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Precambrian Research 224 (2013) 397– 411

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Precambrian Research

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eochemistry of gabbros and granitoids (M- and I-types) from the Nubian Shieldf Egypt: Roots of Neoproterozoic intra-oceanic island arc

yman E. Mauricea,∗, Bottros R. Bakhita, Fawzy F. Bastab, Ali A. Khiamyc

Geology Department, Faculty of Science, Beni-Suef University, 62517 Beni-Suef, EgyptGeology Department, Faculty of Science, Cairo University, 12613 Giza, EgyptAlexander Nubia Inc., Cairo, Egypt

r t i c l e i n f o

rticle history:eceived 28 April 2012eceived in revised form 8 October 2012ccepted 11 October 2012vailable online 23 October 2012

eywords:ceanic island arcs- and I-type granites

ontinental crustrabian-Nubian Shieldastern Desert

a b s t r a c t

The Neoproterozoic intrusive rocks of the Wadi Ranga area, Nubian Shield of Egypt, comprise gabbrosand granitoids emplaced during oceanic island arc and post-collision stages. The plutonic rocks of theisland arc stage include hornblende gabbros (Dabbah pluton), trondhjemite (Abu Ghalaga pluton) andtonalites with subordinate quartz gabbro and quartz diorite (Reidi and Abu Ghusun plutons), whereas thepost-collision intrusives include granodiorite and monzogranite (Helifi-Hamata pluton). The gabbros andgranitoids of the island arc stage are largely calcic, low-K rocks which have either tholeiitic (gabbro andtrondhjemite) or transitional tholeiitic to calcalkaline nature (tonalites). On the other hand, the granitoidsof the post-collision stage are medium to high-K calcalkaline rocks. All the investigated granitoids aremetaluminous. The spider diagrams, with enrichment in LILE and strong Nb depletion, and the almostflat to slightly LREE-depleted REE patterns of the gabbro and trondhjemite are similar to those of theWadi Ranga low-K tholeiitic basalts and silicic volcanics, respectively, suggesting that the gabbro andtrondhjemite are the plutonic equivalents of the Wadi Ranga immature island arc extrusives, and theywere 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 = 1.71–5.54). The derivationof 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 tonalitesduring a late immature island arc stage. The post-collision granitoids show considerable enrichment inLILE and to a lesser extent in HFSE, slight negative Nb anomaly and strong negative P and Ti anomaliesrelative 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 arccrust and subsequent fractional crystallization. The geochemical characteristics of the trondhjemite andtonalites, 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 K2O, Rb andLREE compared to the M-type calcalkaline intrusives of the North Eastern Desert, implying northwardlydipping subduction zone.

The geochemical similarities between the intrusives of Neoproterozoic and Phanerozoic oceanic islandarcs imply that they share similar style of subduction, which differs from that of the Archaean. The gener-

ation of high SiO2 (up to 74.5 wt%), low K2O (0.56–1.78 wt%) and slightly LREE-depleted trondhjemite inearly immature oceanic island arc setting supports the arc origin of the primitive continental crust. Silicicmagma production through partial melting of the early arc volcanic rocks during the evolution of the arcand the post-collision stage, drives the middle and upper oceanic arc crust towards a composition closerto that of the continental crust. The present study indicates that the intra-oceanic island arcs continuedto play a role in the generation of the continental crust after the Archaean.

∗ Corresponding author.E-mail address: Ayman.Maurice@yahoo.com (A.E. Maurice).

301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.precamres.2012.10.012

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Almost all studies on the characteristics and origin of igneousrocks in oceanic arcs were focused on volcanic rocks simply becausethe exposed oceanic island arc rocks are mostly volcanics. Exposedroots of oceanic island arcs provide an opportunity to understand

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he processes occurring at depth and reveal the characteristics andrigin of intrusive rocks of oceanic island arcs (Kesler et al., 1977;erfit et al., 1980; Whalen, 1985; Kawate and Arima, 1998; Saitot al., 2004; Jagoutz et al., 2009), which are assumed to be the set-ing where continental crust have been created (Rudnick, 1995).ntrusive rocks of oceanic island arcs comprise gabbros and gran-toids. Understanding the formation of granitoids in oceanic arcseads to further understanding of the continental crust formatione.g. Jagoutz et al., 2009).

The Neoproterozoic basement rocks in Egypt are part of therabian-Nubian Shield and are exposed mainly in the Easternesert and the Sinai Peninsula. The juvenile crust in the Easternesert includes four main lithologic units: volcano-sedimentary

uccessions, ophiolites, gneissic core complexes and granitoidntrusions. Granitoids constitute more than half of the Egyptianasement complex. They can, in general, be classified into older andounger granitoids (El-Ramly, 1972) based on their compositionnd age. The older granitoids comprise trondhjemites, tonalites,ranodiorites and rarely granites whereas the younger granitoidsre predominated by granites. The older granitoids formed dur-ng convergence and collision, whereas the younger granitoids

ere emplaced during the post-collision and anorogenic stagese.g. Moghazi, 2002; El-Sayed et al., 2002; Farahat et al., 2007;

oussa et al., 2008; Mohamed and El-Sayed, 2008). The ages ofhe syn- to late-orogenic (older) granitoids vary between 880 and10 Ma, whereas the post- to anorogenic (younger) granitoids weremplaced between 600 and 475 Ma (Bentor, 1985).

Previous studies on the Wadi Ranga arc gabbros and granitoidsocused on field relations and petrographic descriptions. Abu El-us (1991) gave a comprehensive description of the gabbroic andranitoid plutons and classified them petrographically into quartznd hornblende metagabbros, and tonalites. Akaad et al. (1996)escribed the gabbros and granitoids of the Wadi Ranga area asetagabbros and tonalites with subordinate quartz diorite, respec-

ively. They concluded that the metagabbros represent syn-tectonicntrusions while the granitoids are syn-tectonic to late-tectonicntrusions. On Hamata Quadrangle geological map (1997), preparedy the Egyptian Geological Survey and Mining Authority (EGSMA)

n collaboration with the British Geological Survey (BGS), the Wadianga garnitoids are identified as syn- to late-tectonic tonalite andranodiorite, whereas the gabbros are mapped as late- to post-ectonic mafic intrusions.

The granitoids of the oceanic arcs are important in showing thatntermediate granitic rocks can be generated without inventionf continental crust (Pitcher, 1993). Consequently, the origin andvolution of island arc crust is crucial in understanding the gene-is of the continental crust. The plutonic part of the oceanic islandrc rocks of the Wadi Ranga area, Eastern Desert, Egypt, offers anxcellent opportunity to investigate the origin of Neoproterozoicceanic island arc plutonism, which, along with coeval volcanism,ontributed to the creation of Neoproterozoic continental crust.oreover, the variation of the origin of the arc building plutonismith arc evolution can be evaluated and the relation to the Phanero-

oic oceanic island arc and the Neoproterozoic arc plutonism cane investigated.

. Geological setting

The Precambrian rocks of the Wadi Ranga area (Fig. 1) aressentially a Pan-African assemblage comprising a metavolcanicroup, mafic plutonites (the Dabbah arc gabbro and the Abu Gha-

aga within-plate gabbros), granitoids, and molasse clastic rocksHammamat sediments). The Wadi Ranga primitive oceanic islandrc metavolcanic rocks are mafic and felsic lavas and pyroclasticocks (Maurice et al., 2012). The mafic volcanic rocks cover about

search 224 (2013) 397– 411

200 km2 and crop out as two large belts, in the central and southernparts of the study area, and comprise massive and pillow lavas andagglomerates. The felsic volcanic rocks crop out in the eastern partof the study area, covering about 50 km2 and consist essentially ofporphyritic dacite–rhyolite lava flows, lapilli and crystal tuffs andagglomerates.

