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Precambrian Research 182 (2010) 163–174

Contents lists available at ScienceDirect

Precambrian Research

journa l homepage: www.e lsev ier .com/ locate /precamres

eoproterozoic SHRIMP U–Pb zircon ages of silica-rich Dokhan Volcanics in theorth Eastern Desert, Egypt

hristoph Breitkreuza,∗, Hassan Eliwab, Ibrahim Khalafb, Khaled El Gameela,b,enjamin Bühlera, Sergei Sergeevc, Alexander Larionovc, Mamoru Muratad

Geological Institute, TU Bergakademie Freiberg, Freiberg, GermanyGeology Department, Faculty of Science, Minufiya University, Sheben El Kom, EgyptVSEGEI, CIR, St. Petersburg, Russian FederationDepartment of Geosciences, Faculty of Science, Naruto University of Education, National University Corporation, Naruto, Tokushima 772-8502, Japan

r t i c l e i n f o

rticle history:eceived 30 March 2009eceived in revised form 18 May 2010ccepted 22 June 2010

a b s t r a c t

Chronology of Neoproterozoic volcanosedimentary successions remains controversial for many regionsof the Arabian–Nubian Shield, including the Dokhan Volcanics of NE Egypt. New U–Pb zircon SHRIMPages have been obtained for 10 silica-rich ignimbrites and two subvolcanic dacitic bodies, mapped asDokhan Volcanics, from the North Eastern Desert of Egypt. Crystallization ages range between 592 ± 5and 630 ± 6 Ma (Early Ediacaran). Apparently, the late consolidation of the Arabian–Nubian Shield was

eywords:rabian–Nubian Shieldarly Ediacaranastern Desert Basement Complexammamat Sedimentsast African Orogen

accompanied by the evolution of isolated volcanic centres and basin systems which developed during aperiod of approx. 40 Ma, independently in space and time and probably under changing tectonic regimes.The obtained age data together with other previously published reliable ages for Dokhan Volcanics sug-gest two main pulses of volcanic activity: 630–623 Ma and 618–592 Ma. Five samples contain inheritedzircons, with ages of 669, 715–746, 847 and 1530 Ma, supporting models that North Eastern Desert crustis mainly juvenile Neoproterozoic crust.

an-African Orogenic Cycle

. Introduction

Outcrops mapped as Dokhan Volcanics (DVs) are abundant inhe Eastern Desert of Egypt, especially in the northern part; fur-hermore outcrops are known from the Sinai peninsula (Fig. 1).he DVs form a part of the Eastern Desert Basement Com-lex (EDBC) which represents the northwestern segment of therabian–Nubian Shield (ANS).

The studied DVs are intercalated with Hammamat SedimentsHS) constituting a thick sequence of Neoproterozoic volcanosedi-

entary successions in some areas like in Gebel El Urf (Eliwa et al.,010), Gebel El Kharaza, and Wadi Bali. Other areas like Gebel Mon-ol are dominated by DVs (Figs. 1 and 2). With respect to the DVutcrops in the northern EDBC, it is still discussed whether theseolcanics represent relicts of a larger continuous volcanic provincer whether the silica-rich ignimbrites and andesitic lavas formed in

eparated basin systems distributed in space and time which devel-ped under different tectonic regimes. Another unsolved issue ishe relation of the DVs to the abundant Neoproterozoic granitoidomplexes in the EDBC.

∗ Corresponding author.E-mail address: cbreit@geo.tu-freiberg.de (C. Breitkreuz).

301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2010.06.019

© 2010 Elsevier B.V. All rights reserved.

Precise radiometric ages of the volcanic rocks in the EDBC arean essential prerequisite for a consistent reconstruction of the lateconsolidation of the ANS. They facilitate a stratigraphic correla-tion between volcanic centres, a correlation between volcanic andplutonic events, timing of tectonic activity, and a chronostrati-graphic allocation of basin development with its sedimentary fill.Furthermore, precise ages of the volcanic rocks help to constrainchronologically paleoclimatic information gained from the so farfossil-free HS.

Previous ages obtained using different dating techniquesresulted in a wide spread of data with large uncertainties, ofteninterpreted as long period of DV activity (Bentor and Eyal, 1987;Beyth et al., 1994; Jarrar et al., 2003; Stern, 1979; Stern and Hedge,1985; Willis et al., 1988). By applying in situ U–Pb geochronologywe attempted to produce more accurate and reliable age data forDVs collected from many locations in the NED (Fig. 1) and thus toexplore possible regional orogenic and magmatic trends.

