geochemical constraints from the hafaﬁt metamorphic

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Journal of African Earth Sciences 45 (2006) 173–186

Geochemical constraints from the Hafafit MetamorphicComplex (HMC): Evidence of Neoproterozoic back-arc

basin development in the central Eastern Desert of Egypt

H. Abd El-Naby a,b,*, W. Frisch b

a Nuclear Materials Authority, P.O. 530 El-Maadi, Cairo, Egyptb Institut fur Geologie und Palantologie, Universitat Tubingen, Sigwartstr. 10, D-72076 Tubingen, Germany

Received 8 June 2005; received in revised form 23 October 2005; accepted 2 February 2006Available online 31 March 2006

Abstract

The Hafafit Metamorphic Complex (HMC) is a part of the Precambrian belt in the central Eastern Desert of Egypt. Two distinctmetamorphic units were identified: gneisses and amphibolites. The gneisses are subdivided on mineralogical grounds into granitic gneiss,biotite-gneiss, hornblende-gneiss and psammitic gneiss. Using major elements discrimination criteria to discriminate between orthogneissand paragneiss, the granitic gneiss shows igneous origin, whereas biotite-gneiss, hornblende-gneiss and psammitic gneiss show sedimen-tary origin. The mineralogical and chemical compositions of the granitic gneisses indicate that they are tonalitic to trondhjemitic andhave compositions consistent with hydrous partial melting of a mafic source, suggesting subduction-related magmatism. Based on Si,Al and alkali contents of paragneisses, the psammitic gneiss could be classified as metamorphosed lithic arenite, whereas biotite- andhornblende-gneisses are classified as metamorphosed greywacke. Sedimentation may have occurred in a back-arc basin setting with tran-sitional deposition from shallow-marine to terrestrial environment. This sedimentation was probably occurred on a tholeiitic basalticoceanic crust. The amphibolites are subdivided according to mineralogical basis into clinopyroxene-amphibolite, garnet-amphiboliteand garnet-free massive amphibolite. Chemical data of amphibolites shows tholeiitic affinity, which suggests a back-arc geotectonic set-ting. A generation of the leucogranite along thrust zones is related to the late phase of metamorphism of Hafafit rocks. This interpre-tation is supported by the similarity between metamorphic age and granite emplacement age.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Hafafit; Nugrus; Geochemistry; Back-arc; Gneisses; Amphibolites

1. Introduction

The Neoproterozoic evolution of the Arabian–Nubianshield involves: (1) Formation of ophiolites by rifting ofRodinia (Abdelsalam and Stern, 1996). The ages of theophiolites range between 900 and 740 Ma (Ries et al.,1983; Kroner et al., 1992, 1994; Loizenbauer et al., 2001).(2) Development of sutures associated with arc accretionbetween 750 and 650 Ma (Abdelsalam and Stern, 1996;Blasband et al., 2000). (3) Orogenic extension and exhuma-

1464-343X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2006.02.006

* Corresponding author. Present address: King Abdulaziz University,Faculty of Earth Sciences, P.O. Box 80206, Jeddah 21589, Saudi Arabia.

E-mail address: [email protected] (H. Abd El-Naby).

tion of core complexes as constrained by dating of corecomplex exhumation (Fritz et al., 1996), late- to post-tec-tonic magmatic activity (Stern and Hedge, 1985; Hassanand Hashad, 1990) and molasse basin formation (Grothauset al., 1979; Rice et al., 1993; Willis et al., 1988). The pre-vious data suggest close association between tectonic andmagmatic activity between 620 and 580 Ma (Fritz et al.,2002).

Hafafit area represents one of three major domal struc-tures on the Eastern Desert of Egypt (Fig. 1, inset): AbuSwayel (Finger and Helmy, 1998; Abd El-Naby and Frisch,2002), Gabal Meatiq (Sturchio et al., 1983; Blasband et al.,2000; Fowler and Osman, 2001; Loizenbauer et al., 2001)and Hafafit area (El Ramly et al., 1984; Greiling et al.,

mailto:[email protected]

Fig. 1. (a) General geological map of the study area (modified from El Ramly et al., 1993). (b) Detailed geological map of dome A (after El Ramly et al.,1993).

174 H. Abd El-Naby, W. Frisch / Journal of African Earth Sciences 45 (2006) 173–186

1988; Rashwan, 1991; Fowler and El Kalioubi, 2002). Thegeology of Hafafit has long been recognized as highly com-plex (called Hafafit Core Complex), and many theorieshave been proposed for its tectonic evolution. El Ramlyet al. (1984) interpreted the Hafafit gneisses, the associatedgranitoid cores and the Wadi Ghadir melange (Fig. 1a) as aresult of Pan-African convergent and marginal ocean basinprocesses. Hassan and Hashad (1990) interpreted the gra-nitic gneisses at the core of the domes (Fig. 1) as gneissicgranitic intrusions. They named the rock assemblage abovethe core granite and below the psammitic gneiss as a meta-morphosed and deformed ophiolitic melange assemblage,

whereas the psammitic gneiss as a metamorphosed sedi-mentary unit of a quartozo-feldspathic composition.