The arc intrusive rocks include five gabbro and granitoid plu-tons (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 thenorthwestern part of the mapped area, covering an area of about7 km2. Generally, the gabbro is massive, medium to coarse-grainedand greenish black in colour. The Dabbah gabbro pluton is intrudedfrom the north by the Abu Ghusun granitoids with the formationof a hybrid zone of dioritic composition along the contacts. Thelow to medium relief Abu Ghusun pluton (120 km2) occupies thenorthern part of the mapped area and extends about 25 km north-ward outside the study area. The rocks of the Abu Ghusun plutonare essentially tonalites, which are light grey in colour, mediumto coarse grained and generally massive. Abu El-Rus (1991) notedthe frequent occurrence of enclaves in the Abu Ghusun pluton.The Reidi pluton (250 km2) crops out in the southeastern part ofthe study area and extends about 21 km southward outside themapped area. The rocks of this pluton enclose big metavolcanicxenoliths which are occasionally deformed especially along WadiKhashir. Vast areas of the pluton are extensively weathered forminga wide plain. The granitoids of this pluton are essentially mediumto coarse-grained tonalites. More mafic compositions (diorite andrarely gabbro) are present in the northeastern extremity of thepluton. The Abu Ghalaga trondhjemite pluton (8 km2) is the small-est among the granitoids plutons, and exposed along the easternside of Wadi Abu Ghalaga. The rocks of this pluton enclose a fewblack xenoliths of metavolcanic rocks. The Helifi-Hamata pluton(118 km2) occupies the southwestern part of the study area andextends outside the mapped area. This pluton varies in composi-tion from granodiorite in the north to granite in the south. Thecontact between the granodiorite and granite is gradational andcan be traced along the northern flank of Wadi Helifi. The rocks aremedium- to coarse-grained and massive.

Based on the field observations and the geochemical investiga-tion (see below), the granitoids of Abu Ghusun and Reidi plutonswere likely emplaced between the emplacement of Dabbah gabbroand Abu Ghalaga trondhjemite in one hand and the Helifi-Hamatagranitoids 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 fromhornblende gabbro to quartz gabbro. These rocks are commonlycoarse-grained, hypidiomorphic granular and sometimes exhibitadcumulate texture. These rocks consist mainly of plagioclase andbrown hornblende with minor amounts of opaques, biotite, quartzand apatite. Green hornblende, tremolite, actinolite, epidote, chlo-rite and kaolinite are secondary minerals. Plagioclase is extensivelyto completely kaolinitized and rarely altered to epidote. Coarseanhedral brown hornblende contains laths of plagioclase (cumu-lus). The brown hornblende alters to green hornblende, actinoliteand chlorite. Some hornblende crystals are altered to biotite by

magmatic reactions or contain tremolite cores suggesting formerpresence of pyroxene. Opaques form up to 10%, occurring essen-tially as equant grains enclosed in brown hornblende. Anhedralquartz grains occupy the interstitial spaces between plagioclase

A.E. Maurice et al. / Precambrian Research 224 (2013) 397– 411 399

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nd hornblende and in the quartz hornblende gabbro, quartz con-titutes more than 5% of the mode.

Trondhjemite is the main rock in the Abu Ghalaga pluton ands most probably genetically related to the Dabbah gabbros. Its coarse-grained, hypidiomorphic granular, composed of plagio-lase, quartz and hornblende ± k-feldspar. Opaques, apatite andpidote are accessories. Plagioclase occurs as big, subhedral, freshwinned stout prisms and equant crystals that are occasionallyoned. Quartz is present as anhedral simple or complex grains, witherrated boundaries, occupying the interstitial spaces between pla-ioclase, sometimes shows undulose extinction. Medium to smallnhedral grains and prisms of hornblende are strongly pleochroicrom yellow to brownish green to green. Hornblende is rarelyltered to aggregates of biotite flakes. In some samples, few bignhedral orthoclase and microcline grains are engulfing plagio-lase. Anhedral opaques (less than 1%) are enclosed in plagioclase,ornblende and biotite.

The tonalite association comprises the Reidi and Abu Ghusun plu-ons. It is composed mainly of tonalite with subordinate quartzabbro and quartz diorite. The tonalite (Reidi and Abu Ghusun

lutons) is coarse-grained, hypidiomorphic granular, composed oflagioclase, quartz and hornblende. Opaques, biotite and apatitere accessories. The big subhedral to anhedral plagioclase prisms

adi Ranga area. Modified after Maurice et al. (2012).

are slightly kaolinitized and mostly twinned (albite law). Someplagioclases are zoned. Quartz, constituting more than 10% of therock, is present as anhedral interstitial complex grains corrodingplagioclase. Green hornblende occurs as big subhedral prisms andanhedral grains, which occasionally enclose small plagioclase crys-tals. Some of the hornblende crystals are altered to chlorite andopaques or to biotite and epidote. Small anhedral biotite crystalsare associated with hornblende; they are pleochroic from pale yel-lowish brown to dark brown. The opaques constitute up to 5% ofthe rock, and occur as equant grains enclosed in plagioclase andhornblende or as interstitial anhedral grains.

The quartz gabbro (Reidi pluton) is coarse-grained hypidiomor-phic granular, composed of plagioclase, brown hornblende, quartzand opaques. Apatite is accessory whereas chlorite, sericite andkaolinite are secondary minerals. The plagioclase occurs as biganhedral to subhedral twinned prisms as well as small prisms(cumulus phase) enclosed in hornblende, and shows variousdegrees of alteration to sericite and kaolinite. Hornblende is presentas big subhedral prisms and cross-sections, which are stronglypleochroic 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 anhedralgrains enclosed in hornblende or interstitial between hornblende

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nd plagioclase. Quartz occupies the interstitial spaces betweenlagioclase crystals.

The quartz diorite (Reidi pluton) is medium- to coarse-grainedypidiomorphic granular, composed of plagioclase, green to brownornblende and quartz. Opaques, biotite and apatite are acces-ories. Chlorite, kaolinite, sericite and epidote are secondaryinerals. The plagioclase is largely altered to kaolinite and sericite.

oning and twinning (lamellar, pericline and simple) are still dis-layed by some crystals. The hornblende prisms and cross-sectionsccupy the interstitial spaces between the plagioclase crystals andre altered to chlorite, biotite and epidote. Quartz (about 5%) isresent as interstitial anhedral grains. Anhedral grains of opaqueinerals are enclosed in hornblende and rarely in plagioclase.The granodiorite (Heilifi-Hamata pluton) is coarse grained,

ypidiomorphic to allotriomorphic granular, composed of plagio-lase, quartz, biotite and hornblende. Biotite and hornblende are ofearly equal amount. Opaques are accessories. Plagioclase occurss big subhedral crystals as well as medium euhedral to subhe-ral crystals, which are mostly fresh or slightly kaolinitized. Somef the plagioclase crystals are zoned, with broadly altered coresnd fresh rims. Quartz is interstitial to plagioclase in the formf big anhedral complex grains, which sometimes show undula-ory extinction. Big anhedral hornblende prisms and cross-sectionsre strongly pleochroic (yellow to pale green to dark green) andorroded by biotite and occasionally altered to chlorite. Biotite islosely associated with hornblende as medium subhedral prismshich are altered to chlorite along cleavage planes with release

f iron oxides. Equant anhedral grains of opaques are enclosed inlagioclase, hornblende and biotite.

