Precambrian Research

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1 Precambrian Research 224 (2013) Contents lists available at SciVerse ScienceDirect Precambrian Research journa l h omepa g e: Geochemistry of gabbros and granitoids (M- and I-types) from the Nubian Shield of Egypt: Roots of Neoproterozoic intra-oceanic island arc Ayman E. Maurice a,, Bottros R. Bakhit a, Fawzy F. Basta b, Ali A. Khiamy c a Geology Department, Faculty of Science, Beni-Suef University, Beni-Suef, Egypt b Geology Department, Faculty of Science, Cairo University, Giza, Egypt c Alexander Nubia Inc., Cairo, Egypt a r t i c l e i n f o Article history: Received 28 April 2012 Received in revised form 8 October 2012 Accepted 11 October 2012 Available online 23 October 2012 Keywords: Oceanic island arcs M- and I-type granites Continental crust Arabian-Nubian Shield Eastern Desert a b s t r a c t The Neoproterozoic intrusive rocks of the Wadi Ranga area, Nubian Shield of Egypt, comprise gabbros and granitoids emplaced during oceanic island arc and post-collision stages. The plutonic rocks of the island arc stage include hornblende gabbros (Dabbah pluton), trondhjemite (Abu Ghalaga pluton) and tonalites with subordinate quartz gabbro and quartz diorite (Reidi and Abu Ghusun plutons), whereas the post-collision intrusives include granodiorite and monzogranite (Helifi-Hamata pluton). The gabbros and granitoids of the island arc stage are largely calcic, low-k rocks which have either tholeiitic (gabbro and trondhjemite) or transitional tholeiitic to calcalkaline nature (tonalites). On the other hand, the granitoids of the post-collision stage are medium to high-k calcalkaline rocks. All the investigated granitoids are metaluminous. The spider diagrams, with enrichment in LILE and strong Nb depletion, and the almost flat to slightly LREE-depleted REE patterns of the gabbro and trondhjemite are similar to those of the Wadi Ranga low-k tholeiitic basalts and silicic volcanics, respectively, suggesting that the gabbro and trondhjemite are the plutonic equivalents of the Wadi Ranga immature island arc extrusives, and they were derived from mantle source at the early immature island arc stage. Similar to the trondhjemite, the tonalites show LILE enrichment and strong Nb depletions on the MORB-normalized spider diagrams. However, the tonalites have REE patterns which are enriched in LREE (La/Yb = ). The derivation of the tonalites through fractionation of the same magma produced the trondhjemite seems unlikely. Therefore, high degree partial melting of juvenile basaltic arc crust is favoured for the origin of tonalites during a late immature island arc stage. The post-collision granitoids show considerable enrichment in LILE and to a lesser extent in HFSE, slight negative Nb anomaly and strong negative P and Ti anomalies relative to N-MORB. Their REE patterns are LREE-enriched (La/Yb = 5 19), with negative Eu anomaly. These characteristics are consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization. The geochemical characteristics of the trondhjemite and tonalites, and the granodiorite monzogranite classify them as M-type and I-type granitoids, respectively. The partly tholeiitic intrusives of the Wadi Ranga area (South Eastern Desert) have lower K 2 O, Rb and LREE compared to the M-type calcalkaline intrusives of the North Eastern Desert, implying northwardly dipping subduction zone. The geochemical similarities between the intrusives of Neoproterozoic and Phanerozoic oceanic island arcs imply that they share similar style of subduction, which differs from that of the Archaean. The generation of high SiO 2 (up to 74.5 wt%), low K 2 O ( wt%) and slightly LREE-depleted trondhjemite in early immature oceanic island arc setting supports the arc origin of the primitive continental crust. Silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post-collision stage, drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust. The present study indicates that the intra-oceanic island arcs continued to play a role in the generation of the continental crust after the Archaean Elsevier B.V. All rights reserved. 1. Introduction Corresponding author. address: Ayman.Maurice@yahoo.com (A.E. Maurice). Almost all studies on the characteristics and origin of igneous rocks in oceanic arcs were focused on volcanic rocks simply because the exposed oceanic island arc rocks are mostly volcanics. Exposed roots of oceanic island arcs provide an opportunity to understand /$ see front matter 2012 Elsevier B.V. All rights reserved.

2 398 A.E. Maurice et al. / Precambrian Research 224 (2013) the processes occurring at depth and reveal the characteristics and origin of intrusive rocks of oceanic island arcs (Kesler et al., 1977; Perfit et al., 1980; Whalen, 1985; Kawate and Arima, 1998; Saito et al., 2004; Jagoutz et al., 2009), which are assumed to be the setting where continental crust have been created (Rudnick, 1995). Intrusive rocks of oceanic island arcs comprise gabbros and granitoids. Understanding the formation of granitoids in oceanic arcs leads to further understanding of the continental crust formation (e.g. Jagoutz et al., 2009). The Neoproterozoic basement rocks in Egypt are part of the Arabian-Nubian Shield and are exposed mainly in the Eastern Desert and the Sinai Peninsula. The juvenile crust in the Eastern Desert includes four main lithologic units: volcano-sedimentary successions, ophiolites, gneissic core complexes and granitoid intrusions. Granitoids constitute more than half of the Egyptian basement complex. They can, in general, be classified into older and younger granitoids (El-Ramly, 1972) based on their composition and age. The older granitoids comprise trondhjemites, tonalites, granodiorites and rarely granites whereas the younger granitoids are predominated by granites. The older granitoids formed during convergence and collision, whereas the younger granitoids were emplaced during the post-collision and anorogenic stages (e.g. Moghazi, 2002; El-Sayed et al., 2002; Farahat et al., 2007; Moussa et al., 2008; Mohamed and El-Sayed, 2008). The ages of the syn- to late-orogenic (older) granitoids vary between 880 and 610 Ma, whereas the post- to anorogenic (younger) granitoids were emplaced between 600 and 475 Ma (Bentor, 1985). Previous studies on the Wadi Ranga arc gabbros and granitoids focused on field relations and petrographic descriptions. Abu El- Rus (1991) gave a comprehensive description of the gabbroic and granitoid plutons and classified them petrographically into quartz and hornblende metagabbros, and tonalites. Akaad et al. (1996) described the gabbros and granitoids of the Wadi Ranga area as metagabbros and tonalites with subordinate quartz diorite, respectively. They concluded that the metagabbros represent syn-tectonic