Lithos 120 (2010) 379–392

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Geochemical, U–Pb zircon, and Nd isotope investigations of the NeoproterozoicGhawjah Metavolcanic rocks, Northwestern Saudi Arabia

Kamal A. Ali a,⁎, Robert J. Stern a, William I. Manton a, Jun-Ichi Kimura b, Martin J. Whitehouse c,Sumit K. Mukherjee d, Peter R. Johnson e, William R. Griffin a

a Department of Geosciences, University of Texas at Dallas, Richardson, TX 75080, USAb Japan Agency for Marine Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japanc Swedish Museum of Natural History, Laboratory for Isotope Geology, Box 50007, SE-104 05 Stockholm, Swedend BP America Exploration & Production Company, Houston TX, USAe Geological Consultant, 1242 Tenth Street, NW Washington DC 20001-4214, USA

⁎ Corresponding author.E-mail address: alik6588@yahoo.com (K.A. Ali).

0024-4937/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.lithos.2010.08.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 December 2009Accepted 22 August 2010Available online 31 August 2010

Keywords:Arabian–Nubian ShieldNeoproterozoicU–Pb zircon datingNd isotopesGhawjah volcanic rocks

Newgeochemical, Nd-isotope andU–Pb zircondata fromNeoproterozoic volcanic rocks fromWadi Sawawin innorthwestern Saudi Arabia provide important constraints on the evolution of the crust in this part of theArabian–Nubian Shield (ANS). The Ghawjah volcanic rocks range from tholeiitic to calc-alkaline and aremetamorphosed to greenschist facies. U–Pb zircon analyses for Ghawjah andesite yield aweightedmean 206Pb/238U age of 763±25 Ma, indicating that these are some of the oldest rocks of the Midyan terrane. Ghawjahvolcanic rocks aremostlymoderately fractionated, as indicated byMg-numbers between28 and67, Cr between5 to 537 ppm and Ni from 4 to 175 ppm, REE patterns are slightly fractionated [(La/Yb)N=1.2 to 4.0], andmulti-element diagrams show Ba, Sr, Rb and K enrichments and Nb and Ta depletions, typical of modernconvergent-margin igneous rocks. Ghawjah volcanic rocks have positive εNd (+5.4 to +8.2) and a meanmodel age of 0.71 Ga. Ghawjah volcanic rocks are similar to the “Younger Volcanic” rocks from the CentralEastern Desert (CED) of Egypt, in terms of stratigraphic relations, chemical compositions, Nd-isotopiccompositions, and U–Pb zircon ages (~750 Ma), indicating that both were generated by partial melting ofsubduction-modified depletedmantle. The Ghawjah volcanic rocks are interpreted to have formed at ~750 Main an arc setting during an important episode of crust formation.

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1. Introduction

The Arabian–Nubian Shield (ANS) extends from Egypt south toSudan, Eritrea, and Ethiopia on the western flank of the Red Sea and onthe eastern flank from Jordan and Israel south through Saudi Arabia andYemen (Fig. 1a). The ANS is a collage of Neoproterozoic tectonostrati-graphic terranes (Fig. 1a) joinedby ophiolite-decorated sutures (Stoeserand Camp, 1985; Johnson and Woldehaimanot, 2003), and provides agood opportunity for studying how juvenile crust forms and evolves(Kröner et al., 1984). ANS volcano-tectonic terranes are suggested tohave formed by subduction within and around the Mozambique Ocean(Stern, 1994; Sengör and Natal'in, 1996). Terrane formation began at~870 Ma following the breakup of Rodinia and terminated ~620 Mawhen collision between large fragments of East and West Gondwanaclosed the Mozambique Ocean, forming the East African–AntarcticOrogen (EAAO, Stern, 1994; Jacobs and Thomas, 2004; Abd El-Rahmanet al., 2009a, b). The ANS became a stable continental region by the

beginning of Cambrian at ~540 Ma (Johnson and Woldehaimanot,2003). Igneous activity responsible for the formation of ANS volcanicsequences is likely to have occurred all through the ANS's 250 m.y.history. Model ages for upper crustal rocks and for lower crustal andmantle lithospheric xenoliths suggest that the entire 40-km thickness ofthe crust beneath the ANSwas extracted from the mantle between 870and 550 Ma and much crust was extracted between 740 and 870 Ma(Hargrove et al., 2006b).

ANS volcanic sequences range in composition from early tholeiite-dominated, through later calc-alkaline-dominated, to late high-K orshoshonitic suites (Greenwood et al., 1976; Roobol et al., 1983).Several interpretations of tectonic setting have been suggested for theformation of ANS juvenile crust, largely as a result of studying volcanicsequences. Rifting of continental crust or older arc crust followed bysubduction has been suggested (Kemp et al., 1982; Abdelsalam andStern, 1993). Other workers infer an important role for OIB-typeoceanic plateaux (Stein, 2003). However, a preponderance of workersinfer that pre-collision volcanic rocks mostly formed at several intra-oceanic arcs (Bakor et al., 1976; Roobol et al., 1983; Jackson, 1985;Pallister et al., 1988; Kröner et al., 1991; Stern, 1994; Abd El-Rahmanet al., 2009a, b). Recognition of several terranes and the long time span

27˚

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0 50 100 km

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HU: HurghadaSA: SafajahQU: Quseir

ALMA: Marsa AlamAL: Al WajhALM: Al Muwaylih

37˚E

b

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Metavolcanic rocks

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Quseir

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Jeddah

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Asir

Gebeit

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BayudaDesert

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Fig. 1Ba

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Legend

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Fig. 1. (a) Map of the Arabian–Nubian Shield (ANS), divided into regions of oceanic,intermediate and continental settings according to Pb and Nd isotope data (modified fromJohnson andWoldehaimanot, 2003). The location of Fig. 1b is indicated. (b) Location of theNeoproterozoic volcanic rocks in NW Saudi Arabia and Central Eastern Desert of Egyptwith Red Sea closed (modified fromSultan et al., 1993); locations of diamictite and bandediron formations (BIFs) fromStern et al. (2006) andGoldring (1990). The locationof Fig. 2 isindicated.

380 K.A. Ali et al. / Lithos 120 (2010) 379–392

of volcanic activity require that the magmatic evolution of eachterrane prior to accretion be considered separately, and thatconstraints from correlative terranes on the complementary flank ofthe Red Sea be considered (Fig. 1a). In this context, there has beenlittle correlation of ANS volcanic sequences between Arabia and NEAfrica, in spite of the fact that similar units are well-exposed on bothsides of the Red Sea.

The Arabian part of the ANS is known as the Arabian Shield and isdivided into at least five tectonostratigraphic terranes (Fig. 1a; Stoeserand Camp, 1985): Afif, Asir, Ar Ryan, Hijaz and Midyan, with someworkers (e.g. Johnson and Woldehaimanot, 2003) recognizing threemore terranes (Jiddah, Ad Dawadimi and Khida). All terranes (except

Khida; Stacey and Agar, 1985; Stoeser and Frost, 2006) are inferred tocomprise Neoproterozoic oceanic arcs, including fore-arc and/or back-arc crust and ophiolites (Dilek and Ahmed, 2003; Stoeser and Frost,2006. The Midyan terrane, the site of this study is in the northernmostArabian Shield (Fig. 1a).

