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Arabian Journal of Geosciences ISSN 1866-7511Volume 7Number 2 Arab J Geosci (2014) 7:693-709DOI 10.1007/s12517-013-0854-0

Spinels, Fe–Ti oxide minerals, apatites,and carbonates hosted in the ophiolites ofEastern Desert of Egypt: mineralogy andchemical aspects

Abdel-Aal M. Abdel-Karim, WaheedI. Elwan, Hassan Helmy & Shymaa A. El-Shafey

1 23

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ORIGINAL PAPER

Spinels, Fe–Ti oxide minerals, apatites, and carbonates hostedin the ophiolites of Eastern Desert of Egypt: mineralogyand chemical aspects

Abdel-Aal M. Abdel-Karim & Waheed I. Elwan &

Hassan Helmy & Shymaa A. El-Shafey

Received: 8 July 2012 /Accepted: 21 January 2013 /Published online: 3 March 2013# Saudi Society for Geosciences 2013

Abstract Spinels, Fe–Ti oxide minerals, apatites, and car-bonates hosted in ophiolitic serpentinites and metagabbrosof Gabal Garf (southern ED) and Wadi Hammariya (centralED) of Egypt are discussed. Microscopic and electron probestudies on these minerals are made to evaluate their texturaland compositional variations. Alteration of chromites led toform ferritchromite and magnetite; rutile–magnetite inter-growths and martite are common in serpentinites. Fine trillisexsolution of ilmenite–magnetite and ilmenite–hematite andintergrowth of rutile–magnetite and ilmenite–sphene arerecorded. Composite intergrowth grains of titanomagne-tite–ilmenite trellis lamellae are common in metagabbros.The formation of ilmenite trellis and lamellae in magnetiteand titanomagnetite indicate an oxidation process due toexcess of oxygen contained in titanomagnetite; trappedand external oxidizing agents. This indicates the high PH2O

and oxygen fugacity of the parental magma. The sulfidesminerals include pyrrhotite, pyrite and chalcopyrite. Basedon the chemical characteristics, the Fe–Ti oxide from theophiolitic metagabbros in both areas corresponds to ilmen-ite. The patites from the metagabbros are identified as fluor-apatite. Carbonates are represented by dolomites in serpen-tinites and calcite in metagabbros. Spinel crystals in serpen-tinites are homogenous or zoned with unaltered cores of Al-spinel to ferritchromit and Cr-magnetite toward the alteredrims. Compared to cores, the metamorphic rims are enriched

in Cr# (0.87–1.00 vs. 0.83–0.86 for rims and cores, respec-tively) and impoverished in Mg# (0.26–0.48 vs. 0.56–0.67)due to Mg–Fe and Al (Cr)–Fe3+ exchange with the sur-rounding silicates during regional metamorphism rather thanserpentinization process. The Fe–Ti oxides have beenformed under temperature of ~800 °C for ilmenite. Al-spinels equilibrated below 500–550 °C, while the alteredspinel rims correspond to metamorphism around 500–600 °C.Geochemical evidence of the podiform Al-spinels suggest agreenschist up to lower amphibolite facies metamorphism (at500–600 °C), which is isofacial with the host rocks. Al-spinelcores do not appear to have re-equilibrated completely withthe metamorphic spinel rims and surrounding silicates, sug-gesting relic magmatic composition unaffected by metamor-phism. The composition of Al-spinel grains suggest anophiolitic origin and derivation by crystallization of boniniticmagma that belonging to a supra-subduction setting couldform either in forearcs during an incipient stage of subductioninitiation or in back-arc basins.

Keywords Ophiolites . Gabal Garf . Wadi Hammariya .

Eastern Desert . Egypt . Non-silicateminerals .Mineralogy .

Chemical aspects

Introduction

Spinels, ilmenites and carbonates are common in ultramaficrocks and some of them often used as a petrogenetic indi-cator (Dick and Bullen 1984; Barnes and Roeder 2001).However, post-crystallization re-equilibration, alteration,and metamorphism complicate the petrogenetic interpreta-tion of their compositions. This is especially important forspinel in serpentinites. Generally, serpentinites occurring in

A. M. Abdel-Karim (*) :W. I. Elwan : S. A. El-ShafeyGeology Department, Faculty of Science,Zagazig University, Zagazig, Egypte-mail: abdelaalabdelkarim@hotmail.com

H. HelmyGeology Department, Faculty of Science,Minia University, Minia, Egypt

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orogenic belts have been subjected to three alteration pro-cesses, i.e., serpentinization, carbonatization, and regionalmetamorphism. Metamorphic modifications of spinel havebeen frequently described by some authors (Zhou et al.1996; Barnes and Roeder 2001; Proenza et al. 2008;Gonzálea-Jimènez et al. 2009). Characteristic features ofspinels hosted in the Pan-African ophiolites of Egypt aregiven by Ahmed et al. (2001); Farahat (2008); Ahmed(2007); Hamdy and Lebda (2011).

Ophiolitic serpentinites and metagabbros are commonlydistributed in the Pan-African belt of Egypt. This belt,exposed in Eastern Desert of Egypt, is part of theArabian–Nubian Shield that constitutes the northern sectorof the East African Orogen (Stern 2002). Serpentinites werederived principally from harzburgite and a few lherzolitesubtype ophiolites in the Eastern Desert of Egypt (Abdel-Karim and Ahmed 2010). These ophiolites are frequentlydismembered and consequently the serpentinites are presentas isolated masses. In addition, variably sized ophioliticrocks, including serpentinites and metagabbros, are com-mon in the widespread Eastern Desert mélange and meta-sediments. The ophiolities have a strong geochemicalaffinity to marginal basin tectonic settings (Farahat et al.2004; Abdel-Karim and Ahmed 2010).

The present study focuses on accessory spinel, ilmen-ite, magnetite, apatite, and carbonate hosted in ophioliticserpentinites and metagabbros of Wadi Hammariya (WH;central ED) and Gabal Garf (GG; southern ED) of Egyptto shed light on their mineralogy and chemical aspectswhich provide a good opportunity to assess their texturaland compositional variations with serpentinization andmetamorphism.

Field geology and petrography

The ophiolitic serpentinites

The serpentinite rocks of GG constitute one of the principalrock units distinguished in the field and cover an area ofabout 400 km2 (Fig. 1). The masses occur as lenses andsheet-like bodies of homogeneous massive serpentinite. Themassive serpentinites are antigorite, lizardite, and/or chrys-tile consist mainly of fine grained, dark green to grayish-green and show mesh texture. The talc–carbonate rocks arecomposed mainly of talc with remnants of antigorite andchrysotile minerals and exhibit creamy talcose touch. Thestudied serpentinites have been treated as parts of dismem-bered ophiolite suites. Large masses of talc carbonate rocksalong the shear zone between the serpentinite and otherophiolitic members are also recorded. They exhibit whitealteration specks of talc carbonate rocks particularly nearthrust contact.

Ultramafic rocks are common among the mélangeassociation in WH area (8 km2). They are representedby peridotite–serpentinites. The peridotite–serpentinitesare massive coarse-grained, slightly serpentinized, andcut by calcite–magnesite veinlets. Subordinate dyke-likebodies (about 50-cm-thick and a few meters long) ofhighly deformed metapyroxenite–serpentinites occur asnearly parallel dykes disposed in a SW direction, in thecentral part of WH. These rocks possess a greenishbrown weathered surface. Alteration of serpentinites intodark brown and buff colored talc–carbonate rocks israther common and these occur either as scatteredpatches or sheet-like bodies along shear zones and faultplanes within the serpentinite masses.

Microscopic studies of the serpentinites of GG and WHshow that they comprise massive and sheared varietiestogether with quartz carbonate rocks. The massive serpen-tinites include peridotite- and metapyroxenite–serpentinitesand proper serpentinites.