Several models have been postulated to explain theexhumation of Hafafit Metamorphic Complex (HMC).Fritz et al. (2002) interpreted the exhumation of HMC asa result of orogen-parallel extension during convergence.Fowler and El Kalioubi (2002) interpreted the HafafitComplex as a result of fold interference patterns involvingmultiply deformed sheath folds.

Understanding the geochemical characteristic of theHafafit unit helps in recognizing its tectonic setting andreconstructing the evolution of a former plate margin. In

H. Abd El-Naby, W. Frisch / Journal of African Earth Sciences 45 (2006) 173–186 175

this paper we report the geochemical data of granitic gneis-ses, paragneisses, amphibolites and garnetiferous leucogra-nites, which is a useful guide to constrain the formertectonic setting of the Hafafit Metamorphic Complex.

2. Geological setting and petrography

Hafafit area consists of five granite-cored domes (domeA to dome E, Fig. 1a). These domes are composed of med-ium grade gneisses and are separated from the overlyinglow grade metamorphic rocks by low angle thrust zones.The rock assemblages in Hafafit area could be grouped intotwo main units which are separated by Nugrus Thrust. Theeastern unit (Nugrus unit) is composed mainly of low grademica-schists and metavolcanics. This unit is associated withremnants of ophiolitic altered ultramafic and metagabbros.The western unit (Hafafit unit) forms Hafafit domes andincludes from core to rim (Fig. 2a): granite gneiss of tona-litic and trondhjemitic composition, banded amphibolitewhich is overthrusted by ultramafic rocks, alternatingbands of biotite- and hornblende-gneiss and the psammiticgneiss at the rim of the domal structure. In some parts, theamphibolite is associated with metagabbro. Both unitshave been intruded by undeformed leucogranites, espe-cially along thrust zones.

Mica-schists are composed mainly of quartz + gar-net + muscovite + biotite + plagioclase ± chlorite ± opaque(Fig. 2b). These rocks are strongly microfolded as shown inFig. 2c. Most of these rocks are substantially altered, withsecondary chlorite replacing biotite and sericite replacingplagioclase. The metavolcanics are encountered mainly atthe northern border of HMC. They are composed mainlyof greyish green,1 fine to medium grained meta-andesiteswhich are porphyritic in some places. They are composedof hornblende and/or actinolite-tremolite and plagioclasewith variable proportions of epidote, chlorite and carbonatedepending upon the degree of alteration (El Ramly et al.,1993).

The cored-granite gneisses are moderately foliated, witha well-developed granoblastic-polygonal texture. Locally,mylonitic texture is observed in strongly deformed varie-ties. Two main types of granite gneiss occur. The predom-inant is tonalitic gneiss, consisting of plagioclase, quartz,biotite and, locally, garnet. The other type is trondhjemiticin composition. These rocks show a well-developed milli-metre-spaced gneissic banding. In some places, the tona-lites are invaded by numerous thin pegmatitic veinlets.

The amphibolites form irregular lens-shaped bodiesoverlain by altered ultramafic. At the southern part ofthe dome A, the amphibolites are overlained by psammiticgneiss of Gabal Hafafit. Based on their mineral constitu-ents, amphibolites could be classified into clinopyroxene-amphibolite, garnet-amphibolite and massive amphibolite.

1 For interpretation of color in Figs. 2–4 and 9, the reader is referred tothe web version of this article.

The clinopyroxene-amphibolite is fine-grained and com-posed of amphibole + clinopyroxene + plagioclase with lit-tle quartz and iron oxides. It shows thin alternating bands(few millimeters to one centimeter) of dark grey (amphi-bole-rich) and dark green (clinopyroxene-rich). Thestrongly foliation of clinopyroxene-amphibolites (Fig. 2d)could be related to the first deformational event, whichled to a metamorphic banding and may be synchronouswith the alternating bands of biotite- and hornblende-gneisses under amphibolite facies condition. At the latestage of this deformational event, the banding and foliationwere deformed by folding as shown in Fig. 2d. This event ispreviously described by El Ramly et al. (1993). They con-cluded that the structural framework of the Hafafit areacould be a result of four main deformational events: earlyfoliation and folding (D1), thrusting and folding (D2),regional thrusting and folding (D3) and a late phase ofgravitational deformation (D4). Details of these eventshave been discussed by El Bayoumi and Greiling (1984),Kroner et al. (1987), Greiling et al. (1988) and Rashwan(1991).

