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Magmatic unmixing in spinel from Late Precambrian Concentrically-ZonedMafic-Ultramafic Intrusions, Eastern Desert, Egypt

Ahmed Hassan Ahmed, Hassan Mohamed Helmy, Shoji Arai, MasakoYoshikawa

PII: S0024-4937(07)00274-5DOI: doi: 10.1016/j.lithos.2007.11.009Reference: LITHOS 1707

To appear in: LITHOS

Received date: 21 March 2007Revised date: 25 October 2007Accepted date: 27 November 2007

Please cite this article as: Ahmed, Ahmed Hassan, Helmy, Hassan Mohamed, Arai, Shoji,Yoshikawa, Masako, Magmatic unmixing in spinel from Late Precambrian Concentrically-Zoned Mafic-Ultramafic Intrusions, Eastern Desert, Egypt, LITHOS (2007), doi:10.1016/j.lithos.2007.11.009

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Magmatic unmixing in spinel from Late Precambrian 1

Concentrically-Zoned Mafic-Ultramafic Intrusions, Eastern 2

Desert, Egypt 3

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Ahmed Hassan Ahmed1, Hassan Mohamed Helmy2, Shoji Arai3, Masako 5

Yoshikawa2 6

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1. Geology Department, Faculty of Science, Helwan University, Cairo, 8

Egypt 9

2. Institute of Geothermal Sciences, Kyoto University, Japan 10

3. Department of Earth Sciences, Kanazawa University, 920-1192, Japan 11

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Corresponding author: Hassan M. Helmy 15

Institute of Geothermal Sciences, Kyoto University, Japan 16

E mail: hmhelmy@yahoo.com 17

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Abstract 24

Spinel is widespread in the ultramafic core rocks of zoned late Precambrian 25

mafic-ultramafic complexes from the Eastern Desert of Egypt. These complexes; 26

Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues 27

of Alaskan-type complexes, they are not metamorphosed although weakly altered. 28

Each intrusion is composed of a predotite core enveloped by pyroxenites and 29

gabbros at the margin. Silicate mineralogy and chemistry suggest formation by 30

crystal fractionation from a hydrous magma. Relatively high Cr2O3 contents are 31

recorded in pyroxenes (up to 1.1 wt.%) and amphiboles (up to1.4 wt.%) from the 32

three plutons. The chrome spinel crystallized at different stages of melt evolution; 33

as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles 34

and late-magmatic intercumulus phase. The intercumulus chrome spinel is 35

homogenous with narrow-range of chemical composition, mainly Fe3+-rich spinel. 36

Spinel inclusions in clinopyroxene and amphibole reveal a wide range of Al (27 - 37

44 wt.% Al2O3) and Mg (6 – 13 wt.% MgO) contents and are commonly zoned. 38

The different chemistries of those spinels reflect various stages of melt evolution 39

and re-equilibration with the host minerals. The early cumulus chrome spinel 40

reveals a complex unmixing structures and compositions. Three types of unmixed 41

spinels are recognized; crystallographically oriented, irregular and complete 42

separation. Unmixing products are Al-rich (Type I) and Fe3+-rich (Type II) spinels 43

with an extensive solid solution between the two end members. The compositions 44

of the unmixed spinels define a miscibility gap with respect to Cr-Al-Fe3+, 45

extending from the Fe3+-Al join towards the Cr corner. Spinel unmixing occurs in 46

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response to cooling and the increase in oxidation state. The chemistry and grain 47

size of the initial spinel and the cooling rate control the type of unmixing and the 48

chemistry of the final products. Causes of spinel unmixing during late-magmatic 49

stage are analogous to those in metamorphosed complexes. The chemistry of the 50

unmixed spinels is completely different from the initial spinel composition and is 51

not useful in petrogenetic interpretations. Spinels from oxidized magmas are 52

likely to re-equilibrate during cooling and are not good tools for genetic 53

considerations. 54

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Keywords: Unmixed spinel, late-magmatic, re-equilibration, Alaskan-type, 56

mafic-ultramafic, Egypt 57

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1. Introduction 59

Spinels are important accessory minerals in all mafic-ultramafic magmas 60

of various tectonic settings (e.g., Irvine, 1965, 1967; Hamlyn and Keays, 1979; 61

Dick and Bullen, 1984; Barnes and Roeder, 2001, and many others). They are the 62

main repository of Cr2O3 in mafic-ultramafic rocks, and host other elements as 63

major constituents like Mg, Al, and Fe. Based on geochemical basis, spinels are 64

classified to three series (Deer et al., 1966) according to the dominant trivalent 65

ion; Spinel series (Al+3-dominant), Magnetite series (Fe+3-rich) and Chromite 66

series (Cr+3-rich). Significant solid-solution compositional variations occur 67

naturally within and between the three spinel series. The modification of spinel 68

chemistry is commonly accompanied by textural diversity, like exsolutions and 69

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alteration. The various geological and physicochemical parameters which may 70

influence the texture and chemistry of spinel have been dealt with in many 71

publications (e.g., Barnes, 2000 and references there in). Modification of primary 72

spinel compositions in cumulate rocks is possible through three mechanisms; 1) 73

the reaction of primary spinel with the residual melt (e.g., Henderson, 1975; 74

Henderson and Wood, 1981), 2) exsolution of initial spinel in response to cooling 75

(Sack and Ghiorso, 1991), and 3) sub-solidus mineral-mineral reactions (Loferski 76

and Lipin, 1983, Candia and Gaspar, 1997; Barnes, 2000). The spinel chemistry 77

thus depends on the chemical evolution of the magma at the post-cumulus stage 78

and the post-magmatic processes; like hydrous alteration and metamorphism. 79

Systematic changes in spinels chemistry with progressive alteration (Burkhard, 80

1993) and metamorphism (Evans and Frost, 1975) are proved. 81

Unmixed spinel phases of different chemical compositions have been 82

widely noted. Unmixed spinels were described from many metamorphosed mafic-83

ultramafic complexes (Muir and Naldrett, 1973; Loferski and Lipin, 1983; Eales 84

et al., 1988; Zakrezewski, 1989; Van der Veen and Maaskant, 1995; Barnes and 85

Zhong-Li Tang, 1999), one Alpine-type peridotite (Tamura and Arai, 2005) and 86

Alaskan-type complexes (Garuti et al., 2003; Krause et al., 2007). Unmixing 87

patterns in spinel vary from simple to complex intergrowths and from single to 88

multiple stages of exsolutions (e.g., Haggerty, 1991). The simple feature is large 89

Fe-rich blebs at the center of Al-rich spinel grains or granules at their outer 90

margins (e.g. Fig. 2 of Loferski and Lipin, 1983). Complex unmixing comprises 91

the presence of two or three types of Fe-rich exsolutions in a matrix of Al-rich 92

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spinel (e.g., Fig. 4 of Eales et al., 1988). In this case, the Fe-rich exsolutions form 93

either very fine irregular vermicular bodies or crystallographically oriented 94

lamellae at the center of the initial spinel grains (e.g. Fig. 4a-d of Tamura and 95

Arai, 2005). In few cases, the Al-rich spinel is found as small exsolutions in a Fe-96

rich matrix (e.g., Fig. 3 of Loferski and Lipin, 1983). When present, the Al-rich 97

lamellae are commonly crystalographically oriented (e.g., Fig. 5 of Eales et al., 98

