Enrichment from plume interaction in the generation of Neoproterozoic arc rocks in northern Eritrea: implications for crustal accretion in the southern Arabian–Nubian Shield

Chemical Geology 184 (2002) 167 – 184 www.elsevier.com/locate/chemgeo

Enrichment from plume interaction in the generation of Neoproterozoic arc rocks in northern Eritrea: implications for crustal accretion in the southern Arabian–Nubian Shield Mengist Teklay a,b,1, Alfred Kro¨ner a,*, Klaus Mezger b,2 a

Institut fu¨r Geowissenschaften, Universita¨t Mainz, 55099 Mainz, Germany b Max-Planck-Institut fu¨r Chemie, Postfach 3060, 55020 Mainz, Germany Received 19 October 1999; accepted 3 August 2001

Abstract The Neoproterozoic volcano – sedimentary – plutonic associations in Eritrea are part of the Arabian – Nubian Shield (ANS). In the Nakfa region, northern Eritrea, low-grade metavolcanic rocks consist of calc-alkaline crystal tuffs, lapilli tuffs and basic and felsic lava flows. These are intruded by pre/syn- to late-tectonic calc-alkaline intrusive rocks ranging in composition from gabbro to granite. They have element characteristics and MORB-normalized geochemical patterns similar to modern arc lavas. These similarities include enrichment in large ion lithophile elements and depletion in high-field strength elements. SHRIMP U – Pb analyses of zircons from a metarhyolite yielded a concordant age of 854 ± 3 Ma, suggesting that arc magmatism in Eritrea began at about 850 Ma. Initial eNd values range from 4.8 to 5.7 These values preclude significant contributions from a long-lived crustal source and indicate that the sources for these melts were juvenile. Furthermore, the remarkably restricted initial eNd values clearly suggest an intra-oceanic tectonic setting. These data support models that show the southern extension of the ANS in northern Eritrea as an oceanic island arc system. Compared to metavolcanic rocks of similar ages from southern Sudan, the metavolcanic rocks from Nakfa display lower initial eNd values at similar LILE/HFSE ratios. This is interpreted that the source of the Nakfa metavolcanic rocks was enriched prior to the onset of formation of subduction-related magmas. This enrichment process may be related to the presence of plume-related magmatism that affected the mantle source of the Nakfa metavolcanic rocks prior to subduction. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Arabian – Nubian Shield; Eritrea; Mantle source; Metavolcanic rocks; Neoproterozoic; Plume magmatism

1. Introduction The currently most popular model for the initial formation of continental crust involves the production

*

Corresponding author. Present address: Department of Earth Sciences, University of Asmara, P.O. Box 1020, Eritrea. 2 Present address: Institut fu¨r Geowissenschaften, Universita¨t Mu¨nster, Correustr. 24, Germany. 1

of new continental crust in island arcs. However, recent ideas for crustal growth and evolution have also emphasized the importance of accretion of oceanic plateaux and enrichment of the uppermost mantle by plume material throughout Earth’s history (Ben Avraham et al., 1981; Nur and Ben Avraham, 1982; Storey et al., 1991; Stein and Hofmann, 1994; Abbott and Mooney, 1995; Puchtel et al., 1997). During the Neoproterozoic, many major orogens have formed, including the Arabian – Nubian Shield (ANS) (Windley,

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 ( 0 1 ) 0 0 3 5 9 - X

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1995). Hence, understanding of the correct palaeoenvironment for the formation of ANS rocks is important for the understanding of crust formation in Neoproterozoic times. The ANS constitutes the northern sector of the East African Orogen (Stern, 1994) and is an important PanAfrican orogenic belt exposed in the Arabian Peninsula, Sinai, Eastern Desert of Egypt, the Red Sea Hills of Sudan, Eritrea and Ethiopia (Fig. 1). Isotopic evidence indicates that the core region of the ANS comprises juvenile crust produced from 870 to at least 690 Ma (Stern, 1994). This was followed by terminal continental collision, which resulted in the consolidation of East and West Gondwana. The formation of so much juvenile continental crust in the ANS has implications for the global continental growth rates in the Neoproterozoic (Reymer and Schubert, 1984, 1986). The oldest juvenile supracrustal sequences of the ANS are found in south-western Arabia (Kro¨ner et al., 1992), in Sudan (Reischmann, 1986; Reischmann et al., 1992) and in Eritrea (Teklay, 1997). Lateral crustal growth through arc accretion has been proposed as the main mechanism for the evolution of the Neoproterozoic ANS (Stoeser and Camp, 1985; Kro¨ner et al., 1987, 1991; Abdel-Rahman, 1995). However, the high crustal growth in the ANS compared to the growth rate throughout the Phanerozoic led Reymer and Schubert (1984, 1986) to suggest that island arc accretion (at a rate comparable to the present-day rate) alone could not account for the amount of crustal material produced in the ANS. This led to the suggestion that plume-related magmatism played an important role in the evolution of the shield (Reymer and Schubert, 1984, 1986). Stein and Hofmann (1994) related the formation of the juvenile crust in the ANS to a major upwelling event in the mantle. The oldest volcanic rocks in the ANS are oceanic tholeiitic basalts erupted from about 900 to 870 Ma (Bentor, 1985). These old volcanic rocks lack a ‘‘subduction signature’’, e.g. the Wadi Sadiyah tholeiites in Saudi Arabia (Reischmann et al., 1984); the Shemegui tholeiites in Eritrea (De Souaza Filho and Drury, 1998) were interpreted to have formed at volcanic passive margins (Stern, 1994) or in oceanic plateaux (Stein and Goldstein, 1996; Teklay, 1997). Stein and Goldstein (1996) suggested that a plumerelated mantle reservoir, which generated an oceanic plateau in the earliest phases of ANS magmatism, has

subsequently been transformed into continental crust and lithospheric mantle via subduction and calc-alkaline magmatism. Isotopic characteristics of post-orogenic late Proterozoic dykes and Phanerozoic alkali basalts from the northern ANS were interpreted to result from melting of this fossilized enriched lithospheric mantle (Stein and Hofmann, 1992; Stein et al., 1997; Kessel et al., 1998). Several efforts have been made to estimate growth rates over the approximately 300 Ma long tectonomagmatic history of the ANS (Reymer and Schubert, 1984, 1986). These estimates range from about 20% to nearly 80% of the global Phanerozoic crustal growth rate (Stern, 1994 and references therein), the range largely indicates the uncertainties about how much of the ANS crust is juvenile and the appropriate growth mechanism model followed by the respective authors. Hence, understanding the age and composition of the oldest juvenile arc supracrustal sequences in the ANS is important for the identification of processes by which continental crust was produced. We present results of U –Pb SHRIMP single zircon dating, geochemical and Nd isotopic systematics for Neoproterozoic metavolcanic rocks from northern Eritrea which provide important information for a better understanding of crustal growth and evolution in the ANS.