The garnet-amphibolite is abundant in Gabal Hafafit andin the area to the east of dome C. It consists of amphibole +plagioclase + garnet + quartz ± clinopyroxene + iron oxides(Fig. 2e). The garnet is coarse grained (up to 5 mm in dia-meter). The massive amphibolite is free from garnet andclinopyroxene and found as lenses within the gneisses. Itis composed mainly of amphibole + plagioclase + quartz +iron oxides. At the eastern side of Nugrus thrust, theamphibolites are associated with metagabbros. Thesemetagabbros probably pertain to the calc-alkaline meta-gabbros associating Hafafit gneisses (El Ramly et al.,1993). They are dark green in color, medium-grained andcomposed of highly saussuritized plagioclase crystals andhornblende which is altered partly to pale green actinoliteor chlorite.

The ultramafic rocks are found as small masses in thecore of the northern Hafafit dome (dome A, Fig. 1b) over-lying amphibolites. The contact between them seems to betectonic where there is no sign for intrusive contact. Theyare massive dark grey, brown or green in color and repre-sented mainly by serpentinized dunite and pyroxenite. Lessaltered samples are composed mainly of olivine in a meshtexture of serpentine, whereas highly altered samples arecomposed of serpentine, talc, chlorite, tremolite and cal-cite. The clastic sediments in this domain which gave riseto the biotite schists and psammitic gneisses were depositedin a basin associated with an active continental margin.This basin is floored by an old oceanic crust now repre-sented by the fore-mentioned ultramafic-mafic rocks(Rashwan, 1991).

The alternating bands of biotite-rich gneiss and horn-blende-rich gneiss are encountered between the amphibo-lites at the core of the domal structure and the psammiticgneiss at the rim. The biotite-gneiss is composed essentiallyof quartz, biotite, plagioclase, and zircon. Garnet isobserved in some samples. Chlorite and epidote are

Fig. 2. (a) Rock assemblages in Hafafit dome A. (b) Secondary electron image of large garnet porphyroblast in mica-schist. (c) Photomicrograph showingmicro-folded mica-schist (PPL). (d) Strongly foliated and folded clinopyroxene-amphibolites. (e) Photomicrograph showing garnet porphyroblast ingarnet-amphibolite (PPL).


occasionally present as alteration phases. The hornblende-gneiss consists mainly of hornblende, plagioclase andquartz with some opaques. The psammitic gneiss formsthe summits of the main mountain masses in the area(e.g. Gabal Hafafit and Gabal Migif). It is composed essen-tially of quartz, potash feldspar, plagioclase with minoramphibole, biotite, epidote and zircon. In Wadi Abu Rus-heid, the psammitic gneiss is mylonitized and dissected byseveral shear zones. It is highly metasomatized and reflects

high radioactive anomalies. Secondary uranium minerali-zation is found in the altered zone of the mylonitic psam-mitic gneiss. It occurs as stains along crevices andfracture surfaces and as acicular crystals filling cavities.Uranophane is the most abundant uranium mineral asdeduced from the EDX pattern (Fig. 3).

The latest Pan-African activity in the mapped area isrepresented by a suite of leucogranites and minorintrusions of felsite and aplite which intruded the Hafafit

Fig. 3. Secondary electron image showing uranophane crystal separated from Abu Rushied paragneisses and their EDX spectrum.


gneisses and the ophiolitic assemblage (El Ramly et al.,1993). Elongated narrow intrusive leucogranite is foundin the eastern part of the mapped area and parallel to theNugrus Thrust (Fig. 1a). To the other side of NugrusThrust, there is other elongated granitic belt forming GabalNugrus. Generally, these leucogranites are garnet-bearingand developed along thrust faults, e.g. garnetiferous gran-ites to the south of Nugrus Thrust, Gabal Nugrus, GabalEl-Faliq and Gabal Mudargag (to the northwest extensionof the mapped area). The leucogranites are composedmainly of quartz, orthoclase, oligoclase, garnet and biotite.Several small lens-like plutonic masses of leucogranites arefound also in Wadi Abu Rushied. They are medium- tocoarse-grained, garnet-bearing and contain abundant ofmetasedimentary xenoliths. Large K-feldspar phenocrystsare also observed in some localities. Geochronological con-straints show that the emplacement ages of the leucogra-nites are between 594 ± 12 and 610 ± 20 Ma (Moghaziet al., 2004).