intrusions while the granitoids are syn-tectonic to late-tectonic intrusions. On Hamata Quadrangle geological map (1997), prepared by the Egyptian Geological Survey and Mining Authority (EGSMA) in collaboration with the British Geological Survey (BGS), the Wadi Ranga garnitoids are identified as syn- to late-tectonic tonalite and granodiorite, whereas the gabbros are mapped as late- to posttectonic mafic intrusions. The granitoids of the oceanic arcs are important in showing that intermediate granitic rocks can be generated without invention of continental crust (Pitcher, 1993). Consequently, the origin and evolution of island arc crust is crucial in understanding the genesis of the continental crust. The plutonic part of the oceanic island arc rocks of the Wadi Ranga area, Eastern Desert, Egypt, offers an excellent opportunity to investigate the origin of Neoproterozoic oceanic island arc plutonism, which, along with coeval volcanism, contributed to the creation of Neoproterozoic continental crust. Moreover, the variation of the origin of the arc building plutonism with arc evolution can be evaluated and the relation to the Phanerozoic oceanic island arc and the Neoproterozoic arc plutonism can be investigated. 2. Geological setting The Precambrian rocks of the Wadi Ranga area (Fig. 1) are essentially a Pan-African assemblage comprising a metavolcanic group, mafic plutonites (the Dabbah arc gabbro and the Abu Ghalaga within-plate gabbros), granitoids, and molasse clastic rocks (Hammamat sediments). The Wadi Ranga primitive oceanic island arc metavolcanic rocks are mafic and felsic lavas and pyroclastic rocks (Maurice et al., 2012). The mafic volcanic rocks cover about 200 km 2 and crop out as two large belts, in the central and southern parts of the study area, and comprise massive and pillow lavas and agglomerates. The felsic volcanic rocks crop out in the eastern part of the study area, covering about 50 km 2 and consist essentially of porphyritic dacite rhyolite lava flows, lapilli and crystal tuffs and agglomerates. The arc intrusive rocks include five gabbro and granitoid plutons (Dabbah, Abu Ghusun, Reidi, Abu Ghalaga and Helifi-Hamata), which intrude the primitive oceanic island arc metavolcanic group. The Dabbah gabbro pluton occurs around Wadi El-Dabbah in the northwestern part of the mapped area, covering an area of about 7 km 2. Generally, the gabbro is massive, medium to coarse-grained and greenish black in colour. The Dabbah gabbro pluton is intruded from the north by the Abu Ghusun granitoids with the formation of a hybrid zone of dioritic composition along the contacts. The low to medium relief Abu Ghusun pluton (120 km 2 ) occupies the northern part of the mapped area and extends about 25 km northward outside the study area. The rocks of the Abu Ghusun pluton are essentially tonalites, which are light grey in colour, medium to coarse grained and generally massive. Abu El-Rus (1991) noted the frequent occurrence of enclaves in the Abu Ghusun pluton. The Reidi pluton (250 km 2 ) crops out in the southeastern part of the study area and extends about 21 km southward outside the mapped area. The rocks of this pluton enclose big metavolcanic xenoliths which are occasionally deformed especially along Wadi Khashir. Vast areas of the pluton are extensively weathered forming a wide plain. The granitoids of this pluton are essentially medium to coarse-grained tonalites. More mafic compositions (diorite and rarely gabbro) are present in the northeastern extremity of the pluton. The Abu Ghalaga trondhjemite pluton (8 km 2 ) is the smallest among the granitoids plutons, and exposed along the eastern side of Wadi Abu Ghalaga. The rocks of this pluton enclose a few black xenoliths of metavolcanic rocks. The Helifi-Hamata pluton (118 km 2 ) occupies the southwestern part of the study area and extends outside the mapped area. This pluton varies in composition from granodiorite in the north to granite in the south. The contact between the granodiorite and granite is gradational and can be traced along the northern flank of Wadi Helifi. The rocks are medium- to coarse-grained and massive. Based on the field observations and the geochemical investigation (see below), the granitoids of Abu Ghusun and Reidi plutons were likely emplaced between the emplacement of Dabbah gabbro and Abu Ghalaga trondhjemite in one hand and the Helifi-Hamata granitoids on the other. 3. Petrography The plutonic rocks in the Wadi Ranga area comprise gabbros, trondhjemites, tonalites with quartz gabbros and quartz diorites (tonalite association), granodiorites and granites. The gabbroic rocks (Dabbah pluton) range in composition from hornblende gabbro to quartz gabbro. These rocks are commonly coarse-grained, hypidiomorphic granular and sometimes exhibit adcumulate texture. These rocks consist mainly of plagioclase and brown hornblende with minor amounts of opaques, biotite, quartz and apatite. Green hornblende, tremolite, actinolite, epidote, chlorite and kaolinite are secondary minerals. Plagioclase is extensively to completely kaolinitized and rarely altered to epidote. Coarse anhedral brown hornblende contains laths of plagioclase (cumulus). The brown hornblende alters to green hornblende, actinolite and chlorite. Some hornblende crystals are altered to biotite by magmatic reactions or contain tremolite cores suggesting former presence of pyroxene. Opaques form up to 10%, occurring essentially as equant grains enclosed in brown hornblende. Anhedral quartz grains occupy the interstitial spaces between plagioclase

3 A.E. Maurice et al. / Precambrian Research 224 (2013) Fig. 1. Geological map showing the different rock units of Wadi Ranga area. Modified after Maurice et al. (2012). and hornblende and in the quartz hornblende gabbro, quartz constitutes more than 5% of the mode. Trondhjemite is the main rock in the Abu Ghalaga pluton and is most probably genetically related to the Dabbah gabbros. It is coarse-grained, hypidiomorphic granular, composed of plagioclase, quartz and hornblende ± k-feldspar. Opaques, apatite and epidote are accessories. Plagioclase occurs as big, subhedral, fresh twinned stout prisms and equant crystals that are occasionally zoned. Quartz is present as anhedral simple or complex grains, with serrated boundaries, occupying the interstitial spaces between plagioclase, sometimes shows undulose extinction. Medium to small anhedral grains and prisms of hornblende are strongly pleochroic from yellow to brownish green to green. Hornblende is rarely altered to aggregates of biotite flakes. In some samples, few big anhedral orthoclase and microcline grains are engulfing plagioclase. Anhedral opaques (less than 1%) are enclosed in plagioclase, hornblende and biotite. The tonalite association comprises the Reidi and Abu