2. Geologic setting and petrography Based on satellite image interpretation, Drury and Berhe (1993) and Drury et al. (1994) divided the Neoproterozoic basement of Eritrea into three terranes separated by major shear zones (Fig. 2). These are, from west to east, the Barka, Hagar and Nakfa terranes. The Hagar terrane is separated from the Nakfa terrane by the Adobha Shear zone and is interpreted to represent a Neoproterozoic oceanic plateau (Teklay, 1997). The Nakfa area, which is part of the Nakfa terrane (Fig. 2), comprises supracrustal rocks intruded by pre/syn-tectonic granodioritic and granitic bodies and syn- to late-tectonic gabbroic, syeno-dioritic and granitic intrusions (Fig. 2). Metasedimentary units crop out in the central part and consist mainly of interlayers of basic schists (biotite, biotite-hornblende schists) and felsic schists (quartz –feldspar schists). The western part is predom-

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Fig. 1. Tectonic sketch map of the Arabian–Nubian Shield (modified after Abdelsalam and Stern, 1996). N shows approximate location of the Nakfa area.

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Fig. 2. Geological map of the Nakfa area. Inset map shows the three terranes in Eritrea after Drury et al. (1994), (BSZ = Barka Shear Zone, ASSZ = Adobha Strike – Slip Shear Zone).

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inantly metavolcanic, consisting of well preserved metamorphosed massive crystal tuffs, lappili tuffs and basic felsic lava flows. Felsic lava flows occur as interlayers, up to 5 m thick, usually forming prominent ridges. These are typically porphyritic with partially to totally sericitized and/or epidotized euhedral laths of feldspar and quartz phenocrysts with a groundmass consisting of feldspar, quartz, white micas and epidote. Accessory minerals are chlorite, carbonates, apatite, titanite, zircon, biotite and opaque minerals. Basic metavolcanic rocks are both aphanitic and porphyritic with phenocrysts of plagioclase and pseudomorphs of basic minerals. Basic metavolcanic rocks are rare in the north but, together with mediumto coarse-grained basic hypabyssals and intrusives, they are the dominant rock type in the south. The metabasalts are composed of partially to wholly epidotized and/or sericitized plagioclase, epidote, chlorite, and actinolite. The metavolcanic units have the characteristic of subareal deposits: poorly sorted, angular clasts, high proportion of fragmental rocks and absence of bedding, graded bedding, stratification, pillowed lavas and interbedded sedimentary rocks. The typical mineral assemblage in the metabasites is actinolite –chlorite – epidote –albite and indicates that the Nakfa supracrustal rocks were metamorphosed under greenschist facies conditions.

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tralia, using the high-resolution ion-microprobe SHRIMP II (De Laeter and Kennedy, 1998). The analytical procedures are outlined in Compston et al. (1992), Claoue´-Long et al. (1995) and Nelson (1997). Zircons were mounted in epoxy, together with chips of the standard zircon CZ3, and polished until sectioned in half. Analyses of samples and standards were alternated to allow assessment of Pb + /U + discrimination. Raw data reduction and error assessment followed the method described by Nelson (1997). Common Pb corrections have been applied using the 204 Pb-correction method and assuming the isotopic composition of Broken Hill, since common Pb is thought to be surface-related (Kinny, 1986). The analytical data are presented in Table 2. Errors given on individual analyses are based on counting statistics and are at the 1-sigma level. Errors for pooled analyses are at 2-sigma or the 95% confidence level. Sm and Nd concentrations as well as the isotopic composition of Nd were determined on Finnigan-MAT 261 mass spectrometers at the Max-Planck-Institut fu¨r Chemie in Mainz. The method is described in White and Patchett (1984). All Nd measurements are normalized to 146 Nd/144Nd = 0.7219. The mean value for the La Jolla standard obtained during the course of this study are 143 Nd/144Nd = 0.511843 ± 14(N = 31).

4. Geochemistry 3. Analytical procedures Both major and trace elements were analyzed by X-ray fluorescence spectrometry (XRF) on fused discs and powder pellets, respectively, at the Department of Geosciences, University of Mainz. Analytical procedures are detailed in Laskowski and Kro¨ner (1984). The reproducibility for major elements is better than 5% and for most trace elements better than 10%. Rare earth elements and additional trace elements were determined on selected samples on an inductively coupled plasma mass spectrometer (ICPMS) at the Department of Geochemistry, University of Go¨ttingen. Analytical procedures are detailed in Mu¨nker (1998). Relative errors (reproducibility) are better than 10% for all elements except for Sc, which is less than 20%. The analytical data are presented in Table 1. U –Pb analyses for zircons from a metarhyolite were performed at Curtin University in Perth, Aus-

As indicated by their mineralogy, the Nakfa metavolcanic rocks were affected by greenschist facies regional metamorphism. Assuming that metamorphism was isochemical, except for the introduction of volatiles, analyses of metavolcanic rocks are recalculated on a volatile-free basis. This facilitates comparison of samples with different loss on ignition (LOI) values. Nakfa metavolcanic rocks range in composition from basalt to rhyolite (Table 1). Their low alkali and TiO2 contents and low Nb/Y and Zr/P2O5 ratios show a subalkaline affinity and follow a calc-alkaline trend on an AFM diagram (not shown here). The SiO2 concentrations in the metavolcanic rocks exhibit a bimodal distribution with medians at about 50% and 70%. On MgO vs. major element chemical variation diagrams, a linear relationship is indicated between felsic and basic volcanic rocks (Fig. 3). A linear relationship is also observed for the felsic and basic

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Table 1 Chemical analyses of Nakfa metavolcanic rocks as determined by XRF and ICP-MS Sample No. Rock type

ERN 070A Basalt

ERN 064B Basalt

Major elements (wt.%, XRF) SiO2 50.94 49.76 TiO2 1.38 0.97 Al2O3 15.75 19.28 11.44 10.47 Fe2O3a MnO 0.22 0.15 MgO 5.17 5.44 CaO 8.84 10.01 Na2O 3.96 3.52 K2O 1.88 0.19 P2O5 0.42 0.22 LOI 8.04 3.17 Total 99.98 100.03 Trace elements (ppm, XRF) V 273 267 Cr 89 48 Co 48 43 Ni 31 15 Cu 17 37 Zn 106 76 Ga 17.1 19.8 Rb 190 3 Sr 398 596 Y 35 14 Zr 70 12 Nb 4 Ba 147 67 Pb Th U Trace elements (ppm, ICP-MS) Sc n.d. Nb 0.8 Cs 0.1 La 4 Ce 10 Pr 1.5 Nd 8.1 Sm 2 Eu 1.2 Gd 2.7 Tb 0.43 Dy 2.4 Ho 0.53 Er 1.2 Tm 0.22 Yb 1.3 Lu 0.21 Hf 0.4 Pb n.d.

