Paleozoic alkaline volcanism: geochemistry and petrogenesis of Um Khors and Um Shaghir trachytes of the central Eastern Desert, Egypt

Page 1

ORIGINAL PAPER

Paleozoic alkaline volcanism: geochemistry and petrogenesisof Um Khors and Um Shaghir trachytes of the centralEastern Desert, Egypt

Moustafa E. Gharib & Mohamed A. Obeid &

Ahmed H. Ahmed

Received: 30 April 2010 /Accepted: 3 October 2010 /Published online: 26 October 2010# Saudi Society for Geosciences 2010

Abstract The Um Khors and Um Shaghir trachyte (UKTand UST) plugs and sheets represent two conspicuousoutcrops of Paleozoic alkaline volcanism in the centralEastern Desert of Egypt. The trachyte magmatisms eruptedalong the Pan-African NW-trending shear zone (302±15 and273±15 Ma, respectively) and intruded the Late Proterozoicrocks of the studied area. The trachyte rocks consist mainlyof sanidine, anorthoclase, albite, and quartz with a noticeableamount of aegirine–augite, aegirine, hedenbergite, andarvedsonite. The studied trachytes are moderately evolvedin composition (with 62–67.5 wt.% SiO2) and exhibit alimited compositional range in most of the major elements.They are alkaline in nature and considered as silica-oversaturated rocks. The rare earth elements (REE) patternsare somewhat uniform and highly fractionated, beingenriched in light REE over heavy REE and show prominentnegative Eu anomalies. The UKT and UST are enriched inhigh field strength elements Nb, Zr, and Y, consistent withtypical within-plate alkaline magmatisms of extensionaltectonic regimes. They were generated through the fractionalcrystallization of mantle-derived magmas. Although the USTis younger than the UKT, they show approximately similarchemical compositional ranges of the most major and traceelements, with somewhat higher MgO, Cr, Ni, and Bacontents in the former. This may argue against the evolutionof the UST via a continuous fractional crystallization of the

residual magmatic melt of the UKT. Thus, the UKT and USTare genetically related but could be emplaced through twovarious magmatic pulses of the same parent source (i.e.,asthenospheric mantle source) at different times. Theascending magmatisms were subjected to variable significantdegrees of crustal contamination during their generation.

Keywords Paleozoic alkaline volcanism . Um Khors . UmShaghir . Trachyte . Asthenospheric mantle . Eastern Desert .

Egypt

Introduction

The Arabian–Nubian Shield (ANS) extends over most ofNE Africa and the Arabian Peninsula, covering an area ofabout 3 × 106 km2. The ANS crust is considered to havedeveloped during the Pan-African event (950–550 Ma;Kröner 1985) by rapid subduction along a series of islandarcs (Stoeser 1986). It was stabilized by accretion andsubsequent sweeping together of this arc system. Towardthe end of the Pan-African event (650–550 Ma), the calc-alkaline arc-related magmatism was replaced by calc-alkaline to alkaline post-orogenic magmatism (Stern 1981;Abdel-Rahman 1995). From the end of the Pan-Africanorogeny until the Tertiary, the ANS was extensivelyintruded by alkaline to peralkaline, basic to acidic, plutonicto volcanic rocks (ca. 600–30 Ma; Harris 1982). Theseigneous rocks represent the surface manifestations ofwithin-plate or A-type magmatism during the anorogenictectonic–magmatic phase of the Arabian–Nubian Shield.

Most of the Egyptian alkaline rocks are located in theEastern Desert and considered part of its basementcomplex. They comprise a wide variety of alkaline granites,

M. E. Gharib (*) :A. H. AhmedGeology Department, Faculty of Science, Helwan University,Cairo, Egypte-mail: [email protected]

M. A. ObeidGeology Department, Faculty of Science, El Fayoum University,El Fayoum, Egypt

Arab J Geosci (2012) 5:53–71DOI 10.1007/s12517-010-0212-4

Page 2

ring complexes, volcanic suites, and ring dykes. ThePhanerozoic alkaline volcanic suites constitute <0.1% ofthe whole basement rocks of the Eastern Desert. They wereemplaced along reactivated Pan-African fractures or preex-isting lithospheric weakness zones (Meneisy 1986). ThePhanerozoic volcanisms erupted through three phases ofactivation (Meneisy and Abdel Aal 1984). These volcanicphases belong to Paleozoic (395–233 Ma), Mesozoic (191–74 Ma), and Tertiary (48–15 Ma). Paleozoic and Mesozoicvolcanics are diverse in their chemical and eruptivecharacteristics,

Volcanicity of the Paleozoic Era in the central andsouthern parts of the Eastern Desert occur as plugs, sheets,dykes, cones, and little sills. Based on the isotopic agedating, Meneisy (1990) suggested three main volcanicepisodes for the Paleozoic volcanics in Egypt, including:the Late Carboniferous, Permian, and Permo-Triassicperiods. These volcanics comprise a wide variety of rocktypes: basalts, andesites, trachytes rhyolites, bostonites, andlatites (Aly and Moustafa 1984).

There are only few detailed mineralogical, geochemical,and tectono-magmatic studies carried out on the Paleozoicvolcanisms of the central Eastern Desert (e.g., Hassan et al.1997; Ghoneim et al. 1998: Heikal 2003). This paperintroduces new mineralogical and geochemical data on thePaleozoic trachyte plugs and sheets of the Um Khors–UmShaghir area. The aim of the present study was to outlinethe petrological, mineralogical, and geochemical character-istics of these Paleozoic trachyte rocks and to discuss theirpetrogenetic implications.

Relationships to Pan-African tectonic structures

The African continent has widespread occurrences of alkalineprovinces throughout the Phanerozoic. The alkaline provincesin NE Africa consist of anorogenic granites, ring complexes,dyke swarms, and alkaline volcanisms (Fig. 1a). ThePaleozoic magmatisms of the Nubian Shield (554–245 Ma)are represented by diverse igneous activities of the ringcomplexes and alkaline volcanisms (Hashad 1980). Thedistribution of the ring complexes in the Eastern Desert isstructurally controlled by N 60° E crustal block faults andshear zones and N 30° W deep-seated tectonic zones relatedto the opening of the Red Sea (Garson and Krs 1976). TheUm Khors–Um Shaghir area lies along a major Pan-AfricanNW shear zone that prevailed during the geodynamicevolution of the Arabian–Nubian Shield (Fig. 1b). The UmKhors trachyte (UKT) and Um Shaghir trachyte (UST)erupted through a long period (302 and 273 Ma, respective-ly; El-Manharawy 1972) of crustal extension around thePaleozoic boundaries due to crustal relaxation, cooling, andfracturing of the newly formed Pan-African crust. They

represent the Paleozoic volcanic activations that followed thePan-African tectonic events in the Nubian Shield. The UKTand UST subsequently erupted along the reactivated Pan-African fractures and shear zones of the central EasternDesert, which are marked by anorogenic alkaline magma-tisms during the Phanerozoic Era.

Field observations

The UKT and UST plugs and sheets are exposed in thecentral Eastern Desert, south of Qift-Quseir road (Fig.1b).Field investigations of the Um Khors–Um Shaghir areawere carried out by Aly (1973), Akaad and Shazly (1977),El-Desuky (1978), Heikal (2003), and Joseph (2007). Thestudied area has moderate to high mountainous rockscomprising a series of Late Proterozoic rocks, which wereextruded by the UKT and UST masses. The LateProterozoic rocks consist of serpentinites, metasediments,metavolcanics, metagabbro-diorite, tonalite–granodiorite,Hammamat sediments, and granites (Fig. 2). The serpen-tinites form huge and isolated lenses in the western side ofthe area. The metasediments are composed of schist,metagreywackes, and metamudstones. The metavolcanicsare mainly composed of basalt and andesite. The metagab-bro–diorite rocks are made up of gabbro, diorite, monzo-gabbro, and monzodiorite (Essawy and Shazly 1978). Thetonalite–granodiorite rocks are massive, medium- to coarse-grained, pale pink in color, and slightly deformed. Theinvestigated trachytes represent the youngest rock unit inthe studied area. The UKT and UST form two major plugs(approx. 2.5 km2) with small sheets and dykes. The UKTplug dips outward in all directions and possesses invariablesharp contact with the metagabbro–diorite rocks. The othersmall trachyte sheets are distributed in the north directionalong Wadi Um Khors. The UST plug intrudes the tonalite–granodiorite rocks. It shows an elongated curved shape andextends for about 3.5 km toward E–W trend. It dips 65° Sat the foothill, whereas the uppermost parts of the plug arevery steep. The trachyte rocks are massive, fine-grained,and vary in color from reddish brown to buff. Theysometimes show porphyritic and trachytic textures. TheUm Khors trachyte has Rb/Sr isochron age of about 302±15 Ma, whereas the Um Shaghir trachyte yields isotopicage of about 273±15 Ma (El-Manharawy 1972).

Petrography

Microscopically, the UKT and UST rocks consist essential-ly of sanidine, albite, anorthoclase, quartz, aegirine–augite,and arvedsonite. Augite is only recognized in the UKT,whereas the more sodic pyroxene mineral (i.e., aegirine) is

54 Arab J Geosci (2012) 5:53–71

Page 3

recorded in the UST. Iron oxides, apatite, and zircon are themain accessories. Secondary minerals are sericite, claymineral, and chlorite. The rocks are mainly fine, whereporphyritic and trachytic textures are not common.

