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Lithos 104 (200

Magmatic unmixing in spinel from late Precambrian concentrically-zonedmafic–ultramafic intrusions, Eastern Desert, Egypt

Ahmed Hassan Ahmed a, Hassan Mohamed Helmy b,⁎, Shoji Arai c, Masako Yoshikawa b

a Geology Department, Faculty of Science, Helwan University, Cairo, Egyptb Institute of Geothermal Sciences, Kyoto University, Japan

c Department of Earth Sciences, Kanazawa University, 920-1192, Japan

Received 21 March 2007; accepted 27 November 2007Available online 14 December 2007

Abstract

Spinel is widespread in the ultramafic core rocks of zoned late Precambrian mafic–ultramafic complexes from the Eastern Desert of Egypt.These complexes; Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues of Alaskan-type complexes, they are notmetamorphosed although weakly altered. Each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin.Silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma. Relatively high Cr2O3 contents are recordedin pyroxenes (up to 1.1 wt.%) and amphiboles (up to 1.4 wt.%) from the three plutons. The chrome spinel crystallized at different stages of meltevolution; as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles and late-magmatic intercumulus phase. Theintercumulus chrome spinel is homogenous with narrow-range of chemical composition, mainly Fe3+-rich spinel. Spinel inclusions inclinopyroxene and amphibole reveal a wide range of Al (27–44 wt.% Al2O3) and Mg (6–13 wt.% MgO) contents and are commonly zoned. Thedifferent chemistries of those spinels reflect various stages of melt evolution and re-equilibration with the host minerals. The early cumuluschrome spinel reveals a complex unmixing structures and compositions. Three types of unmixed spinels are recognized; crystallographicallyoriented, irregular and complete separation. Unmixing products are Al-rich (Type I) and Fe3+-rich (Type II) spinels with an extensive solidsolution between the two end members. The compositions of the unmixed spinels define a miscibility gap with respect to Cr–Al–Fe3+, extendingfrom the Fe3+–Al join towards the Cr corner. Spinel unmixing occurs in response to cooling and the increase in oxidation state. The chemistry andgrain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products. Causes of spinel unmixingduring late-magmatic stage are analogous to those in metamorphosed complexes. The chemistry of the unmixed spinels is completely differentfrom the initial spinel composition and is not useful in petrogenetic interpretations. Spinels from oxidized magmas are likely to re-equilibrateduring cooling and are not good tools for genetic considerations.© 2007 Elsevier B.V. All rights reserved.

Keywords: Unmixed spinel; Late-magmatic; Re-equilibration; Alaskan-type; Mafic–ultramafic; Egypt

1. Introduction

Spinels are important accessory minerals in all mafic–ultra-mafic magmas of various tectonic settings (e.g., Irvine, 1965,1967; Hamlyn and Keays, 1979; Dick and Bullen, 1984; Barnesand Roeder, 2001, and many others). They are the main repo-sitory of Cr2O3 in mafic–ultramafic rocks, and host other ele-

⁎ Corresponding author.E-mail address: hmhelmy@yahoo.com (H.M. Helmy).

0024-4937/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2007.11.009

ments as major constituents like Mg, Al, and Fe. Based ongeochemical basis, spinels are classified to three series (Deeret al., 1966) according to the dominant trivalent ion; Spinelseries (Al+3-dominant), Magnetite series (Fe+3-rich) andChromite series (Cr+3-rich). Significant solid-solution composi-tional variations occur naturally within and between the threespinel series. The modification of spinel chemistry is commonlyaccompanied by textural diversity, like exsolutions and alter-ation. The various geological and physicochemical parameterswhich may influence the texture and chemistry of spinel have

86 A.H. Ahmed et al. / Lithos 104 (2008) 85–98

been dealt with in many publications (e.g., Barnes, 2000 andreferences there in). Modification of primary spinel composi-tions in cumulate rocks is possible through three mechanisms; 1)the reaction of primary spinel with the residual melt (e.g.,Henderson, 1975; Henderson andWood, 1981), 2) exsolution ofinitial spinel in response to cooling (Sack and Ghiorso, 1991),and 3) subsolidus mineral–mineral reactions (Loferski and Lipin,1983; Candia and Gaspar, 1997; Barnes, 2000). The spinelchemistry thus depends on the chemical evolution of the magmaat the post-cumulus stage and the post-magmatic processes; likehydrous alteration and metamorphism. Systematic changes inspinels chemistry with progressive alteration (Burkhard, 1993)and metamorphism (Evans and Frost, 1975) are proved.

Unmixed spinel phases of different chemical compositionshave been widely noted. Unmixed spinels were described frommany metamorphosed mafic–ultramafic complexes (Muir andNaldrett, 1973; Loferski and Lipin, 1983; Eales et al., 1988;Zakrezewski, 1989; Van der Veen and Maaskant, 1995; Barnesand Zhong-Li Tang, 1999), one Alpine-type peridotite (Tamuraand Arai, 2005) and Alaskan-type complexes (Garuti et al.,2003; Krause et al., 2007). Unmixing patterns in spinel varyfrom simple to complex intergrowths and from single to mul-tiple stages of exsolutions (e.g., Haggerty, 1991a). The simplefeature is large Fe-rich blebs at the center of Al-rich spinelgrains or granules at their outer margins (e.g. Fig. 2 of Loferskiand Lipin, 1983). Complex unmixing comprises the presence oftwo or three types of Fe-rich exsolutions in a matrix of Al-richspinel (e.g., Fig. 4 of Eales et al., 1988). In this case, the Fe-richexsolutions form either very fine irregular vermicular bodies or

Fig. 1. Location map of the zoned mafic–ultramafic complexes in the Eastern De

crystallographically oriented lamellae at the center of the initialspinel grains (e.g. Fig. 4a–d of Tamura and Arai, 2005). In fewcases, the Al-rich spinel is found as small exsolutions in a Fe-rich matrix (e.g., Fig. 3 of Loferski and Lipin, 1983). Whenpresent, the Al-rich lamellae are commonly crystalographicallyoriented (e.g., Fig. 5 of Eales et al., 1988; Fig. 3 of Loferski andLipin, 1983). Loferski and Lipin (1983) noted compositionaldifferences between exsolution types. Commonly, the composi-tions of the different types of exsolutions define a miscibilitygab with respect to Cr–Al–Fe3+, extending from the Fe3+–Aljoin towards the Cr corner (Loferski and Lipin, 1983, Ealeset al., 1988; Tamura and Arai, 2005). In many cases, spinelexsolution is attributed to metamorphic processes. The uppertemperature limit of spinel unmixing was postulated from thepeak metamorphic temperatures at about 600 °C (e.g. Loferskiand Lipin, 1983).

In this contribution we describe intercumulus and cumulusunmixed spinel from three unmetamorphosed zoned (Alaskan-type) mafic–ultramafic complexes from the Eastern Desert ofEgypt (Fig. 1). The general characteristics of these complexescommon with Alaskan-type complexes are discussed elsewhere(Helmy and El Mahallawi, 2003; Farahat and Helmy, 2006).Spinels from these complexes show complex exsolution pat-terns and are interesting in studying such exsolution in unme-tamorphosed intrusions. In this work we discuss the relation ofspinel chemistry to the evolution of magma in each complexand the possible reasons and mechanisms of spinel unmixing.The complex unmixing is attributed to late-magmatic changesin temperature and oxidation state, analogous to metamorphic

sert of Egypt; transverse tectonic fractures from Garson and Shalaby (1976).