3. Analytical methods

A total of thirty one representative samples were col-lected for whole-rock chemical analysis. The samples col-lected from the banded clinopyroxene amphibolitesinclude both dark grey (amphibole-rich) and dark greenbands (clinopyroxene-rich). All samples were crushed in asteel jaw crusher to 3-cm-sized pieces. Fresh pieces wereselected, cleaned and crushed again to 3-mm pieces. Theproducts of the last crushing were then pulverised in anagate mill. Loss on ignition (LOI) was determined by heat-ing powdered samples at 850 �C for 3 h. Whole rock majorand some selected trace and rare earth elements were ana-lysed using the X-ray fluorescence (XRF) technique at thelaboratories of the Mineralogical Institute, TubingenUniversity, Germany. Absolute accuracy has been assessedby comparison with international reference materials ana-lyzed along with the samples and is generally better than2%. The results of these chemical analyses are given in

Table 1. JEOL JXA-8900RL instrument at the Universityof Tubingen, Germany, was used to identify some radioac-tive mineral grains. Analytical conditions were 15-kV accel-erating voltage, 10–20-nA beam current, 1–2-lm beamdiameter and 10–20 s counting time.

4. Geochemistry

4.1. Geochemistry of paragneisses

Fig. 4a is a discrimination diagram (after Demant, 1992)to distinguish between sedimentary and magmatic originsof the gneisses. It shows that the cored granitic gneiss fallin the orthogneiss field, whereas psammitic gneiss, biotite-and hornblende gneisses fall in the paragneiss field.

Paragneiss samples are plotted in (Na + Ca)/(Na + Ca + K) versus Si/(Si + Al) (atomic proportions)in Fig. 4b which defines compositional fields for varioussedimentary rocks (Wintsch and Kvale, 1994). Most ofthe psammitic gneiss fall within the lithic arenite with somesamples plot in the arkose to subarkose fields. On the otherhand, biotite- and hornblende-gneisses fall within the fieldof greywacke. Sedimentation may have occurred in a back-arc basin setting with deposition of the greywacke in mar-ine environment. This sedimentation was probablyoccurred on a tholeiitic basaltic oceanic crust. This is indi-cated from the association of amphibolite representing oce-anic crust with biotite- and hornblende gneisses. Plotting ofmost of the psammitic gneisses in the lithic arenite fieldmay reflect deposition of materials of different sources ina fluvial to shallow-marine environment. Whereas plottingof some other psammitic gneiss samples in the arkose tosubarkose fields may indicate a marked transition fromsedimentation in shallow-marine environment to sedimen-tation in a terrestrial environment.

The depositional environment of paragneiss is con-strained from Fig. 4c (Roser and Korsch, 1986). It suggestssedimentation in an active continental margin setting, withthe exception of two samples which plot in the passive

Table 1Major, trace and some rare earth elements of the Hfafit rocks

Sample Psammitic gneiss Biotitet-gneiss Hornblende-gneiss Granitic gneiss

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16

SiO2 72.39 76.99 91.55 77.29 77.18 86.89 76.16 77.133 65.99 78.76 61.14 52.64 66.71 68.35 69.69 77.39TiO2 0.615 0.293 0.035 0.027 0.029 0.07 0.034 0.199 1.037 0.227 0.398 1.351 0.455 0.22 0.399 0.287Al2O3 11.49 11.01 4.065 11.09 11.06 5.068 11.08 10.51 12.05 10.93 14.54 18.02 17.17 17.81 15.86 11.99Fe2O3 4.834 3.239 1.224 1.261 1.229 2.149 1.606 2.348 8.28 2.541 8.416 8.854 2.734 1.653 2.191 3.429MnO 0.096 0.063 0.019 0.054 0.066 0.029 0.06 0.046 0.144 0.086 0.152 0.14 0.042 0.033 0.038 0.066MgO 0.904 0.65 0.124 0.205 0.716 0.876 0.993 1.064 2.39 1.185 3.976 4.515 1.699 0.707 1.056 0.898CaO 1.838 0.181 0.02 0.036 0.007 0.004 0.065 3.13 3.139 2.042 9.087 10.06 5.488 4.358 3.031 4.08Na2O 4.006 4.209 1.738 5.071 5.203 2.289 5.442 3.412 2.994 3.045 1.636 3.673 4.501 5.337 5.335 2.746K2O 3.662 4.111 1.082 3.517 3.652 1.783 3.442 0.933 2.697 1.42 0.592 0.348 0.862 1.061 1.302 0.108P2O5 0.131 0.023 0.011 0.041 0.025 0.01 0.034 0.04 0.391 0.02 0.076 0.168 0.13 0.075 0.116 0.034CO2 0.46 0.23 0.16 0.86 0.49 0.27 0.44 0.37 0.92 0.75 0.62 0.63 0.91 0.61 0.36 0.13Total 100.4 100.9 100 99.45 99.65 99.43 99.35 99.18 100 101 100.6 100.4 100.7 100.2 99.37 101.1

Trace elements (ppm)