Sanidine is the most common mineral. It forms euhedralprismatic microcrystals (up to 0.5 mm in length) and is

arranged in a subparallel fluxional way, giving the trachytictexture (Fig. 3a). Most of these crystals exhibit simpletwinning and are slightly altered to sericite and koalinite.Sanidine sometimes occurs as microlites (approx. 0.1 mm).Albite is present as minute laths (0.3 × 0.2 mm) or asmicrophenocrystals reaching up to 0.8 mm. The micro-

Tertiary-Recent Alkaline Volcanism

Ring-complex province

Craton

30 O W

30 O N

30 O E0 O

Romanche FZ

T A L

Chain FZ

Charcot FZ

C A L

N

A

B

N Studied area (Fig. 2 )

0 100 km

Trachyte plug

Ring complex

Deep-seated fracture

Tectonic zone

S i n a iGulf of Suez

Najd shear zone

Qena

Qift

Aswan

Bernice

Quseir

E a s t e r n

D e s e r t

R E D

S E A

Riv

er

N

ile

Arabian Shield

Shear zone

Tertiary-Recent Alkaline Volcanism

Ring-complex province

Craton

30 O N

30 O EO

0 O0 O Romanche FZ

T A L

Chain FZ

Charcot FZ

C A L

N

A

B

N Studied area (Fig. 2 )

28 O28 O

26 O26 O

24 O24 O

00 100 km

Trachyte plug

Ring complex

Deep-seated fracture

Tectonic zone

S i n a iGulf of Suez

Najd shear zone

Qena

Qift

Aswan

Bernice

Quseir

E a s t e r n

D e s e r t

R E D

S E A

Riv

er

N

ile

Arabian Shield

34 OO 38 OO

34 O34 O 36 O36 O

Shear zone

Fig. 1 a Distribution of themajor alkaline provinces inNorth Africa. The CentralAfrican Lineament (CAL) andthe Trans-African Lineament(TAL) and their relation to thetransform faults of theEquatorial Atlantic Ocean arealso shown (after Schandelmeierand Pudlo 1990). b Locationmap of the Um Khors–UmShaghir area and some mainring complexes in Egypt inrelation to major structuraltrends of the Arabian–NubianShield (structural lines are fromGarson and Krs, 1976)

Arab J Geosci (2012) 5:53–71 55

Page 4

phenocrystals are commonly twinned with intense alteration.They are embedded along the groundmass, forming aprophyritic texture (Fig. 3b). Quartz occurs as subhedral grains(approx. 0.3 mm across) filling the interstices between thefeldspar minerals. It is also found as fine inclusions in someK-feldspar microlites. Arvedsonite occurs as anhedral crystals(approx. 0.3 mm across) and minute aggregates. It iscommonly fresh and strongly pleochroic from yellowishgreen to deep green. Some arvedsonite crystals are partiallycorroded and replaced by iron oxides along their cleavage(Fig. 3c). Augite present as subhedral microcrystals (approx.0.08 mm) in the groundmass of the UKT. Augite is usuallypale green in color and frequently rimmed by thin film ofarvedsonite, giving a corona texture (Fig. 3d). Aegirine–augiteforms short subhedral to anbhedral crystals (up to 0.1 mmacross) scattered in the groundmass. They are slightlypleochroic and show high interference colors. Some aegir-ine–augite crystals are replaced in places by iron oxides(Fig. 3e). In the UST, the aegirine forms subhedral crystalsand often occurs in close association with the other maficminerals (Fig. 3f). Apatite and zircon are rarely disseminatedin the groundmass. Apatite is present as minute needles.

Zircon forms euhedral prisms which sometimes show zoning.In polished sections, iron oxides are represented by titano-magnetite, magnetite, and hematite. They occur in two formsof different origins: fine-grained subhedral crystals of primaryorigin and as scattered shapeless aggregates, most probablyalteration products of the preexisting mafic minerals. Titano-magnetite is pale gray with a faint brownish tint and is weaklyanisotropic. It has moderate reflectivity with no internalreflection (Fig. 3g). Magnetite crystals have a pale gray colorwith a faint pinkish tint. They are isotropic with no internalreflection. Most of the magnetite grains are partially altered tohematite. Hematite is bluish white with moderate reflectivityand blood red internal reflection. Secondary hematite iscommon after arvedsonite and aegirine–augite (Fig. 3h).

Geochemistry

Mineral chemistry

Analysis of the pyroxene, amphibole, and feldspar in thestudied trachytes (Tables 1, 2, and 3) was carried out using

33 O 55 /

N

G. Um Shaghir

LEGEND

0 1 km

Trachytes

Granites

25 O

55 /

Hammamat sediments

Tonalite-granodiorite

Metagabbro-diorite

Metavolcanics

W. Um KhorsMetasediments

G. Um Khors

Serpentinites

Fig. 2 Geologic map of the UmKhors–Um Shaghir area, centralEastern Desert of Egypt(modified after Akaad andNoweir 1980)

56 Arab J Geosci (2012) 5:53–71

Page 5

a JEOL electron probe micro-analyzer JXA-8800 hosted atthe Cooperative Research Center of Kanazawa University,Japan. Analytical conditions were 15-kV acceleratingvoltage, 20-nA probe current, and 3-μm beam diameter.Counting time was 10 s for all major and minor elements.Ferrous and ferric Fe were calculated using the equation ofDroop (1987).

Pyroxene minerals are well represented in the studiedtrachytes. Representative analyses of the pyroxene micro-crystals in both UKT and UST (out of 40 probe analyses)

are shown in Table 1. Pyroxenes show a wide composi-tional range, varying in terms of En–Fs–Wo end memberfrom 92% to 28% within the Um Khors trachyte and from43% to 1% in the Um Shaghir trachyte. On the basis of themineralogical classification diagrams of Morimoto (1989),the pyroxenes of the UKT consist of hedenbergite andaegirine–augite, but that of the UST varies from aegirine–augite to aegirine (Fig. 4a, b).

Selected analyses of some amphibole crystals and theiratomic ratios are given in Table 2. All the investigated

Augite

Arvedsonite

a b

dc

e f

hg

Fig. 3 a Photomicrographshowing a trachytic texture inthe UKT, crossed Nicols. bPhotomicrograph of an albitephenocrystal exhibitingporphyritic texture in the UKT,polarized light.c Photomicrograph ofarvedsonite crystal partiallycorroded and replaced by ironoxides along their cleavage inthe UST, polarized light.d Photomicrograph of augitecrystal rimmed by thin film ofarvedsonite, giving coronatexture in the UKT, polarizedlight. e Photomicrograph ofaegirine–augite crystal highlyaltered to iron oxides (UST),polarized light. f Photomicro-graph of aegirine crystals inassociation with arvedsonite inthe UST, polarized light.g Photomicrograph of primarytitanomagnetite and hematitecrystals embedded in thegroundmass of UKT, incidentlight. h Photomicrograph ofsecondary hematite crystalwhere the alteration of the maficcrystal starts from the grainboundary, incident light

Arab J Geosci (2012) 5:53–71 57

Page 6

crystals are sodic amphiboles in the sense of Leake et al.(1997), where NaB≥1.5 and (Na+K)A≥0.5. They have anMg/(Mg+Fe2+) ratio ranging from 0.01 to 0.04 andindicating Fe-rich types. According to the amphibolenomenclature of Leake et al. (2003), arvedsonite is theonly amphibole mineral occuring in the studied trachytes(Fig. 5).

Feldspars occur mainly as K-feldspar microcrystals andminute plagioclase laths in the studied trachytes. The chemicalcompositions of some representative alkali feldspars andplagioclase are listed in Table 3. The feldspars of the studiedtrachytes have nearly the same compositional range in mostelement oxides. On the ternary An–Ab–Or diagram of Deeret al. (1966) (Fig. 6), the investigated K-feldspars consist ofsanidine and anorthoclase, whereas the plagioclase crystals

have albite (Ab99–92) composition. The K-feldspar crystalsare more or less rich in Na content (Ab%=65–28).Following the An–Ab–Or isothermal system at 1 kb ofFuhrman and Lindsley (1988), the studied feldspars werecrystallized at about 750°C.

Whole rock geochemistry

Out of 40 trachyte samples, 24 were carefully chosen fromthe UKT (11) and UST (13) for the chemical analyses toavoid the effects of secondary alteration. Major oxides andtrace elements were determined using a fully automatizedX-ray fluorescence spectrometer (XRF) on fused glass discsand pressed powder pellets, respectively. Analytical preci-sion is better than ±1% for major oxides and ±5% for most

Table 1 Representative major oxide compositions of the different pyroxene minerals in the Um Khors and Um Shaghir trachytes, central EasternDesert, Egypt

Trachytes Um Khors Um Shaghir

Sample No. K4 K4 K5 K5 K5 K11 K11 S6 S6 S12 S12 S12 S12

En–Fs–Wo pyroxene Aegrine–augite Aegrine–augite Aegrine

SiO2 47.42 47.22 48.88 48.69 51.74 49.97 50.24 49.48 49.08 50.65 52.06 52.71 51.39

TiO2 0.45 0.40 0.50 0.38 0.53 0.76 0.45 0.44 0.69 0.31 0.40 1.80 0.83

Al2O3 0.76 0.74 0.81 0.30 0.92 0.40 0.22 0.22 0.26 0.28 0.50 0.44 0.81

Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

FeOt 26.51 27.26 26.24 28.82 26.44 29.09 29.36 28.99 29.63 29.96 29.18 30.27 31.64

MnO 1.24 1.25 1.28 1.17 0.95 1.01 0.97 0.90 0.87 0.73 0.69 0.60 0.76

MgO 1.95 1.40 1.55 0.28 0.50 0.19 0.15 0.17 0.20 0.15 0.00 0.00 0.04

CaO 20.56 20.51 14.55 12.51 11.00 9.88 7.46 12.01 11.23 8.69 3.73 1.00 0.87

Na2O 1.15 1.22 6.22 6.90 7.21 8.97 10.63 7.68 7.93 9.20 13.39 12.71 13.60

Sum 100.04 100.00 100.03 99.05 99.93 100.28 99.50 99.89 99.89 99.97 99.95 99.53 99.95

Atomic ratios based on 6 oxygen

Si 1.93 1.93 1.92 1.93 1.99 1.93 1.93 1.94 1.92 1.95 1.94 1.97 1.93

Ti 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.02 0.01 0.01 0.05 0.02

Al 0.04 0.04 0.03 0.01 0.04 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.04

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fe+3 0.84 0.87 0.42 0.44 0.32 0.28 0.16 0.37 0.38 0.29 0.00 0.04 0.04