The object of this study is to understand the age and tectonicsetting of a well-preserved volcanic sequence in NW Arabia andevaluate correlation of the sequence with volcanic rocks of similar agein Egypt. We report new geochemical data, U–Pb zircon ages and Nd-isotope analyses for Ghawjah volcanic rocks, and use the data tocorrelate themwith the Younger Volcanic rocks (YMV; Stern, 1981) inthe Central Eastern Desert (CED) of Egypt, recently studied by K.A. Aliet al. (2009). In addition, there is a controversy about the significanceof pre-Neoproterozoic xenocrystic zircons in otherwise juvenile ANScrust (Kröner et al., 1991; Hargrove et al., 2006a, b; B.H. Ali et al.,2009; K.A Ali et al., 2009), and integrated study of this apparentlyjuvenile volcanic sequence provides an opportunity to address thisproblem.

2. Geological setting

The Wadi Sawawin study area (Fig. 2) is in NW Saudi Arabia,between 27˚45 and 28˚00′N and 35˚40 to 36˚00 E, in theMidyan Terrane.Rock units include volcanic rocks (Ghawjah Formation), depositionallyoverlain by sedimentary rocks including Banded Iron Formation (BIF) ofthe Silasia Formation. The Ghawjah and Silasia Formations are part ofthe Za'amGroup (Davies and Grainger, 1985). The Za'amGroup, namedafterWadi Za'ambyAlabouvette and Pellaton (1979), is one of themostextensive Precambrian rock units in NW Saudi Arabia but has not beendirectly dated. It is intruded by the Duba Complex (conventional multi-grain U–Pb zircon age=710±5 Ma; Hedge, 1984) and containsdiamictite, constrained to have been deposited ~750 Ma (Ali et al.,2010), suggesting that the Za'am Group formed ~750 Ma.

The Ghawjah Formation is composed of basalt, porphyriticandesite and subordinate dacite, metamorphosed to greenschistfacies. Its base is not observed and its thickness is therefore unknown,but the Japanese Geological Survey (J.G.S, 1977) identified N1100 m ofvolcanic rocks beneath the Silasia Formation. The Silasia Formation iscomposed mainly of tuffaceous sedimentary rocks and tuff and isintruded by subconcordant diabase sills. The upper part of the SilasiaFormation contains BIF asmuch as 90 m thick. The Silasia Formation isunconformably overlain by the ~600 Ma Minaweh Formation (silicicand intermediate lavas and pyroclastic rocks, subordinate tuffaceoussedimentary rocks; Al-Rehaili, 1982).

The Za'am Group in Wadi Sawawin is well-preserved, with lesspenetrative deformation than is observed elsewhere, maybe due toarmoring of the region by strong BIF deposits, which may have servedto deflect strong tectonic deformation from the region. Davies andGrainger (1985) inferred that Ghawjah and Silasia formations are infault contact, but earlier mapping in connection with BIF exploration(Ashworth and Mackenzie, 1975; Harris, 1978) suggested the contactis depositional, as we observed in the field. We recognized no faultingat the contact between the Ghawjah volcanic rocks and SilasiaFormation; instead it is transitional from coarse-grained beds at thebase to fine-grained sedimentary beds at the top, consistent with adepositional contact.

3. Analytical techniques

The following is a brief synopsis of analytical procedures; furtherdetails are presented in Appendix A. Forty-seven volcanic samples andone sedimentary sample (SW2) were selected for geochemical studies(Fig. 2). Sampleswereprepared at theUniversity of Texas atDallas (UTD)and analyzed for chemical composition at Shimane University in Japan(Table A1; Appendix A). All sampleswere analyzed forwhole-rockmajorand trace element (14 elements) concentrations by XRF and 28 trace

0 5 10 Km

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Sawawin Complex

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Ghawjah Formation

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.

.Unconformity

Unconformity

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.

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# GA-19763 ± 25 Ma

# SW-2

# SWT-5648 ± 17 Ma

# SW-9

# SWC-1662 ± 2 Ma

GW1-GW15

GW16-GW28

SW2-GW22

Fig. 2. Geological map of Wadi Sawawin, NW Arabia (modified from Goldring, 1990), showing the volcanic rocks (Ghawjah Formation) and sedimentary rocks (Silasia Formation)are intruded by a diorite complex. Locations of samples analyzed for geochemistry are indicated with boxes. Locations of geochronological samples: GW19, SW9, SW2, SWC1 & SWT5analyzed during this study are indicated ‘#’.

381K.A. Ali et al. / Lithos 120 (2010) 379–392

elements by ICP-MS. JB-1a, JA-3, JG-1a and JB-2 were used as referencestandards to determine the accuracy of major and trace elements (TableA2; Appendix A). Mg# was calculated assuming all Fe as Fe2+.

Five samples comprising two Ghawjah volcanic rocks (GW19 andSW9), a Silasia sedimentary sample (SW2), one sample (SWT5) froma felsic sill beneath the BIF, and a diorite sample (SWC1) from theintrusive Sawawin Complex were selected for U–Pb zircon geochro-nology (Fig. 2). The samples were crushed and zircons separated atUTD. Cathodoluminescence images were obtained for all zircons usingscanning electron microscope, following which the zircons wereanalyzed for U–Th–Pb concentrations and isotopic compositions(Table A3; Appendix A). Three samples (SW9, SW2 and SWT5)

Table 1Sm–Nd concentration and isotopic data for samples from Wadi Sawawin (Ghawjah volcani

Sample Lithology Sm (ppm) Nd (ppm) 147Sm/144Nd 143N

GW1 Bas. andesite 1.79 6.07 0.178 0.5GW2 Bas. andesite 1.88 6.51 0.174 0.5GW3 Bas. andesite 1.50 5.32 0.170 0.5GW5 Bas. andesite 1.36 4.33 0.190 0.5GW6 Bas. andesite 1.56 5.40 0.174 0.5GW7 Andesite 1.33 4.90 0.164 0.5GW8 Andesite 1.36 4.60 0.179 0.5GW9 Andesite 1.68 5.97 0.170 0.5GW10 Bas. andesite 2.67 9.73 0.165 0.5GW11 Andesite 1.37 4.52 0.183 0.5GW12 Bas. andesite 3.03 10.56 0.173 0.5GW13 Basalt 3.24 14.42 0.136 0.5GW14 Pyroclastic 2.58 9.20 0.169 0.5GW18 Dacite 5.50 21.12 0.157 0.5GW19 Andesite 4.69 19.92 0.142 0.5GW20 Dacite 3.45 12.69 0.164 0.5GW26 Basalt 2.24 8.55 0.158 0.5GW27 Basalt 2.43 8.89 0.165 0.5GW28 Andesite 2.23 9.16 0.147 0.5SW9 Diabase 6.32 23.56 0.162 0.5

All isotopic analyses conducted at UT Dallas on a Finnigan Mat 261 solid-source instrument.based on DePaolo (1981). Errors reported for isotopic ratios are 2σ. εNd(t) values were calcula0.000014. Accepted TDM ages are shown in bold whereas rejected values are show in italic.

were analyzed in 2005 and 2006 using a sensitive high mass-resolution ionmicroprobe with reverse geometry (SHRIMP-RG) at theSUMAC facility at Stanford University and two samples (GW19 andSWC1) were analyzed in 2007 using the CAMECA IMS 1270 at theSwedish Museum of Natural History in Stockholm.

