inversion A summary of published ages from the Ethiopian of the Tsaliet-Tambien basin in the course of final clo- basement (e.g. Asrat et al., 2001) shows significant sim- sure and collision along the EAO could be a plausible ilarities with the Enticho detrital zircon ages and lends source for the formation of the diamictites as well as for credence to our crustal evolutionary model for Tigrai. their subsequent folding and metamorphism. However, Each of the detrital zircon peaks we observed can be because the exact age of North Ethiopia diamictites is matched with dated Ethiopian basement. A strength of currently not well constrained and because the setting of detrital zircon geochronology is in providing a quasi- tectonic processes affecting this area is not fully under- quantitative indications of the scale and intensity of stood, more work is required in order to assess the link each of the igneous phases monitored, although biased between EAO tectonic evolution and the origin of the towards zircon-rich sources. In this respect, we note diamictites. that out of all dated rocks in Ethiopia (e.g. Asrat et al., 2001, and references therein), only a handful yielded 2.13. Implications for ANS crustal evolution 0.75–0.65 Ga U–Pb zircon ages, consistent with this period being characterized by reduced igneous activ- Detrital zircon geochronology is a powerful tool for ity, and by subsidence and marine sedimentation of the investigating the crustal evolution of source terranes. The Tambien Group. Negash diamictite and the Shiraro sequence are sed- A summary of radiometric ages for ANS ophiolites imentologically immature and were probably derived (e.g. Stern et al., 2004) shows a spread from 0.87 to from a limited drainage basin. However, the Enticho 0.74 Ga, overlapping the time period defined by us sandstone overlies a regional peneplain atop of the Pre- for island-arc volcanism and strengthening the notion 104 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 presented by us whereby this was the major crustal at CEREGE (France). Comments by two anonymous building phase in Ethiopia. Moreover, a similar timing of reviewers helped to improve this manuscript and are arc volcanism (particularly in the 0.8–0.76 Ga interval) greatly appreciated. is observed also in pre-Neoproterozoic terranes at the border and outside ANS such as in Yemen (Whitehouse References et al., 1998) and Madagascar (Kroner,¨ 2001), indicating igneous activity was then widespread within and beyond Abdelsalam, M.G., Stern, R.J., 1996. Sutures and shear zones in the the ANS. Arabian–Nubian Shield. J. Afr. Earth Sci. 23, 289–310. The detrital zircon ages of the Ordovician Enticho Abdelsalam, M.G., Liegeois,´ J.P., Stern, R.J., 2002. The Saharan Metacraton. J. Afr. Earth Sci. 34, 119–136. sandstone probably represent crustal ages of both north- Anderson, U.B., Ghebreab, W., Teklay, M., 2006. Crustal evolution and ern Ethiopia (Tigrai) as well as the southern, eastern and metamorphism in east central Eritrea, south-east Arabian–Nubian western parts of Ethiopia where high grade rocks are Shield. J. Afr. Earth Sci. 44, 45–65. abundant. The originally extensive nature of Ordovician Arkin, Y., Beyth, M., Dow, D.B., Levitte, D., Haile, T., Hailu, T., 1971. exposures of the high-grade Mozambique sequences, Geological map of Mekele Sheet area ND 37-11 Tigrai Province, 1:250,000. Geological Survey of Ethiopia. and their potential role as a source for Enticho detritus, Alene, M., Raffini, R., Sacchi, 2000. Geochemistry and geotec- is indicated by a small outcrop of Paleozoic sandstone tonic setting of Neoproterozoic rocks from northern Ethiopia mapped above high-grade basement rocks in