Ba 898 764 3.4 68 28 22 18 267 682 525 75 123 229 254 242 35.5Co 8 1.7 1.4 9.9 11 2.4 15 1.2 15 3.1 23 33 6.9 1.4 4.4 7.6Cr 18 8.4 11 44 35 11 37 10 86 13 68 99 28 11 14 18Rb 49 45 184 661 979 314 946 18 57 29 8.3 5.1 22 24 43 2.6Sr 102 18 1.8 12 0 1.6 2.7 144 226 118 139 398 519 340 333 159V 36 8.9 1.9 8 10 0.4 14 9.9 47 4 245 196 47 17 26 45Y 159 215 15 133 90 4.1 234 23 92 39 17 25 14 12 13 0Zn 186 188 110 1075 89 204 514 17 139 46 71 74 37 22 32 12Zr 946 1197 293 5863 3294 112 4563 160 513 175 36 69 164 159 170 219Nb 37 49 150 0 173 118 217 0 21 0 0 0 0 0 0 0Pb 32 32 2.6 421 228 4.7 42 3.9 16 9.3 6.1 2.2 8.3 4.9 8 3Th 15 14 28 225 130 3.9 136 2.3 8.7 1.8 1.1 0.8 3.4 9.4 3.7 0U 2.8 1.9 1.1 55 27 2.9 31 0 1.3 0 0 0 1.9 2.8 2.2 3.6

REE (ppm)

La 77 187 31 34 41 30 42 38 50 27 53 44 52 66 55 39Ce 168 230 0 97 0 0 91 0 73 0 0 0 0 54 30 0Nd 104 207 13 59 36 3.8 60 8.7 68 13 5.1 21 9.2 30 19 5.9Sm 15 22 0 0 0 3.5 6.1 0 9.4 0 0 2.2 3 4.2 2.6 0Eu 1.4 1.7 0 0 0 0 0 0.3 1.4 0.3 0.6 1.2 1.3 1.1 0.9 0.5Yb 14 19 0.9 10 7 0.5 21 1.7 8.2 3.5 1.7 2.4 0.9 0.7 0.8 0.7

Granitic gneiss Leucogranites Clinopyroxene amphibolites Garnet amphibolites

H17 H18 H19 H20 H21 H22 H23 H24 H25 H26 H27 H28 H29 H30 H31SiO2 59.71 73.37 67.06 77.08 76.51 75.27 75.91 74.91 75.56 50.67 48.86 52.18 57.25 44.68 47.77TiO2 0.637 0.385 0.318 0.244 0.06 0.127 0.16 0.17 0.09 0.42 0.449 0.329 0.534 0.401 0.336Al2O3 18.93 14.66 14.2 10.83 12.93 13.84 13.36 14.21 13.01 16.67 18.67 16.04 14.12 15.03 12.24Fe2O3 4.869 2.572 5.809 3.432 1.051 1.075 1.32 1.11 1.6 8.586 4.869 9.32 11.04 12.42 7.516MnO 0.087 0.038 0.124 0.033 0.013 0.043 0.022 0.05 0.13 0.137 0.097 0.13 0.144 0.181 0.147MgO 2.697 0.758 1.985 0.109 0.112 0.345 0.177 0.41 0.22 8.604 8.265 4.662 5.774 10.42 15.51CaO 7.472 2.609 6.143 0.395 0.257 1.649 1.42 1.2 0.76 11.12 15.12 11.8 8.93 13.42 12.79Na2O 4.49 5.189 3.495 2.683 4.979 4.058 2.88 3.65 3.93 2.693 2.427 3.894 1.186 1.822 1.332K2O 0.424 1.089 0.681 4.998 3.774 3.394 4.35 4.12 3.86 0.357 0.162 0.515 0.325 0.191 0.549P2O5 0.127 0.125 0.065 0.022 0.014 0.04 0.05 0.06 0.03 0.029 0.018 0.158 0.06 0.017 0.015

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CO2 0.59 0.37 0.47 0.57 0.57 0.44 0.32 0.55 0.41 1.02 1.09 1.13 0.69 1.19 2.04Total 100 100.2 100.3 100.4 100.3 100.3 99.96 100.4 99.6 100.3 100 100.1 100.1 99.8 100.2

Trace elements (ppm)

Ba 113 840 83 341 62 765 441 632 73 216 13 209 22 113 83.5Co 14 3.2 14 2.8 0 0.6 0.8 1.1 1.8 34.5 29 29 34 47 52.4Cr 24 7.5 30 16 5.7 2.7 9 7 6 233 888 86 200 229 866Rb 4.4 17 4.7 79 85 150.2 160 155 149 5.2 3.9 9.6 12 4.2 10Sr 527 690 155 37 19 210 66 90 70 354 178 226 126 108 123V 95 25 160 5.7 3.3 7.5 11 10 8 251 123 197 302 459 120Y 0 0 17 165 90 19.3 117 80 32 0 0 29 9.2 12 0Zn 54 52 50 179 116 31 47 33 83 61 31 84 84 90 45Zr 133 267 64 1315 189 150 140 156 170 33 21 76 19 15 17Nb 0 0 0 46 81 8.4 6 8.7 19 0 0 0 0 0 0Pb 1.6 20 6.3 33 11 15 22 32 21 0 1.3 1.5 6 0.5 1.6Th 0 14 2.7 16 7.6 12 14 9 20 0 0 0 0 0 0U 1.8 3.8 0 0 0 0 3.8 1.6 4 1 0.7 0 0 0 1