87A.H. Ahmed et al. / Lithos 104 (2008) 85–98

changes. It is shown that late- and post-magmatic unmixing ofinitial spinel produce spinels end members of completely dif-ferent chemistries than the initial spinel and, therefore givemisleading petrogenetic meanings. Throughout this manuscript“spinel” refers to a spinel-group mineral generally expressed as(Mg, Fe2+) (Cr, Al, Fe3+)2 O4 (e.g., Haggerty, 1991b).

2. Geologic background and locations

The basement complex of Egypt forms the western part ofthe Arabian–Nubian shield which is considered a juvenile ter-rane formed in convergent plate boundaries through the for-mation of intra-oceanic island arc system, subsequent oceanclosure, amalgamation of the arc complexes and accretion tocontinental crust (e.g. Gass, 1982; Stern, 2002). These pro-cesses took place during the Pan-African orogenic event (800–600 Ma, Hassan and Hashad, 1990).

Ultramafic–mafic rocks comprise about 5% of all Precam-brian outcrops in the Eastern Desert of Egypt (Dixon, 1979).They are of two types; ophiolitic and intrusive complexes. Theophiolitic (dismembered) ultramafic rocks represent remnantsof the oceanic crust that coexisted with the intra-oceanic islandarcs. The ophiolitic ultramafic rocks recur along major thrustzones (Shackleton, 1994) and are severely serpentinized andcontain chromitite lenses. The intrusive ultramafic–mafic com-plexes form small, elliptical outcrops located along major frac-ture zones trending ENE (Garson and Shalaby, 1976) and arecommonly concentrically zoned (Helmy and El Mahallawi,2003). The concentrically zoned complexes are not metamor-phosed and sometimes contain Cu–Ni–PGE mineralization(e.g., Helmy and Mogessie, 2001; Helmy, 2004). Recently,Yoshikawa et al. (unpublished data) estimated Sm/Nd and Rb/Sr ages of 673±10 Ma of two of these complexes; GabbroAkarem and Genina Gharbia. These zoned complexes areconsidered Precambrian analogues of Alaskan-type complexesand represent the roots of island arcs (Helmy and El Mahallawi,2003; Farahat and Helmy, 2006; Helmy et al., 2007).

Three of these complexes; Gabbro Akarem, Genina Gharbiaand Abu Hamamid are studied for their spinel mineralogy andchemistry. These complexes are located in the south EasternDesert (Fig. 1) in an area dominated by island arc volcano-sedimentary rocks, calc-alkaline arc granites and back-arc basinophiolites (Stern and Hedge, 1985). Previous petrologicalstudies (Helmy and El Mahallawi, 2003; Farahat and Helmy,

Fig. 2. Geologic map of Gabbro Ak

2006) suggest uncontaminated arc parental magmas of thesecomplexes.

3. Field relations and rock petrography

Gabbro Akarem (GA) intrusion is located about 130 km eastof Aswan. The intrusion is 8 km by 1.5 km in plan, formed ofplagioclase-hornblendite (at the margins), olivine-plagioclasehornblendite and amphibole-bearing peridotite (at the core), indecreasing order of abundance. The peridotite bodies areoriented and elongated in an ENE direction, following theregional trend of the intrusion (Fig. 2). The contact between thedifferent rock units is gradational. Petrography of the GAintrusion is described in detail by Helmy and El Mahallawi(2003), and only a brief summary is given here.

The GA rock units show primary magmatic silicate miner-alogy with a faint degree of secondary alteration. Core peri-dotites are characterized by fresh to slightly altered olivine (50–70% modal) up to 3 mm in size, about 12% modal ortho- andclinopyroxenes and intercumulus amphibole (up to 25%modal),about 2% spinel, and 7% sulfides (pyrrhotiteNpentlandite≥chalcopyrite). Amphibole coronas between olivine and plagio-clase are very common. The modal abundance of spinel de-creases from the core (4%) to the rim (b1%) of the intrusion.

The Genina Gharbia (GG) mafic–ultramafic intrusion islocated about 40 km southeast of GA (Fig. 1). It covers an areaof 9 km long and 3.5 km wide and comprises gabbros (in themargin), pyroxenites, amphibole pyroxenites, and amphibole-bearing peridotite (in the core). This association intruded Pre-cambrian metasedimentary and island arc volcanic rocks (ElMahallawi, 1996). The intrusion is non-metamorphosed buthighly affected by faulting and shearing where most of theoriginal magmatic contacts have been obliterated. The differentrocks are characterized by high modal content of intercumulusand corona (Helmy et al., in press) amphibole and abundantbiotite and apatite indicating hydrous nature of the parentmagma (Helmy, 2004). Massive and disseminated Cu–Nisulfide ores are found in amphibole gabbro and peridotite.

The Abu Hamamid (AH) mafic–ultramafic complex is lo-cated about 100 km to the west of the Red Sea intrudingvolcanic rocks (Shadli Volcanics: 710 Ma, Stern et al., 1991).The AH complex is an elliptical body of 1.5 km long and 500 min the maximum width. The geologic contacts with the sur-rounding volcanic rocks are hidden below thick valley sediments.

arem complex (Carter, 1975).

88 A.H. Ahmed et al. / Lithos 104 (2008) 85–98

This intrusion comprises a peridotite core enveloped by wehrlite,clinopyroxenite and gabbro outward (Farahat and Helmy, 2006).The peridotite core bodies occur as small rounded outcropsaligned in an ENE direction parallel to the direction of elongationof the intrusion. Peridotites composed of olivine (40–60%),amphibole (up to 20%) clinopyroxene (up to 15%) and spinel (2–3%). The AH rock units show a higher degree of alteration,specially the core rocks relative to the other complexes. As thecomplex is intruding non-metamorphosed island arc volcanics,this extensive hydrous alteration is related to late-magmatic fluids(Farahat and Helmy, 2006).

3.1. Analytical techniques

Silicate and oxide mineral chemistry were carried out at theCentre for Co-operative Research of Kanazawa University,Japan, using a JEOL JXA-8800 electron probe micro analyzer.The analytical conditions were a 25-kV accelerating voltage,20 nA probe current and 3-µm probe diameter. To avoidcontamination, relatively large areas were selected to analyzeunmixed spinels. The raw data were corrected using an on-lineZAF program. The ferric and ferrous ions were calculatedassuming spinel stoichiometry. Standards used for oxides andsilicate minerals are natural minerals; quartz for Si, eskolite forCr, fayalite for Fe, wollastonite for Ca, jadeite for Na, corundumfor Al, periclase for Mg, manganosite for Mn, and nickel oxidefor Ni. X-ray maps were made at the Institute of Earth Science,

Table 1Representative electron microprobe analyses of silicate minerals from Gabbro Akar