Peridotite serpentinites are composed of olivine, ensta-tite, and antigorite as main constituents besides carbo-nates and opaques as minor amounts. They show mesh,hour glass and bastite textures. Olivine and enstatiteappears as relics which are partially serpentinized.Antigorite occurs as fibro-lamellar aggregates exhibitingplumose structure. Carbonate occurs as interstitial patchesmostly associated by disseminated grains of opaques.Opaques are represented by chromites and its alterationproducts. Metapyroxenite–serpentinites consist mainly ofclinopyroxene, antigorite, and orthopyroxene with de-creasing order of abundance besides subordinate amountsopaques. They show allotriomorphic granular texture.Clinopyroxene is mainly augite partially altered to ortho-pyroxene. Serpentinites consist principally of antigorite,lizardite, chrysotile with subordinate amounts of magnetite,calcite, opaques, and pyroxene relics. Talc occurs as colorlesspatches partially replaced by carbonate. Pyroxene relics arerepresented by clinopyroxene (augite). Calcite occurs as euhe-dral to subhedral crystals showing pronounced twinkling andstrong birefringence.

Sheared serpentinites in both areas are less commonthan the massive type. They have the same composition,but the minerals are commonly arranged in subparallelalignment producing the schistosity. They include carbo-nates chlorite schists, asbestos talc schists, tremolite talcschists, tremolite–actinolite chlorite schists, serpentinite brec-cia (ophicarbonate), quartz carbonate rocks (listwaenites).Carbonate chlorite schists consist mainly of carbonate, quartzand chlorite. They exhibit schistose texture. Chloriteoccurs in form of dense fibrolamellar aggregates in abanded nature. These bands are alternative with other onesof carbonate and quartz aggregates. Asbestos- and tremo-lite talc schists consist wholly of talc with subordinate

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Fig. 1 shows: a location mapof studied ophioliteoccurrences, b Geologic map ofGabal Garf (Abdel-Karim et al.2001), c Geologic map of WadiHammariya area (Akaad andAbu El Ela 2002) withmodifications

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asbestos or tremolite. They possess a pronounced parallelarrangement of the individual talc flakes and asbestos ortremolite fibers. Tremolite–actinolite chlorite schists consistchiefly of tremolite–actinolite and chlorite associated withopaques. They exhibit schistose texture. Tremolite–actino-lite crystals are fresh and no incipient serpentinization wasobserved. Chlorite occurs as irregular grains of pale greencolor and week birefringence. Quartz carbonate rocks(listwaenites) consist mainly of quartz, carbonate (calciteand dolomite), and subordinate amounts of opaques.Calcite occurs as twinkled rhombs forming curl-likeveins. Carbonate occurs as fine brownish aggregatesforming fish bone skeletal structure.

Ophiolitic metagabbros and amphibolites

Ophiolitic metagabbros and amphibolites of GG are distribut-ed as small scattered lensoidal masses and occur as scatteredmega shear pods within the serpentinites. They range from 1 to5 km in length and from 0.5 to 3 km in width. The masses areoriented in arch form trending NE–SW direction. The

metagabbros are partially subjected to extensive shearing.They are partially dissected by irregular joints and crackswhich filled by quartz veins which associated by gold. TheGG metagabbros are oftenly thrusted over the metavolcanicsand are, in turn, overthrusted by serpentinites. Along thetectonic contact, a thin layer is observed. The lithology of thislayer is quite similar to the sheared metagabbro. The metagab-bros are intruded by syn-tectonic granites with sharp contact.

Ophiolitic metagabbros and amphibolites of WH are en-countered in the southwestern corner of the map area. All ofthe serpentinite and gabbroid masses are tectonically enclosedwithin or thrust over the island arc rock assemblage. Thegabbroic mass covers about 1.2 km2 and traversed by WadiUmm Qarati lying to the NE of the large serpentinite mass.The gabbroid mass occurring in southwestern corner of themap area, covers an area of about 5.2 km2. These massesoccur as large size blocks or thrust sheets. The amphibolitesoccur as small (40–80 m) unmappable bodies, associated withand grading into the metagabbros. They are medium andrarely coarse grained, hard massive dark green rocks spottedwith white clots of altered plagioclase.

Fig. 1 (continued)

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The metagabbros in GG and WH consist principally ofplagioclase, hornblende and pyroxene besides subordinateamount of epidote, chlorite, apatites, sphene and opaques.Plagioclase (An45) is partly altered to epidote and kaolinite.Pyroxene occurs as subhedral to anhedral augite crystals. Itis altered to hornblende and chlorite or tremolite–actino-lite. Epidote is represented by zoisite and clinozoisite.Sphene occurs as irregular cleavage grains associatedwith iron oxides or intergrowth within plagioclase.Calcite forms coarse euhedral grains scattered throughoutthe rock exhibiting obvious twinkling. In some samples itoccurs as subhedral twinkled crystals superimposed ontop of each digit forming veins cutting across the rock.Apatite constitutes needle shape and exhibits weekbirefringence.

Amphibolites in both areas are subordinate rocks, close-ly associated with metagabbros. Their textures indicate thatthey may be originated from the metagabbros. They con-sist essentially of more than 80 % amphiboles (tremolite–actinolite) and Ca-plagioclase with minor amounts of chlo-rite, quartz. Calcite and iron oxides are subordinateamounts. Tremolite–actinolite association occur as elongatepale green xenomorphic crystals sometimes mostly withpreferred orientation. Plagioclase occurs as interstitialanhedral crystals associated with minor quartz. Most pla-gioclase crystals are untwined and strongly altered tozoisite and saussurite or replaced by carbonate.

Ore mineralogy

The ophiolitic serpentinites

1.1. Spinel in serpentinites varies from 2–6.5 % (WH) upto 1.2–9.5 % (GG) of the whole rocks. It includesunaltered Al-spinel and altered ferritchromit and Cr-magnetite (Table 1). Unaltered spinel is the predomi-nant ore minerals.

Al-spinel in GG occurs as isotropic irregular zonedfractured grains of gray color and red internal reflec-tion. The core of the grain is darker than the outer rimwhich is lighter gray in color and has higher reflec-tance. The core is identified as remnant of unaltered(primary) Al-spinel whereas the outer rim identi-fied as Cr-magnetite (Fig. 2a). The alteration startsfrom grain boundaries and extends along the majorcracks of fractured grains. The fractures are eitherfilled with serpentine minerals or magnetite. Thealteration of spinel to highly reflective rims hasbeen reported by several authors (e.g., Takla et al.1975; Farahat 2008).

Al-spinel in WH forms homogeneous grains withinternal reflection of a distinctive reddish brown to browncolor. It is usually irregular, fractured and deformed.Altered brecciate spinel grains are usually common.Some grains are altered along their fractures andmargins.The fractures and margins are replaced by either ferrit-chromit or Cr-magnetite rims which exhibit lighter graycolor and higher reflectance. This is usually attributed tothe effects of low to medium grade metamorphism up tolower amphibolite facies (McEdulff and Stumpfl 1991;Farahat 2008). Spinel in talc–carbonate rocks, occurs assubhedral to euhedral grains. Owing to serpentinizationand carbonatization processes, irregular fractured graygrains of ferritchromite accompanied by lighter graycrystals of Cr-magnetite are observed (Fig. 2b).

Magnetite in GG is recorded in serpentinites andmetapyroxenite. It occurs as an alteration rim mantledspinel (Fig. 2a, b). It shows lighter gray color andhigher reflectance than spinel. Sometimes, magnetiteoccurs as veinlets filling the fractures or as irregulargray grains with white and higher reflectance martite.Martite is formed when the cleavages of magnetite arereplaced by hematite which exhibit whiter color andcherry red internal reflection, a feature indicates a risingtemperature during metamorphism. Magnetite is oftenaltered to geothite and limonite forming colloform tex-ture. In WH, magnetite forms euhedral crystal as analteration product of oxidization of pyrite. Magnetiteappears to be formed by complete replacement of chro-mites or as veinlets associated with spinel specks. Freshmagnetite sometimes forms subhehral pyramidal andprismatic shape grains. Cross-cut veinlets of magnetiteenriched in rutile (ilmeno–rutile) are often seen.