1988; Fig. 3 of Loferski and Lipin, 1983). Loferski and Lipin (1983) noted 99

compositional differences between exsolution types. Commonly, the 100

compositions of the different types of exsolutions define a miscibility gab with 101

respect to Cr-Al-Fe3+, extending from the Fe3+-Al join towards the Cr corner 102

(Loferski and Lipin, 1983, Eales et al., 1988; Tamura and Arai, 2005). In many 103

cases, spinel exsolution is attributed to metamorphic processes. The upper 104

temperature limit of spinel unmixing was postulated from the peak metamorphic 105

temperatures at about 600ºC (e.g. Loferski and Lipin, 1983). 106

In this contribution we describe intercumulus and cumulus unmixed spinel 107

from three unmetamorphosed zoned (Alaskan-type) mafic-ultramafic complexes 108

from the Eastern Desert of Egypt (Fig. 1). The general characteristics of these 109

complexes common with Alaskan-type complexes are discussed elsewhere 110

(Helmy and El Mahallawi, 2003; Farahat and Helmy, 2006). Spinels from these 111

complexes show complex exsolution patterns and are interesting in studying such 112

exsolution in unmetamorphosed intrusions. In this work we discuss the relation of 113

spinel chemistry to the evolution of magma in each complex and the possible 114

reasons and mechanisms of spinel unmixing. The complex unmixing is attributed 115

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to late-magmatic changes in temperature and oxidation state, analogous to 116

metamorphic changes. It is shown that late- and postmgamatic unmixing of initial 117

spinel produce spinels end members of completely different chemistries than the 118

initial spinel and, therefore give misleading petrogenetic meanings. Throughout 119

this manuscript “spinel” refers to a spinel-group mineral generally expressed as 120

(Mg, Fe2+)(Cr,Al,Fe3+)2 O4 (e.g., Haggerty, 1991). 121

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2. Geologic background and locations 123

The basement complex of Egypt forms the western part of the Arabian-124

Nubian shield which is considered a juvenile terrane formed in convergent plate 125

boundaries through the formation of intra-oceanic island arc system, subsequent 126

ocean closure, amalgamation of the arc complexes and accretion to continental 127

crust (e.g. Gass, 1982; Stern, 2002). These processes took place during the Pan-128

African orogenic event (800-600 Ma, Hassan and Hashad, 1990). 129

Ultramafic-mafic rocks comprise about 5 % of all Precambrian outcrops in 130

the Eastern Desert of Egypt (Dixon, 1979). They are of two types; ophiolitic and 131

intrusive complexes. The ophioltic (dismembered) ultramafic rocks represent 132

remnants of the oceanic crust that coexisted with the intra-oceanic island arcs. 133

The ophiolitic ultramafic rocks recur along major thrust zones (Shackleton, 1994) 134

and are severely serpentinized and contain chromitite lenses. The intrusive 135

ultramafic-mafic complexes form small, elleptical outcrops located along major 136

fracture zones trending ENE (Garson and Shalaby, 1976) and are commonly 137

concentrically zoned (Helmy and El Mahallawi, 2003). The concentrically zoned 138

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complexes are not metamorphosed and sometimes contain Cu-Ni-PGE 139

mineralization (e.g., Helmy and Mogessie, 2001; Helmy, 2004). Recently, 140

Yoshikawa et al. (in preparation) estimated Sm/Nd and Rb/Sr ages of 673±10 Ma 141

of two of these complexes; Gabbro Akarem and Genina Gharbia. These zoned 142

complexes are considered Precambrian analogues of Alaskan-type complexes and 143

represent the roots of island arcs (Helmy and El Mahallawi, 2003, Farahat and 144

Helmy, 2006, Helmy et al., 2007). 145

Three of these complexes; Gabbro Akarem, Genina Gharbia and Abu 146

Hamamid are studied for their spinel mineralogy and chemistry. These complexes 147

are located in the south Eastern Desert (Fig. 1) in an area dominated by island arc 148

volcano-sedimentary rocks, calc-alkaline arc granites and back-arc basin 149

ophiolites (Stern and Hedge, 1985). Previous petrological studies (Helmy and El 150

Mahallawi, 2003; Farahat and Helmy, 2006) suggest uncontaminated arc parental 151

magmas of these complexes. 152

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3. Field relations and rock petrography 154

Gabbro Akarem (GA) intrusion is located about 130 km east of Aswan. 155

The intrusion is 8 km by 1.5 km in plan, formed of plagioclase-hornblendite (at 156

the margins), olivine-plagioclase hornblendite and amphibole-bearing peridotite 157

(at the core), in decreasing order of abundance. The peridotite bodies are oriented 158

and elongated in an ENE direction, following the regional trend of the intrusion 159

(Fig. 2). The contact between the different rock units is gradational. Petrography 160

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of the GA intrusion is described in detail by Helmy and El Mahallawi (2003), and 161

only a brief summary is given here. 162

The GA rock units show primary magmatic silicate mineralogy with a 163

faint degree of secondary alteration. Core peridotites are characterized by fresh to 164

slightly altered olivine (50-70 % modal) up to 3 mm in size, about 12 % modal 165

ortho- and clinopyroxenes and intercumulus amphibole (up to 25 % modal), about 166

2 % spinel, and 7 % sulfides (pyrrhotite>pentlandite=chalcopyrite). Amphibole 167

coronas between olivine and plagioclase are very common. The modal abundance 168

of spinel decreases from the core (4 %) to the rim (< 1 %) of the intrusion. 169

The Genina Gharbia (GG) mafic-ultramafic intrusion is located about 40 170

km southeast of GA (Fig.1). It covers an area of 9 km long and 3.5 km wide and 171

comprises gabbros (in the margin), pyroxenites, amphibole pyroxenites, and 172

amphibole-bearing peridotite (in the core). This association intruded Precambrian 173

metasedimentary and island arc volcanic rocks (El Mahallawi, 1996). The 174

intrusion is non-metamorphosed but highly affected by faulting and shearing 175

where most of the original magmatic contacts have been obliterated. The different 176

rocks are characterized by high modal content of intercumulus and corona (Helmy 177

et al., submitted) amphibole and abundant biotite and apatite indicating hydrous 178

nature of the parent magma (Helmy, 2004). Massive and disseminated Cu-Ni 179

sulfide ores are found in amphibole gabbro and peridotite. 180

The Abu Hamamid (AH) mafic-ultramafic complex is located about 100 181

km to the west of the Red Sea intruding volcanic rocks (Shadli Volcanics: 710 182

Ma, Stern et al., 1991). The AH complex is an elliptical body of 1.5 km long and 183

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500 m in the maximum width. The geologic contacts with the surrounding 184

volcanic rocks are hidden below thick valley sediments. This intrusion comprises 185

a peridotite core enveloped by wehrlite, clinopyroxenite and gabbro outward 186

(Farahat and Helmy, 2006). The peridotite core bodies occur as small rounded 187

outcrops aligned in an ENE direction parallel to the direction of elongation of the 188

intrusion. Peridotites composed of olivine (40-60 %), amphibole (up to 20 %) 189

clinopyroxene (up to 15 %) and spinel (2-3 %). The AH rock units show a higher 190

degree of alteration, specially the core rocks relative to the other complexes. As 191

the complex is intruding non-metamorphosed island arc volcanics, this extensive 192

hydrous alteration is related to late-magmatic fluids (Farahat and Helmy, 2006). 193

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Analytlical techniques 195

Silicate and oxide mineral chemistry was carried out at the Centre for Co-196

operative Research of Kanazawa University, Japan, using a JEOL JXA-8800 197

electron probe micro analyzer. The analytical conditions were a 25-kV 198

accelerating voltage, 20 nA probe current and 3-µm probe diameter. To avoid 199

contamination, relatively large areas were selected to analyze unmixed spinels. 200

The raw data were corrected using an on-line ZAF program. The ferric and 201

ferrous ions were calculated assuming spinel stoichiometry. Standards used for 202

oxides and silicate minerals are natural minerals; quartz for Si, eskolite for Cr, 203

fayalite for Fe, wollastonite for Ca, jadeite for Na, corundum for Al, periclase for 204

