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Precambrian Research 171 (2009) 1–22

Contents lists available at ScienceDirect

Precambrian Research

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

eochemistry, Nd isotopes and U–Pb SHRIMP zircon dating of Neoproterozoicolcanic rocks from the Central Eastern Desert of Egypt: New insights into the750 Ma crust-forming event

amal A. Ali a,∗, Robert J. Sterna, William I. Mantona, Jun-Ichi Kimurab, Hossam A. Khameesc

Department of Geosciences, University of Texas at Dallas, 800 West Campbell Rd, Richardson, TX 75080, USADepartment of Geoscience, Shimane University, Matsue City, 690-8504, JapanNuclear Materials Authority, P.O. Box 530, El Maadi, Kattamyia, Egypt

r t i c l e i n f o

rticle history:eceived 18 March 2008eceived in revised form 2 February 2009ccepted 1 March 2009

eywords:rabian-Nubian ShieldeoproterozoicHRIMP dating

a b s t r a c t

Submarine tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed togreenschist facies, are exposed at Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt.These metavolcanic sequences are representative of early stages of Neoproterozoic crust formation in thenorthern part of the Arabian-Nubian Shield. The magmas were primitive to moderately fractionated, asindicated by average Mg# (54), Cr (191 ppm) and Ni (69 ppm). Their REE patterns are slightly fraction-ated [(La/Yb)N = 1.3–3.9], and multi-element diagrams show Ba, Sr, K and Rb enrichments and Nb and Tadepletions, typical of modern convergent margin igneous rocks. Tectonic discrimination diagrams alsoshow affinities with modern convergent margin magmas, suggesting that Wadi El Dabbah lavas formed

d isotopesetavolcanics

in a volcanic arc that experienced cryptic crustal contamination, whereas Wadi Kareim lavas formed ina back-arc basin. Positive initial �Nd (+5.1 to +8.9) and Nd model ages (0.64–0.79 Ga) indicate that thecrust is juvenile and was extracted from a depleted mantle source. Wadis Kareim and El Dabbah metavol-canic rocks are dated at ∼750 Ma (U–Pb SHRIMP) but contain abundant pre-Neoproterozoic zircons whichmight be derived from underlying sediments or perhaps older continental crust. These results indicatethat a major ∼750 Ma crust-forming event involving ophiolite generation and eruption of juvenile melts

matio

was important for the for

. Introduction

The Arabian-Nubian Shield (ANS—Fig. 1) began to form ∼870 Mago and was established ∼620 Ma ago when convergence betweenast and west Gondwana fragments closed the Mozambique Oceanlong the East Africa-Antarctic Orogen (EAAO—Stern, 1994; Jacobsnd Thomas, 2004). Juvenile Neoproterozoic crust (1000–542 Ma)f the Arabian-Nubian Shield is well exposed on the flanks of theed Sea, making it an excellent place to study Neoproteozoic pro-esses of crustal growth and obduction–accretion tectonics (Kröner,985). However, the tectonic setting and age of metavolcanic rocksn Egypt are poorly understood. Neither has any modern, high-uality trace element or isotopic data been presented for these

ocks.

The ANS is characterized by four main lithologic components:uvenile arc supracrustal sequences, ophiolites, gneissic core com-lexes, and granitoid intrusions (Abdel Naby et al., 2002; Shalaby

∗ Corresponding author. Tel.: +1 972 408 6680; fax: +1 972 883 2537.E-mail addresses: kamal8865@hotmail.com, kaa042000@utdallas.edu (K.A. Ali).

301-9268/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2009.03.002

n of the crust.© 2009 Elsevier B.V. All rights reserved.

et al., 2005). These were intercalated by thrusting during accre-tion and by ∼600 Ma left-lateral transcurrent movement along theNajd and other NW-striking shear zones, particularly in the cen-tral part of the Eastern Desert of Egypt (CED; Sultan et al., 1988);a major unconformity formed about 600 Ma. This polyphase andpervasive deformation requires that pre-unconformity supracrustalsequences and ophiolities be studied in isolated blocks, as done herefor two occurrences of CED metavolcanic rocks.

Metavolcanic rocks in Egypt were described by El-Ramly (1972)as Geosynclinal Shadli metavolcanics. The term “Shadli metavol-canics” is still used occasionally, but most workers now use theinformal division proposed by Stern (1981) of Older Metavolcanics(OMV), Younger Metavolcanics (YMV) and Dokhan volcanics. OMVand YMV are best developed in the Central Eastern Desert ofEgypt (CED), where they lie below the unconformity, separatingolder basement units and ∼600 Ma Hammamat sediments and

Dokhan volcanics. The Dokhan volcanics are medium- to high-Kcalc-alkaline lavas which erupted during post-orogenic extension(Moghazi, 2003) or in a continental arc setting (Khalil, 1997). TheDokhan volcanics are largely restricted to the Northeastern Desertof Egypt and are not discussed further here.

2 K.A. Ali et al. / Precambrian Research 171 (2009) 1–22

Fig. 1. (A) Map of the Arabian-Nubian Shield (dashed ellipsoid) (modified from Stern et al., 2006), showing the location of the study areas and regions where pre-Neoproterozoiccrust is found. Ages for pre-Neoproterozoic crustal tracts are from Whitehouse et al. (1998), Sultan et al. (1994), Agar et al. (1992), Kröner and Sassi (1996), and Stern et al.( tavolc1 issikat( Kolet US

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1994). The location of figure B is indicated. (B) Location of the Neoproterozoic me985), showing locations discussed in the text: (1) G. Gattar, (2) El-Bula, (3) G. El-M9) Um Geigh and Um Selimat, (10) El Sakari, Um Khariga and El Igl El Iswid, (11)uweyel, and (17) G. Um Ara. The location of Fig. 2A is indicated.

Khalil (1997) described the OMV as ocean-related volcanics andhe YMV as island arc related volcanics. The OMV are associatedith CED ophiolites (Stern et al., 2004); these are low-K tholeiitic

asalts, often pillowed and overlain by immature wackes repre-enting deep-water turbidites. The metasediments include minoranded iron formation (BIF) and diamictite (Ali et al., in press).tern (1981) interpreted the metasediments to be overlain by lowrade metamorphic lavas ranging in composition from tholeiiticasalt to calc-alkaline andesite of the YMV, but this is an over-implification, because at Wadi Kareim the metasediments overlyhe YMV. Instead, it appears that the metasediments and YMVere erupted and deposited approximately contemporaneously.ere we investigate the YMV rocks of Wadi Kareim and Wadi Elabbah (Fig. 2) in the Central Eastern Desert of Egypt. We useajor and trace element geochemistry, Nd isotope data and U–Pb

HRIMP zircon dating to evaluate the age, composition, and signifi-ance of the Younger Metavolcanics in the Central Eastern Desert ofgypt.

. Previous work

Khalil (1997) presented a comprehensive overview and syn-hesis of OMV and YMV occurrences, including a geochemical

ompilation of 200 OMV analyses and 254 YMV analyses. Therere still two confusing points about the YMV in the Central Easternesert of Egypt that need to be understood. First, the age relation-

hips of YMV and OMV are not clear, as discussed further below.econd, the stratigraphic significance of the Shadli metavolcanics

anic rocks in the Central Eastern Desert of Egypt (modified from Stern and Hedge,, (4) Nakhil, (5) Fawakhir, (6) Abu Mureiwa, (7) W. Arak and Massar, (8) W. Mahdaf,m Kharit, (12) W. Ghadir, (13) Zabara, (14) Um Samiuki (15) W. Hodein, (16) Abu

is confusing in the context of YMV and OMV; some workers con-sider the Shadli to be equivalent to OMV (El-Shazly and El-Sayed,2000), whereas others describe it as equivalent to YMV (El-Gabyet al., 1988). Part of the confusion results from the fact that YMVand OMV apply to the CED whereas the type locality of the Shadlimetavolcanics lies in the SE Desert. The Shadli metavolcanic rocksmay be unrelated to OMV and YMV; they were interpreted by Sternet al. (1991) to have erupted at a magmatic rift.

A large part of the confusion about the relationship between theOMV, YMV and Shadli metavolcanic sequences results from a lackof high-precision geochronology. There are some previous age data,but these yielded conflicting results. The only radiometric age forthe Shadli metavolcanics comes from the study of Stern et al. (1991)who presented a Rb–Sr whole-rock isochron age of 712 ± 24 Mafor metavolcanic rocks around Um Samiuki (Fig. 1B). Rhyodaciticmetavolcanics at Wadi Kreiga in the SED yielded a Rb–Sr whole-rock age of 768 ± 31 Ma (Stern and Hedge, 1985); these two agestogether suggest that metavolcanics in the SED are ∼750 Ma old.The age of the OMV has been established by zircon dating of twoophiolite occurrences in the CED, 746 ± 19 Ma for a plagiogranitefrom the Ghadir ophiolite (Pb–Pb zircon evaporation age by Kröneret al., 1992) and 736.5 ± 1.2 Ma for a gabbro sample from the Fawkhirophiolite (U–Pb TIMS zircon age by Andresen et al., in press). Thesetwo localities are at opposite margins of the CED and indicate that

the OMV is ∼740 Ma old or younger.

The age of the YMV in the CED is less clear. Stern and Hedge(1985) identified metavolcanics in Wadi El Mahdaf as YMV and pre-sented a Rb–Sr whole-rock age of 622 ± 6 Ma, interpreted to be theage of eruption and cooling. Stern and Hedge (1985) also reported

K.A. Ali et al. / Precambrian Research 171 (2009) 1–22 3

Fig. 2. (A) Structural map of Eastern Desert of Egypt showing the distribution of basins and core complexes (modified from Fritz and Messner, 1999; Shalaby et al., 2005).T l DabbG of theE tudy.

awianHtOabHn

he location of figure B is indicated. (B) Geological map of Wadi Kareim and Wadi Eeologic map of Wadi Kareim, Central Eastern Desert of Egypt, showing the locationl Dabbah, showing the location of the metavolcanic samples obtained during this s

Rb–Sr errorchron age of 632 ± 28 Ma for YMV metavolcanics inadis Arak and Massar and suggested this might be a compos-

te age, consisting of a low Rb/Sr (andesitic) suite of 690 ± 56 Mand a high Rb/Sr (rhyodacitic) suite with an age of 580 ± 29 Ma, butoted that these also defined errorchrons. The presence of “post-ammamat” Atalla felsite in the Arak-Zeidun region may explain

he mixed age. It is unlikely that ∼110 Ma separated eruption of the

MV and YMV because these have experienced similar deformationnd metamorphic histories and no unconformities older than thatetween the deformed metavolcanics and the overlying ∼600 Maammamat conglomerates are known from the CED. The recog-ition by Andresen et al. (in press) of felsic metavolcanics in the

ah, Central Eastern Desert, Egypt. The locations of figures C and D are indicated. (C)metavolcanic sampling area obtained during this study. (D) Geologic map of Wadi

CED yielding a U–Pb single zircon TIMS age of 748 ± 3 Ma furthersuggests that the Rb–Sr age estimates for the YMV are erroneouslylow. What is clearly needed is to determine that the ages of non-ophiolitic (YMV) metavolcanics in the CED are U–Pb zircon ages sothat the various metavolcanic units of Egypt can be distinguishedand compared.