1.2. Fe–Ti oxides in serpentinites vary from 5.5–7 % (GG)up to 4.6–10.5 % (WH) of the whole rocks. The Fe–Tioxide minerals include magnetite, ilmenite, and titano-magnetite (Table 1). Magnetite is the predominant Fe–Ti oxide ore minerals (they are observed in both studiedareas). Rutile–magnetite intergrowths in WH serpentin-ites and martite in GG serpentinites are recorded.Titanomagnetite in GG displays light gray color withpinkish tint and moderate reflectance. It occurs as par-allel streaks, irregular grains (Fig. 2c) and blades alongthe cleavage planes of clinopyroxene. In places, irregu-lar grains of magnetite with fine network of ilmenite areobserved. Ilmenite in GG is only recorded in chloriteschist. It forms about 15 % of the total rock volume.Ilmenite occurs as irregular anhedral granules exhibit-ing grayish color with brownish shade and moderatereflectance. Hematite in GG is recorded in quartz car-bonate rocks, occurs as an alteration product of magne-tite. It exhibits irregular aggregates of whiter color andcherry red internal reflection. Hematite inWH occurs as

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Table

1Distributionof

opaque

mineralsandtheirintergrowths

Area

Sam

ple

number

Petrographic

nomenclature

Spinel

Sulphides

Ilmenite

Magnetite

Hom

og.

spine

Zoned

chromite

Hom

og.

magnetit

Hem

atitized

magnetit

Titano-

magnetite

Magnetite-

ilmenite

intergrowth

Rutile-

magnetite

intergrowth

Martite

Normal

ilmenite

Hem

atite-

ilmenite

exsolutio

n

Ilmenite-

sphene

intergrowth

Pyrite

Chalco-

pyrite

Pyrrhotite

Bornite

Gabal

Garf

1/2

Per.serpentinite

−XXXX

−−

−−

−−

−−

−X

−−

−

26/3

Metapyroxenite

−−

XXX

−XX

−−

−−

−−

X−

−−

13/1

Lizardite

serpentin

iteXXX

−XX

−−

−−

XX

−−

−−

−−

14/3

XX

xXXX

−−

−−

−−

−−

XX

−−

−

26/1

XXX

XX

−XX

XX

−−

−−

−−

−−

−−

10/1

Chrys.

Serpentinite

XX

XXXX

−−

XXX

−−

−−

−−

−−

−−

26/2

Antigorite

serpentin

iteXXX

XXX

XXX

−−

−−

−−

−−

XX

−−

−

26/7

XXX

XXX

XXX

−XX

−−

−−

−−

−−

XX

−

36/3

Qz-carbon.rock−

−−

−−

XX

−−

−−

−−

−−

−xx

22/4

Antigorite

chlorite

schist

−−

−−

−−

−−

XXXX

−−

−−

−−

33/1

Metagabbros

−−

−−

−XX

−XX

XXX

XX

−XX

−−

−

39−

−XX

−−

−−

−XXX

XXX

−XX

−−

−

20/2

Amphibolite

−−

−−

−−

−−

−−

XXX

X−

−−

WadiHam

mariya

256

Per.serpentinite

XXXX

−−

−−

−−

−−

−−

XX

XX

−−

205

Antigorite

chrysotile

serpentin

ite

XX

XXX

XXX

−−

−−

−−

−−

−−

X−

206

XX

XXX

−−

−−

−−

−−

−−

−−

−

227

XXXX

−−

−−

−−

−−

−−

−−

XX

−

249

−−

XX

−−

−XXX

−−

−−

XX

XX

−−

242

Talc-carbonate

rock

XXX

−XXX

XX

−−

X−

−v−

−XX

−−

−

251

Qz-carbonaterock

XX

−X

XXX

−−

−−

−−

−XX

−X

−

258

Metagabbros

−−

−XX

XX

−−

−XXX

−−

−−

−−

264

−−

−XXX

XXX

−X

−−

X−

XX

XX

−

267

−−

−XXX

XXX

XXX

−−

−−

−−

−−

−

(xxxx)

dominant,(xxx)common

,(xx)

fairly

common

,(x)rare,(−)absent,Per:perido

tite,Chrys:chrysotile,carbon

:carbon

ate

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irregular grains of higher reflectance and cherry redinternal reflection, mostly after magnetite, probablyindicates an overprint by intensive metamorphism.

1.3. Sulphides vary from 2.5–4.4 % (GG) to 5.6–6.5 %(WH) of the whole rocks. The sulfides minerals in-clude pyrrhotite, pyrite and chalcopyrite (Table 1).Pyrrhotite and pyrite are the predominant sulphideminerals. Pyrite is the most common secondary sul-phide phase. Chalcopyrite is less abundant. Sulphideminerals in GG occur as subordinate amounts repre-sented by pyrrhotite and pyrite. Pyrrhotite occurs asirregular grains with bluish shade and moderate reflec-tance. Sometimes, it is mantled by titanomagnetite whichreflects the reduction of titanomagnetite to pyrrhotite as aresult of alteration process. Pyrite occurs as isotropicsubrounded grains associated with magnetite or as inde-pendent anhedral grains disseminated throughout therock. It exhibits yellowish white color and high reflec-tance. Isotropic irregular grains bornite with orange colorand high reflectance in quartz carbonate rocks is alsoobserved. Sulphides in WH are represented by smalldisseminated subhedral to anhedral grains of chalcopy-rite, pyrite, sphalerite and pyrrhotite. Chalcopyrite formssubhedral yellow crystals. It is embedded in the serpenti-nized groundmass of the dark gray color.Pyrite occurs aseuhedral grains of bright yellow color and high reflec-tance. Pyrite is partially oxidized to magnetite, particu-larly along their margins. Pyrrhotite occurs also aseuhedral grains of pale yellow color and lower reflec-tance than pyrite. It is partially replaced by magnetite.

The ophiolitic metagabbros

2.1. Fe–Ti oxides are the essential opaque minerals in thestudied metagabbros. They vary from 1.5–2.5 % (GG)up to 4.3–7.5 % (WH) of total rock volume. They arerepresented by magnetite, titano–magnetite, and ilmen-ite with less abundant amounts ilmenite–hematite.Fine trillis exsolution of ilmenite–magnetite in WHmetagabbros and ilmenite–hematite in GGmetagabbrosare observed. Rutile–magnetite and ilmenite spheneintergrowth and in WH and GG metagabbros are alsorecorded.Magnetite in GG forms subhedral to anhedralgrains, pale gray in color with brownish tint of mediumreflectivity but higher than ilmenite reflectivity.Sometimes, scattered small homogeneous magnetitegrains without any exsolution are surrounded by sili-cate. Alteration and replacement textures of magnetiteinto martite are also observed. Magnetite in WH occursas euhedral or irregular grains. It shows the distinctivecolloform structure due to the partial alteration to goe-thite and limonite. It exhibit various intergrowth texturewith ilmenite such as fine network lamellae exsolution

and myrmakitic textures. Similar textures were reportedon the older gabbros of the Eastern Desert (Takla et al.1981). Titanomagnetite in WH occurs as irregular toskeletal shaped crystals with pinkish tint. Compositeintergrowth grains of titanomagnetite–ilmenite trellislamellae are recorded (Fig. 2d). Ilmenite in GG is thedominant opaque minerals and constitutes about 80 %by volume of total opaques. Sometimes ilmenites withminor sulphides represent the main opaque minerals.Homogeneous single phase ferri-ilmenite free from anyexsolutions is the most frequent type of ilmenite. Itoccurs as subhedral and anhedral discrete grains whichare entirely surrounded by silicate. In most cases, thediscrete grains show rugged edges and finger protrusionspiercing the surrounding silicates. Ilmenite inWH occursas irregular homogenous grains of light gray color andmoderate reflectance. In places, hematite–ilmenite net-work exsolution is observed. Exsolution of fine networkintergrowth of ilmenite–magnetite lamellae in titano-magnetite is also recorded (Fig. 2e). Ilmenite sometimesreplaces by sphene (Fig. 2f). Rutile in WH is Fe-richrutile (ilmeno–rutile), occurs as rhombic crystals exhib-iting light gray color and low reflectance.