Based on Neoproterozoic Nd model ages (Stern, 2002) theANS is generally accepted as juvenile crust which was formed

by amalgamation of intra-oceanic arc/back arcs with subordinatemicrocontinental plates (e.g. El Sayed et al., 1999; Gass, 1981;Kröner, 1991; Kusky et al., 2003; Pallister et al., 1988; Stern etal., 1988; Stern, 1994; Stoeser and Camp, 1985). There is still nogeneral consensus on the evolution of the ANS from the accretion

164 C. Breitkreuz et al. / Precambrian R

Fig. 1. Regional map of the Egyptian Eastern Desert and Sinai basement complexes.The Dokhan Volcanics (DVs) are displayed as black coloured areas within the remain-ing dark grey undifferentiated basement areas (modified from Moghazi, 2003). Theapproximate boundaries between the North Eastern Desert (NED), Central EasternDesert (CED) and South Eastern Desert (SED) according to Greiling et al. (1994) areshown as dashed lines. Approximate areas of the Phanerozoic sedimentary coverdisconformably overlying the basement complex are light grey patches. The num-bsr

po2itN

2

2

ai((r

ers 1, 2, 3 and 4 refer to sampling localities of dated rocks outside Fig. 2 (Ras Gharibegment, dashed box). The dated samples and their U–Pb ages are enframed withinectangles.

hase to the subsequent cratonization during the collision of partsf proto-East and proto-West Gondwana (Abdeen and Greiling,005; Jacobs and Thomas, 2004). In the present study dating of

nherited zircons and old relict cores has been carried out in ordero further illuminate the chronostratigraphic structure of theeoproterozoic EDBC crust.

. Geological setting

.1. Outline on previous studies

The EDBC represents the north western part of the ANS. Stern

nd Hedge (1985) recognized three distinctive basement terrainsn the Eastern Desert of Egypt; namely North Eastern DesertNED), Central Eastern Desert (CED) and South Eastern Desert (SED)Fig. 1). Based on radiometric ages and distribution of diagnosticock units in the Eastern Desert, they stated that the oldest rocks

esearch 182 (2010) 163–174

occur in the SED, while the youngest rocks occur in the NED. Cryoge-nian volcanism is represented by ophiolitic and arc-related volcanicrocks (850–620 Ma; Hashad, 1980; Stern and Hedge, 1985), whichwere subjected to low-grade regional metamorphism and areknown as the “Older and Younger Metavolcanics” of the EDBC (Aliet al., 2009; Stern, 1981).

The NED is characterized by the abundance of Ediacarannon-metamorphic volcanosedimentary successions, includinginterfingering DVs and HS (Fig. 1; Willis et al., 1988). Furthermore,granitoid complexes are abundant in the NED, whereas there isa general lack of ophiolites. The Precambrian granitoids in Egyptare represented by two main stages: the old stage, known as“Older Granitoids” (OG), comprises calc-alkaline syn-tectonic(900–700 Ma) diorite, tonalite, trondhjemite and granodioriteintrusions, while during the younger stage (690–520 Ma), knownas “Younger Granitoids” (YG), late- to post-tectonic granodiorites,granites and alkali granites formed (El Gaby et al., 1988; Fleck etal., 1980; Jackson et al., 1984; Stern and Hedge, 1985; Stern andGottfried, 1986; Stoeser and Elliot, 1980).

The intermontane HS, named after the type locality in WadiHammamat (Akaad and Noweir, 1980), consist of coarse alluvialconglomerates, fluvial sandstones and lacustrine pelites, which arepartially volcanoclastic in composition (Eliwa et al., 2010; Grothauset al., 1979; Holail and Moghazi, 1998). Willis et al. (1988) usedeight HS samples to calculate a Rb–Sr age of 585 ± 15 Ma, whichthey interpreted to approximate closely the time of sedimentation.SHRIMP U–Pb ages of detrital zircons from HS in the NED yieldeda minimum depositional age of 585 ± 13 Ma (Wilde and Youssef,2002).

The term “Dokhan Volcanics” is after the type locality of GebelDokhan in the NED (Fig. 1). Here, DVs constitute a 1200 m thicksequence of acidic to intermediate lava flows, pyroclastics, ign-imbrites, and hypabyssal porphyries that grade downwards intoepizonal A-type granite plutons (Akaad and Noweir, 1980; El Ramly,1972; El Shazly, 1977; Stern and Gottfried, 1986). In the NED, fieldrelations indicate that granitoids both pre- and post-dates the DVs(e.g. Eliwa et al., 2010). However, some radiogenic ages also suggesta synchronous formation of DVs and granitoids.

