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Chemie der Erde 71 (2011) 247–266

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Chemie der Erde

journa l homepage: www.e lsev ier .de /chemer

ost-collisional magmatism in the northern Arabian-Nubian Shield:he geotectonic evolution of the alkaline suite at Gebel Tarbush area,outh Sinai, Egypt

sam Sadek Farahata, Mokhles Kamal Azerb,∗

Geology Department, Faculty of Science, Minia University, El-Minia 61519, EgyptGeological Science Department, National Research Centre, Cairo, Egypt

r t i c l e i n f o

rticle history:eceived 21 March 2010ccepted 10 June 2011

eywords:rabian-Nubian Shieldinaiost-collisional-type rocksing complex

a b s t r a c t

Post-collisional alkaline magmatism (∼610–580 Ma) is widely distributed in the northern part of the Neo-proterozoic Arabian-Nubian Shield (ANS), i.e. the northern part of the Egyptian Eastern Desert and Sinai.Alkaline rocks of G. Tarbush constitute the western limb of the Katharina ring complex (∼593 ± 16 Ma)in southern Sinai. This suite commenced with the extrusion of peralkaline volcanics and quartz syenitesubvolcanics intruded by syenogranite and alkali feldspar granite. The mineralogy and geochemistry ofthese rocks indicate an alkaline/peralkaline within-plate affinity. Quartz syenite is relatively enrichedin TiO2, Fe2O3, MgO, CaO, Sr, Ba and depleted in SiO2, Nb, Y, and Rb. The G. Tarbush alkaline suite mostlikely evolved via fractionation of mainly feldspar and minor mafic phases (hornblende, aegirine) froma common quartz syenite parental magma, which formed via partial melting of middle crustal rocks ofANS juvenile crust. Mantle melts could have provided the heat required for the middle crustal melting.The upper mantle melting was likely promoted by erosional decompression subsequent to lithospheric

delamination and crustal uplift during the late-collisional stage of the ANS. Such an explanation couldexplain the absence or scarce occurrence of mafic and intermediate lithologies in the abundant late-to post-collisional calc-alkaline and alkaline suites in the northern ANS. Moreover, erosion related tocrustal uplift during the late-collision stage could account for the lack or infrequent occurrence of olderlithologies, i.e. island arc metavolcanics and marginal basin ophiolites, from the northern part of the ANS.

. Introduction

The late- to post-collisional stages (post ∼650 Ma) in thevolution of the extreme northern part of the Neoproterozoicrabian-Nubian Shield (ANS), i.e. the northern part of the Egyp-

ian Eastern Desert and Sinai Peninsula, were characterized by vastntrusion of calc-alkaline and alkaline/peralkaline granitoids andheir associated volcanics, constituting ∼80% of the basement out-rops. Understanding the petrogenetic and geotectonic evolutionf these rocks has, therefore, a particular geodynamic interest ast provides an outstanding opportunity to learn how continentalrust forms. The ANS juvenile crust is exposed around the Redea in Saudi Arabia, Sinai, the Eastern Desert of Egypt and Red

ea Hills of Sudan and as far south as Eritrea and Ethiopia. It ishe northern continuation of the Mozambique belt, and together,hey have been referred to as the East African Orogen (Stern, 1994).

∗ Corresponding author.E-mail address: mokhles72@yahoo.com (M.K. Azer).

009-2819/$ – see front matter © 2011 Elsevier GmbH. All rights reserved.oi:10.1016/j.chemer.2011.06.003

© 2011 Elsevier GmbH. All rights reserved.

The ANS represents an excellent example of the Pan-African oro-genic cycle that has long been recognized as a period of majorcrustal accretion (Kröner, 1984), where many island-arc/marginalbasin terranes were brought together (Gass, 1981; Kröner et al.,1991; Reischmann and Kröner, 1994; Kusky et al., 2003) to formthe crystalline basement of the African continent as part of the lateNeoproterozoic supercontinent Gondwana. The ANS is one of thelargest tracts of juvenile continental crust of Neoproterozoic ageon Earth (Patchett and Chase, 2002). The stabilization of the ANSoccurred before the development of an extensive peneplain in mid-Cambrian times (∼520 Ma) and was exhumed in Neogene time asa consequence of Red Sea rifting and flank uplift.

The ANS juvenile crust comprises metamorphosed volcano-sedimentary rocks, ophiolite suites and calc-alkaline metagabbro-diorite-granitoid rocks formed during the pre-collisional (island-arc) stage (∼820–700 Ma). The collisional stage (670–630 Ma)

is characterized by the abundances of weakly deformed calc-alkaline gabbros and granodiorites. The Dokhan volcanics (NEEgypt) together with undeformed calc-alkaline granitoids andalkaline/peralkaline granitoids are formed during late- to post-

248 E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266

Fig. 1. (a) Simplified geological map of the late Proterozoic rocks in south Sinai (modified after Be’eri-Shlevin et al., 2009a) showing the location of the detailed study areashown in the part figure ‘c’. Inset shows the location of the Sinai in the northernmost ANS Neoproterozoic exposures of Eastern Africa and Western Arabia. (b) Schematicg ion ofa

ca1e

(eeatdaCr

eological map of the Katharina ring complex (Katzir et al., 2007a) showing the locatrea (modified after Katzir et al., 2007a).

ollisional stage (630–580 Ma) of the ANS crust evolution (Sternnd Hedge, 1985; Stern, 1994; Abdel-Rahman, 1995; Moghazi et al.,998; Jarrar et al., 2003; Mohamed and El-Sayed, 2008; Moussat al., 2008; Farahat et al., 2007; 2011).

In Sinai, calc-alkaline (630–590 Ma) and alkaline/peralkaline608–580) igneous activity overlap in time (Beyth et al., 1994; Alit al., 2009; Be’eri-Shlevin et al., 2009a; Eyal et al., 2010; Moragt al., 2011). In the field, the alkaline rocks are devoid of xenolithsnd are rarely intruded by dykes, while the calc-alkaline rocks con-ain many xenoliths of different compositions and cut by various

ykes. Nevertheless, alkaline rocks always intruded into the calc-lkaline ones. Geochemically, the alkaline rocks have low Al2O3,aO, MgO, Ba and Sr, and high Na2O, K2O, Nb, Ga, Y and REE contentselative to the calc-alkaline rocks. Despite numerous studies and

the detailed study area shown in the part figure ‘c’. (c) Geological map of G. Tarbush

their broad distribution in south Sinai, the origin and geotectonicevolution of calc-alkaline and alkaline magmatism are still contro-versial. G. Tarbush area is a good example of alkaline/peralkalinerocks which intruded and/or extruded into the calc-alkaline ones. Inthis work, we first present new field, mineralogical and geochem-ical data for late Neoproterozoic alkaline igneous rocks exposedhere. We use the new data to examine the genetic relationshipsbetween different components of the alkaline/peralkaline suite,and provide geologic and geochemical constraints on the natureof the magma source, the role of magmatic differentiation, and

possible genetic relationships with calc-alkaline magmatism. Wethen integrate these results with published geochemical and iso-topic data to discuss the significance of the alkaline/peralkalinemagmatism in the geotectonic evolution of the northernmost ANS.

E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266 249

(Cont

2

btagtLaias

wdA1iq(

Fig. 1.

. Geological outline

Neoproterozoic basement exposed in south Sinai is dominatedy late- to post-orogenic granitoid rocks intruded into mediumo highly metamorphosed metavolcanic rocks together with para-nd ortho-gneisses (Fig. 1a). El-Gaby (1975) proposed that theseranitoids constitute one continuous series, and that the alkalineo peralkaline granites are related to minor subvolcanic intrusions.ater, El-Gaby and Ahmed (1980) noted that the highly fraction-ted calc-alkaline, two feldspar granite of G. Ma,in in south Sinais intruded by the less fractionated quartz syenite of G. Goza, andccordingly they considered the latter as independent magma con-tituting an alkaline to peralkaline granite series.

The investigated alkaline rocks in G. Tarbush area represent theestern limb of Katharina ring complex (Fig. 1b) which formeduring the transition from compressional to extensional stage ofNS evolution (Stern et al., 1984; Bentor, 1985; Kröner et al.,

987; Genna et al., 2002; Meert, 2003; Katzir et al., 2007a). They

nclude peralkaline volcanics, a number of sub-volcanic bodies ofuartz syenite porphyry, syenogranite and alkali feldspar graniteFig. 1c). The outcrop area is dissected by a number of narrow val-

inued )

leys, most of which are structurally controlled. Country rocks ofthe alkaline suite include calc-alkaline volcanics, gabbro-diorite,granodiorite and granitic gneisses. Granitic gneiss represents theoldest unit in the area and is related to the Solaf metamor-phic belt. The calc-alkaline volcanics extend beyond the southernboundary of the mapped area. They are unconformably overlainby alkaline volcanics and intruded by granodiorite and gabbro-diorite, while quartz syenite and alkali feldspar granites intrudethe alkaline volcanics. Calc-alkaline volcanics constitute a thickvolcano-sedimentary succession made up of intermediate to acidlava flows and pyroclastics alternating with conglomerates andarkose.

The alkaline volcanics constitute a stratified sequence of pyro-clastics, ignimbrites together with typical rhyolite flows. Thepyroclastics include pyroclastic breccia, lithic and crystal tuffs.Pyroclastic breccia consists of angular rock fragments attain 10 cmin diameter and are set in a tuffaceous matrix. Ignimbrites are

highly welded and are formed of oriented lenticular vitric clastsshowing the filamentous nature of fiamme. Quartz syenites occuras ring dykes that were probably introduced along a pre-existingfracture related to cauldron subsidence that formed the Katha-

250 E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266

(Conti

rgv

puwtgscsragScg

Fig. 1.

ina ring complex. They are intruded with sharp contacts intoranitic gneisses, granodiorite, calc-alkaline volcanics and alkalineolcanics.