Ilmenite–hematite fine trillis exsolution intergrowthis widely distributed in the present metagabbros. Thealteration and replacement of ilmenite has been takenplace through the regional metamorphism processes oras a result of latter hydrothermal solutions and weath-ering (Basta and Takla 1968). In places, ilmenite con-tains titano–hematite exsolution which is believed to beresulted from continues and progressive nature of theunmixing processes while the titano–hematite exsolu-tion bodies have grown in situ by absorbing Fe2O3 fromthe adjacent ferri-ilmenite (Edwards 1938). Exsolutionlamellae of hematite in titanomagnetite is due to marti-tization of titanomagnetite is recorded (Fig. 2g).

2.2. Sulphides in present metagabbros are represented bypyrite (GG); pyrite, chalcopyrite and pyrrhotite (WH).Pyrite in both areas, occurs as subrounded fine grains ofpale yellow color and high reflectance, sometimes ispartially replaced by chalcopyrite. Chalcopyrite exhib-its pure yellow color and lower reflectance than pyrite.Pyrrhotite occurs as subhedral grains of bluish tint. It isoften reduced from magnetite due to the effect of alter-ation process. Some grains of pyrrhotite are partiallyreplaced by magnetite (Fig. 2h).

Mineral chemistry

Twenty three rock samples from the present ophioliticserpentinites and metagabbros were selected and polished

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for studying the minerals (their textures in the differentrock types are given in Table 1). CAMEBAX-electronprobe microanalyses of the non-silicate minerals from thetwo localities were carried out in the TechnischeUniversity, Berlin, Germany. The analytical conditionswere a 25-kV accelerating voltage, 20 nA probe currentand 3 μm probe diameter. The raw data were correctedusing an on-line ZAF program. The ferric and ferrousirons of minerals were calculated assuming minerals stoi-chiometry. The analytical results of 62 spots (12 spinels,22 ilmenites, 14 apatites, and 14 carbonates) are given inTables 2, 3, 4, and 5.

Results

The derivation and evolution of the magma from which theophiolitic metagabbros and serpentinites was formed has been

thoroughly documented (Zimmer et al. 1995; Abdel-Karim etal. 2001, 2008). Moreover, the spinels compositions and theirrelations with magma evolution are discussed herein. Thestages of relative magmatic evolution of all the studied sam-ples can be roughly worked out using the correlation ofrelative contents of elements in the present minerals.

Al-spinels

Representative 12 spot chemical analyses of 4 samples fromGG and WH serpentinites and their chemical formulaebased on 32 oxygens are listed in Table 2. The Cr# isCr/(Cr+Al) atomic ratio. The Mg# is Mg/(Mg+Fe2+) atomicratio for spinel.

The analyzed spinels show obvious zoning in GG ser-pentinites, whereas as those of WH appear to be homoge-nous chromites. The core of the grains is darker than theouter light colored rim and has higher reflectance.

Al- spinel Cr-Mag.

52µm

Al- spinelCr-Mag.

20µm

26µm

Ilmenite

Ti-mag.

20µm

Il. /mag.

Ti-mag.

20µm

Ilmenite

Sphene

26µm

Hem. /Ti-mag.

13µm 26µm

a b

c d

e f

g h

Fig. 2 Photomicrographsshowing: a euhedral and banhedral zoned grains of Al-spinel core and Cr-magnetiterim, in massive (GG) andsheared (WH) serpentinite. cIrregular grain oftitanomagnetite, serpentinite,GG. d Composite intergrowthgrains of titanomagnetite-ilmenite trellis lamellae,metagabbro, WH. e Exsolutionintergrowth of ilmenite-magnetite lamellae intitanomagnetite, metagabbros,WH. f Ilmenite replaces bysphene, metagabbros, GG. gExsolution lamellae of hematite(white) in titanomagnetite dueto martitization oftitanomagnetite, metagabbros,WH. h Euhedral grain ofpyrrhotite partially replaced bymagnetite, metagabbro, WH

700 Arab J Geosci (2014) 7:693–709

Author's personal copy

Cr–Fe3+–Al triangular plot of Steven’s (1944; Fig. 3a)distinctively characterize the two zones with apparentcompositional gaps in between. The spinel cores of GGand fresh spinel spots of WH are Al-rich and lie alongthe Cr-Al join. Due to extensive Al loss, relative to Cr,spinels of the outer rims lie along the Cr–Fe3+ join.Moreover, the outer rims of GG serpentinites are Cr-magnetite, nearly devoid of Al. The low-alumina spinelresulting from alteration is often called ferritchromite.The Cr# [Cr/(Cr+Al)] of metamorphic magnetite is highcompared to those of primary magnetite because at thestage of magnetite crystallization from mafic magmathere is little Cr but still significant amount of Al.Therefore, primary magnetite plots along the Al–Fe3+

join of the Al–Cr–Fe3+ rather than the Cr–Fe3+ join asthe metamorphic magnetite (Barnes and Roeder 2001). Itis indicates that the spinel rims have the metamorphicorigin, including ferritchromite and Cr-magnetite.

Plotting of the analyzed chromites on the Al–Cr–Fe+3 dia-gram of Musallam et al. (1998) and Arai et al. (2004) (Fig. 3b)show that the Al-spinel cores of GG and fresh spinel spots ofWH display podiform, whereas the altered rims of GG serpen-tinites exhibit again Cr-magnetite and Fe-chromite composi-tion. On the Cr2O3 versus Al2O3 variation diagram, the freshspinel of both areas have composition comparable to ophioliticpodiform chromites from the central (WH) and southern (GG)Eastern Desert of Egypt (Ahmed et al. 2001) (Fig. 3c). Thetextural and compositional variations of spinel during meta-morphism are generally related to the metamorphic grade (e.g.,Barnes 2000; Farahat 2008). During serpentinization thin Cr-magnetite or ferritchromite rims starts to develop due to therelease of iron from olivine (Arai et al. 2006). Cr and Mg showenrichment in the core (12.98–13.07) (4.30–5.08) and progres-sive loss towards the rim (1.92–3.07) (2.04–3.67), whereasFe+3 enrichment is mainly observed from core (0.52–1.12) torim (5.98–14.07) (Table 1). This heterogeneous nature of the

Table 2 Chemical analysis and structural formulae of spinels from serpentinites

Area Gabal Garf Wadi Hammariya

S. No. 1/2 26/7 205 227

Spots Core-1 Rim-1 Core-2 Rim −2 Core-1 Rim-1 Core-2 Rim-2 1 2 1 2

SiO2 0.001 0.02 0.000 0.001 0.011 0.040 0.000 0.012 0.000 0.000 0.000 0.020

TiO2 0.097 0.052 0.080 0.009 0.051 0.000 0.000 0.048 0.000 0.000 0.140 0.000

Al2O3 7.642 0.078 7.666 0.033 7.228 0.016 6.220 3.777 6.760 7.440 7.340 8.210

Cr2O3 61.451 30.152 61.525 12.855 61.978 8.279 60.850 39.373 59.230 61.050 62.132 60.780

FeO 17.165 53.38 14.714 73.942 14.498 78.627 18.320 42.054 18.430 18.100 17.540 17.670

MnO 1.248 2.644 1.504 0.743 1.82 0.589 1.040 1.425 1.0320 0.890 0.700 0.770

MgO 10.726 7.013 12.827 4.549 12.856 4.542 11.920 8.752 11.760 12.34 13.43 12.780

CaO 0.000 0.059 0.000 0.000 0.007 0.000 0.000 0.011 0.000 0.000 0.100 0.000

Na2O 0.063 0.023 0.000 0.000 0.060 0.095 0.000 0.040 0.010 0.000 0.010 0.000

K2O 0.043 0.017 0.019 0.037 0.080 0.024 0.010 0.037 0.020 0.000 0.040 0.000

Total 98.436 93.438 98.335 92.169 98.589 92.212 98.360 95.530 97.242 99.820 101.43 100.23