Individual zircon analyses are plotted as two-sigma error ellipses onTera-Wasserburg concordia diagrams (Tera and Wasserburg, 1972).Ages are 206Pb/238U for those b1000 Ma and 207Pb/206Pb for thoseN1000 Ma. This procedure is followed because of low abundances of207Pb as a consequence of the relatively short half-life of its 235U parent,making 207Pb/206Pb ages less precise than 206Pb/238U ages for youngzircons (Black et al., 2003). In contrast, lead loss from old zircons shifts

c rocks), NW Saudi Arabia.

d/144Nd Initial εNd 143Nd/144Nd (initial) Model age (Ga)

12910±0.000024 7.10 0.512018 0.6112849±0.000029 6.27 0.511975 0.7812844±0.000035 6.57 0.511991 0.7212951±0.000012 6.78 0.512000 0.6412871±0.000014 6.76 0.511997 0.7012868±0.000023 7.67 0.512047 0.5712914±0.000027 7.15 0.512019 0.6012881±0.000031 7.34 0.512029 0.6012811±0.000028 6.37 0.511981 0.7612865±0.000042 5.75 0.511948 0.9012835±0.000023 6.11 0.511967 0.8112684±0.000017 6.81 0.512004 0.7212910±0.000019 7.95 0.512061 0.5012733±0.000009 5.69 0.511945 0.8512756±0.000013 7.58 0.512043 0.6312754±0.000021 5.42 0.511931 0.9012813±0.000020 7.16 0.512020 0.6612780±0.000042 5.85 0.511953 0.8412761±0.000014 7.25 0.512025 0.6612887±0.000008 8.22 0.512075 0.50

Trace element concentrations determined at Shimane University, Japan. Model ages areted for an age of 763 Ma. Mean value of La Jolla standards is 143Nd/144Nd=0.512843±

382 K.A. Ali et al. / Lithos 120 (2010) 379–392

238U/206Pb without affecting 207Pb/206Pb (Compston et al., 1984),making 207Pb/206Pb ages more reliable for old zircons. Analyticalprocedures are described in further detail in Appendix A.

Nd isotopic analyses were performed on 20 whole rock volcanicsamples using the MAT 261 mass spectrometer at UTD (Table 1).Analytical runs consisted of 10 blocks of 10 scans each for unknownsand three analyses of the La Jolla Nd standard (mean 143Nd/144Nd=0.511843±0.000014). Analytical procedures are describedin further detail in Appendix A.

4. Results

4.1. Petrography

Primary igneousandsedimentary rock termswill beusedbut it shouldbe understood that all rocks are metamorphosed and strictly speakingrequire the prefix “meta”. The Ghawjah volcanic rocks share manypetrographic features (Table A4; Appendix A). All of the studied rockshave beenmetamorphosed from low tohigh greenschist facies (Fig. 3), soplagioclase typically has altered cores and less altered rims and pyroxene

a

c

e

b

d

f

Fig. 3. Photomicrographs (cross-polarized light) of volcanic rock samples fromWadi Sawawin,NGA18; (e) Diabase SW8; and (f) Diorite SWC1. Act=actinolite; Chl=chlorite; CPX=clinopyr

is partly altered to actinolite and chlorite. Calcite occurs as irregularpatches or in veins. Amygdules are filled with chlorite, calcite and quartz,and accessory minerals are epidote, apatite, zircon and Fe-oxides.

4.2. Geochemical results

Major element data (Table A1; Appendix A) show that Ghawjahvolcanic rocks (Fig. 4a) are bimodal with respect to silica and includeboth calc-alkaline and tholeiitic suites (cf., Miyashiro, 1974). Fig. 4bshows that the Ghawjah volcanic rocks are basalts, basaltic andesites,andesites and dacites (cf., Winchester and Floyd, 1977). Ghawjahvolcanic rocks show wide ranges of SiO2 (49.3–70.5 wt.%; Fig. 4b),TiO2 (0.36–3.0 wt.%), Al2O3 (10.79–18.75 wt.%), K2O (0.08–2.32 wt.%),and MgO (1.18–11.17 wt.%) contents and Mg# (26.6–67.4). Onesample is a tuff, more siliceous than the volcanic rocks (SiO2N76.5%;Fig. 4b). Some of the Ghawjah volcanic rocks are quite primitive, withMg# up to 67.4, up to 175 ppm Ni and up to 537 ppm Cr (Fig. 4c, d).The most primitive Ghawjah volcanic rocks are calc-alkaline. None ofthe primitive samples contains abundant phenocrysts, so bulk rock

WSaudiArabia: (a) Basalt SW21; (b)Basaltic–andesiteGA1; (c)AndesiteGA19; (d)Daciteoxene; Hb=hornblende; And=andesine; Qz=quartz and Plg=plagioclase.

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70

cGhawjah calc-alkalineGhawjah tholeiiteGhawjah daciteCED tholeiite CED calc-alkaline

Fig. 4. Geochemical characteristics of major elements and compatible trace elements for volcanic rocks from Wadi Sawawin, NW Saudi Arabia. (a) FeO/MgO vs. SiO2 classificationdiagram of Miyashiro (1974); (b) Zr/TiO2 vs. SiO2 classification diagram of Winchester and Floyd (1977); (c) Plot of Mg# vs. Ni (ppm); (d) Plot of Mg# vs. Cr (ppm). Geochemicaldata of Central Eastern Desert (CED) of Egypt volcanic rocks shown in grey and dashed fields are from K.A. Ali et al. (2009).

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compositions are reasonable approximations of magmatic composi-tions for these samples.

Trace-element data (Table A1; Appendix A) indicate the followinggeochemical features: (1) REE patterns (Fig. 5a, b, c) are slightly LREE-enriched [(La/Yb)N=1.23–4.03], although three samples are outsidethis range [(La/Yb)N=0.8, 5.26 and 9.4]. REE patterns for tholeiitesand calc-alkaline can be distinguished by lower HREE abundances incalc-alkaline rocks. All Ghawjah volcanic rocks show Eu/Eu* from 0.6to 1.4 (Table A1; Appendix A). Tholeiitic samples show negative Euanomalies, whereas calc-alkaline samples show both negative topositive Eu anomalies. All Ghawjah volcanic rocks show negative topositive Ce anomalies (Ce/Ce*=0.8 to 1.2); (2) Multi-elementdiagrams (Fig. 5d, e, f) show considerable scatter in the alkali metalsand alkaline earths. These patterns show Cs, Ba, Sr, Rb and Kenrichments and Nb, Ta and Ti depletions, however some tholeiiticsamples are relatively depleted in Sr; and (3) Ghawjah volcanicsamples have La/Nb generally N1.2 (1.2 to 6).

4.3. U–Pb zircon geochronology results

4.3.1. Ghawjah volcanic rocksGW19 is a porphyritic andesite (Table A4; Appendix A; Fig. 2).

Zircon cathodoluminescence imaging suggests an igneous origin(Fig. 6), although geochronology found both Neoproterozoic andpre-Neoproterozoic grains. Neoproterozoic zircons are euhedral,elongated, have well-developed growth zoning (e.g., grains Z1, Z2),

and have rare cores mantled by overgrowths (e.g. grains Z3, Z4). Pre-Neoproterzoic zircons are rounded and resemble detrital and/ormetamorphic zircons (e.g., grains Z5, Z6). One analysis was conductedfor each of fifteen zircons. The results range from 323 to 2587 Ma(Table A3; Appendix A; Fig. 7a), but five data points were excludedfrom further age calculation. Of these, one came from a zircon thatyields a 206Pb/238U age of 323 Ma with high U contents (1095 ppm);two zircons yielding pre-Neoproterozoic 207Pb/206Pb ages (2124 and2587 Ma) are interpreted as xenocrysts; and two zircons arediscordant and yield young 206Pb/238U ages (629 and 680 Ma). Theremaining ten data points yield a weighted mean 206Pb/238U age of763±25 Ma (±95% conf., MSWD=0.28; Fig. 7a), which we interpretas the time of Ghawjah volcanism.