ERN 069A Basalt

ERN 068A Basalt

ERN 022A Basalt

ERN 017B Basalt

ERN 011B Basalt

ERN 063B Andesite

ERN 019C Andesite

49.74 0.96 16.57 10.83 0.17 6.89 11.66 2.33 0.50 0.33 2.33 99.46

50.06 1.25 16.29 11.94 0.18 6.28 9.56 3.80 0.32 0.31 2.52 99.66

50.22 0.76 16.60 10.75 0.16 7.96 9.02 2.01 2.33 0.19 2.70 99.41

51.55 1.19 17.21 11.75 0.21 4.86 8.67 3.47 0.70 0.40 3.02 99.59

53.29 1.65 14.72 13.40 0.20 4.35 6.94 4.45 0.69 0.30 2.07 99.73

55.86 1.04 16.60 10.95 0.19 3.97 8.11 2.78 0.24 0.26 3.72 100.28

57.25 1.40 15.60 9.71 0.20 3.28 5.74 4.02 1.81 0.97 2.75 100.04

260 144 55 47 85 87 16.4 14 539 18 44 3 153

249 151 64 79 26 90 16.3 6 534 25 80 5 197

255 233 50 79 101 93 16.6 57 616 18 45 2 747

279 48 50 16 155 126 19.1 10 716 23 60 4 493

360 7 60 9 51 120 20.2 12 369 27 89 3 289

260 14 53 7 40 114 18.3 5 698 16 43 2 171

114 8 45 3 2 135 19.8 39 405 33 100 5 501

41 1.9 0.4 7 17 2.5 12 3.2 1.1 3 0.47 2.9 0.58 1.7 0.24 1.6 0.23 1.1 2.9

33 5.4 0.2 9 23 3.3 16.3 4.2 1.4 4 0.65 4.3 0.83 2.4 0.35 2.3 0.34 1.7 5.3

34 1.7 0.5 7 16 2.2 10.9 2.6 0.9 2.3 0.36 2.3 0.46 1.3 0.2 1.2 0.2 1.2 5.9

32 3 0.1 11 26 3.7 17.7 4.3 1.3 4 0.6 3.7 0.73 2.1 0.29 2 0.3 1.5 8.1

n.d. 3 0.4 9 21 2.8 13.5 3.7 1.5 4.6 0.79 4.5 1.03 2.8 0.37 2.5 0.37 2 n.d.

25 6.9 0.5 13 33 4.5 21.8 5.8 1.9 5.4 0.83 5.4 1.08 3 0.45 2.9 0.45 2.5 5.6

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Table 1 (continued) Sample No. Rock type

ERN 070A Basalt

ERN 069A Basalt

ERN 068A Basalt

ERN 022A Basalt

ERN 017B Basalt

ERN 011B Basalt

Trace elements (ppm, ICP-MS) Th 0.03 U 0.03

0.7 0.3

0.4 0.2

1 0.4

1.1 0.5

0.9 0.4

Sample Nr. Rock type

ERN 001C Dacite

ERN 081B Dacite

ERN 005A Dacite

ERN 017C Dacite

ERN 004A Dacite

ERN 009A Dacite

Major elements (wt.%, XRF) SiO2 64.54 65.84 TiO2 0.62 1.08 15.56 14.75 Al2O3 Fe2O3a 5.65 6.30 MnO 0.10 0.17 MgO 2.43 1.63 CaO 5.21 3.08 Na2O 3.65 5.03 K2O 2.05 1.77 0.19 0.37 P2O5 LOI 1.45 1.44 Total 100.03 99.62

66.17 0.69 16.17 4.83 0.12 2.20 2.03 5.33 2.21 0.25 1.98 100.01

66.85 0.68 16.00 5.05 0.17 1.22 4.36 4.01 1.44 0.22 1.74 100.23

68.66 0.87 14.71 4.36 0.13 0.97 2.76 5.28 2.06 0.20 1.02 100.02

69.63 0.78 15.24 3.83 0.15 0.65 1.93 6.11 1.51 0.17 2.23 100.04

69.49 0.88 14.34 4.34 0.13 0.87 1.88 5.99 1.88 0.19 1.70 100.13

70.33 0.60 14.54 3.60 0.12 0.88 2.88 4.61 2.27 0.16 0.93 100.16

Trace elements (ppm, XRF) V 120 69 Cr 33 9 Co 95 40 Ni 5 3 Cu 36 3 Zn 67 134 Ga 16.4 19.1 Rb 47 36 Sr 472 295 Y 18 39 Zr 111 167 Nb 6 9 Ba 846 741 Pb 10 8 Th 5.3 2 U 2.1 0.9

70 14 69 n.d. 16 113 17.1 45 223 30 156 9 813 8 2.7 1

20 8 63 n.d. 2 84 17.1 28 448 19 86 6 967

31 11 63 11 3 81 17.8 41 323 39 176 8 724 6.5 2.8 1.4

27 12 63 n.d. 3 82 17.7 27 318 39 187 10 715 8.5 1.8 1.1

33 12 43 n.d. 2 110 16.6 26 202 41 203 10 747 4.5 1.9 0.8

40 11 68 n.d. 11 67 16.0 43 339 30 175 8 765

ERN 070B Dacite

ERN 064B Basalt

ERN 020A Dacite

Trace elements (ppm, ICP-MS) Sc 16 Nb 9.1 Cs 0.6 La 18 Ce 46 Pr 6.1 Nd 27.6 Sm 6.8 Eu 1.9 Gd 6 Tb 0.95 Dy 6.1

ERN 063B Andesite

ERN 019C Andesite 1.3 0.5

14 5.3 0.9 12 27 3.5 14.7 3.5 1.5 3 0.47 3 (continued on next page)

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Table 1 (continued) Sample No. Rock type

ERN 070B Dacite

ERN 020A Dacite

ERN 001C Dacite

Trace elements (ppm, ICP-MS) Ho 1.26 Er 3.6 Tm 0.57 Yb 4 Lu 0.6 Hf 4 Pb 7 Th 2.5 U 1 Sample Nr. Rock type

ERN 013A Dacite

ERN 021A Dacite

ERN 081B Dacite

ERN 005A Dacite

ERN 017C Dacite

ERN 004A Dacite

ERN 009A Dacite

0.62 1.7 0.27 1.9 0.3 1.8 4.8 1.8 0.8 ERN 068C Rhyolite

ERN 015A Rhyolite

ERN 026A Rhyolite

ERN 019D Rhyolite

ERN 019A Rhyolite

ERN 054C Rhyolite

Major elements (wt.%, XRF) SiO2 70.56 71.88 TiO2 0.57 0.47 14.79 14.26 Al2O3 Fe2O3a 3.12 2.87 MnO 0.13 0.10 MgO 0.79 0.85 CaO 2.30 1.75 Na2O 5.38 5.50 K2O 2.22 2.21 P2O5 0.14 0.11 LOI 0.86 1.28 Total 99.94 99.96

73.24 0.39 13.08 3.18 0.08 1.43 3.32 4.98 0.15 0.12 1.32 100.15

72.82 0.55 13.72 2.91 0.10 0.57 1.45 4.83 2.95 0.09 0.76 99.98

72.84 0.41 14.20 2.43 0.10 0.52 1.96 4.90 2.54 0.09 0.90 100.35

76.41 0.31 13.02 1.88 0.05 0.20 0.99 5.92 1.18 0.04 1.65 100.22

76.66 0.09 13.49 0.85 0.05 0.09 0.44 4.58 3.74 0.02 1.11 100.28

82.06 0.29 12.05 0.82 0.01 0.49 0.12 0.23 3.88 0.04 1.69 100.04

Trace elements (ppm, XRF) V 24 37 Cr 6 8 Co 96 88 Ni n.d. n.d. Cu 1 8 Zn 71 72 Ga 17.9 16.3 Rb 42 34 Sr 296 269 Y 37 29 Zr 183 183 Nb 9 8 Ba 757 958 Pb 10 11 Th 2.7 2.8 U 1.6 1.9