Structural formulae of Al spinels based on 32 oxygen atoms

Si 0.000 0.006 0.000 0.000 0.003 0.012 0.000 0.003 0.000 0.000 0.000 0.005

Ti 0.020 0.012 0.016 0.002 0.010 0.000 0.000 0.010 0.000 0.000 0.027 0.000

Al 2.422 0.027 2.397 0.012 2.256 0.006 1.967 1.254 2.157 2.304 2.224 2.518

Cr 13.066 7.024 12.905 3.068 12.977 1.968 12.912 8.770 12.676 12.685 12.631 12.504

Fe+3 0.520 8.933 0.673 12.930 0.799 14.067 1.124 5.984 1.179 1.011 1.109 0.967

Fe+2 3.340 4.219 2.592 5.736 2.411 5.697 2.987 3.924 2.993 2.967 2.663 2.877

Mn 0.284 0.66 0.338 0.190 0.408 0.150 0.236 0.340 0.237 0.198 0.152 0.170

Mg 4.300 3.081 5.074 2.047 5.076 2.036 4.770 3.676 4.746 4.835 5.148 4.958

Ca 0.000 0.019 0.000 0.000 0.002 0.000 0.000 0.003 0.000 0.000 0.028 0.000

Na 0.033 0.013 0.000 0.000 0.031 0.055 0.000 0.022 0.005 0.000 0.005 0.000

K 0.015 0.006 0.006 0.014 0.027 0.009 0.003 0.013 0.007 0.000 0.013 0.000

Total 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000

Mg# 0.563 0.422 0.662 0.263 0.678 0.263 0.615 0.484 0.613 0.620 0.659 0.633

Cr# 0.840 1.000 0.840 1.000 0.852 0.997 0.868 0.875 0.855 0.846 0.850 0.832

Arab J Geosci (2014) 7:693–709 701

Author's personal copy

Table

3Chemical

analysisandstructural

form

ulaof

ilmenites

Gabal

Garf

WadiHam

mariya

Serpentinites

Metagabbros

Metagabbros

S.No

22/4

14/1

Av.

23/3

42Av.

258

264

267

Av.

Spo

ts1

23

45

12

31

23

41

21

23

45

61

1

TiO

255

.00

55.70

55.82

56.51

56.18

56.35

54.35

55.20

55.89

56.32

54.97

55.68

55.38

54.67

55.67

55.45

48.96

50.79

49.81

53.89

49.39

49.62

53.85

43.96

50.03

Cr 2O3

0.03

40.011

0.00

00.00

00.04

30.00

00.04

00.00

00.01

60.08

90.00

00.02

20.01

30.04

70.00

90.03

0.01

50.00

00.00

40.03

70.00

00.02

70.22

60.09

10.05

Al 2O3

0.00

00.00

00.00

80.02

00.00

00.01

00.00

40.00

50.00

60.00

00.03

10.00

00.00

00.00

00.00

60.00

60.03

00.01

60.00

00.01

80.00

60.00

00.03

30.00

10.01

3

FeO

44.99

44.52

45.85

45.04

45.33

45.36

43.7

45.29

45.01

43.8

44.7

45.23

44.24

44.39

44.42

44.46

49.78

47.56

49.02

45.67

48.78

47.28

44.83

52.56

48.19

MnO

0.71

70.82

70.87

30.76

50.81

60.83

00.86

80.83

40.81

61.29

71.14

41.18

21.12

11.12

01.09

71.16

01.50

11.74

91.63

71.63

81.37

61.35

81.97

31.70

01.617

MgO

0.115

0.17

70.14

90.16

60.14

20.115

0.05

30.15

20.13

40.26

50.35

80.28

80.25

60.15

10.18

50.25

10.05

80.05

50.00

70.02

40.01

60.05

30.04

90.17

30.054

CaO

0.00

00.00

00.00

00.00

90.03

80.06

60.17

20.01

20.02

90.06

30.01

50.03

90.07

00.13

50.03

70.06

00.02

40.115

0.00

70.04

00.03

70.03

30.03

30.01

30.038

SiO

20.02

20.00

00.00

00.00

00.01

60.01

30.06

30.00

40.01

50.03

60.03

90.02

60.01

70.03

60.00

70.02

70.01

80.02

50.03

80.00

60.04

20.01

40.02

80.01

70.02

4

Na 2O

0.00

00.00

00.03

40.00

00.01

40.06

60.00

00.00

90.01

50.00

00.02

50.00

00.01

20.00

30.01

20.00

90.02

70.00

00.00

00.00

00.00

00.04

20.00

00.00

00.00

9

K2O

0.00

50.00

00.00

40.03

20.03

40.00

90.01

70.00

00.01

30.00

00.00

00.00

00.02

80.00

00.00

90.00

60.00

90.02

80.011

0.01

00.02

00.00

00.01

20.01

30.01

3

Total

100.9

101.2

102.7

102.5

102.6

102.8

99.30

101.5

100.9

101.9

101.3

102.4

101.1

100.5

101.5

101.5

100.4

100.3

100.5

101.3

99.66

98.42

101.0

98.53

100.0

Structuralform

ulaof

ilmenite

basedon

6ox

ygen

atom

s

Ti

2.04

82.06

02.04

32.06

32.05

42.05

62.05

52.04

42.04

42.06

62.04

02.04

22.05

32.04

42.05

72.05

01.89

31.94

41.91

62.01

31.91

61.93

92.01

51.77

31.92

8

Cr

0.00

10.00

00.00

00.00

00.00

20.00

00.00

20.00

00.00

10.00

30.00

00.00

10.00

10.00

20.00

00.00

10.00

10.00

00.00

00.00

10.00

00.00

10.00

90.00

40.00

2

Al

0.00

00.00

00.00

00.00

10.00

00.00

10.00

00.00

00.00

00.00

00.00

20.00

00.00

00.00

00.00

00.00

00.00

20.00

10.00

00.00

10.00

00.00

00.00

20.00

00.00

1

Fe

1.86

31.83

11.86

61.82

81.84

31.84

01.83

81.86

51.86

41.78

81.84

31.84

41.82

41.84

51.82

41.82

82.14

02.02

42.09

61.89

72.10

42.05

41.86

52.35

72.06

4

Mn

0.03

00.03

40.03

60.03

10.03

40.03

40.03

70.03

50.03

40.05

40.04

80.04

90.04

70.04

70.04

60.04

80.06

50.07

50.07

10.06

90.06

00.06

00.08

30.07

70.07

0

Mg

0.00

80.01

30.011

0.01

20.01

00.00

80.00

40.011

0.01

00.01

90.02

60.02

10.01

90.011

0.01

40.01

80.00

40.00

40.00

10.00

20.00

10.00

40.00

40.01

40.00

4

Ca

0.00

00.00

00.00

00.00

00.00

20.00

30.00

90.00

10.00

20.00

30.00

10.00

20.00

40.00

70.00

20.00

30.00

10.00

60.00

00.00

20.00

20.00

20.00

20.00

10.00

2

Si

0.00

10.00

00.00

00.00

00.00

10.00

10.00

30.00

00.00

10.00

20.00

20.00

10.00

10.00

20.00

00.00

10.00

10.00

10.00

20.00

00.00

20.00

10.00

10.00

10.00

1

Na

0.00

00.00

00.00

30.00

00.00

10.00

60.00

00.00

10.00

10.00

00.00

20.00

00.00

10.00

00.00

10.00

10.00

30.00

00.00

00.00

00.00

00.00

40.00

00.00

00.00

1

K0.00

00.00

00.00

00.00

20.00

20.00

10.00

10.00

00.00

10.00

00.00

00.00

00.00

20.00

00.00

10.00

00.00

10.00

20.00

10.00

10.00

10.00

00.00

10.00

10.00

1

Total

3.95

13.93

93.95

73.93

63.94

53.94

33.94

53.95

63.95

53.93

33.95

93.95

83.94

73.95

63.94

33.94

94.10

64.05

54.08

43.98

54.08

44.06

03.97

94.22

54.07

1

702 Arab J Geosci (2014) 7:693–709

Author's personal copy

Table

4Chemical

analysisandstructural

form

ulae

ofapatitesfrom

metagabbros

Area

G.Garf

W.Ham

mariya

S.No

4233

/1Av.