Ghusun plutons. It is composed mainly of tonalite with subordinate quartz gabbro and quartz diorite. The tonalite (Reidi and Abu Ghusun plutons) is coarse-grained, hypidiomorphic granular, composed of plagioclase, quartz and hornblende. Opaques, biotite and apatite are accessories. The big subhedral to anhedral plagioclase prisms are slightly kaolinitized and mostly twinned (albite law). Some plagioclases are zoned. Quartz, constituting more than 10% of the rock, is present as anhedral interstitial complex grains corroding plagioclase. Green hornblende occurs as big subhedral prisms and anhedral grains, which occasionally enclose small plagioclase crystals. Some of the hornblende crystals are altered to chlorite and opaques or to biotite and epidote. Small anhedral biotite crystals are associated with hornblende; they are pleochroic from pale yellowish brown to dark brown. The opaques constitute up to 5% of the rock, and occur as equant grains enclosed in plagioclase and hornblende or as interstitial anhedral grains. The quartz gabbro (Reidi pluton) is coarse-grained hypidiomorphic granular, composed of plagioclase, brown hornblende, quartz and opaques. Apatite is accessory whereas chlorite, sericite and kaolinite are secondary minerals. The plagioclase occurs as big anhedral to subhedral twinned prisms as well as small prisms (cumulus phase) enclosed in hornblende, and shows various degrees of alteration to sericite and kaolinite. Hornblende is present as big subhedral prisms and cross-sections, which are strongly pleochroic from yellow to brown to yellowish brown, and is altered to chlorite with release of iron oxides along cleavage planes. Opaques constitute about 10% of the rock, and occur as anhedral grains enclosed in hornblende or interstitial between hornblende

4 400 A.E. Maurice et al. / Precambrian Research 224 (2013) and plagioclase. Quartz occupies the interstitial spaces between plagioclase crystals. The quartz diorite (Reidi pluton) is medium- to coarse-grained hypidiomorphic granular, composed of plagioclase, green to brown hornblende and quartz. Opaques, biotite and apatite are accessories. Chlorite, kaolinite, sericite and epidote are secondary minerals. The plagioclase is largely altered to kaolinite and sericite. Zoning and twinning (lamellar, pericline and simple) are still displayed by some crystals. The hornblende prisms and cross-sections occupy the interstitial spaces between the plagioclase crystals and are altered to chlorite, biotite and epidote. Quartz (about 5%) is present as interstitial anhedral grains. Anhedral grains of opaque minerals are enclosed in hornblende and rarely in plagioclase. The granodiorite (Heilifi-Hamata pluton) is coarse grained, hypidiomorphic to allotriomorphic granular, composed of plagioclase, quartz, biotite and hornblende. Biotite and hornblende are of nearly equal amount. Opaques are accessories. Plagioclase occurs as big subhedral crystals as well as medium euhedral to subhedral crystals, which are mostly fresh or slightly kaolinitized. Some of the plagioclase crystals are zoned, with broadly altered cores and fresh rims. Quartz is interstitial to plagioclase in the form of big anhedral complex grains, which sometimes show undulatory extinction. Big anhedral hornblende prisms and cross-sections are strongly pleochroic (yellow to pale green to dark green) and corroded by biotite and occasionally altered to chlorite. Biotite is closely associated with hornblende as medium subhedral prisms which are altered to chlorite along cleavage planes with release of iron oxides. Equant anhedral grains of opaques are enclosed in plagioclase, hornblende and biotite. The granites (Heilifi-Hamata pluton) are of two types: hornblende- and biotite-granites. The hornblende granite is coarse grained, allotriomorphic granular, composed of K-feldspar, plagioclase, quartz and hornblende. Opaques and epidote are accessories. Plagioclase occurs as big subhedral prisms, fresh or slightly kaolinitized, sometimes zoned, occasionally corroded and encrusted by microcline. K-feldspar is represented by microcline and orthoclase perthite. Microcline is present as small anhedral crystals corroded by quartz and rarely hosts albite forming vein-type perthite. Orthoclase perthite occurs as few equant crystals enclosing and corroding plagioclase. Quartz is present as big anhedral interstitial grains corroding the feldspars. Few big anhedral strongly pleochroic hornblende prisms (brownish yellow to dark green to dark greenish brown) occupy the interstitial spaces between and corrode plagioclase crystals. Few anhedral grains of opaques are closely associated with hornblende or rarely enclosed in plagioclase. The biotite granite is similar in composition to hornblende granite with presence of biotite instead of hornblende. 4. Analytical techniques Whole-rock XRF and ICP-MS analyses were performed at the Institute of Geochemistry and Petrology, ETH-Zurich, Switzerland. Major element compositions were determined using a wave-length dispersive X-ray fluorescence spectrometer (WD-XRF, Axios, PANalytical). Trace and rare earth elements were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP- MS). The details of analytical procedures are similar to those described by Basta et al. (2011) and Maurice et al. (2012). 5. Geochemistry The results of chemical analyses for 18 samples from the Wadi Ranga gabbros and granitoids are given in Appendix A. These include 3 samples from the Dabbah pluton, 4 samples from the Reidi pluton, 2 samples from the Abu Ghusun pluton, 3 samples from the Abu Ghalaga pluton and 6 samples from the Helifi-Hamata pluton Classification Following the Q -ANOR classification scheme of Streckeisen and Le Maitre (1979), the Dabbah samples plot in the gabbro and quartz gabbro fields, the Reidi and Abu Ghusun samples in the quartz gabbro and tonalite fields, the Abu Ghalaga samples in the tonalite and granodiorite fields, whereas the Helifi-Hamata samples fall in the granodiorite and monzogranite fields (Fig. 2A). In the normative Ab-An-Or plot (Fig. 2B), the granitoids of the Reidi and Abu Ghusun plot in the field of tonalite, whereas those of Abu Ghalaga and Helifi-Hamata fall in the trondhjemite field, and the granodiorite and granite fields, respectively. The Dabbah gabbro and the Reidi, Abu Ghusun and Abu Ghalaga granitoids are classified largely as low-k rocks, while the Helifi-Hamata granitoids belong to medium- to high-k granitoids (Fig. 2C). The modified alkali-lime index (MALI) vs. SiO 2 diagram (Fig. 2D) of Frost et al. (2001), reveals that the Dabbah gabbro and the Reidi, Abu Ghusun and Abu Ghalaga granitoids are largely calcic, whereas the granitoids of Helifi-Hamata pluton range from calcic to calc-alkalic. On the alkali-silica diagram (Fig. 2E) of Kuno (1969), the Dabbah