66 22 83 2 21 65 12.5 4 312 13 117 5 53 5.5 2.7 1.3

27 14 80 2 5 50 14.4 52 230 35 230 11 974 7.5 3.2 1.6

19 9 97 n.d. 4 84 16.0 54 326 25 161 7 825 19 3.9 1.2

9 14 90 n.d. 5 32 13.1 22 173 39 230 10 540 6.5 2.9 1.6

9 14 87 n.d. 2 33 21.7 85 125 5 80 31 162 19 11.6 4.3

21 9 160 n.d. 1 20 17.3 85 18 26 181 8 802 4 6.4 1.9

Trace elements (ppm, ICP-MS) Sc 9 Nb 8.1 Cs 0.3 La 21 Ce 48 Pr 6

7 4.8 0.1 14 31 3.5

7 7.4 0.9 18 42 5

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Table 1 (continued) Sample No. Rock type

ERN 013A Dacite

ERN 021A Dacite

Trace elements (ppm, ICP-MS) Nd 24.8 Sm 5.4 Eu 1.4 Gd 4.5 Tb 0.72 Dy 4.4 Ho 0.93 Er 2.7 Tm 0.45 Yb 3.2 Lu 0.49 Hf 3.9 Pb 12 Th 3.7 U 1.4

ERN 068C Rhyolite

ERN 015A Rhyolite

13.9 2.7 0.7 2.2 0.34 2 0.42 1.3 0.2 1.5 0.23 2.5 4.6 3.1 1.1

ERN 026A Rhyolite

ERN 019D Rhyolite

ERN 019A Rhyolite

ERN 054C Rhyolite

20.5 4.4 1.2 3.9 0.62 3.9 0.83 2.4 0.39 2.8 0.44 3.8 18.9 4.1 1.5

Volatile-free calculated, original analyses total given for reference; blanks, not determined; n.d., not detected. a Total iron as Fe2O3.

volcanic rocks when trace elements are plotted against the immobile and incompatible element Zr (Fig. 3). Linear chemical variations between comagmatic lavas are a result of magmatic processes like partial melting, fractional crystallization, assimilation fractional crystallization (contamination) or magma mixing. The absence of correlations between isotope ratios of Sr and Nd and indices of fractionation (e.g. SiO2) argues against any significant assimilation of an older crustal component. Furthermore, a progressive and marked decrease in Cr and Ni (compatible elements) abundance (Fig. 3) suggests that the Nakfa metavolcanic rocks, for the most part, are related by fractional crystallization processes rather than by different extents of partial melting. In Fig. 3, a notable change in slope at about 4% MgO occurs for the elements Ti, Fe, and V. The same

change in slope is observed also for P and Sc (Table 1). Such change in slope is generally explained as indicating the entry of a new mineral phase during crystal fractionation. This will be more apparent where the chemistry of the extract is reflected in the plotting parameters. Thus, this new phase could be a Ti, Fe, and V-bearing phase such as ilmenite, titanomagnetite or hornblende. The change in slope of SiO2 at about the same MgO value suggests that this phase is a silicate mineral, probably an amphibole. The basic volcanic rocks have slightly enriched REE patterns and show a slightly positive Eu anomaly, suggesting minor plagioclase accumulation (Fig. 4). (Ce/Yb)N (the subscript N denotes chondrite-normalized) ranges between 2.59 and 3.45. The nearly flat REE patterns of the basic volcanic rocks suggest that residual garnet during melting and/or fractional

Table 2 SHRIMP II analytical data for Zircons from a metarhyolite (sample ERN 026A) Grainspot

U (ppm)

Th (ppm)

Comma 206%

208

Pb/206Pb

207

Pb/206Pb

206

Pb/238U

207

Pb/235U

Apparent age 206/238

207/235

207/206

1-1 2-1 3-1

41 79 357

17 36 47

1.6 0.3 0.03

0.1255 ± 25 0.1393 ± 39 0.0402 ± 35

0.0675 ± 55 0.0677 ± 18 0.0584 ± 16

0.1255 ± 17 0.1417 ± 18 0.0884 ± 11

1.17 ± 10 1.32 ± 4 0.71 ± 2

762 854 546

786 856 546

854 ± 178 859 ± 55 545 ± 60

Ratios refer to radiogenic Pb. Uncertainties are at the standard error. Data for grains 1 to 3 represent means of seven mass scans. a Common 206Pb/total 206Pb from the observed 204Pb.

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Fig. 3. Fenner diagrams for selected major and trace elements and plot of some trace elements against Zr (right) for Nakfa metavolcanic rocks. The change in slope for Ti, Fe, and V at about 4% MgO suggests the introduction of Fe – Ti – V bearing phase(s) as a fractionating phase(s) at this point.

crystallization was not responsible for the low abundances of the heavy rare earth elements (HREE). The felsic volcanic rocks have more fractionated patterns with (Ce/Yb)N ratio ranging between 2.97 and 5.35 (Fig. 4). Enriched REE patterns reflect extensive fractional crystallization. Most samples show a small negative Eu anomaly indicating removal of plagioclase. The concave middle and heavy rare earth patterns in the felsic volcanic rocks suggest that amphibole and/or clinopyroxene has been an important phase. Such patterns are also common in modern felsic volcanic arc rocks (White and Patchett, 1984). N-MORB normalized spidergrams of the Nakfa basic volcanic rocks show the distinctive arc geochemical signature (Fig. 5). The spidergram patterns are similar with those from modern island arc basalts (Pearce, 1982; Kay, 1984; Thompson et al., 1984; Saunders et al., 1991), which show enrichment in

LILE (Rb, Ba, K) and depletion in HFSE (Nb, Zr, Hf). The regularity of patterns among the Nakfa samples reflects little or no mobilization of LILE. The basic volcanic rocks have Ce/Yb ratios lower than 15 which places them in the low Ce/Yb group of Hawkesworth et al. (1993). Low Ce/Yb arc suites occur in relatively primitive oceanic arc systems and exhibit a strikingly restricted range in Sr, Nd and Pb isotope ratios, often over considerable distances (Hawkesworth et al., 1993). Such a restricted range of initial Sr and Nd isotope ratios is also observed for Nakfa metavolcanic rocks (see below), further indicating that these rocks formed in an oceanic island arc environment. 5. Geochronology Three zircon grains from metarhyolite sample ERN 026A were analyzed for U –Pb ratios, and the results

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Fig. 4. Chondrite-normalized rare earth elements for basic (top) and felsic (below) volcanic rocks. Normalizing values from Boyton (1984).

are presented in Table 2 and on the Concordia diagram of Fig. 6. Two light brown, short prismatic zircons of igneous origin yielded a concordant age of 854 ± 3 Ma, and one grain gave a concordant age of 546 ± 7 Ma (Table 2 and Fig. 6). The 854 ± 3 Ma age,