Syenogranites and alkali feldspar granites of G. Tarbush areaertain to the Katharina pluton that represents the youngest rocknits of the Katharina ring complex. They intruded older rock unitsith sharp contacts, without any evidence of thermal effect. At

he northeastern boundary of the mapped area the syenograniterades vertically into alkali feldspar granite. The transition fromyenogranite to alkali feldspar granite is poorly discerned in out-rop and only recognized by lower plagioclase content. At theouthern boundary the alkali feldspar granite grades up into moreeddish perthite granite. Syenogranite and alkali feldspar granitere devoid of xenoliths and are rarely intruded by dykes. These

ranites were named as Iqna granite by Bentor et al. (1972). In southinai, the Iqna granites and alkaline/peralkaline granites possessharacteristics of within-plate, A-type granites (Azer, 2006). A-typeranites of Katharina area gave Rb–Sr ages of 560 ± 10 Ma (Bielski,

nued ).

1982). However, recent ages, 596 ± 18 Ma (Abu Anbar et al., 1999),593 ± 16 (Katzir et al., 2007a) and 583 ± 6 Ma (Be’eri-Shlevin et al.,2009a), indicate that the Katharina ring complex formed during theANS post-collisional stage (∼610–580 Ma).

3. Petrography

G. Tarbush alkaline rocks are represented by alkaline volcanics,quartz syenite porphyry and granites. Sixteen representative sam-ples of the granitic rocks were point-counted for modal analyses.They are syenogranite and alkali feldspar granite on the QAP modaldiagram (Fig. 2; Streckeisen, 1976). Petrographic descriptions of thedifferent G. Tarbush alkaline rocks are given hereinafter.

3.1. Alkaline volcanics

G. Tarbush rhyolite, ignimbrites and pyroclastics are exclu-sively subaerial and unmetamorphosed. The rhyolite phenocrysts

E.S. Farahat, M.K. Azer / Chemie d

Quartzdiorite

P

Q

A

QuartzMonzodiorite

Tonalite

Granodiorite

onzr

niteM

oga

Quartzmonzonite

i

Syen

ogra

nte

Quartzsyenite

kAl

alife

ldsp

ar

quar

tzsy

enite

Syenite Monzonite Monzodiorite Gabbro

Alka

life

dspa

r gra

nite

lAlkali feldspar graniteSyenogranite

Fig. 2. Ternary plot of modal quartz-alkali feldspar-plagioclase for the granites ofG

(cqamamtbabaag(n

flmimfaas

td4tfma(apcm

range of natural and synthetic minerals was used as standards.

. Tarbush area (Streckeisen, 1976).

5–25%) occur either as discrete crystals or as glomerophyriclusters of crystals. Phenocrysts include abundant K-feldspar anduartz, together with minor albite, sodic pyroxene and/or sodicmphibole. The phenocrysts are set in a microcrystalline ground-ass showing micrographic and spherulitic textures. Quartz occurs

s subhedral to euhedral phenocrysts, 0.3–2.5 mm across, com-only showing perfect square or hexagon cross sections. Albite

ypically occurs as prismatic microphenocrysts and exsolvedatches or stringers within some K-feldspar host crystals andlong their peripheries forming perthitic texture. Alkali amphi-ole is strongly pleochroic (deep greenish blue to pale yellow)nd occurs as moss-like patchy aggregates. Pyroxene (0.1–1.5 mmcross) occurs as microphenocrysts or as acicular aggregates in theroundmass. Rhyolite has very low contents of opaque minerals<1%) represented mainly by magnetite and rare ilmenite. Mag-etite mostly contains ilmenite lamellae (Fig. 3a).

Ignimbrites are rhyolitic and show regular orientation ofattened lenticular glass particles (fiamme) embedded in a ground-ass formed of acid volcanic glass, pumice and glass shards of

rregular shape. The groundmass encloses crystals and crystal frag-ents of quartz and K-feldspars together with scarce felsic rock

ragments. The fiamme are occasionally sinuous and their bound-ries are penetrated by crystal fragments as a result of compactionnd welding. The groundmass of most of the examined ignimbriteshows fluidal structure.

Fallout pyroclastics cover the entire spectrum from pyroclas-ic breccia to fine (ash) tuffs. Pyroclastic breccia are poorly sorted,ominated by angular to subangular felsic volcanic rock fragments,0–60 mm across, and embedded in a tuffaceous matrix. Crystaluffs contain abundant crystal fragments and minor rhyolitic rockragments. The crystal fragments are quartz and feldspars with

inor amounts of altered mafics and opaque minerals. Banded tuffsre very fine grained, composed of alternating varicolored laminae<2.0 mm thick) and thin beds (<15 mm thick). The fine laminaere formed of very fine-grained, vitreous-ashy material, with tinyarticles of quartz and feldspars. The thin beds contain abundant

oarser crystals and crystal fragments of quartz, feldspars and rareafics set in a tuffaceous groundmass.

er Erde 71 (2011) 247–266 251

3.2. Quartz syenite

Quartz syenites are porphyritic and composed mainly of alkalifeldspar, plagioclase, quartz, mafic minerals and Fe–Ti oxides setin a fine-grained groundmass of the same composition. Alkalifeldspar is the most common mineral and is mostly turbid andstained with pale, reddish brown material. Plagioclase occursas both large phenocrysts and small laths in the groundmass.Quartz is present as small anhedral crystals or in the groundmass.The ferromagnesian phases include amphibole, clinopyroxene andbiotite. Amphibole occurs as either individual discrete crystals orgreenish-brown pleochroic rims around pyroxene (Fig. 3b). Onthe basis of habit and composition, two types of clinopyroxeneare recognized; a colorless to light gray clinopyroxene occur-ring as subhedral crystals and a yellow-greenish type occurringas rims around the first type. Biotite forms small tabular crys-tals and fine interstitial flakes occupying interspaces betweenother minerals. Accessory minerals are zircon, sphene and apatite(Fig. 3c).

The opaque minerals (1–3 vol.%) in the quartz syenite are mainlytitanomagnetite together with rare ilmenite and fine specks ofpyrite. The ilmenite is represented by discrete homogeneous grainsassociated with the mafic minerals (Fig. 3d) or as sandwich inter-growth with titanomagnetite (Fig. 3e); this can be produced byeither oxidation or primary crystallization (Haggerty, 1991). Rarediscrete grains of ilmenite contain fine hematite exsolution (Fig. 3f).This type of exsolution was interpreted by Basta (1959) to be aresult of unmixing of hematite-ilmenite solid solution at tempera-tures above 600 ◦C

3.3. Granites

G. Tarbush granitic rocks are medium- to coarse-grainedwith a hypidiomorphic inequigranular texture; these vary fromsyenogranite to alkali feldspar granite (Fig. 2). They consist ofK-feldspar (48–64%), quartz (30–39%), plagioclase (2–16%) andbiotite (<2%). Plagioclase and mafics abundances decrease fromsyenogranite to alkali feldspar granite. Apatite, titanite, zircon andiron oxides are accessories, while chlorite, epidote and calcite aresecondary minerals. K-feldspar is the most common mineral and ismostly turbid, especially in the alkali feldspar granite. Quartz occursas anhedral sutured crystals as well as small interstitial crystals;locally it forms graphic, granophyric and myrmekitic intergrowths.Plagioclase occurs as euhedral to subhedral tabular crystals, whereits content ranges from 9 to 16 vol.% in the syenogranite, butdecreases (as albite) to 2 vol.% in the alkali feldspar granite. Biotiteis the dominant mafic mineral in the granite varieties; rare horn-blende occurs as very fine crystals in the syenogranite. Biotiteoccurs as small prismatic crystals or as fine flakes in the intersticesbetween the other constituents, variably altered to Fe-oxides andsecondary mica. The studied granites have low contents of opaqueminerals (0.5–1.5 vol.%); represented mainly by homogenous mag-netite and rare ilmenite.

4. Analytical techniques

Mineral analyses and backscattered electron images were doneat the Institute of Earth Sciences (Mineralogy and Petrology), KarlFranzens University, Graz, Austria using a JEOL-JSM 6310 scan-ning electron microscope with Oxford ISIS EDX and MicrospecWDX (15 kv, 6 nA, Na with TAP, all other elements on EDX). A

The analyses were normally done at a magnification of spot mode.The matrix correction was calculated with ZAF-correction pro-gram. Fe2+–Fe3+ redistribution from electron microprobe analyses

252 E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266

Fig. 3. Backscattered-electron (BSE) images. (a) Magnetite crystals showing fine trellis-intergrowths with ilmenite in rhyolite, (b) amphibole rim around clinopyroxenei ilmet tite ex

ie

lsae

n quartz syenite, (c) euhedral apatite crystal in quartz syenite, (d) homogeneousitanomagnetite and ilmenite in quartz syenite, and (f) ilmenite showing fine hema

s made using the charge balance equation of Droop (1987) forstimating Fe3+.

Twenty-four most fresh rock samples representing the main

ithologies of the alkaline rocks at G. Tarbush area (10 quartzyenites, 5 volcanics and 9 granites) were selected for chemicalnalysis of major and trace elements. Whole-rock major and tracelement contents were obtained with a fully automated X-ray flu-

nite associated with biotite in quartz syenite, (e) sandwich intergrowth betweensolution in quartz syenite.

orescence (XRF) spectrometer at the Institute of Earth Sciences(Mineralogy and Petrology), Karl Franzens University, Graz, Austriaand the Saudi Arabia Geological Survey. Fused glass discs using

Li2B4O7 flux were used for major oxides, whereas pressed pow-der pellets were used for trace element determinations. Loss onignition (LOI) was determined by heating powdered samples for1 h at 1000 ◦C. The analytical precision and accuracy is better than

mie d

3e

5

p

5

asa

TR

E.S. Farahat, M.K. Azer / Che

% of the amount present for major elements and 10% for tracelements.

. Mineral chemistry

Representative microprobe analyses of the major mineralhases from the investigated rocks are given in Tables 1–5.

.1. Feldspars

Representative feldspar analyses from quartz syenite and per-lkaline rhyolite are listed in Table 1. Feldspars in the quartzyenite are plagioclase and alkali-feldspars. Plagioclase varies fromlbite to andesine (An3.6−32.5), while alkali-feldspars are ortho-

able 1epresentative composition of feldspars in the quartz syenite and peralkaline rhyolite.