gabbro and the Reidi and Abu Ghusun granitoids are tholeiitic to calc-alkaline, the Abu Ghalaga trondhjemites are tholeiitic and the Helifi-Hamata granitoids are calc-alkali. The tholeiitic nature of the Abu Ghalaga trondhjemite and Dabbah gabbro, the transitional tholeiitic to calc-alkaline character of the Reidi and Abu Ghusun granitoids and the calc-alkaline affinity of the Helifi-Hamata granitoids are evident in the Zr vs. Y diagram (Fig. 2F) of Barrett and MacLean (1994). The alumina saturation index (ASI) [ASI = molar ratio Al 2 O 3 /(CaO + Na 2 O + K 2 O)] varies between 0.80 and 1.02 for all the investigated granitoid rocks reflecting their metaluminous character Spider diagrams and REE patterns N-MORB-normalized (normalization values after Pearce, 1983) spider diagram of the Dabbah gabbro is characterized by slight enrichment in LILE, almost flat HFSE which are depleted relative to N-MORB and a strongly negative Nb anomaly (Fig. 3A). This pattern is similar to that of the Wadi Ranga low-k tholeiitic basalts (Maurice et al., 2012). The chondrite-normalized REE patterns of the Dabbah gabbro (Fig. 3B) are almost flat (La/Yb = ). Europium displays either a weak or strong positive anomaly. Except for the strong Eu anomaly (due to plagioclase accumulation) and the relatively higher La/Yb values, these patterns are similar to the REE patterns of Wadi Ranga low-k tholeiitic basalts (La/Yb = , Maurice et al., 2012). The Abu Ghalaga trondhjemite has MORB-normalized patterns (Fig. 3C) which display enrichment in LILE, pronounced negative Nb, P and Ti anomalies and flat HFSE which have values more or less similar to those of N-MORB. This pattern is largely similar to the MORB-normalized pattern of Wadi Ranga felsic volcanic rocks (Maurice et al., 2012). The Abu Ghalaga trondhjemite REE patterns (Fig. 3D) are slightly LREE-depleted (La/Yb = ) with Eu anomalies that range from slightly positive to strongly negative. It is worth noting that the sample with the largest LREE-depletion is the one displaying the small positive Eu anomaly. The REE patterns of trondhjemite are similar to those of Wadi Ranga felsic volcanic rocks (La/Yb = , Maurice et al., 2012), but the former has higher REE abundances. The spider diagrams of Reidi and Abu Ghusun granitoids (Fig. 4A and C, respectively) exhibit enrichment in LILE and pronounced Nb troughs. The high field strength elements Ce, P, Zr, Hf and Sm are almost flat with values similar to N-MORB normalization values,

5 A.E. Maurice et al. / Precambrian Research 224 (2013) Fig. 2. Geochemical classifications of the Wadi Ranga intrusive rocks. (A) Normative Q -ANOR classification diagram (after Streckeisen and Le Maitre, 1979); (B) An-Ab-Or normative classification of silicic plutonic rocks (after Barker, 1979), fields of the Phanerozoic oceanic island arc intrusive rocks based on data from Whalen (1985), Kawate and Arima (1998), Saito et al. (2004) and Perfit et al. (1980); (C) SiO 2 vs. K 2O diagram (after Le Maitre, 2002), WRMV and WRFV are fields of Wadi Ranga oceanic island arc mafic and felsic volcanics (Maurice et al., 2012), respectively; (D) Na 2O + K 2O CaO vs. SiO 2 diagram (after Frost et al., 2001); (E) Na 2O + K 2O vs. SiO 2 diagram (after Kuno, 1969); (F) Zr vs. Y diagram (after Barrett and MacLean, 1994).

6 402 A.E. Maurice et al. / Precambrian Research 224 (2013) Fig. 3. MORB-normalized spider diagrams and chondrite-normalized REE patterns for the Dabbah gabbros (A and B) and the Abu Ghalaga trondhjemite (C and D). The spider diagrams and REE patterns of the island arc mafic and felsic volcanic rocks of Wadi Ranga (Maurice et al., 2012) are shown. MORB and chondrite normalization values after Pearce (1983) and Sun and McDonough (1989), respectively. while the normalized values of Ti, Y and Yb are lower than one and generally decrease from Ti through Y to Yb in the Reidi granitoids. The Reidi and Abu Ghusun granitoids display REE patterns (Fig. 4B and D), which are LREE-enriched (La/Yb = ) and almost without Eu anomalies. The MORB-normalized patterns of Helifi-Hamata granitoids (Fig. 4E) show considerable enrichment in K, Rb, Ba and Th and lesser degree of enrichment in HFSE, except Y and Yb which have normalized values slightly higher or lower than N-MORB normalization values. These patterns lack the pronounced Nb anomaly, which characterizes the patterns of gabbro and other granitoids, but they display strongly negative P and Ti anomalies. The REE patterns of Helifi-Hamata granitoids (Fig. 4F) display strong enrichment in LREE relative to HREE (La/Yb = ), mostly with small negative Eu anomaly, and the LREE are more fractionated than HREE. 6. Discussion 6.1. Tectonic setting The Dabbah gabbro and the Reidi, Abu Ghusun and Abu Ghalaga granitoids are low-k rocks enriched in LILE and show pronounced Nb depletion on spider diagrams. Magmas with these characters are generally believed to be generated in subduction-related environments. The Helifi-Hamata granitoids, on the other hand, are medium- to high-k rocks which have calc-alkaline affinity similar to magmas generated in convergent-margin settings (Wilson, 1989), but lack the pronounced Nb anomaly characteristic of subduction-related magmas. Consequently, we believe that these rocks were formed during or shortly after arc collision. Applying the Nb SiO 2 tectonic discrimination diagram of Pearce and Gale (1977), the Dabbah gabbro and the Reidi, Abu Ghusun and Abu Ghalaga granitoids occupy the field of volcanic arc magmas, whereas the Helifi-Hamata granitoids plot in the area of overlap between volcanic arc and within-plate magma fields (Fig. 5A). In the Rb vs. Y + Nb tectonic discrimination diagram (Fig. 5B) of Pearce (1996) the granitoids of the Reidi, Abu Ghusun and Abu Ghalaga plutons largely plot in the volcanic arc granite (VAG) field, whereas the Helifi-Hamata granitoids plot in the field of post-collision granites (post-colg) Arc maturity The low K 2 O and Rb contents of the Reidi, Abu Ghusun and Abu Ghalaga granitoids along with their tholeiitic (Abu Ghalaga pluton) and transitional tholeiitic-calcalkaline (Reidi and Abu Ghusun plutons) nature indicate that these rocks were generated during the early stages of subduction, i.e. an immature island arc. On the

7 A.E. Maurice et al. / Precambrian Research 224 (2013) Fig. 4. MORB-normalized spider diagrams and chondrite-normalized REE patterns for the Reidi (A and B) and Abu Ghusun (C and D) tonalites, and the Helifi-Hamata granodiorite-granites (E and F). MORB and chondrite normalization values after Pearce (1983) and Sun and McDonough (1989), respectively. other hand, the relatively high K 2 O and Rb contents of the Helifi- Hamata granitoids and their calcalkaline nature suggest that these rocks were generated in a thicker crust, probably shortly after the collision stage. Using the La/Yb vs. Th/Yb diagram (Fig. 6A) the Dabbah gabbro and the Reidi, Abu Ghusun and Abu Ghalaga granitoids plot within or near the fields of primitive island arc and island arc, whereas the Helifi-Hamata granitoids plot in the fields of island arc and continental margin arc. Brown et al. (1984) classified the granitoidbearing arcs into: (1) primitive island and continental arcs, broadly the M-type category of calcic, metaluminous granitoids; (2) normal continental arcs, the I-type calcalkaline metaluminous