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which is identical to the Sm – Nd isochron age (see below), is interpreted to reflect crystallization of the original rhyolite. This interpretation is further supported by the following arguments: (i) evaporation of one single zircon from a pre/syn-tectonic granodiorite that intrudes these supracrustal sequences (Fig. 2) yielded a mean 207Pb/206Pb age of 838 ± 2 Ma (Teklay, 1997); (ii) evaporation of four inherited single zircons from a syeno-diorite from the Granitic Batholith (Fig. 2) yielded 207 Pb/ 206 Pb ages varying between 832 and 846 Ma, with a mean age of 841 Ma (Teklay, 1997). These pre-intrusion zircons could be inherited from  850 Ma plutonic rocks coeval with the volcanism and; (iii) this age is similar to zircon ages obtained for similar supracrustal sequences farther north in the Tokar terrane, southern Sudan (Kro¨ner et al., 1991). Ryolites from the Tokar terrane yielded Pb – Pb zircon evaporation ages of 840– 854 Ma (Kro¨ner et al., 1991). The 546 ± 7 Ma age, which is lower than the emplacement ages of the granitoids intruding the metavolcanic rocks (see Fig. 2), is difficult to interpret. A possible explanation is that there was a thermal event at about 550 Ma responsible for the growth of new zircon in the metarhyolite. The nature of this event is not clear, besides no magmatism at around this age is found in the Nakfa area as well as in adjacent areas (e.g. to the south in central Eritrea, Teklay, 1997; to the north in the Tokar area, Sudan, Kro¨ner et al., 1991). However, ages of  550 Ma have been reported for late- to post-tectonic plutons farther north in Sudan (Stern and Kro¨ner, 1993) and farther

Fig. 5. N-MORB normalized spider diagrams for Nakfa basic metavolcanic rocks. Normalizing values from Hofmann (1988).

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Fig. 6. Concordia diagram for SHRIMP II analyses of three zircons grains from metarhyolite sample ERN 026A. Data boxes for each analyses are defined by standard errors in 207Pb/235U, 206Pb/238U and 207Pb/206Pb.

south in northern (Tadesse et al., 1997) and western Ethiopia (Ayalew et al., 1990).

6. Nd isotope systematics The Nd isotopic data are presented in Table 3. In a plot of 147Sm/147Nd vs. 143Nd/144Nd (Fig. 7), the data points are reasonably well aligned, and the regression line corresponds to an age of 837 ± 140 Ma (MSWD = 2.6). The large uncertainty in the age is

mostly due to the limited range in 147Sm/144Nd, which ranges from 0.13 to 0.17. However, this imprecise age is within error of the U –Pb age of 854 ± 3 Ma for zircons from a metarhyolite (see above), suggesting that the Sm –Nd system was not greatly affected by the low-grade regional metamorphism. Given the coherence, it is also likely that all the metavolcanic rocks had the same or similar source. In addition, the absence of a linear correlation between the ratios 143 Nd/144Nd and 1/Nd (approximating 1/144Nd) precludes any mixing of components for the generation

Table 3 Nd isotope data for Nakfa metavolcanic rocks Sample Nr.

Rock type

Sm (ppm)

Nd (ppm)

147

143

eNda

ERN 011B ERN 013A

Basalt Dacite

3.799 6.233

13.751 27.036

0.1670 0.1394

5.4 5.2

ERN ERN ERN ERN ERN ERN ERN

Basalt Andesite Dacite Rhyolite Basalt Basalt Basalt

4.289 5.564 5.392 4.590 2.150 4.160 3.073

17.510 21.802 25.213 21.141 7.511 16.121 11.809

0.1481 0.1543 0.1293 0.1312 0.1731 0.1560 0.1573

0.512748 0.512585 0.512543 0.512657 0.512679 0.512535 0.512525 0.512765 0.512657 0.512663 0.512636

a

017B 019C 021A 026A 064B 068A 069A

Initial values and ratios calculated to the crystallization age.

Sm/144Nd

Nd/144Nd

5.7 5.4 5.4 5.0 5.1 4.8 4.8

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Fig. 7. Nd isotope data plotted on

of the metavolcanic rocks, in agreement with the chemical and isochron data. Initial eNd values for the metavolcanic rocks, calculated for their crystallization age of 854 Ma, are given in Table 3 and are all similar, ranging from 4.8 to 5.7.

7. Discussion The U – Pb zircon age of 854 ± 3 Ma for the metarhyolite, supported by a similar, but less precise, age obtained from the Sm – Nd isotopic system, suggests that arc volcanism in northern Eritrea, and thus in the southern Nubian Shield, began around 850 Ma ago and was coeval with initial arc evolution in the Sudan (Reischmann et al., 1992; Reischmann and Kro¨ner, 1994) and in south-western Saudi Arabia (Kro¨ner et al., 1992). The high positive initial eNd values reflect PanAfrican juvenile (mantle-derived) volcanism, without significant contribution from older continental crust, in the generation of the Nakfa metavolcanic rocks. Furthermore, the remarkably restricted initial eNd values, extending only to 0.9 eNd units (Table 3), support an intra-oceanic origin, which is also supported by the absence of rocks with continental affinities. Such narrow Nd isotope ratios are reported for modern oceanic arc volcanic rocks. For example,

147

Sm/144Nd vs.

179

143

Nd/144Nd.

Nd isotope ratios for volcanic rocks from the Mariana arc have a spread of 1.9 eNd units (Woodhead, 1989); from the Kermadec arc about 2.7 eNd units (Gamble et al., 1993) and from the South Sandwich arc about 4.3 eNd units (Pearce et al., 1995). In contrast, volcanic rocks from modern active continental margins have a much greater range in Nd isotope ratios (Hawkesworth et al., 1993). Thus, the isotope data, combined with the trace element characteristics, suggests an oceanic arc setting for the Nakfa metavolcanic rocks. In order to understand and interpret the Nd isotope data, the Nakfa metavolcanic rocks are compared with data from the 750 Ma Gerf ophiolite of the northern Red Sea Hills, Sudan, the only ophiolite with NMORB chemistry so far identified in the Nubian Shield (Zimmer et al., 1995). Fig. 8 shows the low initial eNd values of the Nakfa metavolcanic rocks compared to the Gerf ophiolite. If the Gerf Nd isotope data are considered to represent the Neoproterozoic depleted mantle under the Nubian Shield, then the shift towards lower eNd for the Nakfa metavolcanic rocks, in the absence of underlying crust, may be interpreted as contributions from both subducted sediments and altered oceanic crust. The low Nd isotope ratios of modern intra-oceanic arc systems, free from the effects of contamination with continental crust, are interpreted similarly (e.g. Mariana arc, Woodhead, 1989).

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Fig. 8. Plot of initial isotope ratios expressed as epsilon values against time for Nakfa metavolcanic rocks, metavolcanic rocks from Sudan (Reischmann and Kro¨ner, 1994) and the Gerf ophiolite (Zimmer et al., 1995). Depleted mantle evolution line (after Goldstein et al., 1984) and Chondritic Uniform Reservoir are labelled DM and CHUR, respectively.