SW9 is a diabase (Table A4; Appendix A; Fig. 2). Cathodolumines-cence images of zircons (Fig. 6) reveal internal structures rangingfrom rounding suggestive of detrital zircons (e.g., grains Z9, Z10) andxenocrystic cores with rim overgrowths suggestive of magmaticzircons (Corfu et al., 2003, e.g., grains Z7, Z8). One analysis wasconducted for each of 11 zircons, with results ranging from 604 to2693 Ma (Table A3; Appendix A; Fig. 7b). Three zircons yielded pre-Neoproterozoic ages (1113, 2417, and 2693 Ma; grains Z11, Z12) andthree others yielded younger ages ~600 Ma. The latter grains displayvarious degrees of roundness and are not zoned but containsignificant common Pb or are discordant (e.g. grain Z13). The youngages probably reflect Pb loss or perhaps fluid-related processes: onesuch grain shows cracks under transmitted light (grain Z14). Spot

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CsRbBaTh U NbTa K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y YbLu

Ghawjah dacitef

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Roc

k/C

hond

rites

Roc

k/P

rimiti

ve M

antle

Fig. 5. Rare Earth Element (REE) and trace element diagrams for the analyzed volcanic samples (Ghawjah Formation) from Wadi Sawawin, NW Saudi Arabia. (a) Chondrite-normalized REE patterns for calc-alkaline samples; (b) Chondrite-normalized REE patterns for tholeiitic; (c) Chondrite-normalized REE patterns for dacite samples; (d) Primitivemantle-normalized trace element diagram for calc-alkaline samples; (e) Primitive mantle-normalized trace element diagram for tholeiitic samples; and (f) Primitive mantle-normalized trace element diagram for dacite samples. (Trace element and REE data for CED of Egypt volcanic rocks from K.A. Ali et al. (2009); all normalization values for chondriteand primitive mantle are those of Sun and McDonough (1989).

384 K.A. Ali et al. / Lithos 120 (2010) 379–392

(2.1) records high common 206Pb (4.7%) and shows reversediscordance (Fig. 7b). Filtering out zircons with pre-Neoproterozoicages, discordance, and elevated common 206Pb (N1%), the remainingdata points (n=4) do not yield a reliable age (Fig. 7b).

4.3.2. Silasia formation sedimentary rocksSW2 is a tuffaceous sedimentary rock or wacke (Table A4;

Appendix A; Fig. 2) intercalated with BIF and overlying the Ghawjahvolcanic rocks. Cathodoluminescence images of zircons (Fig. 6) showrounding suggestive of detrital zircons and/or metamorphism (e.g.grains Z17, Z18), and some grains show patchy textures suggestingmetamorphic crystallization (Whitehouse et al., 1998; e.g. grain Z16).Other grains show fine-scale zoning, typical of igneous zircons (e.g.grain Z20) and xenocrystic cores with overgrowths (e.g. grain Z19).One analysis was conducted for each of thirteen zircons, with resultingranging from 330 to 2130 Ma (Table A3; Appendix A; Fig. 8a). Of these,7 zirconsyieldpre-Neoproterozoic agesand2yieldpost-Neoproterozoicages, perhaps reflecting Pb loss or fluid-related processes. Filtering outthese zircons as well as those that have high common 206Pb (N1%) andare discordant, or give pre-Neoproterozoic ages, leaves only three datapoints (580, 731, 765 Ma), which do not yield a reliable age. The seven

zircons with pre-Neoproterozoic ages, ranging from 1009 to 2130 Ma,were possibly derived from older crust or detritus but, as discussedbelow, the abundance of pre-Neoproterozoic zircons in otherwisejuvenile igneous rocks in the region suggests that pre-Neoproterozoiccrust is not necessarily the only source.

4.3.3. Sawawin complexSWC1 is a coarse-grained diorite (Table A4; Appendix A) intruded

into Ghawjah and Silasia Formations (Fig. 9). Cathodoluminescenceimaging of this sample (Fig. 6; grains Z21 to Z26) shows zircons thatare euhedral, elongated and have fine-scale zoning. One analysis wasconducted for each of the twenty zircons and the results arepresented in Table A3 (Appendix A) and Fig. 8b. Three data pointsare omitted because two are discordant and the one might be aninherited grain. The remaining 17 analyses yielded a concordia age of661.5±2.3 Ma (±95% conf., MSWD=2.3; Fig. 8b) that we interpretas the time of intrusion of the Sawawin Complex.

4.3.4. Sawawin felsic sillSWT5was collected froma felsic conformable body, within the BIF

in Wadi Sawawin (Fig. 2). We originally collected this sample

744 Ma752 Ma

733 Ma 782 Ma

2124 Ma

728 Ma

2693 Ma

1113 Ma

641 Ma

820 Ma739 Ma

2587 Ma

726 Ma

1495 Ma

Z1 Z2 Z3 Z4 Z5

Z7 Z8 Z10

Z11 Z12 Z13 Z14 Z15

Z9

Z16

1119 Ma 2109 Ma

Z17 Z18

1211 Ma

Z19

633 Ma

Z20

731 Ma

654 Ma 665 Ma

Z23

630 Ma

Z24

659 Ma

Z25

665 Ma

Z26

668 Ma

Z28

647 Ma

Z29

658 Ma

Z30

640 Ma

Z31

641 Ma

Z32

664 Ma

Z21 Z22

Z27

638 Ma

Z6

Fig. 6. Cathodoluminescence images of zircons for the volcanic, sediment and intrusive samples from the Wadi Sawawin, NW Saudi Arabia analyzed during this study: (Z1 to Z6)typical Neoproterozoic and pre-Neoproterozoic zircons from andesite sample (GW19); (Z7 to Z14) typical Neoproterozoic and Paleoproterozoic zircons from diabase sample (SW9);(Z15 to Z20) typical Neoproterozoic and Paleoproterozoic zircons from tuffaceous sedimentary sample (SW2); (Z21 to Z26) typical Neoproterozoic igneous zircons from dioritesample (SWC1); and (Z27 to Z32) typical Neoproterozoic igneous zircons from felsic tuff sample (SWT5). Note that Z14 is transmitted light image for grain Z13. Locations of ion-microprobe areas are shown by white circles, scale is 100 μm.

385K.A. Ali et al. / Lithos 120 (2010) 379–392

because we thought it was a tuff but its young age (648±17 Ma;Fig. 8c) indicates that it must be a sill. Cathodoluminescence images(Fig. 6; grains Z27 to Z32) show internal structures includingelongation and well-developed growth zoning. Twenty four zirconswere analyzed. Five data points were omitted from the agecalculation because they are either discordant, have high U contents,or have high common Pb. The remaining 19 points cluster to yield aweighted mean 206Pb/238U age of 648±17 Ma (±95% conf.,MSWD=0.20; Fig. 8c). This age is indistinguishable from theSawawin complex diorite (SWC1=661.5±2.3 Ma) and is inter-preted to date intrusion of a felsic sill contemporaneous with theSawawin Complex igneous activity.

4.4. Nd isotopes

Epsilon Nd at 763 Ma ranges from +5.4 to +8.2 (mean=+6.8) forGhawjah volcanic rocks (Table 1; Fig. 10). Nd model ages (Nd TDM) werecalculated for the individual samples using the DePaolo model (1981)because it ismoreappropriate than themodel ofGoldsteinet al. (1984) forcalculating crustal-extraction ages generated in arc-like tectonic settings(Dickin, 2005). Nd TDMmodel ages for samples with 147Sm/144Nd≤0.165are regarded as meaningful (Stern, 2002). Ten samples of mafic,intermediate, and felsic rock pass this filter (bold ages in Table 1) andyield TDM ages from 0.50 to 0.90 Ga, averaging 0.71 Ga.