8 404 A.E. Maurice et al. / Precambrian Research 224 (2013) Fig. 5. Tectonomagmatic diagrams for the intrusive rocks of Wadi Ranga area. (A) Nb vs. SiO 2 diagram (after Pearce and Gale, 1977); (B) Rb vs. Y + Nb diagram (after Pearce et al., 1984), VAG, volcanic arc granite, ORG, ocean ridge granite, WPG, within-plate granite, syn-colg, syn-collision granite, post-colg, post-collision granite. The field of post-collision granites after Pearce (1996). The fields of oceanic island arc mafic and felsic volcanics of the Wadi Ranga are based on data from Maurice et al. (2012). Fig. 6. Tectonomagmatic diagrams to deduce the type and maturity of the magmatic arc that produced the intrusive rocks of the Wadi Ranga area. (A) La/Yb vs. Th/Yb diagram (after Condie, 1989); (B) Rb/Zr vs. Nb diagram (after Brown et al., 1984), the field of the oceanic island arc felsic volcanics of Wadi Ranga is based on data from Maurice et al. (2012) Origin to peraluminous suites; (3) mature continental arcs, which are alkali-calcic and peraluminous, and often termed S-type. The metaluminous and largely calcic nature of the Reidi, Abu Ghusun and Abu Ghalaga granitoids suggest that they belong to primitive island arc granitoids of Brown et al. (1984). Additionally, the Reidi, Abu Ghusun and Abu Ghalaga granitoids fall in or close to the field of primitive island arcs and continental arcs, while the granitoids of Helifi-Hamata pluton plot near the normal continental arc field on the Rb/Zr vs. Nb diagram (Fig. 6B) of Brown et al. (1984). The primitive island arc origin of the Dabbah gabbro and the Reidi, Abu Ghalaga and Abu Ghusun granitoids is supported by the fact that the gabbro and granitoids have geochemical characteristics similar to those of Phanerozoic oceanic island arcs (see below) such as New Britain and Izu-Bonin. That the granitoids plot close to the field of Wadi Ranga felsic volcanics (Figs. 2C and 6B), which are believed to be formed in a primitive oceanic island arc (Maurice et al., 2012), lends credence to this conclusion. The Dabbah gabbros are characterized by low K 2 O values, HFSE contents lower than N-MORB values (Fig. 3A) and REE patterns which are either flat or slightly LREE-enriched (Fig. 3B). These characteristics are comparable to those of the Wadi Ranga low-k tholeiitic basalts which were produced by partial melting of ultradepleted mantle source as a consequence of addition of subducted slab-derived fluids to an overlying mantle wedge (Maurice et al., 2012). This leads us to suggest that the Dabbah gabbros represent the plutonic equivalents of the Wadi Ranga low-k tholeiites. However, the differences between the gabbros and tholeiites of the Wadi Ranga area (the slightly higher La/Yb, lower HREE, and variable Eu anomaly in some gabbros) are attributed to fractional crystallization and/or accumulation of phases such as hornblende and plagioclase. Tonalitic and/or trondhjemitic magmas could be formed by (1) fractional crystallization of mantle-derived melts, (2) partial melting of hydrous basaltic arc lower crust or (3) partial melting of

9 A.E. Maurice et al. / Precambrian Research 224 (2013) subducted basaltic oceanic crust (Saito et al., 2004 and references therein). Beard (1995) proposed that the plausible mechanisms for the genesis of low-k island arc dacites and tonalites include dehydration melting of amphibolitized arc crust and low-pressure (less than 200 MPa) fractionation of hydrous basaltic magmas or fractionation of relatively dry arc basaltic magmas at any pressure. Partial melting origin is favoured for tonalitic plutons emplaced at pressures 200 MPa, whereas fractionation origin is favoured for dacitic magmas having very low concentrations of incompatible trace elements (Beard, 1995). The trondhjemites of the Abu Ghalaga pluton have low K 2 O ( wt%) and Rb (5 25 ppm) contents at comparatively high SiO 2 contents ( wt%), spider diagrams with enrichment in LILE and HFSE values similar to those of N-MORB (Fig. 3C) and REE patterns which are slightly LREE-depleted (Fig. 3D). These characteristics are comparable to those of the low-k dacite and rhyolite of Wadi Ranga. Accordingly, we suggest that the Abu Ghalaga trondhjemites are the plutonic equivalents of the Wadi Ranga silicic volcanics, which were produced by fractionation of low-k tholeiitic basalts (Maurice et al., 2012). This is supported by the fact that Abu Ghalaga is the smallest pluton among the other plutons in the study area. The P and Ti troughs are consistent with fractionation of apatite and Fe Ti oxides and/or hornblende. By analogy with the mafic and silicic volcanic rocks of the Wadi Ranga area, we propose that clinoproxene, amphibole as well as plagioclase fractionation was responsible for the derivation of trondhjemite from its parental mafic magma. This is supported by the presence of clinopyroxene and plagioclase phenocrysts in the mafic volcanic rocks and hornblende in the Dabbah gabbros. The relatively more LREE-depleted patterns of trondhjemites (La/Yb = ) compared to the silicic volcanics (La/Yb = ; average = 1.01, Maurice et al., 2012) of Wadi Ranga may be attributed to loss of minor fluid phase or fractionation of minor phases, which preferentially scavenge LREE. The tonalites of the Reidi and Abu Ghusun plutons are characterized by low K 2 O ( wt%) and Rb (10 27 ppm) contents, which are comparable to those of Abu Ghalaga trondhjemite. Their MORB-normalized spider diagrams (Fig. 4A and C) are enriched in LILE and have HFSE which are either close to or lower than the N-MORB normalization values, suggesting derivation from depleted mantle source impregnated with LILE through subduction component. In contrast with the slightly LREE-depleted patterns of the Abu Ghalaga trondhjemite, the REE patterns of the Reidi and Abu Ghusun granitoids (Fig. 4B and D) are LREE-enriched (La/Yb = ) and almost without Eu anomalies, suggesting that the derivation of the Reidi and Abu Ghusun granitoids (with lower SiO 2 contents, wt%) through fractionation of the same magma produced the Abu Ghalaga trondhjemite (SiO 2 = wt%) seems unlikely. Therefore, we propose that the parental magmas of the Reidi and Abu Ghusun plutons were produced through partial melting of the arc lower basaltic crust, which has a composition more