3. Geology of the study areas

Two areas were studied in detail, Wadi Kareim and Wadi Dabbah(Fig. 2). These were previously studied by Stern (1981). These twoareas are separated by 10–15 km, which is mostly occupied by the

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K.A. Ali et al. / Precambr

areim Basin (Fig. 2A) of Hammamat sediments (Fritz and Messner,999). Previous studies demonstrate that the larger region is a broadynform between infrastructure exposures at Gabal Meatiq to theW and Gabal El Sibai on the SE, and the metavolcanic sequencesxposed at Kareim and Dabbah are likely to be quasi-continuousnder the Kareim Basin.

.1. Wadi Kareim

The Wadi Kareim area (Fig. 2C) is located between latitudes5◦54′ and 25◦58′N, and longitudes 34◦00′ and 34◦05′E and is acces-ible by desert track from the Qift-Quesir asphalt road. The volcanicocks are overlain by Atud diamictite, immature metasediments,nd BIF (Fig. 3; Ali et al., in press). Younger metavolcanic rocks athe base of the section are ∼100 m thick and are thrust over youngerammamat sediments to the south marking the northern marginf the Kareim Basin (Fig. 2A).

.2. Wadi El Dabbah

The Wadi El Dabbah study area (Fig. 2D) is located betweenatitudes 25◦46′ and 25◦52′N and longitudes 34◦07′ and 34◦10′End is accessible by desert track from the Red Sea coastal high-ay or from the Qift-Quesir asphalt road. Rock units around Wadi

l Dabbah include serpentinite and talc-carbonate, metavolcanicocks, metasediments and banded iron formation, Hammamatediments, and granite. The metavolcanics lie structurally above

erpentinite and talc carbonate. The metavolcanics are overlain byuffaceous metasediments which intercalated with the BIF (Fig. 3);he metasediments and metavolcanics are unconformably overlainy Hammamat sediments marking the southern flank of the Ham-amat basin (Fritz and Messner, 1999; Fig. 2A). The metavolcanic

ig. 3. Generalized stratigraphic section of (A) Wadi El Dabbah and (B) Wadi Kareim, witositions of samples analyzed for geochemistry and geochronology are indicated. Sample

search 171 (2009) 1–22

rocks are intruded by a range of plutons, ∼700 Ma and younger, inthe region around Gebel Sibai (Bregar et al., 2002).

4. Analytical techniques

Stratigraphic sections (Fig. 3A and B) show the positions of sam-ples collected for geochemistry and geochronology. The followingis a brief synopsis of analytical procedures; further details are pre-sented in Appendix A (supplementary materials). Samples wereprepared at the University of Texas at Dallas (UTD) and analyzed forchemical composition using XRF and ICP-MS facilities at ShimaneUniversity in Japan. Tables 1 and 2 show the results of the X-ray flu-orescence and ICP-MS analyses of 41 samples. Zircons separated forgeochronology at UTD were cathodoluminescent imaged and ana-lyzed for U–Th–Pb concentrations and isotopic compositions usingthe high mass-resolution ion microprobe with reverse geometry(SHRIMP-RG) at the SUMAC facility co-managed by U.S. Geologi-cal Survey and Stanford University Department of Geological andEnvironmental Sciences. A total of 106 spots on 104 zircons wereanalyzed for U–Pb ages. Analytical results are listed in Table B1 (sup-plementary materials) and summarized in Table 3. Whole-rockNd isotopic analyses were performed for 19 metavolcanic samplesusing the MAT 261 mass spectrometer at UTD and the results arepresented in Table 4.

5. Petrography and stratigraphy

The metavolcanic rocks can be subdivided into metabasalt,meta-andesite, meta-pyroclastic rocks, and tuffaceous metased-iments, metamorphosed to greenschist facies. Fig. 4 showsrepresentative photomicrographs for the metavolcanic rocksdescribed below.

h more detailed stratigraphic sections for the metavolcanic rocks. The stratigraphics also studied for U–Pb zircon geochronology are indicated with boxes.

K.A. Ali et al. / Precambrian Research 171 (2009) 1–22 5

Table 1Geochemical data (major and trace element) of the Central Eastern Desert, Egypt.

Sample Wadi El Dabbbah

D2 D4 D5 D6 D10 D13 D14 D15 D16 D17 D18 D19 D20 D21

SiO2 51.80 50.37 69.60 52.34 52.95 75.15 50.74 48.22 48.83 54.71 50.02 53.01 58.87 50.26TiO2 1.10 1.17 0.82 1.28 1.14 0.44 0.76 0.78 0.94 0.73 0.80 0.52 0.37 1.00Al2O3 17.65 17.31 16.18 17.24 16.31 13.42 16.05 16.91 16.33 14.21 16.25 17.35 15.30 17.41Fe2O3 13.20 11.07 3.99 10.76 9.83 4.05 10.60 11.36 10.20 8.86 10.19 10.45 8.42 10.41MnO 0.12 0.18 0.05 0.13 0.14 0.06 0.15 0.16 0.17 0.17 0.16 0.18 0.12 0.16MgO 9.53 4.49 1.28 5.44 4.26 1.28 8.99 9.49 9.18 8.41 8.30 5.05 5.33 8.36CaO 2.18 11.79 3.30 8.89 12.16 1.50 10.82 11.56 11.73 8.79 11.71 10.65 8.80 10.27Na2O 3.95 2.32 3.86 3.25 2.42 5.33 1.39 1.37 1.84 3.69 2.41 2.31 2.31 2.28K2O 1.03 0.07 1.83 0.16 0.02 0.50 0.04 0.06 0.42 0.52 0.13 0.51 0.35 0.06P2O5 0.18 0.18 0.17 0.16 0.16 0.07 0.09 0.09 0.12 0.08 0.08 0.10 0.07 0.12Total 100.7 98.9 101.1 99.7 99.4 101.8 99.6 100.0 99.8 100.2 100.1 100.1 99.93 100.3LOI 5.40 3.67 4.44 6.93 5.70 1.32 3.65 3.89 4.15 2.09 2.36 1.46 1.44 3.71Mg# 58.9 44.5 38.9 50.0 46.2 38.5 62.7 62.3 64.1 65.3 61.7 48.9 55.6 61.4

Ba 360.4 67.1 314.2 53.0 50.9 163.3 39.4 46.6 144.5 149.8 52.9 190.5 203.6 42.9Ce 8.6 22.4 40.3 15.2 15.3 39.7 10.0 13.3 16.6 11.7 12.7 14.1 12.5 15.8Cr 211.0 80.8 6.8 297.2 355.0 33.5 335.7 419.8 325.8 348.6 471.0 71.8 35.2 237.9Ga 16.5 18.2 18.4 16.2 15.7 15.4 15.7 17.7 15.9 11.3 17.3 18.2 14.4 17.9Nb 3.0 5.1 6.3 4.3 3.8 5.7 2.5 2.6 2.5 2.3 2.5 2.7 2.2 3.8Ni 59.2 27.5 5.7 110.1 90.5 16.4 115.4 155.5 129.2 113.7 147.4 20.1 17.9 106.7Pb -0.2 0.3 3.9 2.3 2.1 7.0 1.3 1.6 2.5 2.1 0.6 1.7 2.7 1.0Rb 21.2 6.4 37.2 6.2 4.0 10.8 5.7 2.9 18.9 22.9 5.4 10.9 13.9 2.5Sc 48.1 41.5 13.8 44.7 42.3 14.3 38.2 38.4 42.1 37.5 38.6 44.1 29.6 39.6Sr 154.1 360.4 188.9 617.4 402.6 99.1 330.1 328.1 246.6 224.1 372.4 396.3 284.9 327.6Th 2.3 4.6 4.4 2.9 3.3 4.9 3.8 2.4 4.3 3.1 2.0 2.2 3.7 1.7V 365.6 344.0 106.0 344.5 302.3 74.4 295.0 292.8 314.8 276.2 294.2 330.7 227.4 306.5Y 18.4 22.9 12.4 18.2 18.8 25.9 14.4 16.4 20.6 15.3 15.3 9.4 10.1 18.0Zr 55.3 79.4 172.7 71.2 65.4 121.7 45.5 44.8 51.4 42.6 47.1 48.2 45.2 55.4Sample Wadi El Dabbah Wadi Kareim

D22 D23 D24 D25 D26 D27 K11-1 K11-2 K11-3 K11-4 K11-5 K11-6 K11-7 K11-8

SiO2 57.77 47.43 56.44 50.40 59.49 54.66 59.06 62.27 54.51 71.12 58.63 48.96 53.24 49.02TiO2 1.01 1.58 0.71 1.68 0.40 0.44 0.73 1.44 1.45 0.77 1.83 1.33 1.18 0.96

Al2O3 15.68 15.21 15.43 18.73 15.71 19.73 15.99 15.26 15.77 13.35 14.98 17.62 15.47 18.89Fe2O3 9.10 13.73 10.78 12.13 7.47 9.46 7.02 7.83 8.53 5.71 9.90 11.05 10.78 9.18MnO 0.12 0.18 0.22 0.16 0.17 0.18 0.10 0.14 0.17 0.12 0.20 0.18 0.16 0.14MgO 5.22 6.95 6.19 5.57 3.74 3.66 6.76 3.06 5.13 1.10 3.76 5.18 4.51 8.92CaO 7.34 11.51 5.71 6.88 10.27 8.20 6.65 4.59 9.07 3.46 4.59 12.02 10.99 8.24

Na2O 3.78 2.26 4.77 3.82 2.63 2.64 3.80 4.93 4.75 3.76 4.55 2.92 3.43 2.12K2O 0.21 0.38 0.10 0.72 0.23 0.87 0.24 0.90 0.48 1.52 1.23 0.35 0.24 2.58

P2O5 0.23 0.10 0.27 0.23 0.07 0.10 0.12 0.33 0.15 0.18 0.57 0.11 0.12 0.08Total 100.4 99.3 100.6 100.3 100.2 99.9 100.5 100.8 100.0 101.1 100.2 99.7 100.1 100.1

LOI 2.22 2.38 1.43 4.37 1.36 1.09 4.09 3.56 3.08 4.43 2.28 6.02 2.97 4.85Mg# 53.2 50.1 53.2 47.6 49.8 43.3 65.6 43.6 54.3 27.6 42.9 48.1 45.3 65.8

