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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. Beyth c , N. Miller b , M.O. McWilliams d
a
Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Geosciences Department, University of Texas at Dallas, Richardson, TX 75083-0688, USA
c Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel
Department of Geological and Environmental Sciences, Stanford University, CA 94305-2115, USA
b
d
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.
E-mail address: avigad@vms.huji.ac.il (D. Avigad).
0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.precamres.2006.12.004
Inasmuch as Rodinia rifted apart during the Neoproterozoic (e.g. Wang, 2003; Weil, 2004), great parts
D. Avigad et al. / Precambrian Research 154 (2007) 88–106
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 Gondwana (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 significance requires understanding the orogenic processes that
shaped this region. Here, we present U–Pb SHRIMP
dating of detrital zircons from Cryogenian diamictites
and Lower Paleozoic sandstone to temporally constrain
crustal evolution and orogeny in north Ethiopia (southern Arabian Nubian Shield). We integrate these results
with dating of igneous rocks and use these data to further
clarify the properties of Cryogenian diamictites in this
region, and attempt to place them within the history of
Neoproterozoic crust formation and orogeny.
2. Geological setting
The East African Orogen (EAO; Fig. 1, Stern, 1994)
formed during Neoproterozoic time by closure of the
Mozambique Ocean. It comprises two major segments:
the Arabian–Nubian Shield (ANS) in the north, and the
Mozambique Belt in the south. ANS is juvenile Neoproterozoic crust, its growth involved intra-oceanic arc
volcanism and perhaps accretion of oceanic plateaux
(Bentor, 1985; Stern, 1994; Stein and Goldstein, 1996;
Stern, 2002; Johnson and Woldehaimanot, 2003). Arc
terranes were welded together beginning about 780 Ma
and then tectonically thickened as a result of convergence between the East Sahara Metacraton in the west
(Abdelsalam et al., 2002), and a number of continental
89
Fig. 1. The Neoproterozoic East African Orogen and the
Arabian–Nubian Shield (after Stern, 2002). Approximate location of
Fig. 2 is marked.
fragments in the east (e.g. the Afif-Abas terranes in Arabia and Azania in East Africa; Collins and Pisarevsky,
2005) soon after 630 Ma (Katz et al., 2004). Convergence related to the progressive amalgamation of East
Gondwana and its docking on the SE margins of the
EAO continued until 550 Ma (Meert, 2003; Collins and
Pisarevsky, 2005)) leading to strike slip faulting, lateral
displacements and northward extrusion (Bonavia and
Chorowicz, 1993; Jacobs and Thomas, 2004).
The ANS (Fig. 1) is dominated by supracrustal
metavolcanics including volcaniclastics and immature
sediments mostly metamorphosed in the greenschist
facies, variously deformed and intruded by granites, gabbros, and dikes. Geochemical and isotopic signatures
indicate that these rocks are dominantly mantle-derived
juvenile crust (Stern, 2002; Stoeser and Frost, 2006).
In Ethiopia, the ANS merges with the Mozambique
Belt which is the southern half of the EAO and which
accommodated the most intense collision between East
and West Gondwana fragments (e.g. Stern, 1994).
The Mozambique Belt exposes higher temperature and
pressure suites with abundant amphibolite and granulitefacies metamorphic rocks and gneiss terranes.
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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 protholiths (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
overlying Tambien Group (Beyth, 1972). The Tsaliet
Group is several kilometers thick and represents an arc
volcano-sedimentary sequence (Teklay, 1997; Alene et
al., 1998; Tadesse-Alemu, 1998; Tadesse et al., 1999)
or an arc–back–arc system (Teklay, 2006), whereas the
overlying Tambien Group (Miller et al., 2003; Alene et
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
surrounded by Tsaliet Group metavolcanics. The presence of marine carbonates low in the Tambien Group
sequence indicates that by the time the Tambien Group
was deposited, large portions of the arc complex lay
below sea level. A complete shutdown of regional arc
volcanism is inferred from the absence of interbedded
lavas or tuffs from the Tambien carbonate and from the
overlying clastics and diamictites.
A synthesis of available crystallization ages (mainly
zircon U–Pb and Pb–Pb) on Tsaliet arc crust in Ethiopia
(e.g. Tadesse et al., 1999; Alene et al., 2000; Tadesse et
al., 2000) shows that igneous activity occurred mainly
Fig. 3. Schematic geologic columnar section of Tigrai region showing
main rock units. Two major Neoproterozoic units are distinguished:
Tsaliet Group (metamorphosed arc volcanics and syntectonic granitoid intrusions) and overlying metasediments of the Tambien Group.
Diamictites comprise the top of the Tambien Group. The entire Neoproterozoic section is pierced by post-tectonic Mereb-type granitoids.
Ordovician (Enticho) sandstone and associated Endaga Arbi tillites
overly the peneplained basement.
at 820–740 Ma, but older (854 ± 3 Ma) arc volcanics
occur in neighboring Eritrea (Teklay, 1997; Teklay et
al., 2003; Anderson et al., 2006). Thus, Tsaliet Group
arc volcanism – and the most important episode of
crust building – lasted in this region from ∼0.85 to
0.74 Ga.
The Tambien Group section has been studied by
us at Negash and in Mai Kenetal synclinoria (Fig. 2).
At Negash it contains (from base to top): ∼1.2 km
of shallow marine carbonate and mudrocks (now calcareous slate), overlain by 250 m of finely laminated
black limestone (deposited in a relatively deeper water
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.
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D. Avigad et al. / Precambrian Research 154 (2007) 88–106
transitional sequence of thinly bedded black limestone,
calcareous slate, and non-calcareous slates, culminating
with a diamictite.
The diamictite, ca. 200 m thick, is pinched within
the Negash synclinoria (Fig. 4 transect B); it is weakly
metamorphosed and displays vertical layering parallel
to a pervasive N–S schistosity. It has been interpreted as
glacigenic because it contains a wide range of matrixsupported clasts of many rock types that appear to be
dropstones (see also Miller et al., 2003; Beyth et al.,
2003). Yet, striated and/or faceted pebbles were not
observed by us and the interpretation of the diamictites as glacigenic is therefore not unequivocal. Clasts,
angular, subrounded or slightly elongated, include felsic
volcanics, fine-grained black limestone and dolomites,
low-grade semipelitic sediments, rare volcanic conglomerates, pink granite and vein quartz, and are usually
up to 10 cm in size. Some clasts appear to have been
affected by low grade metamorphism prior to incorporation into the diamictite, their presence reflects a
metamorphic phase precedent to the regionally observed
collision-related metamorphism. Cap carbonates do not
exist in the area as all carbonate units lie stratigraphically below the Negash and Shiraro diamictites
(e.g. Fig. 3).
Miller et al. (2003) demonstrated that most Tambien carbonates have 87 Sr/86 Sr between 0.7050 and
0.7067, the maximum observed in the black limestone,
which is also characterized by heavy δ13 C (+3 to +6‰).
At Negash (Fig. 2), the transition sequence below the
diamictite shows significantly lighter δ 13 C (≤−2 to
−9‰), similar to that observed elsewhere for Cryogenian glacial transitions (e.g. Hoffman and Schrag, 2000).
Miller et al. (2003) found that the C- and Sr-isotope
composition of the Tambien are most consistent with an
age range of 720–750 Ma, broadly corresponding to the
Sturtian glaciation. Farther chemostratigraphic investigations on Tambien carbonates were recently published
by Alene et al. (2006) who generally accepted a Sturtian
age for the overlying diamictite.
The Shiraro diamictite (Beyth, 1972; Tadesse, 1996a)
is exposed in the lowlands of westernmost Tigrai, straddling the border with Eritrea (Figs. 2 and 4). Possibly
equivalent sediments in westernmost Eritrea are known
as the Gulgula Group (Teklay et al., 2003). The Shiraro
diamictite is widely distributed but is not well described.
Tambien-like carbonates are locally exposed below it.
It contains polymictic conglomerates, arkosic sandstone
and siltstone and is weakly metamorphosed, locally
folded and cleaved by strain slip, and pressure solution
is abundant. Clasts range up to 8 cm, mostly rounded and
sorted, and usually comprise low-grade (Tsaliet-like)
volcanics, various phyllites, granites, quartz pegmatite
and chert. Carbonate clasts were not observed. Cross
bedding indicates fluvial transport towards the SW.