REE (ppm)

La 53.9 110 50 103 72 63 77 90 56 60 56 31 56 52 41Ce 0 150 0 228 32 46 47 112 29 0 0 0 0 0 0Nd 16.7 47 9.5 120 54 20 44 48 22 3.6 5.4 15 3.4 1.6 3.1Sm 0 8.3 0 17 8.4 4.4 12 8 15 2.8 0 3.6 2.9 1.6 0.9Eu 1.2 2.1 0.5 1.4 0.6 0.7 0.9 1.2 0.5 1.2 0.5 1 0.8 0.8 0.6Yb 0.9 0.8 1.6 14.1 8.6 1.5 4.9 1.6 6 0.8 0.5 2.7 1 1.2 0.3

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Fig. 5. (a) AFM plot of granitic gneisses, leucogranites and amphibolites of Hafafit area. Line shows boundary between tholeiitic and calc-alkaline rocksfrom Irvine and Baragar (1971). (b) Nomenclature of the granitoids of the Hafafit area judging from their normative mineral composition (afterO’Connor, 1965). (c) SiO2 versus A/CNK (A/CNK = molar Al2O3/CaO + Na2O + K2O) (Clark, 1992). (d) K2O–SiO2 for Hafafit granitic gneisses andleucogranites. Fields of partial melts of different rock sources are after Gerdes et al. (2000) and references therein.


4.2. Geochemistry of granitic gneisses and leucogranites

The calc-alkaline character of Hafafit granitic gneissesand leucogranites is indicated from the AFM diagram(Fig. 5a). This diagram is commonly used to distinguishbetween tholeiitic and calc-alkaline differentiation trendsin a magma series (Irvine and Baragar, 1971). Normativemineral composition of Hafafit granitic samples is plottedin Orthoclase-Albite-Anorthite diagram (Fig. 5b). It classi-fies the granitic gneiss into tonalite and trondhjemite,whereas leucogranite plots in the field of granite.

On the SiO2–A/CNK diagram (Fig. 5c), most of graniticgneiss samples plot in the I-type metaluminous field indict-ing an igneous source for these rocks, whereas the leucog-ranite plots in the S-type peraluminous field suggestingpartial melting of metasedimentary source. The possiblemagma source of the studied granitic samples could beinferred from Fig. 5d. It shows that the granitic gneisscould be derived from partial melting of amphibolitesource, whereas leucogranite was formed by partial meltingof metagreywackes and metapellites.

Trace element concentrations of the studied graniticgneiss are normalized to the Oceanic Ridge Granite valuesas proposed by Pearce et al. (1984). In the diagram shownin Fig. 6, two groups of elements can be distinguished. Thefirst group includes the large ion lithophile elements(LILEs) Rb, Ba, K, Th, U, Sr and La that are highlyenriched relative to Oceanic Ridge Granite. The secondgroup consists of the high field strength elements (HFSEs)Ce, Nb, Nd, Sm, Zr, Eu, Y and Yb that show values closeto or less than one in most samples. The strong differencebetween LILEs and HFSEs excludes an oceanic ridge ori-gin of the Hafafit granitic gneiss. The LILEs enrichmentcharacterizes volcanic arcs, within-plate and collision gran-ites. The second group of elements correlates with granitesof volcanic arc and collision settings. Based on the tectonicdiscrimination diagrams proposed by Batchelor and Bow-den (1985), the granitic gneisses represent pre-plate colli-sion pluton (subduction regime), whereas theleucogranite is classified as intrusive in a syn-collision set-ting reflecting restricted range of S-type and anatecticgranites (Fig. 7).

Fig. 6. Ocean Ridge Granite-normalized multi-element diagrams for the Hafafit granitic gneisses. Normalization values from Sun and Mcdonough (1989).

Fig. 7. R1–R2 diagram for the Hafafit granitic gneisses and leucogranites(after Batchelor and Bowden, 1985).


4.3. Geochemistry of amphibolites

On the diagram of K2O versus SiO2 for arc magmaticseries (Peccerillo and Taylor, 1976), the amphibolite sam-

Fig. 8. (a) Weight percent K2O versus SiO2 plot for Hafafit amphibolites. Serie(Winchester and Floyd, 1977).

ples plot in the low-K tholeiite series (Fig. 8a). The tholei-itic character of Hafafit amphibolite is also indicated on theAFM diagram of Fig. 5a (Irvine and Baragar, 1971). Usinga discrimination diagram based on the relation of the ratioZr/TiO2 versus SiO2 (Winchester and Floyd, 1977), thestudied amphibolites are classified as subalkaline basalts(Fig. 8b).