Fe+2 0.05 0.06 0.43 0.51 0.53 0.65 0.78 0.57 0.59 0.67 0.91 0.90 0.95

Mn 0.04 0.04 0.04 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.02 0.02

Mg 0.12 0.09 0.09 0.02 0.03 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00

Ca 0.89 0.89 0.61 0.53 0.45 0.40 0.30 0.50 0.47 0.35 0.14 0.04 0.03

Na 0.09 0.10 0.47 0.53 0.54 0.67 0.79 0.58 0.60 0.69 0.97 0.93 0.98

Total 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

End members (%)

Ac 7.10 7.71 43.85 51.11 56.41 64.68 76.43 56.24 57.69 67.08 93.45 94.64 94.57

Jd 1.78 1.74 1.81 0.68 2.14 0.89 0.48 0.49 0.57 0.63 1.07 1.00 1.75

Q=(En+Fs+Wo) 91.12 90.55 54.34 48.21 41.45 34.43 23.09 43.26 41.74 32.29 5.48 4.36 3.68

En 6.18 4.45 5.80 1.10 2.15 0.80 0.68 0.68 0.81 0.65 0.00 0.00 0.22

Fs 47.06 48.66 55.08 63.56 63.83 69.11 74.93 64.89 66.77 72.44 85.93 95.94 96.39

Wo 46.76 46.89 39.12 35.34 34.02 30.08 24.39 34.43 32.42 26.92 14.07 4.06 3.40

58 Arab J Geosci (2012) 5:53–71

Page 7

trace elements. Loss of ignition (LOI) was determined byheating powdered samples for 2 h at 1,100°C. Chemicalanalyses were carried out at the Institute of Mineralogy,Salzburg University, Austria. Six trachyte samples, three foreach area, were selected for rare earth elements (REE)analysis by ICP-MS at the Acme Analytical LaboratoriesLtd., Canada. Whole rock and REE analyses for thetrachyte samples are given in Tables 4, 5, and 6,respectively.

Major and trace element characteristics

The investigated Paleozoic trachytes have SiO2 contentwhich ranges from 62% to 67.5%. They exhibit a limitedcompositional range in most of the major elements: 14.2–17.1% Al2O3, 3.21–4.77% Fe2O3, 0.03–0.7% MgO, 0.84–

3.2% CaO, 5.57–7.6% Na2O, and 4.1–5.37% K2O (Tables 4and 5). In contrast, they show a wide compositional rangein Ni (15–127 ppm), Ba (11–302 ppm), and Zr (620–1,431 ppm). Their composition conveniently occurs on thetrachyte field of the binary Zr/TiO2–SiO2 diagram (Fig. 7a,after Winchester and Floyd 1977). The calculated CIPWnorms and some geochemical parameters for the UKT andUST are shown in Table 7. The trachyte samples have Qznorm range from 0.13 to 12.2, reflecting that they are silica-oversaturated rocks. On the total alkalis vs. silica diagram(Fig. 7b), the analyzed trachytes lie within the alkaline fieldand the Qz normative character is also evident as comparedwith the continental oversaturated volcanics of the Came-roon, West Africa (Fitton, 1987). On the primitive mantle-normalized trace element plot (Fig. 8a), using normalizationvalues of Sun and McDonough (1989), the UKT and UST

Table 2 Representative major oxide compositions of the different amphibole minerals in the Um Khors and Um Shaghir trachytes, centralEastern Desert, Egypt

Trachytes Um Khors Um Shaghir

Sample No. K5 K11 K11 K11 K11 K11 K11 S6 S6 S6 S6 S6 S12 S12

Arvedsonite Arvedsonite

SiO2 48.50 48.12 48.28 48.41 48.30 48.51 48.61 47.75 48.50 47.28 48.20 47.70 47.82 47.80

TiO2 0.31 3.17 2.61 2.54 3.00 2.65 2.18 1.13 0.92 2.39 0.65 0.91 0.73 0.71

Al2O3 1.08 0.17 0.18 0.16 0.18 0.18 0.13 1.53 0.97 1.58 1.05 1.40 1.30 1.26

FeOt 34.00 33.90 34.38 34.00 33.15 33.98 33.95 35.18 34.19 34.12 34.70 34.80 34.80 34.30

MnO 1.49 1.28 1.29 1.26 1.30 1.23 1.20 1.34 2.56 2.18 1.63 1.43 1.56 1.58

MgO 0.66 0.11 0.04 0.05 0.07 0.19 0.24 0.31 0.28 0.18 0.15 0.17 0.28 0.28

CaO 1.89 1.11 0.98 0.96 1.06 1.25 1.23 2.85 0.65 1.22 2.30 2.54 2.78 2.82

Na2O 7.63 8.15 7.80 8.00 7.95 7.90 7.55 6.50 8.00 7.15 6.89 7.24 7.20 7.10

K2O 1.44 1.49 1.41 1.35 1.39 1.36 1.54 1.46 1.54 1.59 1.56 1.37 1.30 1.28

Total 97.00 97.50 96.97 96.73 96.40 97.25 96.63 98.05 97.61 97.69 97.13 97.56 97.77 97.13

Structural formulae based on 23 oxygen

Si 7.81 7.79 7.81 7.86 7.88 7.84 7.88 7.62 7.73 7.53 7.78 7.69 7.70 7.74

Al iv 0.19 0.03 0.03 0.03 0.03 0.03 0.02 0.29 0.18 0.30 0.20 0.27 0.25 0.24

Ti 0.04 0.39 0.32 0.31 0.37 0.32 0.27 0.14 0.11 0.29 0.08 0.11 0.09 0.09

Al vi 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fe3+ 0.76 0.44 0.63 0.50 0.30 0.46 0.56 0.91 1.12 1.12 0.81 0.70 0.70 0.63

Fe2+ 3.82 4.15 4.02 4.11 4.22 4.13 4.05 3.79 3.44 3.43 3.87 3.99 3.98 4.02

Mn 0.20 0.18 0.18 0.17 0.18 0.17 0.16 0.18 0.35 0.29 0.22 0.20 0.21 0.22

Mg 0.16 0.03 0.01 0.01 0.02 0.05 0.06 0.07 0.07 0.04 0.04 0.04 0.07 0.07

Ca 0.33 0.19 0.17 0.17 0.19 0.22 0.21 0.49 0.11 0.21 0.40 0.44 0.48 0.49

Na 2.38 2.48 2.45 2.52 2.51 2.47 2.37 2.01 2.47 2.21 2.16 2.26 2.25 2.23

K 0.30 0.31 0.29 0.28 0.29 0.28 0.32 0.30 0.31 0.32 0.32 0.28 0.27 0.26

Total 16.00 15.98 15.91 15.96 15.99 15.97 15.91 15.80 15.90 15.74 15.87 15.98 15.99 15.98

(Ca+Na)(B) 2.01 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00

Na(B) 1.68 1.81 1.83 1.83 1.81 1.78 1.79 1.51 1.89 1.79 1.60 1.56 1.52 1.51

(Na+K) (A) 1.00 0.98 0.91 0.96 0.99 0.97 0.91 0.80 0.90 0.74 0.87 0.98 0.99 0.98

Mg/(Mg+Fe2) 0.04 0.01 0.00 0.00 0.00 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.02 0.02

Arab J Geosci (2012) 5:53–71 59

Page 8

Tab

le3

Representativemajor

oxidecompo

sitio

nsof

thevariou

sfeldspar

mineralsin

theUm

Kho

rsandUm

Shagh

irtrachy

tes,centralEastern

Desert,Egy

pt

Trachytes

Um

Kho

rsUm

Shagh

ir

Sam

pleNo.

K4

K4

K5

K5

K4

K4

K5

K5

S6

S12

S6

S12

S6

S6

S12

S12

Albite

Ano

rtho

clase

Sanidine

Albite

Ano

rtho

clase

Sanidine

SiO

268

.04

64.75

62.14

67.05

65.93

65.36

67.17

65.21

67.60

67.86

68.96

68.83

65.60

65.46

66.01

65.97

TiO

2–

–0.11

–0.04

–0.02

––

––

–0.01

––

–

Al 2O3

18.56

15.80

14.85

17.64

17.78

17.90

17.66

17.37

18.52

18.76

18.64

18.08

17.96

17.80

17.42

17.48

FeO

t0.33

5.07

7.75

1.13

0.67

0.33

0.93

1.55

0.35

0.16

0.24

0.55

0.55

0.45

1.02

0.61

MnO

0.01

0.30

0.31

–0.01

––

0.01

––

––

0.02

0.01

0.02

–

MgO

0.01

–0.05

––

–0.02

0.10

0.02

0.04

––

––

––

CaO

0.03

0.13

0.39

0.01

0.03

0.13

0.30

0.05

0.07

0.01

0.02

0.02

0.08

0.04

0.01

–

Na 2O

12.71

9.70

9.35

8.82

7.73

5.50

7.32

7.41

11.87

12.98

7.45

10.02

5.37

3.33

4.43

3.05

K2O

0.41

4.24

5.06

5.34

7.75

10.66

5.56

8.41

1.49

0.18

4.70

2.45

10.41

12.91

10.95

12.81

Sum

100.1

99.99

100.0

99.99

99.94

99.88

98.98

100.1

99.92

99.99

100.0

99.95

100.0

100.0

99.86

99.92

Atomic

ratio

sbasedon

8ox

ygen

Si

2.99

2.97

2.92

3.01

2.99

2.99

3.02

2.98

2.99

2.99

3.04

3.04

2.99

3.01

3.02

3.03

Ti

––

––

––

––

––

––

––

––

Al

0.96

0.85

0.82

0.93

0.95

0.96

0.94

0.93

0.97

0.97

0.97

0.94

0.97

0.96

0.94

0.94

Fe

0.01

0.19

0.30

0.04

0.03

0.01

0.03

0.06

0.01

0.01

0.01

0.02

0.02

0.02

0.04

0.02

Mn

–0.01

0.01

––

––

––

––

––

––

–

Mg

––

––

––

–0.01

––

––

––

––

Ca

–0.01

0.02

––

0.01

0.01

––

––

––

––

–

Na

1.08

0.86

0.85

0.77

0.68

0.49

0.64

0.66

1.02

1.11

0.64

0.86

0.47

0.30

0.39

0.27

K0.02

0.25

0.30

0.31

0.45

0.62

0.38

0.49

0.08

0.01

0.26

0.14

0.61

0.76

0.64

0.75

End

mem

bers

(%)

An

0.13

0.58

1.67

0.04

0.12

0.57

1.40

0.22

0.30

0.04

0.10

0.10

0.36

0.19

0.02

0.00

Ab

97.80

77.20

72.52

71.49

60.20

43.69

62.03

57.11

92.11

99.05

70.60

86.04

43.78

28.08

38.07

26.54

Or

2.07

22.22

25.81

28.47

39.68

55.74

36.56

42.67

7.59

0.91

29.30

13.86

55.86

71.73

61.91

73.46

–Not

determ

ined

60 Arab J Geosci (2012) 5:53–71

Page 9

show significant negative anomalies in Ba, Sr, and TiO2,which indicate an ancestry involving plagioclase and Fe–Tioxide fractionation. However, the marked high Nb contentin both of the UST and UKT is typical of within-platemagmas (Wilson 1989).