Oxide wt.% Gabbro Akarem (GA) Genina Gharbia

Ol Opx Cpx Amph Ol Op

Sample no. GA155 GA151 GA159 GA143 GG239 GG

SiO2 40.53 55.17 52.72 43.61 39.10 55TiO2 b0.05 0.18 0.17 1.62 b0.05 b0Al2O3 b0.05 3.07 3.03 13.32 b0.05 1Cr2O3 b0.05 0.41 0.44 1.22 b0.05 0FeO 13.62 9.63 5.83 7.33 16.53 10MnO 0.25 0.19 0.16 0.13 0.23 0MgO 45.41 30.18 16.22 16.33 44.02 30CaO 0.01 1.34 20.47 11.38 0.03 0Na2O b0.05 b0.05 0.39 2.06 b0.05 b0K2O b0.05 b0.05 b0.05 0.36 b0.05 b0NiO 0.08 b0.05 b0.05 b0.05 0.19 0Total 99.94 100.23 99.45 97.40 100.11 99O 4 3 3 23 4 3Si 1.011 0.967 0.970 6.283 0.990 0Ti 0.000 0.002 0.002 0.176 0.000 0Al 0.000 0.063 0.066 2.261 0.000 0Cr 0.000 0.006 0.006 0.139 0.000 0FeO 0.284 0.141 0.090 0.882 0.350 0Mn 0.005 0.003 0.002 0.015 0.005 0Mg 1.687 0.788 0.445 3.504 1.661 0Ca 0.000 0.025 0.403 1.755 0.001 0Na 0.000 0.001 0.014 0.576 0.000 0K 0.001 0.001 0.000 0.067 0.001 0Ni 0.001 0.000 0.000 0.000 0.003 0Mg# 86 85 83 80 83 84aOl olivine, Opx orthopyroxene, Cpx clinopyroxene, Amph amphibole, Mg# = 100

Karl-Franzens University, Graz, Austria using OXFORD-energy dispersive detector (EDX), model 6687. In Speedmapshigh resolution (512×400 pixels) is selected, after setting theenergy on a characteristic line (Cr K alpha, Al K alpha and Fe Kalpha) three frames are integrated to accumulate the numberof X-ray quants. The image holds for each pixel the x andy-coordinates and z (the brightness = number of acquired X-rayquants). The integration time was set as 1 min per frame and theexposure time as 4 h. In each X-ray map, the higher the con-centration of an element the brighter the pixel is.

3.2. Silicate mineral chemistry

Silicatemineral chemistry of ultramafic–mafic rocks fromGA,GG and AH was discussed in detail by Helmy and El Mahallawi(2003), Helmy (2004), and Farahat and Helmy (2006),respectively. Here, a brief summary of silicate mineralogy isprovided. Selected microprobe analyses of silicate minerals arelisted in Table 1.

Olivine, orthopyroxene, clinopyroxene, amphibole and pla-gioclase are the main primary silicates coexisting with spinel inthe studied plutons. All the mafic minerals show a similar trendof variation in Mg number (Mg#) in the three plutons, the Mg#(100 Mg/Mg+Fe2+) decrease from the core to the margins.Olivine at GA is more magnesian (Mg# 87-69) than at GG (Mg#85-73) and AH (Mg# 81-74). In all localities, olivine has Cr2O3

contents below the EMP detection limit (ca 0.05 wt.%).

em, Genina Gharbia and Abu Hamamid mafic–ultramafic intrusionsa

(GG) Abu Hamamid (AH)

x Cpx Amph Ol Opx Cpx Amph

234 GG223 GG250 AH23 AH27 AH27 AH17

.33 52.67 43.73 39.35 55.34 53.39 44.68

.05 0.55 0.83 b0.05 0.21 0.75 0.93

.62 3.21 12.57 0.02 0.65 2.45 11.81

.19 0.68 1.56 b0.05 b0.05 0.52 0.33

.53 3.70 6.92 18.15 12.55 5.32 8.45

.20 0.08 0.19 0.25 0.47 0.06 0.10

.93 16.89 17.02 41.71 29.91 16.32 17.69

.57 20.11 11.36 0.09 0.51 20.75 11.49

.05 0.67 2.41 b0.05 b0.05 0.33 2.35

.05 b0.05 0.46 0.02 b0.05 0.12 0.50

.06 b0.05 0.05 0.21 b0.05 b0.05 0.07

.48 98.60 97.10 99.84 99.66 100.02 98.403 23 4 3 3 23

.979 0.968 6.330 1.005 0.987 0.975 6.404

.000 0.008 0.090 0.000 0.003 0.010 0.100

.034 0.069 2.144 0.001 0.014 0.053 1.995

.003 0.010 0.178 0.000 0.000 0.008 0.037

.156 0.057 0.837 0.388 0.187 0.081 1.013

.003 0.001 0.023 0.005 0.007 0.001 0.012

.815 0.462 3.670 1.588 0.795 0.444 3.777

.011 0.396 1.761 0.002 0.010 0.406 1.764

.001 0.024 0.675 0.000 0.000 0.012 0.653

.000 0.001 0.084 0.001 0.000 0.003 0.091

.001 0.000 0.005 0.004 0.000 0.000 0.00789 81 80 81 85 79

Mg/(Mg+Fe2+).

89A.H. Ahmed et al. / Lithos 104 (2008) 85–98

Clinopyroxenes from the three plutons are classified asdiopsides. The Al2O3 content of clinopyroxene varies from 3.0to 4.0wt.% inGA, from2.6 to 3.2wt.% inGGand about 3.0wt.%in AH intrusions (Table 1). Clinopyroxene from the core rocks ofGA is more magnesian (Mg# 90) than that in GG (Mg# 87) andAH (Mg# 85). Cr2O3 contents in clinopyroxenes from the corerocks of the three plutons are relatively high; 1.1, 0.7, 0.8 wt.%, atGA, GG and AH, respectively.

Orthopyroxene, when exists, in the ultramafic cores of thethree intrusions is of enstatite composition. It has high Al2O3

content (3.1, 1.9 and 0.7 wt.%, in GA, GG and AH intrusions,respectively). Cr2O3 contents of orthopyroxenes from the corerocks of GA and GG are similar (0.6 wt.%), the Cr content ofAH orthopyroxenes is below the EMP detection limit.

Amphiboles of the ultramafic cores have constantly high Mg#(88, 86 and 86, at GA, GG and AH, respectively) and Cr2O3

contents (up to 1.4, 1.1, 0.4wt.%, at GA,GG andAH, respectively).

4. Spinel petrography and chemistry

4.1. Spinel petrography

More than 65 polished thin sectionswere studied from the threecomplexes; all the rock units in each pluton were represented.Although accessory grains of spinel with different optical pro-perties are common, no massive chromitite was found so far.

In general, spinels are muchmore abundant in ultramafic corerocks forming less than 2% modal. The following descriptions

Fig. 3. Photomicrograph (A) and back-scattered electron images (BSE) (B–D) of sorthopyroxene (Opx), sample GG234, Genina Gharbia intrusion. B Small spinel clu(Ol), sample GA153, Gabbro Akarem intrusion. C Large intercumulus spinel associaAkarem intrusion. D Small contiguous Mg–Al-spinel (Type I) and Fe-spinel (Typeolivine, sample GA146, Gabbro Akarem intrusion.

are for spinel fromperidotites. Spinelwas observed as inclusions inolivine, pyroxenes and amphiboles and as intercumulus phase. GAand GG spinels are more abundant than AH spinels, the latercommonly form small grains (b50 microns) in altered silicates.

