Characterization of the thermally metamorphosed mantle–crust transition zone of the Neoproterozoic...
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Characterization of the thermally metamorphosed mantle–crust transition zone of the Neoproterozoic ophiolite at Gebel Mudarjaj, south Eastern Desert, Egypt Ahmed Hassan Ahmed a, ⁎, Moustafa E. Gharib a , Shoji Arai b a Geology Department, Faculty of Science, Helwan University, Cairo, Egypt b Department of Earth Sciences, Kanazawa University, Kanazawa, Japan abstract article info Article history: Received 30 November 2011 Accepted 19 February 2012 Available online 27 February 2012 Keywords: MTZ Thermal metamorphism Gebel Mudarjaj Egypt A suite of mantle–crust transition zone (Moho transition zone = MTZ) rocks are exceptionally well exposed in Gebel Mudarjaj area, southeastern desert of Egypt. The MTZ rocks were thermally metamorphosed by younger granitic intrusion, forming mafic–ultramafic hornfels with characteristic metamorphic mineral as- semblages. The MTZ rocks are remarkably thin (30–50 m thick) and are composed mainly of dunites, trocto- lites, gabbroic rocks and pyroxenite masses overlying a basal serpentinized mantle harzburgite section. The Cr# of spinels of the basal serpentinized harzburgites and the MTZ dunites are on average 0.76 and 0.74, re- spectively, which is consistent with the range for arc peridotite spinels. The melt in equilibrium with these MTZ rocks is compositionally similar to boninitic magmas produced by high degrees of partial melting. The basal harzburgites and MTZ dunites have been produced by 19–23% mantle melting, and are compositionally similar to supra-subduction zone (SSZ) peridotites. The mantle melt in equilibrium with pyroxenites was formed after 16–17% partial melting, which subsequently reacted with the lower crustal gabbroic rocks to produce pyroxenites. The occurrence of pyroxenite masses at the crust–mantle boundary suggests a medium- to high-pressure accumulation of pyroxenes in mid- to lower crustal magma chambers. The original MTZ rocks were partially or fully hydrated, prior to the granitic intrusion, during the regional metamorphism, tectonic disruption and emplacement as various fragments of a dismembered ophiolite, to form antigorite- bearing serpentinized mafic–ultramafic rocks. Progressive metamorphic assemblages then overprinted the primary features due to the contact metamorphism of the MTZ rocks. The resultant metamorphic mineral as- semblages are: (1) olivine + anthophyllite + tremolite ± chlorite ± talc (in the basal serpentinites), (2) oliv- ine + enstatite ± chlorite (in the MTZ dunites), and (3) olivine + aluminous spinel (pleonaste) + chlorite + magnetite ± enstatite (in the troctolites). The peak of thermal metamorphism was about 650°–700 °C and b 7 kb, equivalant to the upper amphibolite facies. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Several ophiolite complexes, particularly those of Phanerozoic age, preserve a complete sequence, resembling that of the ocean floor, namely pillow lavas, sheeted dikes, layered gabbros and upper mantle peridotites from the top to the base (Coleman, 1977). Many older ophiolite complexes are severely fragmented and dismembered, and lack one or more of the above-mentioned rock units (Nicolas, 1989). The scarcity of these old ophiolites enhances their importance in un- derstanding geotectonic processes at the end of the Precambrian. The difficulty in studying these old ophiolites arises from their complicat- ed postmagmatic histories including intense serpentinization, region- al deformation and contact metamorphism that have destroyed many of the primary characteristics of the rocks. Most, if not all, of the Pro- terozoic ophiolite complexes have been invaded by later granitic intrusions of various compositions. This leads to thermal prograde/ retrograde metamorphism overprinting the original characteristics of the mafic–ultramafic rocks. One of the scientifically interesting parts of the ophiolite complex is the Moho transition zone (MTZ) that represents a transition between the upper mantle and the lower crust. The MTZ lies between the uppermost mantle peridotites and layered gabbros of the lower crust (Benn et al., 1988; Karson and Collins, 1984). The MTZ consists mainly of various proportions of tec- tonized harzburgite, dunite, troctolite, pyroxenites, gabbro, and wehrlite (e.g., Akizawa and Arai, 2009; Boudier and Nicolas, 1995; Jousselin and Nicolas, 2000; Melcher and Meisel, 2004; Tamura and Arai, 2006). Plagioclase-rich leucocratic rocks (plagiogranites) can also be found in the MTZ of many ophiolitic complexes (e.g., Amri et al., 1996; Koepke et al., 2007; Rao et al., 2004; Yaliniz et al., 2000). In the case of primary MTZ, mantle dunite is gradually changed to layered gabbro by increasing the frequency of gabbro sills (gabbro- in-dunite relation), that was formed at a spreading center. However, a secondary MTZ is formed when the dunite (or wehrlite) subsequently intruded into the base of layered gabbro (dunite-in-gabbro relation), Lithos 142-143 (2012) 67–83 ⁎ Corresponding author at: King Abdulaziz University, Faculty of Earth Sciences, Jed- dah, Saudi Arabia. Tel.: +966 053 058 7113. E-mail address: [email protected] (A.H. Ahmed). 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.02.014 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos
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Lithos 142-143 (2012) 67–83
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Characterization of the thermally metamorphosed mantle–crust transition zone ofthe Neoproterozoic ophiolite at Gebel Mudarjaj, south Eastern Desert, Egypt
Ahmed Hassan Ahmed a,⁎, Moustafa E. Gharib a, Shoji Arai b
a Geology Department, Faculty of Science, Helwan University, Cairo, Egyptb Department of Earth Sciences, Kanazawa University, Kanazawa, Japan
⁎ Corresponding author at: King Abdulaziz Universitydah, Saudi Arabia. Tel.: +966 053 058 7113.