Tadesse (1996b) divided northern Ethiopia into
six blocks tectonically interleaved with two maficultramafic ophiolitic belts (Fig. 2). Tectonic boundaries
and structures within the intervening blocks trend N to
NNE, and E- or W-verging thrusts display significant
oblique slip (Tadesse et al., 1997). The age of the ophiolites is not known but in Axum area (e.g. Tadesse et
al., 2000) the ophiolitic belt is penetrated by syntectonic granitoid dated to 0.75–0.79 Ga (Tadesse et al.,
2000) implying arc terranes were sutured by then. A second orogeny took place after Tambien Group deposition,
producing folding, thrusting and brittle–ductile lateral
displacements (Tadesse, 1996b) and probably reflects
major squeezing and tightening of the EAO during final
collision. The structure of the Neoproterozoic complex
is dominated by upright, NNE-trending, tight folds and
layering that often dips steeply (Fig. 4). As indicated
by the orientation of boudin necks, stretching lineations
and microfractures, approximately E–W shortening was
accompanied by nearly N–S horizontal extension. The
exposed Tsaliet-Tambien sequence was metamorphosed
to greenschist facies. The Shiraro sequence also displays
low-grade metamorphism and faint ductile deformation.
Stretched pebbles are elongated SW–NE, and fold axes
measured at two localities trend 045◦ , somewhat oblique
to the boundary of adjacent blocks.
Together with the Shiraro diamictite, the entire
metamorphic sequence at Tigrai was intruded by postorogenic 600–620 Ma Mereb granites (Miller et al.,
2003; Asrat et al., 2004; Fig. 2), a somewhat older suit
(640–620 Ma) was recognized in Eritrea (Teklay et al.,
2001). Similar post-tectonic granitoids are characteristic of the entire ANS, and their abundance increases
northward (e.g. Bentor, 1985, and references therein).
2.2. Ordovician cover sequence: Enticho sandstone
The earliest Phanerozoic sedimentary sequence covering the Neoproterozoic basement in the region is
the Ordovician Enticho sandstone (Garland, 1980;
Figs. 2 and 3). It comprises cross-stratified quartz sandstone that locally contains conglomerate layers, and may
be partially glacial at its base (Saxena and Assefa, 1983;
Kumulainen et al., 2006). The unconformity at its base
is a remnant of the Afro-Arabian peneplain that records
the beveling of Neoproterozoic orogens across North
Africa and Arabia (Avigad et al., 2003). The Enticho
sandstone is a part of a widespread Cambro-Ordovician
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
the East African Orogeny (Avigad et al., 2003). Starting
in Middle Cambrian time, the sedimentary cover spread
progressively from the north (in present coordinates)
and blanketed the entire northern margin of Gondwana
(Garfunkel, 2002), ultimately reaching Yemen (e.g. the
Wajid sandstone) and Ethiopia in Ordovician time. A
regional synthesis indicates that along the entire north
Gondwana margin these sands were transported from
a southern provenance (Avigad et al., 2005 and references therein). Our reconnaissance study along the
Adigrat-Axum road revealed transport directions trending between NE and NW indicating an overall northward
sense, but contrasting cross bedding directions were
measured locally (Mekelle-Adigrat road). The Enticho
sandstone is among the southernmost Early Paleozoic
sandstone units in the Middle East and North Africa,
and it was probably deposited near the headwaters of the
Early Paleozoic drainage system. The detrital zircon age
spectrum of this unit should highlight the crustal com-
position of the southern Arabian–Nubian Shield and of
the northern Mozambique Belt. Farther south along the
East African Orogen, drainage must have been directed
southward to yield the Table Mountain Group and other
Early Paleozoic siliciclastics of South Africa and (then)
adjacent regions (Burke et al., 2003) so that the Enticho
sandstone must have a rather proximal provenance. In
view of the proximal provenance of Enticho sandstone in
Ethiopia, its mineralogical maturity (quartz sandstone)
probably reflects the intense chemical weathering that
prevailed over north Gondwana at this time (Avigad et
al., 2005).
2.3. Scope of the present work
Here we use zircon U–Pb SHRIMP geochronology to
illustrate key events in the geological and environmental history of Neoproterozoic Ethiopia, with a particular
effort to define the paleogeographic setting and age of
the glaciogenic rocks.
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D. Avigad et al. / Precambrian Research 154 (2007) 88–106
Detrital zircons from the Negash diamictite were
dated (from both, clasts and matrix) to constrain its depositional age and the glaciation it may represent, to detect
whether any far-traveled material is present and to define
the crustal composition of the provenance. A principal
goal was to constrain the age of the Negash diamictite so
that we could place it within the history of globally recognized Neoproterozoic glacial intervals. Ideally, ash beds
intercalated in the section would be targets for dating,
but we have found none at Negash. Instead we treated
the diamictite as a detrital sequence, separated zircons
from a mélange of rock fragments and their matrix and
infer that deposition must postdate the youngest zircon
found. Because the Tigrai volcano-sedimentary section
is Neoproterozoic in age, this strategy allows us to detect
far-traveled exotic material because this would likely be
much older. Similar targets were defined for the Shiraro diamictite although a priori the sequence does not
unequivocally indicate that it formed by the action of
glaciers.
Detrital zircon ages from the Enticho sandstone provide a novel perspective on the evolution of Ethiopia’s
Neoproterozoic basement and on Cambro-Ordovician
paleogeography because these randomly sample sources
across a wide drainage system. The detrital zircon age
spectrum thus should reflect the age of the dominant
zircon-producing orogenic processes that shaped the
region.
Additionally, in order to augment the existing
chronology of Tsaliet arc volcanism, a syn-tectonic
micro-granite and a felsic metavolcanic sill from the
top of the Tsaliet Group were dated to establish the age
of igneous activity and to search for inherited zircons.
A sample of post-tectonic Mereb granite that cuts the
folded and imbricate structure of the Tambien and Tsaliet
groups in the Mai Kenatal region was also dated.
For the most part, our results from igneous rocks,
diamictite, and sandstone yield a remarkably consistent
history of crust formation in the region.
2.4. Analytical procedures
Zircons from 5 to 10 kg samples were isolated using
standard separation techniques. Representative zircon
fractions of all samples, usually between 64 and 250 ␮m,
were mounted in epoxy, polished, coated with gold and
scanned by cathodoluminescence imaging. U–Th–Pb
analyses were made with the SHRIMP RG (Reverse
Geometry) of the USGS-Stanford University facility,
using the calibrated SL 13 standard. Analytical spots
∼30 ␮m in diameter were sputtered using ∼10-nA O2 −
primary beam. The primary beam was rastered across the
analytical spot for 90 s before analysis to reduce common
Pb, and the resulting analyses showed that 204 Pb is generally <0.01% of the total Pb. Isotope ratios were calibrated
against AS57, with an assumed age of 1099 Ma (Paces
and Miller, 1993). Each spot analysis was the average
of five scans through nine mass-stations. Common lead
was estimated using the method of Stacey and Kramers
(1975) and was generally low. Data processing and plotting were performed using Squid and Isoplot (Ludwig,
1994). Zircons yielding concordant ages (less than 10%
discordancy) were usually selected for presentation and
206 Pb/238 U ages were plotted in histograms. Analytical
data is presented in Tables 1a and 1b.
2.5. Detrital zircon geochronology of the Negash
diamictite
Two diamictite samples, ∼10 kg each, were taken
from Negash syncline north of Mekelle (#T4-3-3;
13.83541◦ N; 39.61569◦ E). Zircons were separated from
both samples including matrix and clasts and were analysed together. The detrital zircon ages are shown on a
Concordia diagram and in Fig. 5a and as a histogram of
spot ages in Fig. 6. We eliminated data that were more
than 10% discordant or that contained elevated levels of
common Pb.
Fig. 6A demonstrates that the sources for the
Negash diamictite were principally Neoproterozoic
rocks (<1.0 Ga). The age spectrum is characterized by
a peak at ∼0.8 Ga with submodes at 0.75 and 0.86 Ga.
The youngest concordant zircon is 0.75 Ga and seven
zircons lie in the 0.96–1.03 Ga bin. No 0.89–0.95 Ga
zircons were detected.
The 0.75—0.82 Ga interval represents igneous activity and volcanic accretion in the southern ANS but
0.87 Ga rocks are not reported from Tigrai. These ages
are similar to slightly old zircon cores reported from
the Ghedem high-grade gneisses in east Eritrea (e.g.
Anderson et al., 2006) and from intrusive rocks in SE
Sudan (Kröner et al., 1991). Similar ages are also known
in south Ethiopia where the oldest dated zircon yielded
0.87 Ga (Yibas et al., 2002). Our data suggest that the
Negash diamictite was derived mainly from proximal
sources with no distinguishable outside-ANS components, except for a few ∼1 Ga zircons, the source of
which is not straightforward identified (see discussion
below). This is consistent with field and petrographic
observations that show the diamictite is composed of
low-grade volcanic fragments and fine-grained carbonates with a few granitoid pebbles. A suitable source for
most of these fragments is present in the underlying
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.