or less similar to the low-k tholeiitic basalts of Wadi Ranga. Their origin through partial melting of lower arc crust is supported by the relatively large size of Reidi and Abu Ghusun plutons compared to Abu Ghalaga pluton. The magmas of low-k tonalitic rocks are generated by significant degrees (20 50%) of partial melting of amphibolite of low-k tholeiitic composition as indicated by experimental work (e.g. Beard and Lofgren, 1991; Rapp and Watson, 1995; Nakajima and Arima, 1998). In an intra-oceanic setting, intermediate rocks (>62 wt% SiO 2 ) could be produced by dehydration melting of low-k amphibolite at pressure below 700 MPa (Beard, 1995). The granitic and tonalitic melts produced by low-pressure dehydration melting of low-k amphibolites have Al 2 O 3 contents similar to those of island arc granitoids whereas low-pressure hydration melting produce melts with significantly higher Al 2 O 3 contents (Beard and Lofgren, 1991). The crust thickness of the primitive intra-oceanic island arc that produced the Wadi Ranga rocks was estimated to be about 8 km (Maurice et al., 2012), which means that the maximum pressure within the lower crust of such arc was about 3 kbar (maximum pressures within the crust of most island arcs and ocean basins are of the order of 4 8 kbar, Gill, 1981 in Beard and Lofgren, 1991). Accordingly, we propose that the parental magmas of the Reidi and Abu Ghusun low-k tonalites (SiO 2 mostly 62 wt%) were generated through substantial degrees of partial melting of a juvenile amphibolitized arc crust, with composition similar to the low-k tholeiitic basalt of the Wadi Ranga area, at low pressure. Underplating of the amphibolitized lower arc crust with hot basaltic magmas can provide the heat source required for partial melting of such mafic crust (Petford and Gallagher, 2001) and generation of tonalitic melt. The Helifi-Hamata granitoids have higher K 2 O ( wt%), Rb ( ppm) and Zr (mostly ppm) compared with the arc granitoids of the Wadi Ranga area (K 2 O = wt%; Rb = 5 27 ppm; Zr = ppm). They are notably enriched in LILE and most of HFSE compared to N-MORB (Fig. 4E), have LREEenriched patterns with negative Eu anomalies (Fig. 4F) and are similar to post-collision granites. Low and high-pressure partial melting experiments prove that low-al 2 O 3 granitic melts are produced by lower degrees of partial melting (compared with tonalitic melts) of low-k mafic crust (Beard and Lofgren, 1991; Rapp et al., 1991; Rapp and Watson, 1995), and K 2 O contents decrease with increasing the amount of the melt due to increase in temperature (Beard and Lofgren, 1991). The geochemical characteristics of Helifi-Hamata post-collision granitoids, including their high SiO 2 contents (69 74 wt%), are consistent with derivation of their parental magmas by lower degrees of partial melting of arc lower crust of basaltic composition at greater depth due to increased arc crust thickness as a consequence of arc arc collision. Partial melting of the lower part of the overthickened island arc crust is considered as an important process for generation of the Archaean granitoids (e.g. Nagel et al., 2012). The relatively fractionated REE patterns of the Helifi-Hamata granitoids may reflect small amount of garnet in their source. During amphibolite melting, garnet becomes stable at pressures between 8 and 10 kbar and above (Beard, 1995 and references therein). Consequently, we suggest that the parental magmas of the Helifi-Hamata granitoids were generated at the base of at least 25 km thick crust. These parental magmas were evolved through fractional crystallization of apatite and Fe Ti oxides and/or hornblende as indicated by P and Ti troughs in the MORB-normalized patterns. The negative Eu anomaly in REE patterns reflects plagioclase fractionation or/and residual plagioclase in the source. Similar to the metabasalts of Wadi Ranga, the studied granitoids have low or sub-chondritic Nb/Ta ratios (mostly between 3.32 and 15.09). Foley et al. (2002) proposed that during partial melting of low-mg amphibolites, amphiboles with Mg# less than 70 can cause low Nb/Ta ratios in coexisting tonaliticgranodioritic melts. On the other hand, Xiong (2006) suggested that amphibole cannot impart substantial Nb Ta fractionation to tonalitic-granodioritic melts. Moreover, Rapp et al. (2003) and Xiong (2006) concluded that the low or sub-chondritic Nb/Ta ratios in tonalitic-granodioritic melts are most likely inherited from their source rocks, i.e. amphibolites and eclogites with sub-chondritic Nb/Ta ratios. Accordingly, the sub-chondritic Nb/Ta ratios in the Wadi Ranga tonalites and granites provide additional evidence that the Wadi Ranga low-k tholeiitic basalts (with sub-chondritic Nb/Ta ratios) are plausible sources for generation of their parental magmas. Several studies indicate the important role of amphibole in the differentiation of arc magmas (e.g. Davidson et al., 2007; Alonso- Perez et al., 2009; Kartzmann et al., 2010; Larocque and Canil, 2010;

10 406 A.E. Maurice et al. / Precambrian Research 224 (2013) Fig. 7. Zr/Sm vs. SiO 2 plot to evaluate the role of hornblende fractionation in the evolution of the arc magmas produced the oceanic island arc intrusive rocks of the Wadi Ranga area. See text for explanation. Dessimoz et al., 2012). The variation of Zr/Sm with SiO 2 (Fig. 7) in the arc intrusive rocks of the Wadi Ranga suggests that amphibole played an essential role in the evolution of these rocks. Although small probably due to fractionation of zircon from trondhjemite, the increase of Zr/Sm from gabbro to trondhjemite and Ti trough in trondhjemite spider diagrams can be related to amphibole fractionation (Thirlwall et al., 1994; Jagoutz et al., 2009). Similarly, the increase in Zr/Sm with SiO 2 in the Reidi and Abu Ghusun tonalites supports amphibole fractionation from their parental magmas. We therefore propose that the differentiation of the magma produced the Reidi and Abu Ghusun tonalites was largely dominated by amphibole. On the other hand, amphibole and plagioclase fractional crystallization was the main process in the evolution of gabbro and trondhjemite as indicated by the presence of positive (gabbro) and negative (trondhjemite) Eu anomalies in their REE patterns (Fig. 3B and D). The early fractionation of amphibole (poor in silica) and delay of plagioclase (richer in silica) fractionation push the derivative liquids to silica-rich composition over a short fractionation interval (Jagoutz et al., 2009 and references therein). The high SiO 2 content of the Abu Ghalaga trondhjemite can therefore be attributed to early and extensive fractionation of hornblende. This is supported by the fact that hornblende is the essential ferromagnesian mineral in the hornblende gabbro Typology of granitoids The granitic rocks are classified