southwest (Arabian–Nubian shield). Gondwana Res. 3 (3), 333–347. Ethiopia (∼80 km south of Gambella; Geological Map of Alene, M., Jenkin, G.R.T., Leng, M.J., Fiona Darbyshire, D.P., Ethiopia, 1:200,0000; 1966). Additional support comes 2006. The Tambien Group, Ethiopia: an early Cryogenian (c. 800–735 Ma) Neoproterozoic sequence in the Arabian–Nubian from cooling ages from these rocks (Asrat et al., 2001 Shield. Precambrian Res. 147, 79–99. and references therein; Mock et al., 1999; Ghebreab Asrat, A., Barbey, P., Gleizes, G., 2001. The Precambrian geology of et al., 2005) and from southern Ethiopia (Yibas et al., Ethiopia: a review. Afr. Geosci. Rev. 8, 271–288. 2002) showing the orogenic edifice cooled by late Cam- Asrat, A., Barbey, P., Ludden, J.N., Reisberg, L., Gleizes, G., Ayalew, brian time. Being derived from all Ethiopian basement D., 2004. Petrology and isotope geochemistry of the Pan-African Negash Pluton, Northern Ethiopia: Mafic-Felsic Magma Inter- intervals, including the high-grade Mozambique Belt, actions during the Construction of Shallow-level Calc-alkaline the detrital zircon age spectra gain extra importance in Plutons. J. Petrol. 45, 1147–1179. that they confirm that volumetrically significant, pristine Avigad, D., Kolodner, K., McWilliams, M., Persing, H., Weissbrod, T., pre-Neoproterozoic crust does not exist in Ethiopia. The 2003. Origin of northern Gondwana Cambrian sandstone revealed presence of a small but not negligible amount of 1.1 Ga by detrital zircon SHRIMP dating. Geology 31 (3), 227–230. Avigad, D., Sandler, A., Kolodner, K., Stern, R.J., McWilliams, and 1.9–2.5 Ga detrital zircons in the Enticho sandstone M., Miller, N., Beyth, M., 2005. Mass-production of Cambro- (ca. 10% of the analysed zircons) may reflect inheritance Ordovician quartz-rich sandstone as a consequence of chemical from an older component incorporated at the source of weathering of Pan-African terranes: Environmental implications. the arc magmas (e.g. Teklay et al., 1998) or derivation Earth Planet. Sci. Lett. 240, 818–826. from terranes flanking ANS such as the Abas of Yemen Ayalew, T., Bell, K., Moore, J.M., Parrish, R.R., 1990. U–Pb and Rb–Sr geochronology of the Western Ethiopian Shield. Geol. Soc. Am. (e.g. Whitehouse et al., 1998). Bull. 102, 1309–1316. Bentor, Y.K., 1985. The crustal evolution of the Arabo-Nubian massif Acknowledgements with special reference to the Sinai peninsula. Precambrian Res. 28, 1–74. Our study in Ethiopia is funded by USA–Israel Bina- Beyth, M., 1972. The geology of central western Tigrai, Ethiopia. Ph.D. Thesis. University of Bonn, 155 pp. tional Science Foundation (BSF). We thank D. Kuster¨ Beyth, M., Avigad, D., Wetzel, H., Matthews, A., Berhe, S.M., 2003. (Mekelle University) and Kiros Mehari (Hisana Min- Crustal exhumation and indications for Snowball Earth in the East ing) for hospitality and assistance during our work in African Orogen: north Ethiopia and east Eritrea. In: Kusky, T., Tigrai and T. Tadesse for providing helpful comments Abdelsalam, M.G., Tucker, R., Stern, R.J. (Eds.). Evolution of the on the manuscript. F. Mazdab and B. Wiegand (USGS) East African and Related Orogens, and Assembly of Gondwana. Precambrian Res. 123, 187–201. guided us on the ion probe and their help is greatly appre- Bonavia, F.F., Chorowicz, J., 1993. Northward expulsion of the Pan- ciated. T. Hurgrove is thanked for analysing igneous African of northeast Africa guided by reentrant zone of the rock samples T3-11-10 and T3-17-01. We also thank A. Tanzanian craton. Geology 20, 1023–1026. Abraham, chief geologist of the Ethiopian Geological Burke, K., McGregor, D.S., Cameron, N.R., 2003. African Petroleum Survey for support and M. Alene from the University Systems: Four Tectonic Aces in the past 600 million years. The Geological Society of London, pp. 1–20 (special publication 207). of Addis Ababa for joining us on the 2004 field trip. Collins, A.S., Pisarevsky, S.A., 2005. Amalgamating eastern Gond- D.A. acknowledges hospitality and discussions with P. wana: The evolution of the Circum-Indian orogens. Earth Sci. Rev. Henry, O. Bellier and P. Affaton while on a sabbatical 71, 229–270. D. Avigad et al. / Precambrian Research 154 (2007) 88–106 105