The DVs from many outcrops have been investigated petrolog-ically and geochemically since the 1980s (Fig. 1; Abdel Rahman,1996; Abu EL-Leil et al., 1990; Basta et al., 1980; El Gaby et al.,1988, 1990, 1991, 2002; El Sayed et al., 2004; Eliwa, 2000; Eliwaet al., 2006; Mohamed et al., 2000; Stern and Gottfried, 1986). Inaddition, volcanosedimentary facies analysis was applied to betterdefine the volcanic style and the depositional environment for theDVs (first results presented by Eliwa et al., 2010). These studiesshow that the DVs have been formed in subaerial environmentsand that they have medium- to high-K calc-alkaline affinities. Inaddition they exhibit a dominance of porphyritic texture and acomposition of andesite, quartz andesite, dacite, rhyodacite andrhyolite. The DVs are generally non-metamorphosed. However,in some outcrops (e.g. Wadi Um Had and Wadi Kid area) thevolcanosedimentary successions were subjected to low-pressuremetamorphism (El Gaby et al., 1991; Kamal El Din, 1986).

Several studies have been carried out to reveal the magma gen-esis of the DVs and concluded that there was a significant role offractional crystallization coupled with minor crustal contaminationin the evolution of basaltic magma. However, interpretations of tec-tonic setting of the s are still controversial, especially whether theyhave been formed (I) in a subduction-related environment (AbdelRahman, 1996; El Gaby et al., 1988, 1990; Hassan and Hashad, 1990;

Ragab, 1987), (II) in a rift-environment associated with extensionafter crustal thickening (Fritz et al., 1996; Stern, 1994; Stern et al.,1984, 1988; Willis et al., 1988), or (III) during transition betweensubduction and extension (Eliwa et al., 2006; Mohamed et al., 2000;Ressetar and Monard, 1983).

C. Breitkreuz et al. / Precambrian Research 182 (2010) 163–174 165

Fig. 2. Simplified geological map of the Ras Gharib segment of the EDBC between latitudes 27◦45′ and 28◦00′N featuring sample sites and ages of DV from this study; mapbased on new field data and satellite images (modified after Francis, 1972; Osman et al., 2001; Mohamed and El-Sayed, 2008; Daas, 2009; Eliwa et al., 2010).

Table 1Summary of SHRIMP results for DV from the northern segment of Eastern Desert Basement Complex (data of the present study).

Sample Location Coordinates Rock type No. of spotsa Emplacement age (Ma)206Pb/238U–207Pb/235U (I)207Pb/206Pb–238U/206Pb (II)

Age of relict cores (Ma)

SQ 1 Gebel Samr El-Qaa 28◦13′47′′N Subvolcanic rhyolite 13 592.0 ± 5.0 (I) 669 ± 2332◦25′45′′E 592.1 ± 5.0 (II)

594.2 ± 5.3 (I)b

594.5 ± 5.3 (II)b

GK 4 Wadi Abu Hammad 27◦56′08′′N Ignimbrite 1 628 ± 7 18 spots between32◦51′46′′E 715–746, 1530 ± 17

JK 23 Wadi Abu Hammad 27◦56′17′′N Ignimbrite 10 623.7 ± 4.5 (I) 847 ± 1132◦51′29′′E

JK 37 Gebel El Kharaza 27◦55′56′′N Ignimbrite 11 630 ± 6 (I, II) Not found32◦52′04′′E

GK 8 Gebel El Kharaza 27◦55′44′′N Ignimbrite 10 605 ± 6 (I) 746 ± 1432◦52′06′′E 606.0 ± 5.7 (II) 746 ± 15

JM 21 Wadi Abu Mu’aymil 27◦51′42′′N Ignimbrite 10 617.0 ± 7.0 (I, II) Not found32◦51′25′′E

GU 172 Gebel El Urf 27◦48′30′′N Ignimbrite 9 616.0 ± 5.4 (I) 735 ± 2232◦51′05′′E 617.0 ± 5.4 (II)

622.3 ± 6.0 (I)c

622.5 ± 6.0 (II)c

GU 197 Gebel El Urf 27◦48′39′′N Ignimbrite 8 615 ± 4 Not found32◦56′21′′E

GM 1 Gebel Mongal 27◦50′38′′N Subvolcanic dacite 11 618 ± 4 (I) Not found33◦02′03′′E

WD 1 Wadi Dib 27◦42′33′′N Ignimbrite 9 615.9 ± 2.8 (I) Not found33◦03′46′′E

WB 1 Wadi Bali 27◦23′38′′N Ignimbrite 10 608 ± 4 (I) Not found33◦34′09′′E

GN 2 Gebel Nuqara 26◦41′55′′N Ignimbrite 9 601 ± 4 (I) Not found33◦53′15′′E

a Used for calculation of emplacement age; (I) Wetherill, (II) Tera-Wasserburg.b Excluding one slightly younger spot.c Excluding two slightly younger spots.