4025

826

7Av.

Spo

ts1

11

12

12

34

56

78

9

P2O5

43.879

42.923

43.401

43.591

42.499

43.297

42.949

41.619

43.556

42.656

41.477

43.486

41.546

42.679

42.539

42.658

SiO

20.110

0.16

60.13

80.10

50.13

20.114

0.19

20.14

70.16

40.12

60.12

00.13

40.117

0.18

40.118

0.13

8

TiO

20.00

00.04

10.02

10.00

40.06

00.00

00.08

40.00

00.01

90.00

00.02

70.00

00.00

00.02

30.00

00.01

8

FeO

0.15

60.09

40.12

50.19

70.16

60.20

60.17

50.15

40.25

90.10

50.117

0.25

20.05

50.09

30.10

10.15

7

MnO

0.08

10.01

50.04

80.07

10.07

60.08

70.00

00.08

30.15

00.09

80.05

70.08

90.06

30.13

00.08

90.08

3

MgO

0.02

90.00

00.01

50.02

80.07

40.02

70.06

30.03

20.08

10.08

10.08

00.09

20.04

40.10

10.05

20.06

3

CaO

53.407

53.181

53.294

53.376

52.683

52.968

52.827

52.795

52.586

52.699

52.611

53.348

53.313

53.489

53.178

52.989

Na 2O

0.00

00.02

40.01

20.04

10.02

10.00

00.08

00.16

80.15

60.05

00.06

10.12

40.10

90.12

10.05

80.08

2

K2O

0.00

00.00

00.00

00.011

0.00

00.00

00.01

50.00

00.00

50.00

00.01

90.00

00.00

00.00

90.02

40.00

7

F2.49

53.51

83.00

72.51

74.37

42.97

93.57

34.28

73.12

54.40

14.70

12.47

34.45

33.17

93.66

13.64

4

Total

100.16

99.962

100.06

99.941

100.09

99.678

99.958

99.285

100.10

100.21

699

.27

99.998

99.700

100.00

899

.820

99.840

Structuralform

ulaof

apatitesbasedon

12ox

ygen

atom

s

Si

0.00

90.01

30.011

0.00

80.011

0.00

90.01

60.01

20.01

30.01

00.01

00.011

0.01

00.01

50.01

00.011

Ti

0.00

00.00

00.00

00.00

00.00

00.00

00.01

0.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

0

Fe

0.01

00.00

60.00

80.01

30.011

0.01

40.01

20.011

0.01

70.00

70.00

80.01

70.00

40.00

60.00

70.01

0

Mn

0.00

50.00

10.00

30.00

50.00

50.00

60.00

00.00

60.01

00.00

70.00

40.00

60.00

40.00

90.00

60.00

6

Mg

0.00

30.00

00.00

20.00

30.00

90.00

30.00

80.00

40.01

00.01

00.01

00.011

0.00

50.01

20.00

60.00

8

Ca

4.56

04.60

94.58

54.57

34.60

54.57

34.57

74.66

94.52

14.59

84.67

24.57

24.70

74.62

94.63

44.611

Na

0.00

00.00

40.00

20.00

60.00

30.00

00.01

30.02

70.02

40.00

80.01

00.01

90.01

70.01

90.00

90.01

3

K0.00

00.00

00.00

00.01

0.00

00.00

00.00

20.00

00.00

10.00

00.00

20.00

00.00

00.00

10.00

20.00

2

F1.31

31.80

01.52

91.27

32.25

71.51

81.82

72.23

81.58

62.26

72.46

41.25

12.32

11.62

41.88

31.87

6

P2.96

12.93

92.95

2.95

32.93

52.95

42.94

02.90

92.95

92.94

12.911

2.94

52.89

92.91

82.92

92.93

3

Total

8.86

39.37

29.09

8.84

49.83

69.07

79.40

59.87

69.14

19.84

810

.091

8.83

29.96

79.23

39.48

69.47

0

Arab J Geosci (2014) 7:693–709 703

Author's personal copy

Table

5Chemical

analysisandstructural

form

ulaof

thecarbon

ates

Area

Serpentinites

Metagabbros

Rocktype

W.Ham

mariya

G.Garf

Aver.

W.Ham

mariya

G.Garf

Aver.

1/2

205

14/1

26/7

258

264

Spo

ts1

23

45

67

12

12

12

3

SiO

20.00

40.04

60.02

70.02

10.04

00.03

20.00

00.02

30.48

20.07

50.02

80.01

20.03

40.05

00.00

50.02

6

TiO

20.01

80.05

10.02

20.04

70.02

50.00

00.00

00.00

00.00

00.01

80.00

00.00

00.00

00.00

00.00

00.00

0

Al 2O3

0.00

00.00

00.00

00.00

50.00

10.01

80.00

80.01

00.011

0.00

60.00

90.00

00.00

00.01

60.00

50.00

6

FeO

0.42

50.23

70.29

00.44

90.63

70.13

10.10

60.14

70.28

60.30

10.52

20.55

80.30

40.22

30.23

10.36

8

MnO

0.07

00.19

60.16

10.12

10.15

10.36

20.116

0.15

60.05

50.15

40.39

60.23

90.20

70.09

50.22

10.23

2

MgO

20.814

21.118

20.818

21.144

20.852

21.266

20.662

21.642

20.839

21.017

0.16

90.21

50.09

80.15

40.20

90.16

9

CaO

29.942

30.319

30.064

29.943

29.899

29.869

30.378

29.297

29.775

29.943

56.708

56.946

57.619

56.868

57.510

57.130

Na 2O

0.00

20.00

00.00

50.04

30.04

90.00

00.00

70.02

20.00

00.01

40.03

20.05

70.00

00.01

00.02

80.02

5

K2O

0.00

00.00

00.00

00.00

00.011

0.00

50.00

50.00

00.00

00.00

20.00

70.00

20.00

80.00

00.00

00.00

3

CO2

48.435

48.995

48.391

48.789

48.798

48.705

48.310

48.297

48.723

48.604

42.871

41.531

41.328

42.749

41.209

41.138

Cr 2O3

0.00

00.02

80.00

40.01

60.06

50.02

20.02

80.00

00.00

20.01

80.00

00.03

30.00

00.01

70.00

00.01

Total

99.710

100.99

99.782

100.58

100.53

100.41

99.620

99.594

100.17

100.15

100.74

99.593

99.656

100.18

99.418

99.107

Structuralform

ulaof

carbon

ates

basedon

6ox

ygen

atom

s

Si

0.00

00.00

10.00

10.00

10.00

10.00

10.00

00.00

00.01

50.00

20.00

10.00

00.00

10.00

20.00

00.00

1

Al

0.00

00.00

10.00

10.00

10.00

10.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

0

Ti

0.00

00.00

00.00

00.00

00.00

00.00

10.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

10.00

00.00

0

Fe

0.011

0.00

60.00

70.011

0.01

60.00

30.00

30.00

40.00

70.00

80.01

50.01

60.00

90.00

60.00

70.011

Mn

0.00

20.00

50.00

40.00

30.00

40.00

90.00

30.00

40.00

10.00

40.011

0.00

70.00

60.00

30.00

60.00

7

Mg

0.95

10.95

20.95

10.95

80.94

50.96

40.94

60.98

80.94

50.95

60.00

80.011

0.00

50.00

80.01

00.00

9

Ca

0.98

30.98

30.98

70.97

50.97

40.97

30.99

90.96

10.97

00.97

82.03

72.08

52.114

2.05

02.114

2.10

6

Na

0.00

00.00

00.00

00.00

30.00

30.00

00.00

00.00

10.00

00.00

10.00

20.00

40.00

00.00

10.00

20.00

2

K0.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

0

C2.02

62.02

42.02

42.02

42.02

62.02

32.02

42.02

02.02

32.02

41.96

31.93

81.93

21.96

41.93

01.93

2

Cr

0.00

00.00

10.00

00.00

00.00

20.00

10.00

00.00

00.00

00.00

00.00

00.00

0.00

00.00

00.00

00.00

0

Total

3.97

33.97

33.97

53.97

63.97

23.97

53.97

53.97

83.96

13.97

34.03

74.06

14.06

74.03

54.06

94.06

8

Ca/(Ca+Mg+

Fe)

0.50

50.50

60.50

70.50

20.50

30.50

20.51

30.49

20.50

50.50

40.98

90.98

70.99

30.99

30.99

20.99

1

704 Arab J Geosci (2014) 7:693–709

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chromite composition is definitely related to the low tempera-ture hydrothermal alteration (Mukherjee et al. 2010).