(%)
206 Pb
Negash diamictite #T4-3-3
DIAM1-13
0.89
DIAM1-3
0.82
DI AM 1-30
0.56
DIAM2-3
0.20
DIAM1-11
0.37
DI AM 1-47
0.27
DIAM1-19
0.20
DIAM2-2
0.83
DIAM2-18
0.42
DI AM 1-27
0.26
DI AM 1-25
0.38
DIAM2-15
0.22
DI AM 1-33
0.27
DI AM 1-46
0.52
DI AM 1-44
0.18
DIAM2-20
0.27
DI AM 1-37
0.37
DI AM 1-35
0.19
DIAM2-5
0.19
DIAM1-16
0.16
DIAM2-13
0.12
DIAM2-9
0.14
DIAM2-17
0.84
DIAM1-10
0.00
DI AM 1-39
0.15
DI AM 1-42
0.22
DIAM2-16b
0.66
DIAM1-8
0.39
DI AM 1-40
0.17
DIAM1-21
0.00
DI AM 1-24
0.11
DIAM1-2
0.00
DI AM 1-32
0.27
DIAM2-21
0.32
DIAM1-41
0.00
DIAM2-10
0.14
DI AM 1-20
0.13
DIAM1-4
0.09
DIAM2-11
0.20
DI AM 1-26
0.15
DI AM 1-38
0.15
DIAM2-7
0.13
DI AM 1-23
0.22
DI AM 1-34
0.33
DIAM1-31
0.00
DIAM2-8
0.12
DIAM1-7
0.00
DI AM 1-36
0.21
DIAM1-9
0.31
DIAM1-15
0.00
DIAM1-5
0.00
DIAM1-12
0.12
DI AM 1-45
0.11
DIAM1-6
0.00
DIAM2-14
0.60
DIAM2-6
0.00
U
(ppm)
Th
(ppm)
232 Th/
Rad 206 Pb
(ppm)
206 Pb/238 U
238 U
48
101
168
226
70
79
233
181
77
190
102
260
137
277
210
199
208
218
239
252
280
130
310
281
159
96
116
310
224
226
353
137
125
99
131
114
586
300
184
145
402
142
150
56
126
327
226
114
71
243
84
490
284
425
237
216
14
89
84
107
37
35
89
101
70
114
38
142
42
132
87
67
159
114
120
94
77
49
119
118
91
61
37
124
120
93
295
112
25
45
58
35
184
101
72
144
103
114
140
14
86
197
86
48
36
171
41
292
163
312
215
88
0.30
0.91
0.51
0.49
0.54
0.47
0.39
0.57
0.94
0.62
0.38
0.56
0.32
0.49
0.43
0.35
0.79
0.54
0.52
0.39
0.29
0.39
0.40
0.43
0.59
0.65
0.33
0.41
0.55
0.42
0.86
0.84
0.20
0.47
0.46
0.32
0.32
0.35
0.41
1.02
0.26
0.83
0.96
0.25
0.71
0.62
0.39
0.44
0.52
0.73
0.51
0.62
0.59
0.76
0.94
0.42
5.1
11.7
21.0
24.8
8.9
10.1
26.5
17.4
8.5
21.0
12.4
29.3
16.4
31.8
23.4
23.0
25.7
26.9
25.5
31.0
33.7
15.1
34.2
29.8
18.1
11.6
16.9
37.5
25.4
27.7
50.2
16.4
18.6
11.7
14.7
16.1
62.3
32.4
21.2
17.8
45.2
16.6
17.2
8.0
18.0
37.3
25.8
14.1
8.9
28.8
10.1
57.4
35.5
47.4
25.5
25.6
743.5
811.4
878.2
778.5
892.7
906.8
803.6
681.5
781.6
781.9
857.3
796.3
841.0
808.8
787.7
812.5
868.3
865.2
755.2
863.0
846.6
820.2
775.2
750.7
800.7
852.0
1004.1
849.9
801.2
860.2
989.5
838.8
1032.9
831.5
791.9
978.6
752.0
761.9
808.5
861.2
791.5
820.4
806.9
983.0
995.6
803.0
804.7
866.5
876.4
832.6
844.8
821.9
873.8
785.8
753.4
831.4
±1σ
age (Ma)
14.1
10.4
9.1
6.5
13.8
13.6
7.1
7.7
13.1
7.6
11.5
6.2
9.5
6.2
7.1
8.3
8.0
7.7
6.1
7.1
6.1
8.8
7.0
5.9
7.2
11.4
12.9
6.2
6.9
7.6
5.9
9.1
11.8
12.3
9.1
10.9
4.2
5.9
7.2
9.6
4.3
8.6
7.4
16.0
11.7
5.3
6.9
10.9
13.1
6.9
11.9
4.7
6.8
5.0
6.3
8.5
Disc
(%)
−38
−28
−27
−26
−26
−23
−22
−21
−19
−18
−16
−16
−16
−16
−13
−12
−11
−10
−10
−10
−10
−10
−8
−8
−8
−7
−7
−7
−7
−6
−5
−4
−4
−4
−4
−4
−3
−3
−2
−2
−1
−1
−1
1
1
1
1
1
3
3
4
5
5
5
5
7
238 U/
206 Pb
8.19
7.46
6.88
7.83
6.77
6.66
7.57
8.93
7.76
7.78
7.04
7.62
7.20
7.48
7.71
7.45
6.94
6.97
8.05
6.99
7.14
7.38
7.78
8.12
7.57
7.08
5.91
7.09
7.56
7.02
6.03
7.21
5.75
7.25
7.66
6.10
8.08
7.97
7.47
6.99
7.65
7.36
7.48
6.05
5.99
7.53
7.52
6.93
6.88
7.25
7.13
7.33
6.88
7.70
8.01
7.25
Error
(%)
207Pb/
206Pb
Error
(%)
1.9
1.3
1.1
0.8
1.6
1.5
0.9
1.2
1.7
1.0
1.4
0.8
1.2
0.8
0.9
1.0
0.9
0.9
0.8
0.8
0.7
1.1
0.9
0.8
0.9
1.4
1.3
0.7
0.9
0.9
0.6
1.1
1.2
1.5
1.2
1.1
0.6
0.8
0.9
1.1
0.6
1.1
0.9
1.6
1.2
0.7
0.9
1.3
1.5
0.8
1.4
0.6
0.8
0.6
0.8
1.1
0.0633
0.0659
0.0655
0.0609
0.0644
0.0646
0.0623
0.0655
0.0644
0.0630
0.0661
0.0637
0.0650
0.0663
0.0638
0.0655
0.0678
0.0666
0.0638
0.0663
0.0657
0.0652
0.0705
0.0625
0.0651
0.0672
0.0759
0.0686
0.0655
0.0661
0.0712
0.0659
0.0742
0.0686
0.0646
0.0717
0.0646
0.0647
0.0672
0.0685
0.0665
0.0673
0.0676
0.0747
0.0726
0.0672
0.0662
0.0700
0.0665
0.0677
0.0684
0.0688
0.0688
0.0667
0.0709
0.0689
4.2
2.8
2.3
2.0
3.4
3.2
2.0
2.3
3.2
2.3
3.1
1.8
2.5
1.8
2.1
2.0
2.0
1.9
1.9
1.7
1.6
2.5
1.8
1.8
2.1
2.9
2.2
1.6
2.0
1.9
1.2
2.3
2.2
2.7
2.6
2.3
1.3
1.8
2.0
2.3
1.2
2.3
2.0
4.3
2.3
1.5
1.9
2.8
3.2
1.7
3.1
1.2
1.7
1.4
1.9
1.8
96
D. Avigad et al. / Precambrian Research 154 (2007) 88–106
Table 1a (Continued )
±1σ
U
(ppm)
Th
(ppm)
232 Th/
238 U
Rad 206 Pb
(ppm)
0.42
0.09
0.07
0.00
0.00
1.48
1.27
76
90
516
97
235
163
693
41
50
120
62
121
54
733
0.56
0.58
0.24
0.67
0.53
0.34
1.09
9.6
12.5
52.6
11.1
24.4
14.5
65.4
886.5
958.7
721.0
805.2
734.1
621.3
659.9
13.3
14.6
4.2
10.7
6.4
6.2
4.2
9
10
11
12
15
24
30
Shiraro arkose #T4-15-1
SHIRA1-49
0.85
SHIRA2-18
0.83
SHIRA1-38
0.49
SHIRA1-16
0.51
SHIRA2-23
0.43
SHIRA1-24
0.36
SHIRA1-28
1.10
SHIRA2-11
0.71
SHIRA1-39
0.44
SHIRA2-25
0.38
SHIRA1-36
0.46
SHIRA2-14
0.45
SHIRA1-3
0.42
SHIRA1-30
0.11
SHIRA2-4
0.30
SHIRA2-26
0.47
SHIRA1-25
0.61
SHIRA1-23
0.16
SHIRA2-19
0.31
SHIRA1-4
0.14
SHIRA1-55
0.25
SHIRA1-19
0.28
SHIRA2-3
0.30
SHIRA1-45
0.23
SHIRA1-33
0.68
SHIRA1-8
0.00
SHIRA1-7
0.13
SHIRA1-14
0.09
SHIRA1-47
0.07
SHIRA1-18
0.56
SHIRA1-12
0.25
SHIRA2-24
0.31
SHIRA1-60
0.13
SHIRA1-5
0.14
SHIRA1-6
0.00
SHIRA1-15
0.00
SHIRA1-40
0.32
SHIRA1-9
0.00
SHIRA1-32
0.23
SHIRA1-34
0.00
SHIRA1-29
0.00
SHIRA1-48
0.33
SHIRA2-17
0.40
SHIRA1-20
0.40
SHIRA1-17
0.00
SHIRA1-21
0.00
SHIRA2-4
0.15
SHIRA1-22
0.06
SHIRA2-7
0.00
43
71
149
78
82
53
69
87
90
213
170