The mineral assemblage of the amphibolites (i.e., amphi-bole + plagioclase + clinopyroxene + garnet ± quartz +opaques) is characteristic of the upper amphibolite facieswith temperatures around 600–700 �C and pressuresaround 6–8 kb (Bucher and Frey, 1994). In Such condi-tions, the major elements may have been mobilized. So,selected trace elements are often used to give informationabout the tectonic setting of such highly metamorphosedrocks. Among the trace elements, the high field strengthelements (HFSEs) including REE, are the most immobile.Fig. 9 shows the normalized multi-element diagram. Itcompares the rock chemistry of the studied amphiboliteswith mid-ocean ridge basalts (the MORB-normalized formproposed by Pearce (1982)). In typical magmatic arcs, the

s boundaries are after Peccerillo and Taylor (1976). (b) Log Zr/TiO2–SiO2

Fig. 9. MORB-normalized multi-element diagrams for the Hafafit amphibolites. Normalization values from Sun and Mcdonough (1989).


abundance of the LILEs (Rb, Ba, K, Th, U, Sr and La) iscontrolled by the fluid phase and consequently shows var-iable enrichment. The HFSEs (Ce, Nb, Nd, Sm, Zr, Eu, Yand Yb) are a function of the chemistry of the source andcrystal/melt processes and are normally depleted relative tothe LILEs (Rollinson, 1993). The amphibolites are variably

Fig. 10. (a) Ti–V (Shervais, 1982), ARC = Island Arc Basalts, OFB = Ocean F(Pearce and Cann, 1973). LKT: low potassium tholeiites, CAB: Calc-Alkaline1973).

enriched in LILEs and depleted in HFSEs (Fig. 9). Thissuggests a mantle source similar to those for E-MORBSand BABBs (Back-Arc Basin Basalts). Field evidence forthe association of these amphibolites with paragneissesand for intruding with tonalitic to trondhjemitic magmas(granitic gneiss) precludes an origin in a mid-ocean ridge

loor Basalts. (b) Nb–SiO2 diagram (after Pearce and Gale, 1977). (c) Zr–TiBasalts, OFB: Ocean Floor Basalts. (d) Zr–Sr/2-Ti/100 (Pearce and Cann,

Fig. 11. (a) Plot of Na8 versus Fe8; (b) Plot of Ba/La versus Ti8 for Hafafit amphibolites (after Taylor and Martinez, 2003). Na8 = [Na2O + 0.115 (8-MgO)]/[1 + 0.133 (6-MgO)]; Fe8 = [FeO + 8-MgO]/[1 + 0.25 (8-MgO)] Ti8 = (TiO2) (MgO)1.7/34.3.


environment; therefore, we conclude that amphiboliticmagmas are BABBs associated with back-arc basin devel-opment during convergent process.

Because Ti and V are HFSEs and thought to be rela-tively immobile under conditions of high-grade metamor-phism (Rollinson, 1993), we plot Ti versus V for theHafafit amphibolites (Fig. 10a). They plot in the arc fieldwhich is well-correlated with the tectonic setting as sug-gested by TiO2–Zr diagram (Fig. 10b), where all amphibo-lite samples fall into the volcanic-arc field. The island-arccharacter of the studied amphibolites could also be inferredfrom Fig. 10c and d, where they plot in the filed of low-ktholeiites and island arc basalts. This geochemical signatureis consistent with an ensialic back-arc basin origin accord-ing to Saunders and Tarney (1991) who stated that ‘‘basaltsfrom ensialic basins often show strong arc-likecharacteristics’’.

Plot of Na8 versus Fe8 (after Taylor and Martinez,2003) supports the conclusion of volcanic arc setting forHafafit amphibolites (Fig. 11a). Similarities between thechemistry of the Hafafit amphibolites and composition oflavas from active BAB spreading centers include arc-likecomponents is evidenced by low Na8, Ti8 and Fe8 andthe reversal relationship between Ti8 and Ba/La (Fig. 11b).

5. Discussion and conclusion

The Hafafit area represents one of the important suturezones in the Eastern Desert of Egypt. It comprises twomain units, the western Hafafit unit and the eastern Nugrusunit. The study area is characterized by a distinctive rela-tionship between sedimentation (greywackes and lithic are-nite to arkose rocks in back-arc basin whichmetamorphosed later to paragneisses), tectonic deforma-tion (thrusting and folding), metamorphism (amphibolitefacies in Hafafit unit, and greenschist facies in Nugrusunit), and magmatism (arc-related and collision-relatedgranitoids). These processes lead to development of basins(Fig. 12), mountain building and eventual exhumation of

HMC. Such features are characteristics of orogenic beltswhich are located at convergent plate boundaries and alonglines of arc collision.