5. Discussion

5.1. Alteration

Although Ghawjah volcanic rocks preserve their original igneoustexture, they show evidence of alteration and metamorphism, andwere conceivably affected by oceanic hydrothermal processes. Thequestion of which elements were redistributed during alteration iscrucial for interpreting geochemical results. LOI at 1050 °C can be usedto help monitor the extent of element redistribution because mostgreenschist facies minerals (calcite, actinolite, and chlorite) containsignificant structural CO2 and H2O – which dominate ignition losses –because greenschist facies metamorphism typically involves largefluxes of water as a result of seafloor hydrothermal activity. Suchalteration can leach as well as introduce significant quantities of fluid-mobile elements such as Na2O, K2O, Rb, Sr, MgO and CaO. LOI in theGhawjah volcanic samples mostly varies from ~0.72 to 4.1 wt.%(mean=2.73%), but 3 samples have higher LOI (5.1 to 12.9 wt.%);suggesting that these samples are highly altered. Greenschistalteration in volcanic rocks has much less effect on REE, HFSE, Crand Ni (Polat and Hofmann, 2003).

Another criteria used to assess the effect of alteration on theGhawjah volcanic rocks follows the suggestion of Polat and Hofmann(2003) that samples with LOIN6 wt.% and Ce/Ce* ratio greater than1.1 and less than 0.9 should be considered variably altered. All

600

1000

1400

1800

2200

0.04

0.08

0.12

0.16

0.20

0.04

0.08

0.12

0.16

0.20

0 4 8 12 16 20 24238U/206Pb

207 P

b/20

6 Pb

data-point error ellipses are 2σ

830

0.058

0.060

0.062

0.064

0.066

0.068

0.070

7.2 7.6 8.0 8.4 8.8

Mean=763±25 Ma (95%conf.)Wtd by data-pt errs only, 0 of 10 rej.MSWD = 0.28, probability = 0.98

790770

750730

710

810

data-point error ellipses are 2σ

GW19a

2800

2400

2000

1600

1200

800

0 2 4 6 8 10 12

b

270 /

bP20

6 Pb

238U/206Pb

data-point error ellipses are 2σ

2693 Ma

2417 Ma

1113 Ma

SW9

Fig. 7. U–Pb concordia diagrams for geochronology data for volcanic samples: (a) SampleGW19 (andesite); and (b) Sample SW9 (diabase). Error ellipses are 2σ; weighted averageage errorsquoted at 95% confidence.Analytical datapresented inTablesA1andA3; sampleinformation is given in Table A4 (Appendix A).

238U/ 206Pb

238U/ 206Pb

2000

1600

1200

800

0.04

0.06

0.08

0.10

0.12

0.14

1 3 5 7 9 11 13

data-point error ellipses are 2σ

data-point error ellipses are 2σ

data-point error ellipses are 2σ

# SW2Wadi Sawawin metasediment(Silasia Formation)

a

270 P

b/ 20

6 Pb

270 P

b/ 20

6 Pb

238U/ 206Pb

270 P

b/ 20

6 Pb

2130 Ma2109 Ma

1411 Ma

1495 Ma

1085 Ma

1009 Ma

1119 Ma

SWT51600

1200

800

0.04

0.06

0.08

0.10

0.12

0.14

2 4 6 8 10 12

Mean= 648±17 Ma (95% conf.)Wtd by data-pt errs only, 0 of 19 rej.MSWD= 0.20, probability = 1.000

error bars are 2σ

c

Concordia Age = 661,5 ± 2.3 Ma(2σ, decay-const. errs included)

MSWD (of concordance) = 2.3,Prob. (of concordance) = 0.13

SWC1

b

640700

0.040

0.044

0.048

0.052

0.056

0.060

0.064

0.068

8.7 8.9 9.1 9.3 9.5 9.7 9.9

660680

Fig. 8. U–Pb concordia diagrams for geochronolgy data from sedimentary and intrusivesamples: (a) Sample SW2(tuffaceous sediment); (b) Sample SWC1(diorite); and (c) SampleSWT5 (felsic tuff). Error ellipses are 2σ; weighted average age errors quoted at 95%confidence. Analytical data presented in Tables A1 and A3; sample information is given inTable A4 (Appendix A).

386 K.A. Ali et al. / Lithos 120 (2010) 379–392

Ghawjah volcanic rocks have Ce/Ce* from 0.9 to 1.1, but three samples(GA20, GA19, GA17) have Ce/Ce* of 0.77, 1.15 and 1.16 respectively(Table A1; Appendix A). This could reflect dehydration of a subductedslab, resulting in oxygenated and the generation of low-Ce slab-derived fluids (Pearce, 1983) or carbonate-rich alteration (Rollinson,1993). Because of this uncertainty, Ce/Ce* was not used inpetrogenetic interpretation.

5.2. Tectonic setting and petrogenesis

Ghawjah volcanic samples with high Mg# and Ni and Cr contentsare inferred to have formed by melting of mantle peridotite anderuption without significant fractionation (Best and Christiansen,2001), implying short residence times of magma in the crust. Thestrongly positive and restricted range of εNd of the Ghawjah volcaniclavas (+5.4 to +8.2; mean=+6.8) indicate an asthenosphericsource (Dickin, 2005). Their flat HREE patterns indicate partialmelting of plagioclase- or spinel-peridotite in the uppermost mantle,b80 km deep (Rudnick et al., 2004; Fig. 5), suggesting magma genesisin a region of thin lithosphere (b80 km thick). The presence andextensive melting of asthenospheric mantle at such shallow depths isseen today only beneath mid-ocean ridges (Connolly et al., 2009),intra-oceanic convergent margins (Dilek and Furnes, 2009), andperhaps beneath Large Igneous Provinces (Kerr, 1994), and one of

these tectonic settings is also likely to apply to the formation of theGhawjah magmas.

On the mantle-normalized trace element diagrams (Fig. 5d, e, f),most samples show Cs, Ba, Sr, Rb and K enrichments and Nb, Ta, and Ti

Diorite

Diabase

Duba Complex

Upper unit

Middle unit (BIF)

Lower unit

Volcanic rocksS

ilasi

a

For

mat

ion

Intr

usiv

e

Uni

ts

Gha

wja

h

For

mat

ion

300 m

200 m

100 m

0 m

Scale for

Silasia Formation

Felsic Sill

648 + 17 Ma_

Tuffaceous sedimentary

rocks and tuffs

Approximate thickness: 1000m

Pyroclastics, basalts,

andesites, dacites

Approximate thickness: ~1100m

Jaspilitic BIFs and

ferrugenous tuffs

Approximate thickness: 5-90m

Tuffs, tuffaceous

sedimentary rocks

Approximate thickness: 120-200m

Gha

wja

h F

orm

atio

nS

ilasi

a F

orm

atio

n

_

_

aM

3.2+

5.1_

aM

3.2+

5.166_

Unconformity

aM

5+017

aM

52+

367

Fig. 9. Generalized stratigraphic section of Wadi Sawawin, NW Saudi Arabia, showing that the volcanic rocks (Ghawjah Formation, 763±25 Ma) are overlain conformably bysedimentary rock (Silasia Formation), which is intercalated with banded iron formation (BIF). The BIF is accompanied by diabase sills and a tuffaceous felsic sill (648±17 Ma).Ghawjah and Silasia Formations are intruded by the diorite complex (662±2.3 Ma). Ages indicated are from this study and Hedge (1984).

387K.A. Ali et al. / Lithos 120 (2010) 379–392

depletions, apart from some tholeiitic samples that are relativelydepleted in Sr, and all samples have La/NbN1.2 (1.2 to 6.0), typical ofan intra-oceanic island arc setting (Straub, 1995). These arecharacteristics of arc magmas.