95
90
184
58
77
74
136
61
294
82
268
123
175
68
36
303
240
321
71
133
325
293
141
264
86
102
163
82
191
233
204
141
53
85
65
161
145
143
16
21
86
51
44
26
36
76
51
235
132
37
30
234
37
59
44
37
24
310
31
141
64
87
39
18
157
227
227
34
82
206
189
96
3
48
71
104
37
126
128
156
60
27
43
27
82
116
91
0.40
0.31
0.59
0.67
0.55
0.50
0.53
0.90
0.59
1.14
0.80
0.40
0.34
1.32
0.65
0.80
0.62
0.28
0.41
1.09
0.40
0.54
0.54
0.51
0.59
0.51
0.54
0.98
0.73
0.50
0.64
0.66
0.67
0.71
0.01
0.57
0.73
0.66
0.47
0.68
0.57
0.79
0.44
0.52
0.53
0.43
0.53
0.83
0.66
5.4
8.7
18.7
9.9
10.1
7.1
8.7
10.4
11.2
26.4
21.8
11.0
11.3
23.1
7.2
8.7
9.3
15.7
7.3
35.3
9.9
33.5
14.2
21.9
8.5
4.6
36.7
29.3
36.4
9.1
16.1
35.9
35.3
16.9
30.4
10.7
12.7
20.4
10.8
24.2
29.9
25.5
17.3
6.4
9.9
7.9
19.3
18.3
16.9
888.3
850.6
884.2
893.5
868.0
925.4
879.3
834.4
874.3
867.4
896.4
819.3
881.1
883.5
868.0
799.7
870.3
810.0
837.8
844.9
850.1
877.7
817.3
877.9
870.2
899.6
851.4
858.1
801.5
893.9
848.8
779.1
847.4
844.7
811.3
880.3
877.9
878.0
921.5
887.6
901.1
874.7
857.8
848.0
820.1
862.2
841.2
883.3
832.8
16.7
15.0
9.3
13.0
14.0
16.2
13.3
13.3
11.7
8.7
8.6
12.4
11.6
8.4
13.8
13.2
12.7
8.9
15.8
6.3
11.9
6.7
9.0
8.6
13.9
18.9
6.2
7.3
5.8
13.5
9.4
6.5
6.2
9.0
6.5
12.4
11.0
8.8
13.1
8.2
7.5
7.9
10.8
15.0
11.0
13.5
8.2
9.3
8.7
−47
−31
−31
−29
−23
−22
−22
−21
−18
−16
−16
−16
−15
−15
−14
−14
−13
−13
−13
−13
−12
−11
−11
−11
−11
−10
−9
−9
−8
−7
−7
−7
−7
−7
−7
−7
−6
−6
−5
−5
−5
−4
−4
−4
−3
−3
−3
−2
−2
Spot name
Comm.
(%)
206 Pb
DI AM 1-22
DIAM2-16
DIAM1-17
DI AM 1-28
DI AM 1-43
DIAM2-4
DIAM2-19
206 Pb/238 U
age (Ma)
Disc
(%)
238 U/
Error
(%)
207Pb/
206Pb
Error
(%)
6.79
6.20
8.43
7.49
8.25
9.69
9.09
1.5
1.6
0.6
1.3
0.9
1.0
0.6
0.0682
0.0756
0.0654
0.0691
0.0673
0.0780
0.0790
3.2
2.6
1.3
2.9
2.0
2.2
1.1
6.82
7.10
6.84
6.76
6.96
6.51
6.82
7.23
6.90
6.95
6.71
7.38
6.84
6.84
6.95
7.57
6.91
7.49
7.21
7.16
7.11
6.87
7.40
6.87
6.90
6.70
7.09
7.04
7.57
6.70
7.11
7.78
7.13
7.15
7.47
6.85
6.85
6.87
6.51
6.79
6.68
6.87
7.01
7.09
7.38
7.00
7.17
6.82
7.26
1.9
1.8
1.1
1.5
1.7
1.8
1.5
1.6
1.4
1.0
1.0
1.5
1.3
1.0
1.6
1.7
1.5
1.1
1.9
0.8
1.4
0.8
1.1
1.0
1.6
2.1
0.7
0.9
0.7
1.5
1.1
0.9
0.7
1.1
0.8
1.4
1.3
1.0
1.5
0.9
0.9
0.9
1.3
1.8
1.4
1.6
1.0
1.1
1.1
0.0631
0.0667
0.0640
0.0647
0.0654
0.0660
0.0711
0.0677
0.0666
0.0668
0.0679
0.0664
0.0674
0.0651
0.0666
0.0665
0.0692
0.0641
0.0662
0.0649
0.0660
0.0673
0.0661
0.0670
0.0704
0.0660
0.0661
0.0660
0.0644
0.0711
0.0673
0.0661
0.0664
0.0664
0.0644
0.0664
0.0690
0.0665
0.0699
0.0670
0.0675
0.0696
0.0701
0.0695
0.0656
0.0670
0.0677
0.0674
0.0663
4.2
3.3
2.3
3.2
3.0
4.9
3.2
3.0
2.9
1.9
2.1
2.9
2.8
2.1
3.6
3.2
3.4
2.5
3.6
1.6
3.4
1.6
2.5
2.1
4.8
4.6
1.6
1.9
1.6
3.2
2.4
1.6
1.6
2.3
1.8
3.0
2.6
2.2
2.9
2.0
1.8
1.9
2.3
3.8
3.1
3.5
2.3
2.2
2.3
206 Pb
D. Avigad et al. / Precambrian Research 154 (2007) 88–106
97
Table 1a (Continued )
±1σ
U
(ppm)
Th
(ppm)
232 Th/
Rad 206 Pb
(ppm)
206 Pb/238 U
238 U
50
130
387
167
142
132
321
143
215
161
186
132
174
227
184
219
139
99
140
165
224
156
96
37
86
153
98
250
75
25
64
27
104
334
116
126
111
335
34
149
79
160
94
150
144
143
156
68
77
50
143
204
89
28
22
37
112
73
281
32
13
48
0.56
0.82
0.89
0.72
0.92
0.87
1.08
0.25
0.71
0.51
0.89
0.73
0.89
0.65
0.80
0.73
0.50
0.80
0.37
0.89
0.94
0.59
0.30
0.61
0.44
0.76
0.77
1.16
0.44
0.56
0.77
5.9
21.0
43.0
20.8
17.5
14.9
38.6
17.7
26.5
19.7
22.2
16.1
22.1
34.0
21.9
27.0
16.2
11.2
15.9
19.7
23.5
18.7
12.2
4.3
9.3
17.5
11.8
27.2
9.2
2.9
7.0
834.5
1107.6
783.3
871.4
867.3
795.6
844.6
866.0
861.9
857.2
838.3
855.0
890.8
1032.6
833.9
862.5
822.4
792.0
800.6
836.6
739.2
842.8
884.1
812.5
762.6
800.4
837.0
766.7
865.0
821.2
764.4
15.1
11.4
6.0
8.6
9.4
8.4
5.9
9.2
7.5
8.9
7.8
9.3
8.4
8.4
8.1
7.3
8.8
11.6
8.7
8.2
6.3
8.5
11.6
17.3
11.0
8.1
12.4
6.2
12.2
20.0
14.5
−2
−1
−1
−1
−1
0
0
0
1
1
2
2
2
2
3
3
3
3
4
5
5
6
7
8
8
8
11
12
14
17
39
Enticho sandstone (Ordovician) #T3-21-1
Ordo1-59*
0.00
42
67
Ordo1-27*
0.00
36
38
Ordo1-55*
0.08
164
124
Ordo1-24*
0.16
48
43
Ordo1-54*
0.00
128
122
Ordo1-13
0.00
49
45
Ordo1-58
0.10
149
36
Ordo1-7
0.32
212
165
Ordo1-37
0.00
54
45
Ordo1-29
0.00
73
59
Ordo1-33
0.55
67
42
Ordo1-44
0.00
209
78
Ordo1-39
0.34
86
75
Ordo1-35
0.18
218
139
Ordo1-28
0.00
116
66
Ordo1-1
0.00
56
49
Ordo1-52
0.00
130
107
Ordo1-41
0.24
99
75
Ordo1-6
0.31
111
106
Ordo1-38
0.10
83
73
Ordo1-21
0.81
75
28
Ordo1-32
0.11
184
212
Ordo1-15
0.07
129
83
Ordo1-20
1.11
36
31
Ordo1-53
0.53
58
41
1.63
1.08
0.78
0.92
0.98
0.96
0.25
0.80
0.86
0.83
0.65
0.39
0.90
0.66
0.59
0.92
0.85
0.78
0.99
0.91
0.38
1.19
0.67
0.88
0.73
17.3
13.0
55.6
14.8
38.4
7.6
22.9
29.4
7.2
9.6
8.5
24.0
9.8
24.9
13.1
6.3
14.5
11.0
12.3
9.2
8.2
20.4
14.2
4.0
6.3
2526.1
2572.5
2096.6
2059.6
1851.2
1108.7
979.3
958.3
926.0
912.4
884.4
808.9
800.3
802.2
798.4
796.9
789.0
781.8
784.3
782.5
770.2
783.1
773.1
775.4
765.7
23.8
27.2
14.3
39.8
21.1
64.8
40.6
8.4
15.3
13.0
13.2
7.0
10.5
6.7
9.2
13.1
8.6
8.3
11.6
10.7
9.6
7.2
8.2
16.4
12.6
0
14
−2
5
−4
3
−8
1
17
−5
−3
−7
12
3
−9
−10
−13
7
−11
−6
42
−14
7
−9
30
Spot name
Comm.