Garfunkel, Z., 1999. History and paleogeography during the Pan- Kroner,¨ A., Linnebacher, P., Stern, R.J., Reischmann, T., Manton, W., African orogen to stable platform transition: reappraisal of the Hussein, I.M., 1991. Evolution of Pan-African island arc assem- evidence from Elat area and the northern Arabian–Nubian Shield. blages in the southern Red Sea Hills, Sudan, and in southwestern Israel J. Earth Sci. 48, 135–157. Arabia as exemplified by geochemistry and geochronology. Pre- Garfunkel, Z., 2002. Early Paleozoic sediments of NE Africa and Ara- cambrian Res. 53, 99–188. bia: Products of continental-scale erosion, sediment transport, and Kumulainen, R.A., Uchman, A., Woldehaimanot, B., Kreuser, T., Ghir- deposition. Israel J. Earth Sci. 51, 135–156. may, S., 2006. Trace fossil evidence from the Adigrat Sandstone Garland, C.R., 1980. Geological map of Adigrat Sheet area ND 37-7 for an Ordovician glaciation in Eritrea, NE Africa. J. Afr. Earth Tigrai Province. Geological Survey of Ethiopia. Sci. 45, 408–420. Ghebreab, W., Talbot, C.J., Page, L., 2005. Time constraints on Ludwig, K.R., 1994. Isoplot—a plotting and regression program for exhumation of the East Africa Orogen from field observations radiogenic–isotope data. US Geological Survey, pp. 91–445. and 40Ar/39Ar cooling ages of low-angle mylonites in Eritrea, NE Meert, J., 2003. A synopsis of events related to the assembly of eastern Africa. Precambrian Res. 139, 20–41. Gondwana. Tectonophysics 362, 1–40. Hargrove, U.S., Stern, R.J., Kimura, J.-I., Manton, W.I., Johnson, Miller, N.R., Alene, M., Sacci, R., Stern, R.J., Kroner,¨ A., Conti, A., P.R., 2006. How juvenile is the Arabian–Nubian Shield? Evidence Zuppi, G., 2003. Significance of the Tambien Group (Tigrai, N. from Nd isotopes and pre-Neoproterozoic inherited zircon in the Ethiopia) for Snowball Earth Events in the Arabian–Nubian Shield. Bi’r Umq suture zone, Saudi Arabia. Earth Planet. Sci. Lett. 252, Precambrian Res. 121, 263–283. 308–326. Miller, N., Schilman, B., Avigad, D., Stern, R.J., Beyth, M., 2006. Hoffman, P.F., Schrag, D.P., 2000. Snowball Earth. Sci. Am. 282, Neoproterozoic Snowball Earth – the northern Ethiopia record. 62–75. Abstract presented in the “Snowball Earth 2006” conference, Hoffman, K.H., Condon, D.J., Bowiring, S.A., Crowley, J.L., 2004. Monte Verita, Switzerland, pp. 74–75. U–Pb zircon date from the Neoproterozoic Ghaub forma- Mock, C., Arnaud, N.O., Cantagrel, J.-M., Yirgu, G., 1999. tion, Namibia: constrains on Marinoan glaciation. Geology 32, 40Ar/39Ar thermochronologyof the Ethiopian and Yemeni base- 817–820. ments: reheating related to the Afar plume? Tectonophysics 314, Jacobs, J., Thomas, R.J., 2004. Himalayan-type indenter-escape 351–372. tectonics model for the southern part of the late Neoproterozoic- Paces, J.B., Miller, J.D., 1993. U–Pb ages of the Duluth complex and early Paleozoic East African-Antarctic orogen. Geology 32, related mafic intrusions, northeastern Minnesota. geochronologic 721–724. insights into physical petrogenetic, paleomagnetic and tectono- Jarrar, G.H., Wachendorf, H., Zellmer, D., 1991. The Saramuj Con- magnetic processes associated with the 1.1 