E-mail address: [email protected] (A.H. Ahmed)
0024-4937/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.lithos.2012.02.014
a b s t r a c t
a r t i c l e i n f oArticle history:Received 30 November 2011Accepted 19 February 2012Available online 27 February 2012
Keywords:MTZThermal metamorphismGebel MudarjajEgypt
A suite of mantle–crust transition zone (Moho transition zone = MTZ) rocks are exceptionally well exposedin Gebel Mudarjaj area, southeastern desert of Egypt. The MTZ rocks were thermally metamorphosed byyounger granitic intrusion, forming mafic–ultramafic hornfels with characteristic metamorphic mineral as-semblages. The MTZ rocks are remarkably thin (30–50 m thick) and are composed mainly of dunites, trocto-lites, gabbroic rocks and pyroxenite masses overlying a basal serpentinized mantle harzburgite section. TheCr# of spinels of the basal serpentinized harzburgites and the MTZ dunites are on average 0.76 and 0.74, re-spectively, which is consistent with the range for arc peridotite spinels. The melt in equilibrium with theseMTZ rocks is compositionally similar to boninitic magmas produced by high degrees of partial melting. Thebasal harzburgites and MTZ dunites have been produced by 19–23% mantle melting, and are compositionallysimilar to supra-subduction zone (SSZ) peridotites. The mantle melt in equilibrium with pyroxenites wasformed after 16–17% partial melting, which subsequently reacted with the lower crustal gabbroic rocks toproduce pyroxenites. The occurrence of pyroxenite masses at the crust–mantle boundary suggests amedium- to high-pressure accumulation of pyroxenes in mid- to lower crustal magma chambers. The originalMTZ rocks were partially or fully hydrated, prior to the granitic intrusion, during the regional metamorphism,tectonic disruption and emplacement as various fragments of a dismembered ophiolite, to form antigorite-bearing serpentinized mafic–ultramafic rocks. Progressive metamorphic assemblages then overprinted theprimary features due to the contact metamorphism of the MTZ rocks. The resultant metamorphic mineral as-semblages are: (1) olivine+anthophyllite+tremolite±chlorite±talc (in the basal serpentinites), (2) oliv-ine+enstatite±chlorite (in the MTZ dunites), and (3) olivine+aluminous spinel (pleonaste)+chlorite+magnetite±enstatite (in the troctolites). The peak of thermal metamorphism was about 650°–700 °Cand b7 kb, equivalant to the upper amphibolite facies.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Several ophiolite complexes, particularly those of Phanerozoic age,preserve a complete sequence, resembling that of the ocean floor,namely pillow lavas, sheeted dikes, layered gabbros and upper mantleperidotites from the top to the base (Coleman, 1977). Many olderophiolite complexes are severely fragmented and dismembered, andlack one or more of the above-mentioned rock units (Nicolas, 1989).The scarcity of these old ophiolites enhances their importance in un-derstanding geotectonic processes at the end of the Precambrian. Thedifficulty in studying these old ophiolites arises from their complicat-ed postmagmatic histories including intense serpentinization, region-al deformation and contact metamorphism that have destroyed manyof the primary characteristics of the rocks. Most, if not all, of the Pro-terozoic ophiolite complexes have been invaded by later granitic
, Faculty of Earth Sciences, Jed-
.
rights reserved.