Alkali feldspar

Quartz syenite

SiO2 66.10 65.52 66.45 67.05TiO2 0.16 0.27 0.23 0.17Al2O3 19.43 18.62 17.68 17.99Cr2O3 0.07 0.00 0.01 0.00FeO 0.18 0.30 0.21 0.34MnO 0.03 0.00 0.00 0.00MgO 0.27 0.00 0.00 0.00CaO 0.33 0.08 0.29 0.22Na2O 1.25 0.80 3.88 3.72K2O 14.47 15.07 11.11 11.27Total 102.29 100.66 99.86 100.76Strcutural formula based on 8 oxygensSi 2.967 2.994 3.023 3.022Ti 0.005 0.009 0.008 0.006Al 1.028 1.003 0.948 0.956Cr 0.002 0.000 0.000 0.000Fe2+ 0.007 0.011 0.008 0.013Mn 0.001 0.000 0.000 0.000Mg 0.018 0.000 0.000 0.000Ca 0.016 0.004 0.014 0.011Na 0.109 0.071 0.342 0.325K 0.828 0.878 0.645 0.648Or 86.9 92.2 64.4 65.8Ab 11.4 7.4 34.2 33.1An 1.7 0.4 1.4 1.1

Plagioclase

Quartz syenite

SiO2 60.88 60.68 63.56 61.91 66.64TiO2 0.29 0.14 0.38 0.46 0.07Al2O3 22.89 24.05 21.10 21.52 18.52Cr2O3 0.05 0.00 0.00 0.04 0.00FeO 0.46 0.62 0.32 0.42 0.30MnO 0.00 0.04 0.03 0.03 0.11MgO 0.00 0.00 0.00 0.00 0.00CaO 5.70 6.73 3.73 4.56 0.75Na2O 7.69 7.66 8.28 6.82 6.06K2O 0.78 0.32 1.66 2.93 7.89Total 98.74 100.24 99.06 98.69 100.34Strcutural formula based on 8 oxygensSi 2.749 2.705 2.851 2.809 2.992Ti 0.010 0.005 0.013 0.016 0.002Al 1.218 1.263 1.115 1.151 0.980Cr 0.002 0.000 0.000 0.001 0.000Fe2+ 0.017 0.023 0.012 0.016 0.011Mn 0.000 0.002 0.001 0.001 0.004Mg 0.000 0.000 0.000 0.000 0.000Ca 0.276 0.321 0.179 0.222 0.036Na 0.673 0.662 0.720 0.600 0.527K 0.045 0.018 0.095 0.170 0.452Ab 67.7 66.1 72.4 60.5 51.9An 27.7 32.1 18 22.4 3.6Or 4.6 1.8 9.6 17.1 44.5

er Erde 71 (2011) 247–266 253

clase/sanidine with minor anorthoclase (Fig. 4a). Feldspars of theperalkaline rhyolite are mainly sanidine; rare albite intergrowthsoccur in perthite phenocrysts. Sanidine shows mean compositionof Or97. Albitic plagioclase is Ab99.1 to Ab99.5. Alkali feldspars areunzoned, indicating equilibrium crystallization.

5.2. Pyroxene

Representative pyroxene analyses from quartz syenite and per-alkaline rhyolite are listed in Table 2. Clinopyroxene in the quartz

syenite has variable Ca, Ti and Al contents. Pyroxenes in the quartzsyenite are classified mainly as augite with minor diopside (Fig. 4b;Morimoto et al., 1988). However, the sodic pyroxene in the peral-kaline rhyolite is aegirine and aegirine–augite (Fig. 4c), which have

Rhyolite

65.11 65.24 66.87 65.540.02 0.00 0.04 0.02

16.82 16.80 17.46 16.330.00 0.01 0.09 0.060.23 0.06 0.10 0.770.00 0.03 0.03 0.000.00 0.00 0.00 0.020.08 0.06 0.05 0.000.27 0.26 0.26 0.41

15.67 15.98 16.32 15.5498.20 98.44 101.22 98.69

3.055 3.056 3.047 3.0660.001 0.000 0.001 0.0010.930 0.927 0.937 0.9000.000 0.000 0.003 0.0020.009 0.002 0.004 0.0300.000 0.001 0.001 0.0000.000 0.000 0.000 0.0010.004 0.003 0.002 0.0000.025 0.024 0.023 0.0370.938 0.955 0.948 0.927

97.1 97.3 97.4 96.12.5 2.4 2.3 3.90.4 0.3 0.3 0

Rhyolite

62.58 61.40 69.90 70.03 70.180.40 0.30 0.00 0.04 0.00

21.46 22.94 17.73 18.66 18.410.01 0.06 0.00 0.00 0.000.31 0.46 1.11 0.52 0.550.00 0.02 0.00 0.09 0.000.00 0.00 0.00 0.00 0.284.14 6.92 0.02 0.00 0.056.66 6.92 11.63 11.69 11.573.10 1.53 0.14 0.10 0.05

98.66 100.55 100.53 101.13 101.09

2.831 2.738 3.048 3.028 3.0330.014 0.010 0.000 0.001 0.0001.144 1.206 0.911 0.951 0.9380.000 0.002 0.000 0.000 0.0000.012 0.017 0.040 0.019 0.0200.000 0.001 0.000 0.003 0.0000.000 0.000 0.000 0.000 0.0180.201 0.331 0.001 0.000 0.0020.584 0.598 0.983 0.980 0.9700.179 0.087 0.008 0.006 0.003

60.6 58.9 99.1 99.4 99.520.8 32.5 0.1 0 0.218.6 8.6 0.8 0.6 0.3

254 E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266

Table 2Representative composition of pyroxenes from the quartz syenite and peralkaline rhyolite.

Quartz syenite

SiO2 51.56 51.29 52.36 52.11 53.23 52.53 51.54 49.11 51.18 52.14 52.25 50.73TiO2 0.63 0.68 0.60 0.74 0.01 0.66 0.67 1.26 0.36 0.74 0.63 0.77Al2O3 1.02 1.04 1.37 1.93 0.40 1.59 1.39 5.47 1.17 2.19 1.47 2.05Cr2O3 0.00 0.05 0.02 0.00 0.05 0.03 0.00 0.00 0.00 0.03 0.00 0.02Fe2O3 0.91 1.20 0.00 0.00 0.00 0.00 6.00 6.10 1.93 1.58 2.34 3.91FeO 9.35 9.34 9.44 9.55 12.64 9.77 4.00 10.81 12.83 8.84 8.53 6.58MnO 0.91 0.91 0.73 0.60 0.99 0.75 0.80 0.77 1.28 0.67 0.93 0.60MgO 15.07 14.92 15.27 14.85 10.13 14.43 18.09 14.62 12.64 15.89 15.99 16.22CaO 17.49 17.63 18.23 17.88 21.80 17.81 17.80 10.03 17.63 17.99 17.91 17.74Na2O 0.54 0.54 0.46 0.60 0.26 0.56 0.50 1.44 0.44 0.46 0.44 0.50K2O 0.06 0.00 0.00 0.00 0.02 0.02 0.02 0.88 0.08 0.00 0.03 0.07Total 97.54 97.60 98.48 98.26 99.53 98.15 100.81 100.49 99.54 100.53 100.53 99.19Strcutural formula based on 6 oxygensSi 1.967 1.959 1.972 1.965 2.030 1.984 1.887 1.832 1.954 1.926 1.934 1.898Ti 0.018 0.020 0.017 0.021 0.000 0.019 0.018 0.035 0.010 0.021 0.018 0.022Al 0.046 0.047 0.061 0.086 0.018 0.071 0.060 0.240 0.053 0.095 0.064 0.090Cr 0.000 0.002 0.001 0.000 0.002 0.001 0.000 0.000 0.000 0.001 0.000 0.001Fe3+ 0.026 0.034 0.000 0.000 0.000 0.000 0.165 0.171 0.055 0.044 0.065 0.110Fe2+ 0.298 0.298 0.297 0.301 0.403 0.309 0.122 0.337 0.410 0.273 0.264 0.206Mn 0.029 0.029 0.023 0.019 0.032 0.024 0.025 0.024 0.041 0.021 0.029 0.019Mg 0.857 0.849 0.857 0.835 0.576 0.812 0.987 0.813 0.719 0.875 0.882 0.904Ca 0.715 0.721 0.736 0.722 0.890 0.721 0.698 0.401 0.721 0.712 0.710 0.711Na 0.040 0.040 0.034 0.044 0.019 0.041 0.035 0.104 0.033 0.033 0.032 0.036K 0.003 0.000 0.000 0.000 0.001 0.001 0.001 0.042 0.004 0.000 0.001 0.003

Peralkaline rhyolite

SiO2 51.88 51.84 50.41 50.86 52.64 50.49 52.95 50.75TiO2 0.40 0.31 0.70 0.84 0.26 0.79 0.30 1.14Al2O3 0.00 0.17 0.83 1.10 0.39 0.66 0.87 1.09Cr2O3 0.00 0.00 0.00 0.03 0.11 0.05 0.02 0.00Fe2O3 27.52 25.29 13.04 12.15 25.72 14.95 17.16 13.99FeO 6.40 7.73 21.63 22.16 7.56 18.82 11.68 20.45MnO 0.38 0.72 1.66 1.54 0.61 1.72 1.69 1.84MgO 0.00 0.04 1.78 1.60 0.16 1.96 6.18 1.73CaO 1.86 3.89 1.09 0.90 3.76 2.33 1.51 1.43Na2O 11.48 10.51 6.46 6.55 10.73 6.58 7.25 6.59K2O 0.00 0.01 1.01 1.14 0.05 1.16 1.19 1.29Total 99.93 100.50 98.61 98.88 101.99 99.51 100.80 100.29Strcutural formula based on 6 oxygensSi 2.018 2.013 2.040 2.049 2.011 2.020 2.022 2.018Ti 0.012 0.009 0.021 0.025 0.007 0.024 0.009 0.034Al 0.000 0.008 0.040 0.052 0.018 0.031 0.039 0.051Cr 0.000 0.000 0.000 0.001 0.003 0.002 0.001 0.000Fe3+ 0.806 0.739 0.397 0.368 0.739 0.450 0.493 0.418Fe2+ 0.208 0.251 0.732 0.747 0.241 0.630 0.373 0.680Mn 0.013 0.024 0.057 0.053 0.020 0.058 0.055 0.062Mg 0.000 0.002 0.107 0.096 0.009 0.117 0.352 0.103Ca 0.078 0.162 0.047 0.039 0.154 0.100 0.062 0.061

.512

.059

nt

5

amb1pdlisodF

Na 0.866 0.791 0.507 0K 0.000 0.000 0.052 0

ot been reported previously, to the best of our knowledge, fromhe Neoproterozoic alkaline volcanics in south Sinai.