Ba 148.5 116.6 99.6 133.1 100.5 187.1 88.0 281.3 149.5 303.5 245.0 100.4 99.6 350.1Ce 27.7 16.5 76.5 28.9 12.6 8.1 22.0 55.3 21.2 91.7 56.3 14.0 14.6 14.3Cr 148.1 89.4 18.1 15.0 61.9 15.8 338.4 68.5 133.7 20.7 59.9 284.4 229.4 281.0Ga 16.4 16.2 17.9 22.8 14.8 18.0 14.3 21.3 15.1 21.5 19.8 17.5 14.1 16.4Nb 3.6 3.9 6.6 7.1 3.2 1.5 5.3 9.8 2.9 13.0 8.7 3.4 2.6 1.9Ni 67.0 38.8 9.8 12.6 21.8 9.1 148.2 29.4 75.3 10.2 22.6 74.9 56.6 115.1Pb 1.8 3.1 1.4 0.5 2.5 1.5 0.7 6.0 5.7 2.5 4.4 0.8 1.7 0.0Rb 7.6 10.5 3.7 28.8 58.1 16.5 8.1 10.5 10.8 30.9 17.5 7.4 5.5 45.7Sc 33.1 49.1 32.0 33.2 26.4 30.6 26.3 26.4 29.2 17.2 29.9 35.4 33.9 32.4Sr 267.5 343.1 202.2 302.8 272.1 382.5 199.8 228.3 350.8 156.9 291.0 285.0 229.4 265.0Th 2.8 3.7 5.4 1.5 1.7 2.3 3.3 3.2 3.7 4.8 5.9 0.0 0.8 2.5V 234.5 742.0 103.3 392.2 160.5 205.7 160.1 161.7 196.9 73.5 224.0 228.8 223.6 202.6Y 28.3 18.8 74.7 28.7 10.0 9.7 14.4 47.2 29.4 68.5 46.4 27.3 25.9 16.0

Zr 95.9 62.3 320.5 102.3 57.0 37.7 106.2 284.8 145.6 462.5 241.7 104.6 94.2 71.7Sample Wadi Kareim

K11-9 K11-10 K11-11 K11-13 K12-1 K12-2 K12-3 K12-4 K12-4 K12-6 K12-8 K4-B K4-D

SiO2 51.41 58.21 51.2 55.95 51.05 49.90 50.49 48.82 49.26 60.86 61.43 49.38 69.62TiO2 1.53 3.15 2.13 1.94 2.16 1.98 2.51 1.23 2.14 1.24 1.20 2.12 0.64Al2O3 16.69 14.62 13.92 14.17 15.68 18.15 16.24 17.52 17.18 16.28 15.99 17.18 14.88Fe2O3 9.47 11.99 13.50 10.77 11.92 11.74 12.52 9.42 12.18 8.24 8.61 11.60 6.11MnO 0.17 0.09 0.22 0.19 0.16 0.18 0.18 0.16 0.18 0.13 0.13 0.16 0.13MgO 6.65 2.76 6.20 5.28 6.15 5.72 6.30 6.50 8.13 3.96 4.35 6.38 0.63CaO 9.58 4.14 7.47 6.81 9.21 7.53 6.90 12.90 7.43 4.55 4.06 8.39 3.18Na2O 3.27 4.42 3.61 3.81 3.24 3.69 4.30 2.44 3.05 4.52 4.00 4.12 5.40K2O 0.65 0.51 0.76 0.71 0.25 0.99 0.34 0.60 0.64 0.60 0.74 0.24 0.45P2O5 0.15 0.28 0.21 0.25 0.22 0.21 0.29 0.08 0.22 0.22 0.23 0.22 0.13Total 99.6 100.2 99.2 99.9 100.0 100.1 100.1 99.6 100.4 100.6 100.7 99.8 101.2LOI 4.22 6.06 2.66 7.17 4.03 3.77 4.59 6.50 4.20 4.78 4.08 4.53 1.57Mg# 58.2 31.3 47.6 49.2 50.5 49.1 49.9 57.8 56.9 48.7 50.0 52.2 16.9

6 K.A. Ali et al. / Precambrian Research 171 (2009) 1–22

Table 1 (Continued )

Sample Wadi Kareim

K11-9 K11-10 K11-11 K11-13 K12-1 K12-2 K12-3 K12-4 K12-4 K12-6 K12-8 K4-B K4-D

Ba 202.6 127.7 184.0 207.6 116.1 272.5 183.5 145.0 206.4 213.0 200.9 158.4 240.5Ce 21.5 32.6 29.9 29.8 23.9 17.1 36.7 8.2 23.7 50.0 48.7 18.2 100.4Cr 315.2 8.8 34.3 154.4 98.8 88.1 72.0 291.3 100.2 86.4 76.8 100.0 22.5Ga 16.6 22.6 18.0 18.0 18.3 19.2 21.4 16.6 20.2 19.7 19.1 17.5 25.4Nb 2.4 3.5 3.3 3.8 2.7 2.7 5.2 1.2 2.6 7.6 7.5 3.3 9.4Ni 40.6 7.4 17.0 17.6 45.6 54.9 22.7 132.7 58.4 42.3 45.9 53.0 7.3Pb 0.1 2.3 1.5 0.6 1.8 1.4 2.1 3.8 1.6 0.8 0.1 2.9 7.9Rb 13.3 8.7 13.2 16.5 6.0 17.5 5.7 10.6 11.8 13.3 14.6 5.7 5.7Sc 46.4 56.0 44.7 40.3 37.6 38.1 37.0 37.5 41.5 19.2 19.4 37.3 16.4Sr 428.7 192.1 322.2 269.4 452.3 490.5 514.4 282.6 350.1 377.2 291.0 346.8 276.3Th 4.3 2.7 5.2 3.6 0.5 1.7 2.2 1.0 0.8 3.5 3.5 1.6 5.2V 296.1 539.1 414.2 321.2 324.2 304.6 349.9 232.4 029.9 161.2 158.3 327.0 29.9YZ

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28.0 38.1 39.1 40.7 31.7 28.6r 125.2 234.9 180.4 193.4 143.9 136.2

ajor (wt%) and trace (ppm) elements analyzed by XRF (in water free basis).

The metabasalt is dominated by altered plagioclase laths inocks with ophitic texture and as phenocrysts in porphyritic rocks.ypically, plagioclase has altered cores and less altered rims andyroxene is altered to actinolite and chlorite. Clinopyroxene occu-ies the interstices between plagioclase laths. Clinopyroxene isltered to actinolite and chlorite. Carbonate alteration occurs asalcite patches and veins; locally it reaches up to ∼20% of the rock,ut such intensely altered rocks were avoided. Amygdules are filledith chlorite, calcite and quartz, and accessory minerals are epi-ote, apatite, zircon and Fe-oxides.

The meta-andesite consists of plagioclase, actinolite, clinopy-oxene, quartz, chlorite and calcite. Plagioclase forms moderatelyltered phenocrysts, clinopyroxene occurs as relicts, actinoliteccur as prismatic replacements of clinopyroxene, quartz isnhedral, and amygdules are filled with chlorite, calcite and quartz.alcite occurs as patches and in veins, and accessory minerals arepidote, apatite and Fe-oxides.

Pyroclastic and volcaniclastic rocks are poorly sorted and con-ist of volcanic fragments in a fine-grained matrix of plagioclase,hlorite, quartz and Fe-oxides.

The tuffaceous metasediments are intercalated with bandedron formation, which demonstrate that the succession formed in aubmarine environment. These are greenish meta-mudstones con-isting of anhedral fine-grained quartz, anhedral calcite patchesn a matrix consisting of clay, chlorite, and Fe-oxides. Thinections are described in detail in Table B2 (supplementary mate-ials).

. Bulk rock geochemistry

.1. Major and trace element compositions

The least altered samples were selected for geochemical studyn the basis of thin section examination. Major and trace ele-ent (14 elements) X-ray fluorescence analyses for 41 metavolcanic

amples from Wadi Kareim and Wadi El Dabbah are presented inable 1. The question of which elements have been redistributeduring alteration is crucial for interpreting results. Loss on igni-ion (LOI) at 1050 ◦C can be used to help monitor the extentf element redistribution because most greenschist-facies miner-ls (calcite, actinolite, and chlorite) contain significant structuralO2 and H2O – which dominate ignition losses – resulting fromhe fact that greenschist-facies metamorphism typically involves

arge fluxes of water as a result of seafloor hydrothermal activity.uch alteration can leach and/or introduce significant quantities ofuid-mobile elements such as Na2O, K2O, Rb, Sr, MgO and CaO.OI in Kareim and Dabbah samples varies from ∼1.3 to 7.1 wt%Mean = 3.72 wt%). LOI does not vary systemically with the most

41.4 22.2 30.8 36.9 38.0 33.2 74.7225.0 78.8 146.6 259.8 262.5 144.8 245.6

fluid-mobile major elements: Na2O, K2O and CaO (Fig. S1, sup-plementary materials). We conclude that some major and traceelements have been modestly affected by greenschist-facies meta-morphism but, with the exception of those samples with intensecarbonation or abundant veins (excluded on this basis from geo-chemical analysis), the original magmatic compositions are largelypreserved.

On the basis of the highest Mg#, and highest Ni and Cr concen-trations (Fig. 5A and B), 32 samples were selected for analysis of 28trace elements by ICP-MS (Table 2).

Major element data confirm that Wadi Kareim and Wadi El Dab-bah metavolcanic rocks are mainly basalts, basaltic andesites andandesites with subordinate dacite (Fig. 5C) following the classifi-cation of Le Maitre et al. (1989). Some of these are quite primitive,with Mg# (100× molar ratio of Mg to Mg+Fe) up to 65.8 and highNi (up to 156 ppm) and Cr (up to 471 ppm; Fig. 5A and B). Sam-ples with high Mg# and Ni and Cr contents are inferred to haveformed by melting mantle peridotite and erupted without signif-icant fractionation, further implying short residence times in thecrust. Two samples are tuff/tuffaceous metasediment (K11-4 andD13), more siliceous than the metavolcanics (SiO2 > 70%, Fig. 5D).The metavolcanic rocks show a wide range of TiO2 (0.52–3.15%),Al2O3 (13.35–18.89%), K2O (0.07–2.58%), and MgO (0.63–9.53%)contents and Mg# (65.8–16.5). These are calc-alkaline and tholei-itic with respect to the classification of Miyashiro (1974) (Fig. 5E).The Le Maitre et al. (1989) plot of K2O against SiO2 shows thatthe Kareim and Dabbah metavolcanics are low to medium-K suites(Fig. 5F).

Trace element data show that these rocks have the follow-ing geochemical features: (1) The REE patterns (Fig. 6A–C) areslightly fractionated [(La/Yb)N = 1.3–3.9], although two sampleshave (La/Yb)N = 0.75 and 5.5; Wadi Kareim samples show positiveEu and Ce anomalies, whereas Wadi El Dabbah samples show neg-ative Eu and Ce anomalies. Heavy REE patterns are flat, indicatingthat garnet did not control elemental partitioning during melt-ing or fractionation; (2) multi-element diagrams (Fig. 6D–F) showBa, Sr, K and Rb enrichments and Nb, Ta, and Ti depletions; and(3) samples have La/Nb generally >1.1 (1.1—4.5) and low averageTa/Yb (0.07) and Th/Yb (0.4). Overall, Wadi Kareim REE and otherincompatible elements are enriched relative to Wadi El Dabbahsamples.

6.2. Trace element discrimination diagrams

Fig. 7 shows eight different tectonic discrimination diagrams,where analyses of mafic volcanic rocks from Wadi Kareim and WadiEl Dabbah are plotted and tectonic setting inferred. Results fromeach of these diagrams are discussed below, and Table 5 summa-

ian Re

ro

frttt

TG

S

LBRYZNSCBLCPNSEGTDHETYLHTTPTU

S

LBRYZNSCBLCPNSEGTDHETYLHTTPTU

T

K.A. Ali et al. / Precambr

izes these results; suites with mixed tectonic settings are given inrder from dominant to subordinate.