Mg, manganosite for Mn, and nickel oxide for Ni. X-ray maps were made at the 205

Institute of Earth Science, Karl-Franzens University, Graz, Austria using 206

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OXFORD-energy dspersive detector (EDX), model 6687. In Speedmaps high 207

resolution (512x400 pixels) is selected, after setting the energy on a characteristic 208

line (Cr K alpha, Al K alpha and Fe K alpha) three frames are integrated to 209

accumulate the number of X-ray quants. The image holds for each pixel the x and 210

y-coordinates and z (the brightness = number of aquired X-ray quants). The 211

integration time was set as one minute per frame and the exposure time as 4 212

hours. In each X-ray map, the higher the concentration of an element the brighter 213

the pixel is. 214

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Silicate mineral chemistry 216

Silicate mineral chemistry of ultramafic-mafic rocks from GA, GG and 217

AH was discussed in detail by Helmy and El Mahallawi (2003), Helmy (2004), 218

and Farahat and Helmy (2006), respectively. Here, a brief summary of silicate 219

mineralogy is provided. Selected microprobe analyses of silicate minerals are 220

listed in Table 1. 221

Olivine, orthopyroxene, clinopyroxene, amphibole and plagioclase are the 222

main primary silicates coexisting with spinel in the studied plutons. All the mafic 223

minerals show a similar trend of variation in Mg number (Mg#) in the three 224

plutons, the Mg# (100Mg/Mg + Fe2+) decrease from the core to the margins. 225

Olivine at GA is more magnesian (Mg# 87-69) than at GG (Mg# 85-73) and AH 226

(Mg# 81-74). In all localities, olivine has Cr2O3 contents below the EMP 227

detection limit (ca 0.05 wt.%). 228

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Clinopyroxenes from the three plutons are classified as diopsides. The 229

Al2O3 content of clinopyroxene varies from 3.0 to 4.0 wt % in GA, from 2.6 to 230

3.2 wt % in GG and about 3.0 wt. % in AH intrusions (Table 1). Clinopyroxene 231

from the core rocks of GA is more magnesian (Mg# 90) than that in GG (Mg# 87) 232

and AH (Mg# 85). Cr2O3 contents in clinopyroxenes from the core rocks of the 233

three plutons are relatively high; 1.1, 0.7, 0.8 wt. %, at GA, GG and AH, 234

respectively. 235

Orthopyroxene, when exists, in the ultramafic cores of the three intrusions 236

is of enstatite composition. It has high Al2O3 content (3.1, 1.9 and 0.7 wt. %, in 237

GA, GG and AH intrusions, respectively). Cr2O3 contents of orthopyroxenes from 238

the core rocks of GA and GG are similar (0.6 wt. %), the Cr content of AH 239

orthopyroxenes is below the EMP detection limit. 240

Amphiboles of the ultramafic cores have constantly high Mg# (88, 86 and 241

86, at GA, GG and AH, respectively) and Cr2O3 contents (up to 1.4, 1.1, 0.4 242

wt.%, at GA, GG and AH, respectively). 243

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4. Spinel petrography and chemistry 245

4.1. Spinel petrography 246

More than 65 polished thin sections were studied from the three complexes; all 247

the rock units in each pluton were represented. Although accessory grains of 248

spinel with different optical properties are common, no massive chromitite was 249

found so far. 250

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In general, spinels are much more abundant in ultramafic core rocks forming less 251

than 2 % modal. The following descriptions are for spinel from peridotites. Spinel 252

was observed as inclusions in olivine, pyroxenes and amphiboles and as 253

intercumulus phase. GA and GG spinels are more abundant than AH spinels, the 254

later commonly form small grains (< 50 microns) in altered silicates. 255

Two types of spinel are optically observed; homogenous and unmixed 256

“exsolved” spinel. The homogenous spinel occurs in three textures: 257

1) Individual grains enclosed in pyroxenes, amphibole and sulfides. The 258

individual spinel grains range in size from 0.2 mm down to < 50 microns across 259

and occur as clustered octahedra or small rounded crystals enclosed within 260

pyroxenes (Fig. 3a), amphibole (Fig. 3b) and sulfides. Optically, this spinel is 261

homogenous although electron microprobe step scans across some relatively large 262

grains revealed weak compositional zoning (Fig. 6). 263

2) Small (< 50 microns) grains hosted in chlorite (and serpentine). All 264

small spinel grains in AH complex are of this type. Optically and chemically, all 265

grains are homogenous, no sub-microscopic pores were observed. Some grains 266

show alteration along cracks and margins into hematite. The optical and chemical 267

homogeneity of this spinel and absence of any sub-microscopic pores suggest that 268

it is the result of re-crystallization. 269

3) Intercumulus large spinel (Fig. 3c). The intercumulus spinel occurs only 270

in sulfide-bearing samples at GG and GA but not at AH, where it fills interstices 271

between olivine and pyroxene grains. 272

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The chemistry of homogenous spinel in each area varies from one sample 273

to another and within the same thin section, according to the host mineral. Spinel 274

inclusions in clinopyroxene and amphibole are Al and Mg-rich, while those 275

hosted in chlorite and intercumulus spinel are mainly Fe3+-rich. 276

The majority of spinel grains hosted by olivine and orthopyroxene at GA 277

and GG are unmixed “exsolved” despite the degree of alteration of the rock. 278

Variable degrees of unmixing are observed between two spinel types; dark gray 279

and light gray (Fig. 3d, 4a). Figures 4 and 5 illustrate the unmixed spinel textures; 280

the two end-members; dark gray (Al-rich) and light gray (Fe-rich) are referred to 281

as Type I and II, respectively. Type I spinel was not found as individual grains, 282

but always in association with Type II spinel which may suggest that both are the 283

result of exsolution from initially homogenous spinel of intermediate 284

composition. This feature distinguishes Type I spinel from the homogenous Al-285

Mg-rich spinel hosted individually in clinopyroxene and amphibole. According to 286

the degree and pattern of unmixing, three sub-types are recognized: 287

1) Crystallographically oriented exsolutions in the inner parts of the large 288

initial spinel, Type II spinel forms either well-developed crystals or a network 289

of exsolutions within the (110) planes of the host forming a “cloth texture” 290

(Fig. 4a). Some of the exsolutions continue to the outer margins but get larger 291

(Fig. 4b). Initial spinel crystals showing this type are usually euhedral and 292

large (>150 microns in diameter). The uniform distribution of the exsolutions 293

in the parental spinel may suggest an initial homogenous chemical 294

composition of this spinel. This type was not observed in AH spinels. 295

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2) Irregularly exsolved blebs of Type I spinel leaving areas of homogenous 296

Type II. This sub-type occurs in the outer margin or along cracks in the initial 297

large and small spinel grains (Fig. 4a-d). Type II exsolutions may form 298

symplectic or myrmekitic texture of very small irregular blebs in Type I (Fig. 299

4c). All spinel grains with crystallographically oriented exsolution center, 300

have an irregular unmixing margin (Figs. 4a, b and 5). 301

3) Complete separation of Type I and II spinels with well-defined boundaries 302

in between. Initial spinel grains showing this type are commonly hosted by 303

olivine. More than 90% of the grains showing complete separation are < 100 304

microns in diameter. In this texture, the two spinels are contiguous to one 305

another in small clusters (see Fig. 3d). 306

The three sub-types of unmixed spinels could be observed in one polished 307

thin section, and even within the same grain. 308

Figure 5 is a back-scattered electron image and element distribution of an 309

unmixed spinel grain hosted in olivine from Genina Gharbia. Similar distribution 310

and variation in exsolution sizes is observed in many unmixed grains from GG 311

and GA. Elemental distribution maps show many interesting features: 1) The 312

crystallographically oriented growth of Type II spinel lamellae created a halo of 313

Al-enrichment in the host spinel immediately surrounding lamellae, 2) The 314

groundmass of the zone with small crystallographically oriented Type II 315

exsolutions is rich in Al (and slightly in Cr), and 3) the zone with no Type II 316

exsolutions, has a uniform composition but with relatively lower Al (and Cr) 317

contents indicated by the light-gray color in the back-scattered electron image. 318