.3. Biotite

The biotites are analyzed only from quartz syenite (Table 3). Thenalyzed biotite crystals have restricted compositional range andostly are iron-rich, siliceous biotites (annite). The composition of

iotites reflects the composition of the parent magma (Nachit et al.,985; Abdel-Rahman, 1994). Abdel-Rahman (1994) used the com-osition of igneous biotites to infer host magma compositions. Heefined three compositionally distinct fields: (1) biotites in alka-

ine (mostly anorogenic or extension-related) suites are mostlyron-rich and siliceous, (2) biotites in the peraluminous suites are

iderophyllitic; and (3) biotites in calc-alkaline, subduction-relatedrogenic suites are moderately enriched in MgO. On the biotiteiscrimination diagrams (MgO–FeO(t)–Al2O3 and Al2O3 vs. FeO(t);ig. 4d), the studied biotites plot in the field of alkaline suite and

0.795 0.510 0.537 0.5080.002 0.059 0.058 0.065

do not overlap with other fields indicating their chemically distinctnature.

5.4. Amphiboles

Amphiboles (Table 4) represent the sole ferromagnesian min-eral present in the quartz syenite and are distinguished intoprimary and secondary varieties. In general the secondary amphi-boles are after clinopyroxenes, which is consistent with thepresence of clinopyroxene relics. According to the classification ofLeake (1997), the primary amphiboles are Fe–Mg–Mn and calcic,whereas the secondary amphiboles are calcic. Primary amphibolesare cummingtonite, edinite and hornblende, whereas secondaryamphiboles are mainly hornblende and subordinate edinite.

5.5. Fe–Ti oxides

Fe–Ti oxides were analyzed only from the quartz syenite(Table 5). Fe–Ti oxides make up less than 3% of quartz syenite

E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266 255

Table 3Representative composition of biotite in the quartz syenite.

SiO2 36.63 37.22 37.33 37.35 37.43 36.80 36.85 35.95 35.95TiO2 3.68 3.75 3.17 2.99 3.55 3.33 3.69 3.13 3.72Al2O3 10.89 10.96 10.18 11.32 10.62 10.18 10.84 10.71 10.99Cr2O3 0.02 0.00 0.12 0.00 0.00 0.06 0.00 0.05 0.04FeO 23.92 23.03 23.35 23.21 22.62 23.89 23.41 22.19 23.91MnO 0.33 0.53 0.21 0.16 0.40 0.26 0.53 0.36 0.36MgO 8.96 9.58 9.60 10.22 10.49 8.50 8.73 9.79 8.53CaO 0.00 0.00 0.14 0.07 0.05 0.00 0.05 0.46 0.01Na2O 0.03 0.06 0.06 0.10 0.06 0.04 0.06 0.12 0.06K2O 9.04 9.03 8.74 9.01 8.44 9.02 8.58 8.44 9.11Total 93.50 94.16 92.90 94.43 93.66 92.08 92.74 91.20 92.68Strcutural formula based on 22 oxygensSi 5.86 5.88 5.98 5.87 5.91 5.98 5.92 5.86 5.82Ti 0.44 0.45 0.38 0.35 0.42 0.41 0.45 0.38 0.45Al 2.05 2.04 1.92 2.10 1.98 1.95 2.05 2.06 2.10Cr 0.00 0.00 0.02 0.00 0.00 0.01 0.00 0.01 0.01Fe2+ 3.20 3.04 3.13 3.05 2.99 3.25 3.14 3.02 3.24Mn 0.05 0.07 0.03 0.02 0.05 0.04 0.07 0.05 0.05Mg 2.14 2.26 2.29 2.40 2.47 2.06 2.09 2.38 2.06Ca 0.00 0.00 0.02 0.01 0.01 0.00 0.01 0.08 0.00Na 0.01 0.02 0.02 0.03 0.02 0.01 0.02 0.04 0.02K 1.85 1.82 1.79 1.81 1.70 1.87 1.76 1.75 1.88

SiO2 37.52 37.42 34.43 36.55 35.01 35.57 35.34 33.79 36.45TiO2 3.41 2.90 2.95 3.12 3.76 3.45 3.68 2.66 3.48Al2O3 10.55 10.03 13.54 10.97 11.66 11.56 11.92 12.75 11.60Cr2O3 0.00 0.03 0.00 0.04 0.00 0.10 0.00 0.05 0.04FeO 21.99 23.23 26.36 26.23 27.37 25.05 25.66 26.18 25.86MnO 0.37 0.28 0.46 0.38 0.18 0.21 0.61 0.44 0.32MgO 10.27 9.80 11.32 11.09 8.23 10.06 9.92 11.02 9.11CaO 0.08 0.26 0.20 0.04 0.10 0.09 0.08 0.11 0.09Na2O 0.10 0.04 0.06 0.08 0.03 0.05 0.02 0.04 0.04K2O 8.99 8.65 5.08 8.42 8.21 7.79 7.74 5.90 8.67Total 93.28 92.64 94.40 96.92 94.55 93.93 94.97 92.94 95.66Strcutural formula based on 22 oxygensSi 5.95 6.01 5.409 5.635 5.624 5.666 5.590 5.435 5.735Ti 0.41 0.35 0.348 0.362 0.454 0.413 0.438 0.322 0.412Al 1.97 1.90 2.507 1.993 2.207 2.170 2.222 2.417 2.151Cr 0.00 0.00 0.000 0.005 0.000 0.013 0.000 0.006 0.005Fe2+ 2.92 3.12 3.463 3.043 3.676 3.337 3.394 3.521 3.402Mn 0.05 0.04 0.061 0.050 0.024 0.028 0.082 0.060 0.043Mg 2.43 2.34 2.650 2.548 1.970 2.388 2.339 2.642 2.136Ca 0.01 0.05 0.034 0.007 0.017 0.015 0.014 0.019 0.015Na 0.03 0.01 0.018 0.024 0.009 0.015 0.006 0.012 0.012

ataipoc

6

TiaAai77M

caa

K 1.82 1.77 1.018 1.656

nd mainly represented by titanomagnetite and ilmenite. Most ofhe titanomagnetites contain high TiO2 (3.28–8.93 wt.%) and smallmounts of Mn replaced Fe2+. Ilmenite is nearly pure and enrichedn MnO (2.52–8.46 wt.%). Deer et al. (1992) indicated that pyro-hanite (MnTiO2) may be important in ilmenite due to substitutionf Mn for Fe2+ with increasing oxygen fugacity under magmaticonditions.

. Geochemistry

Chemical analyses of G. Tarbush alkaline rocks are given inable 6. In terms of major element compositions, quartz syen-tes show a narrow range of silica content (63.61–66.87 wt.%)nd contain high abundances of total alkalis (9.94–10.76 wt.%),l2O3 (15.96–17.41 wt.%) and CaO (1.57–2.38 wt.%) relative to otherlkaline rocks of G. Tarbush. The syenogranite shows very lim-ted compositional variations with SiO2 ranging from 74.51 to4.82 wt.%, while alkali feldspar granite have SiO2 ranging from5.98 to 77.01 wt.%. Quartz syenite have low and relatively constantg# [100 molar Mg/(Mg + Fe)], which ranges from 25 to 29.

In terms of trace element compositions, quartz syenites are

haracterized by relatively high abundance of Ba (1847–2869 ppm)nd Sr (212–368 ppm) relative to the other alkaline rocks in therea. The low Ba contents of the more felsic rocks (rhyolite,

1.682 1.583 1.562 1.210 1.740

syenogranite and alkali feldspar granite) is due to fractionation ofalkali feldspar (Blundy and Wood, 1991), while the extreme highBa contents in the quartz syenite almost reflect alkali feldspar accu-mulations. G. Tarbush alkaline rhyolites are chemically similar, richin Rb (144–218 ppm), Nb (38–50 ppm), Zr (542–679 ppm), and Y(51–86 ppm) relative to the quartz syenite. On the other hand, G.Tarbush alkali feldspar granite has relatively high abundances ofNb (37–46 ppm), Rb (273–363 ppb) and Y (51–60 ppm), and low Ba(53–249 ppm) and Sr (20–65 ppm) as compared to syenogranite.

Quartz syenites are quartz-normative (8.23–21.35%) with fewsamples characterized by normative corundum. None of theanalyzed samples, however, contain normative nepheline char-acteristic of undersaturated rocks. The occurrence of normativeacmite in the rhyolite (sample T4) attests to its high alkaline nature.On the Q/(F/)-ANOR diagram (Fig. 5a; Streckeisen and Le Maitre,1979), the quartz syenite almost plots in the syenite field whereasthe alkaline volcanics occupy the alkali rhyolite/granite field. In har-mony with petrographic classification, granitoid rocks plot in thealkali granite and syenogranite fields.

High agpaitic index (AI = 0.81–1.06; Table 6) of the studied rocks

clearly indicate their alkaline nature (Liégeois and Black, 1987;Liégeois et al., 1998). The alkaline affinity of the silica-rich rocksis further confirmed on the diagram of Sylvester (1989), which dis-criminates between alkaline, calc-alkaline and highly fractionated

256 E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266

Table 4Representative composition of amphiboles in the quartz syenite.