Two types of spinel are optically observed; homogenous andunmixed “exsolved” spinel. The homogenous spinel occurs inthree textures:

1) Individual grains enclosed in pyroxenes, amphibole andsulfides. The individual spinel grains range in size from0.2 mm down to b50microns across and occur as clusteredoctahedra or small rounded crystals enclosed withinpyroxenes (Fig. 3A), amphibole (Fig. 3B) and sulfides.Optically, this spinel is homogenous although electronmicroprobe step scans across some relatively large grainsrevealed weak compositional zoning (Fig. 6).

2) Small (b50 microns) grains hosted in chlorite (andserpentine). All small spinel grains in AH complex are ofthis type. Optically and chemically, all grains are homo-genous, no sub-microscopic pores were observed. Somegrains show alteration along cracks and margins intohematite. The optical and chemical homogeneity of thisspinel and absence of any sub-microscopic pores suggestthat it is the result of re-crystallization.

3) Intercumulus large spinel (Fig. 3C). The intercumulus spineloccurs only in sulfide-bearing samples at GG and GA but notat AH, where it fills interstices between olivine and pyroxenegrains.

pinels and associated silicates. A Euhedral spinel cubes (Spl) included withinsters (white grains) in large amphibole (Amph) associated with cumulus olivineted with cumulus olivine and magmatic sulfides (Sul), sample GA155, GabbroII) associated with a sulfide grain, all are embedded in slightly serpentinized

Fig. 4. BSE images of unmixed spinel types. A “Cloth texture” of Type II (light gray) in Type I (dark gray) spinel (large grain) and coarse-grained irregular unmixing of Type Iand Type II spinels (small spinel grains). The white stars are the 50 µm analysed areas, sample GA161, Gabbro Akarem intrusion. B Crystallographically oriented Type IIexsolutions in Type I spinel, note the enlargement of exsolutions towards the margin and the development of irregular unmixing along crack and margins, sample GA155,Gabbro Akarem intrusion. C Irregular unmixing of Type I and Type II spinels, sample GG264, Genina Gharbia intrusion. D Close-up of the white rectangle area in (C).

90 A.H. Ahmed et al. / Lithos 104 (2008) 85–98

The chemistry of homogenous spinel in each area variesfrom one sample to another and within the same thin section,according to the host mineral. Spinel inclusions in clino-

Fig. 5. BSE image and element (Al, Fe, Cr) distribution maps of an unmixed spi

pyroxene and amphibole are Al and Mg-rich, while thosehosted in chlorite and intercumulus spinel are mainly Fe3+-rich.

nel from Genina Gharbia intrusion, sample GG250, see text for explanation.

91A.H. Ahmed et al. / Lithos 104 (2008) 85–98

The majority of spinel grains hosted by olivine and ortho-pyroxene at GA and GG are unmixed “exsolved” despite thedegree of alteration of the rock. Variable degrees of unmixingare observed between two spinel types; dark gray and light gray(Figs. 3D, 4A). Figs. 4 and 5 illustrate the unmixed spineltextures; the two end members; dark gray (Al-rich) and lightgray (Fe-rich) are referred to as Type I and II, respectively. TypeI spinel was not found as individual grains, but always in asso-ciation with Type II spinel which may suggest that both are the

Table 2Summary of the chemical composition of spinels from Gabbro Akarem, Genina Gh

Spinel type Homogeneous spinel Un

Host mineral Amph Cpx Chl + Srp Intercumulus Cr

Ty

Gabbro Akarem n=6 n=19 n=8 n=TiO2 Range 0.1–0.2 0.1–0.5 0.2–0.6 0.1

Average 0.2 0.4 0.1Al2O3 Range 40.9–44.0 25.1–39.6 _ 1.9–5.3 31

Average 42.7 31.7 2.8 33Cr2O3 Range 16.3–18.0 18.9–25.8 10.5–16.0 20

Average 17.3 22.1 12.2 21MgO Range 12.0–21.4 5.7–11.6 _ 1.1–1.7 7.9

Average 12.3 8.6 1.3 8.8Mg# Range 53–55 30–50 6–10 37

Average 54 40 7 40Cr# Range 20–22 24–39 67–80 27

Average 21 32 75 30Fe3+# Range 4–9 11–18 65–78 11

Average 5 13 75 13

Genina Gharbia n=7 n=10 n=6 n=TiO2 Range 0.1–0.12 b0.1 0.8–2.4 0.4

Average 0.1 b0.1 1.5 0.7Al2O3 Range 32.3–44.2 36.2–48.7 0.3–19.2 20

Average 36.6 40.9 5.8 27Cr2O3 Range 14.4–22.3 17.8–25.4 17.3–27.3 21

Average 19.4 20.4 20.6 24MgO Range 8.6–12.6 8.6–13.3 0.8–5.1 6–

Average 10.3 10.7 1.8 7Mg# Range 39–53 38–55 1–26 32

Average 45 46 8 34Cr# Range 19–31 15–32 62–90 30

Average 26 25 85 37Fe3+# Range 8–16 5–9 18–90 14

Average 13 6 55 18

Abu Hamamid n=4 n=6 n=32TiO2 Range 0.2–0.6 1.0–5.2 0.3–6.7

Average 0.4 2.7 2.7Al2O3 Range 22.8–31.7 7.0–9.7 1.9–13.1

Average 26.5 8.6 6.6Cr2O3 Range 17.2–21.5 17.9–22.6 5.3–24.6

Average 19.1 20.2 14.0MgO Range 3.1–6.2 0.8–3.2 0.1–3.4

Average 4.9 1.9 0.9Mg# Range 17–31 5–19 0–19

Average 25 11 5Cr# Range 27–39 58–63 38–84

Average 34 61 62Fe3+# Range 15–26 45–52 38–89

Average 21 48 60

Mg#=100Mg/(Mg+Fe2+), Cr#=100Cr/(Cr+Al), Fe3+#=100Fe3+/(Fe3++Al+Cr).

result of exsolution from initially homogenous spinel of inter-mediate composition. This feature distinguishes Type I spinelfrom the homogenous Al–Mg-rich spinel hosted individually inclinopyroxene and amphibole. According to the degree andpattern of unmixing, three sub-types are recognized:

1) Crystallographically oriented exsolutions in the innerparts of the large initial spinel, Type II spinel forms eitherwell-developed crystals or a network of exsolutions

arbia and Abu Hamamid

mixed spinel

yst. oriented Irregular separation Complete separation

pe I Type II Type I Type II Type I Type II

8 n=8 n=7 n=9 n=8 n=7–0.13 0.3–1.0 b0.1 0.3–0.7 0.1–0.2 0.1–0.7

0.5 b0.1 0.5 0.1 0.3.2–36.9 1.1–3.6 31.9–42.7 2.6–10.4 31.2–34.1 0.4–2.8.9 2.6 39.1 4.2 33.1 1.7.4–22.5 12.5–17.9 17.1–25.6 11.0–17.1 21.2–22.4 4.6–14.3.3 14.6 20.3 13.3 21.6 9.5–9.9 1.0–1.4 8.9–12.4 1.2–3.9 7.9–9.1 0.6–1.2