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. 319

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4.2 Spinel chemistry 321

Extensive electron microprobe analyses were performed on spinels from 322

different textures. The following compositional data are for spinel from 323

peridotites. The initial chemical composition of the unmixed spinel was estimated 324

by calculating the average of area analyses made on fine-grained unmixed spinels 325

of various stages of unmixing. For irregularly and completely unmixed spinels, 326

average compositions were calculated from the EMP analyses of Type I and II 327

spinels. The later approach is not completely justified as the modal abundance of 328

both spinel types vary from one grain to another. However, the difference 329

between the estimated initial compositions using both approaches is small (+/- 3 330

%). The estimated initial spinel compositions of the unmixed spinels of GA, GG 331

and AH are plotted together with EMP point analyses on Figures 7 and 8. A 332

summary of the compositional variations of various spinel types and 333

representative EMP analyses are listed in Tables 2-5. Spinels in the three studied 334

localities display considerable compositional variations from high-Mg-Al to high-335

Fe varieties. The important geochemical relationships are illustrated in Figures 6 336

through 8. In the following paragraphs important geochemical features among 337

spinels of the three plutons are summarized: 338

1) Homogenous spinel is generally Al-rich (up to 44 wt.% Al2O3, Table 2) 339

relative to Type I (up to 37 wt.%) of unmixed spinel. Large homogenous 340

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grains in clinopyroxene and amphibole are commonly zoned with a slight 341

increase in Al and Mg and decrease in Cr towards the margin (Fig. 6). 342

2) Homogenous spinels hosted in pyroxenes show wide range of 343

compositions (Table 2). The Mg# of homogenous spinel hosted in 344

amphibole from GA (average 54) is higher than GG (average 45) and AH 345

(average 25). 346

3) When present, the intercumulus spinel is Fe3+-rich (average Fe3+# 75 and 347

55 at GA and GG, respectively) and Al-Mg poor (Figs. 7, 8). 348

4) Despite differences in spinel textures, AH spinels show similar chemical 349

trends to those of GA and GG. AH spinels are Fe3+-Cr -Ti rich and have 350

the lowest Mg# (< 25) relative to spinels from GA (< 54) and GG (< 46) 351

with small miscibility gabs between the exsolved types (Figs. 7, 8). 352

Spinels hosted in chlorite show the widest range of compositions (Table 353

2). 354

5) Wide miscibility gabs exist between the different spinel types at GA, 355

with relatively Fe3+-rich nature; the crystallographically oriented spinels 356

show the widest miscibility gab (Fig. 8). 357

6) The estimated initial compositions of unmixed spinels in all areas are 358

Fe3+-rich relative to homogenous spinels. Both unmixed and homogenous 359

spinels plot in the field of Alaskan-type complexes on the Mg# vs Fe3+# 360

diagram (Fig. 7). 361

362

5. Discussion 363

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Previous studies (Helmy and El Mahlallawi, 2003; Helmy et al, 2007, 364

Farahat and Helmy, 2006) have shown that GA parental magma is more mafic 365

(Mg# 66) than the GG (Mg# 61) and AH (Mg# 59) magmas and that the magmas 366

underwent fractional crystallization and accumulation. This may explain the 367

overall compositional differences between spinels from the three plutons, 368

especially Mg#, Al2O3 and TiO2 contents. The low Mg# and high TiO2 contents 369

of spinel from AH reflect the evolved nature of the parental melt. The 370

compositions of cumulus spinel inclusions in olivine, pyroxenes and amphibole 371

and the intercumuls spinel reflect compositions of the parent melts at various 372

evolutionary stages. The general Al enrichment of spinel hosted in clinopyroxene 373

and amphibole (Table 2) is due to the early fractionation of olivine and 374

orthopyroxene. Homogenous spinel inclusions in amphibole were either formed 375

directly from the late interstitial melt or were early formed, and later 376

reequilibrated with this melt. Spinel zoning, revealed by the decrease of Cr2O3 377

and the increases of both MgO and Al2O3 towards the margin (Fig. 6) supports 378

subsolidus re-equilibration. 379

Both ortho- and clinopyroxenes have relatively high Al2O3 and Cr2O3 380

contents and are likely to influence the Cr-Al ratio of the enclosed spinel. The 381

increase of Al and Mg and decrease of Cr towards the margin of zoned spinel 382

inclusions in pyroxenes indicate subsolidus modifications. Similar observations 383

and interpretation were made by Henderson and Wood (1981) on spinel from 384

layered intrusions in Scotland. It is to stress here that subsolidus chemical changes 385

are strongly dependent upon the nature of a crystal’s immediate environment. 386

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As stated above, almost all the unmixed spinel grains occur as inclusions 387

in cumulus olivine and orthopyroxene, indicating that it was originally primary 388

cumulus phase. Reaction with interstitial melt is not likely, but equilibration with 389

host olivine and pyroxene is possible. Olivine from the studied plutons has Al and 390

Cr contents below the EMP detection limit (Table 1); it has no influence on the 391

Cr-Al ratio of coexisting spinel. The Cr-numbers of unmixed spinels in olivine 392

depends largely on the initial composition of the spinel and the degree of 393

unmixing. As the initial composition of cumulus spinel hosted in olivine is likely 394

to be similar within the same rock sample, intermediate compositions of unmixed 395

spinels indicate low degrees of unmixing while formation of two end members; 396

Al-Mg- and Fe3+-rich indicate complete unmixing (separation). Changes in the 397

Mg# of unmixed spinel hosted in olivine are also likely (Hatton and Von 398

Gruenwaldt, 1985; Kepezhinskas et al., 1993). The exchange of Mg and Fe2+ ions 399

between olivine and spinel is temperature sensitive (Irvine, 1967; Evans and 400

Frost, 1975). Due to the wide-range of temperatures experienced by the spinel-401

olivine pairs, we expect strong Mg and Fe2+ ions re-equilibration between olivine 402

and spinel. 403

5.1. Reasons and mechanisms of spinel unmixing 404

Unmixed spinels are described from metamorphosed gabbroic intrusions 405

(Muir and Naldrett, 1973; Loferski and Lipin, 1983; Eales et al., 1988; 406

Zakrezewski, 1989; Van der Veen and Maaskant, 1995; Barnes and Zhong-Li 407

Tang, 1999), Alpine type peridotite (Tamura and Arai, 2005) and Alaskan-type 408

complexes (Pushkarev et al., 1999; Garuti et al., 2003 and Krause et al., 2007). 409

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The different geological environments and post-magmatic processes that affected 410

these intrusions made it difficult to attribute this feature to any specific cause. 411

However, subsolidus modification was suggested to be the main reason of spinel 412

unmixing (e.g. Muir and Naldrett, 1973; Frost, 1975; Evans and Frost, 1975). All 413

complexes containing the unmixed spinel show evidence of Fe3+-enrichment 414

during metamorphism or magma crystallization. This fact may suggest that Fe-415

rich spinel unmixing results from changes in temperature and oxidation state. 416

It is noteworthy that the unmixed spinels observed in this study are found 417

in non-metamorphosed mafic-ultramafic intrusions. Although AH rocks 418

underwent hydrous alteration, there is clear geological evidence that this 419

alteration is of late-magmatic origin (Farahat and Helmy, 2006). The 420

serpentinization degree at GA and GG is very low; all silicate minerals are 421

preserved, moreover there is no evidence for de-serpentinization. It is, therefore, 422

assumed that spinel exsolutions and chemical trends are mainly the result of 423

unmixing of initially homogeneous spinel phase developed during magma 424

cooling. This could have happened during the emplacement of the arc plutons at 425

the shallower levels in the sub-arc crust. 426

427

Reasons of unmixing: Spinel composition is sensitive to changes in bulk 428

chemistry, temperature, oxygen fugacity, and fluid composition (Irvine, 1967). As 429

most of the unmixed spinels from the three plutons are hosted by olivine and 430

orthopyroxene, the effect of bulk chemistry and fluid composition are relatively 431

limited (e.g., Henderson, 1975). Spinel crystal fractionation is unlikely to have an 432