Plot of the analyzed spinel on the Cr–Fe+3–Al diagram(Evans and Frost 1975; Frost 1991; Suita and Streider 1996;Barnes and Roeder 2001) (Fig. 3d) shows that the fresh spinelof both areas are analogous to that overprinted by greenschistfacies whereas the altered rims chromites of GG serpentinitesare similar to that underwent the lower amphibolite faciesmetamorphism. A feature indicates that the studied chromitesunderwent a transitional greenschist to lower amphibolitefacies. The studied spinel exhibit boninitic affinity whereplotted on Mg# versus Cr# diagrams (Fig. 3e). The boniniticaffinity indicates a supra-subduction setting (Dick and Bullen1984; Bonatti and Michael 1989). On the Al2O3 versus TiO2

tectonic discrimination diagram of Kamenetsky et al. (2001)(Fig. 3f), the fresh (primary) chromites fall in the supra-subduction zone and arc-low Ti fields.

Ilmenites

Based on the chemical characteristics, it can distinguish theanalyzed Fe–Ti oxides from the ophiolitic metagabbros inboth areas and serpentinites in Gabal Garf into ilmenite (Fig.4). The ilmenite is characterized by high TiO2 and low FeOcontents with respect to that of magnetite. Moreover, theilmenite is characterized by similar TiO2 and low FeO con-tents as compared with rutile (Fig. 4).

MnO and FeO contents in ilmenite of WH metagabbrosare higher than ilmenite of GG serpentinites and metagab-bro. The Cr2O3 content in ilmenite in GG is higher than WHmetagabbros. Moreover, the WH serpentinites have a higherFeOt and a lower MnO contents with respect to that of themetagabbros.

Apatites

The representative analyses of apatites from the metagab-bros of both areas in Eastern Desert (Table 4) imply thatthey are identified as fluor-apatite. They contain appreciableamounts of fluorine (average 3.007 in GG and average3.642 in WH). The apatites compositions in the metagab-bros of both areas are analogous and characterized by aver-age 43.4 & 42.7 P2O5% and average 53.3 & 53 CaO%respectively, as the main bulk oxides forming minerals.The average of total cation of apatite based on one formulaunit has 9.1 (GG) and 9.4 (WH) which are consistent withthe Ca5(PO4)3F formula.

Carbonate minerals

Carbonates in serpentinites occur as an alteration product afterserpentine minerals. They are represented by dolomites. Theanalyzed dolomites from WH and GG are quite similar. In

dolomite {[Ca00.974–0.987 Mg0.945–0.958 Fe0.006–0.016 Mn0.002–0.005 (CO3)12)] in WH and [Ca00.961–0.999 Mg0.945–0.988Fe0.003–0.007 Mn0.001–0.009 (CO3)12)] in GG}, the major compo-nents MgO and CaO have a minimal range (20.81–21.18 wt%MgO and 29.90–30.31 wt% CaO in WH and 20.66–21.64wt% MgO and 29.30–30.39 wt% CaO in GG) indicative ofvery small variation in Ca/(Ca+Mg+Fe) ratios (atoms per for-mula unite, a.f.u) (0.502–0.507 in WH, 0.492–0.513 in GG).Meanwhile, the minor elements such as FeO, MnO, TiO2 andCr2O3 have a wide variation (Table 5). This variation is con-sistent with data given by El-Shibiny et al. (2005) on thedolomite of marbles from Sol Hamid mélange, SED ofEgypt. The carbonates analyzed from metagabbros are repre-sented by calcite. The analyzed calcite from both areas is alsosimilar. In calcite {[Ca02.037–2.111 Mg0.005–0.011 Fe0.009–0.015Mn0.006–0.011 (CO3)12)] in WH and [Ca02.050–2.114 Mg0.008–0.010 Fe0.006–0.007 Mn0.003–0.006 (CO3)12)] in GG}, the variationin the concentration of major components CaO is significantlysmall (56.71–57.62 wt.% CaO inWH and 56.82–57.51 in GG)as well as a very small variation in Ca/(Ca+Mg+Fe) ratios(a.f.u) (0.989–0.987 in WH, 0.992–0.993 in GG), while theminor elements particularly MnO and Cr2O3 have a widevariation in concentration (Table 5). The occurrence of calcitein ophiolitic metagabbros may be related to either dioriticgabbroic dikes injection in ophiolitic metagabbros or to hydro-thermal alteration processes. The analyzed CO2 in both types ofcarbonate do not give amatch in the analysis because it is easilyeliminated during analysis but it can be recalculated stiocho-meterically corresponding to the total.

Discussion

The details of variation in the major element chemistry ofspinel in the podiform chrome ores of the Eastern Desert ofEgypt imply early magmatic fractionation at the primarysource, which apparently caused the differentiation intoAl-spinel. The geochemical data obtained from spinel, hostrocks (Abdel-Karim, in prep.) and minerals during thisstudy will now be discussed in favor of pervasive mantlemetasomatism triggered by a subduction zone.

a. GeothermometryOne of the factors controlling the development of Fe3+-

enriched spinels is the diminishing size of the miscibilitygap between chromite core and magnetite and ferritchro-mite rim with increasing temperature (Barnes 2000).Figure 5(a) shows the spinel stability limits for chromiteand magnetite in equilibrium with Fo90 olivine (Sack andGhiorso 1991). There is a complete solid solution fromchromite to magnetite at 600 °C. A significant miscibilitygap widens rapidly below 600 °C. The investigatedchrome-spinel cores plot within the primary magmatic field

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and do not appear to be completely re-equilibrated with themetamorphic spinel rim, i.e., indicating relic magmaticcomposition not affected by metamorphism. However, themetamorphic spinel rims lie along the Cr-Fe3+ join withadditional compositional (miscibility?) gap between theferritchromite and Cr-magnetite, corresponding to meta-morphism around ~500–600 °C. Barnes (2000) suggestedthat the maximum Cr content of the rims is controlled byequilibrium with chromite cores across the miscibility gapbetween chromite and magnetite. Cr-spinel equilibratedbelow ~500–550 °C, retains original Cr-content, butMg/(Mg+Fe2+) values are substantially lowered by Fe-Mg exchange with silicates and/or carbonates. The overallchemical results of ilmenite are illustrated by tie lines inFig. 5(b) to show equilibrium of the oxides in the triangle

Fig. 3 Chemical features of thestudied spinels: a Cr-Fe+3-Aldiagram (after Stevens 1944), bAl-Cr- Fe+3 diagram (afterMussallam et al. 1998 and Araiet al. 2004), c Cr2O3 versusAl2O3 diagram for the studiedfresh chromite. Compositionalfields of podiform chromititesfrom the central (CED) andsouthern (SED) parts of theEastern Desert of Egypt (afterAhmed et al. 2001) are givenfor comparison. The podiformand stratiform fields are fromBonavia et al. (1993), d Cr-Fe+3-Al diagram. The solvuscurve (dashed line) and fields ofdifferent metamorphic facies forCr-spinel phases are fromPurvis et al. (1972), Evans andFrost (1975), Suita and Streider(1996), and Barnes and Roeder(2001). e Mg# versus Cr#diagram (after Kepezhinskas etal. 1993). f Al2O3 versus TiO2

diagram (after Kamenetsky etal. 2001)

Fig. 4 FeO (wt%) versus TiO2 (wt%) variation diagram showing thechemical features of the studied ilmenite

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TiO2–FeO–Fe2O3 under variable temperature conditions.These values were obtained qualitatively from typicalanalyses of oxide grains, as much as possible includingminor exsolution lamellae, and will be followed up bymore precise estimates based on modal analysis of oxidegrains. On this diagram, the analyzed ilmenite plots onthe ferranilmenite line formed by continuous solid solu-tion above 800 °C.