Primary amphibole

SiO2 45.43 45.96 44.81 44.72 49.30 44.37 46.61 44.53 50.32TiO2 1.39 1.31 1.23 1.29 0.63 1.29 0.98 1.38 0.05Al2O3 5.50 5.75 5.69 5.82 3.09 6.98 5.10 5.90 0.37Cr2O3 0.00 0.00 0.04 0.07 0.00 0.04 0.00 0.01 0.00Fe2O3 4.85 0.00 5.59 4.25 9.73 0.09 11.73 0.00 1.91FeO 13.80 17.87 14.41 18.97 12.04 22.67 11.13 23.05 26.93MnO 0.67 0.76 0.86 0.96 1.13 0.90 1.07 0.88 3.45MgO 10.24 11.34 9.45 6.70 10.36 8.42 9.41 7.73 10.89CaO 9.90 9.86 9.91 9.99 10.13 9.91 9.78 10.14 2.37Na2O 1.92 1.95 1.89 1.44 0.74 1.50 1.12 1.13 0.13K2O 0.92 0.91 0.95 0.92 0.40 0.84 0.64 0.81 0.02Total 94.62 95.71 94.83 95.13 97.56 97.01 97.57 95.56 96.43Strcutural formula based on 23 oxygensSi 7.033 7.067 6.973 7.050 7.349 6.893 7.000 7.035 7.827Ti 0.162 0.151 0.144 0.153 0.071 0.151 0.111 0.164 0.006Al 1.003 1.042 1.043 1.081 0.543 1.278 0.903 1.098 0.068Cr 0.000 0.000 0.005 0.009 0.000 0.005 0.000 0.001 0.000Fe3+ 0.565 0.000 0.655 0.504 1.092 0.011 1.325 0.000 0.223Fe2+ 1.786 2.298 1.875 2.500 1.501 2.945 1.397 3.045 3.502Mn 0.088 0.099 0.113 0.128 0.143 0.118 0.136 0.118 0.454Mg 2.363 2.599 2.192 1.574 2.302 1.950 2.106 1.820 2.525Ca 1.642 1.624 1.652 1.687 1.618 1.649 1.573 1.716 0.395Na 0.576 0.581 0.570 0.440 0.214 0.452 0.326 0.346 0.039K 0.182 0.178 0.189 0.185 0.076 0.166 0.123 0.163 0.004

Secondary amphibole

SiO2 47.51 45.27 45.52 46.75 46.10 46.77 46.56 48.86 44.68 45.27 46.16 45.48 46.75 46.61TiO2 1.14 1.00 1.36 0.79 1.16 1.12 0.83 1.11 1.34 1.22 1.04 1.03 0.90 0.66Al2O3 5.02 4.90 5.93 5.26 5.22 5.12 5.04 4.21 6.20 6.58 5.34 5.48 4.18 4.34Cr2O3 0.03 0.07 0.00 0.00 0.00 0.00 0.09 0.00 0.00 0.04 0.00 0.00 0.00 0.02Fe2O3 7.39 7.25 7.71 9.99 7.01 7.36 8.00 1.54 9.62 0.19 0.38 0.51 0.00 0.58FeO 9.95 10.77 11.81 9.24 11.38 12.05 10.54 9.99 12.41 21.70 18.94 18.70 19.63 18.25MnO 0.67 0.60 0.64 0.95 0.70 0.82 0.70 0.42 0.96 0.80 0.99 0.83 1.12 0.93MgO 12.39 11.16 10.82 11.42 11.10 10.76 11.21 14.37 9.33 9.28 11.46 11.30 10.70 11.08CaO 9.84 9.46 9.71 9.58 9.59 9.73 9.57 15.80 9.94 10.07 9.77 9.93 9.96 10.16Na2O 1.90 1.92 1.81 1.71 1.94 1.81 1.75 0.45 1.49 1.36 1.47 1.51 1.30 1.05K2O 0.93 0.90 1.01 0.78 0.91 0.90 0.77 0.35 0.91 0.88 0.74 0.86 0.56 0.56Total 96.77 93.30 96.32 96.46 95.10 96.45 95.06 97.10 96.87 97.39 96.29 95.63 95.10 94.24Strcutural formula based on 23 oxygensSi 7.081 7.050 7.092 7.018 7.047 7.070 7.093 7.124 6.808 6.960 7.084 7.039 7.273 7.274Ti 0.128 0.117 0.159 0.089 0.133 0.127 0.095 0.122 0.154 0.141 0.120 0.120 0.105 0.077Al 0.882 0.899 0.926 0.931 0.940 0.912 0.905 0.723 1.113 1.192 0.966 1.000 0.766 0.798Cr 0.004 0.009 0.000 0.000 0.000 0.000 0.011 0.000 0.000 0.005 0.000 0.000 0.000 0.002Fe3+ 0.829 0.849 0.000 1.128 0.806 0.838 0.917 0.424 1.103 0.022 0.044 0.060 0.000 0.068Fe2+ 1.240 1.402 1.539 1.159 1.454 1.524 1.343 0.795 1.580 2.790 2.430 2.420 2.554 2.382Mn 0.085 0.079 0.084 0.121 0.091 0.105 0.090 0.052 0.124 0.104 0.129 0.109 0.148 0.123Mg 2.752 2.590 2.513 2.555 2.529 2.424 2.545 3.124 2.119 2.127 2.621 2.607 2.481 2.577Ca 1.571 1.578 1.621 1.541 1.570 1.576 1.562 2.468 1.622 1.659 1.606 1.646 1.660 1.699

0.5170.150

coimslshr

tcAitgim(

Na 0.549 0.580 0.547 0.498 0.575 0.530K 0.177 0.179 0.201 0.149 0.177 0.174

alc-alkaline granites with SiO2 > 68 wt.% (Fig. 5b). The occurrencef sodic pyroxene and/or sodic amphibole in the rhyolite furtherndicates their peralkaline character. Moreover, large increases in

ost incompatible elements (such as Rb, Y and Nb) from quartzyenite to more felsic rocks are related to increasing peralka-inity (Weaver et al., 1990). The chemical characteristics of theyenogranite and alkali feldspar granite indicate that these areighly differentiated and share many features of A-type graniticocks.

G. Tarbush alkaline rocks exhibit features characteristic of A-ype granites. Whalen et al. (1987) used plots of Ga/Al versusertain major and trace elements to clearly distinguish between-type granites and other granite types (M-, I- and S-type gran-

tes). Quartz syenites and alkaline volcanics of G. Tarbush plot inhe field of A-type granites on Ga/Al vs. (Na2O + K2O) and Zr dia-

rams (Fig. 5c and d). Unfortunately, Ga content is not determinedn the studied syenogranite and alkali feldspar granite. Further-

ore, the G. Tarbush alkaline rocks fall in the A2 subtype of Eby1992) on the Rb/Nb vs. Y/Nb plot (Fig. 5e), which was interpreted

0.127 0.440 0.405 0.437 0.453 0.392 0.3180.065 0.177 0.173 0.145 0.170 0.111 0.111

as a crustal-derived magma of post-orogenic setting. The A1 sub-type represents, however, a mantle-derived magma of anorogenicsetting.

Selected major and trace elements are plotted against SiO2, usedas a differentiation index (Fig. 6a and b). The studied alkaline rocksshow consistent trends, especially with regard to the trace elementswhich are less sensitive to alteration (Sr, Nb, Y, Ba and Rb), witha compositional gap between the quartz syenite and more felsicrocks. The lack of compositions in the range 67–71 wt.% SiO2 indi-cates a silica gap. The composition gap between the A-type rocksin south Sinai is attributed to the presence of two main cycles ofmagmatic activity separated by a period of magmatic quiescence(Azer, 2006). In the quartz syenite, Al2O3, Fe2O3, TiO2, MgO, CaO,and P2O5 concentrations decrease with increasing SiO2 content.Regarding trace elements, Nb, Y and Rb increase whereas Sr and Ba

decrease with increasing SiO2 content. Overall, the G. Tarbush alka-line volcanics and granites are characterized by a regular and clearenrichment in Rb, Nb and Y, whereas Ba, Sr, Co and Ni are depleted,implying their high evolved nature.

E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266 257

Table 5Representative composition of Fe–Ti oxides in the quartz syenite.

Titanomagnetite

SiO2 0.60 0.55 0.43 0.36 0.30 0.20 0.53 0.37 0.48 0.42 0.37TiO2 6.59 5.01 8.93 3.28 4.66 5.01 4.64 4.12 4.54 4.03 4.49Al2O3 1.42 0.21 0.04 0.15 0.28 0.36 1.00 0.23 1.15 1.03 0.93Cr2O3 0.07 0.00 0.08 0.00 0.07 0.07 0.05 0.01 0.00 0.06 0.06Fe2O3 52.73 57.93 50.30 61.78 58.54 57.82 56.49 58.02 55.33 56.45 55.76FeO 37.59 35.96 38.97 34.23 35.10 35.29 34.27 34.10 34.62 33.58 33.87MnO 0.45 0.57 0.81 0.29 0.32 0.41 0.32 0.33 0.39 0.42 0.85MgO 0.00 0.00 0.00 0.00 0.12 0.05 0.65 0.00 0.00 0.16 0.00CaO 0.00 0.00 0.00 0.11 0.02 0.00 0.00 0.10 0.00 0.09 0.00Total 99.45 100.23 99.56 100.20 99.41 99.21 97.95 97.28 96.51 96.24 96.34Strcutural formula based on 4 oxygensSi 0.023 0.021 0.016 0.014 0.012 0.008 0.020 0.015 0.019 0.017 0.015Ti 0.189 0.144 0.257 0.094 0.135 0.145 0.135 0.122 0.134 0.120 0.133Al 0.064 0.009 0.002 0.007 0.013 0.016 0.046 0.011 0.053 0.048 0.043Cr 0.002 0.000 0.002 0.000 0.002 0.002 0.002 0.000 0.000 0.002 0.002Fe3+ 1.511 1.661 1.449 1.777 1.693 1.676 1.642 1.716 1.640 1.678 1.659Fe2+ 1.197 1.146 1.247 1.094 1.128 1.137 1.107 1.121 1.140 1.109 1.120Mn 0.015 0.018 0.026 0.009 0.010 0.013 0.010 0.011 0.013 0.014 0.028Mg 0.000 0.000 0.000 0.000 0.007 0.003 0.037 0.000 0.000 0.009 0.000Ca 0.000 0.000 0.000 0.005 0.001 0.000 0.000 0.004 0.000 0.004 0.000

Ilmenite

SiO2 0.26 0.31 0.33 0.27 0.35 0.42 0.39 0.32TiO2 50.28 51.46 52.00 50.77 50.66 50.17 50.49 50.36Al2O3 0.17 0.00 0.14 0.36 0.24 0.26 0.23 0.13Cr2O3 0.00 0.00 0.18 0.00 0.05 0.00 0.00 0.00Fe2O3 5.11 2.66 1.64 4.44 4.53 3.96 1.35 2.86FeO 41.30 40.58 42.69 42.69 43.42 42.03 37.12 38.75MnO 4.17 5.99 4.08 3.25 2.52 3.49 8.46 6.83MgO 0.00 0.00 0.16 0.00 0.00 0.03 0.00 0.00CaO 0.00 0.00 0.04 0.00 0.00 0.00 0.14 0.00Total 101.29 101.00 101.25 101.77 101.77 100.36 98.19 99.25Strcutural formula based on 3 oxygensSi 0.006 0.008 0.008 0.007 0.009 0.011 0.010 0.008Ti 0.943 0.967 0.973 0.947 0.945 0.948 0.973 0.963Al 0.005 0.000 0.004 0.011 0.007 0.008 0.007 0.004Cr 0.000 0.000 0.004 0.000 0.001 0.000 0.000 0.000Fe3+ 0.096 0.050 0.031 0.083 0.085 0.075 0.026 0.055Fe2+ 0.861 0.848 0.888 0.885 0.901 0.883 0.796 0.824Mn 0.088 0.127 0.086 0.068 0.053 0.074 0.184 0.147