Rare earth element patterns

REE analytical data and the chondrite-normalized plot(chondrite values after Evensen et al. 1978) of representa-tive samples of the investigated trachytes are shown inTable 6 and Fig. 8b. The UKT and UST have high REEcontents (720 and 761 ppm, respectively, on average). Theirchondrite-normalized REE patterns show a nearly onedistinct spectrum. They are in general highly fractionated[(La/Lu)N=12.83 and 14.51 on average, respectively] witha negative Eu anomaly (Eu/Eu*=0.31 and 0.52 on average,respectively). The high light rare earth elements (LREE)contents and the negative Eu anomaly in the trachyterocks assure the important role of fractional crystalliza-tion during their magmatic evolution. The negative Euanomaly also reflects significant fractional crystallizationof plagioclase and alkali feldspar in the studied trachytes.The REE patterns of the UKT and UST are enriched inLREE [(La/Sm)N=4.86 and 4.78 on average, respectively]relative to heavy rare earth elements (HREE) [(Gd/Lu)N=1.21 and 1.30 on average, respectively]. In many felsicigneous rocks, the LREE decrease rapidly in abundancewith fractionation, indicating that they are not incompat-ible (Miller and Mittlefeld 1982). However, the REEpatterns of the UKT and UST have a marked fractionatedLREE and a slightly fractionated HREE.

Sanidine

Anorthoclase

Albite

Oligioclase

OrAb

An

750º C

Fig. 6 Compositional variation of the plagioclase and K-feldsparminerals in the studied trachytes (after Deer et al. 1966). Symbols asin Fig. 4a

0

0.5

1

6.57.07.58.0

Mg

/Mg

+Fe2

+

Si

Ferro-eckermannite

(VIAl>Fe3+)

Eckermannite(VIAl>Fe3+)

B(Fe2+, Mg2+, Mn2+, Li< 2.5; B(Na)>1.5A(Na+K)>50

Ferric-ferronybdite(VIAl<Fe3+)

Ferronybdite(VIAl>Fe3+)

Arvedsonite(VIAl<Fe3+)

Ferric-nybdite(VIAl<Fe3+)

Nybdite(VIAl>Fe3+)

Obertiite (Ti>o,5OH+F+Cl<1)

__

Fig. 5 Classification diagram of the amphiboles in the UKT and USTtrachytes (after Leake et al. 2003). Symbols as in Fig. 4a

a Um Khors Um Shaghir

8080

2020

Jadetite Aegirine

Aegirine-augite

Omphacite

Ca-Mg-FePyroxenes

NaAl2Si2O6 (Jd) NaFe3+Si2O6 (Ac)

Wo,En.Fs

Clinoenstatite Clinoferrosilite

Pigeonite

Augite

Diopside

Hedenbergite

En

Wo

Fs

b

Fig. 4 a Compositional variation of the pyroxene minerals (Morimoto1989) in the UKT and UST (En enstatite, Fs ferrosilite, Wowollastonite, Jd jadeite, Ae aegirine). b Clinopyroxene classificationof the studied trachytes on the base of quadrilateral components (afterMorimoto 1989)

Arab J Geosci (2012) 5:53–71 61

Page 10

Table 4 Whole rock chemical composition of the trachyte rocks at the Um Khors area, central Eastern Desert, Egypt

Trachytes Um Khors

Sample No. K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11

Major oxides (wt.%)

SiO2 64.41 67.50 64.03 63.17 64.55 64.34 64.12 66.93 67.44 63.63 64.22

TiO2 0.16 0.17 0.17 0.18 0.16 0.15 0.17 0.19 0.18 0.18 0.16

Al2O3 16.55 14.70 16.7 16.5 17.07 17.1 16.81 14.2 14.44 15.60 16.91

Fe2O3a 3.35 3.81 3.54 3.6 3.35 3.21 3.58 3.34 3.96 3.71 3.38

MnO 0.13 0.12 0.14 0.15 0.12 0.10 0.15 0.14 0.12 0.17 0.13

MgO 0.03 0.26 0.04 0.10 0.07 0.05 0.05 0.14 0.20 0.07 0.09

CaO 0.86 1.09 0.90 2.00 0.95 1.14 0.97 1.92 0.84 1.50 0.92

Na2O 7.60 5.91 7.25 7.02 7.43 7.36 7.34 6.38 6.15 6.50 7.31

K2O 4.89 4.63 4.94 4.87 4.98 5.00 5.06 4.44 4.50 5.02 4.94

P2O5 0.11 0.13 0.11 0.13 0.10 0.08 0.13 0.15 0.13 0.16 0.11

LOI 1.04 0.78 1.19 1.74 0.99 0.87 0.93 1.91 1.12 2.74 0.91

Total 99.13 99.10 99.01 99.46 99.77 99.40 99.31 99.74 99.08 99.28 99.08

Trace elements (ppm)

As 6 1 2 – 4 1 3 – 2 1 11

Ba 49 11 51 70 68 52 52 42 34 52 56

Be 54.9 – – – – – – 85.1 – – 45.9

Ce 359 313 315 330 383 200 383 358 309 435 313

Cl 258 370 210 472 282 303 326 912 266 286 263

Co 1 2 2 2 2 2 2 1 1 1 2

Cr 30 25 54 64 37 26 42 17 42 27 40

Cs 9 9 8 9 7 6 9 8 8 10 8

Cu 14 17 15 17 14 10 17 22 17 19 15

Er 9 10 9 10 8 7 9 13 10 11 8

F 679 889 868 838 961 495 1032 549 832 547 198

Ga 32 34 32 33 31 32 32 33 34 32 33

Gd 15 17 16 17 15 11 19 16 17 22 16

Hf 29 35 28 28 24 19 33 45 37 37 28

La 155.6 119 124 132 101 74 151 160.8 115 158 139.5

Mo 5 5 6 6 5 5 7 5 5 7 5

Nb 195 248 197 211 164 114 244 286 237 276 195

Nd 115 122 129 137 107 82 148 110 113 165 100

Ni 22 23 33 57 32 21 33 15 23 22 27

Pb 9 10 12 11 15 4 15 13 7 15 5

Pr 35 47 43 46 38 29 53 35.7 43 57 31

Rb 115 176 115 116 112 107 119 175 177 143 116

Sm 23 18 15 17 10 12 30 22 18 15 21

Sn 7 – – – – – – 11 – – 10

Sr 29 30 23 53 24 28 24 28 29 37 25

Ta 12.5 – – – – – – 18.9 – – 15.1

Th 18 28 17 19 13 11 22 33 27 26 16

U 11 10 7 4 8 6 8 9 10 3 4

V 4 7 5 7 5 5 6 5 4 4 5

W 1.2 2 2 2 2 1 2 5.7 2 2 2.4

Y 72 95 75 81 64 45 95 108 98 106 74

Yb 8 7 6 7 5 4 4 11 8 8 9

Zn 148 237 165 171 142 112 191 211 234 205 184

Zr 898 1,156 904 961 777 620 1,046 1,304 1,165 1,234 881

– Not determineda Total iron as Fe2O3

62 Arab J Geosci (2012) 5:53–71

Page 11

Table 5 Whole rock chemical composition of the trachyte rocks at the Um Shaghir area, central Eastern Desert, Egypt

Trachytes Um Shaghir

Sample No. S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13

Major oxides (wt.%)

SiO2 64.84 62.02 64.29 65.50 63.40 62.80 63.60 62.85 64.25 64.63 64.90 63.13 62.99

TiO2 0.16 0.20 0.16 0.16 0.16 0.21 0.17 0.16 0.16 0.16 0.16 0.19 0.21

Al2O3 15.50 15.73 16.38 15.77 15.70 15.57 16.00 15.20 16.09 16.01 15.49 15.87 16.04

Fe2O3a 3.90 4.18 3.60 3.72 4.11 4.77 4.16 4.23 3.77 4.16 4.05 4.21 4.46

MnO 0.20 0.20 0.12 0.10 0.14 0.21 0.19 0.18 0.17 0.17 0.17 0.18 0.18

MgO 0.17 0.22 0.27 0.58 0.40 0.16 0.63 0.70 0.30 0.16 0.19 0.15 0.17

CaO 2.01 1.92 1.37 1.11 1.96 1.76 1.49 3.20 1.32 1.04 1.61 1.82 1.47

Na2O 6.14 6.40 6.01 6.46 6.19 6.63 5.95 5.57 6.85 7.14 6.72 6.87 6.51

K2O 4.50 5.30 5.37 4.10 4.58 4.57 4.71 4.30 4.59 4.52 4.34 4.45 4.93

P2O5 0.16 0.17 0.15 0.18 0.17 0.16 0.18 0.22 0.17 0.17 0.17 0.13 0.14

LOI 2.18 2.75 1.94 1.56 2.22 2.30 2.14 2.40 1.57 1.40 1.72 2.95 2.33

Total 99.76 99.09 99.66 99.24 99.03 99.14 99.22 99.01 99.24 99.56 99.52 99.95 99.43