b. Grade of metamorphismThe chromites rims of Gabal Garf serpentinites have

evidence of alteration, which characterize by enrich-ment of FeO (42.05–78.63) and loss of Al2O3 (0.02–3.78) compared with those of their fresh cores (14.50–18.32) and (6.22–7.66) (Table 1). According to Evansand Frost (1975) and Suita and Streider (1996), suchchemical variation is typified chromite overprinted bygreenschist facies. The ophiolitic serpentinites under-went progressive metamorphism (Regional) causing al-teration of Al-rich chromite to intermediate Fe+3-richchromites (Ferrichromite and Cr-magnetite). This trans-ferred the peridotites from the greenschist facies to thelower amphibolite facies at T=500–550 °C.

c. Petrogenetic evolutionThe Al-spinel in most cases survives serpentinization

and metamorphism and can be used as the only reliablekey mineral of the primary mantle lithology even inaltered Precambrian ultramafic rocks (e.g., Liipo et al.1995). The Cr# of spinel of spinel peridotite is basicallycontrolled by the degree of partial melting (Dick andBullen 1984; Arai and Yurimoto 1994). The degree ofpartial melting, including second-stage melting (Duncanand Green 1980), enhances the Cr# of spinel in theperidotite restite (Dick and Bullen 1984; Arai andYurimoto 1994). The melting style of the upper mantleis possibly different from a tectonic setting to another,which gives rise to the difference of degree of melting of

peridotite depending on the tectonic setting; arc, plumeor mid-ocean ridge (Dick and Bullen 1984; Arai andYurimoto 1994). The highly depleted peridotite withhigh-Cr# (>0.7) spinel can be produced either at themantle wedge beneath arcs or at the plume relatedwithin-plate mantle (e.g., Pearce et al. 1984; Arai andYurimoto 1994; Ishiwatari et al. 2003). The former high-Cr# spinel is similar to the Cr# spinel in the studied Al-chromite (0.8–1.0 in GG and > 0.8 in WH) (Table 2)which suggests a similar conclusion.

Spinel grains from several ophiolites, such as those inAl Ays of Saudi Arabia, Troodos in Cyprus, NewCaledonia and Oman (Lorand and Cottin 1987) andsome ophiolite occurrences in the Southeastern Desertof Egypt (Saleh 2006) contain volatile and fluid inclu-sions. In such environment, H2O, K2O-rich fluids, andpossible silicate melts derived from the subducted slabcan be added to the overlying mantle wedge (Roberts1988), playing an important role in enrichment of vola-tiles. Magmas of boninitic affinity (Fig. 3e) are general-ly believed to be parental to high-Cr chromite. Fromsuch magmas, large volumes of chromite and olivinemay be fractionated (Roberts 1988). The boninitic af-finity indicates a supra-subduction setting (Dick andBullen 1984; Bonatti and Michael 1989). Boninitic af-finity of some Eastern Desert ophiolitic rocks has beenrecognized by El-Sayed et al. (1999), Abdel Aal et al.(2003) and Hamdy and Lebda (2011). These authorsinferred a back arc or an interarc basin origin based onthe chemical composition of the ophiolitic rocks. Supra-subduction ophiolites could form either in forearcs dur-ing an incipient stage of subduction initiation or in back-arc basins. High Cr# of chromite from serpentinites inthe central Eastern Desert of Egypt has been interpretedas formed in forearc basins rather than back-arc basins,which are relatively difficult to emplace (Stern et al.2004; Azer and Stern 2007; Hamdy and Lebda 2011).

Fig. 5 a Spinel compositionsplotted on their stability limits ofSack and Ghiorso (1991),calculated for equilibrium withFo90 olivine and b TiO2–FeO–Fe2O3 solid system diagramshowing the composition andapproximate equilibrium tie lines(dashed lines) between analyzedilmenite and magnetite fromophiolitic metagabbros andserpentinites in two areas inEastern Desert (modified afterBuddington and Lindsley 1964;Broska et al. 2003)

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Conclusion

The present petrographic and chemical investigations car-ried out on non-silicate minerals in the vicinity of centraland southern Eastern Desert, offered new data about theorigin and genesis of the opaques related to the ophioliticserpentinites and metagabbros. Microscopic and electronmicroprobe studies made it possible to obtain the composi-tion of the minerals, their textures and genesis as well ascondition of formation and metamorphism. This investiga-tion can be summarized as follows:

Complete replacement of chromites led to form magne-tite which is also found as cross-cut veinlets or fresh pris-matic grains. Rutile-magnetite intergrowths and martite arecommon in serpentinites. Fine trillis exsolution of ilmenite–magnetite and ilmenite–hematite and intergrowth of rutile–magnetite and ilmenite–sphene are recorded in both areas.Composite intergrowth grains of titanomagnetite–ilmenitetrellis lamellae are common in metagabbros. The formationof ilmenite trellis and lamellae in magnetite and titanomag-netite indicates an oxidation process due to excess of oxy-gen contained in titanomagnetite; trapped and externaloxidizing agents. This indicates the high PH2O and oxygenfugacity of the parental magma. The sulfides minerals in-clude pyrrhotite, pyrite and chalcopyrite.

Textural and compositional features of the Al-spinelssuggest a greenschist up to lower amphibolite facies meta-morphism (at 500–600 °C), which is isofacial with thecountry rocks. Al-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rimsand surrounding silicates, indicating relic magmatic compo-sition not affected by metamorphism.

The Fe-Ti oxides have been formed under temperature of~800 °C for ilmenite, while the sulfide assemblage is crystal-lized below 600 °C, with final re-equilibration temperatureabove 140 °C.

Based on the chemical characteristics, the analyzed Fe–Tioxides from the ophiolitic metagabbros in both areas correspondto ilmenite. SiO2 and FeOt, and MnO contents in ilmenitedecrease, while Cr2O3 increases with increasing TiO2, probablyas a function of temperature and composition. The anal-yses of apatites from the metagabbros are identified asfluor-apatite. They contain appreciable amounts of fluo-rine (average 3.0–3.6).

The chemical results of ilmenite show equilibrium of thepresent oxides under variable temperature conditions. Thesevalues plot on the ferranilmenite line formed by continuoussolid solution above 800 °C. The investigated Al-spinel coresexhibit a primary magmatic relicts that appear to be unaffectedby metamorphism which equilibrated below 500 °C.However, the metamorphic spinel rims lie along the Cr–Fe3+

join, corresponding to metamorphism around 500–600 °C.Al-spinel equilibrated below 500–550 °C, retains original

Cr-content, but Mg/(Mg+Fe2+) values are substantially low-ered by Fe–Mg exchange with silicates and/or carbonates.

Carbonates in serpentinites of both areas are quite similar,represented by dolomites, having a composition {[Ca00.974–0.987Mg0.945–0.958 Fe0.006–0.016 Mn0.002–0.005 (CO3)12)] in WH and[Ca00.961–0.999 Mg0.945–0.988 Fe0.003–0.007 Mn0.001–0.009(CO3)12)] in GG. The carbonates from metagabbros are repre-sented by calcite, {[Ca02.037–2.111 Mg0.005–0.011 Fe0.009–0.015Mn0.006–0.011 (CO3)12)] in WH and [Ca02.050–2.114 Mg0.008–0.010 Fe0.006–0.007 Mn0.003–0.006 (CO3)12)] in GG}.

Mineral chemistry of Al-spinels suggest that the hostophiolitic serpentinites and metagabbros exhibit a boniniticaffinity magma belonging to a supra-subduction settingcould form either in forearcs during an incipient stage ofsubduction initiation or in back-arc basins.

Acknowledgments The authors acknowledge Professor Dr. M AbuAnbar, Faculty of Sciences, Tanta University and Associate Professor M.Azer, National Research Centre, Egypt for valuable comments, ProfessorDr. Abdullah M. Al-Amri (Editor-in-Chief) for handling the manuscript.

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