0.0000.000

7

mct1tToitpmV2cs1Gorouf

Mg 0.000 0.000 0.006Ca 0.000 0.000 0.001

. Discussion and conclusions

Controversy currently exists regarding origin of alkaline mag-atism of northern ANS. It is widely accepted that during the

losing stage of Pan-African orogeny, the calc-alkaline magma-ism was replaced by a post-orogenic alkaline magmatism (Bentor,985; Beyth et al., 1994). Two post-orogenic associations are dis-inguished in many alkaline provinces in Africa (Bowden, 1985).he early association is related to the terminal phases of majorrogenies (i.e. not strictly anorogenic), while the later associations related to progressive uplift, long-term doming and rifting (i.e.rue anorogenic). A variety of petrogenetic models have been pro-osed for the A-type magmatism of ANS. Many workers accept aantle origin with a significant crustal contribution (e.g. Stern andoegeli, 1987; Beyth et al., 1994; Kessel et al., 1998; Mushkin et al.,003; Azer, 2006; Samuel et al., 2007; Jarrar et al., 2008). Othersonsidered the A-type suites as anatectic melts of various crustalources (e.g. Clemens et al., 1986; Creaser et al., 1991; King et al.,997; Abdel-Rahman, 2006; Ali et al., 2009). The present data for. Tarbush alkaline rocks coupled with the available isotopic dataf A-type rocks in south Sinai can furnish, to some extent, crite-

ia for their petrogenetic interpretation. This includes the naturef their source magma, the proposed protolith(s), the conditionsnder which melts were extracted and the extent of subsequentractional crystallization.

0.000 0.001 0.000 0.0000.000 0.000 0.004 0.000

7.1. Geotectonic affinities

Previous studies dealing with the tectonic model of alkalinemagmatism in Sinai and their counterparts in neighboring areasproposed controversial views. Bentor (1985) suggested that theserocks, at the type locality of G. Saint Katharine, were erupted dur-ing a non-orogenic period under tensional stresses, block faultingand differential uplift while Abdel Maksoud et al. (1993) and AbdelKhalek et al. (1994) considered them as rift-related. In the Elat area(in Negev), no evidence of rifting is reported by Garfunkel (1999)and Mushkin et al. (1999) during the emplacement of late Pro-terozoic alkaline magmatism. In central Wadi Araba (southwestJordan), the uppermost Proterozoic volcanic activity (553 ± 11 Ma)composed of rhyolite flows and tuffs displaying rift-related geo-chemical affinities (Jarrar, 1992). G. Tarbush alkaline rocks haveoversaturated and alkaline nature, no undersaturated rocks orbasalts are recorded. It is well known that fully developed intra-continental rifts are accompanied by significant basalt volcanismand an alkaline (oversaturated to undersaturated) igneous suite(Black et al., 1985; Wilson, 1994; Menuge et al., 2002).

Field studies revealed that the alkaline rocks of G. Tarbush rep-

resent the westernmost outcrops of the Katharina ring complex insouth Sinai; the most prominent example of ring complexes in thenorthern ANS. According to Bentor (1985), it is a part of the alka-line batholithic phase characterized by the emplacement of several

258E.S.Farahat,M

.K.A

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(2011)247–266

Table 6Major and trace element compositions of the alkaline rocks at G. Tarbush area.

Sample Quartz syenite Rhyolite Tuffs Alkali feldspar granitea Syenogranitea

202A 203A 206A 207A 208A 217A T1 T3 T10 T19 T4 T13 T14 210B T7 AG-1 AG-4 AG-7 AG-8 AG-11 S-10 S-30 S-40 S70

SiO2 65.28 64.08 63.61 65.87 64.06 64.01 65.14 63.94 66.07 66.87 76.15 71.86 72.63 74.70 70.98 75.98 76.67 76.73 76.05 77.01 75.03 74.51 74.82 74.72Al2O3 16.78 17.31 17.14 16.72 17.41 17.33 16.95 17.39 16.29 15.96 11.74 14.21 13.38 14.11 14.82 12.51 12.44 12.34 12.45 12.24 12.74 13.11 13.08 13.20Fe2O3 3.42 3.55 3.54 3.10 3.45 3.54 3.37 3.59 3.26 3.19 2.44 2.29 2.70 0.71 2.17 1.47 1.30 1.38 1.35 1.17 1.36 1.52 1.48 1.40MnO 0.12 0.12 0.11 0.11 0.13 0.12 0.11 0.11 0.12 0.11 0.11 0.05 0.08 0.04 0.05 0.06 0.05 0.05 0.04 0.03 0.05 0.05 0.06 0.05MgO 0.63 0.72 0.71 0.56 0.69 0.70 0.60 0.72 0.61 0.53 0.12 0.23 0.12 0.06 0.63 0.11 0.05 0.04 0.06 0.04 0.15 0.25 0.20 0.21CaO 1.91 2.15 2.08 1.57 1.87 2.16 2.01 2.38 1.79 1.64 0.31 0.33 0.56 0.35 0.97 0.46 0.39 0.36 0.40 0.39 0.53 0.68 0.72 0.62Na2O 5.19 5.43 5.11 5.20 5.08 5.28 5.25 5.31 4.75 5.01 4.57 4.41 4.53 5.16 4.84 4.02 3.95 3.87 3.91 3.89 4.01 3.81 3.74 3.86K2O 5.25 4.90 5.18 5.55 5.30 5.22 5.28 5.14 5.19 5.37 4.56 5.64 4.67 4.36 4.19 4.50 4.48 4.42 4.47 4.46 5.07 4.99 4.96 5.03TiO2 0.69 0.77 0.78 0.65 0.79 0.79 0.72 0.79 0.64 0.60 0.17 0.39 0.33 0.11 0.36 0.14 0.08 0.09 0.11 0.10 0.15 0.20 0.18 0.19P2O5 0.22 0.26 0.26 0.18 0.27 0.23 0.22 0.26 0.18 0.17 0.03 0.09 0.07 0.05 0.11 0.05 0.04 0.03 0.04 0.02 0.06 0.06 0.06 0.06LOI 0.43 0.51 1.16 0.62 1.01 0.37 0.40 0.37 0.56 0.44 0.37 0.62 0.64 0.47 0.91 0.52 0.46 0.40 0.45 0.43 0.62 0.43 0.39 0.50Total 100.24 100.14 100.04 100.46 100.43 100.11 100.38 100.37 99.76 100.17 100.71 100.33 99.92 100.22 100.21 99.82 99.91 99.71 99.33 99.78 99.77 99.61 99.68 99.83Mg# 27 29 29 27 29 28 26 28 27 25 9 16 8 15 37 13 7 5 8 6 18 25 21 23Trace elements (ppm)Ba 2364 2869 2667 2168 2795 2584 2502 2842 2004 1847 91 944 591 491 954 108 53 71 63 66 249 370 264 287Co 0 4 0 3 3 0 0 0 4 3 0 5 0 4 5 4 2 2 3 1 3 4 5 4Cr 2 19 0 23 3 15 21 0 16 6 18 5 18 20 16 13 8 9 11 14 16 21 14 18Cu 1 2 4 34 3 51 31 1 2 3 14 3 2 42 1 13 11 9 12 8 11 17 15 13Ga 24 22 24 22 23 23 22 21 22 23 26 25 25 20 20 – – – – – – – – –Hf 8 8 8 9 8 8 9 8 10 10 17 13 13 3 6 – – – – – – – – –Nb 22 19 20 24 20 20 21 19 24 26 50 38 44 26 17 43 39 45 41 46 37 31 33 35Ni 4 5 3 7 4 7 7 3 4 4 8 4 3 6 8 7 6 5 6 3 6 7 5 6Pb 15 12 50 11 18 12 15 13 14 15 20 65 42 5 15 26 19 17 21 14 31 27 24 21Rb 74 59 68 79 74 55 71 59 79 91 218 150 144 77 130 290 319 357 305 363 273 216 237 234Sc 8 10 8 7 11 10 8 9 8 8 0 5 2 2 1 – – – – – – – – –Sr 286 368 337 241 326 344 299 346 247 212 22 68 76 52 321 41 15 23 20 20 65 90 84 79Th 10 7 8 12 8 8 10 7 13 13 31 21 24 5 18 – – – – – – – – –U 3 3 4 3 3 2 3 2 2 4 10 7 8 4 4 – – – – – – – – –V 15 14 18 12 18 13 13 14 15 13 12 9 11 11 23 7 4 6 5 3 13 19 15 13Y 33 28 31 34 31 28 31 29 35 38 86 51 62 40 38 55 54 60 52 59 51 38 43 40Zn 92 85 122 97 101 109 98 84 97 89 146 138 147 52 46 51 34 46 37 29 22 29 31 25Zr 275 289 298 363 296 292 328 292 367 388 679 542 584 112 257 160 149 151 152 162 180 171 178 175

ASI 0.95 0.95 0.96 0.96 0.99 0.95 0.94 0.93 0.97 0.94 0.90 1.02 0.99 1.02 1.04 1.01 1.03 1.04 1.04 1.02 0.98 1.01 1.02 1.02AI 0.85 0.82 0.82 0.87 0.81 0.83 0.85 0.82 0.83 0.88 1.06 0.94 0.94 0.94 0.84 0.92 0.91 0.90 0.91 0.92 0.95 0.89 0.88 0.89Rb/Ba 0.03 0.02 0.03 0.04 0.03 0.02 0.03 0.02 0.04 0.05 2.41 0.16 0.24 0.16 0.14 2.69 5.97 5.01 4.83 5.53 1.10 0.58 0.90 0.81Rb/Sr 0.26 0.16 0.20 0.33 0.23 0.16 0.24 0.17 0.32 0.43 9.95 2.22 1.90 1.47 0.41 7.07 21.27 15.52 15.25 18.15 4.20 2.40 2.82 2.96Zr/Nb 12.6 15.3 15.3 14.9 14.7 14.9 15.5 15.4 15.6 14.9 13.5 14.3 13.3 4.3 15.5 3.7 3.8 3.3 3.7 3.5 4.9 5.5 5.4 5.0Ba/Nb 108.4 151.8 136.7 88.8 138.4 131.8 118.0 150.4 85.3 71.0 1.8 25.0 13.5 19.0 57.5 2.5 1.4 1.6 1.5 1.4 6.7 11.9 8.0 8.2Rb/Nb 3.4 3.1 3.5 3.3 3.6 2.8 3.3 3.1 3.3 3.5 4.3 4.0 3.3 3.0 7.8 6.7 8.2 7.9 7.4 7.9 7.4 7.0 7.2 6.7K/Nb 1999.5 2151.3 2203.9 1889.6 2178.1 2211.7 2068.7 2259.4 1833.0 1714.2 751.7 1237.5 884.3 1401.6 2093.8 868.7 953.6 815.4 905.0 804.9 1137.5 1336.2 1247.7 1192.1

a Samples analyzed in Saudi Arabia.