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essential role in the formation of unmixed spinels since the textures of the 433

unmixing vary greatly. Unmixing must have formed in response to changes in 434

temperature and oxygen fugacity. The parent arc magmas of zoned complexes in 435

Egypt, and worldwide, are hydrous and oxidized (e.g., Claeson and Meurer, 436

2004). The hydrous nature is reflected in the dominance of amphibole and locally 437

biotite in the different lithologies within the intrusions. The development of 438

amphibole reaction coronas, a feature commonly attributed to hydrous reaction 439

between olivine and plagioclase during cooling (Lamoen, 1979), is supporting 440

evidence. Such hydrous magma is likely to solidify over a wide temperature 441

range, with concomitant change in composition and over a long period. It is 442

commonly accepted that the subsolidus reactions are enhanced by dominance of 443

fluids, which act as a transporting medium for chemical exchange (O’Hara, 1993). 444

Although fluids might have no direct impact on spinel unmixing, they must have 445

lowered the temperature of final solidification of magma. Prolonged cooling, 446

consequently, enhances the unmixing process. Sack and Ghiorso (1991) 447

calculated solvus lines for spinel coexisiting with olivine at different Fo contents. 448

The compositional data of unmixed spinels from the three plutons fits well with 449

these solvus lines (Fig. 8). 450

The increase in Fe3+ relative to other trivalent cations leads to general 451

enrichment of spinel in total Fe and decrease in Mg# (Power et al., 2000). The 452

ratio Cr/(Cr+Fe3+) determines whether a spinel exsolves during cooling or not; 453

low ratios increase the possibility of exsolution (e.g. Burkhard, 1993). The overall 454

Fe3+ enrichment of the estimated initial composition of unmixed spinel (Figs. 7 455

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and 8) relative to the homogenous spinel supports this suggestion. The general 456

enrichment of total Fe will led to the formation of “inverse” spinel instead of 457

“normal” spinel (Lindsley, 1976); in inverse spinel, Fe2+ is stabilized in the 458

octahedral site instead of the tetrahedral site. The dominance of Fe3+-rich spinel at 459

AH spinel may be attributed to the formation of inverse spinel by complete re-460

crystallization of the initial normal spinel under high oxygen activity. The 461

alteration of AH spinels into hematite along cracks and crystal margins support 462

this suggestion (e.g. Burkhard, 1993). In conclusion, spinel unmixing in the 463

studied plutons is the result of magma slow cooling and increase in oxygen 464

fugacity. 465

Unmixing mechanism: As discussed above, olivine has negligible effect 466

on the Cr# (100Cr/Cr+Al) of unmixed spinel, i.e., unmixing is the result of 467

element redistribution within the initial spinel. Regardless of texture, the 468

unmixing mechanism is the same; the differences in shape and size of exsolutions 469

may reflect variable rates of cooling, or local differences in the chemistry of the 470

initial spinel. In all of the unmixed spinel crystals and regardless of pattern of 471

unmixing, Type II spinel is the exsolving phase. When Type II spinel exsolves 472

along crystallographic directions, it forms a network in Type I spinel (Figs 4, 5); 473

chemical separation is more efficient (Type II crystallographically oriented spinel 474

contains lower Al, Cr and Mg and higher Fe contents relative to the irregularly 475

exsolved, Table 2 and Fig. 8). The crystallographically oriented unmixing is 476

characteristic of slow cooling (Buddington and Lindsley, 1964). The enrichment 477

of Type II crystallographically oriented spinel in Ti (up to 3.6 wt.% TiO2) may 478

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suggest that this type starts at high temperature (Buddington and Lindsley, 1964) 479

in response to an increase in oxygen activity, as the solvus is intersected on 480

cooling in a way analogous to ulvöspinel exsolution from magnetite. The solvus 481

lines of Sack and Ghiroso (2001) 482

Irregular Type II blebs have been initiated by heterogeneous nucleation at 483

the outer margins of spinel grains leading to the formation of external granules 484

(Fig. 4). The restriction of the irregular unmixing to the margins and along cracks, 485

suggests that this type is later and occurs at lower temperature and possibly higher 486

oxidation state. The enlargement of crystallographically oriented exsolutions at 487

the margin of spinel is in support of this inference. The concentration of Type II 488

blebs at the edge and the existence of Type I clear haloes devoid of such blebs 489

(Fig. 5) may indicate that the blebs migrate to the margins after exsolution. 490

Accordingly, complete separation is considered as an advanced stage of irregular 491

unmixing. As the exsolutions will maintain their outline after their formation, the 492

irregular blebs and complete separation must have formed under high fluid 493

activity to allow large velocity of migration and large surface tension. Complete 494

separation of Type I and II spinels is in response of long-life unmixing processes. 495

This may explain the absence of the crystallographically oriented exsolutions in 496

the AH spinels. 497

The miscibility gaps in spinel prism were comprehensively discussed in 498

Sack and Ghiorso (1991) based on thermodynamic calculations at different 499

temperatures and different forsterite content of the coexisting olivine. The small 500

miscibility gab in spinel reported by Van der Veen and Maaskant (1995) was 501

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attributed to the high temperature of crystallization (650ºC). Relatively large 502

miscibility gaps of crystallographically oriented exsolutions at GA and GG (Fig. 503

8) may indicates re-equilibration under low temperatures (<650ºC), although 504

unmixing might have started at much higher temperatures (Buddington and 505

Lindsley, 1964). In general, the change from small to large miscibility gabs of 506

unmixed spinels from the studied plutons (Fig. 8) indicates re-equilibration over a 507

wide range of temperatures during cooling. The compositions of unmixed spinel 508

types from the studied areas plot very close to the calculated (Sack and Ghiorso, 509

1991) and suggested (Loferski and Lipin, 1983) solvus lines at 600ºC (Fig. 8). It 510

is, however, difficult to make accurate quantitative estimates of the starting and 511

closing temperatures of unmixing. 512

In conclusion, the chemistry (the total Fe content) and grain size of initial 513

spinel, the oxidation state and the cooling rate are the important factors 514

influencing the type of unmixing and the chemistry of final products. In all cases 515

mentioned above, we suggest that the exsolutions, (small or larger blebs), are of 516

late-magmatic origin, as they show distinct petrographic differences from the low-517

temperature ferrit-chromite that occurs as alteration products of chromite grains in 518

ultramafic rocks (Eales et al., 1988). 519

520

5.2. Petrogenetic significance of spinel in Alaskan-type complexes 521

The composition of primary spinel reflects the degree of partial melting that the 522

mantle experienced while producing the chromium spinel-bearing rock. Various 523

tectonic settings of mafic-ultramafic magmas were successfully characterized by 524

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using the spinel chemistry. Accordingly, spinel has been used as a petrogenetic 525

indicator in many studies, and it was possible to distinguish the tectonic setting of 526

the magma, the degree of partial melting (e.g., Irvine, 1965, 1967; Dick and 527

Bullen, 1984, Arai, 1992, Barnes and Roeder, 2001). The success in using spinel 528

chemistry to estimate the degree of partial melting was based on the selection of 529

primary spinel which did not undergo serious re-equilibration except for Mg-Fe 530

redistribution. Any major modification in the spinel chemistry either by the 531

reaction with the interstitial melt or subsolidus equilibration limits the role of 532

spinel as a petrogenetic indicator. As has been presented above, spinel may form 533

at different evolutionary stages of the melt, thus its composition can not be used 534

to constrain the primary melt composition, even if its chemistry has not been 535

modified. Moreover, the chemistry of unmixed spinel end members is completely 536

different from the initial spinel composition; will give misleading petrogenetic 537

meanings. 538

According to the data presented here (Fig. 7) and by Barnes and Roeder 539

(2001), the diagnostic features of spinels from Alaskan-type complexes are the 540

overall Fe3+-enrichment and presence of both Fe3+#- and Mg#-rich compositions 541

within the same sample. While the overall enrichment of the initial spinel in Fe3+ 542

is an original feature attributed to the oxidized nature of arc magmas (the parent 543

magma of Alaskan-type complexes), the existence of Fe3+- and Mg#-rich spinels 544

is a secondary feature attributed to initial spinel unmixing. Barnes and Roeder 545