Zr/Y–Zr diagram (Fig. 7A; Pearce and Norry, 1979): Most samples

rom Wadi El Dabbah plot in the field for volcanic arc (subduction-elated) basalts (VAB), whereas most Wadi Kareim samples plot inhe within-plate and mid-ocean ridge basalt (MORB) fields; onlyhree samples plot in the VAB field, and two of these overlap withhe MORB field (Fig. 7A).

able 2eochemical data (trace and rare earth element) of the Central Eastern Desert, Egypt.

ample Wadi El Dabbbah

D2 D6 D10 D14 D15 D16 D17 D18

i 21.41 15.80 6.68 15.73 19.50 17.62 5.96 7.14e 0.29 0.39 0.33 0.32 0.36 0.34 0.34 0.31b 18.46 4.08 0.69 1.46 1.95 17.53 21.86 3.79

17.86 15.34 16.94 14.78 16.02 22.54 15.10 14.72r 54.1 25.3 41.7 35.1 32.9 45.4 36.9 40.7b 1.79 3.34 3.40 1.4 1.28 1.64 1.39 1.45b 0.08 0.31 0.16 0.74 0.69 0.86 0.46 0.86s 0.57 0.17 0.03 0.13 0.3 0.81 0.89 0.36a 360 38 29 18 22 145 145 42a 4.58 6.46 7.06 5.18 5.18 7.44 4.82 4.56e 10.7 15.0 15.9 10.9 11.1 15.5 10.0 10.2r 1.45 1.99 2.08 1.49 1.51 2.01 1.35 1.40d 6.95 9.32 9.53 6.92 6.83 9.05 6.25 6.46m 2.13 2.43 2.44 1.86 1.97 2.47 1.79 1.87u 0.74 0.94 0.92 0.81 0.81 0.85 0.59 0.73d 2.71 2.84 3.03 2.32 2.51 3.14 2.34 2.36b 0.48 0.48 0.50 0.41 0.43 0.53 0.41 0.41y 3.22 3.25 3.36 2.83 2.89 3.63 2.78 2.77o 0.68 0.66 0.70 0.59 0.62 0.80 0.59 0.59r 1.90 1.74 1.90 1.62 1.72 2.22 1.65 1.63m 0.29 0.26 0.29 0.26 0.26 0.34 0.26 0.26b 1.91 1.71 1.87 1.67 1.73 2.23 1.74 1.69u 0.29 0.23 0.28 0.25 0.26 0.34 0.27 0.26f 1.41 0.91 1.24 1.05 1.07 1.39 1.13 1.20a 0.10 0.18 0.18 0.07 0.07 0.08 0.06 0.06l 0.11 0.02 0.01 0.02 0.03 0.23 0.24 0.04b 1.19 1.80 1.08 1.68 1.89 2.06 2.13 2.64h 0.62 0.72 0.68 0.81 0.87 1.06 0.75 0.74

0.19 0.21 0.20 0.22 0.27 0.32 0.20 0.22

ample Wadi Kareim

K11-1 K11-3 K11-6 K11-7 K11-8 K11-9 K11-11 K11-13

i 11.98 8.89 11.78 10.18 9.75 16.29 7.26 8.61e 0.77 1.14 0.64 0.73 0.46 0.72 1.00 1.00b 4.34 7.45 5.16 3.69 42.42 10.37 8.10 10.97

14.54 28.85 26.59 24.77 16.49 27.00 42.47 39.60r 99 144 95 69 59 105 146 143b 4.45 2.98 2.54 4.37 1.27 0.672 2.68 3.67b 0.20 0.78 0.43 0.48 0.46 0.99 1.23 0.73s 0.16 0.10 0.37 0.10 1.18 0.73 0.85 0.56a 67 111 79 84 350 199 210 159a 11.07 7.46 4.63 4.73 3.10 5.66 8.56 9.53e 23.0 20.0 13.0 12.6 8.9 16.6 24.9 27.2r 2.78 2.93 2.02 1.94 1.40 2.59 3.70 3.81d 11.27 14.82 10.35 9.88 7.25 13.28 19.28 19.10m 2.59 4.24 3.07 3.03 2.24 3.93 5.49 5.21u 0.87 1.56 1.10 1.32 0.99 1.6 1.88 1.64d 2.81 5.03 3.89 3.90 2.84 4.89 6.66 6.44b 0.44 0.85 0.66 0.67 0.47 1.81 1.11 1.06y 2.81 5.49 4.44 4.38 3.10 5.18 7.00 7.03o 0.55 1.12 0.91 0.92 0.63 1.06 1.44 1.38r 1.51 3.03 2.52 2.48 1.68 2.82 3.88 3.65m 0.22 0.46 0.37 0.38 0.26 0.43 0.58 0.55b 1.45 2.95 2.44 2.43 1.65 2.77 3.65 3.67u 0.22 0.44 0.37 0.37 0.25 0.40 0.55 0.53f 2.60 3.46 2.25 1.96 1.48 2.51 3.43 3.49a 0.28 0.18 0.13 0.12 0.08 0.13 0.19 0.24l 0.03 0.05 0.03 0.02 0.21 0.06 0.04 0.07b 1.94 5.28 2.51 2.60 0.56 1.25 0.95 1.38h 1.77 0.68 0.57 0.52 0.19 2.6 0.39 0.85

0.66 0.27 0.18 0.20 0.08 0.11 0.16 0.37

race and rare earth element (ppm) analyzed by ICP-MS.

search 171 (2009) 1–22 7

Ti/Y–Nb/Y diagram (Fig. 7B; Pearce, 1982): The high Ti/Y andNb/Y ratios for WPB reflect enriched mantle sources relativeto those of VAB and MORB and/or the role of residual garnet

(Rollinson, 1993); note that no evidence for garnet control is seenin Kareim or El Dabbah REE patterns. Wadi Kareim and Wadi ElDabbah samples overlap the fields for volcanic arc and MORB.No samples plot in the WPB field, in contrast to results on theZr/Y vs. Zr diagram (Fig. 7A). This may be due to the rule of

D19 D20 D21 D22 D23 D24 D25 D26

4.50 5.50 20.96 9.93 8.15 8.68 53.48 4.700.44 0.56 0.47 0.69 0.41 0.98 0.65 0.818.80 10.52 0.75 4.36 7.14 1.99 23.38 3.58

10.19 10.89 16.45 26.61 22.67 81.53 31.48 11.7936.5 37.5 27.3 57.4 52.4 381.1 79.6 48.2

1.27 1.16 2.86 2.99 2.82 7.42 5.27 2.030.13 0.17 0.28 0.29 0.38 0.07 0.37 0.210.37 0.40 0.19 0.35 0.27 0.18 1.26 0.28

176 243 19 142 105 82 99 875.70 3.98 5.56 8.90 5.87 31.82 10.07 4.41

12.6 8.8 12.4 21.7 13.35 82.1 21.99 9.401.67 1.23 1.73 2.98 1.78 8.52 3.02 1.297.44 5.55 8.10 14.20 8.36 39.76 13.39 5.971.78 1.51 2.32 3.81 2.37 9.33 3.55 1.540.62 0.51 0.98 1.13 0.94 2.56 1.36 0.521.80 1.78 2.90 4.51 3.08 11.27 4.32 1.730.28 0.29 0.50 0.76 0.53 1.91 0.74 0.291.73 1.91 3.27 5.06 3.47 13.78 5.01 1.880.36 0.40 0.69 1.05 0.74 2.74 1.04 0.390.98 1.13 1.87 2.87 2.01 7.82 2.80 1.100.16 0.18 0.28 0.45 0.31 1.27 0.43 0.171.04 1.21 1.85 2.86 1.97 8.64 2.82 1.200.16 0.19 0.28 0.43 0.29 1.38 0.43 0.191.19 1.20 0.95 1.89 1.36 8.62 2.48 1.220.04 0.05 0.14 0.18 0.14 0.67 0.28 0.060.09 0.07 0.01 0.04 0.06 0.02 0.19 0.031.67 1.91 1.47 2.45 2.72 1.40 1.64 3.281.17 0.61 0.58 1.25 0.71 4.56 1.27 0.780.38 0.26 0.16 0.44 0.19 1.11 0.35 0.32

K12-1 K12-2 K12-3 K12-4 K12-5 K12-6 K12-8 K4-B

12.94 18.63 7.22 10.85 18.92 9.86 8.63 18.930.66 0.63 1.14 0.35 0.67 1.39 1.31 0.853.48 14.68 4.47 9.48 10.57 10.99 12.37 4.80

27.86 29.51 43.37 20.91 30.12 37.36 38.29 32.41122 75 113 32 165 226 249 163

2.30 2.05 4.26 0.67 2.47 7.07 6.60 2.720.41 0.56 1.19 0.32 0.55 0.20 0.31 0.500.14 0.61 0.26 0.68 0.44 0.56 0.35 0.14

72 301 173 134 189 225 245 1516.05 5.43 11.80 2.26 6.01 16.16 17.09 6.68

16.7 16.2 33.9 7.41 17.6 40.90 42.67 18.92.77 2.51 4.96 1.36 2.79 4.98 5.22 2.98

14.20 13.67 25.59 3.5 14.65 22.52 23.52 15.854.41 4.02 6.28 2.55 4.32 5.51 5.69 4.711.69 1.57 2.18 0.98 1.55 1.60 1.56 1.785.35 4.96 7.12 3.50 5.31 6.06 6.32 5.680.90 1.81 1.14 0.61 0.88 0.99 1.02 0.945.67 5.24 7.29 4.01 5.71 6.41 6.47 5.981.17 1.04 1.43 0.83 1.15 1.30 1.30 1.233.10 2.82 3.76 2.23 3.11 3.50 3.55 3.210.47 0.41 0.56 0.34 0.46 0.54 0.54 0.482.95 2.64 3.61 2.16 3.02 3.48 3.56 3.150.44 0.40 0.52 0.32 0.45 0.52 0.54 0.463.28 2.88 3.45 1.48 3.33 5.89 6.12 3.550.16 0.15 0.28 0.04 0.16 0.48 0.48 0.160.03 0.12 0.04 0.08 0.08 0.08 0.10 0.030.99 2.04 2.17 1.75 1.33 2.61 1.43 1.330.49 0.48 0.68 0.2 0.49 2.68 2.66 0.520.22 0.21 0.28 0.05 0.22 1.03 1.04 0.24

8 K.A. Ali et al. / Precambrian Research 171 (2009) 1–22

Table 3Geochronological summary for Wadi Kareim and Wadi El Dabbah metavolcanic and meta-sediment samples.

Location Sample No. Latitude N Longitude E Lithology Pre-Neoproterozoic ages (Ma) Concordant ages

Wadi Kareim K11-6 25◦56′29′′ Basalt 1250 ± 10 (conc.)34◦02′22′′ 1002 ± 8 (conc.)

2060 ± 11 (disc.)1039 ± 24 (disc.)

Wadi Kareim K11-8 25◦56′31′′ Basalt 1023 ± 3 (conc.)34◦02′22′′ 1060 ± 6 (conc.)

1012 ± 7 (conc.)2730 ± 12 (disc.)

Wadi Kareim K11-9 25◦56′31′′ Gabbro 2456 ± 12 (conc.) 730 ± 22 (Mean)34◦02′22′′ 2474 ± 13 (conc.)