E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266 259

Clinoenstatite Clinoferrosillite

Pigeonite

Augite

Diopside Hedenbergite

En Fs

Wob.

Sani

dine

Anrth

ocas

e

ol

Albite Ol o la eig c s Andesine

Labradorite BytowniteA

rthno

ite

Ab An

OrPeralkaline rhyolite

Quartz syenitea.

Jadeite Aegirine

Omphacite Aegirine-Augite

Quad

Jd

WEFc.

Ae

d.

(t)FeO

Alkaline

5 15 25 355

10

15

20

25O

Al

32

Peraluminous

Calc-alkaline

F b andv 94).

rav

pTNriiomTscmfao

c

ig. 4. (a) Or–Ab–An ternary diagram of the analyzed feldspars (Deer et al., 1992), (s. Al2O3 biotite discriminant diagram for the analyzed biotites (Abdel-Rahman, 19

ing complexes of quartz-monzonite, quartz syenite and high-levellkali granites that intrude the alkali rhyolites of earlier Katharinaolcanics.

The tectonic environment of G. Tarbush alkaline rocks is inter-reted by an integrated approach using all the gained information.he present study showed that the alkaline rocks cross-cut lateeoproterozoic calc-alkaline rocks. The intrusion of the alkaline

ocks was preceded by the intrusion of dyke swarms of vary-ng compositions that cut across the calc-alkaline rocks but arentruded by the alkaline rocks. The dyke swarms formed as a resultf extension and mark the switch from calc-alkaline to alkalineagmatism (Friz-Töpfer, 1991). Therefore, the alkaline rocks of G.

arbush formed most likely during a post-orogenic period. Thistage is characterized by the radical change from late-orogenicalc-alkaline magmatism to post-orogenic alkaline/peralkalineagmatism. The post-orogenic setting of the investigated rocks is

urther supported by both their A2-subtype characteristics (Fig. 5e)

nd ages (∼593–583 Ma) which are coeval with the ANS post-rogenic stage (∼608–580 Ma).

The overall geochemical characteristics of the studied rocks areonsistent with a within-plate tectonic setting. G. Tarbush alka-

c) classification diagrams of clinopyroxenes (Morimoto et al., 1988), and (d) FeO(t)

line rocks are plotted in the within-plate field of Nb vs. Y tectonicdiscrimination diagram (not shown) of Pearce et al. (1984). Ithas been established that the geochemical characteristics of cer-tain magmatic minerals (e.g. biotite, amphibole and pyroxene)may be used to identify the magmatic affinity and tectonic envi-ronments of their host rocks. Biotites of the quartz syenite aremostly iron-rich and similar to biotites of alkaline igneous rocks(Fig. 4d; Abdel-Rahman, 1994). In addition, the presence of sodicamphibole and pyroxene in rhyolites indicates their peralkalineaffinities.

7.2. Petrogenesis

ANS alkaline igneous rocks are generally regarded as the prod-uct of either extensive fractional crystallization of mantle-derivedmafic magmas (e.g. Stern and Gottfried, 1986; Bonin, 2007) or par-tial melting of various crustal sources (e.g., Clemens et al., 1986;

Creaser et al., 1991; King et al., 1997; Abdel-Rahman, 2006; Farahatet al., 2007; Ali et al., 2009). However, due to the contrasting evi-dences in favor or against each of the two models, the mantle originwith crustal contribution is sometimes suggested for alkaline gran-

260 E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266

10

100

1000

3000

Ga/Al10 122 4 6 8

rZ

I-, S- andM-typegranites

A-type granite

d.

6

8

10

12

10 122 4 6 8Ga/Al

a+K

ON

O 22

I-, S- andM-typegranites

A-type granite

c.

0 2 4 6 8 10 120.5

1.0

1.5

2.0

100*(MgO+FeO +TiO )/SiO(t) 2 2

Calc-alkaline

Alkaline

i l f c ona

H gh y ra tted

icalc-alkaline

+aO

)/((A

l OC

23

F+

O+

O)

eON

aK

t( )2

2

b.

10 20 30 400

10

20

30

40

50

ANOR

/Q

/F

Quartz syenite

RhyoliteTuffs

Alkali feldspar graniteSyenogranite

Alkaligranite

Monzogranite

Granodiorite

Syenogranite

Quartzsyenite Quartz

monzoniteQuartzalkali feldspar

syenite

a.

A1

A2

0.1 1 100.5

10

30

b/b

RN

Y/Nb

e.

Fig. 5. (a) Q/(F/)-ANOR diagram for normative classification of G. Tarbush alkaline rocks (Streckeisen and Le Maitre, 1979), (b) (Al2O3 + CaO)/(FeO(T) + Na2O + K2O) versus100(MgO + FeO(T) + TiO2)/SiO2 diagram for distinguishing between calc-alkaline, highly fractionated calc-alkaline granites and alkaline granites (Sylvester, 1989), (c) Ga/Alvs. (Na2O + K2O) diagram of G. Tarbush alkaline rocks (Whalen et al., 1987), (d) Ga/Al vs. Zr diagram of G. Tarbush alkaline rocks (Whalen et al., 1987), and (e) Rb/Nb versusY/Nb diagram of the alkaline rocks of G. Tarbush (Eby, 1992); A1: A-type granitoids with an OIB-type source. A2: A-type granitoids with crustal derived magmas.

E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266 261

0.0

1.0

2.0

3.0

4a

.0

FeO 2

3

0.0

0.2

0.4

0.6

0.8

MgO

0.0

1.0

2.0

3.0

CaO

60 70 8010

12

14

16

18

20

AlO 2

3

60 70 800.0

0.3

0.6

0.9

TiO2

SiO

0.0

0.1

0.2

0.3

PO 2

5

ace (b

i2

mge2latctBn1a

SiO2

Fig. 6. The relation between SiO2 and some major (a) and tr

tes in Sinai (e.g. Azer, 2007; Samuel et al., 2007; Be’eri-Shlevin et al.,009b).

Of the above two possible models, fractional crystallization ofantle-derived mafic magmas is the most advocated for the ori-

in of ANS A-type granites (e.g. Stern and Voegeli, 1987; Beytht al., 1994; Kessel et al., 1998; Mushkin et al., 2003; Jarrar et al.,008) based mainly on geochemical modeling. However, many geo-

ogic, geochemical, and isotopic characteristics of the alkaline suitesrgue against such a model. For instance, Eyal et al. (2010) statedhat the geochemical modeling of the fractional crystallization pro-ess calls for assumptions that make the results unconvincing. Also,he ratios of the highly incompatible trace elements (e.g. Zr/Nb,

a/Nb, Rb/Nb and K/Nb; Table 6) have averages lying within orear the values currently in use for the continental crust (Weaver,991; Wedepohl, 1994). Low Mg# of the alkaline rocks of G. Tarbushccompanied by low concentrations of Cr (0–23) and Ni (3–8) fur-

2

) elements of G. Tarbush alkaline rocks. See legend in Fig. 5.

ther exclude mantle origin of the studied rocks. Significantly, theabsence or scarce occurrence of mafic lithologies in the alkalinesuites argues against extensive fractional crystallization of maficmagma. The generalization that nine parts of mafic magma frac-tionates to produce only one part felsic magma (Winter, 2001)makes it difficult to explain how such great quantities of felsicmagma in south Sinai can result from fractional crystallization ofmafic magma because so little of the parent can be found. Moreover,intermediate compositions are missing even when mafic parentalmelts can be found. Moreover, the marked difference in magmatic�18O values between alkaline silicic and mafic rocks within thenorthern ANS argues against co-genetic relationships (Katzir et al.,

2007b; Be’eri-Shlevin et al., 2009b, 2010).

Although unlikely originated by fractional crystallization ofmantle parent magma, the geochemical characteristics of theinvestigated alkaline rocks indicate their evolution via such a pro-

262 E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266

10

20

30

40

50

60

Nb

20

30

40

50

60

70

80

90

Y

100

200

300

400

500

600

700

ZrR

b

60 70 800.0

100

200

300

400

60 70 800.0

1000

2000

3000

Ba

0.0

100

200

300

400b

Sr

(Conti

ctKdpcsawtcsitseGaae

SiO2

Fig. 6.

ess from a common parent magma, probably quartz syenite. Onhe CaO/Al2O3 vs. D.I (D.I = norm content Qz + Or + Ab) and Rb vs.2O/Rb plots (Fig. 7a and b), the studied alkaline rocks exhibit oneistinct compositional trend consistent with origination from onearent magma. The role of feldspar fractionations is indicated byovariation between Sr and Ba (Fig. 7c), both of which decreaseystematically from quartz syenite to alkali feldspar granite. Srnd Ba show strong compatible behavior, which is in agreementith plagioclase fractionation during the magmatic evolution of

he suite. The role of feldspars fractionation is further indicated byovariation in Rb/Sr and Rb/Ba ratios which increase with progres-ive fractionation (Fig. 7d). Increasing Rb/Ba ratios are especiallyndicative of alkali feldspar fractionation (Henderson, 1982). Sys-ematic decrease in Fe2O3 and MgO with increasing silica (Fig. 6a)uggests that fractionation of primary ferromagnesian silicate min-rals played a role in developing the compositional variation in the

. Tarbush alkaline suite. MgO vs. Rb/Sr and TiO2 vs. Rb/Sr vari-tion diagrams (Fig. 7e and f) suggest that amphibole, pyroxenend Fe–Ti oxides fractionation were also important during magmavolution.