Relative involvement of mantle and slab-derived fluids can beevaluated geochemically using trace element diagrams (Pearce, 1983;Pearce and Peate, 1995). On Nb/Yb versus Y/Yb and Zr/Yb plots(Fig. 11a and b), for example, any contribution from slab-derived fluidresults in displacement from the mantle array defined by mantle-derived oceanic basalts (mid-ocean ridge basalts, MORB and oceanicisland basalts, OIB). All Ghawjah volcanic samples plot within themantle array, indicating that the HFSE and HREE were not importantcomponents in the subduction related flux, similar to the situationfound by Abd El-Rahman et al. (2009a, b) for arc crust in the Fawakhirarea of the Eastern Desert. On Nb/Yb versus La/Yb plot, (Fig. 11c),most Ghawjah volcanic rocks are confined to the mantle array butsome samples are displaced towards the upper boundary or above themantle array. These features suggest that LREE were added to themantle wedge from the subducted slab but not in significant amounts(Abd El-Rahman et al., 2009a, b). On Ta/Yb versus Th/Yb plot(Fig. 11d), all Ghawjah volcanic rocks are displaced above the mantlearray. These higher ratios indicate contribution of Th from thesubducted slab. Because Th is not mobile in hydrous fluids,subduction-related input occurs when Th from melted subductedsediment is released into the mantle source of the arc melts (Pearce,2008). This hybrid source partially melts to yield primitive magmaswith Th/Yb that lie above but parallel to the MOR-OIB array. Thedisplacement above the mantle array is greater for more depletedmantle sources (Pearce, 2008). Because HFSE (Nb, Zr) and HREE are

less affected by subduction derived flux, they can used to address thenature of the mantle wedge (Abd El-Rahman et al., 2009a, b).

In general, the trace-element characteristics of the Ghawjah volcanicrocks indicate that themantle source of themagmaswas affected by Th-enriched fluids, which is today characteristic of subduction-related arcmagmas. In particular, it is likely that subducted sediment had animportant role in the observed enrichment of LIL elements (Kay, 1984;White and Durpre, 1986; Wyllie et al., 1989; Obeid, 2006).

Basedon these traceelement compositions, and the strongly positiveand restricted range of εNd, we conclude that the Ghawjah volcanicrocks formed in an intra-oceanic arc setting. A similar conclusion wasreached for the origin of correlative units in the CED of Egypt (K.A. Aliet al., 2009). It should be noted that the direction of subductionresponsible for the Za'am group/CED-YMV arc system remains to beelucidated, although Fig. 1b suggests an ~E-W trend of the subductionzone parallel to the Yanbu suture. Stoeser and Camp (1985) in an earliershield-widemodel, suggested subduction along the Yanbu suture at thejoin between the Midyan and Hijaz terranes was directed towards thesoutheast, whereas Kröner (1985) invoked a pattern of subduction tothe NW.

5.3. Source of pre-Neoproterozoic zircons

The present study, combined with the results of K.A. Ali et al. (2009),Ali et al. (2010), indicates that pre-Neoproterzoic zircons are common in~750 MaGhawjahvolcanic rocks andequivalent volcanic rocks in theCEDofEgypt. Igneous rock sampleGW19contains twoxenocrystic zircons thatyield a concordant 207Pb/206Pb age of 2124±11Ma and a discordant207Pb/206Pb age of 2587±10 (TableA3;AppendixA; Fig. 7a). Igneous rock

0 500 1000 1500 2000-20

-15

-10

-5

0

5

10

U-Pb zircon age (Ma)

DM (Goldstein et al., 1984)

DM (DePaolo, 1981)

Khida Terrane

ANS juvenile Neoproterozoic crust

Saharan Metacraton

b

εNd

εNd

500 600 700 800 900 10003

4

5

6

7

8

9

10

DM (Goldstein et al., 1984)

DM (DePaolo, 1981)Mean εNd (T) = +6.8

N = 20

Ghawjah tholeiite

CED volcanics (N = 19)

Ghawjah calc-alkalineGhawjah dacite

ANS juvenile crust ( N = 157)

U-Pb zircon age (Ma)

a

0 5 10 15 20 25 30 35 400.5116

0.5117

0.5118

0.5119

0.5120

0.5121

0.5122

Nd (ppm)

Goldstein

DePaolo

MeanKhida terrane

0.5

0.50.25

0.25

c

Initi

al 14

3 Nd/

144 N

d

Fig. 10. Plot of epsilon Nd versus crystallization age: (a) Neoproterozoic volcanic rocksanalyzed during this study, and Neoproterozoic volcanic rocks from previous studies;(b) Plot for all data of the Proterozoic rocks of the Arabian–Nubian Shield from this andprevious studies. The reference line for chondritic uniform reservoir (CHUR) and thedepleted mantle evolution curves of DM DePaolo (1981) and DM Goldstein et al.(1984). Data points for CED volcanic samples from K.A. Ali et al. (2009); the field forKhida terrane from Stacy and Hedge (1984); Agar et al. (1992); the field for SaharanMetacraton from Küster et al. (2008); and the field for the ANS juvenile Neoproterozoiccrust from Hargrove et al. (2006b), B.H. Ali et al. (2009), K.A. Ali et al. (2009), Moussa etal. (2008); Zimmer et al. (1995); Stoeser and Frost (2006); Woldemichael et al. (inpress); Liégeois and Stern (2010); and Claesson et al. (1984); and (c) Plot of initial143Nd/144Nd vs. Nd concentration forWadi Sawawin, NWSaudi Arabia volcanic samplesat 763 Ma, in which pre-Neoproterozoic inheritance occurs. The trajectories are shownfor ideal mixtures between the mean composition for samples from Khida terrane(Saudi Arabia) and themean for DePaolo (1981) and Goldstein et al. (1984). This fails todetect significant mixing of ancient continental crust with mantle-derived magmas.

388 K.A. Ali et al. / Lithos 120 (2010) 379–392

sample SW9 contains three pre-Neoproterozoic zircons (1113, 2417,and2693 Ma, Fig. 7b). For equivalent volcanic rocks in the CED of EgyptK.A. Ali et al. (2009) reported that 28 of 77 (36%) zircons analyzedhave ages N900 Ma. These results add to a growing number of studiesthat document the presence of xenocrystic pre-Neoproterozoiczircon in ANS igneous rocks that on the basis of mantle-like initialisotopic compositions are otherwise thought to be juvenile (Pallisteret al., 1988; Sultan et al., 1990; Kennedy et al., 2004, 2005; Hargroveet al., 2006a; Küster et al., 2008; B.H. Ali et al., 2009; K.A. Ali et al.,2009; Be'eri-Shlevin et al., 2009a, b; Andresen et al., 2009).

The question of the source of inherited zircons in otherwisejuvenile ANS crust arises also in the CED, Egypt, where K.A. Ali et al.(2009) showed that the igneous rocks in Wadi Kareim and Wadi ElDabbah contain abundant xenocrystic zircons as well as juvenileisotopic characteristics, similar to the cases observed by Hargrove etal. (2006b) and the present study. Well beyond the ANS, a similarproblematic combination of abundant xenocrystic zircons and juvenileisotopic characteristics has beenobserved for theCentralAsianOrogenicBelt by Kröner et al. (2007), who inferred “substantial reworking of oldcrust despite seemingly primitive Nd isotopic characteristics”. Thisproblem is not confined to Neoproterozoic rocks, because oceanicbasalts and gabbros from theMid-Atlantic Ridge, known tobeonly a fewmillion years old and far from any continental crust, contain ancientzircon (Pilot et al., 1998).