(2001) compiled the spinel chemical data from mafic and ultramafic complexes of 546

different geological settings and defined a new field of Alaskan-type complexes 547

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(Fig. 7). The new field of Alaskan-type complexes extends from Fe3+#-rich to 548

Mg#-rich and overlaps that of layered intrusions. The spinel chemical data 549

presented in this work confirm the Alaskan-type field suggested by Barnes and 550

Roeder (2001). It is likely that spinels formed from oxidized magmas, e.g., those 551

from Alaskan-type complexes, are not good candidates for chemical classification 552

of rocks as they usually undergo chemical re-equilibration or form at different 553

stages of melt fractionation and crystallization. 554

555

6. Conclusions 556

Spinel in Alaskan-type complexes is not always an early cumulate phase, 557

its initial composition widely varies according to its place in the paragenetic 558

sequence. The slow cooling of the oxidized parent magmas of Alaskan-type 559

complexes lead to extensive subsolidus equilibration and unmixing of spinel, 560

equivalent to changes happen during metamorphism. Chemical re-equilibration 561

may happen without textural modification of spinel. The type of unmixing is 562

controlled mainly by the chemistry of the initial spinel, cooling rate and oxidation 563

state. The two end members resulting from unmixing during re-equilibration have 564

compositions completely different from the initial spinel, being Al- and Fe3+-rich. 565

This difference in chemistry depends up on the degree of re-equilibration and the 566

chemistry of the host mineral of spinel. The late-magmatic changes in spinel 567

chemistry make it difficult to locate spinel compositions along a spinel-olivine 568

solvus and result in an extended compositional field of Alaskan-type complexes 569

in diagrams using spinel chemistry to define the tectonic settings. The broad range 570

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of compositions defined by spinels from Alaskan-type complexes is the result of 571

various degrees of unmixing and/or formation at diffenet evolutionary stages of 572

melt solidification. In addition to the degree of re-equilibration, the success of 573

spinel as a petrogenetic indicator will depend up on its place in the paragenetic 574

sequence. 575

576

Acknowledgments 577

The analytical work presented in this paper was made during a research visit of 578

the first two authors to the Department of Earth Sciences of Kanazawa University 579

founded by the latter Institute. Karl Ettinger, Institute of Earth Sciences, Karl-580

Franzens University, Graz, Austria is thankd for helping with elements X-ray 581

maps. AH Ahmed and HM Helmy would like to thank Akihiro Tamura for many 582

discussions and help during their stay in Kanazawa. Two anonymous reviewers 583

are thanked for their helpful suggestions that significantly improved the 584

manuscript. 585

586

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735 736

Figure captions 737 738 Figure 1. Location map of the zoned mafic-ultramafic complexes in the Eastern 739

Desert of Egypt; transverse tectonic fractures from Garson and Shalaby (1976). 740

741

Figure 2. Geologic map of Gabbro Akarem complex (Carter, 1975). 742

743

Figure 3. Photomicrograph (A) and backscattered electron images (BSE) (B – D) 744

of spinels and associated silicates. A Euhedral spinel cubes (Spl) included within 745

orthopyroxene (Opx), sample GG234, Genina Gharbia intrusion. B Small spinel 746

clusters (white grains) in large amphibole (Amph) associated with cumulus 747

olivine (Ol), sample GA153, Gabbro Akarem intrusion. C Large intercumulus 748

spinel associated with cumulus olivine and magmatic sulfides (Sul), sample 749

GA155, Gabbro Akarem intrusion. D Small contiguous Mg-Al-spinel (Type I) 750

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and Fe-spinel (Type II) associated with a sulfide grain, all are embedded in 751

slightly serpentinized olivine, sample GA146, Gabbro Akarem intrusion. 752

753

Figure 4. BSE images of unmixed spinel types. A “Cloth texture” of Type II 754

(light gray) in Type I (dark gray) spinel (large grain) and coarse-grained irregular 755

unmixing of Type I and Type II spinels (small spinel grains). The white stars are 756

the 50 µm analysed areas, sample GA161, Gabbro Akarem intrusion. B 757

Crystallographically oriented Type II exsolutions in Type I spinel, note the 758

enlargement of exsolutions towards the margin and the development of irregular 759

unmixing along crack and margins, sample GA155, Gabbro Akarem intrusion. C 760

Irregular unmixing of Type I and Type II spinels, sample GG264, Genina Gharbia 761

intrusion. D Close-up of the white rectangle area in (C). 762

763

Figure 5. BSE image and element (Al, Fe, Cr) distribution maps of an unmixed 764

spinel from Genina Gharbia intrusion, sample GG250, see text for explanation. 765

766

Figure 6. Electron microprobe step scans on two optically homogenous spinels 767

from Genina Gharbia (GG244) and Gabbro Akarem (GA153). 768

769

Figure 7. Fe3+# (100Fe3+/(Fe3+ + Cr + Al)) – Mg# (100Mg/Mg + Fe2+) variation 770

diagram of spinels from Gabbro Akarem (GA), Genina Gharbia (GG) and Abu 771

Hamamid (AH) intrusions. Discriminating fields of Alpine-type complexes, 772

stratiform complexes and South East (SE) Alaskan-type complexes (Irvine, 1967) 773

and Alaskan-type complexes worldwide (Barnes and Roeder, 2001) are presented 774

for comparison. The large red symbols are the estimated initial compositions (see 775

text for explanation). 776

777

Figure 8. Ternary diagram of Cr-Al-Fe3+ atomic ratios of spinels from Gabbro 778

Akarem (GA), Genina Gharbia (GG) and Abu Hamamid (AH) intrusions. The 779

large red symbols are the estimated initial compositions. CS dashed line is a 780

calculated solvus for spinel coexisting with Fo80 olivine at 600ºC (Sack and 781

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Ghiorso, 1991) and SS is a suggested solvus at 600ºC for unmixed spinels from 782

the Red Lodge district (Loferski and Lipin, 1983). 783

784

785

786

787

788

Table captions: 789

790

Table 1. Representative electron microprobe analyses of silicate minerals from 791 Gabbro Akarem, Genina Gharbia and Abu Hamamid mafic-ultramafic 792 intrusions*. 793 794 Table 2. Summary of the chemical composition of spinels from Gabbro Akarem, 795 Genina Gharbia and Abu Hamamid. 796 797 Table 3. Representative electron microprobe analyses of Gabbro Akarem spinels. 798 799 Table 4. Representative electron microprobe analyses of Genina Gharbia spinels. 800 801 Table 5. Representative electron microprobe analyses of Abu Hamamid spinels.802

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Table 1. Representative electron microprobe analyses of silicate minerals from Gabbro Akarem, Genina Gharbia

and Abu Hamamid mafic-ultramafic intrusions*

Oxide wt. % Gabbro Akarem (GA)

Genina Gharbia (GG)

Abu Hamamid (AH)

Ol Opx Cpx Amph Ol Opx Cpx Amph Ol Opx Cpx Amph Sample no. GA155 GA151 GA159 GA143 GG239 GG234 GG223 GG250 AH23 AH27 AH27 AH17