Trace elements (ppm)

As – 4 – – 4 5 – 1 4 3 – 5 1

Ba 59 176 59 74 53 218 46 91 59 65 61 203 302

Be 58.1 – – – – – 60.1 – – – – – 47.7

Ce 311 316 352 373 357 307 469 343 346 394 371 248 308

Cl 819 587 634 398 613 263 448 604 830 391 375 797 384

Co 5 4 4 5 5 4 4 3 3 3 3 4 5

Cr 133 121 122 124 103 107 92 76 97 98 120 123 125

Cs 7 7 10 9 9 9 10 8 9 8 9 6 8

Cu 24 22 20 25 23 20 23 23 22 23 24 17 19

Er 11 10 11 11 11 9 11 12 11 11 11 8 10

F 1,078 1,271 1,438 1,078 1,212 1,242 315 832 1,321 1,564 1,486 1,043 786

Ga 32 29 32 32 32 30 33 32 33 33 32 30 28

Gd 14 17 16 18 18 16 19 19 18 19 18 13 13

Hf 43 37 38 46 45 33 42 45 44 42 42 27 29

La 132.9 134 136 137 145 124 213.1 134 139 145 132 93 139.1

Mo 8 7 8 7 7 6 6 7 6 7 10 6 6

Nb 270 242 237 245 272 223 276 275 299 221 252 193 189

Nd 104 137 138 143 144 127 146 139 141 144 154 100 99

Ni 127 118 113 119 97 104 92 74 96 96 114 108 119

Pb 14 8 10 9 10 11 8 9 10 8 12 8 9

Pr 32 48 48 51 53 43 47 48 50 54 49 34 32

Rb 140 121 137 116 133 107 133 121 126 135 128 100 109

Sm 20 14 20 18 28 16 29 29 12 23 28 15 21

Sn 10 – – – – – 10 – – – – – 7

Sr 37 45 26 36 29 36 30 29 32 20 28 50 61

Ta 18.1 – – – – – 18.9 – – – – – 12.1

Th 29 23 22 25 28 20 29 26 24 27 26 14 18

U 11 10 11 5 5 4 7 7 5 8 8 9 5

V 6 5 7 6 6 6 4 9 5 4 5 7 10

W 3 2 2 2 2 2 3.5 2 2 2 2 1 2.3

Y 66 79 82 88 89 79 95 90 87 90 97 62 68

Yb 8 6 4 7 8 7 11 7 4 8 8 5 7

Zn 243 177 134 163 188 169 196 206 167 202 178 141 157

Zr 1,334 1,231 1,160 1,431 1,379 1,132 1,360 1,410 1,344 1,318 1,359 886 970

– Not determineda Total iron as Fe2O3

Arab J Geosci (2012) 5:53–71 63

Page 12

Discussion

Tectonic regime and magma source

The tectonic setting of the studied trachytes is considered inlight of their chemical data to understand their geologicalhistory and the nature of any genetic process during theirevolution. In most of Phanerozoic alkaline volcanic, Rb, U,Th, Nb, Ta, Y, Zr, and REE are relatively abundant andtheir rock varieties commonly exhibit geochemical traits ofwithin-plate magmatism (White and Urbanczyk 2001).They are enriched in alkalis, Nb, Y, Zr, Rb, Ga, U, Th, aswell as LREE, relative to HREE (Tables 4, 5, and 6). In thisrespect, the trace element contents of the UKT and USThave chemical characteristics which resemble many anoro-genic silica-oversaturated alkaline suites (i.e., Eby 1990;Abdel-Rahman and El-Kibbi 2001). Sources of within-platemagma could be: asthenospheric mantle, lithosphericmantle, or the crust that the magma passes to reach thesurface. However, no published experimental study ofcrustal melting has produced peralkaline melts (Scailletand Macdonald 2003; White et al. 2009).

In general, the various sources of trachytic rocks aresummarized as follows: (1) fractional crystallization ofmantle-derived magma, either during major lithosphericextension or when a mantle plume impinges the base of

the lithosphere (e.g., Campbell and Griffiths 1990;Mohamed 2001; Abdel-Rahman 2002; Peccerillo et al.2007; Godwin et al. 2009, White et al. 2009); (2)interaction of mantle-derived magmas with crustal materi-als to produce trachytic melts (Duncan et al. 1986;Davidson and Wilson 1989); and (3) mixing of indepen-dent mafic and felsic magmas (Gourgaud and Maury1984; Ulrych et al. 2003, 2006). However, there is noevidence in favor of magma mixing as being theresponsible process in the magmatic evolution of thestudied trachyte rocks. The field, petrographic, andgeochemical characteristics of the UKT and UST, includ-ing the absence of any acidic or basic microgranularxenoliths, their homogeneous petrographic nature, and thelimited major and trace element variations, do not supportthe magma mixing model. Thus, the two remainingpossible sources for the investigated trachytes are themantle-dominated magma source and interaction ofmantle-derived magma with continental crust materials.

Many alkaline suites represent fractionates of dominantmantle-derived magma (Eby 1990) characterized by lowinitial 87Sr/86Sr values (approx. 0.704; Faure and Powell1972) and low Y/Nb ratios. The UKT and UST are silica-oversaturated, have rather high Cr and Ni contents (Cr=133–17 ppm and Ni=127–15 ppm, respectively), andexhibit fractionated REE patterns (Fig. 8b) besides the

Trachytes Um Khurs Um Shaghir

Sample No. K1 K8 K11 S1 S7 S13

Rare earth elements (ppm)

La 155.6 160.8 139.5 132.9 213.1 139.1

Ce 359 358 313 311 469 308

Pr 35.3 35.7 30.9 32.4 47.2 31.8

Nd 115 110 100 104 146 99.3

Sm 22.5 21.9 21.2 20 29.1 20.9

Eu 1.87 1.51 1.44 2.17 3.02 3.15

Gd 14.7 15.9 15.5 13.7 19.3 13.3

Tb 2.58 3.05 2.78 2.58 2.54 2.55

Dy 14.4 18.6 16.2 14.9 19.8 14.4

Ho 2.73 3.79 3.27 2.78 3.6 2.54

Er 7.81 10.34 9.85 7.76 10.6 7.09

Tm 1.15 1.72 1.43 1.21 1.58 1.16

Yb 7.81 10.72 9.02 7.99 10.9 7.09

Lu 1.02 1.44 1.29 1.14 1.32 0.97

Sum 741.47 753.47 665.38 654.53 977.06 651.35

(La/Lu)N 15.77 11.54 11.18 12.05 16.68 14.80

(La/Sm)N 4.84 5.14 4.61 4.55 5.13 4.66

(Gd/Lu)N 1.40 1.07 1.16 1.16 1.42 1.33

Eu/Eu* 0.33 0.27 0.26 0.42 0.41 0.60

Table 6 REE compositions ofthe Um Khors and Um Shaghirtrachytes, central Eastern Desert,Egypt

64 Arab J Geosci (2012) 5:53–71

Page 13

moderate initial 87Sr/86Sr ratios (0.7117 and 0.7095,respectively; Hashad et al. 1981). On the Zr/Nb vs. Y/Nbplot (Fig. 9a), the investigated samples show a narrowvariation range of Zr/Nb (4.29–5.96) and low Y/Nbratios similar to oceanic island basalts (OIB) that arederived from mantle source since Zr and Nb behave as veryincompatible elements in alkaline magmas (Macdonald et al.1987). Moreover, the Y/Nb ratios for both the UKT and USTare low values (<0.5, Table 7 and Fig. 9a), a featuretypifying many alkaline suites (e.g., alkaline rocks of OsloGraban and Ras ed Dom syenites and granites of Sudan)which represent fractionates of mantle-derived magmas(O’Halloran 1985).

The depth and nature of the mantle source (eitherasthenosphere or subcontinental ancient lithosphere) are

the dominant factors that control the overall characteristicsof mantle-derived melts. According to Smith et al. (1999),the high field strength elements like Nb are depleted in thelithospheric mantle relative to the LREE (such as La). Thehigh Nb/La ratios (approximately >1) indicate an OIB-likeasthenospheric mantle source for basaltic magmas, whereaslower ratios (<0.5) reveal a lithospheric mantle source. TheUKT and UST have Nb/La and La/Yb ratios of 1.41 and19.81 (for the UKT) and 1.8 and 21 (for the UST),respectively (Table 7 and Fig. 9b), similar to thosecharacteristics of the asthenospheric mantle (OIB-like)source.

Are the UST and UKT genetically related?

Geochemical characteristics of UKT show somewhat lowFe2O3, MgO, Ba, Ni, and Cr relative to UST (Tables 4 and5). These features are also supported by more depletion inBa and the highly negative Eu anomalies in some samplesof the former (Fig. 8a, b). On the other hand, mineralogicalstudies of pyroxene in the two areas indicate the presenceof Ca pyroxene (hedenbergite) besides the commonaegirine–augite in some samples in the UKT area, featuresnot present in pyroxene of UST which represented mainlyaegirine–augite with some aegirine.

Zirconium is a high field strength element thatremains highly incompatible throughout the trachyterocks and has been used successfully as a differentiationindex in many similar rock suites. Variation plots of Nband Y vs. Zr are presented in Fig. 10a, b. The Nb and Ycontents have positive correlations against Zr in both theUKT and UST with a remarked consistency in theirfractionation trends, reflecting that these trachyte rocksmay be genetically related. However, the general linearvariation trends of the Nb and Y in the UKT and USTcould be interpreted as indicating two separate fractionalcrystallization processes.