SiO2

nued ).

Given that the investigated alkaline rocks cannot be generatedfrom mantle-derived mafic magma by fractional crystallizationleave formation via partial melting of crustal materials as the mostplausible mechanism. Some petrogenetic models concluded thatA-type granitic melts are generated by melting of the lower crust,which include granulitic lower crust (Collins et al., 1982; Chappellet al., 1987), charnokitic lower crust (Landenberger and Collins,1996), or tonalitic crust (Cullers et al., 1981; Creaser et al., 1991). Inthe Eastern Desert of Egypt, Abdel-Rahman (2006) suggested thatthe A-type granitic magma has been formed by high degrees ofbatch partial melting of metasomatized Pan-African calc-alkalinerocks. Moreover, Farahat et al. (2007) concluded that ANS A-typegranites are better modeled by partial melting of tonalitic middlecrustal source represented by the least fractionated rocks of the pre-collisional (island-arc) calc-alkaline tonalite–granodiorite suites.It is interesting noting that the modeled K2O (5.55–5.03 wt.%),

Sr (171–146 ppm), Rb (140–117 ppm) and Nb (20–15 ppm) val-ues after 20–30% partial melting of tonalitic middle crustal source(Farahat et al., 2007) generally match those of the G. Tarbush quartzsyenite (5.55–4.90 wt.%, 368–212, 91–55 and 26–19 ppm, respec-

E.S. Farahat, M.K. Azer / Chemie der Erde 71 (2011) 247–266 263

75 80 85 90 95 1000.00

0.10

0.20

D.I.=Qz-Or+Ab

CaO

/AlO 2

3a.

0 100 200 300 4000.00

0.02

0.04

0.06

0.08

0.10

Rb

KO

/Rb

2

b.B

a

0.0 100 200 300 4000.00

500

1000

1500

2000

2500

3000

3500

Sr

Sr

c.

0 1 2 3 4 5 6 70

13

25

Rb/Ba

Rb/

Sr

d.

0.0 0.2 0.4 0.6 0.8

0.0

10

20

/

MgO

Rb/

Sr

e.

Rb/

Sr

0.0 0.2 0.4 0.6 0.80.0

10

20

TiO

f.

s in th

tmAasi

Fig. 7. Some major and trace elements relationship

ively). Similarly, Eyal et al. (2010) suggested partial melting ofonzodioritic middle crustal source for the generation of Sinai

-type granites. Such models find further geologic, geochemical,nd isotopic evidences. The partial melting process explains thecarce mafic rocks and the absence of intermediate compositionn the alkaline plutonic suites. Moreover, it could account for the

2

e alkaline suite of G. Tarbush. See legend in Fig. 5.

systematic variations in many major and trace element contentsof calc-alkaline and alkaline suites from the ANS that led some

authors (e.g. El-Gaby, 1975) to suggest that the granitoid rocks fromthe ANS represent one continuous series. Moreover, the generationof the alkaline granites via partial melting of the pre-collisionalcalc-alkaline granitoids finds additional support from the relative

2 mie der Erde 71 (2011) 247–266

aStDeIa2fmtzgoTgARn+sm�t2

lwiaThcdw

7n

goDtaqghn2HpoAliphcio

(eea

Fig. 8. Cartoons showing the tectonomagmatic evolution of the ANS juvenile crustfrom syn- to post-collisional stages. The crust and mantle thickness used are not toscale. See text for further explanation.

64 E.S. Farahat, M.K. Azer / Che

bundances of the two granitoid types in the Eastern Desert andinai. The A-type granites become increasingly important relativeo calc-alkaline igneous rocks from the southern part of the Easternesert of Egypt (1 to 4) through the central part (1 to 1) to the north-rn part of the Eastern Desert and Sinai (12 to 1) (Bentor, 1985).n addition, rare inherited zircons with ages ∼740 and ∼1790 Mare recently recognized in the A-type granites from Sinai (Ali et al.,009). The presence of inherited zircons of high ages in A-type rocksrom south Sinai indicates either contamination during ascent or

ost likely crustal origin of these rocks. According to Ali et al. (2009)he weighted mean 206Pb/238U age of 741 ± 8 Ma for two inheritedircons grains in the Wadi Ghazala syenogranite from Sinai sug-ests derivation from the ‘older’ granites. However, a wide rangef isotopic data indicates that A-type melts are mantle-derived.he mantle signature in the A-type rocks can be attributed to theenerated by partial melting of juvenile and/or metasomatized Pan-frican ANS crust (e.g. Haapala et al., 2007; Farahat et al., 2007).ecently, Morag et al. (2011) argued that the alkaline rocks fromorthern ANS are characterized by positive average εHf(t) values of7 to +8, which are consistent with both derivation from a mantleources and reworking of the young juvenile crust, implying re-elting of the preceding island-arc crust. Moreover, the elevated

18O of some A-type granites from northern ANS was interpretedo indicate derivation from crustal reservoirs (Be’eri-Shlevin et al.,009b).

In summary, the investigated A-type rocks were generated mostikely via partial melting of ANS middle juvenile arc crustal source

ith subsequent fractionation of feldspars and mafic minerals dur-ng evolution. The heat required for such partial melting can beccounted for by mantle-derived, volatile-rich, melt intraplating.he volatiles from the upper mantle were important agents ofeat transfer, and sufficient for metasomatising and anatexing therustal rocks. These melts are related to decompression meltingue to erosional uplifting following lithospheric delamination asill be discussed in detail hereinafter.

.3. Regional considerations of post-collisional magmatism inorthern ANS

Late- to post-collisional calc-alkaline to alkaline/peralkalineranitoids are widely distributed, constituting ∼80 of the basementutcrops, in the extreme northern part of the ANS, i.e. north Easternesert and Sinai. This likely attests to the high crustal growth rate of

he ANS crust formation that cannot be explained solely by simplerc–arc accretion model (e.g. Stein and Hofmann, 1994). Conse-uently, understanding the origin of these rocks has significanteodynamic implications. The high crustal growth rate of the ANSas been attributed to either contribution of pre-Pan African conti-ental components (e.g. Dixon and Golombek, 1988; Farahat et al.,004; Hargrove et al., 2006) or plume interaction (e.g. Stein andofmann, 1994; Stein, 2003; Farahat, 2006). The contribution ofr-Neoproterozoic crust has been recently specified based mainlyn the increasing recognition of old, xenocrystic zircon in juvenileNS igneous rocks. However, Liégeois and Stern (2010) have chal-

enged this interpretation since inherited zircons can be introducedn juvenile magmas through several ways without requiring thearticipation of ancient crust. Recently, some authors related suchigh crustal growth rate to asthenospheric uprise due to obliqueonvergence during the pre-collisional tectonic setting and follow-ng lithospheric delamination during late-to post collisional stagef the ANS crust evolution (e.g. Farahat et al., 2007).

The transition from calc-alkaline (∼630–590 Ma) to alkaline

∼610–580 Ma) magmatism during ANS crust evolution has beenxplained in terms of lithospheric delamination process (Farahatt al., 2007, 2011; Avigad and Gvirtzman, 2009). The latter mech-nism involves lithospheric removal subsequent to crustal and

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A

E.S. Farahat, M.K. Azer / Che

antle thickening and subduction break-off during the colli-ional (Fig. 8a), i.e. the period of maximum convergence, stage∼670–630 Ma). Such lithospheric removal results in upwellinghe hot lighter asthenosphere and crustal uplifting, up to ∼3 kmn the northern ANS (Avigad and Gvirtzman, 2009). The astheno-pheric uprise likely caused extensive melting of the remainingithosphere (Fig. 8b). The melts produced underplate the lowerrust to promote its partial melting and generate the late-collisionalalc-alkaline granitoids (Farahat et al., 2011). This large scale melt-ng could accounts to the wide distribution of the late-collisionalalc-alkaline magmatism (∼630–590 Ma) in the northern ANS.

During the post-collisional crustal extensional stage∼610–580 Ma), the effect of lithospheric delamination, andhus asthenospheric uprise, likely diminishes. The upper mantle-erived melts produced in turn as a result of decompressionalelting due to erosional uplifting (Fig. 8d). Such melts intraplate

he middle crustal levels, facilitated by the abundance of strike-slipaults and shear zones during the post-collisional crustal exten-ional stage, to promote their partial melting. Under such conditionolatile could influx into crust from deeper sources. The middleNS crust at this stage was formed likely of island arc calc-alkalineranitoids. The common occurrence of core complexes, strike-slipaults, shear zones and dyke swarms attest to abundances ofhe inferred extensional structure during this stage of ANS crustvolution. We could speculate that during the transition from late-ollisional to early post-collisional stage (∼610–590 Ma), i.e. withhe onset of extensional structure, mantle-derived melt underplatend also intraplate the crust, i.e. both calc-alkaline (∼630–590 Ma)nd alkaline (610–580) magmatism could form coevally (Fig. 8c).s we early mentioned, the investigated G. Tarbush alkaline rocksre part of the Katharina ring complex. The early mentioned ages ofhe Katharina ring complex [596 ± 18 Ma (Abu Anbar et al., 1999),93 ± 16 (Katzir et al., 2007a) and 583 ± 6 Ma (Be’eri-Shlevin et al.,009a)] clearly indicate a post-collisional setting. It is importanto mention that, erosion related to crustal uplift during the late-ollision stage could be responsible for the lack or infrequentccurrence of older lithologies, i.e. island arc metavolcanics andarginal basin ophiolites, from the northern part of the ANS.

cknowledgments

The authors are greatly indebted to Prof. Stern, University ofexas at Dallas, U.S.A. for his critical reading and valuable com-ents that improved this contribution. E.S. Farahat would like to

hank Profs. C. Hauzenberger and G. Hoinkes for putting some ofhe analytical facilities of the Institute of Earth Sciences, Graz Uni-ersity, Austria at his disposal. An anonymous reviewer is thankedor critical reviews that considerably improved this manuscript.

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