The granites (Heilifi-Hamata pluton) are of two types:ornblende- and biotite-granites. The hornblende granite is coarserained, allotriomorphic granular, composed of K-feldspar, plagio-lase, quartz and hornblende. Opaques and epidote are accessories.lagioclase occurs as big subhedral prisms, fresh or slightly kaolin-tized, sometimes zoned, occasionally corroded and encrusted by

icrocline. K-feldspar is represented by microcline and orthoclaseerthite. Microcline is present as small anhedral crystals corrodedy quartz and rarely hosts albite forming vein-type perthite. Ortho-lase perthite occurs as few equant crystals enclosing and corrodinglagioclase. Quartz is present as big anhedral interstitial grainsorroding the feldspars. Few big anhedral strongly pleochroic horn-lende prisms (brownish yellow to dark green to dark greenishrown) occupy the interstitial spaces between and corrode plagio-lase crystals. Few anhedral grains of opaques are closely associatedith hornblende or rarely enclosed in plagioclase. The biotite gran-

te is similar in composition to hornblende granite with presencef biotite instead of hornblende.

. Analytical techniques

Whole-rock XRF and ICP-MS analyses were performed at thenstitute of Geochemistry and Petrology, ETH-Zurich, Switzerland.

ajor element compositions were determined using a wave-lengthispersive X-ray fluorescence spectrometer (WD-XRF, Axios, PAN-lytical). Trace and rare earth elements were analyzed using laserblation inductively coupled plasma mass spectrometry (LA-ICP-S). The details of analytical procedures are similar to those

escribed by Basta et al. (2011) and Maurice et al. (2012).

. Geochemistry

The results of chemical analyses for 18 samples from the Wadianga gabbros and granitoids are given in Appendix A. These

nclude 3 samples from the Dabbah pluton, 4 samples from the Reidi

search 224 (2013) 397– 411

pluton, 2 samples from the Abu Ghusun pluton, 3 samples from theAbu Ghalaga pluton and 6 samples from the Helifi-Hamata pluton.

5.1. Classification

Following the Q′-ANOR classification scheme of Streckeisen andLe Maitre (1979), the Dabbah samples plot in the gabbro and quartzgabbro fields, the Reidi and Abu Ghusun samples in the quartz gab-bro and tonalite fields, the Abu Ghalaga samples in the tonalite andgranodiorite fields, whereas the Helifi-Hamata samples fall in thegranodiorite and monzogranite fields (Fig. 2A). In the normativeAb-An-Or plot (Fig. 2B), the granitoids of the Reidi and Abu Ghusunplot in the field of tonalite, whereas those of Abu Ghalaga andHelifi-Hamata fall in the trondhjemite field, and the granodioriteand granite fields, respectively.

The Dabbah gabbro and the Reidi, Abu Ghusun and Abu Gha-laga granitoids are classified largely as low-K rocks, while theHelifi-Hamata granitoids belong to medium- to high-K granitoids(Fig. 2C). The modified alkali-lime index (MALI) vs. SiO2 diagram(Fig. 2D) of Frost et al. (2001), reveals that the Dabbah gabbro andthe Reidi, Abu Ghusun and Abu Ghalaga granitoids are largely calcic,whereas the granitoids of Helifi-Hamata pluton range from cal-cic to calc-alkalic. On the alkali-silica diagram (Fig. 2E) of Kuno(1969), the Dabbah gabbro and the Reidi and Abu Ghusun gran-itoids are tholeiitic to calc-alkaline, the Abu Ghalaga trondhjemitesare tholeiitic and the Helifi-Hamata granitoids are calc-alkali. Thetholeiitic nature of the Abu Ghalaga trondhjemite and Dabbah gab-bro, the transitional tholeiitic to calc-alkaline character of the Reidiand Abu Ghusun granitoids and the calc-alkaline affinity of theHelifi-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 Al2O3/(CaO + Na2O + K2O)] varies between 0.80and 1.02 for all the investigated granitoid rocks reflecting theirmetaluminous character.

5.2. Spider diagrams and REE patterns

N-MORB-normalized (normalization values after Pearce, 1983)spider diagram of the Dabbah gabbro is characterized by slightenrichment in LILE, almost flat HFSE which are depleted relative toN-MORB and a strongly negative Nb anomaly (Fig. 3A). This patternis similar to that of the Wadi Ranga low-K tholeiitic basalts (Mauriceet al., 2012). The chondrite-normalized REE patterns of the Dab-bah gabbro (Fig. 3B) are almost flat (La/Yb = 0.99–1.70). Europiumdisplays either a weak or strong positive anomaly. Except for thestrong Eu anomaly (due to plagioclase accumulation) and the rel-atively higher La/Yb values, these patterns are similar to the REEpatterns of Wadi Ranga low-K tholeiitic basalts (La/Yb = 0.49–1.03,Maurice et al., 2012).

The Abu Ghalaga trondhjemite has MORB-normalized patterns(Fig. 3C) which display enrichment in LILE, pronounced negativeNb, P and Ti anomalies and flat HFSE which have values moreor less similar to those of N-MORB. This pattern is largely simi-lar to the MORB-normalized pattern of Wadi Ranga felsic volcanicrocks (Maurice et al., 2012). The Abu Ghalaga trondhjemite REE pat-terns (Fig. 3D) are slightly LREE-depleted (La/Yb = 0.65–0.89) withEu anomalies that range from slightly positive to strongly negative.It is worth noting that the sample with the largest LREE-depletion isthe one displaying the small positive Eu anomaly. The REE patternsof trondhjemite are similar to those of Wadi Ranga felsic volcanicrocks (La/Yb = 0.58–1.34, Maurice et al., 2012), but the former hashigher REE abundances.

The spider diagrams of Reidi and Abu Ghusun granitoids (Fig. 4Aand C, respectively) exhibit enrichment in LILE and pronounced Nbtroughs. The high field strength elements Ce, P, Zr, Hf and Sm arealmost flat with values similar to N-MORB normalization values,

A.E. Maurice et al. / Precambrian Research 224 (2013) 397– 411 401

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-Ornormative classification of silicic plutonic rocks (after Barker, 1979), fields of the Phanerozoic oceanic island arc intrusive rocks based on data from Whalen (1985), Kawateand Arima (1998), Saito et al. (2004) and Perfit et al. (1980); (C) SiO2 vs. K2O diagram (after Le Maitre, 2002), WRMV and WRFV are fields of Wadi Ranga oceanic island arcmafic and felsic volcanics (Maurice et al., 2012), respectively; (D) Na2O + K2O–CaO vs. SiO2 diagram (after Frost et al., 2001); (E) Na2O + K2O vs. SiO2 diagram (after Kuno,1969); (F) Zr vs. Y diagram (after Barrett and MacLean, 1994).

402 A.E. Maurice et al. / Precambrian Research 224 (2013) 397– 411

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ig. 3. MORB-normalized spider diagrams and chondrite-normalized REE patterns

iagrams and REE patterns of the island arc mafic and felsic volcanic rocks of Wadiearce (1983) and Sun and McDonough (1989), respectively.

hile the normalized values of Ti, Y and Yb are lower than one andenerally decrease from Ti through Y to Yb in the Reidi granitoids.he Reidi and Abu Ghusun granitoids display REE patterns (Fig. 4Bnd D), which are LREE-enriched (La/Yb = 1.71–5.54) and almostithout Eu anomalies.