Several lines of evidence are consistent with the interpre-tation that the two trachyte rocks of the studied area aregenetically related to a dominant source component. Forexample: (1) The UKT and UST have somewhat similarmineralogical and chemical compositions, (2) the twovolcanic rocks were extruded in the same tectonic setting,(3) they have geochemical parameters corresponding to eachother (Table 7), and (4) the normalized incompatible traceelement and REE patterns are closely similar for the twovolcanic eruptions, except the highly negative anomaly ofBa and Eu in some samples of UKT, which probablyindicates high fractional crystallization.

The transition element Ni is commonly incorporatedinto ferromagnesian minerals. It usually concentrates inmafic minerals during the early fractionated magmaticmelts. The Ni content in the UKT (94–127 ppm) and

40

50

60

70

80

0.001 0.01 0.1 1 10Zr/TiO2

SiO

2 (%

)

Rhyolite

Trachyte

Comendite/Pantellerite

Phonolite

Subalkalinebasalt

Rhyodacite

Andesite

a

0

4

8

12

16

20

40 50 60 70 80SiO2 (%)

Na 2

O +

K2O

(%)

Subalkaline

AlkalineQz normvolcanics

Ne norm volcanics

b

Fig. 7 a Nomenclature diagram of the UKT and UST rocks(Winchester and Floyd 1977). b Total alkalis vs. SiO2 plot (Irvineand Baragar 1971) illustrating the alkaline nature of the studiedtrachytes. The line shaded area represents the continental over-saturated volcanics from the Cameroon (Fitton 1987). Symbols as inFig. 4a

Arab J Geosci (2012) 5:53–71 65

Page 14

Tab

le7

CIPW

norm

sandsomegeochemical

parametersof

thestud

iedtrachy

tesamples

attheUm

Kho

rs–U

mShagh

irarea,centralEastern

Desert,Egy

pt

Trachytes

Um

Khors

Um

Shaghir

Sample

No.

K1

K2

K3

K4

K5

K6

K7

K8

K9

K10

K11

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

S12

S13

CIPW

norm

s

Qz

2.00

11.93

1.31

0.13

0.52

0.03

0.76

11.91

12.21

3.87

0.68

7.95

2.86

5.35

8.05

5.19

2.36

5.55

6.86

4.04

3.79

5.52

2.57

2.01

Or

28.95

27.43

29.24

28.83

29.48

29.59

29.95

26.31

26.66

29.73

29.24

26.65

29.60

31.79

24.28

27.12

27.05

27.89

25.46

27.18

26.77

25.70

26.34

29.18

Ab

57.87

48.91

58.37

57.73

60.05

60.09

58.26

48.27

49.17

52.25

59.45

49.53

50.30

48.97

53.48

50.56

54.62

49.02

45.34

55.50

57.15

55.48

55.77

53.94

An

–0.47

––

––

––

––

–2.71

1.46

2.84

2.54

2.47

–3.73

4.71

0.89

––

0.84

0.58

Ns

1.32

–0.55

0.06

0.46

0.97

0.67

0.70

0.48

0.44

0.38

––

––

–0.16

––

–0.49

0.06

––

Di

2.57

2.80

2.57

7.18

3.21

3.55

3.10

6.84

2.50

5.72

3.43

4.53

5.01

1.27

0.64

4.28

5.71

2.04

7.84

2.79

2.11

4.69

5.58

4.53

Hy

4.22

5.41

4.55

2.37

3.96

3.48

4.38

2.20

5.64

3.29

3.96

4.55

5.83

5.88

7.15

5.52

5.29

7.43

4.74

5.54

6.18

4.72

4.38

5.36

Cm

0.03

0.02

0.05

0.06

0.03

0.02

0.04

0.01

0.02

0.02

0.03

0.11

0.10

0.10

0.11

0.09

0.09

0.08

0.06

0.08

0.08

0.10

0.10

0.11

Il0.30

0.32

0.32

0.34

0.30

0.28

0.32

0.36

0.34

0.34

0.30

0.30

0.38

0.30

0.30

0.30

0.40

0.32

0.30

0.30

0.30

0.30

0.36

0.40

Hl

0.04

0.06

0.03

0.03

0.05

0.05

0.05

0.15

0.04

0.05

0.04

0.14

0.10

0.10

0.07

0.10

0.04

0.07

0.10

0.14

0.06

0.06

0.13

0.06

Ap

0.26

0.31

0.26

0.31

0.24

0.19

0.31

0.36

0.31

0.36

0.26

0.38

0.40

0.36

0.43

0.40

0.38

0.43

0.52

0.40

0.40

0.40

0.31

0.33

Fl

0.24

0.32

0.31

0.35

0.17

0.40

0.18

0.29

0.18

0.03

0.02

0.39

0.46

0.54

0.38

0.44

0.45

0.06

0.26

0.48

0.58

0.55

0.38

0.27

Zr

0.18

0.23

0.18

0.19

0.16

0.12

0.21

0.26

0.23

0.25

0.18

0.27

0.25

0.23

0.29

0.28

0.23

0.27

0.28

0.27

0.26

0.27

0.18

0.19

Total

97.98

98.21

97.74

97.58

98.63

98.77

98.23

97.66

97.78

96.35

97.97

97.51

96.75

97.73

97.72

96.75

96.78

96.89

96.47

97.61

98.17

97.85

96.94

96.96

Geochem

ical

parameters

Na 2O+

K2O

12.49

10.54

12.19

11.89

12.41

12.36

12.40

10.82

10.65

11.52

12.25

10.64

11.15

11.38

10.56

10.77

11.20

10.66

9.87

11.44

11.66

11.06

11.32

11.44

Fe#

9994

9997

9898

9996

9598

9796

9593

8791

9787

8693

9696

9796

K/Rb

353

218

357

348

369

388

353

211

211

291

353

267

343

325

293

286

354

294

295

302

278

281

369

375

Rb/Sr

3.97

5.87

52.19

4.67

3.82

4.96

6.25

6.1

3.86

4.64

3.78

2.69

5.27

3.22

4.59

2.97

4.43

0.94

3.94

6.75

4.57

21.79

Y/Nb

0.37

0.38

0.38

0.38

0.39

0.39

0.39

0.38

0.41

0.38

0.38

0.24

0.33

0.35

0.36

0.33

0.35

0.34

0.33

0.29

0.41

0.38

0.32

0.36

Zr/Nb

4.61

4.66

4.59

4.55

4.74

5.44

4.29

4.56

4.92

4.47

4.52

4.94

5.09

4.89

5.84

5.07

5.08

4.93

5.13

4.49

5.96

5.39

4.59

5.13

Th/Nb

0.09

0.11

0.09

0.09

0.08

0.1

0.09

0.12

0.11

0.09

0.08

0.11

0.91

0.09

0.1

0.1

0.09

0.11

0.1

0.08

0.12

0.1

0.07

0.1

Nb/La

1.25

2.08

1.59

1.60

1.62

1.54

1.62

1.78

2.06

1.75

1.40

2.03

1.81

1.74

1.79

1.88

1.80

1.30

2.05

2.15

1.52

1.91

2.08

1.36

La/Yb

19.92

17.00

20.67

18.86

20.20

18.50

37.75

15.00

14.38

19.75

15.47

16.63

22.33

34.00

19.57

18.13

17.71

19.55

19.14

34.75

18.13

16.50

18.60

19.62

Nb/U

17.73

24.80

28.14

52.75

20.50

19.00

30.50

31.78

23.70

92.00

48.75

24.55

24.20

21.55

49.00

54.40

55.75

39.43

39.29

59.80

27.63

31.50

21.44

37.80

Ce/U

32.64

31.30

45.00

82.50

47.88

33.33

47.88

39.78

30.90

19.75

78.25

28.27

31.60

32.00

74.60

71.40

76.75

67.00

49.00

69.20

49.25

46.38

27.56

61.60

Fe#

Iron

number

66 Arab J Geosci (2012) 5:53–71

Page 15

UST (15–57 ppm) is not in the same range, where theUST have higher Ni relative to the UKT (Tables 4 and5). Based on a geochronological study (El-Manharawy1972), the UKT and UST have isotopic ages of about 302±15 and 273±15 Ma, respectively. In this regard, the USTis younger than the UKT of about 25 Ma, but showsapproximately similar chemical compositional ranges ofthe most major and trace elements with higher Cr and Nicontents (Tables 4 and 5). This may also argue against theevolution of the UST via a continuous fractional crystal-lization of the residual magmatic melt of the UKT, whichwill be expected to become much lower in some traceelement contents, particularly Cr, Ni, Sr, Rb, and Ba,through such fractionation process. Thus, the UKT andUST are genetically related but could be emplacedthrough two various magmatic pulses of the same parentsource (i.e., asthenospheric mantle source) at differenttimes. Such mantle source was subsequently subjected totwo significant fractional crystallization processes toyield the UKT and UST plugs.