166 C. Breitkreuz et al. / Precambrian Research 182 (2010) 163–174

F GharG samp

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in volcanosedimentary successions that accumulated in Ediacaranextensional basins. These successions unconformably overlie OG

ig. 3. Schematic lithological profiles of volcanosedimentary successions in the Rasebel El Kharaza, (C) S of Gebel El Kharaza and (D) Gebel El Urf (Eliwa et al., 2010);

.1.1. Previous geochronological studies of the Dokhan VolcanicsDVs)

Stern (1979) calculated a Rb–Sr age of 602 ± 12 Ma for theVs exposed along the Qena-Safaga Road (Fig. 1). Stern andedge (1985) dated 24 samples in the EDBC including lava, ign-

mbrite, dykes and plutonic rocks using both whole-rock Rb–Srnd U–Pb zircon techniques. Ten samples of volcanic rocks fromhe Gebel Dokhan area yielded a Rb–Sr whole-rock isochron agef 592 ± 13 Ma. Eleven andesitic lava and dyke samples fromebel Nuqara gave a Rb–Sr age of 686 ± 28 Ma. In a review paper,essetar and Monard (1983) presented a chronologic sequence ofhe major rock units in the EDBC, quoting after Stern (1979) ange range from 620 to 560 Ma for the DVs. Abdel Rahman andoig (1987) measured a seven-point Rb–Sr whole-rock isochronhich gave age of 620 ± 16 Ma for the DVs of the Gebel Elharaza area (Fig. 1). Wilde and Youssef (2000) carried out SHRIMP–Pb zircon dating on two andesite samples from Gebel Dokhanhich yielded weighted average 206Pb/238U ages of 593 ± 13 and

02 ± 9 Ma.

Segev (1987) recalculated previously published Rb–Sr age data

anging from 535 to 523 Ma for Neoproterozoic volcanic rocks crop-ing out in southern Israel, northeastern Sinai, and southwestern

ordan. He concluded that the alkaline rhyolitic volcanism had ange range of 536–523 Ma marked by the termination of the alkaline

ib segment (Fig. 2): (A) Wadi Abu Hammad area NE of Gebel El Kharaza, (B) East ofle position and radiogenic ages are indicated; profiles have different scales.

batholithic phase IV. In Saudi Arabia, Fleck et al. (1980) reportedRb–Sr ages of 620 ± 90 Ma for andesite lava (Juqjuq Formation),612 Ma1 for rhyolitic ignimbrite and 593 ± 53 Ma for dacite lava(Halaban Formation). In Sudan, Vail (1990) reported an age rangefrom 660 to 550 Ma for numerous syn-orogenic to late-orogenicvolcanic rocks, similar to the DVs in Egypt. This compilation revealsa clustering of previously published DV ages between 590 and620 Ma, however, with large uncertainties.

2.2. Geology of the studied areas in the North Eastern Desert

The studied areas in the NED stretch between Ras Gharib city inthe north and Safaga city southward (Fig. 1). Neoproterozoic crustalcomponents of the cited areas in the NED include metavolcanics,metagabbro–diorite complexes, OG, DV, HS, YG, and abundantdykes. In the investigated locations, the metavolcanics representthe oldest rock unit intruded by OG. The DV and HS interfinger

and metavolcanics and have been intruded by YG with sharp con-tacts (for Gebel El Urf area see Eliwa et al., 2010). A brief outline

1 Two-point isochron with no error given in Fleck et al. (1980).

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f the investigated areas, arranged from north to south, is givenelow.

.2.1. Gebel Samr El-QaaThe Gebel Samr El-Qaa area is located to the north of the Ras

harib-Sheikh Fadl asphalt road (Fig. 1; Eliwa, 2000). Three unitsf porphyritic rocks have been observed at the sampling site, fromhich a highly porphyritic red variety has been collected (SQ 1;

able 1). The sampled rocks show sharp intrusive contacts and are

robably subvolcanic intrusions.