Ga mid-continent rift glomerate: Evolution of a Pan-African molasse sequence from system. J. Geophys. Res. 98, 13997–14013. southwest Jordan. N. Jb. Geol. Palaontol. Mh. 6, 335–356. Saxena, G.N., Assefa, G., 1983. New evidence on the age of the glacial Johnson, P.R., 2003. Post-amalgamation basins of the NE Arabian rocks of northern Ethiopia. Geol. Mag. 120 (6), 549–554. shield and implications for Neoproterozoic III tectonism in the Sommer, H., Kroner,¨ A., Hauzenberger, C., Muhongo, S., 2005. northern East African Orogen. Precambrian Res. 123, 321–337. Reworking of Archaean and Palaeoproterozoic crust in the Mozam- Johnson, P.R., Woldehaimanot, B., 2003. Development of the bique belt of central Tanzania as documented by SHRIMP zircon Arabian–Nubian Shield: Perspectives on accretion and defor- geochronology. J. Afr. Earth Sci. 43, 447–463. mation in the northern East African Orogen and assembly Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead of Gondwana. In: Yoshida, M., Windley, B.F., Dasgupta, S. isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, (Eds.), Proterozoic East Gondwana: Supercontinent Assembly and 207–221. Breakup, vol. 206. Geological Society, London, pp. 289–325 (Spe- Stern, R.J., 1994. Arc assembly and continental collision in the Neopro- cial Publication). terozoic East African Orogen: implications for the consolidation Katz, O., Beyth, M., Miller, N., Stern, R., Avigad, D., Basu, A., Anbar, of Gondwanaland. Annu. Rev. Earth Planet 22, 319–351. A., 2004. A Late Neoproterozoic (∼630 Ma) Boninitic Suite from Stern, R.J., 2002. Crustal evolution in the East African Orogen: a southern Israel: implications for the consolidation of Gondwana- Neodymium Isotopic perspective. J. Afr. Earth Sci. 34, 109–117. land. Earth Planet. Sci. Lett. 218, 475–490. Stern, R.J., Johnson, P.R., Kroner,¨ A., Yibas, B., 2004. Neoproterozoic Kazmin, V., 1973. Geological Map of Ethiopia (scale 1:2,000,000). ophiolites of the Arabian–Nubian shield. In: Kusky, T.M. (Ed.), Geological Survey of Ethiopia, Addis Ababa. Precambrian Ophiolites and Related Rocks, vol. 13. Developments Kazmin, V., Shiferau, A., Balcha, T., 1978. The Ethiopian basement: in Precambrian Geology, pp. 95–128. stratigraphy and possible manner of evolution. Geol. Rundsch. 67, Stern, R.J., Avigad, D., Miller, N.R., Beyth, M., 2006. Geological Soci- 531–546. ety of Africa presidential review: evidence for the snowball Earth Kebede, T., Kloetzli, U.S., Koeberl, C., 2001. U/Pb and Pb/Pb zir- Hypothesis in the Arabian–Nubian Shield and the East African con ages from granitoid rocks of Wallagga area: constraints on Orogen. J. Afr. Earth Sci. 44, 1–20. magmatic and tectonic evolution of Precambrian rocks of western Stein, M., Goldstein, S.L., 1996. From plume head to continental Ethiopia. Mineral. Petrol. 71, 251–271. lithosphere in the Arabian–Nubian shield. Nature 382, 773–778. Kolodner, K., Avigad, D., McWilliams, M., Wooden, J.L., Weissbrod, Stein, M., 2003. Tracing the plume material in the Arabian–Nubian T., Feinstein, S., 2006. Provenance of north Gondwana Cambrian- Shield. Precambrian Res. 123, 223–234. Ordovician sandstone: U–Pb SHRIMP dating of detrital zircons Stoeser, D.B., Frost, C.D., 2006. Nd, Pb, Sr, and O isotopic char- from Israel and Jordan. Geol. Mag. 143, 367–391. acterization of Saudi Arabian Shield terranes. Chem. Geol. 226, Kroner,¨ A., 2001. The Mozambique belt of East Africa and Madgascar: 163–188. Significance of zircon and Nd model ages for Rodinia and Gond- Tadesse-Alemu, A., 1998. Geochemistry of Neoproterozoic granitoids wana supercontinent formation and dispersal. South Afr. J. Geol. from the Axum area, northern Ethiopia. J. Afr. Earth Sci. 27 (3–4), 104, 151–166. 437–460. 106 D. Avigad et al. / Precambrian Research 154 (2007) 88–106