The MORB-normalized patterns of Helifi-Hamata granitoidsFig. 4E) show considerable enrichment in K, Rb, Ba and Th andesser degree of enrichment in HFSE, except Y and Yb which haveormalized values slightly higher or lower than N-MORB normal-

zation values. These patterns lack the pronounced Nb anomaly,hich characterizes the patterns of gabbro and other granitoids, but

hey display strongly negative P and Ti anomalies. The REE patternsf Helifi-Hamata granitoids (Fig. 4F) display strong enrichment inREE relative to HREE (La/Yb = 5–19.30), mostly with small negativeu anomaly, and the LREE are more fractionated than HREE.

. Discussion

.1. Tectonic setting

The Dabbah gabbro and the Reidi, Abu Ghusun and Abu Ghalagaranitoids are low-K rocks enriched in LILE and show pronouncedb depletion on spider diagrams. Magmas with these charac-

ers are generally believed to be generated in subduction-related

Dabbah gabbros (A and B) and the Abu Ghalaga trondhjemite (C and D). The spider (Maurice et al., 2012) are shown. MORB and chondrite normalization values after

environments. The Helifi-Hamata granitoids, on the other hand, aremedium- to high-K rocks which have calc-alkaline affinity simi-lar to magmas generated in convergent-margin settings (Wilson,1989), but lack the pronounced Nb anomaly characteristic ofsubduction-related magmas. Consequently, we believe that theserocks were formed during or shortly after arc collision.

Applying the Nb–SiO2 tectonic discrimination diagram of Pearceand Gale (1977), the Dabbah gabbro and the Reidi, Abu Ghusun andAbu Ghalaga granitoids occupy the field of volcanic arc magmas,whereas the Helifi-Hamata granitoids plot in the area of overlapbetween volcanic arc and within-plate magma fields (Fig. 5A). Inthe Rb vs. Y + Nb tectonic discrimination diagram (Fig. 5B) of Pearce(1996) the granitoids of the Reidi, Abu Ghusun and Abu Ghalagaplutons largely plot in the volcanic arc granite (VAG) field, whereasthe Helifi-Hamata granitoids plot in the field of post-collision gran-ites (post-COLG).

6.2. Arc maturity

The low K2O and Rb contents of the Reidi, Abu Ghusun and Abu

Ghalaga granitoids along with their tholeiitic (Abu Ghalaga plu-ton) and transitional tholeiitic-calcalkaline (Reidi and Abu Ghusunplutons) nature indicate that these rocks were generated duringthe early stages of subduction, i.e. an immature island arc. On the

A.E. Maurice et al. / Precambrian Research 224 (2013) 397– 411 403

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ig. 4. MORB-normalized spider diagrams and chondrite-normalized REE patternranodiorite-granites (E and F). MORB and chondrite normalization values after Pea

ther hand, the relatively high K2O and Rb contents of the Helifi-amata granitoids and their calcalkaline nature suggest that these

ocks were generated in a thicker crust, probably shortly after theollision stage.

Using the La/Yb vs. Th/Yb diagram (Fig. 6A) the Dabbah gabbrond the Reidi, Abu Ghusun and Abu Ghalaga granitoids plot within

the Reidi (A and B) and Abu Ghusun (C and D) tonalites, and the Helifi-Hamata983) and Sun and McDonough (1989), respectively.

or near the fields of primitive island arc and island arc, whereasthe Helifi-Hamata granitoids plot in the fields of island arc and

continental margin arc. Brown et al. (1984) classified the granitoid-bearing arcs into: (1) primitive island and continental arcs, broadlythe M-type category of calcic, metaluminous granitoids; (2)normal continental arcs, the I-type calcalkaline metaluminous

404 A.E. Maurice et al. / Precambrian Research 224 (2013) 397– 411

Fig. 5. Tectonomagmatic diagrams for the intrusive rocks of Wadi Ranga area. (A) Nbvs. SiO2 diagram (after Pearce and Gale, 1977); (B) Rb vs. Y + Nb diagram (after Pearceet al., 1984), VAG, volcanic arc granite, ORG, ocean ridge granite, WPG, within-plategranite, syn-COLG, syn-collision granite, post-COLG, post-collision granite. The fieldof post-collision granites after Pearce (1996). The fields of oceanic island arc mafica

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Fig. 6. Tectonomagmatic diagrams to deduce the type and maturity of the magmaticarc that produced the intrusive rocks of the Wadi Ranga area. (A) La/Yb vs. Th/Ybdiagram (after Condie, 1989); (B) Rb/Zr vs. Nb diagram (after Brown et al., 1984), the

Tonalitic and/or trondhjemitic magmas could be formed by (1)

nd felsic volcanics of the Wadi Ranga are based on data from Maurice et al. (2012).

o peraluminous suites; (3) mature continental arcs, which arelkali-calcic and peraluminous, and often termed S-type. Theetaluminous and largely calcic nature of the Reidi, Abu Ghusun

nd Abu Ghalaga granitoids suggest that they belong to primitivesland arc granitoids of Brown et al. (1984). Additionally, the Reidi,bu Ghusun and Abu Ghalaga granitoids fall in or close to the fieldf primitive island arcs and continental arcs, while the granitoidsf Helifi-Hamata pluton plot near the normal continental arc fieldn the Rb/Zr vs. Nb diagram (Fig. 6B) of Brown et al. (1984). Therimitive island arc origin of the Dabbah gabbro and the Reidi, Abuhalaga and Abu Ghusun granitoids is supported by the fact that

he gabbro and granitoids have geochemical characteristics similaro those of Phanerozoic oceanic island arcs (see below) such asew Britain and Izu-Bonin. That the granitoids plot close to theeld of Wadi Ranga felsic volcanics (Figs. 2C and 6B), which are

elieved to be formed in a primitive oceanic island arc (Mauricet al., 2012), lends credence to this conclusion.

field of the oceanic island arc felsic volcanics of Wadi Ranga is based on data fromMaurice et al. (2012).

6.3. Origin

The Dabbah gabbros are characterized by low K2O values, HFSEcontents lower than N-MORB values (Fig. 3A) and REE patternswhich are either flat or slightly LREE-enriched (Fig. 3B). Thesecharacteristics are comparable to those of the Wadi Ranga low-Ktholeiitic basalts which were produced by partial melting of ultra-depleted mantle source as a consequence of addition of subductedslab-derived fluids to an overlying mantle wedge (Maurice et al.,2012). This leads us to suggest that the Dabbah gabbros repre-sent the plutonic equivalents of the Wadi Ranga low-K tholeiites.However, the differences between the gabbros and tholeiites ofthe Wadi Ranga area (the slightly higher La/Yb, lower HREE, andvariable Eu anomaly in some gabbros) are attributed to fractionalcrystallization and/or accumulation of phases such as hornblendeand plagioclase.

fractional crystallization of mantle-derived melts, (2) partial melt-ing of hydrous basaltic arc lower crust or (3) partial melting of

ian Re

sttdttPpdt

(SLptWhviAtawwffocmcMme

twMitdcoa(i(a(tpwilAm(p1ipsplwn

A.E. Maurice et al. / Precambr

ubducted basaltic oceanic crust (Saito et al., 2004 and referencesherein). Beard (1995) proposed that the plausible mechanisms forhe genesis of low-K island arc dacites and tonalites include dehy-ration melting of amphibolitized arc crust and low-pressure (lesshan 200 MPa) fractionation of hydrous basaltic magmas or frac-ionation of relatively dry arc basaltic magmas at any pressure.artial melting origin is favoured for tonalitic plutons emplaced atressures ≥200 MPa, whereas fractionation origin is favoured foracitic magmas having very low concentrations of incompatiblerace elements (Beard, 1995).