1.2 11.1 1.7 8.6 0.9–44 5–8 40–54 7–20 36–43 4–8

7 50 19 40 5–32 73–89 21–35 48–75 30–32 76–88

80 26 70 31 80–16 67–77 4–12 60–76 10–16 70–92

72 7 70 12 80

4 n=4 n=12 n=7 n=8 n=10–1.2 1.8–3.6 0.1–0.5 0.9–1.5 0.1–0.4 0.3–3

2.4 0.3 1.3 0.25 1.5–33 3–6 23–32 8–13 24–34 1.6–7.7.5 4.5 27.5 10.2 28.3 4.5–29 14–20 22–27 23–31 21–26 16.1–25.6.3 17.3 24.3 28.5 23.5 21.28 1.2–1.8 5–8 2–8 5–8 1.1–3.0

1.5 6.2 4 6.5 2.1–38 7–11 25–36 14–43 26–37 2–18

9 28 22 30 10–49 69–78 31–45 57–69 29–42 64–87

73 38 65 36 74–21 52–72 14–22 34–46 13–21 44–72

62 19 39 19 56

n=8 n=8 n=8 n=90.9–1.7 1.1–4.9 0.1–0.6 0.6–3.81.2 1.9 0.2 2.211.7–17.7 7.9–13.1 10.8–21.9 2.8–6.915.9 9.3 15.9 5.523.9–30.5 15.6–24.6 16.4–30.5 16.6–22.928.6 20.8 23.6 18.93.4–6.0 0.5–2.6 0.6–7.5 0.1–1.35.4 1.3 3.75 0.518–31 3–19 4–38 1–828 7 20 350–59 51–64 42–56 64–8055 60 50 7924–40 36–51 24–51 40–6528 45 32 55

92 A.H. Ahmed et al. / Lithos 104 (2008) 85–98

within the (110) planes of the host forming a “clothtexture” (Fig. 4A). Some of the exsolutions continue tothe outer margins but get larger (Fig. 4B). Initial spinelcrystals showing this type are usually euhedral and large(N150 microns in diameter). The uniform distribution ofthe exsolutions in the parental spinel may suggest aninitial homogenous chemical composition of this spinel.This type was not observed in AH spinels.

2) Irregularly exsolved blebs of Type I spinel leaving areasof homogenous Type II. This sub-type occurs in the outermargin or along cracks in the initial large and small spinelgrains (Fig. 4A–D). Type II exsolutions may form sym-plectic or myrmekitic texture of very small irregular blebsin Type I (Fig. 4C). All spinel grains with crystal-lographically oriented exsolution center, have an irregularunmixing margin (Figs. 4A, B and 5).

3) Complete separation of Type I and II spinels with well-defined boundaries in between. Initial spinel grainsshowing this type are commonly hosted by olivine.More than 90% of the grains showing completeseparation are b100 microns in diameter. In this texture,the two spinels are contiguous to one another in smallclusters (see Fig. 3D).

The three sub-types of unmixed spinels could be observed inone polished thin section, and even within the same grain.

Fig. 5 is a back-scattered electron image and element dis-tribution of an unmixed spinel grain hosted in olivine fromGenina Gharbia. Similar distribution and variation in exsolutionsizes is observed in many unmixed grains from GG and GA.

Table 3Representative electron microprobe analyses of Gabbro Akarem spinels

Spinel type Unmixed

Cryst. Oriented Irregular sep.

Sample no. GA155 GA144

Oxide wt.%TiO2 0.13 0.66 0.10 0.58Al2O3 33.68 2.77 31.91 2.67Cr2O3 21.06 17.89 25.58 11.68FeOa 23.68 29.36 23.16 29.38Fe2O3

a 12.01 47.38 9.97 53.52MnO 0.69 0.67 0.83 0.58MgO 8.66 1.31 8.86 1.23NiO b0.05 b0.05 b0.05 0.07Total 99.90 100.02 100.40 99.71

Structural formula based on 4 oxygensTi 0.003 0.022 0.002 0.020Al 1.254 0.144 1.182 0.146Cr 0.526 0.625 0.635 0.430Fe2+ 0.625 1.089 0.613 1.124Fe3+ 0.285 1.581 0.237 1.843Mn 0.018 0.025 0.022 0.023Mg 0.407 0.086 0.415 0.085Ni 0.000 0.000 0.000 0.002Fe3+# 14 67 11 76Mg# 40 7 40 7Cr# 30 81 35 75aCalculated.

Elemental distribution maps show many interesting features:1) The crystallographically oriented growth of Type II spinellamellae created a halo of Al enrichment in the host spinel im-mediately surrounding lamellae, 2) The groundmass of the zonewith small crystallographically oriented Type II exsolutions isrich in Al (and slightly in Cr), and 3) the zone with no Type IIexsolutions, has a uniform composition but with relatively lowerAl (and Cr) contents indicated by the light-gray color in the back-scattered electron image.

4.2. Spinel chemistry

Extensive electron microprobe analyses were performed onspinels from different textures. The following compositionaldata are for spinel from peridotites. The initial chemical com-position of the unmixed spinel was estimated by calculating theaverage of area analyses made on fine-grained unmixed spinelsof various stages of unmixing. For irregularly and completelyunmixed spinels, average compositions were calculated fromthe EMP analyses of Type I and II spinels. The later approach isnot completely justified as the modal abundance of both spineltypes vary from one grain to another. However, the differencebetween the estimated initial compositions using both ap-proaches is small (+/−3%). The estimated initial spinel com-positions of the unmixed spinels of GA, GG and AH are plottedtogether with EMP point analyses on Figs. 7 and 8. A summaryof the compositional variations of various spinel types andrepresentative EMP analyses are listed in Tables 2–5. Spinels inthe three studied localities display considerable compositionalvariations from high-Mg–Al to high-Fe varieties. The important

Homogeneous

Complete sep. In Cpx In Amph Intercumulus

GA146 GA155 GA148 GA141

0.09 0.26 0.08 0.05 0.4633.18 1.09 29.75 40.93 3.1821.32 6.25 22.67 17.54 12.9923.55 29.91 25.13 18.54 29.5411.10 61.32 15.14 8.61 51.780.38 0.30 0.67 0.40 0.608.63 0.80 7.35 12.74 1.230.06 b0.05 0.07 b0.05 0.0698.30 99.92 100.86 98.82 99.84

0.002 0.010 0.002 0.001 0.0161.257 0.063 1.135 1.441 0.1720.542 0.241 0.580 0.414 0.4700.617 1.212 0.690 0.456 1.1210.262 2.236 0.374 0.190 1.7680.010 0.012 0.018 0.010 0.0230.413 0.058 0.354 0.567 0.0840.001 0.000 0.002 0.000 0.00213 88 18 9 7340 5 34 55 730 79 34 22 73

Table 4Representative electron microprobe analyses of Genina Gharbia spinels

Spinel type Unmixed spinel Homogenous spinel

Cryst. Oriented Irregular sep. Complete sep. In Opx In Amph Intercumulus

Sample no. GG250 GG250 GG200 GG200 GG233 GG233 GG244 GG238 GG250

Oxide wt.%TiO2 0.63 2.05 0.16 1.20 0.21 1.24 b0.05 0.02 0.62Al2O3 29.11 3.81 26.65 8.36 27.75 4.81 43.19 39.99 5.26Cr2O3 24.39 15.22 24.21 23.99 23.28 18.89 19.50 18.22 15.98FeOa 25.76 28.72 26.01 28.40 26.27 29.35 21.06 20.81 29.13Fe2O3

a 12.50 46.93 16.14 33.57 15.15 42.64 4.67 9.92 46.47MnO 0.37 0.21 0.38 0.43 0.26 0.32 0.26 0.30 0.65MgO 6.74 1.48 6.41 2.44 6.32 1.40 11.57 11.39 1.73NiO b0.05 b0.05 0.12 0.21 b0.05 0.24 b0.05 b0.05 0.05Total 99.51 98.42 100.08 98.59 99.25 98.89 100.25 100.64 99.88