Pre-Neoproterozoic xenocrystic zircons could have been incorpo-rated in ANS juvenilemagmas in one of fourways (Stern et al., in press):1) by laboratory contamination; 2) by incorporating zircons from in situ,older continental crust beneath the ANS; 3) by sourcing zircons insediment eroded fromflanking tracts of older crust and transported intothe ANS oceanic basin; or 4) by inheriting zircons from the mantle.

The first possibility (laboratory contamination) can never beprecluded but Stern et al. (in press) note that several laboratories andinvestigators independently report similar results, so that artificialcontamination is unlikely to be the most important control onwhether or not pre-Neoproterozoic zircons are present.

The second explanation (involvement of older crust) relates to along-standing controversy. It is well documented that the Khida“terrane” in the southeastern Arabian Shield (Stacey and Hedge, 1984;Agar et al., 1992; Whitehouse et al., 2001) and part of the shield inYemen (Whitehouse et al., 1998) are underlain by pre-Neoproterozoiccrust and that pre-Neoproterozoic crust of the SaharanMetacraton alsoflanks the ANS to thewest (Fig. 1a; Abdelsalam et al., 2002; Küster et al.,2008). The proximity of pre-Neoproterozoic crust is also demonstratedby the Atud/Nuwaybah diamictite of the CED and NW Saudi Arabia(Fig. 1b; Ali et al., 2010). Atud/Nuwaybah diamictite contains abundantpre-Neoproterozoic zircons and is thought to record glaciation (Ali et al.,2010) comparable in age to the Sturtian event (740–660 Ma); suchsediments could be subsequently digested by shallow-level magmas,incorporating older zircons in juvenile Neoproterozoic lavas (Hargroveet al., 2006b). Pre-Neoproterozoic zircons appear to be widelydistributed in clastic sediments of the region, as shown by resultsfrom the Silasia Formation. Nevertheless, there is no isotopic support forthehypothesis that significant amounts of older crust or sedimentsweredigested and incorporated in Neoproterozoic magmas, a point that hasbeen repeatedly demonstrated (Kröner et al., 1992; Zimmer et al., 1995;Hargrove et al., 2006b). For example, Hargrove et al. (2006b) studiedigneous rocks along the Bi'r Umqsuture of SaudiArabia and showed thatany interaction between Neoproterozoic juvenile magmas and pre-Neoproterozoic crust must be very limited, in spite of the presence ofpre-Neoproterozoic zircons. Samples containing pre-Neoproterozoiczircons show slightly lower Nd concentrations, slightly lower initial143Nd/144Nd, and therefore lower εNd(t) than contemporaneoussamples that lack inherited zircon (Hargrove et al., 2006b).

Fig. 10c explores the possibility that primitive, mantle-derivedmagmas were contaminated by older crust or sediments containingpre-Neoproterozoic zircons. Nd concentrations are plotted against initial

.1 1 10 100.01

.1

1

10

Nb/Yb

MORB

OIBc

Y/Y

b

.1 1 10 1001

10

100

MORB

OIB

a

Ghawjah tholeiite

Ghawjah calc-alkaline

Ghawjah dacite

Nb/Yb.1 1 10 100

.1

1

10

100MORB

OIB

b

Nb/Yb

.001 .01 0.1 1.01

.1

1

10

MORB

OIB

Active continetalmargin arcs

Oceanic arcs

d

Ta/Yb

La/Y

b

Th/

Yb

Zr/

Yb

Fig. 11. (a) Y versusNb/Yb plot; (b) Zr versusNb/Yb plot; (c) La versusNb/Yb plot for the Ghawjah volcanic rocks fromNWSaudi Arabia.MORB andOIB represents the position of themid-ocean ridge and oceanic islandmantle source respectively. Themantle array (MORB–OIB) after Green (2006); and (d) Th/Yb versus Ta/Yb diagramwithmantle array after Pearce (1983).

389K.A. Ali et al. / Lithos 120 (2010) 379–392

143Nd/144Nd for the 20 volcanic samples analyzed, along with isotopiccompositions expected for depleted mantle according to DePaolo (1981)and Goldstein et al. (1984) and for the Paleoproterozoic Khida terrane.Mixtures between depleted mantle melts and older (Khida-like) crustshould plot along the mixing lines in Fig. 10c. However, most of theGhawjah volcanic samples plot at high angles to themixing array,with nosystematic variation in initial 143Nd/144Nd. These relations rule out simpletwo componentmixing andwe conclude thatNdmodel ages forGhawjahvolcanic rocks unequivocally show that these crustal additions areoverwhelmingly juvenile, in spite of the presence of a significantproportion of pre-Neoproterozoic zircons in the volcanic rocks.

There is another line of evidence that ANS crust is juvenileNeoproterozoic material. Lower crustal xenoliths have been broughtup by Cenozoic volcanic eruptions and these are overwhelminglygabbroic. There is no isotopic evidence that pre-Neoproterozoicmaterial is hidden at depth. Instead, isotopic compositions of thesexenoliths indicate formation in Neoproterozoic time (McGuire andStern, 1993), as do upper mantle xenoliths also found in Cenozoiclavas (Stern and Johnson, 2010).

The third explanation holds that xenocrystic zircons in ANSjuvenile rocks resulted from the interaction of Neoproterozoicmagmas with terrigenous sediments that contributed old zirconsbut did not cause a significant shift in the Nd isotopic compositions.We have already documented that such zircons are common in fine-grained volcanogenic wackes; about half of the zircons in sedimentarysample SW2 are pre-Neoproterozoic (Fig. 8a) indicates that ancientzircons are not limited to the diamictite. Two CED sedimentarywackes in the CED studied by K.A. Ali et al. (2009) also containabundant pre-Neoproterozoic zircons. Wüst (1989) studied detritalzircons from Neoproterozoic sediments in Egypt; Wadi Miyahsediments yielded 207Pb/206Pb zircon evaporation ages of 2410 Ma,whereas those along Wadi Allaqi yielded ages of 1460, 2400, and2450 Ma. These results indicate that Neoproterozoic diamictite and/or

wackes could have provided the pre-Neoproterozoic zircons seen inthe Ghawjah volcanic rocks and those of the CED of Egypt. Aninterpretation that old zircons in Ghawjah Formation lavas arescavenged from sediments is supported by similar age distributions(Fig. 12a, b) between these lavas (and correlatives in Egypt) with theAtud diamictite studied by Ali et al. (2010; Fig. 12c). Such aninterpretation is also consistent with the rounded nature of many pre-Neoproterozoic zircons isolated from these volcanic rocks.

The final possibility is that pre-Neoproterozoic zircons wereintroduced into the mantle as a result of subduction or delaminationand that the old zircons were preserved even during melting. Thispossibility is considered in detail by Stern et al. (in press). Zircon cansurvive high temperatures up to 1500 °C and more than 600 km deepin the Earth (Tange and Takahashi, 2004), and ancient xenocrysts areknown to exist in youngmantle peridotite (Bea et al., 2001; Liati et al.,2004). Zircons with ages 1.6-1.8 Ga, 208 Ma, 74 Ma and 35-26 Ma, forexample, are documented from spinel peridotite and pyroxenitexenoliths brought up in 80 Ka basinites at Kilbourne Hole, SE NewMexico, USA (Liati et al., 2004).

On balance, we infer that the Nd isotopic composition of Ghawjahvolcanic rocks indicates little interaction with crust which, togetherwith the observed rounding of the xenocrysts and the identification ofpre-Neoproterozoic zircons showing similar ages to those in the Atuddiamictite lead us to tentatively conclude that the interaction ofmagmas with sediments was responsible for minor contamination.