2066 ± 10 (conc.)2351 ± 9 (disc.)2710 ± 14 (disc.)1944 ± 7 (disc.)2020 ± 14 (disc.)

Wadi Kareim K4G 25◦56′39′′ Diabase 1358 ± 16 (disc.) 743 ± 45 (Mean)34◦02′09′′ 2101 ± 34 (conc.)

2667 ± 7 (disc.)2396 ± 7 (disc.)1370 ± 22 (disc.)

Wadi Kareim K4K 25◦56′52′′ Felsic Tuff 769 ± 29 (Mean)34◦02′15′′

Wadi El Dabbah D7 25◦49′54′′ Andesite 2441 ± 17 (conc.)34◦09′06′′ 1330 ± 12 (conc.)

Wadi El Dabbah D25 25◦47′59′′ Diabase 1408 ± 18 (disc.)34◦08′50′′ 1071 ± 14 (conc.)

2572 ± 6 (disc.)2217 ± 24 (conc.)1896 ± 11 (disc.)1509 ± 17 (conc.)

W ◦ ′ ′′ a-sedi

a-sedi

iai1

ac

TS

S

KKKKKKKKKDDDDDDDDDD

Aa

adi El Dabbah D1 25 49 05 Met34◦09′16′′

D13 25◦47′53′′ Met34◦08′00′′

lmenite, rutile or sphene during magma fractionation. Negative Nbnomalies are also characteristic of the continental crust and may

ndicate crustal involvement in magmatic processes (Rollinson,993).

Cr–Y diagram (Fig. 7C; Pearce et al., 1981): This diagram takesdvantage of the observation that arc basalts typically show low Yoncentrations at a given Cr content relative to other basalt types.

able 4m–Nd concentration and isotopic data for samples from Wadis Kareim and El Dabbah, E

ample Lithology Sm (ppm) Nd (ppm) 147Sm/144

4B Basalt 4.71 15.85 0.179411-6 Basalt 3.07 10.35 0.179011-7 Diabase 3.03 9.88 0.185211-8 Basalt 2.24 7.25 0.187111-9 Gabbro 3.93 13.28 0.178712-1 Basalt 4.41 14.19 0.187812-4 Basalt 2.55 7.45 0.207112-5 Basalt 4.32 14.65 0.178212-8 Pyroclastic 5.69 23.52 0.14632 B. andesite 2.13 6.95 0.185210 B. andesite 2.44 9.53 0.154914 Basalt 1.86 6.92 0.162115 Diabase 1.97 6.83 0.174216 Basalt 2.47 9.05 0.164617 Diabase 1.79 6.25 0.173318 Basalt 1.87 6.46 0.175219 B. andesite 1.78 7.44 0.144424 B. andesite 9.32 39.76 0.141825 Diabase 3.54 13.39 0.1601

ll isotopic analysis conducted at UT Dallas on a Finnigan Mat 261 solid-source instrumeges based on DePaolo (1981).

ment 7 data points conc. (1209–2474 Ma);7 data points disc. (1198–2668 Ma)

ment 2627 ± 6 (disc.)2225 ± 25 (conc.)

Most samples from Wadi El Dabbah plot in the VAB field, whereasmost Wadi Kareim samples plot in the MORB field.

Ti–V diagram (Fig. 7D; Shervais, 1982): This diagram takes advan-tage of the fact that Ti and V behave differently in melts becausepartitioning changes with oxygen fugacity, which is sensitive totectonic setting (arc magmas are more oxidizing than MORB andWPB). This diagram also has a field for back-arc basin (BAB) basalts,

astern Desert, Egypt.

Nd 143Nd/144Nd Initial �Nd Model age (Ma)

0.513012 8.97 –0.512893 6.69 –0.512933 6.88 –0.512933 6.69 –0.512960 8.03 –0.512949 6.93 –0.513083 7.67 –0.512957 8.02 –0.512768 7.43 6420.512841 5.08 –0.512776 6.74 7120.512818 6.87 6890.512804 5.42 –0.512796 6.19 7850.512829 5.99 –0.512821 5.66 –0.512689 6.08 7910.512751 7.54 6380.512779 6.30 768

nt. Trace element concentrations determined at Shimane University, Japan. Model

ian Re

aMiB

swsI

F(B

K.A. Ali et al. / Precambr

field that is lacking from many other discrimination diagrams.ost Wadi El Dabbah samples plot in the field of island arc tholei-

tes (IAT), and most Wadi Kareim samples plot between fields forAB basalts and MORB.

Ti–Zr diagram (Fig. 7E; Pearce and Cann, 1973): This diagram

hows that most samples from Wadi El Dabbah plot in the arc fieldith some overlap into the IAT field, whereas most Wadi Kareim

amples plot in or near the MORB field, with some overlap into theAT field.

ig. 4. Photomicrographs (plane-polarized light) of metavolcanic rock samples from WB) Gabbro K11-15; (C) Pyroclastic K11-2; (D) Diabase 12-3; (E) Basaltic-andesite D24; aas = basalt; CPX = clinopyroxene; and Plg = plagioclase.

search 171 (2009) 1–22 9

Cr–Ce/Sr diagram (Fig. 7F; Pearce, 1982) is effective for rocks thatare not too altered because VAB are enriched in Sr relative to Ceand thus have low Ce/Sr and also have low Cr contents. Nearly allWadi El Dabbah samples plot in the VAB field, whereas most WadiKareim samples plot as VAB, with a significant minority plotting in

the MORB + WPB field.

Th–Hf/3–Ta ternary diagram (Fig. 7G; Wood, 1980). All samplesfrom Wadi El Dabbah and Wadi Kareim plot in the field of arcbasalts. The data for Wadi Kareim metavolcanics and Wadi El Dab-

adi Kareim and Wadi El Dabbah, Central Eastern Desert, Egypt: (A) Basalt K12-2;nd (F) Andesite D20. Qz = quartz, Mag = magnetite; Act = actinolite; Chl = chlorite;

10 K.A. Ali et al. / Precambrian Research 171 (2009) 1–22

Fig. 5. Geochemical characteristics of major elements and compatible trace elements for metavolcanic rocks from Wadi El Dabbah and Wadi Kareim: (A) plot of Mg# vs. Ni(ppm); (B) plot of Mg# vs. Cr (ppm); (C) total alkalis-silica classification diagram of Le Maitre et al. (1989); (D) histogram of weight% SiO2 normalized to 100% anhydrous; (E)FeO/MgO vs. SiO2 classification diagram of Miyashiro (1974); (F) K2O vs. SiO2 classification diagram of Le Maitre et al. (1989).

Table 5Tectonic setting summary for Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt.

Discrimination diagram Wadi Kareim Wadi El Dabbah Reference

Zr/Y–Zr (Fig. 7A) Within-plate/MORB/VAB VAB/MORB Pearce and Norry (1979)Ti/Y–Nb/Y (Fig. 7B) VAB/MORB MORB/VAB Pearce (1982)Cr–Y (Fig. 7C) MORB/VAB/Within-plate VAB/MORB/Within-plate Pearce et al. (1981)V–Ti/1000 (Fig. 7D) BABB/MORB IAT/BAB/MORB Shervais (1982)Ti–Zr (Fig. 7E) MORB/IAT IAT/MORB Pearce and Cann (1973)Cr–Ce/Sr (Fig. 7F) VAB/MORB/Within-plate VAB Pearce (1982)Th–Hf/3–Ta (Fig. 7G) VAB VAB Wood (1980)Th/Yb–Ta/Yb (Fig. 7H) VAB Crustal contamination Pearce et al. (1981)

VAB: Volcanic arc basalt; MORB: Mid-ocean ridge basalt; IAT: Island arc tholeiite; BAB: Back-arc basin.

K.A. Ali et al. / Precambrian Research 171 (2009) 1–22 11

Fig. 6. Rare earth element (REE) and trace element diagrams for the analyzed metavolcanic samples from Wadi El Dabbah and Wadi Kareim, Central Eastern Desert of Egypt.(A) Chondrite-normalized diagram for rare earth element for all metavolcanics samples analyzed from Wadi EL Dabbah. (B) Chondrite-normalized diagram for rare earthelement for all metavolcanics samples analyzed from Wadi Kareim. (C) Chondrite-normalized diagram for rare earth element (REE) for the average values of the metavolcanics alized( s analyf and Wm

bc

fmfmbmdtatfDm

amples analyzed from Wadi El Dabbah and Wadi Kareim. (D) Primitive mantle normE) Primitive mantle normalized trace element diagram for all metavolcanic sampleor the average values of the metavolcanic samples analyzed from Wadi El Dabbah

antle reported by Sun and McDonough (1989).)

ah metavolcanics are separated, with Wadi Kareim samples lyinglosest to the MORB field.

Th/Yb–Ta/Yb diagram (Fig. 7H, Pearce et al., 1981) is use-ul because, in the absence of residual garnet (such as these

etavolcanics), Th/Yb changes little during melt generation andractionation and is typically elevated in subduction-related mag-

as. This is because Th is added to the mantle source of arc meltsy the subduction process whereas Ta is not; Yb is used to nor-alize for fractionation between melts. MORB and WPB typically

efine a diagonal field, “the mantle array”, and data that lie abovehis reflects generation in a convergent margin setting (diagonal

rray above and parallel to mantle array) or reflects contamina-ion of mantle-derived magmas by continental crust (vertical arrayrom mantle array). All samples from Wadi Kareim and Wadi ELabbah plot above the “mantle array”, suggesting they are arcagmas or are contaminated by continental crust. Wadi Kareim

trace element diagram for all metavolcanic samples analyzed from Wadi El Dabbah.zed from Wadi Kareim. (F) Primitive mantle normalized diagram for trace elementadi Kareim. (All elements are normalized to the values of chondrite and primitive

samples follow a trend that parallels the mantle array, stronglysuggesting an arc environment, whereas Wadi El Dabbah samplesfollow a vertical trend, strongly suggestive of crustal contamina-tion (Pearce, 2003). This possibility is explored further in the nextsections.