Fractional crystallization

The trachytic magma is unlikely to yield directly fromthe partial melting of the mantle. Most of trachyte rocks

are derived from a mafic magma through high-pressuremantle fractionation (Bonin and Giret 1990). The inves-tigated trachytes have a more evolved geochemical naturenot in equilibrium with their derivation from a primitivemantle melt. The negative Ba, Sr, TiO2, and Eu anomaliesin the UKT and UST, besides the low concentrations ofFe2O3, MgO, and TiO2 and the gradual decrease in LREEfrom La to Sm (Fig. 8a, b and Table 6), indicate that thesetrachyte rocks were derived through significant fraction-ation processes. Such Ba, Sr, and TiO2 anomalies can be

Fig. 9 a Y/Nb vs. Zr/Nb plot for the UKT and UST. The CRZ/OIBfield represents the Kenya Rift Zone and Tristan da Cunha alkalibasalts. CFB continental flood basalts from Parana basin (Wilson1989). b Nb/La vs. La/Yb variation diagram of the UKT and UST.Average oceanic island basalt (OIB) is after Fitton et al. (1991) andaverage lower crust is after Chen and Arculus (1995). Dashed linesseparating fields of the asthenospheric, lithospheric, and mixed mantleare plotted based on data given by Smith et al. (1999). Symbols as inFig. 4a

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

ondr

ite

b

0.1

1

10

100

1000

Rb Ba K2O Nb La Ce Nd Sr Zr Y TiO2

Roc

k/P

rim

ativ

e m

antl

ea

Fig. 8 a Primitive mantle incompatible trace element patterns for theUKT and UST. Normalization values are after Sun and McDonough(1989). b Chondrite-normalized REE patterns for the UKT and UST.Normalization values are after Evensen et al. (1978). Symbols as inFig. 4a

Arab J Geosci (2012) 5:53–71 67

Page 16

attributed to the fractionation of feldspars (Ba and Sr) andtitanomagnetite (Fig. 8a). Besides, the significant negativeEu anomaly of the studied trachytes reflects that plagio-clase is a major fractionation phase during their evolution(Fig. 8b). Moreover, the gradual increase in Nb and Ycontents with the increase of Zr (Fig. 10a, b) supports thefractional crystallization model of the UKT and UST. TheK/Rb ratio as a petrogenetic indicator is known todecrease with increasing fractionation. In the studiedtrachytes, the ratio decreases from 388 to 211 and sinceK content (3.40–4.46%) does not vary widely. Thus, thevariation reflects the degree of rubidium enrichment inthese rocks (Table 7). The K/Rb vs. Rb and Rb/Sr vs. Srplots (Fig. 11a, b) reflect variation trends consistentdominantly with alkali feldspar fractionation to yield theUKT and UST.

Role of crustal contamination

Trace element Nb/U and Ce/U ratios in mafic andintermediate magmas are valuable to identify magmaticsources because they can be effective in distinguishingbetween mantle and crustal components (McDonough1990). The elemental Nb/U and Ce/U ratios of the UKTand UST (Fig. 12) are compared with those of well-

established intraplate oceanic basalt regions (OIB andMORB, Hofmann et al. 1986; Davies et al. 1989). Thestudied trachytes have high Nb/U and Ce/U values (36.5and 54.13 on average, respectively). Besides, most of theirsamples fall within the OIB field, with some samples moreor less shifted toward the average continental crust(Fig. 12). The Th/Nb ratio is also a sensitive index ofthe involvement of continental crust materials during the

10

100

1000

1 10 100

Nb/U

Ce/

U

OIBMORB

Averagecrust

Fig. 12 Nb/U vs. Ce/U variation diagram of the studied trachytes.The oceanic island basalt (OIB), mid-oceanic ridge basalt (MORB),and average crust data are from Hofmann et al. (1986). Symbols as inFig. 4a

100

1000

10 100 1000Rb (ppm)

K/R

b

cpxamp

plag

ksp

bio

a

1

10

10 100Sr (ppm)

Rb

/Sr

ampcpx

bio

Kspplag

b

Fig. 11 Binary variation diagrams of K/Rb vs. Rb (a) and Rb/Sr vs.Sr (b) of the studied trachytes. The fractionation vectors, assumingRayleigh fractionation, show the theoretical effects on the compositionof liquid crystallizing mineral phases. Cpx clinopyroxene, amphamphibole, bio biotite, plag plagioclase, Ksp K-feldspar. Symbols asin Fig. 4a

20

60

100

140

500 1000 1500Zr (ppm)

Y (

pp

m)

b

50

150

250

350

500 1000 1500Zr (ppm)

Nb

(pp

m)

a

Fig. 10 Zr vs. Nb and Y relationships of the UKT and UST. Symbolsas in Fig. 4a

68 Arab J Geosci (2012) 5:53–71

Page 17

evolution of a rock. The studied trachyte samples have aTh/Nb ratio ranging from 0.07 to 0.12 (Table 7), which ishigher than the corresponding mantle value (approx.0.07; Pin et al. 1992), reflecting a significant degree ofcrustal contamination during the evolution of the UKT andUST. The studied trachytes show moderate initial87Sr/86Sr ratios of about 0.7117 and 0.7095, respectively(Hashad et al. 1981). The initial 87Sr/86Sr ratios foroceanic basalts of typical upper mantle origin range from0.7020 to 0.7060 (Faure and Powell 1972) and for rocksof crustal origin are usually higher than 0.7100 (Faure andHurley 1963). Accordingly, the rather moderate initial87Sr/86Sr ratios of the UKT and UST along with positiveK, Rb, REE (La, Ce, Nd) anomalies in multi-elementvariation diagrams indicate that those rocks were derivedfrom a mantle source with a vital role of crustalcontamination.

Conclusions

The Um Khors and Um Shaghir trachytes represent twosignificant outcrops from the Paleozoic volcanic activi-ties of the central Eastern Desert, Egypt. They are mostlyfine-grained, consisting of sanidine, anorthoclase, albite,quartz, aegirine–augite, and arvedsonite. Based on themineral chemistry, pyroxene in the two areas indicatesthe presence of Ca pyroxene (hedenbergite) besides thecommon aegrine–augite in some samples in UKT area,features not present in pyroxene of UST which repre-sented mainly aegrine–augite with some aegirine. Geo-chemically, the studied trachytes are selectively enrichedin most of LILE and HFSE (i.e., Rb, Ga, U, Th, Nb, Y,and Zr). The REE patterns of UST and UKT aresomewhat uniform, with enrichment in LREE overHREE and negative Eu anomalies. They exhibit ageochemical signature of typical within-plate magmathat originated in anorogenic tectonic regime. Therelatively high Cr and Ni contents and fractionatedREE patterns of the investigated trachytes indicate thatthey represent melts derived from a dominant mantlesource. The negative Ba, Sr, TiO2, and Eu anomalies inthe UKT and UST, besides the gradual decrease in LREE,indicate that these rocks were derived through significantfractionation processes. The younger UST (273±15 Ma)show somewhat higher MgO, Cr, Ni, and Ba compared tothe older UKT (302±15 Ma). This may preclude theevolution of the UST through a continuous fractionalcrystallization of the residual magmatic melt of the UKT.The studied trachytes have a narrow variation range ofZr/Nb (4.29–5.96), low Y/Nb (<0.5), and high Nb/La(>1) ratios resembling OIB, which evolve from astheno-spheric mantle source. Thus, the UKT and UST are

genetically related but could be emplaced through twovarious magmatic pulses of the same parent source (i.e.,asthenospheric mantle source) at different times. Thismost probably indicates the repeated nature of thealkaline trachytic magmas over a long period of time(ca. 25 Ma) along lines of the lithosheric weakness in thestudied area. Such mantle source was subsequentlysubjected to two significant fractional crystallizationprocesses to yield the UKT and UST plugs. Theelemental Th/Nb ratio (0.07–0.12) of the studied tra-chytes, besides the moderate initial 87Sr/86Sr ratios of theUKT and UST (0.7117 and 0.7095, respectively) alongwith positive K, Rb, REE (La, Ce, Nd) anomalies in multi-element variation diagrams, substantiates the significantrole of crustal contamination during their generations.

Acknowledgments The authors are grateful to Prof. F. Finger,Salzburg University, Austria for, XRF facilities. Deep thanks to Prof.S. Arai, Kanazawa University, Japan, for the facilities of microprobeanalysis. Two anonymous reviewers are highly thanked for theirconstructive comments to improve the earlier version of themanuscript.

References

Abdel-Rahman AM (1995) Tectonic–magmatic stages of shieldevolution: the Pan-African belt in northeastern Egypt. Tectono-physics 242:223–240

Abdel-Rahman AM (2002) Mesozoic volcanism in the Middle East:geochemical, isotopic and petrogenetic evolution of extension-related alkali basalts from central Lebanon. Geol Mag 139(6):621–640

Abdel-Rahman AM, El-Kibbi M (2001) Anorogenic magmatism:chemical evolution of the Mount El-Sibai A-type complex(Egypt) and implications for the origin of within-plate felsicmagmas. Geol Mag 138:67–85

Akaad MK, Shazly AG (1977) Geology and geological history of theAbu Ziran area, Eastern Desert, Egypt. Delta J Sci 1:53–69

Akaad MK, Noweir, AM (1980) Geology and lithostratigraphy of theArabian Desert Orogenic belt of Egypt between Latitudes 25° 35´and 26° 30´. Institute of Applied Geology, Jeddah, Bull 3:127–135

Aly SM (1973) Geology of the area around Gebel Um Khors, EasternDesert, Egypt. MSc thesis, Tanta University, Tanta, Egypt, p 222

Aly MM, Moustafa ME (1984) Major chemistry statistics characterizingcommon igneous rocks of Egypt. The 9th International Congress forStatistics, Ain Shams University, Cairo, 4, pp 271–313

Bonin B, Giret A (1990) Plutonic alkaline series: Daly gap andintermediate compositions for liquids filling up crustal magmachambers. Schweiz Miner Petrol 70:173–185

Campbell IH, Griffiths RW (1990) Implications of mantle plumestructure for evolution of flood basalts. Earth Planet Sci Lett99:79–93

Chen W, Arculus RJ (1995) Geochemical and isotopic characteristicsof lower crustal xenoliths, San Francisco Volcanic Field, Arizona,U.S.A. Lithos 36:203–225

\Davidson JP, Wilson IR (1989) Evolution of an alkali basalt–trachyte suite from Jabal Marra volcano, Sudan, throughassimilation and fractional crystallization. Earth Planet ScienLett 95:141–160

Arab J Geosci (2012) 5:53–71 69

Page 18

Davies GR, Norry MJ, Geralch DC, Cliff RA (1989) A combinedchemical and Pb–Sr–Nd isotope study of the Azores and CapeVerde hotspots: the geodynamic implications. In: Saunders AD,Norry MJ (eds) Magmatism in ocean basins. Geological SocietySpecial Publication 42, pp 231–256

Deer WA, Howie RA, Zyssman J (1966) An introduction to the rockforming minerals. Longmans, London, p 527

Droop GTR (1987) A general equation for estimating Fe3+

concentrations in ferromagnesian silicates and oxides frommicroprobe analyses using stoichiometric criteria. Miner Mag5:431–435