Structural formula based on 4 oxygensTi 0.015 0.068 0.004 0.037 0.005 0.041 0.000 0.000 0.020Al 1.112 0.199 1.047 0.401 1.087 0.247 1.475 1.403 0.270Cr 0.625 0.533 0.638 0.772 0.611 0.651 0.446 0.428 0.551Fe2+ 0.695 1.012 0.727 0.937 0.724 1.034 0.512 0.523 1.057Fe3+ 0.303 1.488 0.406 0.997 0.376 1.352 0.102 0.224 1.517Mn 0.010 0.008 0.011 0.015 0.007 0.012 0.006 0.008 0.024Mg 0.325 0.098 0.318 0.148 0.313 0.091 0.499 0.505 0.112Ni 0.000 0.000 0.003 0.006 0.000 0.007 0.000 0.000 0.001Fe3+# 32 9 30 14 30 8 49 49 65Mg# 15 67 19 46 18 60 5 11 10Cr# 36 73 38 66 36 73 23 23 67

aCalculated.

Table 5Representative electron microprobe analyses of Abu Hamamid spinels

Spinel type Unmixed Homogeneous

Irregular sep. Complete sep. In Cpx In Amph In chlorite

Sample no. AH40 AH40 AH24 AH24 AHX AH24 AH23 AH40 AH27

Oxide wt.%SiO2 0.48 0.46 0.29 0.62 b0.05 0.15 0.36 0.40 0.56TiO2 1.03 1.43 0.43 1.67 0.98 0.64 0.47 3.35 0.59Al2O3 17.68 9.31 21.89 2.84 7.89 22.82 3.74 8.70 13.13Cr2O3 29.51 22.11 24.07 16.72 20.96 21.53 3.40 14.79 17.05FeOa 24.67 29.96 25.14 30.31 30.71 26.20 30.50 29.04 30.19Fe2O3

a 18.63 32.11 20.75 44.52 35.92 21.16 60.32 39.55 35.26MnO 0.23 1.32 0.31 0.72 1.47 0.36 0.33 0.47 0.58MgO 5.97 1.41 6.31 0.31 0.82 5.64 0.56 1.50 1.86NiO b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05Total 98.21 98.11 99.17 97.72 98.75 98.49 99.68 97.79 99.21

Structural formula based on 4 oxygensSi 0.008 0.008 0.005 0.010 0.000 0.002 0.006 0.007 0.009Ti 0.013 0.017 0.005 0.021 0.01 0.008 0.006 0.042 0.007Al 0.173 0.089 0.215 0.028 0.08 0.224 0.037 0.085 0.129Cr 0.194 0.144 0.158 0.110 0.14 0.142 0.022 0.097 0.112Fe2+ 0.709 0.981 0.723 1.084 1.06 0.752 1.200 0.938 0.990Fe3+ 0.482 0.946 0.537 1.432 1.11 0.546 2.136 1.150 1.040Mn 0.003 0.017 0.004 0.010 0.02 0.005 0.005 0.007 0.008Mg 0.148 0.034 0.156 0.008 0.02 0.140 0.014 0.037 0.046Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe3+# 24 44 26 65 50 26 86 56 47Mg# 31 8 31 2 5 28 3 9 10Cr# 53 62 42 80 64 39 38 53 47aCalculated.

93A.H. Ahmed et al. / Lithos 104 (2008) 85–98

Fig. 6. Electron microprobe step scans on two optically homogenous spinelsfrom Genina Gharbia (GG244) and Gabbro Akarem (GA153).

Fig. 7. Fe3+# (100Fe3+/(Fe3++Cr+ Al)) – Mg# (100 Mg/Mg+ Fe2+) variationdiagram of spinels from Gabbro Akarem (GA), Genina Gharbia (GG) and AbuHamamid (AH) intrusions. Discriminating fields of Alpine-type complexes,stratiform complexes and South East (SE) Alaskan-type complexes (Irvine,1967) and Alaskan-type complexes worldwide (Barnes and Roeder, 2001) arepresented for comparison. The large red symbols are the estimated initialcompositions (see text for explanation).

94 A.H. Ahmed et al. / Lithos 104 (2008) 85–98

geochemical relationships are illustrated in Figs. 6–8. In thefollowing paragraphs important geochemical features amongspinels of the three plutons are summarized:

1) Homogenous spinel is generally Al-rich (up to 44 wt.%Al2O3, Table 2) relative to Type I (up to 37 wt.%) ofunmixed spinel. Large homogenous grains in clinopyr-oxene and amphibole are commonly zoned with a slightincrease in Al and Mg and decrease in Cr towards themargin (Fig. 6).

2) Homogenous spinels hosted in pyroxenes show widerange of compositions (Table 2). The Mg# of homo-genous spinel hosted in amphibole from GA (average 54)is higher than GG (average 45) and AH (average 25).

3) When present, the intercumulus spinel is Fe3+-rich(average Fe3+# 75 and 55 at GA and GG, respectively)and Al–Mg poor (Figs. 7, 8).

4) Despite differences in spinel textures, AH spinels showsimilar chemical trends to those of GA and GG. AHspinels are Fe3+–Cr–Ti rich and have the lowest Mg#(b25) relative to spinels from GA (b54) and GG (b46)with small miscibility gabs between the exsolved types(Figs. 7, 8). Spinels hosted in chlorite show the widestrange of compositions (Table 2).

5) Wide miscibility gabs exist between the different spinel typesat GA, with relatively Fe3+-rich nature; the crystallographi-cally oriented spinels show thewidestmiscibility gab (Fig. 8).

6) The estimated initial compositions of unmixed spinels inall areas are Fe3+-rich relative to homogenous spinels.Both unmixed and homogenous spinels plot in the fieldof Alaskan-type complexes on the Mg# vs Fe3+# diagram(Fig. 7).

5. Discussion

Previous studies (Helmy and El Mahallawi, 2003; Helmyet al., 2007; Farahat and Helmy, 2006) have shown that GAparental magma is more mafic (Mg# 66) than the GG (Mg# 61)and AH (Mg# 59) magmas and that the magmas underwentfractional crystallization and accumulation. This may explain

Fig. 8. Ternary diagram of Cr–Al–Fe3+atomic ratios of spinels from GabbroAkarem (GA), Genina Gharbia (GG) and Abu Hamamid (AH) intrusions. Thelarge red symbols are the estimated initial compositions. CS dashed line is acalculated solvus for spinel coexisting with Fo80 olivine at 600 °C (Sack andGhiorso, 1991) and SS is a suggested solvus at 600 °C for unmixed spinels fromthe Red Lodge district (Loferski and Lipin, 1983).