as I-, S-, A- and M-types (Chappell and White, 1974; Loiselle and Wones, 1979; White, 1979; Pitcher, 1983). This classification is termed alphabet soup (Eby, 1992), MISA notation (Cobbing, 1996) or Alphabetic classification (Frost et al., 2001). The letter classification is criticized essentially due to its genetic assumptions (Bonin, 2007). However, this classification is frequently applied (e.g. Farahat et al., 2007; Clemens et al., 2011). The Reidi, Abu Ghusun and Abu Ghalaga granitoids have the characteristics of M-type granitoids. They are metaluminous and largely calcic with low K 2 O and Rb contents, suggesting generation in oceanic island arc setting. The REE patterns, which are slightly depleted (the Abu Ghalaga trondhjemite) or not Fig. 8. (A) K 2O vs. SiO 2 and (B) Rb vs. SiO 2 plots for the Neoproterozoic oceanic island arc intrusive rocks of Wadi Ranga and Phanerozoic oceanic island arc intrusive rocks. Data sources: 1, Aleutian tholeiitic intrusive rocks (Kay et al., 1983); 2, Aleutian calcalkaline intrusive rocks (Perfit et al., 1980); 3, New Britain arc (Whalen, 1985); 4, Tanzawa Complex, IBM arc (Kawate and Arima, 1998); 5, Izu arc (Saito et al., 2004). significantly enriched (the Reidi and Abu Ghusun tonalities) in LREE (La/Yb = and , respectively) are similar to the REE patterns of M-type granitoids of Modern oceanic island arcs (e.g. New Britain, Whalen, 1985; Izu-Bonin-Mariana, Kawate and Arima, 1998 and Saito et al., 2004). The Helifi-Hamata granitoids have relatively high K 2 O and Rb contents and LREE-enriched REE patterns (La/Yb = ), characteristics which are different from those of the oceanic island arc M-type granitoids (e.g. Saito et al., 2004). The metaluminous and calcalkaline nature of the granitoids of the Helifi-Hamata pluton suggest that these granitoids are akin to the I-type group. Moreover, the high-k character of some samples (Fig. 2C) implies that these rocks belong to the post-collision I-types (Pitcher, 1983). This is also supported by the observation that the Helifi-Hamata granitoids plot in the field of the post-collision granites in the Rb vs. Nb + Y diagram (Fig. 5B) of Pearce (1996).

11 A.E. Maurice et al. / Precambrian Research 224 (2013) Relation to Phanerozoic island arc and Neoproterozoic Arabian-Nubian arc plutonism Fig. 9. Chondrite-normalized REE patterns for the Neoproterozoic oceanic island arc intrusive rocks of Wadi Ranga compared to the Phanerozoic and Neoproterozoic arc plutonic rocks. (A) Ranga gabbro compared with Phanerozoic (New Britain, IBM and Aleutian arcs) and Neoproterozoic gabbros (Shufayyah gabbro, Arabian Shield; Ras Gharib, Nubian Shield); (B) Ranga trondhjemite compared with Phanerozoic (New Britain, IBM, Aleutian and Izu arcs) and Neoproterozoic granitoids (Bustan trondhjemite, Arabian Shield); (C) Ranga tonalite compared with Neoproterozoic granitoids (Bustan and Shufayyah tonalites, Arabian Shield; Um Gheig granodiorite, The Dabbah, Abu Ghalaga, Reidi and Abu Ghusun intrusions have lithologic and geochemical characteristics that are broadly similar to those of plutonic rocks emplaced in Phanerozoic oceanic island arcs. Similar to the granitoids of New Britain and Izu-Bonin-Mariana (IBM) island arcs, the granitoids of the Reidi and Abu Ghusun, and Abu Ghalaga plutons plot in the fields of tonalite and trondhjemite, respectively, on the normative Ab-An-Or plot (Fig. 2B). On the K 2 O vs. SiO 2 and Rb vs. SiO 2 diagrams (Fig. 8A and B), the Dabbah gabbros and the granitoids of Abu Ghalaga, Reidi and Abu Ghusun are akin to the plutonic rocks of New Britain (Whalen, 1985) and IBM (Kawate and Arima, 1998; Saito et al., 2004) island arcs, while they have lower K 2 O and Rb contents compared with Aleutian island arc plutonic rocks (Perfit et al., 1980; Kay et al., 1983). The post-collision granitoids of Helifi-Hamata pluton has K 2 O and Rb contents which are higher than those of New Britain and IBM arcs but comparable or higher than those of the Aleutian island arc tholeiitic and calc-alkaline plutonism. Moreover, the REE patterns of the Dabbah gabbros and the Abu Ghalaga trondhjemite are similar to those of New Britain and IBM island arc gabbro (Fig. 9A) and granitoids (Fig. 9B), respectively, but different from those of the Aleutian island arc gabbro and granitoids, which display enrichment in LREE. These similarities are compatible with those established from comparing the composition of the Wadi Ranga primitive island arc mafic and felsic volcanics with the lavas erupted in modern oceanic island arcs such as South Sandwich and New Britain (Maurice et al., 2012). The geochemical similarities between the intrusive rocks of Neoproterozoic and Phanerozoic oceanic island arcs imply that they share similar style of subduction, which differs from that of the Archaean (e.g. Martin et al., 2005). The composition of the Wadi Ranga island arc plutonic rocks is comparable to that of the Arabian Shield older felsic plutonic rocks, which are believed to have been emplaced within a Neoproterozoic oceanic island arc (Jackson et al., 1984; Jackson, 1986). Jackson (1986) classified the Arabian Shield island arc felsic plutonic rocks into three associations: trondhjemite association (diorite tonalite trondhjemite), tonalite association (mainly quartz diorite tonalite) and granodiorite association (gabbro diorite quartz diorite tonalite with subordinate granodiorite and monzogranite). Based on chemical composition and analogy with the Cenozoic oceanic island arcs, Jackson (1986) believed that the trondhjemite association was emplaced during a late immature island arc stage, whereas the tonalite and granodiorite associations characterize a mature island arc stage. He suggested that early immature island arc plutonism has not yet been discovered. The generally low K 2 O and Rb contents of the Abu Ghalaga, Reidi and Abu Ghusun granitoids suggest that these rocks are similar to the granitoids of the Arabian Shield trondhjemite association (Fig. 10), which generally contain less than 1% K 2 O in the most felsic rocks (72 78 wt%) of this association (Jackson, 1986). In addition, the REE patterns of the Reidi and Abu Ghusun tonalites are broadly similar to those of the Arabian Shield tonalites (Fig. 9C), which are believed to have been emplaced in older immature (Bustan tonalite of the trondhjemite association) and mature (Shufayyah tonalite of the granodiorite association) island arcs (Jackson, 1986). Based on their geochemical characteristics and similarity to plutonic rocks from modern and Neoproterozoic oceanic island arcs, we suggest that the Dabbah gabbros and Abu Ghalaga Nubian Shield). Data sources: Whalen (1985), Kawate and Arima (1998), Saito et al. (2004), Perfit et al. (1980), Jackson et al. (1984), Abdel-Rahman (1990), and El-Sayed et al. (2002).