(%)
206 Pb
SHIRA1-1
SHIRA2-6
SHIRA2-20
SHIRA1-52
SHIRA1-53
SHIRA2-8
SHIRA2-9
SHIRA1-56
SHIRA1-50
SHIRA1-11
SHIRA1-44
SHIRA1-27
SHIRA1-37
SHIRA1-35
SHIRA1-10
SHIRA1-51
SHIRA1-59
SHIRA2-15
SHIRA1-42
SHIRA1-54
SHIRA1-26
SHIRA1-57
SHIRA1-13
SHIRA1-2
SHIRA1-58
SHIRA1-46
SHIRA2-16
SHIRA1-43
SHIRA1-31
SHIRA2-10
SHIRA2-13
0.00
0.00
0.15
0.00
0.14
0.13
0.06
0.13
0.09
0.13
0.34
0.00
0.06
0.00
0.12
0.14
0.21
0.21
0.46
0.00
0.37
0.19
0.00
0.00
0.00
0.18
0.00
0.00
0.75
0.00
0.63
age (Ma)
Disc
(%)
238 U/
Error
(%)
207Pb/
206Pb
Error
(%)
7.24
5.34
7.73
6.91
6.94
7.60
7.14
6.95
6.98
7.02
7.17
7.05
6.75
5.75
7.23
6.99
7.36
7.63
7.59
7.20
8.19
7.16
6.78
7.42
7.94
7.53
7.19
7.89
6.98
7.32
7.80
1.8
1.1
0.8
1.0
1.1
1.1
0.7
1.1
0.9
1.1
0.9
1.1
1.0
0.8
1.0
0.9
1.1
1.5
1.1
1.0
0.9
1.0
1.3
2.2
1.5
1.0
1.5
0.8
1.4
2.5
1.9
0.0665
0.0759
0.0663
0.0678
0.0688
0.0667
0.0677
0.0690
0.0688
0.0690
0.0701
0.0680
0.0688
0.0744
0.0686
0.0675
0.0655
0.0682
0.0632
0.0684
0.0681
0.0672
0.0707
0.0683
0.0666
0.0694
0.0703
0.0677
0.0662
0.0712
0.0807
4.1
2.1
1.5
2.1
2.2
2.3
1.5
2.2
1.9
2.2
2.1
2.3
2.0
1.5
2.0
1.9
2.9
2.8
2.5
2.1
1.9
2.2
2.8
4.7
3.7
2.2
2.7
1.8
3.2
5.3
3.3
2.09
2.39
2.53
2.80
2.86
5.52
5.59
6.22
6.42
6.59
6.77
7.50
7.56
7.52
7.61
7.62
7.71
7.72
7.73
7.75
7.83
7.77
7.84
7.76
7.89
1.7
1.9
0.8
1.7
1.0
1.7
1.0
0.9
1.7
1.4
1.5
0.9
1.3
0.8
1.2
1.7
1.1
1.1
1.5
1.4
1.3
0.9
1.1
2.1
1.7
0.1668
0.1715
0.1305
0.1284
0.1132
0.0765
0.0726
0.0738
0.0759
0.0679
0.0719
0.0642
0.0663
0.0682
0.0635
0.0631
0.0623
0.0690
0.0652
0.0647
0.0701
0.0627
0.0663
0.0718
0.0684
1.4
1.6
0.8
2.2
1.2
3.2
1.9
1.7
3.2
3.0
3.1
1.9
3.0
1.8
2.6
3.8
2.5
2.3
2.8
3.1
2.7
2.1
2.4
4.7
3.6
206 Pb
98
D. Avigad et al. / Precambrian Research 154 (2007) 88–106
Table 1a (Continued )
Spot name
Comm.
(%)
206 Pb
Ordo1-18
Ordo1-31
Ordo1-36
Ordo1-42
Ordo1-4
Ordo1-12
Ordo1-25
Ordo1-26
Ordo1-19
Ordo1-43
Ordo1-45
Ordo1-9
Ordo1-60
Ordo1-50
Ordo1-22
Ordo1-49
Ordo1-17
Ordo1-16
Ordo1-8
Ordo1-30
Ordo1-48
Ordo1-47
Ordo1-5
Ordo1-56
Ordo1-51
Ordo1-40
Ordo1-3
Ordo1-57
Ordo1-10
0.35
0.14
0.15
0.35
0.00
0.34
0.59
0.53
0.40
0.36
0.00
0.44
0.23
0.04
0.67
0.33
0.49
0.00
0.51
0.27
0.00
0.27
0.52
0.50
0.00
0.52
0.36
0.64
0.47
U
(ppm)
Th
(ppm)
232 Th/
238 U
Rad 206 Pb
(ppm)
108
306
144
182
91
127
493
67
87
165
102
132
177
118
76
102
62
87
79
159
49
194
101
119
46
108
186
253
406
39
49
103
76
48
120
361
46
50
84
95
34
145
53
41
120
64
91
193
226
1
112
85
104
52
93
360
158
236
0.37
0.17
0.74
0.43
0.55
0.97
0.76
0.70
0.59
0.53
0.97
0.27
0.85
0.46
0.56
1.21
1.06
1.07
2.52
1.47
0.02
0.60
0.87
0.90
1.17
0.89
2.00
0.65
0.60
11.8
32.7
15.4
19.4
9.6
13.3
50.3
6.8
8.1
15.3
9.2
11.9
15.8
10.5
6.8
9.0
5.5
7.7
6.9
13.8
4.2
16.4
8.5
10.0
3.8
8.8
13.6
16.5
25.3
206 Pb/238 U
±1σ
age (Ma)
767.6
755.8
752.5
753.0
747.4
739.8
719.5
716.5
657.8
659.1
650.2
638.3
634.2
631.7
626.4
630.5
626.3
626.1
623.3
620.0
607.4
604.8
604.8
600.0
592.2
584.1
529.0
462.5
445.8
9.6
5.5
7.9
7.0
10.0
8.2
4.2
10.8
8.9
6.7
8.5
7.2
6.4
7.3
9.2
7.9
10.7
7.5
9.3
6.4
11.2
5.6
7.8
6.9
11.8
7.2
5.1
4.0
3.2
Disc
(%)
−3
−9
6
−12
5
−13
−3
−41
23
−13
−13
−7
20
5
23
−9
−4
−4
−18
−2
2
−11
−36
−20
20
−50
−39
80
60
238 U/
206 Pb
Error
(%)
207Pb/
206Pb
Error
(%)
7.89
8.05
8.05
8.07
8.12
8.22
8.43
8.55
9.29
9.29
9.45
9.58
9.65
9.71
9.68
9.72
9.76
9.81
9.84
9.88
10.12
10.16
10.18
10.24
10.35
10.58
11.73
13.18
13.77
1.3
0.7
1.1
0.9
1.4
1.1
0.6
1.5
1.4
1.0
1.3
1.1
1.0
1.2
1.5
1.3
1.7
1.2
1.5
1.0
1.9
0.9
1.3
1.2
2.0
1.2
1.0
0.8
0.7
0.0669
0.0635
0.0670
0.0644
0.0652
0.0638
0.0674
0.0595
0.0630
0.0620
0.0589
0.0632
0.0630
0.0614
0.0704
0.0617
0.0639
0.0598
0.0616
0.0623
0.0605
0.0603
0.0586
0.0606
0.0632
0.0563
0.0557
0.0723
0.0671
2.8
1.7
2.4
2.2
3.0
2.6
1.3
3.7
3.3
2.5
3.4
2.7
2.5
2.9
3.5
3.2
4.7
3.1
3.8
2.6
4.8
2.4
3.4
3.0
5.0
3.3
2.7
2.0
1.9
Reported ages are based on 207 corrected data. Ages older than 1.0 Ga are reported as 207 Pb/206 Pb 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.