SiO2 40.53 55.17 52.72 43.61 39.10 55.33 52.67 43.73 39.35 55.34 53.39 44.68

TiO2 <0.05 0.18 0.17 1.62 <0.05 <0.05 0.55 0.83 <0.05 0.21 0.75 0.93

Al2O3 <0.05 3.07 3.03 13.32 <0.05 1.62 3.21 12.57 0.02 0.65 2.45 11.81

Cr2O3 <0.05 0.41 0.44 1.22 <0.05 0.19 0.68 1.56 <0.05 <0.05 0.52 0.33

FeO 13.62 9.63 5.83 7.33 16.53 10.53 3.70 6.92 18.15 12.55 5.32 8.45 MnO 0.25 0.19 0.16 0.13 0.23 0.20 0.08 0.19 0.25 0.47 0.06 0.10 MgO 45.41 30.18 16.22 16.33 44.02 30.93 16.89 17.02 41.71 29.91 16.32 17.69 CaO 0.01 1.34 20.47 11.38 0.03 0.57 20.11 11.36 0.09 0.51 20.75 11.49

Na2O <0.05 <0.05 0.39 2.06 <0.05 <0.05 0.67 2.41 <0.05 <0.05 0.33 2.35

K2O <0.05 <0.05 <0.05 0.36 <0.05 <0.05 <0.05 0.46 0.02 <0.05 0.12 0.50

NiO 0.08 <0.05 <0.05 <0.05 0.19 0.06 <0.05 0.05 0.21 <0.05 <0.05 0.07 Total 99.94 100.23 99.45 97.40 100.11 99.48 98.60 97.10 99.84 99.66 100.02 98.40

O 4 3 3 23 4 3 3 23 4 3 3 23 Si 1.011 0.967 0.970 6.283 0.990 0.979 0.968 6.330 1.005 0.987 0.975 6.404 Ti 0.000 0.002 0.002 0.176 0.000 0.000 0.008 0.090 0.000 0.003 0.010 0.100

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Al 0.000 0.063 0.066 2.261 0.000 0.034 0.069 2.144 0.001 0.014 0.053 1.995 Cr 0.000 0.006 0.006 0.139 0.000 0.003 0.010 0.178 0.000 0.000 0.008 0.037

FeO 0.284 0.141 0.090 0.882 0.350 0.156 0.057 0.837 0.388 0.187 0.081 1.013 Mn 0.005 0.003 0.002 0.015 0.005 0.003 0.001 0.023 0.005 0.007 0.001 0.012 Mg 1.687 0.788 0.445 3.504 1.661 0.815 0.462 3.670 1.588 0.795 0.444 3.777 Ca 0.000 0.025 0.403 1.755 0.001 0.011 0.396 1.761 0.002 0.010 0.406 1.764 Na 0.000 0.001 0.014 0.576 0.000 0.001 0.024 0.675 0.000 0.000 0.012 0.653 K 0.001 0.001 0.000 0.067 0.001 0.000 0.001 0.084 0.001 0.000 0.003 0.091

Ni 0.001 0.000 0.000 0.000 0.003 0.001 0.000 0.005 0.004 0.000 0.000 0.007

Mg# 86 85 83 80 83 84 89 81 80 81 85 79 * Ol olivine, Opx orthopyroxene, Cpx clinopyroxene, Amph amphibole, Mg# = 100Mg/(Mg+Fe2+)

803 804 805 806 807 808 809 810 811 812 813 814 815 816

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817 818 819 820 821 822

Table 2. Summary of the chemical composition of spinels from Gabbro Akarem, Genina Gharbia and Abu Hamamid

Spinel type Homogneous

spinel Unmixed spinel

Host mineral Amph Cpx Chl +

Srp Intercumul

us Cryst. oriented Irregular

separation Complete

separation Type I Type II Type I Type II Type I Type II Gabbro Akarem

n = 6 n = 19 n = 8 n = 8 n = 8 n = 7 n = 9 n = 8 n = 7

TiO2 range 0.1 - 0.2 0.1 - 0.5 0.2 - 0.6 0.1 - 0.13 0.3 - 1.0 <0.1 0.3 - 0.7 0.1 - 0.2 0.1 - 0.7

average 0.2 0.4 0.1 0.5 <0.1 0.5 0.1 0.3 Al2O

3

range 40.9 - 44.0

25.1 - 39.6

_ 1.9 - 5.3 31.2 - 36.9

1.1 - 3.6 31.9 - 42.7

2.6 - 10.4 31.2 - 34.1

0.4 - 2.8

average 42.7 31.7 2.8 33.9 2.6 39.1 4.2 33.1 1.7 Cr2O

3

range 16.3 - 18.0

18.9 - 25.8

10.5 - 16.0

20.4 - 22.5

12.5 - 17.9

17.1 - 25.6

11.0 - 17.1

21.2 - 22.4

4.6 - 14.3

average 17.3 22.1 12.2 21.3 14.6 20.3 13.3 21.6 9.5 MgO range 12.0 -

21.4 5.7 - 11.6 _ 1.1 - 1.7 7.9 - 9.9 1.0 - 1.4 8.9 - 12.4 1.2 - 3.9 7.9 - 9.1 0.6 - 1.2

average 12.3 8.6 1.3 8.8 1.2 11.1 1.7 8.6 0.9

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Mg# range 53 - 55 30 - 50 6 - 10 37 - 44 5 - 8 40 - 54 7 - 20 36 - 43 4 - 8

average 54 40 7 40 7 50 19 40 5 Cr# range 20 - 22 24 - 39 67 - 80 27 - 32 73 - 89 21 - 35 48 - 75 30 - 32 76 - 88

average 21 32 75 30 80 26 70 31 80 Fe3+# range 4 - 9 11 - 18 65 - 78 11 - 16 67 - 77 4 - 12 60 - 76 10 - 16 70 - 92

average 5 13 75 13 72 7 70 12 80

Genina Gharbia

n = 7 n = 10 n = 6 n = 4 n = 4 n = 12 n = 7 n = 8 n = 10

TiO2 range 0.1 - 0.12 <0.1 0.8 - 2.4 0.4 - 1.2 1.8 - 3.6 0.1 - 0.5 0.9 - 1.5 0.1 - 0.4 0.3 - 3

average 0.1 <0.1 1.5 0.7 2.4 0.3 1.3 0.25 1.5 Al2O

3

range 32.3 - 44.2

36.2 - 48.7

0.3 - 19.2 20 - 33 3 - 6 23 - 32 8 - 13 24 - 34 1.6 - 7.7

average 36.6 40.9 5.8 27.5 4.5 27.5 10.2 28.3 4.5 Cr2O

3

range 14.4 - 22.3

17.8 - 25.4

17.3 - 27.3

21 - 29 14 - 20 22 - 27 23 - 31 21 - 26 16.1 - 25.6

average 19.4 20.4 20.6 24.3 17.3 24.3 28.5 23.5 21.2 MgO range 8.6 - 12.6 8.6 - 13.3 0.8 - 5.1 6 - 8 1.2 - 1.8 5 - 8 2 - 8 5 - 8 1.1 - 3.0

average 10.3 10.7 1.8 7 1.5 6.2 4 6.5 2.1 Mg# range 39 - 53 38 - 55 1 - 26 32 - 38 7 - 11 25 - 36 14 - 43 26 - 37 2 - 18

average 45 46 8 34 9 28 22 30 10 Cr# range 19 - 31 15 - 32 62 - 90 30 - 49 69 - 78 31 - 45 57 - 69 29 - 42 64 - 87

average 26 25 85 37 73 38 65 36 74

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Fe3+# range 8 - 16 5 - 9 18 - 90 14 - 21 52 - 72 14 - 22 34 - 46 13 - 21 44 - 72