The geochemical data of the paragneisses suggested dif-ferent sedimentary protolithes ranging from lithic areniteto arkose for psammitic gneiss and greywacke for biotite-and hornblende-gneisses. Such variation in sedimentaryprotolithes of paragneisses may attribute to the changingof the depositional environment from shallow marine toa terrestrial environment.

The magmatism in Hafafit area is represented by twomain magma generations. The early mafic magma pro-duced tonalite and trondhjemite (precursor of the graniticgneiss) in the early stage of underplating. During the colli-sion stage, the metasediments underwent anatexis whichled to final magma producing leucogranite which couldbe occurred at the waning stages of medium-grade regionalmetamorphism of HMC. This conclusion is supported bythe isotopic compositions of leucogranites of Hafafit areathat suggest a metasedimentary source for these rocks(Moghazi et al., 2004). The development of leucogranitesin Hafafit and Nugrus area along thrust faults reflects therole of these faults as pathways for these leucogranites.In other word, the shear heating of metasedimentary rocksduring synorogenic thrusting contributed to the leucogra-nite generation.

Abd El-Naby and Frisch (in review) interpreted the Sm/Nd and Rb/Sr ages around 590 Ma as cooling ages fromthe metamorphic thermal peak which was attained around600 Ma or slightly earlier. This geochronological data is inaccordance with the emplacement ages of the leucogranitesin Nugrus area (594 ± 12 and 610 ± 20 Ma, Moghazi et al.,2004). Because of the similarity between the dates of meta-morphism and granite emplacement in Hafafit-Nugrusarea, we propose that both of metamorphic rocks and leu-cogranites are the products of the same event that occurredduring collision and thrusting of Nugrus unit over Hafafitunit. There are many published data that advocated thegeneration of leucogranite along shear zones and then, at

Fig. 12. Sketch diagrams illustrating the development of Hafafit Metamorphic Complex and its tectonic relation with Nugrus unit. (a) > 680 Ma:subduction of the oceanic crust, ophiolite detachment and thrusting in the Wadi Ghadir, with arc volcanism, arc-related plutonism and back-arc basindevelopment in the Hafafit region. (b) �600 Ma: collision and thrusting of Nugrus unit over Hafafit unit, leucogranite intrusion along thrust zones andregional metamorphism.


least partly, migrate along listric faults to higher levels inthe crust due to pressure gradients generated by buoyancyand tectonic stresses (Moghazi et al., 2004; Nabelek et al.,2001; Brown and Solar, 1998a,b; Solar et al., 1998).

Most of amphibolites are tholeiitic in compositionwhich supports their derivation from a mantle peridotitesource by fractional crystallization. This could be expectedin continental rift or back-arc basin tectonic settings (Reidet al., 1987; Moore, 1989; Raith and Meisel, 2001).

The obtained geochemical data are used to construct amodel as shown in Fig. 12. This model is based on Pan-African convergent and back-arc basin processes. ThePan-African tectonic evolution in the Hafafit-Ghadirregion began before 680 Ma ago with the formation ofWadi Ghadir ophiolites (El Ramly et al., 1984). This eventoccurred synchronously with arc volcanism, arc-relatedplutonism and back-arc basin development in the Hafafitzone (Fig. 12a). This suggestion is based mainly on: (i)the geochemical data of the cored granitic gneisses which

indicate that their protoliths are calc-alkaline, metalumi-nous, I-type, and generated in subduction-related environ-ment; (ii) the sedimentary origin in active continentalmargin setting of the psammitic, biotite- and hornblende-gneisses as indicated from their geochemical characteristicsand (iii) trace element concentrations suggest that amphib-olites were formed in an ensialic back-arc setting.

The arc-continent or arc-arc collision took place around600 Ma (Abd El-Naby and Frisch, in review) and wasresponsible for significant overthrust of the Nugrus unitin a northwest-ward direction onto the back-arc basin(Hafafit unit) and for the second regional metamorphismwith medium grade amphibolitic facies in Hafafit unit.Crustal thickening during collision led to a widespread par-tial-melting and leucogranitic plutonism along thrust zones(Fig. 12b).

The suggested model is generally agreeable with the gen-eral tectonic model for the entire Arabian–Nubian shieldwhich involves progressive cratonization by formation of


oceanic crust, subduction, magmatic arc development andcollision between arc complexes to assemble a continentalshield during the period between 900 and 550 Ma (e.g.Frisch and Al Shanti, 1977; Vail, 1985; Kroner et al.,1988; Stern, 1994; Abdel Rahman, 1995; Abd El-Nabyet al., 2000).

Acknowledgements

Our thanks are due to Deutscher Akademischer Aust-auschdienst (DAAD) for supporting the post-doctoral visitof the first author at Tubingen University, Germany. Weare grateful to M. El Ahmadi, Egyptian Nuclear materialsAuthority, for supporting us with field facilities.

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