.2.2. Ras Gharib segmentThe Ras Gharib segment presents a complex part of the

ED where metavolcanic complexes, granitoid intrusions and

ig. 4. Schematic lithological profiles of volcanosedimentary successions in the NED betwsh El Maleha Range and (C) Gebel Nuqara; sample position and radiogenic ages are indiig. 3.

esearch 182 (2010) 163–174 167

Dokhan/Hammamat-type successions crop out (Fig. 2; AbdelRahman, 1996; Daas, 2009; Mohamed and El-Sayed, 2008; Osmanet al., 2001). Lithostratigraphic data and structural results on theGebel El Urf area have been presented by Eliwa et al. (2010). Vol-canosedimentary facies analysis, basin evolution, petrology andisotope geochemistry of the different rock units in the Ras Gharibsegment are being investigated currently, the results of which willbe published separately. The following volcanosedimentary sec-tions of the Ras Gharib segment have been sampled for the present

study (Fig. 3A–D): Wadi Abu Hammad area (NE of Gebel El Kharaza),the hill immediately east of Gebel El Kharaza, the area west andsouth of Wadi Mu’aymil, as well as the area west and east of GebelEl Urf (Eliwa et al., 2010). At Gebel Mongal, two additional unitswere sampled for the present study (Fig. 2).

een 26◦ and 28◦N (for location see Fig. 1): (A) Wadi Dib, (B) Wadi Bali in the southerncated (Table 1); profiles have different scales; for location see Fig. 1; for legend see

168 C. Breitkreuz et al. / Precambrian Research 182 (2010) 163–174

Fig. 5. Selected SEM-CL images of complexly zoned zircon crystals from the DV; circles indicate analytical crater locations (see Appendix A).

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To the NE of Gebel El Kharaza, along Wadi Abu Hammad, a c.0 m thick, non-metamorphic volcanosedimentary succession isxposed dipping gently towards the southeast (Fig. 3A). The succes-ion, here referred to as Wadi Abu Hammad Succession, containshree ignimbrite units intercalated with alluvial clastic sediments.he succession rests with an erosional unconformity on granitoidsnd subvolcanic rocks. The first and the third ignimbrite unit haveeen sampled (GK 4, JK 23).

The steep hill east of Gebel El Kharaza exposes a section thattarts with alluvial sediments, unconformably overlying granodi-rite (Fig. 3B). The 150 m thick section is dominated by a package ofV ignimbrites, the lower- and the uppermost of which have been

ampled (JK 37, GK 8).Presumably closely related to the former, a large outcrop of

volcanosedimentary succession, with a thickness of more than00 m, stretches to the south and the southwest of Gebel El Kharaza

Fig. 2). The Wadi Abu Mu’aymil Succession, named after the wadiouth of Gebel El Kharaza, consists of three major ignimbritic unitseparated by lacustrine to alluvial sediments (Fig. 3C). Sample JM1 has been collected from the third major ignimbrite unit for ageating. Another sample from the basal ignimbrites did not yield

ig. 6. (A–D) Selected U–Pb Concordia diagrams presenting zircon analyses of this studyopulation. Bold black ellipses represent the concordia ages, thin black ellipses are dataext for discussion).

esearch 182 (2010) 163–174 169

sufficient zircons. The Wadi Abu Mu’aymil Succession, which at itsbase also displays an erosional unconformity on metavolcanic andintrusive rocks, terminates with a thick pile of andesite lavas toppedby thin ignimbrites.

Gebel El Urf, located 12 km to the south of Gebel El Kharaza,represents a volcanosedimentary succession with a thickness of2000 m (Figs. 2 and 3D; Eliwa et al., 2010). This succession startswith thick Hammamat-type conglomerates and lacustrine sedi-ments, overlying a granodiorite with an erosional unconformity.The upper part of the Gebel El Urf Succession is dominated by twothick ignimbrite units. From the lower unit samples GU 172 and GU197 were taken for SHRIMP dating.

At Gebel Mongal, located in the eastern part of the Ras GharibSegment, a coarsely porphyritic subvolcanic dacite unit has beensampled (GM 1, Table 1). This subvolcanic unit intruded into a thicksequence of vesicular andesites. Another sample taken from these

andesitic lavas did not yield enough zircons.