The trondhjemites of the Abu Ghalaga pluton have low K2O0.56–1.78 wt%) and Rb (5–25 ppm) contents at comparatively highiO2 contents (73–74.5 wt%), spider diagrams with enrichment inILE and HFSE values similar to those of N-MORB (Fig. 3C) and REEatterns which are slightly LREE-depleted (Fig. 3D). These charac-eristics are comparable to those of the low-K dacite and rhyolite of

adi Ranga. Accordingly, we suggest that the Abu Ghalaga trond-jemites are the plutonic equivalents of the Wadi Ranga silicicolcanics, which were produced by fractionation of low-K tholei-tic basalts (Maurice et al., 2012). This is supported by the fact thatbu Ghalaga is the smallest pluton among the other plutons in

he study area. The P and Ti troughs are consistent with fraction-tion of apatite and Fe–Ti oxides and/or hornblende. By analogyith the mafic and silicic volcanic rocks of the Wadi Ranga area,e propose that clinoproxene, amphibole as well as plagioclase

ractionation was responsible for the derivation of trondhjemiterom its parental mafic magma. This is supported by the presencef clinopyroxene and plagioclase phenocrysts in the mafic vol-anic rocks and hornblende in the Dabbah gabbros. The relativelyore LREE-depleted patterns of trondhjemites (La/Yb = 0.65–0.89)

ompared to the silicic volcanics (La/Yb = 0.58–1.34; average = 1.01,aurice et al., 2012) of Wadi Ranga may be attributed to loss ofinor fluid phase or fractionation of minor phases, which prefer-

ntially scavenge LREE.The tonalites of the Reidi and Abu Ghusun plutons are charac-

erized by low K2O (0.65–1.41 wt%) and Rb (10–27 ppm) contents,hich are comparable to those of Abu Ghalaga trondhjemite. TheirORB-normalized spider diagrams (Fig. 4A and C) are enriched

n LILE and have HFSE which are either close to or lower thanhe N-MORB normalization values, suggesting derivation fromepleted mantle source impregnated with LILE through subductionomponent. In contrast with the slightly LREE-depleted patternsf the Abu Ghalaga trondhjemite, the REE patterns of the Reidind Abu Ghusun granitoids (Fig. 4B and D) are LREE-enrichedLa/Yb = 1.71–5.54) and almost without Eu anomalies, suggest-ng that the derivation of the Reidi and Abu Ghusun granitoidswith lower SiO2 contents, 54.25–66.49 wt%) through fraction-tion of the same magma produced the Abu Ghalaga trondhjemiteSiO2 = 73–74.5 wt%) seems unlikely. Therefore, we propose thathe parental magmas of the Reidi and Abu Ghusun plutons wereroduced through partial melting of the arc lower basaltic crust,hich has a composition more or less similar to the low-K tholei-

tic basalts of Wadi Ranga. Their origin through partial melting ofower arc crust is supported by the relatively large size of Reidi andbu Ghusun plutons compared to Abu Ghalaga pluton. The mag-as of low-K tonalitic rocks are generated by significant degrees

20–50%) of partial melting of amphibolite of low-K tholeiitic com-osition as indicated by experimental work (e.g. Beard and Lofgren,991; Rapp and Watson, 1995; Nakajima and Arima, 1998). In an

ntra-oceanic setting, intermediate rocks (>62 wt% SiO2) could beroduced by dehydration melting of low-K amphibolite at pres-ure below 700 MPa (Beard, 1995). The granitic and tonalitic melts

roduced by low-pressure dehydration melting of low-K amphibo-

ites have Al2O3 contents similar to those of island arc granitoidshereas low-pressure hydration melting produce melts with sig-ificantly higher Al2O3 contents (Beard and Lofgren, 1991). The

search 224 (2013) 397– 411 405

crust thickness of the primitive intra-oceanic island arc that pro-duced the Wadi Ranga rocks was estimated to be about 8 km(Maurice et al., 2012), which means that the maximum pressurewithin the lower crust of such arc was about 3 kbar (maximumpressures within the crust of most island arcs and ocean basinsare of the order of 4–8 kbar, Gill, 1981 in Beard and Lofgren, 1991).Accordingly, we propose that the parental magmas of the Reidi andAbu Ghusun low-K tonalites (SiO2 mostly ≥62 wt%) were gener-ated through substantial degrees of partial melting of a juvenileamphibolitized arc crust, with composition similar to the low-Ktholeiitic basalt of the Wadi Ranga area, at low pressure. Under-plating of the amphibolitized lower arc crust with hot basalticmagmas can provide the heat source required for partial meltingof such mafic crust (Petford and Gallagher, 2001) and generation oftonalitic melt.

The Helifi-Hamata granitoids have higher K2O (2.48–3.98 wt%),Rb (45–123 ppm) and Zr (mostly 140–474 ppm) compared withthe arc granitoids of the Wadi Ranga area (K2O = 0.56–1.78 wt%;Rb = 5–27 ppm; Zr = 42–159 ppm). They are notably enriched in LILEand most of HFSE compared to N-MORB (Fig. 4E), have LREE-enriched patterns with negative Eu anomalies (Fig. 4F) and aresimilar to post-collision granites. Low and high-pressure partialmelting experiments prove that low-Al2O3 granitic melts are pro-duced by lower degrees of partial melting (compared with tonaliticmelts) of low-K mafic crust (Beard and Lofgren, 1991; Rapp et al.,1991; Rapp and Watson, 1995), and K2O contents decrease withincreasing the amount of the melt due to increase in tempera-ture (Beard and Lofgren, 1991). The geochemical characteristicsof Helifi-Hamata post-collision granitoids, including their highSiO2 contents (69–74 wt%), are consistent with derivation of theirparental magmas by lower degrees of partial melting of arc lowercrust of basaltic composition at greater depth due to increased arccrust thickness as a consequence of arc–arc collision. Partial meltingof the lower part of the overthickened island arc crust is consideredas an important process for generation of the Archaean granitoids(e.g. Nagel et al., 2012). The relatively fractionated REE patternsof the Helifi-Hamata granitoids may reflect small amount of gar-net in their source. During amphibolite melting, garnet becomesstable at pressures between 8 and 10 kbar and above (Beard,1995 and references therein). Consequently, we suggest that theparental magmas of the Helifi-Hamata granitoids were generatedat the base of at least 25 km thick crust. These parental magmaswere evolved through fractional crystallization of apatite and Fe–Tioxides and/or hornblende as indicated by P and Ti troughs in theMORB-normalized patterns. The negative Eu anomaly in REE pat-terns reflects plagioclase fractionation or/and residual plagioclasein the source.

Similar to the metabasalts of Wadi Ranga, the studied gran-itoids have low or sub-chondritic Nb/Ta ratios (mostly between3.32 and 15.09). Foley et al. (2002) proposed that during par-tial melting of low-Mg amphibolites, amphiboles with Mg# lessthan 70 can cause low Nb/Ta ratios in coexisting tonalitic-granodioritic melts. On the other hand, Xiong (2006) suggestedthat amphibole cannot impart substantial Nb–Ta fractionation totonalitic-granodioritic melts. Moreover, Rapp et al. (2003) andXiong (2006) concluded that the low or sub-chondritic Nb/Ta ratiosin tonalitic-granodioritic melts are most likely inherited from theirsource rocks, i.e. amphibolites and eclogites with sub-chondriticNb/Ta ratios. Accordingly, the sub-chondritic Nb/Ta ratios in theWadi Ranga tonalites and granites provide additional evidence thatthe Wadi Ranga low-K tholeiitic basalts (with sub-chondritic Nb/Taratios) are plausible sources for generation of their parental mag-

mas.