The provenance of the 0.95–1.1 Ga age zircons is
less obvious. These zircons crystallized during Kibaran
(Grenvillian) magmatism that predated the onset of
Arabian–Nubian Shield activity at ∼0.87 Ga (Stern,
1994). Kibaran rocks are generally not known north of
Tanzania and the presence of these zircons may a priori
indicate a distal provenance, but if this is correct, other
pre-Neoproterozoic zircons would be expected also. Yet,
pre-Kibaran zircons are not detected in the diamictites.
Recent studies of the Cambro-Ordovician sandstone
blanketing the northern Arabian–Nubian Shield show
that 1.1 Ga zircons are a prominent component of the
detrital zircon age spectra of this terrane (Avigad et
al., 2003; Kolodner et al., 2006). Although the possibility that they represent far traveled ice-rafted material
cannot be refuted, Avigad et al. (2003) also raised the
possibility that Kibaran age rocks and/or zircons reside
within ANS, and Hargrove et al. (2006) recently indicated some ANS crustal segments were contaminated
by pre-Neoproterozoic (including Kibaran) material.
Approximately 1.1 Ga xenocrystic zircons were reported
also within Neoproterozoic rocks of southern Ethiopia (a
metarhyolite from Wadera group; Teklay et al., 1998) and
it is thus most likely that the Kibaran zircons detected in
the diamictites have their provenance in similar rocks, in
the vicinity of Tigrai. Therefore, the Kibaran-age detrital
zircons in the diamictites do not represent long-distance
transport.
Based on the age of the youngest concordant zircon,
the Negash diamictite is younger than 0.75 Ga, consistent
with the constraints for the underlying Tsaliet volcanism.
2.6. Detrital zircon geochronology of the Shiraro
sequence
We separated zircons from a pebble-free Shiraro arkosic sandstone (#T4-15-1; 14.404840◦ N;
37.80902◦ E) to determine the source of the basin detritus
and to constrain its depositional age and possible relationship to the Negash diamictite. The detrital zircon
ages are presented on a Concordia diagram (Fig. 5b) and
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.
(%)
206 Pb
U
(ppm)
Th
(ppm)
232 Th/
238 U
Rad 206 Pb
(ppm)
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
S-2.1
0.32
157
64
0.42
17.5
S-4.1
0.37
185
88
0.49
20.6
S-5.1
0.58
146
56
0.39
15.9
S-8.1
0.27
129
67
0.54
14.1
S-10-2
0.29
170
64
0.39
18.8
S-10-4
0.46
130
64
0.51
13.9
S-10-6
0.19
83
40
0.50
9.3
206 Pb/238 U
1S.E.
age (Ma)
812.2
784.6
779.3
766.8
768.4
779.9
756.1
788.5
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
T317-01-4
0.13
147
48
0.33
12.4
601.1
T317-01-6
0.02
560
502
0.93
48.8
622.7
T317-01-7
0.05
384
159
0.43
33.4
622.0
T317-01-8
0.09
360
124
0.36
31.2
617.8
T317-01-10 0.06
323
199
0.64
27.1
601.3
T317-01-11 0.24
463
175
0.39
39.0
603.2
T317-01-14 0.19
964
583
0.62
82.3
609.8
T317-01-1
0.21
253
79
0.32
20.7
587.3
T317-01-9
0.19
303
119
0.40
25.1
593.4
T317-01-2
1.48
455
178
0.40
35.6
552.6
T317-01-5
2.66
299
73
0.25
18.0
429.1
T317-01-12 1.23
385
197
0.53
25.9
480.8
T317-01-13 3.07
327
168
0.53
23.2
495.1
T317-01-15 0.54
449
416
0.96
34.5
550.4
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
AG-2
0.24
106
35
0.34
11.9
789.4
AG-3
0.32
85
24
0.29
9.5
780.9
AG-4
0.25
150
54
0.37
16.6
782.6
AG-5
0.00
74
19
0.27
8.1
776.6
AG-6
0.65
161
64
0.41
17.6
769.3
AG-7
0.13
111
38
0.35
12.6
797.7
AG-8
0.12
163
93
0.59
19.6
843.1
AG-9
0.00
127
43
0.35
13.6
754.9
AG-10
0.00
93
26
0.29
10.4
789.1
AG-11
0.48
133
45
0.35
15.4
817.2
AG-12
0.00
190
78
0.43
19.7
735.3
Disc
(%)
238 U/
206 Pb
Error
(%)
207Pb/
206Pb
Error
(%)
7.6
5.0
7.3
5.2
5.8
7.6
7.8
9.1
−1
−4
3
13
−1
−7
0
−11
7.43
7.70
7.75
7.87
7.88
7.75
8.00
7.70
1.0
0.6
1.0
0.7
0.8
1.0
1.1
1.2
0.0684
0.0680
0.0682
0.0696
0.0670
0.0676
0.0683
0.0638
1.5
1.1
1.2
1.1
1.3
1.2
1.3
2.2
5.3
6.0
5.1
5.2
5.4
5.4
5.1
4.9
5.3
5.3
4.8
4.1
4.4
4.9
4.7
8
−9
−3
−5
−8
3
6
−1
−2
6
−12
48
14
−16
16
9.99
10.20
9.86
9.87
9.92
10.23
10.18
10.06
10.46
10.36
10.98
14.24
12.78
12.11
11.19
0.9
1.0
0.9
0.9
0.9
0.9
0.9
0.8
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.0618
0.0610
0.0603
0.0601
0.0612
0.0604
0.0619
0.0618
0.0613
0.0613
0.0707
0.0768
0.0667
0.0821
0.0629
1.0
2.8
0.8
0.9
1.0
1.6
1.0
0.6
1.1
1.1
1.6
1.1
1.1
1.0
1.0
12.8
12.4
12.9
11.4
13.4
11.2
12.3
12.1
11.5
12.8
12.4
10.4
−26
−4
1
−6
3
−6
1
−4
8
1
−21
4
7.86
7.67
7.74
7.74
7.80
7.85
7.58
7.16
8.03
7.68
7.41
8.27
1.7
1.6
1.7
1.5
1.8
1.5
1.6
1.5
1.6
1.7
1.6
1.4
0.0651
0.0663
0.0680
0.0657
0.0657
0.0686
0.0671
0.0671
0.0663
0.0656
0.0650
0.0647
2.7
2.4
2.7
2.1
2.9
2.0
2.3
1.9
2.2
2.5
2.2
1.8
Reported ages are based on 207 corrected data. Ages older than 1.0 Ga are reported as 207 Pb/206 Pb 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
provenance that is principally ANS crust with practically
no contribution from pre-Neoproterozoic sources (two
zircons yielded Kibaran ages; Fig. 6B). The youngest
concordant grain is 739 ± 6 Ma but the majority of the
detrital zircon ages range between 0.8 and 0.9 Ga, with
a peak at 0.87 Ga. This time interval corresponds to the
early stages of ANS crustal growth, but – as noted for
Negash diamictite – rocks of this age have not been
reported from Tigrai. On the other hand, 0.85 Ga igneous
activity was reported from western Eritrea (Teklay et al.,
2003), and the Gulgula Group contains clasts of this age
(Teklay et al., 2003).
In terms of its zircon age distribution, the Shiraro
sequence resembles the Negash diamictite in that it has
no zircons younger than 0.74 Ga, but the Shiraro contains a greater proportion of detrital zircons older than
0.85 Ga. Just a small proportion of the detrital zircon
inventory of Shiraro potentially represents rocks dated
in the Tsaliet of Tigrai (∼0.82–0.75 Ga). Thus, although
Negash and Shiraro sedimentary sections were derived
from ANS crust and may have been deposited about the
100
D. Avigad et al. / Precambrian Research 154 (2007) 88–106
Fig. 5. Concordia plots for detrital zircons from the Negash diamictite,
Shiraro arkose and Enticho sandstone. 2σ error ellipses.
same time, Shiraro may have been preferentially derived
from western Eritrea basement sources.