Duncan AR, Lindstrom MM, McMillan NJ, Moorbath S, Hoefs J,Haskin LA (1986) Open system magmatic evolution of the TaosPlateau volcanic field, northern New Mexico: 1. The petrologyand geochemistry of the Servilleta basalt. J Geophys Res 9:5999–6028

Eby GN (1990) The A-type granitoids: a review of their occurrenceand chemical characteristics and speculations on their petrogen-esis. Lithos 26:115–134

El-Desuky BT (1978) Petrographical and geochemical studies on thetrachytes south of the Qift-Quseir road, Eastern Desert, Egypt.MSc thesis, Tanta University, Tanta, Egypt, p 200

El-Manharawy MS (1972) Isotopic ages and origin of some uraniumbearing volcanic rocks in Egypt. MSc thesis, Cairo University,Egypt, p 147

Essawy MA, Shazly AG (1978) Petrology of Um Shaghir metagab-bro–diorite complex, Eastern Desert, Egypt. Bull Fac SciMansoura Univ Egypt 6:438–458

Evensen NM, Hamilton PJ, O’Nions RK (1978) Rare-earth abundan-ces in chondritic meteorites. Geochim Cosmochim Acta42:1199–1212

Faure G, Hurley PM (1963) The isotopic composition of strontium inoceanic and continental basalts: application to the origin ofigneous rocks. J Petrol 4:31–50

Faure G, Powell JL (1972) Strontium isotope geology. Springer,Berlin

Fitton JG (1987) The Cameroon line, West Africa: a comparisonbetween oceanic and continental alkaline volcanism. In: FittonJG, Upton GG (eds) Alkaline igneous rocks. Royal SocietySpecial Publication, 30, pp 273–291

Fitton JG, James D, Leeman WP (1991) Basic magmatism associatedwith Late Cenozoic extension in the western United States:compositional variations in space and time. J Geoph Res 96:693–712

Fuhrman ML, Lindsley DH (1988) Feldspar minerals. Amer Miner73:201–215

Garson MS, Krs M (1976) Geophysical and geological evidence of therelationship of Red Sea transverse tectonics to ancient features.Bull Geol Soc Amer 87:169–181

Ghoneim MF, Aly SM, Hassan A, Abu Anbar MM (1998) Maficminerals of some alkaline rocks of Eastern Desert and Sinai,Egypt: a comparative study. The Fifth Conference of Sinai forDevelopment, Saint Catherina, South Sinai, Suez Canal Univer-sity, Ismailia, 5, pp 97–112

Godwin F, Moller A, Carl C, Swisher A, Lindsay J, McHenry, MarkD, Feigenson A, Michael J, Carr A (2009) Petrogenesis ofbasalt–trachyte lavas from Olmoti Crater, Tanzania. J Afr EarthScien 54:127–143

Gourgaud A, Maury RC (1984) Magma mixing in alkaline series: anexample from Sancy volcano (Mont Dore, Massif Central,France). Bull Volcanol 47:827–847

Harris NBW (1982) The petrogenesis of alkaline intrusives fromArabia and northeast Africa and their implications for within-plate magmatism. Tectonophysics 83:243–258

Hashad AH (1980) Present status of geochronological data on theEgyptian basement complex. Inst Appl Geol Bull Jeddah 3:31–46

Hashad AH, Sayyah TA, El-Manharawy MS (1981) Isotopic compo-sition of strontium and origin of Wadi Kareim volcanics, EasternDesert. Egypt J Geol 25:141–147

Hassan A, Ghoneim MF, Abu Anbar MM (1997) Geochemistry andchemistry of alkaline volcanics from the Eastern Desert, Egypt.3rd Conference on Geochemistry, Alexandria University 1, pp203–219

Heikal MT (2003) Model for the origin of the Um Shaghir–Um Khorsalkaline trachyte plugs, central Eastern Desert, Egypt. The 3rdInternational Conference on the Geology of Africa, 1, pp 233–253

Hofmann AW, Jochum KP, Seufert M, White WM (1986) Nb and Pbin oceanic basalts: new constrains on mantle evolution. EarthPlanet Sci Lett 79:33–45

Irvine TN, Baragar WRA (1971) A guide to the chemical classification ofthe common volcanic rocks. Can J Earth Science 8:523–548

Joseph MS (2007) Conjugate shear joints pattern in interpreting thetectonic affecting on the basement rocks south Qift-Quseir road,Central Eastern Desert, Egypt. Second International Conferenceon Geo-resources in the Middle East and North Africa, CairoUniversity, Egypt, Abstract, p 139

Kröner A (1985) Ophiolites and the evolution of tectonic boundariesin the Late Proterozoic Arabian–Nubian Shield of northeastAfrica and Arabia. Precam Res 27:277–300

Leake BE, Birch WD, Gilbert MC, Grice JD, Hawthorne FC, Kato A,Kish HJ, Krivovichev VG, Linthout K, Laird J, Maresh WV,Nickel EH, Rock NMS, Schumacher JC, Smith DC, StephensonNCN, Ungaretti L, Whittaker EJW, Youzhi G (1997) Nomencla-ture of amphiboles. Report of the Subcommitte on Amphibolesof the International Mineralogical Association Commission NewMinerals and Mineral Names. Eur J Miner 9:623–651

Leake BE, Wooley AR, Birch WD, BURKE EAJ, Ferraris G, GriceJD, Hawthorne FC, Kisch HJ, Krivovichev VG, Schumacher JC,Stephenson NCN, Whittaker EJW (2003) Nomenclature ofamphiboles: additions and revisions to the International Miner-alogical Association’s 1997 recommendations. Can Miner41:1355–1362

Macdonald R, Davies GR, Bliss CM, Leat PT, Bailey DK, Smith RL(1987) Geochemistry of high-silica peralkaline rhyolites, Nalva-sha, Kenya Rift Valley. J Petrol 28:979–1007

Meneisy MY (1986) Mesozoic igneous activity in Egypt. QatarUniversity Science Bulletin 6, Qatar

Meneisy MY (1990) Volcanicity. In: Said R (ed) The geology ofEgypt. Balkema, Rotterdam, pp p157–p172

Meneisy MY, Abdel Aal AY (1984) Geochronology of Phanerozoicvolcanic rocks in Egypt. Bulletin of the Faculty of Science, AinShams University, p 25

McDonough WF (1990) Constraints on the composition of thecontinental lithospheric mantle. Earth Planet Sci Lett 101:1–18

Miller CF, Mittlefeld DW (1982) Depletion of light rare-earthelements in felsic magmas. Geology 10:129–133

Mohamed FH (2001) The Natash alkaline volcanic field, Egypt:geochemical and mineralogical inferences on the evolution of abasalt to rhyolite eruptive suite. J VolcanGeotherm Res 105:291–322

Morimoto N (1989) Nomenclature of pyroxenes. Can Miner 27:143–156O’ Halloran DA (1985) Ras ed Dom migrating complex: A-type

granites and syenites from the Bayuda Desert, Sudan. J Afr EarthSci 3:61–75

Peccerillo A, Donati A, Santo B, Orlando C, Yirgu D, Ayalew D(2007) Petrogenesis of silicic peralkaline rocks in the Ethiopianrift: geochemical evidence and volcanological implications. J AfrEarth Sci 48:161–173

Pin C, Cuesta LAO, Ibarguchi JIG (1992) Mantle-derived, EarlyPaleozoic A-type metagranitoids from the NW Iberian massif: Ndisotope and trace element constrains. Bull Geol Soc Fr 163:483–494

70 Arab J Geosci (2012) 5:53–71

Page 19

Scaillet B, Macdonald R (2003) Experimental constraints on therelationships between peralkaline rhyolites of the Kenya RiftValley. J Petrol 44:1867–1894

Schandelmeier H, Pudlo D (1990) The Central-African Fault Zone inSudan—a possible continental transform fault. Berliner GeowissAbh 120A:31–44

Smith E, Isanchez A, Walker JD, Wang K (1999) Geochemistry ofmafic magmas in the Hurricane volcanic field, Utah: implicationsfor small- and large-scale chemical variability of lithosphericmantle. J Geol 107:433–448

Stern RJ (1981) Petrogenesis and tectonic setting of Late Precambrianensimatic volcanic rocks, central Eastern desert of Egypt. PrecamRes 16:195–230

Stoeser DB (1986) Distribution and tectonic setting of plutonic rocksin the Arabian Shield. J Afr Earth Sci 4:21–46

Sun SS, McDonough WF (1989) Geochemical and isotopic system-atics of oceanic basalts: implications for mantle composition andprocesses. In: Saunders AD, Norry MJ (eds) Magmatism in theocean basins. Geological Society of London Special Publication42, pp 313–345

Ulrych J, Stepankova J, Novak JK, Pivec E, Prouza V (2003) Volcanicactivity in Late Variscan basins of northern Bohemia: petrolog-ical and geochemical constrains. Slovak Geol Mag 33:219–235

Ulrych J, Pesek J, Stepankova J, Bosak P, Lloyd FE, Seckendorff V,Lang M, Novak JK (2006) Permo-carboniferous volcanism inlate Variscan continental basins of the Bohemian Massif (CzechRepublic): geochemical characteristic. Chem Erde 66:37–56

White JC, Urbanczyk KM (2001) Origin of a silica-oversaturatedquartz trachyte–rhyolite suite through combined crustal melting,magma mixing, and fractional crystallization: the Leyva Canyonvolcano, Trans-Pecos Magmatic Province, Texas. J VolcanolGeotherm Res 11:155–182

White JC, Parker DF, Ren M (2009) The origin of trachyte andpantellerite from Pantelleria, Italy: insights from major element,trace element, and thermodynamic modeling. J Volcanol Geo-therm Res 179:33–55

Wilson M (1989) Igneous petrogenesis. Unwin Hyman, London, p 457Winchester JA, Floyd PA (1977) Geochemical discrimination of

different magma series and their differentiation products usingimmobile elements. Chem Geol 20:325–343

Arab J Geosci (2012) 5:53–71 71