Several studies indicate the important role of amphibole in thedifferentiation of arc magmas (e.g. Davidson et al., 2007; Alonso-Perez et al., 2009; Kartzmann et al., 2010; Larocque and Canil, 2010;

406 A.E. Maurice et al. / Precambrian Research 224 (2013) 397– 411

Fig. 7. Zr/Sm vs. SiO2 plot to evaluate the role of hornblende fractionation in theevolution of the arc magmas produced the oceanic island arc intrusive rocks of theWadi Ranga area. See text for explanation.

Dtpstitisttacab(itthbTm

6

(P1(dse

clea

Fig. 8. (A) K2O vs. SiO2 and (B) Rb vs. SiO2 plots for the Neoproterozoic oceanic islandarc 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 calc-alkaline 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).

essimoz et al., 2012). The variation of Zr/Sm with SiO2 (Fig. 7) inhe arc intrusive rocks of the Wadi Ranga suggests that amphibolelayed an essential role in the evolution of these rocks. Althoughmall probably due to fractionation of zircon from trondhjemite,he increase of Zr/Sm from gabbro to trondhjemite and Ti troughn trondhjemite spider diagrams can be related to amphibole frac-ionation (Thirlwall et al., 1994; Jagoutz et al., 2009). Similarly, thencrease in Zr/Sm with SiO2 in the Reidi and Abu Ghusun tonalitesupports amphibole fractionation from their parental magmas. Weherefore propose that the differentiation of the magma producedhe Reidi and Abu Ghusun tonalites was largely dominated bymphibole. On the other hand, amphibole and plagioclase fractionalrystallization was the main process in the evolution of gabbrond trondhjemite as indicated by the presence of positive (gab-ro) and negative (trondhjemite) Eu anomalies in their REE patternsFig. 3B and D). The early fractionation of amphibole (poor in sil-ca) and delay of plagioclase (richer in silica) fractionation pushhe derivative liquids to silica-rich composition over a short frac-ionation interval (Jagoutz et al., 2009 and references therein). Theigh SiO2 content of the Abu Ghalaga trondhjemite can thereforee attributed to early and extensive fractionation of hornblende.his is supported by the fact that hornblende is the essential ferro-agnesian mineral in the hornblende gabbro.

.4. Typology of granitoids

The granitic rocks are classified as I-, S-, A- and M-typesChappell and White, 1974; Loiselle and Wones, 1979; White, 1979;itcher, 1983). This classification is termed alphabet soup (Eby,992), MISA notation (Cobbing, 1996) or Alphabetic classificationFrost et al., 2001). The letter classification is criticized essentiallyue to its genetic assumptions (Bonin, 2007). However, this clas-ification is frequently applied (e.g. Farahat et al., 2007; Clemenst al., 2011).

The Reidi, Abu Ghusun and Abu Ghalaga granitoids have the

haracteristics of M-type granitoids. They are metaluminous andargely calcic with low K2O and Rb contents, suggesting gen-ration in oceanic island arc setting. The REE patterns, whichre slightly depleted (the Abu Ghalaga trondhjemite) or not

significantly enriched (the Reidi and Abu Ghusun tonalities) in LREE(La/Yb = 0.65–0.89 and 1.71–5.54, respectively) are similar to theREE patterns of M-type granitoids of Modern oceanic island arcs(e.g. New Britain, Whalen, 1985; Izu-Bonin-Mariana, Kawate andArima, 1998 and Saito et al., 2004).

The Helifi-Hamata granitoids have relatively high K2O and Rbcontents and LREE-enriched REE patterns (La/Yb = 5–19.3), charac-teristics which are different from those of the oceanic island arcM-type granitoids (e.g. Saito et al., 2004). The metaluminous andcalcalkaline nature of the granitoids of the Helifi-Hamata plutonsuggest that these granitoids are akin to the I-type group. More-over, the high-K character of some samples (Fig. 2C) implies thatthese rocks belong to the post-collision I-types (Pitcher, 1983). Thisis also supported by the observation that the Helifi-Hamata gran-

itoids plot in the field of the post-collision granites in the Rb vs.Nb + Y diagram (Fig. 5B) of Pearce (1996).

A.E. Maurice et al. / Precambrian Re

Fig. 9. Chondrite-normalized REE patterns for the Neoproterozoic oceanic islandarc intrusive rocks of Wadi Ranga compared to the Phanerozoic and Neoproterozoicarc plutonic rocks. (A) Ranga gabbro compared with Phanerozoic (New Britain, IBMand 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 (Bustantrondhjemite, Arabian Shield); (C) Ranga tonalite compared with Neoproterozoicgranitoids (Bustan and Shufayyah tonalites, Arabian Shield; Um Gheig granodiorite,

search 224 (2013) 397– 411 407

6.5. Relation to Phanerozoic island arc and NeoproterozoicArabian-Nubian arc plutonism

The Dabbah, Abu Ghalaga, Reidi and Abu Ghusun intrusions havelithologic and geochemical characteristics that are broadly similarto those of plutonic rocks emplaced in Phanerozoic oceanic islandarcs. 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 trond-hjemite, respectively, on the normative Ab-An-Or plot (Fig. 2B).On the K2O vs. SiO2 and Rb vs. SiO2 diagrams (Fig. 8A and B), theDabbah gabbros and the granitoids of Abu Ghalaga, Reidi and AbuGhusun are akin to the plutonic rocks of New Britain (Whalen,1985) and IBM (Kawate and Arima, 1998; Saito et al., 2004) islandarcs, while they have lower K2O and Rb contents compared withAleutian island arc plutonic rocks (Perfit et al., 1980; Kay et al.,1983). The post-collision granitoids of Helifi-Hamata pluton hasK2O and Rb contents which are higher than those of New Britainand IBM arcs but comparable or higher than those of the Aleu-tian island arc tholeiitic and calc-alkaline plutonism. Moreover, theREE patterns of the Dabbah gabbros and the Abu Ghalaga trond-hjemite are similar to those of New Britain and IBM island arcgabbro (Fig. 9A) and granitoids (Fig. 9B), respectively, but differ-ent from those of the Aleutian island arc gabbro and granitoids,which display enrichment in LREE. These similarities are compati-ble with those established from comparing the composition of theWadi Ranga primitive island arc mafic and felsic volcanics with thelavas erupted in modern oceanic island arcs such as South Sandwichand New Britain (Maurice et al., 2012). The geochemical similari-ties between the intrusive rocks of Neoproterozoic and Phanerozoicoceanic 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 plutonicrocks is comparable to that of the Arabian Shield older fel-sic plutonic rocks, which are believed to have been emplacedwithin a Neoproterozoic oceanic island arc (Jackson et al., 1984;Jackson, 1986). Jackson (1986) classified the Arabian Shield islandarc felsic plutonic rocks into three associations: trondhjemiteassociation (diorite–tonalite–trondhjemite), tonalite association(mainly quartz diorite–tonalite) and granodiorite association(gabbro–diorite–quartz diorite–tonalite with subordinate grano-diorite and monzogranite). Based on chemical composition andanalogy with the Cenozoic oceanic island arcs, Jackson (1986)believed that the trondhjemite association was emplaced duringa late immature island arc stage, whereas the tonalite and grano-diorite associations characterize a mature island arc stage. Hesuggested that early immature island arc plutonism has not yetbeen discovered. The generally low K2O and Rb contents of theAbu Ghalaga, Reidi and Abu Ghusun granitoids suggest that theserocks are similar to the granitoids of the Arabian Shield trond-hjemite association (Fig. 10), which generally contain less than1% K2O in the most felsic rocks (72–78 wt%) of this association(Jackson, 1986). In addition, the REE patterns of the Reidi and AbuGhusun tonalites are broadly similar to those of the Arabian Shieldtonalites (Fig. 9C), which are believed to have been emplaced inolder 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 toplutonic rocks from modern and Neoproterozoic oceanic islandarcs, 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-Sayedet al. (2002).