2.7. Detrital zircon geochronology of the Enticho
sandstone
We collected samples of white Enticho sandstone
on the Adwa-Adigrat road (#T3-21-1; 14.27648◦ N;
39.11582◦ E) from a stratigraphic position several tens
of meters above the peneplain surface. The detrital zircon ages from the Enticho sandstone are presented on a
Fig. 6. (A) U–Pb SHRIMP detrital zircon ages (206 Pb/238 U) from
Negash diamictites. Zircons younger than 750 Ma are either discordant or contain elevated 204 Pb. (B) U–Pb SHRIMP (206 Pb/238 U) ages
of detrital zircons from the Shiraro sequence. (C) U–Pb SHRIMP ages
(206 Pb/238 U, ages older than 1.0 Ga are 207 Pb/206 Pb ages based on
204 corrected data) of detrital zircons from the Enticho Ordovician
sandstone. Note change in time scale with respect to A and B.
Concordia diagram (Fig. 5c), a cumulative histogram in
Fig. 6C and a probability plot on Fig. 7.
Figs. 6C and 7 show that the detritus in Enticho
sandstone is dominated by Neoproterozic sources, with
subordinate contributions from older Kibaran crust and
minor contributions (∼10%) from pre-Kibaran sources.
Within the Neoproterozoic two modes are distinguished:
the older mode is at ∼0. 8 Ga with a major concentration 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
1.8, 2.0, 2.2 and 2.6 Ga are not shown. The zircon age distribution is
interpreted as generally reflecting the crustal evolution of the southern ANS in Ethiopia. We suggest that onset of Tambien deposition
best fits the lull defined at around 0.7 Ga because Tambien deposition
marks the complete shutdown of Tsaliet igneous activity and predated
post-tectonic magmatism. Note that the timing of Neoproterozoic East
African Orogeny and crustal thickening (pertaining to the onset of
Gondwana collision) is very similar to the timing of Marinoan Snowball Earth glaciation. S—the timing of Sturtian glaciation, M—the
timing of Marinon glaciation (e.g. Hoffman et al., 2004). Note that
Marinoan glaciation slightly predated the peak of post-orogenic magmatism so that both Sturtian and Marinoan diamictites would share
similar field relation to the major igneous phases that shaped the EAO
in Ethiopia.
has a peak at 0.62 Ga with a concentration between
0.66 and 0.58 Ga. The two major igneous phases were
separated by a ∼100 m.y. interval of reduced igneous
activity with a magmatic lull about 0.69 Ga. We interpret
the older mode to indicate contributions from Tsalietlike crust, whereas the younger mode is a contribution
from abundant late- to post-tectonic ‘Mereb’ granitoids.
While the younger granites make up a smaller proportion of Neoproterozoic crust in the region (∼20% in
Tigrai, e.g. Tadesse-Alemu, 1998; and our Fig. 2), the late
Neoproterozoic peak is almost as large as the early Neoproterozoic peak because the Mereb granites are very
rich in zircon. The histogram contains also a small number of 0.89–0.93 and 0.9–1.1 Ga (Kibaran–Grenvillian)
zircons as well as a few at 1.8, 2.0 and 2.5 Ga. These Paleoproterozoic and Late Archean zircons may come from
old basement remobilized during the Neoproterozoic
such as detected in eastern Ethiopia (Teklay et al., 1998)
and western Ethiopia (e.g. Kebede et al., 2001), or from a
more distal source such as in Yemen (Whitehouse et al.,
1998) where Late Archean–Early Proterozoic crust has
been reworked during the Neoproterozoic and intruded
by abundant 0.76 Ga granitoids. Unlike the local provenance reflected by zircons from the Negash diamictite
and Shiraro sequence, the presence of Archean and Pale-
In order to augment the existing data on the timing
of igneous activity in Tigrai we dated three rock units.
The results of U–Pb zircon geochronology and analytical
data are presented in Table 1b.
2.9. Post-tectonic Mai Kenetal granite
We sampled the post-tectonic granite (#T3-17-01)
from the Mai Kenetal area (Fig. 2) (14.04972◦ N;
38.96864◦ E). Our analyses yield a mean age of
612.3 ± 5.7 Ma, similar to ages of the Mereb granite to
the east (606 ± 0.9; 613.4 ± 0.9; 608 ± 7 Ma, Miller et
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).
By this time, contractional deformation in Tigrai had
ceased, but the currently exposed metamorphic rocks lay
at a depth of 6–8 km (e.g. Asrat et al., 2004).
2.10. Syntectonic aplite microgranite from the
Tsaliet Group
A deformed aplite micro-granite body within the
volcano-sedimentary section of Tigrai (“Tsaliet Granitoid” in Fig. 2; #T4-7-1; 13.89018◦ N; 39.41531◦ E) was
dated to establish the intrusion age of deformed syntectonic intrusives in this area. SHRIMP analysis of 12
zircons detected no inherited zircons and yielded a concordia age of 784 ± 14 Ma (Table 1b). This age is similar
to the range of ∼740–780 Ma obtained for Tsaliet arcrelated igneous activity in Tigrai by previous works. The
age and lack of inherited zircons in this sample support
the notion that crust in this region formed in Neoproterozoic time.
2.11. Metavolcanics/Sill from the top of the Tsaliet
Group
In the Madhane-Alem region, the section is overturned 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, greenish, fine grained volcanic rock (#T3-11-10; 13.94560◦ N;
39.61816◦ E), possibly a sill injected into volcaniclastics ∼20 m below the first massive Tambien carbonate
(Didikama Fm) beds, was dated to establish the age of
igneous activity at the top (and presumably the youngest)
part of the Tsaliet. A mean age of 774.7 ± 5.7 Ma was
derived from a cluster of 8 zircons measured (Table 1b).
We interpret this result as the eruption/intrusion age
of this body. The observation that a syntectonic aplite
and this sill/metavolcanic from different localities yield
similar ages provides further support for the conclusion
that igneous activity was widespread in the region at
0.77–0.78 Ga, and that the Tambien Group is younger.
2.12. On the timing and nature of diamictite
formation
Neither Shiraro nor Negash diamictites contain clasts
or zircons from the widespread and distinctive late
Neoproterozoic post-tectonic ∼610 Ma Mereb granites.
This is consistent with both diamictites being older
than the post-tectonic granitoid intrusions in the region
(Tadesse, 1996a). They thus significantly differ from
late Neoproterozoic molasse basins which are abundant in the northern ANS and which contain abundant
detritus derived from post-orogenic granitoids (Jarrar et
al., 1991; Garfunkel, 1999; Johnson, 2003; Wilde and
Youssef, 2002; Weissbrod and Sneh, 2002).
The youngest concordant zircons in the Negash
diamictite and Shiraro sequence are ∼745 Ma and these
sequences must therefore be younger. Thus, as far as the
zircon data tell the diamictites may have been deposited
at any time between 0.745 and ∼0.62 Ga.
Miller et al. (2003) suggested an age of ca.
0.75–0.72 Ga for the Negash diamictite on the basis of
strontium isotope ratios measured in the underlying Tambien carbonates. Deposition of the diamictites must have
been preceded by thermal contraction and subsidence of
the Tsaliet arc complex (starting ∼0.74 Ga), and by the
deposition of carbonate and clays. The Tsaliet arc may
have been inundated for quite some time to accumulate a 1 km thick Tambien carbonate section. While the
duration is unknown, a few millions of years to several
tens of million years is not an unreasonable estimate.
The time interval defined in Fig. 7 as reduced igneous
activity (starting ∼0.74 Ga) may thus be appropriate for
Tambien carbonate sedimentation. Formation of Negash
diamictite and Shiraro sequences should have taken
place afterwards, but prior to ∼0.62 Ga. Allowing few
tens of million years for Tsaliet thermal subsidence and
Tambien carbonate deposition, the available geochrono-
logical data and geological consideration would favor
the formation of the diamictites starting 0.72–0.70 Ga
(broadly Sturtian or later) but the exact timing cannot be
constrained further.
Additional insights into the origin of the Neoproterozoic glacial record (assuming Tambien diamictites
are indeed glacigenic) may be obtained by considering
the content of the diamictites. Unlike typical tillites in
which far-traveled clasts are usually observed, Tambien
diamictites appear to have been derived from a proximal source. Neither the clast inventory nor the detrital
zircon spectra contain contributions from extra-ANS
pre-Neoproterozoic sources so that an origin from a
continental-scale ice sheet is difficult to envisage. The
diamictite clast lithologies contain Tambien carbonate
and Tsaliet volcanics, indicating that Tambien sedimentation must have been interrupted to deposit the Negash
and Shiraro diamictites. The source area comprising
both Tambien and Tsaliet rocks must have been locally
uplifted and exposed, eroded and re-deposited at the
top of the Tambien sequence to yield the diamictite.
The presence of Tsaliet Group clasts in the diamictite
may farther indicate that the entire Tambien (∼1500 m)
was locally removed by erosion. This implies significant
vertical movements which cannot be accommodated by
sea level drop due to Neoproterozoic glaciation alone.