average 13 6 55 18 62 19 39 19 56

Abu Hamamid

n = 4 n = 6 n = 32 n = 8 n = 8 n = 8 n = 9

TiO2 range 0.2 - 0.6 1.0 - 5.2 0.3 - 6.7 0.9 - 1.7 1.1 - 4.9 0.1 - 0.6 0.6 - 3.8

average 0.4 2.7 2.7 1.2 1.9 0.2 2.2 Al2O

3

range 22.8 - 31.7

7.0 - 9.7 1.9 - 13.1 11.7 - 17.7

7.9 - 13.1 10.8 - 21.9

2.8 - 6.9

average 26.5 8.6 6.6 15.9 9.3 15.9 5.5 Cr2O

3

range 17.2 - 21.5

17.9 - 22.6

5.3 - 24.6 23.9 - 30.5

15.6 - 24.6

16.4 - 30.5

16.6 - 22.9

average 19.1 20.2 14.0 28.6 20.8 23.6 18.9 MgO range 3.1 - 6.2 0.8 - 3.2 0.1 - 3.4 3.4 - 6.0 0.5 - 2.6 0.6 - 7.5 0.1 - 1.3

average 4.9 1.9 0.9 5.4 1.3 3.75 0.5 Mg# range 17 - 31 5 - 19 0 - 19 18 - 31 3 - 19 4 - 38 1 - 8

average 25 11 5 28 7 20 3 Cr# range 27 - 39 58 - 63 38 - 84 50 - 59 51 - 64 42 - 56 64 - 80

average 34 61 62 55 60 50 79 Fe3+# range 15 - 26 45 - 52 38 - 89 24 - 40 36 - 51 24 - 51 40 - 65

average 21 48 60 28 45 32 55 Mg# = 100Mg/(Mg+Fe2+), Cr# = 100Cr/(Cr+Al), Fe3+# =

100Fe3+/(Fe3++Al+Cr)

823 824

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825 826 827 828 829 830 831 832 833 834 835 836 837

Table 3. Representative electron microprobe analyses of Gabbro Akarem spinels

Spinle type Unmixed Homogeneous Cryst. Oriented Irregular sep Complete sep. In Cpx In Amph Intercumulu

s Sample no. GA155 GA144 GA146 GA155 GA148 GA141

Oxide wt.% TiO2 0.13 0.66 0.10 0.58 0.09 0.26 0.08 0.05 0.46

Al2O3 33.68 2.77 31.91 2.67 33.18 1.09 29.75 40.93 3.18

Cr2O3 21.06 17.89 25.58 11.68 21.32 6.25 22.67 17.54 12.99

FeO* 23.68 29.36 23.16 29.38 23.55 29.91 25.13 18.54 29.54 Fe2O3* 12.01 47.38 9.97 53.52 11.10 61.32 15.14 8.61 51.78

MnO 0.69 0.67 0.83 0.58 0.38 0.30 0.67 0.40 0.60

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MgO 8.66 1.31 8.86 1.23 8.63 0.80 7.35 12.74 1.23 NiO <0.05 <0.05 <0.05 0.07 0.06 <0.05 0.07 <0.05 0.06

Total 99.90 100.02 100.40 99.71 98.30 99.92 100.86 98.82 99.84

Structural formula based on 4 oxygens

Ti 0.003 0.022 0.002 0.020 0.002 0.010 0.002 0.001 0.016 Al 1.254 0.144 1.182 0.146 1.257 0.063 1.135 1.441 0.172 Cr 0.526 0.625 0.635 0.430 0.542 0.241 0.580 0.414 0.470

Fe2+ 0.625 1.089 0.613 1.124 0.617 1.212 0.690 0.456 1.121

Fe3+ 0.285 1.581 0.237 1.843 0.262 2.236 0.374 0.190 1.768

Mn 0.018 0.025 0.022 0.023 0.010 0.012 0.018 0.010 0.023 Mg 0.407 0.086 0.415 0.085 0.413 0.058 0.354 0.567 0.084 Ni 0.000 0.000 0.000 0.002 0.001 0.000 0.002 0.000 0.002

Fe3+# 14 67 11 76 13 88 18 9 73

Mg# 40 7 40 7 40 5 34 55 7 Cr# 30 81 35 75 30 79 34 22 73

* calculated 838 839 840 841 842 843

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844 845 846 847 848 849 850 851 852 Table 4. Representative 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 < 0.05 0.02 0.62

Al2O3 29.11 3.81 26.65 8.36 27.75 4.81 43.19 39.99 5.26

Cr2O3 24.39 15.22 24.21 23.99 23.28 18.89 19.50 18.22 15.98

FeO* 25.76 28.72 26.01 28.40 26.27 29.35 21.06 20.81 29.13 Fe2O3* 12.50 46.93 16.14 33.57 15.15 42.64 4.67 9.92 46.47

MnO 0.37 0.21 0.38 0.43 0.26 0.32 0.26 0.30 0.65 MgO 6.74 1.48 6.41 2.44 6.32 1.40 11.57 11.39 1.73

NiO < 0.05 < 0.05 0.12 0.21 < 0.05 0.24 < 0.05 < 0.05 0.05

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Total 99.51 98.42 100.08 98.59 99.25 98.89 100.25 100.64 99.88

Structural formula based on 4 oxygens

Ti 0.015 0.068 0.004 0.037 0.005 0.041 0.000 0.000 0.020 Al 1.112 0.199 1.047 0.401 1.087 0.247 1.475 1.403 0.270 Cr 0.625 0.533 0.638 0.772 0.611 0.651 0.446 0.428 0.551

Fe2+ 0.695 1.012 0.727 0.937 0.724 1.034 0.512 0.523 1.057

Fe3+ 0.303 1.488 0.406 0.997 0.376 1.352 0.102 0.224 1.517

Mn 0.010 0.008 0.011 0.015 0.007 0.012 0.006 0.008 0.024 Mg 0.325 0.098 0.318 0.148 0.313 0.091 0.499 0.505 0.112 Ni 0.000 0.000 0.003 0.006 0.000 0.007 0.000 0.000 0.001

Fe3+# 32 9 30 14 30 8 49 49 65

Mg# 15 67 19 46 18 60 5 11 10 Cr# 36 73 38 66 36 73 23 23 67

* calculated

853 854 855 856 857 858 859

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Table 5. Representative electron microprobe analyses of Abu Hamamid spinels

Spinle 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 <0.05 0.15 0.36 0.40 0.56 TiO2 1.03 1.43 0.43 1.67 0.98 0.64 0.47 3.35 0.59

Al2O3 17.68 9.31 21.89 2.84 7.89 22.82 3.74 8.70 13.13 Cr2O3 29.51 22.11 24.07 16.72 20.96 21.53 3.40 14.79 17.05 FeO* 24.67 29.96 25.14 30.31 30.71 26.20 30.50 29.04 30.19

Fe2O3* 18.63 32.11 20.75 44.52 35.92 21.16 60.32 39.55 35.26 MnO 0.23 1.32 0.31 0.72 1.47 0.36 0.33 0.47 0.58 MgO 5.97 1.41 6.31 0.31 0.82 5.64 0.56 1.50 1.86

NiO <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 Total 98.21 98.11 99.17 97.72 98.75 98.49 99.68 97.79 99.21

Structural formula based on 4 oxygens

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Si 0.008 0.008 0.005 0.010 0.000 0.002 0.006 0.007 0.009 Ti 0.013 0.017 0.005 0.021 0.0123 0.008 0.006 0.042 0.007 Al 0.173 0.089 0.215 0.028 0.0774 0.224 0.037 0.085 0.129 Cr 0.194 0.144 0.158 0.110 0.1379 0.142 0.022 0.097 0.112

Fe2+ 0.709 0.981 0.723 1.084 1.0552 0.752 1.200 0.938 0.990 Fe3+ 0.482 0.946 0.537 1.432 1.1106 0.546 2.136 1.150 1.040 Mn 0.003 0.017 0.004 0.010 0.0207 0.005 0.005 0.007 0.008 Mg 0.148 0.034 0.156 0.008 0.0203 0.140 0.014 0.037 0.046 Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Fe3+# 24 44 26 65 50 26 86 56 47 Mg# 31 8 31 2 5 28 3 9 10 Cr# 53 62 42 80 64 39 38 53 47

* calculated 868

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