95A.H. Ahmed et al. / Lithos 104 (2008) 85–98

the overall compositional differences between spinels from thethree plutons, especially Mg#, Al2O3 and TiO2 contents. Thelow Mg# and high TiO2 contents of spinel from AH reflect theevolved nature of the parental melt. The compositions of cu-mulus spinel inclusions in olivine, pyroxenes and amphibole andthe intercumuls spinel reflect compositions of the parent melts atvarious evolutionary stages. The general Al enrichment of spinelhosted in clinopyroxene and amphibole (Table 2) is due to theearly fractionation of olivine and orthopyroxene. Homogenousspinel inclusions in amphibole were either formed directly fromthe late interstitial melt or were early formed, and later reequi-librated with this melt. Spinel zoning, revealed by the decreaseof Cr2O3 and the increases of both MgO and Al2O3 towards themargin (Fig. 6) supports subsolidus re-equilibration.

Both ortho- and clinopyroxenes have relatively high Al2O3

and Cr2O3 contents and are likely to influence the Cr–Al ratio ofthe enclosed spinel. The increase of Al and Mg and decrease ofCr towards the margin of zoned spinel inclusions in pyroxenesindicate subsolidus modifications. Similar observations and in-terpretation were made by Henderson and Wood (1981) onspinel from layered intrusions in Scotland. It is to stress here thatsubsolidus chemical changes are strongly dependent upon thenature of a crystal's immediate environment.

As stated above, almost all the unmixed spinel grains occur asinclusions in cumulus olivine and orthopyroxene, indicating thatit was originally primary cumulus phase. Reaction with interstitialmelt is not likely, but equilibration with host olivine and pyroxeneis possible. Olivine from the studied plutons has Al and Crcontents below the EMP detection limit (Table 1); it has noinfluence on the Cr–Al ratio of coexisting spinel. The Cr-numbersof unmixed spinels in olivine depends largely on the initialcomposition of the spinel and the degree of unmixing. As theinitial composition of cumulus spinel hosted in olivine is likely tobe similar within the same rock sample, intermediate composi-tions of unmixed spinels indicate low degrees of unmixing whileformation of two end members; Al–Mg- and Fe3+-rich indicatecomplete unmixing (separation). Changes in the Mg# of un-mixed spinel hosted in olivine are also likely (Hatton and VonGruenwaldt, 1985; Kepezhinskas et al., 1993). The exchange ofMg and Fe2+ions between olivine and spinel is temperaturesensitive (Irvine, 1967; Evans and Frost, 1975). Due to the widerange of temperatures experienced by the spinel-olivine pairs, weexpect strong Mg and Fe2+ions re-equilibration between olivineand spinel.

5.1. Reasons and mechanisms of spinel unmixing

Unmixed spinels are described from metamorphosed gab-broic intrusions (Muir and Naldrett, 1973; Loferski and Lipin,1983; Eales et al., 1988; Zakrezewski, 1989; Van der Veen andMaaskant, 1995; Barnes and Zhong-Li Tang, 1999), Alpine-type peridotite (Tamura and Arai, 2005) and Alaskan-typecomplexes (Pushkarev et al., 1999; Garuti et al., 2003 andKrause et al., 2007). The different geological environments andpost-magmatic processes that affected these intrusions made itdifficult to attribute this feature to any specific cause. However,subsolidus modification was suggested to be the main reason ofspinel unmixing (e.g. Muir and Naldrett, 1973; Frost, 1975;Evans and Frost, 1975). All complexes containing the unmixedspinel show evidence of Fe3+-enrichment during metamorphismor magma crystallization. This fact may suggest that Fe-richspinel unmixing results from changes in temperature and oxi-dation state.

It is noteworthy that the unmixed spinels observed in thisstudy are found in non-metamorphosed mafic–ultramafic in-trusions. Although AH rocks underwent hydrous alteration,there is clear geological evidence that this alteration is of late-magmatic origin (Farahat and Helmy, 2006). The serpentiniza-tion degree at GA and GG is very low; all silicate minerals arepreserved, moreover there is no evidence for de-serpentini-zation. It is, therefore, assumed that spinel exsolutions andchemical trends are mainly the result of unmixing of initiallyhomogeneous spinel phase developed during magma cooling.This could have happened during the emplacement of the arcplutons at the shallower levels in the sub-arc crust.

5.1.1. Reasons of unmixingSpinel composition is sensitive to changes in bulk chemistry,

temperature, oxygen fugacity, and fluid composition (Irvine,1967). As most of the unmixed spinels from the three plutons

96 A.H. Ahmed et al. / Lithos 104 (2008) 85–98

are hosted by olivine and orthopyroxene, the effect of bulkchemistry and fluid composition is relatively limited (e.g.,Henderson, 1975). Spinel crystal fractionation is unlikely tohave an essential role in the formation of unmixed spinels sincethe textures of the unmixing vary greatly. Unmixing must haveformed in response to changes in temperature and oxygenfugacity. The parent arc magmas of zoned complexes in Egypt,and worldwide, are hydrous and oxidized (e.g., Claeson andMeurer, 2004). The hydrous nature is reflected in the dominanceof amphibole and locally biotite in the different lithologieswithin the intrusions. The development of amphibole reactioncoronas, a feature commonly attributed to hydrous reactionbetween olivine and plagioclase during cooling (Lamoen,1979), is supporting evidence. Such hydrous magma is likelyto solidify over a wide temperature range, with concomitantchange in composition and over a long period. It is commonlyaccepted that the subsolidus reactions are enhanced by domi-nance of fluids, which act as a transporting medium for chemicalexchange (O'Hara, 1993). Although fluids might have no directimpact on spinel unmixing, they must have lowered the tem-perature of final solidification of magma. Prolonged cooling,consequently, enhances the unmixing process. Sack and Ghiorso(1991) calculated solvus lines for spinel coexisiting with olivineat different Fo contents. The compositional data of unmixedspinels from the three plutons fits well with these solvus lines(Fig. 8).

The increase in Fe3+ relative to other trivalent cations leads togeneral enrichment of spinel in total Fe and decrease in Mg#(Power et al., 2000). The ratio Cr/(Cr+Fe3+) determines whethera spinel exsolves during cooling or not; low ratios increasethe possibility of exsolution (e.g. Burkhard, 1993). The overallFe3+enrichment of the estimated initial composition of unmixedspinel (Figs. 7 and 8) relative to the homogenous spinel supportsthis suggestion. The general enrichment of total Fe will led to theformation of “inverse” spinel instead of “normal” spinel(Lindsley, 1976); in inverse spinel, Fe2+is stabilized in theoctahedral site instead of the tetrahedral site. The dominance ofFe3+-rich spinel at AH spinel may be attributed to the formationof inverse spinel by complete re-crystallization of the initialnormal spinel under high oxygen activity. The alteration of AHspinels into hematite along cracks and crystal margins supportthis suggestion (e.g. Burkhard, 1993). In conclusion, spinelunmixing in the studied plutons is the result of magma slowcooling and increase in oxygen fugacity.

5.1.2. Unmixing mechanismAs discussed above, olivine has negligible effect on the Cr#

(100Cr/Cr+Al) of unmixed spinel, i.e., unmixing is the result ofelement redistribution within the initial spinel. Regardless oftexture, the unmixing mechanism is the same; the differences inshape and size of exsolutions may reflect variable rates ofcooling, or local differences in the chemistry of the initial spinel.In all of the unmixed spinel crystals and regardless of pattern ofunmixing, Type II spinel is the exsolving phase. When Type IIspinel exsolves along crystallographic directions, it forms anetwork in Type I spinel (Figs. 4, 5); chemical separation ismore efficient (Type II crystallographically oriented spinel

contains lower Al, Cr and Mg and higher Fe contents relative tothe irregularly exsolved, Table 2 and Fig. 8). The crystal-lographically oriented unmixing is characteristic of slow cooling(Buddington and Lindsley, 1964). The enrichment of Type IIcrystallographically oriented spinel in Ti (up to 3.6 wt.% TiO2)may suggest that this type starts at high temperature (Buddingtonand Lindsley, 1964) in response to an increase in oxygenactivity, as the solvus is intersected on cooling in a wayanalogous to ulvöspinel exsolution from magnetite. The solvuslines of Sack and Ghiroso (1991).