One of the carbonate clasts identified in the diamictite
yielded (e.g. Miller et al., 2006) negative δ13 C vlaue of
the type identified by Miller et al. (2006) and by Alene
et al. (2006) at the very bottom of the Tambien sequence
(Assem limestone; Mai Kenetal synclinoria). This feature implies reworking of the deepest part of the Tambien
sequence and is consistent with the Tambien basin being
locally uplifted and tectonically inverted to generate the
diamictite.
The current incomplete understanding of the geotectonic evolution of the southern ANS as well as the
absence of a definite age for the diamictite hinder the
recognition of the exact link between ANS tectonics and
the formation of Tigrai diamictites. Two significant tectonic phases are usually reported to have affected the
ANS. The older phase corresponds to the closure of various ocean basins and to the accretion of island arcs
(Abdelsalam and Stern, 1996), and went on in different
ANS segments from ∼0.78 to ∼0.63 Ga. In Ethiopia,
the exact timing of arc suturing and ophiolite emplacement is not well constrained but it appears to predate the
deposition of the Tambien Group. In the Axum area (e.g.
Tadesse et al., 2000; and Fig. 2) the ophiolitic belt is penetrated by syntectonic granitoid dated to 0.75–0.79 Ga
(Tadesse et al., 2000) implying arc terranes were sutured
by then. In western Ethiopia, metamorphism possibly
D. Avigad et al. / Precambrian Research 154 (2007) 88–106
pertaining to arc suturing was reported by Ayalew et
al. (1990) at 0.76 Ga suggesting orogenic movements
related to arc suturing predated diamictite deposition.
Moreover, sutured arcs would be a plausible source for
the low-grade metamorphic clasts observed by us in
Ethiopian diamictites and by Teklay et al. (2003) in the
equivalent Gulgula section of western Eritrea.
A subsequent major phase of crustal shortening and
orogeny affected ANS as well as other parts of the
EAO in relation to the protracted fusion of Gondwana
starting at about 0.63 Ga. Direct evidence from western
Ethiopia include a major basement isotopic rehomogenization of the Rb–Sr system at 632 ± 8 Ma, (e.g. Ayalew
et al., 1990) and the development of zircon growth rim
at 629 Ma (Kebede et al., 2001). This is considered to
be the time of collision over the length of the EAO from
central Tanzania northward (Sommer et al., 2005), but
the ages quoted above probably represent the culmination of crustal thickening and thermal re-equilibration of
the thickened orogen, so contractional tectonics probably commenced earlier. Remarkably, EAO collision
culminated about the same time as Marinoan glaciation at 635 ± 2 Ma (Hoffman et al., 2004). Although
a Marinoan age for the Negash diamictite and Shiraro
sequence is 100 Ma younger than the youngest detrital zircon detected by us in the diamictites, we note
that it is not inconsistent with the detrital zircon data
(see Fig. 7). Marinoan glacial diamictites in Ethiopia, if
there were any, would not be expected to collect zircons younger than 0.75 Ga anyway, because igneous
activity in the region was reduced significantly starting
0.75 Ga and peaked again at 0.63–0.62 Ga, as demonstrated by the Entichio detrital zircon ages. Inversion
of the Tsaliet-Tambien basin in the course of final closure and collision along the EAO could be a plausible
source for the formation of the diamictites as well as for
their subsequent folding and metamorphism. However,
because the exact age of North Ethiopia diamictites is
currently not well constrained and because the setting of
tectonic processes affecting this area is not fully understood, more work is required in order to assess the link
between EAO tectonic evolution and the origin of the
diamictites.
2.13. Implications for ANS crustal evolution
Detrital zircon geochronology is a powerful tool for
investigating the crustal evolution of source terranes. The
Negash diamictite and the Shiraro sequence are sedimentologically immature and were probably derived
from a limited drainage basin. However, the Enticho
sandstone overlies a regional peneplain atop of the Pre-
103
cambrian basement and is mineralogically and texturally
more evolved. The sandstones originated from a wider
drainage area so that they tap an important area of the
underlying ANS basement, and for this reason we think
that the detrital zircon age inventory reflects the surface
age-composition of the Ethiopian zircon-bearing basement in Ordovician times. This sample largely excludes
mafic and ultramafic rocks that usually have low zircon
content and will be biased towards felsic rocks that are
rich in zircon.
Unlike previous studies that (tacitly perhaps, but not
explicitly) assume continuous crustal growth for the
ANS from 870 Ma until the end of Neoproterozoic time,
the detrital zircon ages reveal that in Ethiopia, ANS
zircon-producing igneous activity took place in two
major phases. Island arc volcanism lasted until ∼0.75 Ga
followed by a period of ca. 100 m.y. during which subsidence and deposition of shallow marine carbonates and
mudrocks of the Tambien Group ocurred; this was later
followed by the major phase of collision orogeny and
crustal thickening (culminating at ca. 0.63 Ga) related to
Gondwana fusion. In Tigrai, terminal collision is manifested by upright folding (such as Negash synclinoria),
thrusting and lateral displacements. This was followed
by a phase of late- to post-orogenic igneous activity that
peaked at around 0.62 Ga. The first phase of island arc
volcanism represents crustal growth via the subduction
factory, whereas the later phase of post-tectonic igneous
activity was related to crustal thickening and collision
and as such likely involved melting of previous lower
crust rocks (see also Teklay et al., 2001; Bentor, 1985;
Stein, 2003).
A summary of published ages from the Ethiopian
basement (e.g. Asrat et al., 2001) shows significant similarities with the Enticho detrital zircon ages and lends
credence to our crustal evolutionary model for Tigrai.
Each of the detrital zircon peaks we observed can be
matched with dated Ethiopian basement. A strength of
detrital zircon geochronology is in providing a quasiquantitative indications of the scale and intensity of
each of the igneous phases monitored, although biased
towards zircon-rich sources. In this respect, we note
that out of all dated rocks in Ethiopia (e.g. Asrat et al.,
2001, and references therein), only a handful yielded
0.75–0.65 Ga U–Pb zircon ages, consistent with this
period being characterized by reduced igneous activity, and by subsidence and marine sedimentation of the
Tambien Group.
A summary of radiometric ages for ANS ophiolites
(e.g. Stern et al., 2004) shows a spread from 0.87 to
0.74 Ga, overlapping the time period defined by us
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
building phase in Ethiopia. Moreover, a similar timing of
arc volcanism (particularly in the 0.8–0.76 Ga interval)
is observed also in pre-Neoproterozoic terranes at the
border and outside ANS such as in Yemen (Whitehouse
et al., 1998) and Madagascar (Kröner, 2001), indicating
igneous activity was then widespread within and beyond
the ANS.
The detrital zircon ages of the Ordovician Enticho
sandstone probably represent crustal ages of both northern Ethiopia (Tigrai) as well as the southern, eastern and
western parts of Ethiopia where high grade rocks are
abundant. The originally extensive nature of Ordovician
exposures of the high-grade Mozambique sequences,
and their potential role as a source for Enticho detritus,
is indicated by a small outcrop of Paleozoic sandstone
mapped above high-grade basement rocks in southwest
Ethiopia (∼80 km south of Gambella; Geological Map of
Ethiopia, 1:200,0000; 1966). Additional support comes
from cooling ages from these rocks (Asrat et al., 2001
and references therein; Mock et al., 1999; Ghebreab
et al., 2005) and from southern Ethiopia (Yibas et al.,
2002) showing the orogenic edifice cooled by late Cambrian time. Being derived from all Ethiopian basement
intervals, including the high-grade Mozambique Belt,
the detrital zircon age spectra gain extra importance in
that they confirm that volumetrically significant, pristine
pre-Neoproterozoic crust does not exist in Ethiopia. The
presence of a small but not negligible amount of 1.1 Ga
and 1.9–2.5 Ga detrital zircons in the Enticho sandstone
(ca. 10% of the analysed zircons) may reflect inheritance
from an older component incorporated at the source of
the arc magmas (e.g. Teklay et al., 1998) or derivation
from terranes flanking ANS such as the Abas of Yemen
(e.g. Whitehouse et al., 1998).
Acknowledgements
Our study in Ethiopia is funded by USA–Israel Binational Science Foundation (BSF). We thank D. Küster
(Mekelle University) and Kiros Mehari (Hisana Mining) for hospitality and assistance during our work in
Tigrai and T. Tadesse for providing helpful comments
on the manuscript. F. Mazdab and B. Wiegand (USGS)
guided us on the ion probe and their help is greatly appreciated. T. Hurgrove is thanked for analysing igneous
rock samples T3-11-10 and T3-17-01. We also thank A.
Abraham, chief geologist of the Ethiopian Geological
Survey for support and M. Alene from the University
of Addis Ababa for joining us on the 2004 field trip.
D.A. acknowledges hospitality and discussions with P.
Henry, O. Bellier and P. Affaton while on a sabbatical
at CEREGE (France). Comments by two anonymous
reviewers helped to improve this manuscript and are
greatly appreciated.
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