12 408 A.E. Maurice et al. / Precambrian Research 224 (2013) the Wadi Ranga granitoids. Compared with the Neoproterozoic subduction-related granitoids of the Eastern Desert, the island arc granitoids of Wadi Ranga generally have different geochemical characteristics, especially the Abu Ghalaga trondhjemite. Except Kab Amiri trondhjemite (Central Eastern Desert, Moghazi, 2002), they have characteristically lower K 2 O and Rb contents (Fig. 10) and less LREE-enriched REE patterns (Fig. 9) than the Central and North Eastern Desert granitoids (Ragab et al., 1989; Abdel-Rahman, 1990; Moghazi, 2002; El-Sayed et al., 2002; Farahat et al., 2007), implying that the Central and North Eastern Desert granitoids were emplaced in more mature arcs. However, the general increase of K 2 O, Rb and LREE in the Eastern Desert granitoids from south (Wadi Ranga) to north (Central and North Eastern Desert) can be attributed either to temporal (due to change of arc crust thickness) or spatial (due to change of distance from trench) variation in the chemistry of island arc magmas (Wilson, 1989). The early orogenic diorites and tonalites of the North Eastern Desert (Abdel-Rahman, 1990) have higher K 2 O and Rb contents than the immature island arc granitoids of the Wadi Ranga area (Fig. 10) whereas the gabbros are more LREE-enriched (Fig. 9A). These differences between early orogenic, and probably coeval, plutonic rocks from South and North Eastern Desert favour the spatial variation in the chemistry of arc magmas as a cause for these geochemical differences, however, the temporal variation cannot be ruled out due to the absence of accurate age dates for these rocks Intra-oceanic island arc magmatism and generation of continental crust Fig. 10. (A) K 2O vs. SiO 2 and (B) Rb vs. SiO 2 plots for the Neoproterozoic oceanic island arc intrusive rocks of Wadi Ranga and the Neoproterozoic oceanic island arc intrusive rocks of the Arabian Shield and the Neoproterozoic subduction-related intrusives of the Nubian Shield (Central and North Eastern Desert). Data sources: 1, Ras Gharib intrusive rocks (Abdel-Rahman, 1990), North Eastern Desert; 2 and 3, Kab Amiri (Moghazi, 2002) trondhjemite tonalite and granodiorite, respectively, Central Eastern Desert; 4, El-Bula granitoids (Farahat et al., 2007), Central Eastern Desert; 5, Um Gheig granitoids (El-Sayed et al., 2002), Central Eastern Desert; 6, Wadi Beizah intrusives (Ragab et al., 1989), Central Eastern Desert. The shaded and striped fields are, respectively, the island arc granodiorite and trondhjemite associations of the Arabian Shield (Jackson et al., 1983). trondhjemites represent early immature island arc plutonism, whereas the Reidi and Abu Ghusun tonalites were emplaced in an older immature island arc stage. This interpretation is in harmony with the immature island arc setting established based on the nature of the coeval volcanic rocks (Maurice et al., 2012). Thus, the present study reports the Abu Ghalaga trondhjemite as a classic example of the Neoproterozoic early island arc felsic plutonism. Moreover, we rank it among the most primitive Neoproterozoic felsic plutonism described in the Arabian-Nubian Shield. Geochemical analyses of whole set of elements (including REE) for the Neoproterozoic granitoids of the Eastern Desert, which represents the northern tip of the Nubian Shield, are scarce (e.g. El-Sayed et al., 2002), hampering broad comparison with The continental crust is generally assumed to be created in island arcs (Rudnick, 1995) and agreement exists that the most significant factor in increasing the volume of the continental crust since the Archaean has been the accretion of island arcs along convergent plate boundaries (Müntener et al., 2001). The arc origin of continental crust has been challenged by the observation that continental crust is andesitic in composition (e.g. Rudnick and Gao, 2003) whereas the exposed island arc sections have a basaltic bulk composition (e.g. DeBari and Sleep, 1991). Advances in the geology of the orogenic belts of various ages and Modern oceanic island arcs support the idea that the continental crust is largely created by island arc magmatism (Taira et al., 1998; Leat et al., 2006; Takahashi et al., 2007; Kodaira et al., 2007). Based on a study of the Northern Izu-Bonin arc crust, Taira et al. (1998) proposed that the middle crust of Izu-Bonin arc is plutonic and felsic and has tonalitic composition. They estimated the overall SiO 2 content of the whole arc crust and upper-middle crust at 54% and 60%, respectively, which is comparable to the average continental crust (e.g. Taylor and McLennan, 1985). Intermediate to felsic middle crust is also recorded in the Mariana intra-oceanic island arc (Takahashi et al., 2007). Evidence for the presence of felsic middle crust in other oceanic island arcs such as the Solomons, Aleutians and South Sandwich exists implying that such layer is a common feature of the oceanic island arcs. Even the thinnest arc crust ( 10 km) of Bonin intra-oceanic island arc has felsic to intermediate middle crust (Kodaira et al., 2007). As to the survival of the created arc crust, Taira et al. (1998) found that 70% of the arc crust survived and was added to the overriding plate, which led them to propose that the role of arc magmatic additions to continental crust formation has been underestimated both compositionally and volumetrically. The oceanic island arc granitoids of the Wadi Ranga area are essentially tonalitic in composition, similar to the middle felsic crust of Modern oceanic island arcs such as Izu-Bonin, Mariana, Tonga-Kermadec and South Sandwich (Taira et al., 1998; Leat et al., 2006; Takahashi et al., 2007; Kodaira et al., 2007). The high SiO 2 contents (up to 74.5%) of the Wadi Ranga granitioids and their geochemical characteristics, which indicate generation through island