Within the Nubian Shield, the Eritrean metavolcanic rocks also have low initial eNd values compared to metavolcanic rocks of comparable age from Sudan (Fig. 8). Similarly, Neoproterozoic magmas from the northern ANS show low initial eNd values compared to metavolcanic rocks from the southern ANS (Stein and Goldstein, 1996). Such shift towards lower eNd values in arc magmas may result from contributions of small amounts of old crustal material and/or of both subducted sediments and altered oceanic crust. Alternatively, they may indicate derivation from ‘‘enriched’’ mantle as suggested by Stein and Goldstein (1996) for Neoprotreozoic arc magmas from the northern ANS. The very narrow range of initial eNd values for the Nakfa metavolcanic rocks, combined with their trace element characteristics, suggests an oceanic arc setting. Therefore, no direct contamination from old continental crust could have occurred. One way to assess the degree of involvement of subduction components is by comparing the LILE/HFSE ratios such as Sr/Nd or Ba/Zr (Fig. 9). The Nakfa basic rocks have similar LILE/HFSE ratios to basic metavolcanic rocks of about the same ages from Sudan. Consequently, the shift towards lower eNd values for the Nakfa rocks cannot be attributed to an increase in the contribution of subduction zone components. To further strengthen

this conclusion, leached K-feldspars from Nakfa intrusive rocks are plotted together with intrusive rocks of similar ages from Sudan (Fig. 10). In contrast to the Nd isotope data, Fig. 10 shows that the Pb isotope ratios of leached feldspars from Nakfa intrusive rocks are similar to values of leached feldspars of intrusive rocks from Sudan as reported by Stern and Kro¨ner (1993) and Stern and Abdelsalam (1998). If the relatively low initial eNd values of the igneous rocks from Eritrea with respect to the values reported from Sudan (Fig. 8) are interpreted as an increase in the contribution of subducted sediments that contain a continental component, then one would expect to see this effect more clearly in the Pb isotope ratios (Fig. 10), since these isotope ratios are sensitive indicators for the involvement of older crustal components. However, this is not the case, and this may imply that the relatively low initial eNd values of the Nakfa metavolcanic rocks are largely related to the source of these rocks. As a consequence, it is most likely that the Nakfa rocks are derived from a relatively more enriched source than the age-equivalent metavolcanic rocks from Sudan. The question of whether the enrichment took place before or after the subduction component was added (i.e. was it the slab-derived fluid or OIB-like silicate

M. Teklay et al. / Chemical Geology 184 (2002) 167–184

Fig. 9. Plot showing comparison of the degree of involvement of subduction components in the Nakfa basic metavolcanic rocks (MgO > 4%) and basic metavolcanic rocks (MgO > 4%) of comparable age from Sudan and the 750 Ma Gerf ophiolite. Symbols and source of data as in Fig. 8 and Reischmann (1986).

melts that were responsible for the enrichment of the mantle source from which the Nakfa rocks were derived?), can be resolved by plotting ratios of incompatible elements such as Nb/Yb vs. M/Yb, where M is any incompatible element (Pearce, 1982, 1983). In the Nb/Yb vs. Zr/Nb and Nb/Yb vs. Ti/Yb diagrams (Fig. 11), the Nakfa basic rocks are aligned covariantly

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along the MORB-OIB array. Vectors are shown on this diagram to highlight the main variation component. Fig. 11 indicates that for the Nakfa volcanic rocks Zr, Ti, and Nb are not present in significant contributions in the subduction component. Moreover, the shift towards higher Nb/Yb, Zr/Yb and Ti/Yb ratios than average N-MORB indicates a contribution from an incompatible element-enriched melt as observed in OIB-type mantle sources. Consequently, a relatively enriched mantle source with an additional selective enrichment of LILE induced by slab-derived fluids is suggested for Nakfa rocks. Such an incompatible element-enriched mantle source is only observed in modern active continental margin rocks. This is because of the potential presence of trace element enriched material, inferred to be in the continental mantle lithosphere. However, the restricted range of initial Nd isotope ratios observed for Nakfa volcanic rocks, together with the absence of rocks of continental affinity, rule out an active continental margin setting. There may be areas beneath the oceanic crust where trace element enriched mantle persists at sufficiently shallow levels to be present in the mantle wedge beneath some oceanic arc systems. This would be possible only if subduction took place beneath the source of trace element enriched OIB. Hence, an enrichment of the mantle wedge prior to subduction is postulated for the Nakfa volcanic rocks.

Fig. 10. Pb isotopic composition for leached feldspars from Nakfa intrusive rocks (open circles) compared to leached feldspars from Sudan intrusive rocks (open diamonds). Plot of 206Pb/204Pb vs. 207Pb/204Pb (left) and 206Pb/204Pb vs. 208Pb/204Pb (right). Field I = Group I (oceanic lead), field II = Group II (continental lead) of Stacey et al. (1980), and Gerf ophiolite field from Zimmer et al. (1995). Pb evolution curves for mantle, orogene and upper crust are from Zartman and Doe (1981).

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rocks. Accordingly, the presence of plume-related magmatism may be responsible for the enrichment of the mantle source of the Nakfa metavolcanic rocks prior to subduction. Hence, the Nakfa rocks may have evolved along the edge of an oceanic plateau as suggested by Teklay (1997). Furthermore, a thermal anomaly in the upper mantle caused by a plume head may result in the production of large amounts of juvenile calc-alkaline magmatic rocks over a short period of time. Accordingly, this may explain the rapid crustal growth in the Arabian – Nubian Shield as proposed by Reymer and Schubert (1984, 1986). The implication from this study is that lateral crustal growth through processes of arc and oceanic plateau formation is suggested as the most appropriate model for the evolution of the southern ANS, similar to the Caribbean – Colombian Cretaceous igneous province (Donnelly et al., 1990; Kerr et al., 1997). This demonstrates the importance of plume-generated magmatism for the formation of continental crust in the Neoproterozoic.

8. Conclusions

Fig. 11. Nb/Yb – Zr/Yb (top) and Nb/Yb – Ti/Yb (bottom) diagrams for Nakfa basic ( > 4% MgO) metavolcanic rocks. MORB array after Pearce and Peate (1995), average N-MORB from Hofmann (1988).

Alternatively, the geochemical data can be interpreted to mean that the Neoproterozoic mantle beneath the Nakfa rocks was not as depleted as present-day mantle. Pre-enrichment of the mantle wedge source prior to subduction was also suggested by Stein and Goldstein (1996) for the Neoproterozoic calc-alkaline arc magmas from the northern ANS. Several mechanism may produce incompatible element enrichment in the oceanic mantle lithosphere, including freezing of mantle plume-derived melts (White, 1995; fossil plume model of Stein and Hofmann, 1992). Accordingly, the potential presence of trace element enriched thick oceanic lithosphere in the mantle wedge prior to subduction is inferred. The hypothesis that the calc-alkaline arc rocks from Eritrea are derived from an enriched mantle source best explains the observed geochemical characters of these