7. U–Pb zircon geochronology

We selected nine samples of mafic and intermediate volcanicsfrom Wadi Kareim and Wadi El Dabbah for U–Pb zircon age determi-

nation. Analytical data acquired by SHRIMP-RG for all samples arelisted in Table B1 (supplementary materials). Individual zircon anal-yses are plotted with two-sigma error ellipses on Tera–Wasserburgconcordia diagrams (Tera and Wasserburg, 1972; Figs. 9–11). Therange in ages reported below are 206Pb/238U for <1000 Ma and

1 ian Re

2

rl2

FK(eVb

2 K.A. Ali et al. / Precambr

07Pb/206Pb for >1000 Ma. This procedure is followed because lowelative levels of 207Pb as a consequent of the relatively short half-ife of its 235U parent make 207Pb/206Pb ages less precise than06Pb/238U ages for dating young zircons (Black et al., 2003). In con-

ig. 7. Trace element discrimination diagrams for mafic lavas from different tectonic setareim: (A) Zr/Y–Zr diagram of Pearce and Norry (1979); (B) Ti/Y–Nb/Y diagram of Pearce

1982); (E) Ti–Zr diagram of Pearce and Cann (1973); (F) Cr–Ce/Sr diagram of Pearce (1982t al. (1981). Results are summarized in Table 5 and discussed in text. WPB = within-plateAB = volcanic-arc basalt, BAB = back-arc basalt, IAT = island-arc tholeiitic, OIB = ocean islaasalt.

search 171 (2009) 1–22

trast, lead loss from old zircons tends to shift 238U/206Pb withoutaffecting 207Pb/206Pb (Compston et al., 1984), making 207Pb/206Pba better estimate for dating old zircons. A summary of ages usingthis procedure is presented in Table 3.

tings showing data for samples with SiO2 < 61 wt% from Wadi El Dabbah and Wadi(1982); (C) Cr–Y diagram of Pearce et al. (1981); (D) V–Ti/1000 diagram of Shervais); (G) Th–Hf/3–Ta diagram of Wood (1980); and (H) Th/Yb–Ta/Yb diagram of Pearcebasalt, MORB = mid-ocean ridge basalt, E-MORB = enriched mid-ocean ridge basalt,nd basalt, Thol = tholeiitic, Alk = alkali, Tran = transitional, and C-A bas = calc-alkali

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K.A. Ali et al. / Precambr

.1. Cathodoluminescence imaging of zircons

Magmatic zircons (Fig. 8) typically show well-developed growthoning, sometimes with xenocrystic cores (Corfu et al., 2003). Theoning reflects variations of Si, Zr, Hf, P, Y, REE, U and Th (Hancharnd Rudnick, 1995). Zoning reflects the nature of the crystal–liquidnterface during magmatic zircon growth, reflecting the degree oflemental saturation and the rate of diffusion of the melt (Corfu etl., 2003). Xenocrystic cores are mantled by younger layers of mag-atic zircon, or can be unmantled, subrounded, or rarely euhedral.

ecause there are different U contents between rim and core, dif-erential metamictization expands the high U part and transformshe original colorless zircon into black or pink, making it hard toecognize xenocrystic cores. When a xenocrystic zircon core withow U contents is surrounded by a high U rim, the core appearsighter than the rim; however, in the opposite case, the zircon rim

ill be fractured by core expansion (Corfu et al., 2003). Zircons stud-ed here (Fig. 8) show the features of magmatic zircon listed above,ut some are rounded as expected for detrital zircon or/and may beetamorphically rounded (Fig. 8G and H). However, zircons from

wo tuffaceous metasediment samples from Wadi El Dabbah showrange of morphologies: euhedral, subhedral and rounded, withell developed growth zoning, even for zircons showing older ages

Fig. 8K to T).

.2. Wadi Kareim metavolcanics

Sample K11-9 is a coarse-grained metagabbro (51% SiO2). Onenalysis was conducted for each of 12 zircon grains, which rangen age from 725 to 2710 Ma (Table B1, Fig. 9A). Seven dataoints are pre-Neoproterozoic, which are interpreted to reflectircon inheritance from older crustal material or detrital grainsn sediment. Three concordant youngest grains cluster togetherielded weighted mean 206Pb/238U age of 730 ± 22 Ma (95% conf.,SWD = 0.14; Fig. 9A) which may be represent the eruption age.Sample K11-6 is a metabasalt (49% SiO2), from which 11 zircons

ere analyzed. These range in age from 591 to 2060 Ma (Table B1,ig. 9B). Four data points are pre-Neoproterozoic, which are inter-reted to reflect zircon inheritance from older crustal materialsr detrital grains in sediment. Two concordant grains with ages31 ± 4 Ma and 741 ± 3 Ma probably represents the eruption age.hree young zircons with ages ∼600 Ma display various degreesf roundness and are not zoned (Fig. 8U). We do not understandhe young ages but these probably reflect recent Pb loss or per-aps fluid-related processes. One zircon grain is discordant and twohow high common Pb contents (Table B1, Fig. 9B).

Sample K11-8 is a metabasalt (49% SiO2). One analysis wasonducted for each of 11 zircons, which range in age from21 to 2730 Ma (Table B1, Fig. 9C). Four data points are pre-eoproterozoic, which are interpreted to reflect zircon inheritance

rom older crustal materials or detrital grains in sediment. Theemaining analyses range in age between 621 and 805 Ma (Table B1,ig. 9C) and do not yield a meaningful age. Again the zircons yield-ng young ages ∼600 Ma display various degrees of roundness andre not zoned but contain significant common Pb (Table B1, Fig. 8Vnd W). We do not understand the young ages but these probablyeflect recent Pb loss or perhaps fluid-related processes.

Sample K4G is a metadiabase. One analysis was conducted forach of 16 zircons, resulting in ages that range from 518 to 2667 MaTable B1, Fig. 10A). Five data points are pre-Neoproterozoic, whichre interpreted to reflect zircon inheritance from older crustal

aterials or detrital grains in sediment. One cluster of four concor-

ant grains yielded a weighted mean 206Pb/238U age of 743 ± 45 Ma95% conf., MSWD = 0.019; Fig. 10A); this may represent the erup-ion age. One concordant grain yielded an age of 679 ± 7 Ma,owever this grain is rounded and does not show zoning. Two young

search 171 (2009) 1–22 13

grains with ages ∼550 Ma display various degrees of roundness anddo not reveal zoning overgrowths but show significant common Pb(Table B1, Fig. 8X). We do not understand the young ages but thesemay reflect recent Pb loss or perhaps fluid-related processes.

Sample K4K is a felsic metatuff. Eight analyses were conductedon six zircons which range in age from 653 to 818 Ma (Table B1,Fig. 10B). Two zircon grains were analyzed twice, the first yielded653 and 739 Ma and the second 746 and 818 Ma for rim and core,respectively. Three analyses were omitted because of discordanceor high U content; the remaining five data points yielded a weightedmean 206Pb/238U age of 769 ± 29 Ma (95% conf., MSWD = 0.33;Fig. 10B). We interpret this age to represent the time of volcaniceruption. This is the only sample which did not contain xenocrysticzircons.

7.3. Wadi El Dabbah metavolcanics

Sample D7 is an amygdaloidal meta-andesite. Six analyseswere conducted for six zircon grains, which range in age from734 to 2488 Ma (Table B1, Fig. 11A). Two data points are pre-Neoproterozoic which are interpreted to reflect zircon inheritancefrom older crustal materials or detrital sediments. The remaininggrains vary in age between 734 and 798 Ma and two grains are dis-cordant. The youngest concordant grain yielded 206Pb/238U age of734 ± 7 Ma which may represents the time of eruption.

Sample D25 is a metadiabase (50% SiO2). One analysis wasconducted for each of 14 zircon grains, yielding ages that rangefrom 477 to 2572 Ma (Table B1, Fig. 11B). Six data points are pre-Neoproterozoic, which are interpreted to reflect zircon inheritancefrom older crustal materials or detrital sediments. The remaininganalyses range in age between 477 and 967 Ma (Table B1, Fig. 11B).We do not understand the young ages but these may reflect recentPb loss or perhaps fluid-related processes because zircon grainsdisplay various degrees of roundness (Fig. 8Y) and do not revealzoning overgrowths. Even excluding analyses that are discordantor yield pre-Neoproterozoic ages, the remaining analyses do notyield a meaningful age.

7.4. Wadi El Dabbah metasediments

Sample D1 is a green metamudstone. One analysis was con-ducted for each of 15 detrital zircons, 14 of which range in agefrom 1198 to 2765 Ma, with the remaining grain having an age of678 Ma (Table B1, Fig. 11C). The 678 Ma age is not reliable becausethe O2− ion beam was focused on an inclusion (Fig. 8N). Some of thepre-Neoproterozoic zircons are euhedral and show well-developedgrowth zoning expected for magmatic zircon (Fig. 8P) and othersare rounded, as expected for detrital and inherited grains.

Sample D13 is a metamudstone intercalated with the BIF in WadiEl Dabbah it has by far the highest silica content (75%) of Wadi ElDabbah samples. One analysis was conducted for each of 14 zircons,which range in age from 614 to 2627 Ma (Fig. 11D, Table B1). Onlytwo zircons showed pre-Neoproterozoic ages (2267 and 2627 Ma)and the remaining 12 data points range in age from 614 to 855 Ma.We do not understand the young ages (∼600 Ma), however youngzircons are rounded, as expected for detrital grains or perhaps meta-morphic overgrowths (Fig. 8Z, ZA). These young ages may reflectrecent Pb loss or may be fluid-related processes. Other Neoprotero-zoic zircons are euhedral and show well developed growth zoning(Fig. 8T), but pre-Neoproterozoic zircons are rounded, as expectedfor detrital and inherited grains.

In summary, the effort to determine the ages of the metavolcanicrocks in Wadi Kareim and Wadi El Dabbah did not yield defini-tive results, but suggests that these sequences formed ∼750 Maago. This age is geologically consistent with an age constraint of<754 ± 15 Ma for the overlying Atud diamictite (Ali et al., in press).

1 ian Re

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efn

FtAd

4 K.A. Ali et al. / Precambr

his is considerably older than the Rb–Sr ages of ∼630 Ma obtainedor the Younger Metavolcanics by Stern and Hedge (1985), as pre-iously discussed; the significance of the latter age is clearly inoubt.

These conclusions are especially noteworthy, because Andresent al. (in press) report a U–Pb single zircon TIMS age of 748 ± 3 Maor two zircons from a felsic lava (previously thought to be Dokhan)ear Fawakhir (Fig. 1B), well to the west of Wadis Kareim and

ig. 8. Cathodoluminescence images of zircon grains for the metavolcanic and metasedimerozoic and Archean igneous zircon grains from metavolcanic samples K11-6, K4-K andrchean from tuffaceous samples D1 and D13; and (U to ZA) zircon grains from metavolcanegrees of roundness probably reflect recent Pb loss or may suggest fluid-related process

search 171 (2009) 1–22

El Dabbah. A third zircon grain was discordant, giving a pre-Neoproterozoic 207Pb/206Pb age of 1440 ± 12 Ma and confirmingthe presence of pre-Neoproterozoic zircons in ∼750 Ma old lavas.Andresen et al. (in press) also reported a zircon age of 736.5 ± 1.2 Ma

for a gabbro sample from the Fawakhir ophiolite, an age which isanalytically indistinguishable from the Pb–Pb zircon evaporationage of 746 ± 19 Ma for a plagiogranite from the Ghadir ophiolite(Fig. 1B) near Marsa Alam (Kröner et al., 1992). These results, con-

ent samples analyzed during this study: (A to J) typical Neoproterozoic, Paleopro-D25; (K to T) zircon grains of metasediment Neoproterozoic, Paleoproterozoic andic samples K11-6, K11-8, K4G, D25, and metasediment sample D13 showing various

es. Location of ion-microprobe area is shown by white circles, scale is 100 �m.

K.A. Ali et al. / Precambrian Research 171 (2009) 1–22 15

F A) same rmatio

sfeMtc

gDtdshtc

8

cfT+fzce

ig. 9. U–Pb concordia diagrams for SHRIMP–RG data from metavolcanic samples: (llipses are 2�; weighted average age errors quoted at 95% confidence. Sample info

idered together, indicate that (1) an important episode of crustormation occurred ∼750 Ma in what is today the Central East-rn Desert of Egypt; (2) that CED ophiolites (and associated “Olderetavolcanics”) are in fact slightly younger than at least some of

he Younger Metavolcanics; and (3) pre-Neoproterozoic zircons areommon in mid-Cryogenian lavas, especially basalts.