408 A.E. Maurice et al. / Precambrian Re

Fig. 10. (A) K2O vs. SiO2 and (B) Rb vs. SiO2 plots for the Neoproterozoic oceanicisland arc intrusive rocks of Wadi Ranga and the Neoproterozoic oceanic island arcintrusive rocks of the Arabian Shield and the Neoproterozoic subduction-relatedintrusives of the Nubian Shield (Central and North Eastern Desert). Data sources:1, Ras Gharib intrusive rocks (Abdel-Rahman, 1990), North Eastern Desert; 2 and3, Kab Amiri (Moghazi, 2002) trondhjemite–tonalite and granodiorite, respectively,Central Eastern Desert; 4, El-Bula granitoids (Farahat et al., 2007), Central EasternDesert; 5, Um Gheig granitoids (El-Sayed et al., 2002), Central Eastern Desert; 6, WadiBfit

twamttsMf

Rw(

eizah intrusives (Ragab et al., 1989), Central Eastern Desert. The shaded and stripedelds are, respectively, the island arc granodiorite and trondhjemite associations ofhe Arabian Shield (Jackson et al., 1983).

rondhjemites represent early immature island arc plutonism,hereas the Reidi and Abu Ghusun tonalites were emplaced in

n older immature island arc stage. This interpretation is in har-ony with the immature island arc setting established based on

he nature of the coeval volcanic rocks (Maurice et al., 2012). Thus,he present study reports the Abu Ghalaga trondhjemite as a clas-ic example of the Neoproterozoic early island arc felsic plutonism.oreover, we rank it among the most primitive Neoproterozoic

elsic plutonism described in the Arabian-Nubian Shield.

Geochemical analyses of whole set of elements (including

EE) for the Neoproterozoic granitoids of the Eastern Desert,hich represents the northern tip of the Nubian Shield, are scarce

e.g. El-Sayed et al., 2002), hampering broad comparison with

search 224 (2013) 397– 411

the Wadi Ranga granitoids. Compared with the Neoproterozoicsubduction-related granitoids of the Eastern Desert, the island arcgranitoids of Wadi Ranga generally have different geochemicalcharacteristics, especially the Abu Ghalaga trondhjemite. ExceptKab Amiri trondhjemite (Central Eastern Desert, Moghazi, 2002),they have characteristically lower K2O and Rb contents (Fig. 10)and less LREE-enriched REE patterns (Fig. 9) than the Central andNorth 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 wereemplaced in more mature arcs. However, the general increaseof K2O, Rb and LREE in the Eastern Desert granitoids from south(Wadi Ranga) to north (Central and North Eastern Desert) can beattributed either to temporal (due to change of arc crust thickness)or spatial (due to change of distance from trench) variation in thechemistry of island arc magmas (Wilson, 1989). The early orogenicdiorites and tonalites of the North Eastern Desert (Abdel-Rahman,1990) have higher K2O and Rb contents than the immature islandarc granitoids of the Wadi Ranga area (Fig. 10) whereas the gabbrosare more LREE-enriched (Fig. 9A). These differences between earlyorogenic, and probably coeval, plutonic rocks from South andNorth Eastern Desert favour the spatial variation in the chemistryof arc magmas as a cause for these geochemical differences,however, the temporal variation cannot be ruled out due to theabsence of accurate age dates for these rocks.

6.6. Intra-oceanic island arc magmatism and generation ofcontinental crust

The continental crust is generally assumed to be created inisland arcs (Rudnick, 1995) and agreement exists that the most sig-nificant factor in increasing the volume of the continental crustsince the Archaean has been the accretion of island arcs alongconvergent plate boundaries (Müntener et al., 2001). The arc ori-gin of continental crust has been challenged by the observationthat continental crust is andesitic in composition (e.g. Rudnick andGao, 2003) whereas the exposed island arc sections have a basalticbulk composition (e.g. DeBari and Sleep, 1991). Advances in thegeology of the orogenic belts of various ages and Modern oceanicisland arcs support the idea that the continental crust is largelycreated by island arc magmatism (Taira et al., 1998; Leat et al.,2006; Takahashi et al., 2007; Kodaira et al., 2007). Based on a studyof the Northern Izu-Bonin arc crust, Taira et al. (1998) proposedthat the middle crust of Izu-Bonin arc is plutonic and felsic and hastonalitic composition. They estimated the overall SiO2 content ofthe whole arc crust and upper-middle crust at 54% and 60%, respec-tively, which is comparable to the average continental crust (e.g.Taylor and McLennan, 1985). Intermediate to felsic middle crust isalso recorded in the Mariana intra-oceanic island arc (Takahashiet al., 2007). Evidence for the presence of felsic middle crust inother oceanic island arcs such as the Solomons, Aleutians and SouthSandwich exists implying that such layer is a common feature ofthe oceanic island arcs. Even the thinnest arc crust (∼10 km) ofBonin intra-oceanic island arc has felsic to intermediate middlecrust (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 wasadded to the overriding plate, which led them to propose that therole of arc magmatic additions to continental crust formation hasbeen underestimated both compositionally and volumetrically.

The oceanic island arc granitoids of the Wadi Ranga area areessentially tonalitic in composition, similar to the middle felsiccrust 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 SiO2contents (up to 74.5%) of the Wadi Ranga granitioids and their geo-chemical characteristics, which indicate generation through island

A.E. Maurice et al. / Precambrian Re

Fig. 11. Nb/Yb vs. Th/Yb plot (Pearce, 2008) to compare the affinity of the WadiRanga granitoids (and felsic volcanic rocks) with the felsic rocks of different tectonicsaM

aRsPawtc

ctpoavRacccc

6

o(Moricgiafc(Tgccf

ettings. The fields of felsic rocks from oceanic and continental arcs, and ophiolitesfter Condie and Kröner (2011). The field of Wadi Ranga felsic volcanic rocks afteraurice et al. (2012).

rc magmatism, along with the relatively large volume of the Wadianga oceanic island arc silicic volcanic rocks (Maurice et al., 2012),upport the hypothesis of an arc origin for the continental crust.artial melting of the early arc volcanics, during the evolution of therc and the post-collision stages, produces evolved silicic magmas,hich become even more evolved through fractional crystalliza-

ion, driving the composition of the middle and upper oceanic arcrust towards that of continental crust.