5.4. Correlation of Metavolcanic sequences across the Red Sea andtectonic implications

Zircons from theGhawjah volcanic rocks yield an age of 763±25Mafor the andesite sample (GW19). This is indistinguishable from theU–Pbzircon ageof ~750Maobtained for thevolcanic rocks in theCEDof Egypt(K.A. Ali et al., 2009). The presence of pre-Neoproterozoic zircons in

Central Eastern Desert, Egypt

(Metavolcanics)N = 77

(b)

0

5

10

15

20

25

ConcordantDiscordant

0

2

4

6

8

10

12

14

16

Wadi Sawawin, NW Saudi Arabia(Ghawjah metavolcanics)

N = 25

ConcordantDiscordant

(a)

Ages from this study # GW19 & SW9

Ages from Ali et al. (2009)

Atud diamictite

N = 225

Matrix

ConcordantDiscordant

(c)

0

10

20

30

80

90

U-Pb age (Ma)

Ages from Ali et al. (2010)

500 1000 1500 2000 2500 3000

Fre

quen

cy (

n)

Fig. 12. Histograms of single-grain zircon ages from this study, CED volcanic rocks (K.A. Aliet al., 2009), and Atud diamictite (Ali et al., 2010): (a) Age distributions for zircons fromWadi Sawawin volcanic rocks; (b) Age distributions for zircons from CED volcanic rocks;and (c) Age distributions for zircons from the Atud diamictite. Ages b1000 Ma are 238U/206Pb ages and those N1000 Ma are 207Pb/206Pb ages. Note similarity in xenocrystic zirconages between Ghawjah volcanic rocks and CED volcanic rocks.

Table 2Correlation summary between volcanic rocks in NW Saudi Arabia and CED Egypt.

Correlation factors NW Saudi Arabia CED Egypt

U–Pb age(volcanic rocks)

763±25 Ma ~ 750 Ma

Pre-NeoproterozoicXenocrysts

1.1 to 2.7 Ga (N=5, 19%) 1.0 to 2.7 Ga (N=28, 36%)

Initial Epsilon Nd +5.4 to +8.2 +5.1 to +8.9Mean Epsilon Nd +6.8 +6.8Model age 0.50–0.84 Ga 0.64–0.79 GaMean Model age 0.71 Ga 0.72 GaMagma type Tholeiitic and calc-alkaline Tholeiitic and calc-alkalineTectonic setting Intra-oceanic arc Intra-oceanic arc/back-arcStratigraphicrelationships

Volcanics overlain byBIF-bearing sediments

Volcanics overlain byBIF-bearing sediments

U–Pb zircon ages from this study and K.A. Ali et al. (2009), Model ages based on DePaolo(1981).

390 K.A. Ali et al. / Lithos 120 (2010) 379–392

Ghawjah andesite and diabase and Silasia sedimentary rock is alsocomparable to the inherited zircons seen in the Central Eastern DesertYMV rocks and related sedimentary sequences.

Epsilon Nd at 763 Ma ranges from+5.4 to+8.2 (mean=+6.8) forGhawjah volcanic rocks, identical to the results from CED volcanic rocks(mean=+6.8; K.A. Ali et al., 2009). These compositions at 763 Ma plotnear the depletedmantle models of DePaolo (1981) and Goldstein et al.(1984), which predict εNd of +6.5 and +8.3, respectively (Fig. 10a).

Most of the volcanic rocks fromArabia and Central Desert of Egypt showhighly positive εNd(t), which contrasts with the strongly negative εNd(t) of the Paleoproterozoic Khida terrane (Fig. 10b) as well as the morevariable isotopic composition of the Saharan Metacraton (Küster et al.,2008). The Ghawjah volcanic samples yield TDM ages from 0.50 to0.90 Ga, averaging 0.71 Ga, similar to the mean Nd model age of theCED-YMV samples (0.72 Ga).

Table 2 summarizes the similarities between the volcanicsequences at Wadis Kareim and Dabbah in the CED, Egypt (Fig. 1b),and the Ghawjah volcanic rocks. These sequences are nearly conjoinedwith the Red Sea closed (separated by b100 km) and have similarstratigraphic relationships with BIF, one of the most distinctivesupracrustal units in the ANS. The volcanic sequences themselves arenearly indistinguishable in age, composition, tectonic affinities, andisotopic characteristics, and we conclude that the Ghawjah volcanicrocks of SaudiArabia and theYoungerVolcanic rocks of the CEDof Egyptare correlative units. This confirms the importance of a ~750 Ma crust-forming event in the region (K.A. Ali et al., 2009) comprising oceanicbasin closure at a subduction zone and the creation of juvenile crust. Themore specific paleogeographic questions about the orientation of thecombined Ghawjah-YMV arc-backarc basin system and the direction ofthe relevant subduction are not addressed by the evidence presented inthis paper must await further research.

6. Conclusions

1. Geochemical data for Ghawjah volcanic rocks indicate these aretholeiitic and calc-alkaline and range from primitive basalt to dacite.

2. U–Pb zircon geochronology of the Ghawjah andesite gives aweighted mean 206Pb/238U age of 763±25 Ma.

3. Geochemical data show that Ghawjah tholeiitic and calc-alkalinelavas have strong affinities to those of modern arcs.

4. Nd isotopic data for the Ghawjah volcanic rocks show stronglypositive initial εNd (+6.1 to +8.2) and a mean Nd model age(0.71 Ga) that is similar to the U–Pb zircon crystallization age(763 Ma). This indicates that Ghawjah volcanic rocks representjuvenile additions from asthenospheric mantle to the crust.

5. Pre-Neoproterozoic zircons in the volcanic and sediment rocksalong both sides of the Red Sea in Saudi Arabia and Egypt suggestthat a significant pre-Neoproterozoic component was involved informing the crust of the ArabianNubian Shield. This was not intactolder continental crust but may reflect incorporation of zirconsfrom sediment.

6. Ghawjah volcanic rocks are stratigraphically, geochemically, andisotopically indistinguishable from similar volcanic sequences atWadis Kareim and Dabbah in the Central Eastern Desert of Egypt,and these volcanic sequences are correlative.

391K.A. Ali et al. / Lithos 120 (2010) 379–392

Supplementary data to this article can be found online atdoi:10.1016/j.lithos.2010.08.024.

Acknowledgments

This paper was part of the first author's Ph.D. research at theUniversity of Texas at Dallas and was supported by NSF grant EAR-0509486, a Graduate Research Fellowship from NSF (0714104) andThe Japan Society for Promotion of Science. Special thanks go to Mr.Fayek Kattan, Mr. Saad El Garny and The Saudi Geological Survey(SGS) for their help, including providing a vehicle and other supportin the field. PRJ wishes to thank the USGS Saudi Arabian Mission andthe SGS for his opportunity to learn about the geology of the Arabianshield. We appreciate the assistance of Dr. Joe Wooden and Dr. LevIlyinsky during geochronological studies at the SUMAC facility and theSwedish Museum of Natural History. We thank Shimane Universityfor allowing the first author to use the lab facility during his JSPSfellowship. The NordSIM ion microprobe facility is financed andoperated under an agreement between the research councils ofDenmark, Sweden, and the Geological Survey of Finland and theSwedish Museum of Natural History. Constructive reviews of thismanuscript by Dr. Matt Leybourne and an anonymous reviewer aregratefully acknowledged. We thank Editor-in-Chief Prof. Nelson Ebyfor insightful comments and criticism that improved this manuscript.This is UTD Geosciences contribution number #1133.

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