Tadesse, T., 1996a. The geology of Axum Sheet (ND 37-6). Geological association, serpentinite mudﬂows and across-arc geochemical Survey of Ethiopia. Memoir No. 9, p. 186. trends in Eritrea, southern Arabian–Nubian Shield. Precambrian Tadesse, T., 1996b. Structure across a possible intra-oceanic suture Res. 145, 81–92. zone in the low-grade Pan-African rocks of northern Ethiopia. J. Unrug, R., 1997. Rodinia to Gondwana: the geodynamic map of Gond- Afr. Earth Sci. 23, 375–381. wana supercontinent assembly. GSA Today 7, 2–6. Tadesse, T., Suzuki, K., Hoshino, M., 1997. Chemical Th–U total Pb Veevers, J.J., 2004. Gondwana from 650–500 Ma assembly through isochron age of zircon from the Mareb Granite in northern Ethiopia. 320 Ma merger in Pangea to 185–100 breakup: supercontinental J. Earth Planet. Sci. Negoya Uni. 44, 21–27. tectonics via stratigraphy and radiometric dating. Earth Sci. Rev. Tadesse, T., Hoshino, M., Sawada, Y., 1999. Geochemistry of low- 68, 1–132. grade metavolcanic rocks from the Pan-African of the Axum area, Wang, J., 2003. History of Neoproterozoic rift basins in south China: northern Ethiopia. Precambrian Res. 99, 101–124. imlication for Rodinia break-up. Precambrian Res. 122 (1–4), Tadesse, T., Hoshino, M., Suzuki, K., Iizumi, S., 2000. Nd, Rb–Sr and 141–158. Th–U–Pb zircon ages of syn- and post-tectonic granitoids from the Weil, A., 2004. Paleomagnetism of the Neoproterozoic Chuar Group, Axum area of northern Ethiopia. J. Afr. Earth Sci. 30, 313–327. Grand Canyon Supergroup, Arizona: implication for Laurentia’s Teklay, M., 1997. Petrology, Geochemistry and Geochronology of Neoproterozoic APWP and Rodinia break-up. Precambrian Res. Neoproterozoic Magmatic Arc Rocks from Eritrea: Implications 129, 71–92. for Crustal Evolution in the Southern Nubian Shield, vol. 1. Depart- Wilde, S.A., Youssef,K., 2002. A re-evaluation of the origin and setting ment of Mines, Eritrea, p. 125 (Memoir). of the Late Precambrian Hammamat Group based on SHRIMP Teklay, M., Kroner,¨ A., Mezger, K., Oberhansli, R., 1998. Geo- U–Pb dating of detrital zircons from Gebel Umm Tawat, North- chemistry, Pb–Pb zircon ages and Nd-Sr isotope composition of Eastern Desert, Egypt. J. Geol. Soc. Lond. 159, 595–604. Precambrian rocks from southern and eastern Ethiopia: implica- Weissbrod, T., Sneh, A., 2002. Sedimentology and paleogeography of tions for crustal evolution in East Africa. J. Afr. Earth Sci. 26, the Late Precambrian–Early Cambrian arkosic and conglomeratic 207–227. facies in the northern margins of the Arabo-Nubian Shield Bulletin, Teklay, M., Kroner,¨ A., Mezger, K., 2001. Geochemistry, geochronol- Geological Survey of Israel, vol. 87, 44 pp. ogy and isotope geology of Nakfa intrusive rocks, northern Eritrea: Whitehouse, M., Windley, B.F., Ba-Bttat, M.A.O., Fanning, M., Rex, products of a tectonically thickened Neoproterozoic arc crust. J. D.C., 1998. Crustal evolution and terrane correlation in the east- Afr. Earth Sci. 33, 283–301. ern Arabian-Shield, Yemen: geochronological constraints. J. Geol. Teklay, M., Haile, T., Kroner,¨ A., Asmerom, Y., Watson, J., 2003. Soc. Lond. 155, 281–295. A back-arc Paleotectonic setting for the Augaro Neoproterozoic Yibas, B., Reimond, W.U., Armstrong, R., Koeberl, C., Anhaeusser, magmatic rocks of western Eritrea. Gondwana Res. 6 (4), 629–640. C.R., Phillips, D., 2002. The tectonostratigraphy, granitoid Teklay, M., 2006. Neoproterozoic arc–back–arc system analog to geochronology and geological evolution of the Precambrian of modem arc–back–arc systems: evidence from tholeiite-boninite southern Ethiopia. J. Afr. Earth Sci. 34, 57–84.