Several studies suggest that the Archaean continental crust wasreated in intra-oceanic island arcs (Polat, 2012 and referencesherein). Additionally, Condie and Kröner (2011) proposed that therimary site of production of continental crust was changed fromceanic arcs and oceanic plateaus in the Archaean to continentalrcs thereafter. Plotting of the Wadi Ranga granitoids on the Th/Ybs. Nb/Yb diagram (Fig. 11) indicates that the Neoproterozoic Wadianga trondhjemites and tonalites (and felsic volcanic rocks) arekin to the felsic rocks generated in oceanic arcs, whereas the post-ollision granites are largely similar to felsic rocks developed inontinental arcs. We therefore propose that the intra-oceanic arcsontinued to play a role, at least partly, in the generation of theontinental crust after the Archaean.

.7. Implication for classification of the Egyptian granites

The granitoid rocks of Egypt are classified into syn- to late-rogenic and post-orogenic to anorogenic granitoid assemblagese.g. Moghazi, 2002; El-Sayed et al., 2002; Farahat et al., 2007;

oussa et al., 2008; Mohamed and El-Sayed, 2008), which wereriginally termed older and younger granitoids (El-Ramly, 1972),espectively. The syn- to late-orogenic or older granitoids rangen composition from quartz diorite to monzogranite. They arelacalkaline I-type granitoids (e.g. Farahat et al., 2007), that areenerally described as subduction-related granitoids withoutndicating whether they were emplaced in oceanic island arc orctive continental margin settings. There is increasing evidenceor the absence of contribution of pre-Neoproterozoic continentalrust to the genesis of the Egyptian granitic gneisses and granitoidse.g. Liégeois and Stern, 2010; Augland et al., 2012; Ali et al., 2012).he Sr isotope and the εNd(t) values of the Egyptian gneisses and

ranitoids do not support the involvement of pre-Neoproterozoicrust in the genesis of these rocks. Additionally, the juvenileontinental crust of the Arabian-Nubian Shield is believed to haveormed as a result of accreting intra-oceanic arcs 880–630 Ma

search 224 (2013) 397– 411 409

(e.g. Stern et al., 2010). Consequently, we cast doubt on the activecontinental margin as the setting for the emplacement of thesubduction-related granitoids of Egypt (El-Gaby et al., 1988),unless the subduction occurred under juvenile continental crust.Instead, the subduction-related granitoids of Egypt were mostlyemplaced in oceanic island arcs similar to the coeval granitoids ofthe Arabian Shield (Jackson et al., 1984; Jackson, 1986). Hence, wesuggest classifying the subduction-related granitoids of Egypt toM-type granitoids of immature oceanic arcs (represented by theAbu Ghalaga, Reidi and Abu Ghusun plutons and similar rocks),which are largely calcic and have characteristically low K2O, andI-type granitoids of mature arcs (Central and North Eastern Desertplutons), which are clacalkaline and have higher K2O.

The post-orogenic to anorogenic (younger) granitoids of Egyptare classified as I- and A-types (e.g. El-Sayed, 1998; Moghazi, 2002;El-Sayed et al., 2002; Farahat et al., 2007). The I-type younger gran-ites are clacalkaline (e.g. post-collision Helifi-Hamata granitoids),whereas the A-type granitoids are either subduction-related highlyfractionated calcalkaline or anorogenic rift-related alkaline rocks(e.g. Moghazi, 2002; El-Sayed et al., 2002; Mohamed and El-Sayed,2008).

Collectively, we propose that the granitoids of Egypt can beclassified into M-, I- and A-types. All the syn- to late-orogenic orolder granitoids belong to the M- and the I-types. The syn- to late-orogenic I-type granitoids most probably formed in oceanic islandarc setting and not in an active continental margin (Cordilleran)setting. This is supported by the fact that pre-Neoproterozoic con-tinental crust was not involved in the genesis of the Egyptiangranitoids (e.g. Ali et al., 2012). Thus, the Egyptian I-type granit-oids comprise the Eastern Desert calcalkaline ‘subduction-relatedgranitoids’, which were developed in a mature island arc setting,and the calcalkaline post-collision granitoids (e.g. Helifi-Hamatagranitoids). The Egyptian mature arc and post-collision calcalka-line I-type granitoids are discriminated by their strong and slightNb depletions, respectively, on MORB-normalized spider diagrams.The Egyptian A-type granites are represented by the highly frac-tionated post-collision and the alkaline anorogenic granitoids.

7. Conclusions

The oceanic island arc gabbros and granitoids of Wadi Rangaare largely calcic with low K2O and Rb contents. They have eithertholeiitic (gabbro and trondhjemite) or transitional tholeiitic tocalcalkaline nature (tonalites). These characteristics are similar tothose of the Phanerozoic oceanic island arc intrusives.

The spider diagrams and REE patterns of the gabbro and trond-hjemite are similar to those of the Wadi Ranga low-K tholeiiticbasalts and silicic volcanics, respectively, suggesting that the gab-bro and trondhjemite are the plutonic equivalents of the WadiRanga immature island arc extrusives, and were derived from amantle source at the early immature island arc stage. The tonalites,on the other hand, are believed to have been derived by a highdegree partial melting of juvenile basaltic arc crust during lateimmature island arc stage.

Geochemical data indicate that fractional crystallization ofamphibole and plagioclase played an important role in the evo-lution of the intrusive rocks of Wadi Ranga.

The early subduction partly tholeiitic intrusives of Wadi Ranga(South Eastern Desert) have lower K2O, Rb and LREE than the M-type early subduction calcalkaline intrusives of the North EasternDesert, implying northwardly dipping subduction zone.

The geochemical similarities between the intrusive rocks of theNeoproterozoic and Phanerozoic oceanic island arcs imply that theyshare similar style of subduction, which differs from that of theArchaean (e.g. Martin et al., 2005).

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The generation of high SiO2, low K2O and slightly LREE-depletedrondhjemite in early immature oceanic island arc setting supportshe arc origin of the primitive continental crust.

The present study indicates that the intra-oceanic island arcsontinued to play a role in the generation of the continental crustfter the Archaean.

The post-collision granodiorites–granites of Helifi-Hamata plu-on have higher LILE and more LREE-enriched patterns comparedith the arc granitoids suggesting that they formed by low degrees

f partial melting of an old arc crust.The geochemical characteristics of the trondhjemites and

onalites, and the granodiorites–monzogranites of Wadi Rangalassify them as M- and I-type granitoids, respectively. We there-ore propose to classify the granitoids of Egypt into M-, I- and-types. M-type granitoids (tholeiitic and transitional tholeiitic toalc-alkaline nature, low-K, strong Nb depletion) were developeduring an early immature intra-oceanic island arc stage, whereas

-type granitoids were emplaced during mature island arc (calc-lkaline, moderate- to high-K, Nb depletion) and post-collisiontages (calc-alkaline, moderate- to high-K, small Nb depletion).-type granites (mostly alkaline and peralkaline, occasionallyuorite-bearing, high-K, no Nb depletion) were generated in post-ollision and anorogenic settings.

Additional studies, especially Sr and Sm–Nd isotopes and U–Pbircon dating, are needed to complete the characterization of the

adi Ranga intrusive rocks, which constitute the roots of a Neo-roterozoic intra-oceanic island arc.

cknowledgments

We express our gratitude to Prof. Dr. C. Heinrich, ETH-Zurich,witzerland, for the laboratory facilities. We thank Prof. Dr. Peterlmer and Dr. Markus Wälle for help in the geochemical analyses.rof. Dr. Bernard Bonin and anonymous reviewer are acknowledgedor the thorough review and constructive criticism which improvedhe manuscript. A review by Dr. A. El-Shazly, Marshall University,s also acknowledged.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/.precamres.2012.10.012.

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