Irregular Type II blebs have been initiated by heterogeneousnucleation at the outer margins of spinel grains leading to theformation of external granules (Fig. 4). The restriction of theirregular unmixing to the margins and along cracks, suggeststhat this type is later and occurs at lower temperature andpossibly higher oxidation state. The enlargement of crystal-lographically oriented exsolutions at the margin of spinel is insupport of this inference. The concentration of Type II blebs atthe edge and the existence of Type I clear haloes devoid of suchblebs (Fig. 5) may indicate that the blebs migrate to the marginsafter exsolution. Accordingly, complete separation is consid-ered as an advanced stage of irregular unmixing. As theexsolutions will maintain their outline after their formation, theirregular blebs and complete separation must have formed underhigh fluid activity to allow large velocity of migration and largesurface tension. Complete separation of Type I and II spinels isin response of long-life unmixing processes. This may explainthe absence of the crystallographically oriented exsolutions inthe AH spinels.

The miscibility gaps in spinel prism were comprehensivelydiscussed in Sack and Ghiorso (1991) based on thermodynamiccalculations at different temperatures and different forsteritecontent of the coexisting olivine. The small miscibility gab inspinel reported by Van der Veen and Maaskant (1995) wasattributed to the high temperature of crystallization (650 °C).Relatively large miscibility gaps of crystallographicallyoriented exsolutions at GA and GG (Fig. 8) may indicate re-equilibration under low temperatures (b650 °C), althoughunmixing might have started at much higher temperatures(Buddington and Lindsley, 1964). In general, the change fromsmall to large miscibility gabs of unmixed spinels from thestudied plutons (Fig. 8) indicates re-equilibration over a widerange of temperatures during cooling. The compositions ofunmixed spinel types from the studied areas plot very close tothe calculated (Sack and Ghiorso, 1991) and suggested(Loferski and Lipin, 1983) solvus lines at 600 °C (Fig. 8). Itis, however, difficult to make accurate quantitative estimates ofthe starting and closing temperatures of unmixing.

In conclusion, the chemistry (the total Fe content) and grainsize of initial spinel, the oxidation state and the cooling rate arethe important factors influencing the type of unmixing and thechemistry of final products. In all cases mentioned above, wesuggest that the exsolutions, (small or larger blebs), are of late-magmatic origin, as they show distinct petrographic differencesfrom the low-temperature ferrit-chromite that occurs as alter-ation products of chromite grains in ultramafic rocks (Ealeset al., 1988).

97A.H. Ahmed et al. / Lithos 104 (2008) 85–98

5.2. Petrogenetic significance of spinel in Alaskan-typecomplexes

The composition of primary spinel reflects the degree ofpartial melting that the mantle experienced while producing thechromium spinel-bearing rock. Various tectonic settings ofmafic–ultramafic magmas were successfully characterized byusing the spinel chemistry. Accordingly, spinel has been used asa petrogenetic indicator in many studies, and it was possible todistinguish the tectonic setting of the magma, the degree ofpartial melting (e.g., Irvine, 1965; 1967; Dick and Bullen, 1984;Arai, 1992; Barnes and Roeder, 2001). The success in usingspinel chemistry to estimate the degree of partial melting wasbased on the selection of primary spinel which did not undergoserious re-equilibration except for Mg–Fe redistribution. Anymajor modification in the spinel chemistry either by the reactionwith the interstitial melt or subsolidus equilibration limits therole of spinel as a petrogenetic indicator. As has been presentedabove, spinel may form at different evolutionary stages of themelt, thus its composition cannot be used to constrain the pri-mary melt composition, even if its chemistry has not beenmodified. Moreover, the chemistry of unmixed spinel end mem-bers is completely different from the initial spinel composition;will give misleading petrogenetic meanings.

According to the data presented here (Fig. 7) and by Barnesand Roeder (2001), the diagnostic features of spinels fromAlaskan-type complexes are the overall Fe3+-enrichment andpresence of both Fe3+#- and Mg#-rich compositions within thesame sample. While the overall enrichment of the initial spinelin Fe3+is an original feature attributed to the oxidized nature ofarc magmas (the parent magma of Alaskan-type complexes),the existence of Fe3+- and Mg#-rich spinels is a secondaryfeature attributed to initial spinel unmixing. Barnes and Roeder(2001) compiled the spinel chemical data from mafic andultramafic complexes of different geological settings anddefined a new field of Alaskan-type complexes (Fig. 7). Thenew field of Alaskan-type complexes extends from Fe3+#-richto Mg#-rich and overlaps that of layered intrusions. The spinelchemical data presented in this work confirm the Alaskan-typefield suggested by Barnes and Roeder (2001). It is likely thatspinels formed from oxidized magmas, e.g., those fromAlaskan-type complexes, are not good candidates for chemicalclassification of rocks as they usually undergo chemical re-equilibration or form at different stages of melt fractionation andcrystallization.

6. Conclusions

Spinel in Alaskan-type complexes is not always an earlycumulate phase, its initial composition widely varies accordingto its place in the paragenetic sequence. The slow cooling ofthe oxidized parent magmas of Alaskan-type complexes leadto extensive subsolidus equilibration and unmixing of spinel,equivalent to changes happen during metamorphism. Chemicalre-equilibration may happen without textural modification ofspinel. The type of unmixing is controlled mainly by thechemistry of the initial spinel, cooling rate and oxidation state.

The two end members resulting from unmixing during re-equilibration have compositions completely different from theinitial spinel, being Al- and Fe3+-rich. This difference in chem-istry depends upon the degree of re-equilibration and thechemistry of the host mineral of spinel. The late-magmaticchanges in spinel chemistry make it difficult to locate spinelcompositions along a spinel-olivine solvus and result in anextended compositional field of Alaskan-type complexes indiagrams using spinel chemistry to define the tectonic settings.The broad range of compositions defined by spinels fromAlaskan-type complexes is the result of various degrees ofunmixing and/or formation at different evolutionary stages ofmelt solidification. In addition to the degree of re-equilibration,the success of spinel as a petrogenetic indicator will depend upon its place in the paragenetic sequence.

Acknowledgments

The analytical work presented in this paper was made duringa research visit of the first two authors to the Department ofEarth Sciences of Kanazawa University founded by the latterInstitute. Karl Ettinger, Institute of Earth Sciences, Karl-Franzens University, Graz, Austria is thanked for helping withthe elements' X-ray maps. AH Ahmed and HM Helmy wouldlike to thank Akihiro Tamura for many discussions and helpduring their stay in Kanazawa. Two anonymous reviewers arethanked for their helpful suggestions that significantly improvedthe manuscript. This study was partly supported by Grant-in-Aid for Creative Scientific Research (19GS0211).

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