2.2.3. Wadi DibIn Wadi Dib, 90 km north of Hurghada (Fig. 1), a small vol-

canosedimentary succession has been studied displaying two

. Diagram of sample GK8 (A) displays both the emplacement age and the inheritedused for calculation, and black grey ellipses are data omitted from calculation (see

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gnimbrite units separated by 4 m fluvial sandstones and intrudedy an andesitic sill (Fig. 4A). Sample WD 1 was taken from the upper

ithic-poor ignimbrite.

.2.4. Wadi BaliThe Wadi Bali across Esh El Mellaha, 30 km north of Hurghada,

xposes a complex volcanosedimentary succession (Figs. 1 and 4B).t the base of the succession, andesitic lavas are directly overlainy a thick layer of welded ignimbrite from which sample WB1 wasaken for SHRIMP dating. The ignimbrite itself is overlain by a vol-anoclastic to sedimentary sequence (see also Holail and Moghazi,998).

.2.5. Gebel NuqaraThe preliminary profile of Gebel Nuqara southwest of Safaga

Fig. 1) shows a succession of non-welded ignimbrite followedpward by 15 m thick volcaniclastic mass flow deposits overlainy a welded ignimbrite that has been sampled (GN 2). A highlyesicular andesitic lava lies at the top of the sampled ignimbriteayer (Fig. 4C).

.3. Petrography of the investigated samples

The studied DVs include seven samples of welded ignimbrite,hree samples of non-welded ignimbrite and two samples of sub-olcanic intrusions (Table 1). The ignimbrites are grey, pink andiolet in colour and rhyolitic to dacitic in composition. They con-ist of crystals and crystal fragments of plagioclase, K-feldspar,inor quartz and biotite associated with pumice fragments,

amme and glass shards in addition to lithic fragments. The lithicragments comprise mainly metavolcanic rocks, OG, lavas andre-existing ignimbrites with maximum particle size up to 30 cmcross. All the components are embedded in much finer micro-rystalline, ash-rich matrix. We selected lithic-poor ignimbrite

ig. 7. U–Pb Concordia diagram of zircons from sample GK 4, displaying data from two mepresents the emplacement age for GK 4; however, the separate comprises a large inherain with 1530 Ma not shown here (see Table 1 and Appendix A).

esearch 182 (2010) 163–174

samples in order to restrict the amount of accidental zircons to aminimum.

In the welded ignimbrite varieties (GK 8, GN 2, GU 172, GU 197,JK 23, JM 21, WB 1), the matrix shows well developed weldingaround crystals and lithic fragments. Welding compaction in thestudied samples varies between strongly (parataxitic) and moder-ately welded (eutaxitic). The fiamme reach up to 8 cm length andexhibit occasionally spherulitic crystallization domains which arenot present in the matrix. In the non-welded ignimbrites (GK 4, JK37, WD 1), moderate diagenetic compaction has been observed.

The subvolcanic rocks (GM 1 and GQ 1) are fine-grained and por-phyritic with pink and white colours. They are of dacitic to rhyoliticcomposition that mainly consists of plagioclase, quartz, K-feldsparand mica. These minerals occur as phenocrysts in a fine-grainedmicro- to cryptocrystalline groundmass. The groundmass is com-posed mainly of feldspar, quartz, mica, chlorite, sericite and ironoxides as well as accessory zircon and opaques. Both subvolcanicsamples show porphyritic textures; sample GQ1 shows addition-ally micrographic texture in the groundmass.

3. Analytical procedures

The samples (10–15 kg) were crushed with two jaw crush-ers down to size < 250 �m (additional milling was not necessary).Heavy minerals were separated with Wilfley water table, heavyliquid and magnetic separation procedure. About 50 zircon crys-tals were handpicked from each sample for mounting. The zirconmount preparation, the CL imaging for revealing internal struc-ture and analytical spot positioning and the SHRIMP U–Pb analysis

was carried out at the Centre of Isotope Research of the KarpinskyRussian Geological Research Institute (VSEGEI) in St. Petersburg.

The results were acquired with a secondary electron multi-plier in peak-jumping mode following the standard procedure(e.g. Williams, 1998). A primary molecular oxygen beam has been

easurements; as discussed in the text; field relation suggests that the age of 628 Marited zircon population with an age around 726.5 Ma; data from another inherited

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ocused to produce an analytical spot of c. 25 �m × 20 �m size,he corresponding ion current was −2 to −3 nA. Mass resolutiont 254 AMU (UO) was M/�M > 5000 (1% valley) which allows toesolve isobaric interferences. 1-min raster was used to removehe surface Pb contamination. Each fourth measurement was car-ied out on the U–Pb standard zircon TEMORA (Black et al.,003). The 91500 zircon (Wiedenbeck et al., 1995) was applieds U-concentration standard. Raw results were processed withhe SQUID v1.13a (Ludwig, 2005a) and ISOPLOT/Ex 3.22 (Ludwig,005b) software. Overwhelming majority of analytical spots gave

oncordant ages.