The Nakfa region of northern Eritrea is characterized by well-preserved supracrustal rocks metamorphosed to greenschist facies regional metamorphism. These are similar to volcano-sedimentary rocks found in modern volcanic arcs. The Nakfa metavolcanic rocks are calc-alkaline in nature and display geochemical characteristics similar to lavas associated with destructive plate margins, being enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE). Arc volcanism began at about 850 Ma in Eritrea, suggesting that the oldest juvenile supracrustal sequences of the ANS are found in the southern ANS both in Sudan (Reischmann, 1986; Reischmann et al., 1992) and Eritrea. The Nakfa metavolcanic rocks display a narrow range of eNd values similar to modern oceanic arc rocks. High positive initial Nd indicate that the source for the metavolcanic rocks was juvenile with an insignificant contribution, if any, from an old continental source. The relatively low initial Nd ratios of the Nakfa rocks as compared to age-equivalent metavolcanic rocks from Sudan are interpreted as a result of enrichment of the mantle

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wedge source prior to subduction. This may be due to incorporation of OIB-like melts in the mantle wedge from the plume responsible for the formation of the adjacent Hagar oceanic plateau. Acknowledgements This study was largely funded through a scholarship from the German Academic Exchange Service (DAAD) to M.T. The first author thanks G. Wo¨rner, Go¨ttingen, for the ICPMS analytical work. A.K. acknowledges assistance by A. Nemchin during SHRIMP analysis. Support of the Military Division in Nafka, the University of Asmara and the Department of Mines of Eritrea during fieldwork is acknowledged. We thank Yemane Asmerom for review of an earlier version of the manuscript. The useful and constructive comments and suggestions by J. Baker and an anonymous reviewer helped us to improve the paper significantly. MB References Abbott, D., Mooney, W., 1995. The structural and geochemical evolution of the continental crust: support for the oceanic plateau model of continental growth. Rev. Geophys. 33, 231 – 242 (Supplement). Abdelsalam, M.G., Stern, R.J., 1996. Sutures and shear zones in the Arabian – Nubian Shield. J. Afr. Earth Sci. 28, 289 – 310. Abdel-Rahman, A.M., 1995. Tectonic-magmatic stages of shield evolution: the Pan-African belt in northeast Egypt. Tectonophysics 242, 223 – 240. Ayalew, T., Bell, K., Moore, J.M., Parish, R.R., 1990. U – Pb and Rb – Sr geochronology of the Western Ethiopia Shield. Geol. Soc. Am. Bull. 102, 1309 – 1316. Ben Avraham, Z., Nur, A., Jones, D., Cox, A., 1981. Continental accretion: from oceanic plateaus to allochthonous terranes. Science 213, 47 – 54. Bentor, Y., 1985. The crustal evolution of the Arabian – Nubian Massif with special reference to the Sinai Peninsula. Precambrian Res. 28, 1 – 74. Boyton, W.V., 1984. Geochemistry of the rare earth elements: meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63 – 114. Claoue´-Long, J.C., Compston, W., Roberts, J., Fanning, C.M., 1995. Two Carboniferous ages: a comparison of SHIRMP zircon dating with conventional zircon ages and 40Ar/39Ar analyses. Soc. Sediment. Geol., Spec. Publ. 54, 3 – 21. Compston, W., Williams, I.S., Kirschvink, J.L., Zichao, Z., Guogan, M., 1992. Zircon U – Pb ages for the Early Cambrian time-scale. J. Geol. Soc. London 149, 171 – 184.

183

De Laeter, J.R., Kennedy, A.K., 1998. A double focusing mass spectrometer for geochronology. Int. J. Mass Spectrom. 178, 43 – 50. De Souaza Filho, C.R., Drury, S.A., 1998. A Neoproterozoic suprasubduction terrane in northern Eritrea, NE Africa. J. Geol. Soc. London 155, 551 – 556. Drury, S.A., Berhe, S.M., 1993. Accretion tectonics in northern Eritrea revealed by remotely sensed imagery. Geol. Mag. 130, 177 – 190. Drury, S.A., Kelly, S.P., Berhe, S.M., Collier, R., Abraha, M., 1994. Structures related to Red Sea evolution in northern Eritrea. Tectonics 13, 1371 – 1380. Donnelly, T.W., Beets, D., Carr, M.J., Jackson, T., Klaver, G., Lewis, J., Maury, R., Schellenkens, H., Smith, A.l., Wadge, G., Westercamp, D., 1990. History and tectonic setting of Caribbean magmatism. In: Dengo, G., Case, J.E. (Eds.), The Caribbean Region: Boulder, Colorado, Geol. Soc. Am., The geology of North, pp. 339 – 374. Gamble, J.A., Smith, I.E.M., McCulloch, M.T., Graham, I.J., Kokelaar, B.P., 1993. The geochemistry and petrogenesis of basalts from the Taupo Volcanic Zone and Kermadec Island Arc, SW Pacific. J. Volcanol. Geotherm. Res. 54, 265 – 290. Goldstein, S.L., O’Nions, R.K., Hamilton, P.J., 1984. An Sm – Nd isotopic study of atmospheric dusts and particulates from major river systems. Earth Planet. Sci. Lett. 70, 221 – 236. Hawkesworth, C.J., Gallagher, K., Hergt, J.M., McDermott, F., 1993. Mantle and slab contributions in arc magmas. Annu. Rev. Earth Planet. Sci. 21, 175 – 204. Hofmann, A.W., 1988. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth Planet. Sci. Lett. 90, 297 – 314. Kay, R.W., 1984. Elemental abundances relevant to identification of magam sources. Philos. Trans. R. Soc. London, Ser. A 310, 535 – 547. Kessel, R., Stein, M., Navon, O., 1998. Petrogenesis of late Neoproterozoic dikes in the northern Arabain – Nubian Shield implications for the origin of A-type granites. Precambrian Res. 92, 195 – 213. Kerr, A.C., Tarney, J., Marriner, G.F., Nivia, A., Saunders, A.D., 1997. The Caribbean – Colombian Cretaceous Igneous Province: the internal anatomy of an oceanic plateau. In: Mahoney, J.J., Coffin, M. (Eds.), Large Igneous Provinces: Continental, Oceanic and Planetary Flood Volcanism. Geophys. Mono., vol. 100, pp. 123 – 144. Kinny, P.D., 1986. 3820 Ma zircons from a tonalitic Amitsoq gneiss in the Godthab district of southern West Greenland. Earth Planet. Sci. Lett. 79, 337 – 347. Kro¨ner, A., Greling, R., Reischmann, T., Hussein, I.M., Stern, R.J., Du¨rr, S., Kruger, J., Zimmer, M., 1987. Pan-African crustal evolution in the Nubian segment of northeast Africa. In: Kro¨ner, A. (Ed.), Proterozoic Lithosphere Evolution. Am. Geophy. Union Geodynamics Series 17, Washington DC, pp. 235 – 257. Kro¨ner, A., Linnebacher, P., Stern, R.J., Reischmann, T., Manton, W., Hussein, I.M., 1991. Evolution of the Pan-African island arc assemblages in the southern Red Sea Hills, Sudan, and in southwestern Arabia as exemplified by geochemistry and geochronology. Precambrian Res. 53, 99 – 118.