We note that these results are broadly consistent witheochronologic results for deformation in the region. For the Sibaiome, to the SE of Wadi El Dabbah, Fowler et al. (2007) interpreted

he geochronologic results of Bregar et al. (2002) to indicate thateformation had commenced by ∼700 Ma ago, requiring deformedupracrustal components such as the El Dabbah metavolcanics toave been in place by that time. Deformation around the Mea-iq dome to the NW began much later; Andresen et al. (in press)onsider that this deformation occurred sometime after 630 Ma.

. Nd isotopic compositions

Samarium and neodymium concentrations and Nd isotopicompositions for 19 whole-rock samples of metavolcanic rocks (9rom Wadi Kareim and 10 from Wadi El Dabbah) are presented inable 4. Epsilon Nd values at 750 Ma ranges from +6.7 to +8.9 (mean

7.5) for Wadi Kareim samples and from +5.1 to +7.5 (mean +6.2)or Wadi El Dabbah samples. These results are plotted against theircon age (750 Ma, Fig. 12A). These isotopic compositions are verylose to the depleted mantle model of DePaolo (1981) and Goldsteint al. (1984), which predict �Nd of +6.5 and +8.5, respectively,

ple K11-9 (gabbro); (B) sample K11-6 (basalt); and (C) sample K11-8 (basalt). Errorn is given in Table B2 and analytical data in Table B1 (Supplementary materials).

at 750 Ma. It is noteworthy that the Wadi Kareim metavolcanics,which are geochemically enriched relative to Wadi El Dabbah sam-ples, have higher epsilon-Nd values. The Wadi Kareim samplesgenerally plot above the DePaolo (1981) mantle evolution curve,whereas the Wadi El Dabbah samples generally plot below thiscurve (Fig. 12A). Zimmer et al. (1995) reported �Nd values between+6.5 and +8.8 for the ∼750 Ma igneous rocks of the Gabal Gerf ophi-olite complex. This may suggest that �Nd values of less +6.5 mayreflect a minor amount of crustal contamination. Also plotted onFig. 12A are the �Nd (t) data for some ∼600 Ma A-type granitoidrocks from Egypt (Sultan et al., 1990; Moussa et al., 2008) whichplot below the curve (Fig. 12A).

The age of crustal extraction of the metavolcanic rocks at WadiKareim and Wadi El Dabbah can also be estimated with the Nd meancrustal residence age (tDM) which approximates the time of Sm/Ndfractionation associated with partial melting of depleted mantle toform juvenile ANS crust. The DePaolo (1981) model is used here,because it is more appropriate than the model of Goldstein et al.(1984) for calculating crustal extraction ages generated in arc-liketectonic settings (Dickin, 2005). The 147Sm/144Nd ratio for WadiKareim samples ranges from 0.146 to 0.207 (mean 0.181) and from0.142 to 0.185 (mean 0.164) for Wadi El Dabbah samples. Samples

with high 147Sm/144Nd produce unreliable model ages, so tDM agesshould not be calculated. A review of Nd isotopic compositions inthe ANS by Stern (2002) used 147Sm/144Nd > 0.165 to filter samplesthat are likely to give reasonable Nd model ages from those that arenot.

16 K.A. Ali et al. / Precambrian Research 171 (2009) 1–22

F (A) saa 2 and

Dms1

ata(aoccs

dclmf

9. Discussion

ig. 10. U–Pb concordia diagrams for SHRIMP–RG data from metavolcanic samples:verage age errors quoted at 95% confidence. Sample information is given in Table B

To independently assess the most suitable filter for Wadi Elabbah and Wadi Kareim samples, we plotted tDM ages for theetavolcanic samples against 147Sm/144Nd (Fig. 12B). This clearly

hows that there is much more scatter in tDM ages for samples with47Sm/144Nd > 0.165 than those with 147Sm/144Nd < 0.165, and wedopt this filter here. Nd model ages for seven samples that passhis filter include the metapyroclastic sample from Wadi Kareimnd six mafic and intermediate samples from Wadi El DabbahTable 4, Fig. 12B). These yield tDM ages from 0.64 to 0.77 Ga, aver-ging 0.72 Ga, very close to the eruption age of ∼750 Ma. Similarityf crystallization and Nd model ages is strong evidence of juvenilerust, and there is no hint of the significant participation of olderrust in the Nd isotopic composition of any of the 19 metavolcanicamples analyzed for Nd isotopes.

Fig. 12C explores possible contamination of primitive, mantle-

erived magmas by older continental crust. We plotted the Ndoncentration against initial 143Nd/144Nd for the 19 samples ana-yzed, along with isotopic compositions expected for depleted

antle according to DePaolo (1981) and Goldstein et al. (1984) andor the Paleoproterozoic Khida terrane. Mixtures between depleted

mple K4G (diabase); and (B) sample K4K (felsic tuff). Error ellipses are 2�; weightedanalytical data in Table B1 (Supplementary materials).

mantle melts and older (Khida-like) crust should plot along themixing lines in Fig. 12C but they do not. These relations rule outsimple two component mixing. We conclude that Nd model agesfor Kareim and El Dabbah samples unequivocally show that thesecrustal additions are overwhelmingly juvenile. However, this mustbe reconciled with zircon data indicating the presence of pre-Neoproterozoic material and the geochemical analysis discussed inthe next paragraphs suggesting involvement of continental crust.Kröner et al. (2007) argued that the term “juvenile” cannot bedefined precisely on the basis of Nd isotopic system alone after theyfound that some reworked rocks from Kazakhstan and Mongoliacontain old crustal material with positive �Nd values.

Below we further explore two important questions: (1) Whatwas the tectonic setting that produced Kareim and El Dabbah lavas?and (2) What is the significance of abundant pre-Neoproterozoiczircons in mid-Cryogenian basalts and andesites?

K.A. Ali et al. / Precambrian Research 171 (2009) 1–22 17

F s: (A)m ; weiT

9

ERatEfobiwtpj

9

cWtTazi

ig. 11. U–Pb concordia diagrams for SHRIMP–RG data from metavolcanic sampleetasedimet); and (D) sample D13 (tuffaceous metasediment). Error ellipses are 2�

able B2 and analytical data in Table B1 (Supplementary materials).

.1. Tectonic setting

Using trace element data to assign tectonic setting to the Wadil Dabbah and Wadi Kareim yielded mixed results (Fig. 7, Table 5).esults for Wadi El Dabbah mostly indicate an arc setting withffinities to MORB. The Th/Ta diagram suggests crustal contamina-ion, consistent with abundant pre-Neoproterozoic zircons in Wadil Dabbah. Wadi Kareim lavas, in contrast, show mixed affinitiesor arc, MORB and BAB tectonic environments, with no evidencef crustal contamination seen on a Th/Yb–Ta/Yb diagram. On thisasis, we tentatively conclude that Wadi El Dabbah lavas formed

n a ∼750 Ma arc that experienced some crustal contamination,hereas Wadi Kareim lavas formed in a back-arc basin. We also note

hat these results provide no support for models calling for oceaniclateaus or plume-related sequences to be important in generating

uvenile crust of this part of the ANS (Stein and Goldstein, 1996).

.2. Source of pre-Neoproterozoic xenocrystic zircons

The present study indicates that pre-Neoproterzoic zircons areommon in Neoproterozoic (∼750 Ma) metavolcanic rocks fromadi Kareim and Wadi El Dabbah. Fig. 13A–C summarizes the dis-

ribution of zircon ages from Wadi Kareim and Wadi El Dabbah.hese histograms show that ∼750 to 800 Ma old zircons dominatet both locations but with a very significant proportion of olderircons: 44 of 106 (42%) zircons analyzed have ages >900 Ma. Thiss not the first time that pre-Neoproterozoic xenocrystic zircons

sample D7 (andesite); (B) sample D25 (diabase); and (C) sample D1 (tuffaceousghted average age errors quoted at 95% confidence. Sample information is given in

from the ANS are documented, first shown with conventional TIMSzircon dating of an Egyptian granite by Sultan et al. (1990). Increas-ing use of the high-resolution ion-microprobe for dating individualzircons in the last few years by different investigators has increas-ingly recognized pre-Neoproterozoic zircons in the ANS igneousrocks that are otherwise (because of mantle-like initial isotopiccompositions) thought to be juvenile; to date this work has beencarried out in Saudi Arabia (Kennedy et al., 2004, 2005; Pallisteret al., 1988; Hargrove et al., 2006a). Our results indicate that pre-Neoproterozoic zircons are also abundant in ∼750 Ma igneous rocksin Egypt.

Pre-Neoproterozoic zircons may be not found in all ANS igneousrocks; felsic plutonic rocks tend to lack these xenocrystic zircons(Moussa et al., 2008; Andresen et al., in press), whereas mafic lavasmay carry them in abundance. This presumably reflects differentmagma compositions and different amounts of time that mag-mas and xenocrystic zircons have to interact. Felsic plutonic rockscool slowly, allowing xenocrystic zircons to dissolve in the magma,whereas unfractionated mafic magmas rise rapidly from the siteof melt generation in the mantle through the crust. Magmatic zir-cons are relatively uncommon in such lavas, and xenocrystic zirconshave little time to dissolve in the melt. As a result, the proportion

of xenocrystic zircon is expected to be greatest in mafic lavas andlowest in felsic plutons.

There must be a source of pre-Neoproterozoic zircons beneaththe ANS but it is less clear what this might be. Pre-Neoproterozoicxenocrystic zircons could have originated in one of three ways: (1)

18 K.A. Ali et al. / Precambrian Re

Fig. 12. (A) Plot of epsilon Nd versus crystallization age for Neoproterozoic volcanicrocks analyzed during this study and granitic rocks from previous studies. The refer-ence line for chondritic uniform reservoir (CHUR) and the depleted mantle evolutioncurves of DM DePaolo (1981) and DM Goldstein et al. (1984). Data points for granitesamples from Sultan et al. (1990) and Moussa et al. (2008). (B) Plot of 147Sm/144Nd vs.Nd model age (model of DePaolo, 1981). Samples with 147Sm/144Nd > 0.165 may notyield reliable model ages (Stern, 2002) and are excluded from model age calculations.(C) Plot of initial 143Nd/144Nd vs. Nd concentration for Wadi Wadi Kareim and WadiEl Dabbah volcanic samples at 750 Ma, in which pre-Neoproterozoic inheritanceoccurs. The trajectories are shown for ideal mixtures between the mean compositionfor samples from Khida terrane (Saudi Arabia) and the mean for DePaolo (1981) andGoldstein et al. (1984). This fails to detect significant mixing of ancient continentalcrust with mantle-derived magmas.

search 171 (2009) 1–22

from fragments of in situ continental crust beneath the ANS; (2)from sediments eroded from older crust and transported into theANS oceanic basin; or (3) by processing of sediment subduction, andcontinental material removed from the upper plate by processes ofsubduction erosion.