12 volcanic and subvolcanic samples have been dated with theHRIMP. Each zircon separate has been characterized by SEM andathodoluminescence (CL) imaging (Fig. 5). For each sample con-ordant data of 8–17 spots were used to calculate the crystallization

ig. 8. Diagram showing the radiogenic ages with uncertainties of the present study (seerabia and Sudan; *1, Abdel Rahman and Doig (1987); *2, Wilde and Youssef (2000); *3, S

esearch 182 (2010) 163–174 171

age of the volcanic and subvolcanic rocks. Some results have beenomitted from pooled age calculation. These were interpreted as dis-turbed zircon analyses which could be the result of Pb loss. Thecomplete data set is listed in Appendix A; a summary is given inTable 1.

4. SHRIMP U–Pb results

U–Pb SHRIMP dating of 11 samples yielded well constraint agesranging between 592 ± 5 and 630 ± 6 Ma (Table 1), thus confining

the DVs of the NED to the Early Ediacaran. Concordia diagrams offour samples are displayed in Fig. 6.

The Abu Hammad Succession yielded problematic results(Fig. 3A). The first SHRIMP analytical session on the lowermostignimbrite (GK 4) resulted in a concordant age of 726.8 ± 6.7 Ma

also Table 1), compared with published ages of the Eastern Desert as well as Sauditern and Hedge (1985); *4, Stern (1979); *5, Fleck et al. (1980); *6, Vail (1990).

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efined by 10 spots (10 analyses, Fig. 7 and Appendix A), whichould imply the presence of a non-metamorphosed volcanosed-

mentary succession of Cryogenian age in the Ras Gharib segment.n additional sample (JK 23) from the uppermost ignimbrite of

he Abu Hamad Succession (Fig. 3A) has been dated yielding a wellefined age of 623.7 ± 4.5 Ma (Table 1). Careful mapping and lithos-ratigraphic sectioning through three field seasons confirmed thathe sampled ignimbrites (GK 4 and JK 23) belong to one co-geneticolcanosedimentary succession (Fig. 3A). As a consequence, the27 Ma result of GK 4 was considered to represent an inheritedircon population, and a second run of SHRIMP measurements onhe same mount was carried out in St. Petersburg. As displayedn Fig. 7 and in the Appendix A, GK 4 yielded eight more resultsonfirming the 727 Ma zircon population, and one zircon yieldedconcordant age of 628 ± 7 Ma, an age fitting with the JM 23 agef 624 Ma and supporting the observed field relations. The sampleK 4 contains about 10% of crystal clasts; however, it appears that

ew if any of the zircons in sample GK 4 were igneous, and insteadhe vast majority of zircons are xenocrystic.

Apart from the dominant 727 Ma inherited zircon population,ample GK 4 yielded one spot with an age as old as 1530 Ma (Table 1nd Appendix A). In addition to sample GK 4, inherited grains or oldircon cores have been detected in samples SQ 1, GU 172, GK 8, andK 23 with ages of 669, 735, 746 and 847 Ma, respectively (Table 1nd Fig. 8).

. Discussion and conclusion

The crystallization ages of DVs obtained in the present study arehown in Fig. 8 together with previously published age data fromhe NED and CED as well as from other ANS areas. Our results andhose published by Wilde and Youssef (2000) represent the DVs ofhe entire NED and exhibit a maximum age around 630 Ma that haseen obtained from the area between latitudes 27◦45′ to 28◦00′N.urther to the north and to the south, the SHRIMP U–Pb zircon agesor DVs become younger, as young as 592 Ma.

The obtained ages bear implications for the timing of basin evo-ution during the late ANS consolidation. The dated ignimbrites ofhe Gebel El Urf Succession (Fig. 3D) are underlain by a thick andomplex sedimentary succession (Eliwa et al., 2010), suggesting aasin initiation considerably earlier than 615 Ma. Formation of theV succession east of Gebel El Kharaza started already at 630 Ma. Inontrast, the dated ignimbrite of the Wadi Bali sequence (608 Ma)s overlain by a thick sedimentary succession suggesting a continu-tion of basin fill for some time (Fig. 4B). Considering the precisionf the gained ages and the assumed duration of sedimentation, theED experienced Early Ediacaran intermontane basin developmentrobably over a time span of up to 30, or perhaps at most up to 40illion years. Consequently, it is not appropriate to consider allVs present in the NED as belonging to a single volcanic province.ather, during a period between 630 and 590 Ma, the late consoli-ation of ANS (Beyth et al., 1994; Jacobs and Thomas, 2004; Sternnd Hedge, 1985) was accompanied by the evolution of isolatedolcanic centres and transtensional basin systems which devel-ped independently in space and time and probably under differentectonic regimes.