184

M. Teklay et al. / Chemical Geology 184 (2002) 167–184

Kro¨ner, A., Pallister, J.S., Fleck, R.J., 1992. Age of initial oceanic magmatism in the Late Proterozoic Arabian Shield. Geology 20, 803 – 806. Laskowski, N., Kro¨ner, A., 1984. Geochemical characteristics of Archaean and late Proterozoic to Palaeozoic fine-graind sediments from southern Africa and significance for the evolution of the continental crust. Geol. Rundsch. 74, 1 – 9. Mu¨nker, C., 1998. Nb/Ta fractionation in a Cambrian arc/back arc system, New Zealand; source constraints and application of refined ICPMS techniques. Chem. Geol. 144, 23 – 45. Nelson, D.R., 1997. Compilation of SHIRMP U – Pb zircon geochronology data, 1996. Geol. Surv. Western Australia, Record 1997/2, 89 pp. Nur, A., Ben Avraham, Z., 1982. Oceanic plateaus, the fragmentation of continents, and mountain building. J. Geophys. Res. 87, 3644 – 3661. Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate boundaries. In: Thorpe, R.S. (Ed.), Andesites: Orogenic Andesites and Related Rocks. Wiley, pp. 525 – 548. Pearce, J.A., 1983. Role of the sub-continental lithosphere in magma genesis at active continental margins. In: Hawkesworth, C.J., Norry, M.J. (Eds.), Continental Basalts and Mantle Xenoliths. Nantwich, Shiva, pp. 230 – 249. Pearce, J.A., Peate, D.W., 1995. Tectonic implications of the composition of volcanic arc magmas. Annu. Rev. Earth Planet. Sci. 23, 251 – 285. Pearce, J.A., Baker, P.E., Harvey, P.K., Luff, I.W., 1995. Geochemical evidence for subduction fluxes, mantle melting and fractional crystallization beneath the South Sandwich Island Arc. J. Petrol. 36, 1073 – 1109. Puchtel, I.S., Hofmann, A.W., Jochum, K.P., Mezger, K., Shchipansky, A.A., Samsonov, A.V., 1997. The Kostomuksha greenstone belt, NW Baltic Shield: remnant of a late Archaean oceanic plateau? Terra Nova 9, 87 – 90. Reischmann, T., 1986. Geologie und Genese spa¨tproterozoischer Vulkanite der Red Sea Hills, Sudan. Dissertation, Johannes Gutenberg-Universita¨t, Mainz, Unpublished. Reischmann, T., Kro¨ner, A., Basahel, A., 1984. Petrography, geochemistry, and tectonic setting of metavolcanic sequences from the Al Lith area, southwestern Arabian Shield. IGCP Proj. 164, Proc., Vol. 6, Pan-African crustal evolution in the Arabian Shield, pp. 365 – 379. Jeddah: Faculty of Science, King Abdulaziz Univ. Reischmann, T., Bachtadse, V., Kro¨ner, A., Layer, P., 1992. Geochronology and paleaomagnetism of a late Proterozoic island arc terrane from the Red Sea Hills, north – east Sudan. Earth Planet. Sci. Lett. 114, 1 – 15. Reischmann, T., Kro¨ner, A., 1994. Late Proterozoic island arc volcanics from Gebeit, Red Sea Hills, north – east Sudan. Geol. Rundsch. 83, 547 – 563. Reymer, A., Schubert, G., 1984. Phanerozoic addition rates to the continental crust and crustal growth. Tectonics 3, 63 – 77. Reymer, A., Schubert, G., 1986. Rapid growth of some major segments of continental crust. Geology 14, 299 – 302. Saunders, A.D., Norry, M.J., Tarney, J., 1991. Fluid influence on the trace element compositions of subduction zone magmas. Philos. Trans. R. Soc. London, Ser. A 335, 377 – 392.

Stacey, J.S., Doe, B.R., Roberts, R.J., Delevaux, M.H., Gramlich, J.W., 1980. A lead isotope study of mineralisation in the Saudi Arabian Shield. Contrib. Mineral. Petrol. 74, 175 – 188. Stein, M., Hofmann, A.W., 1992. Fossil plumes beneath the Arabian lithosphere? Earth Planet Sci. Lett. 114, 193 – 209. Stein, M., Hofmann, A.W., 1994. Mantle plumes and episodic crustal growth. Nature 372, 63 – 68. Stein, M., Goldstein, S.L., 1996. From mantle head to continental lithosphere in the Arabian – Nubian Shield. Nature 382, 773 – 778. Stein, M., Navon, O., Kessel, R., 1997. Chromatographic metasomatism of the Arabian – Nubian lithosphere. Earth Planet. Sci. Lett. 152, 75 – 91. Stern, R.J., 1994. Arc assembly and continental collision in the Neoproterozoic East African orogen: implications for the consolidation of Gondwana. Annu. Rev. Earth Planet. Sci. 22, 119 – 151. Stern, R.J., Kro¨ner, A., 1993. Late Precambrian crustal evolution in NE Sudan: isotopic and geochronologic constraints. J. Geol. 101, 555 – 574. Stern, R.J., Abdelsalam, M.G., 1998. Formation of juvenile continental crust in the Arabian Nubian Shield: evidence from granitic rocks of the Nakasib suture, NE Sudan. Geol. Rudsch. 87, 150 – 160. Stoeser, D.B., Camp, V.E., 1985. Pan-African microplate accretion of the Arabian Shield. Geol. Soc. Am. Bull. 96, 817 – 826. Storey, M., Mahoney, J.J., Kroenke, L.W., Saunders, A.D., 1991. Are oceanic plateaus sites of komatiite formation? Geology 19, 376 – 379. Tadesse, T., Suzuki, K., Hoshino, M., 1997. Chemical Th – U – total Pb isochron age of zircon from the Mereb Granite in northern Ethiopia. J. Earth Planet. Sci., Nagoya Univ. 44, 21 – 27. Teklay, M., 1997. Petrology, Geochemistry and Geochronology of Neoproterozoic Magmatic Arc Rocks from Eritrea: Implications for Crustal Evolution in the Southern Nubian Shield Department of Mines, Eritrea, Memoir No. 1, 125 pp. Thompson, R.N., Morrison, M.A., Hendry, G.L., Parry, S.J., 1984. An assessment of the relative role of crust and mantle in magma genesis: an elemental approach. Philos. Trans. R. Soc. London, Ser. A 310, 549 – 590. White, W.M., Patchett, J., 1984. Hf – Nd – Sr isotopes and incompatible element abudances in island arcs: implications for magma origins and crust – mantle evolution. Earth Planet. Sci. Lett. 67, 167 – 185. White, W.M., 1995. Geochemical tracers of mantle processes. Rev. Geophys. 33, 19 – 24 (Supplement). Windley, B.F., 1995. The Evolving Continents, 3rd ed. Wiley, New York, 526 pp. Woodhead, J.D., 1989. Geochemistry of the Mariana arc (Western Pacific): source composition and processes. Chem. Geol. 76, 1 – 24. Zartman, R.E., Doe, B.R., 1981. Plumbotectonics—the model. Tectonophysics 75, 135 – 162. Zimmer, M., Kro¨ner, A., Jochum, K.P., Reischmann, T., Todt, W., 1995. The Gabal Gerf complex: a Precambrian N-MORB ophiolite in the Nubian Shield, NE Africa. Chem. Geol. 123, 29 – 51.