The suggestion that the magmas yielding these volcanic rocksmixed with pre-Neoproterozoic continental crust comes fromprevious work. Abdel-Monem and Hurley (1979) found that multi-grain zircon fractions from a psammitic gneiss in the Sikait areawere strongly discordant but gave an upper concordia interceptat ∼1.8 Ga. Sultan et al. (1990) reported the involvement of pre-Neoproterozoic continental crust in Aswan and Nakhil granites.Hassanen and Harraz (1996) reported strongly negative initial�Nd values and Sm–Nd model ages of 0.96–1.70 Ga for granitesfrom Homerit Mukbid and Homer Akarem, and interpreted thisas evidence for the involvement of pre-Neoproterozoic continen-tal crust. Hassan and Hashad (1990) argued that Mesoproterozoicto Archean crust may exist beneath the Eastern Desert, but there isa little direct geochronological evidence for this suggestion. Theconsistently juvenile nature of lavas as indicated by high posi-tive initial �Nd and Nd model ages that are indistinguishable fromcrystallization ages does not allow significant interaction of WadiEl Dabbah and Wadi Kareim magmas with pre-Neoproterozoiccrust.

Hargrove et al. (2006b) studied igneous rocks along the Bir Umqsuture of Saudi Arabia and argued that interaction between Neopro-terozoic juvenile magmas and pre-Neoproterozoic crust must havebeen very limited. A similar combination of abundant xenocrysticzircons and juvenile isotopic characteristics was observed for theCentral Asian Orogenic Belt in Mongolia by Kröner et al. (2007),who inferred “substantial reworking of old crust despite seeminglyprimitive Nd isotopic characteristics” (p.182).

Another possibility is that xenocrystic zircons were derivedfrom the interaction of Neoproterozoic magmas with terrigenoussediments. Pre-Neoproterozoic zircons are common in the Atuddiamictite (Fig. 13D) which may have been glacially eroded frompre-Neoproterozoic crust to the west and transported by ice-raftinginto what is now the Central Eastern Desert (Ali et al., in press).Such zircons also appear to be common in fine grained metased-iments. The observation that two metasediment samples (D1 andD13) contain pre-Neoproterozoic zircons indicates that ancient zir-cons are not limited to the Atud diamictite but may be common inCED metasediments. This supports the observations of Wüst (1989),who studied detrital zircons from Neoproterozoic sediments in theCED and SED; Wadi Miyah metasediments yielded ages of 2410 Ma,while those along Wadi Allaqi yielded ages of 1460, 2400, and2450 Ma. It is beyond the scope of this study to address whetherolder zircons were transported into the basin or are reworked fromvolcanic rocks containing xenocrytic zircons; further studies areneeded to address this issue. But it is clear that significant magmaticdigestion of these sediments could provide the pre-Neoproterozoiczircons seen in Kareim and El Dabbah metavolcanics.

A comparison of the distribution of pre-Neoproterozoic zir-cons in the metavolcanics studied shows that this is remarkablysimilar to that of the Atud diamictite studied by Ali et al. (inpress) (Fig. 13). Such an interpretation is also consistent withthe rounded nature of many pre-Neoproterozoic zircons isolatedfrom the metavolcanics. However, many of the pre-Neoproterozoiczircon in the metavolcanics are euhedral or subhedral and arecharacterized by well-developed zoning, suggesting that thesegrains were not transported for a great distance from their

source.

A final possibility is that ancient zircons originated by subduc-tion of terrigeneous sediments and continental material removedfrom the upper plate by processes of subduction erosion (von Hueneand Scholl, 1991) whereas zircon survived processing through the

K.A. Ali et al. / Precambrian Research 171 (2009) 1–22 19

F udy: (f D) agea

sm

tTccTBsNmimscf

wrpMgp

ig. 13. Histograms of single-grain SHRIMP-RG zircon ages analyzed during this strom this study, note major peak at ∼750 Ma and secondary peaks >∼ 1.0 Ga; and (ges <1000 Ma are 238U/206Pb ages and those >1000 Ma are 207Pb/206Pb ages.

ubduction zone to become incorporated in subduction-relatedagmas.Fig. 14 (Pearce, 2008) explores the general possibilities of con-

amination using Th-Nb for the crustal component. The ratioh/Nb is very sensitive to the addition of continental crust, espe-ially for melts derived from depleted mantle (Pearce, 2008);rustal contamination is represented by displacement from theh/Yb vs. Nb/Yb mantle array, defined by MORB and Ocean Islandasalt. Nb/Yb is taken to indicate the relative depletion of apinel-peridotite mantle source (high Nb/Yb = less depleted; lowb/Yb = more depleted), and the variations in Th/Yb with Nb/Ybonitors interactions of mantle processes with a variety of crustal

nputs. All data for Wadi Kareim and Wadi El Dabbah plot above theantle array (Fig. 14A), indicating generation of these melts in a

ubduction zone environment (Fig. 14B) or addition of continentalrust, either due to bulk mixing (Fig. 14C) or coupled assimilation-ractional crystallization (AFC; Fig. 14D).

Subduction-related crustal input on these diagrams occurshen Th from subducted sediment and subduction erosion mate-

ial is released into the mantle source of the arc melts. This yieldsrimitive magmas with Th/Yb that lies above but parallel to theOR-OIB array; and the displacement above the mantle array is

reater for more depleted mantle sources (Pearce, 2008). Anotherossibility for contamination is direct interaction of mantle-derived

A) Wadi Kareim, (B) Wadi El Dabbah, (C) Combined distribution of all zircon agesdistributions for zircons from the Atud diamictite (Ali et al., in press). Discordant

magmas with continental crust (Fig. 14C and D); this will alsoincrease Th/Yb, shifting sample compositions from the MORB-OIBarray (Pearce, 2008). In this case, the samples may define a steepertrajectory from the mantle array towards the composition of con-tinental crust (LC–MC–UC triangle in Fig. 14C). For this situationas well, the more depleted magma is displaced more from theMORB-OIB array because its mantle source has lower Nb and Thas well as lower Nb/Yb and Th/Yb. The data for metavolcanic rocksfrom Wadi Kareim and Wadi El Dabbah seem to follow two dif-ferent trends. Both are clearly derived from depleted, low Nb/Ybmantle, although Wadi Kareim samples are derived from a less-depleted mantle source than are Wadi El Dabbah samples. WadiKareim samples mostly lie along a trend that is sub-parallel to themantle array, most consistent with control by subduction-relatedTh enrichment. In contrast, Wadi El Dabbah data define a steepertrend that is most consistent with crustal mixing. Contaminationor AFC interactions of mantle-derived magmas by sediments isnot explicitly considered in these diagrams, but it should behavesimilar to processes shown on Fig. 14C and D. In summary, the

Nd isotopic compositions of lavas indicating little interaction withcrust plus the observed rounded xenocrysts and the common occur-rence of pre-Neoproterozoic zircons with showing similar ages tothat of the Atud diamictite leads us to tentatively conclude thatinteraction of mafic magmas with sediments is responsible for the

20 K.A. Ali et al. / Precambrian Research 171 (2009) 1–22

Fig. 14. Th/Yb–Nb/Yb modeling diagrams for metavolcanic rocks from Wadi El Dabbah and Wadi Kareim (Pearce, 2008): (A, B) illustrates sediment input during subduction;and (C, D) illustrates magma interaction with crustal material during magma ascent through continental crust. Note that both Wadi Kareim and Wadi El Dabbah samples havecompositions above the MORB-OIB array; Wadi El Dabbah samples are derived from a more depleted (lower Nb/Yb) source and is more displaced from the MORB-OIB arraythan Wadi Kareim samples. Dashed lines in B indicates the % of subducted sediment added to mantle showing a range of depletions, demonstrating the shifting of the mantlecomposition from the MORB-OIB array. The trajectories in D explore coupled assimilation-fractional crystallization (AFC) processes for primitive magmas ranging extractedfrom depleted (low Nb/Yb) or enriched (high Nb/Yb) sources, which depend on the variables that include magma composition, crust composition, the relative proportional oft cent of = uppc , PM =

eh

1

(

(

(

(

he magma and the degree of fractional crystallization (F). Note that only a few peror the depleted mantle to shift from the MORB-OIB array (See text for details). UCrust from Rudnick and Fountain, 1995; Rudnick and Fountain (1995; Pearce, 2008)

vidence of crustal interactions in the metavolcanic rocks studiedere.

0. Conclusions

The following conclusions result from our study:

1) Abundant Mesoproterozoic and Archean zircons suggest thatpre-Neoproterozoic material, either in sediment, subductionerosion material or old continental crust, was involved in theformation of Arabian-Nubian Shield.

2) SHRIMP dating of zircons from two localities in the Central East-ern Desert indicate that the metavolcanics at Wadi Kareim andWadi El Dabbah were erupted ∼750 Ma.

3) Wadi El Dabbah and Wadi Kareim metavolcanic rocks were gen-erated by melting of depleted mantle as shown by epsilon Ndat 750 Ma of +5.1 to +9.0 and the fact that Nd model ages aresimilar to U–Pb zircon crystallization ages.

4) A significant episode of crust formation in the form of prim-

itive “arc-like” lavas and ophiolites can now be recognized inthe Central Eastern Desert of Egypt, lasting from 736 ± 1.2 Mato ∼760 Ma, which we informally term the “∼750 Ma crust-forming event”. This is probably also the age of related, highlydeformed and metamorphosed CED basement units, such as

f fractional crystallization coupled with assimilation of continental crust is neededer crust, MC = middle crust, LC = lower crust, P = felsic Phanerozoic and A = Archeanprimordial mantle.

serpentinites and metasediments. Plutonic rocks of this age arenot yet known from the region but are common in the Atuddiamictite (Ali et al., in press) and regions to the east (Hargroveet al., 2006a).

(5) Geochronologic results indicate that the subdivision ofCED metavolcanics into “older metavolcanics” and “youngermetavolcanics” is flawed. These rocks are similar expressionsof the ∼750 Ma crust-forming event, and the ophiolitic basaltspreviously considered as OMV may even be slightly youngerthan mafic-intermediate YMV lavas. A new stratigraphic subdi-vision of Eastern Desert metavolcanics is needed, pending theresults of new field and geochronologic studies.

(6) Geochemical data show that tholeiitic and calc-alkaline mag-mas with strong affinities to lavas from modern arcs (Wadi ElDabbah) and back-arc basins (Wadi Kareim) were importantaspects of the ∼750 Ma crust-forming event. We found no sup-port for models calling for oceanic plateaus or plume-relatedigneous sequences to play an important role in the ∼750 Macrust-forming event in the CED.

Acknowledgments

This paper is part of the first author’s Ph.D. research at theUniversity of Texas at Dallas and was supported by NSF grant EAR-

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K.A. Ali et al. / Precambr

509486, a Graduate Research Fellowship from NSF (0714104) andhe Japan Society for Promotion of Science. Special thanks go to Dr.ani Shalaby and The Nuclear Materials of Egypt (NMA) for theirelp, including providing a vehicle and other support in the field.e appreciate the assistance of Dr. Joe Wooden and Dr. W. R. Griffin

uring geochronlogical studies at SUMAC facility. We thank Shi-ane University for allowing the first author to use the lab facility

uring his fellowship in Japan. We thank Prof. Simon Wilde for hiselpful discussion to modify the geochronology data. Constructiveeviews of this manuscript by Prof. Alfred Kröner and an anonymouseviewer are gratefully acknowledged. This is UTD Geosciences con-ribution number # 1132.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.precamres.2009.03.002.

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