The age range of DVs can be compared with ages obtained fromubvolcanic and plutonic units in the NED and in Sinai. Stern andanton (1987) determined a Rb–Sr age of 591 ± 9 Ma for bimodal

asalt–rhyolite dykes in southern Sinai. Other composite dykes out-

ropping ∼10 km west of Gebel Dokhan yielded an Rb–Sr age of91 ± 12 Ma (Stern and Voegeli, 1987). Moussa et al. (2008) pre-ented a SHRIMP chronological study for granitoids in Abu Harba,attar, Qena-Safaga, Al Missikat and Um Ara areas in the East-rn Desert. The ages here ranged from 652.5 ± 2.6 Ma for the older

esearch 182 (2010) 163–174

granodiorite to 605–595 Ma for the younger granites. Beyth et al.(1994) studied the alkali granite in southern Israel and in Sinai anddetermined Rb–Sr ages ranging between 625 and 610 Ma. Appar-ently the intrusion of YG and of many dyke swarms took placesynchronously with the late stage of the Neoproterozoic intermon-tane basin evolution in the northern ANS. The ages presented hereand other published ages for DVs and intrusive rocks indicate twomain pulses of igneous activity in the northern ANS, 630–623 Maand 618–592 Ma, respectively. However, this suggestion has to beverified by further high precision dating.

As discussed above, sample GK 4 yielded a large populationof inherited zircons (727 Ma) of Cryogenian age. During this time,the Proto-ANS blocks experienced the maximum continental dis-persal stage of the Neoproterozoic supercontinent cycle (Stern,2008). Recently, a number of glacial sedimentary successions havebeen described from the CED (Atud Diamictite; Ali et al., 2010)which have been dated at ∼750 Ma and correlated to the Sturtianglaciation. We did not find any signs of glaciation such as cryotur-bation, diamictites or glacial striae in the alluvial sediments of theAbu Hammad Succession. Based on the displayed data, we con-sider the Abu Hammad Succession to represent Early EdiacaranDokhan/Hammamat-type deposits.

According to a number of previous studies, the NED consistsmainly of juvenile crust generated during the Neoproterozoicorogeny (Kröner et al., 1987; Moussa et al., 2008; Stern, 2002, 2008).This model is supported by low Sri ratios revealed by Rb–Sr stud-ies on EDBC magmatic rocks (e.g. 0.7039–0.7047; Abdel Rahmanand Doig, 1987) indicating predominance of mantle sources withonly minor crustal contamination. On the other hand, Wilde andYoussef (2002) proved that the Hammamat Sediments from theUmm Tawat area contain old detrital zircons with a wide range ofArchean to Neoproterozoic ages (2630–750 Ma). Five of our sam-ples showed old zircons, restricted to the northernmost part of theEDBC (Figs. 1 and 8; Table 1). Most of the inherited ages (669, 727,735 and 746 Ma) are restricted to the Cryogenian, suggesting thatthe magmatic systems which led to the formation of the DVs in theNED mainly originated from mantle sources and experienced onlylimited contamination by ANS crust of Cryogenian age. However,zircons with ages of 847 and 1530 Ma found in DVs of the Ras Gharibsegment reveal the presence of pre-ANS crustal blocks, in places.Alternatively, the Dokhan magmatic systems in the Ras Gharib seg-ment assimilated sediments which contained older detrital zirconsoriginating from beyond the ANS area, as already suggested byWilde and Youssef (2002).

Acknowledgements

Financial support from the German Research Foundation (DFG)is gratefully acknowledged. The authors thank Jacqueline Engel,Michael Steffen, Konrad Stopora and Jens Zimmermann from TUBergakademie Freiberg (TUBAF) for joint field work. We are alsoindebted to Gitta Schneider and Uwe Hoffmann (TUBAF) for theirhelp in preparation of zircon separates. Very careful reviews byJoachim Jacobs and Robert Stern, as well as suggestions from theeditor are highly acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.precamres.2010.06.019.

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