intrusions of various compositions. This leads to thermal prograde/retrograde metamorphism overprinting the original characteristicsof the mafic–ultramafic rocks. One of the scientifically interestingparts of the ophiolite complex is the Moho transition zone (MTZ)that represents a transition between the upper mantle and thelower crust. The MTZ lies between the uppermost mantle peridotitesand layered gabbros of the lower crust (Benn et al., 1988; Karson andCollins, 1984). The MTZ consists mainly of various proportions of tec-tonized harzburgite, dunite, troctolite, pyroxenites, gabbro, andwehrlite (e.g., Akizawa and Arai, 2009; Boudier and Nicolas, 1995;Jousselin and Nicolas, 2000; Melcher and Meisel, 2004; Tamura andArai, 2006). Plagioclase-rich leucocratic rocks (plagiogranites) canalso be found in the MTZ of many ophiolitic complexes (e.g., Amriet al., 1996; Koepke et al., 2007; Rao et al., 2004; Yaliniz et al.,2000). In the case of primaryMTZ, mantle dunite is gradually changedto layered gabbro by increasing the frequency of gabbro sills (gabbro-in-dunite relation), that was formed at a spreading center. However, asecondary MTZ is formedwhen the dunite (or wehrlite) subsequentlyintruded into the base of layered gabbro (dunite-in-gabbro relation),
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that was possibly formed at an off-ridge or a supra-subduction settingduring obduction (e.g., Gahlan et al., 2012; Koepke et al., 2009; Kogaet al., 2001; Uesugi, 2004).
There have been few detailed petrological studies on the Protero-zoic MTZ (cf. Gahlan et al., 2012). However, in this paper we report acomplete association of MTZ rocks, including upper mantle serpenti-nized peridotites, dunites, troctolite/gabbroic rocks, orthopyroxeniteand plagiogranite in a dismembered Neoproterozoic ophiolite com-plex at Gebel Mudarjaj area, south Eastern Desert of Egypt. Chromianspinel is the only primary magmatic mineral that survived metamor-phism, and is used along with detailed petrological characteristics toassess the tectonic setting and parental melt composition of theMTZ rocks. Although the rocks were thermally metamorphosed tovarious extents, we have been able to distinguish primary igneous
Fig. 1. Simplified geological map of the Central, and part of the southern, Eastern Desert ofRed Sea fracture and the locations of the Neoproterozoic ophiolite in the Arabian–Nubian S
characteristics from the metamorphic overprint by using relict tex-tures and mineral compositions. This study disclosed a good examplefor the complicated geologic processes experienced by the MTZ of aProterozoic ophiolite.
2. Regional geology
The Arabian–Nubian Shield of Neoproterozoic age (Patchett andChase, 2002) represents the largest section of juvenile (= mantle de-rived) continental crust on the Earth. It contains remnants of Neopro-terozoic basement in the NE Africa and Arabia, covering huge areas ofseveral countries in the Nubian Shield (Egypt, Sudan, Eritrea and Ethi-opia) and in the Arabian Shield (Jordan, Saudi Arabia and Yemen).Typical ophiolite sequences have been described from many parts of
Egypt (taken from the 1:1 m geological map of Egypt, El Ramly, 1972). Insets show thehield. Location of the study area is marked by the black rectangle.
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the Arabian–Nubian Shield including the Eastern Desert of Egypt (e.g.,Abd El-Rahman et al., 2009a,b; Ahmed et al., 2001; Farahat et al.,2010), Sudan (Hussein et al., 2004), and Saudi Arabia (e.g., Johnsonand Kattan, 2001; Stern et al., 2004).
The Eastern Desert of Egypt represents the main part of the north-western portion of the Arabian–Nubian Shield (Fig. 1). It can be sub-divided into three parts; the northern, central and southern EasternDesert. In the northern part and Sinai, late orogenic granitic plutonsare predominant (e.g. Stern, 1985; Stern and Hedge, 1985), whereasophiolitic and related rocks, if any, are minor remnants (e.g., Clark,1985; Shimron, 1984). Towards the south, the central Eastern Desertis solely built up of ophiolite, ophiolitic mélange and their associatedcalc-alkaline island arc rocks, with subordinate clastic molasses-typesediments and late bimodal volcanics and intrusive granitoids(El Ramly et al., 1993) (Fig. 1). Furthermore, post-tectonic granite in-trusions, which define the end of the Pan-African event, are locallypresent in this central part (e.g., El Ramly and Hermina, 1978; Rieset al., 1983). Further to the south, the southern Eastern Desert is com-posed mainly of medium-grade gneisses (basal units) including con-tinental/shelf facies metasediments exposed in some tectonicwindows (e.g., El Ramly et al., 1984; El-Gaby et al., 1988; Shackletonet al., 1980).
3. Local geology and field observations
The Gebel Mudarjaj area is very close to the Wadi Hafafit culmina-tion, in the northwest of Migif-Hafafit area. It comprises three mainrock assemblages that overlie tectonically the older gneisses (e.g., ElRamly et al., 1993; El-Gharbawi, 1988). They are, from youngest to old-est, (Fig. 2): (1) late Pan-African granitoids and a group of Phanerozoicintrusives, including trachyte dykes, minor acidic intrusives, and lategranitoids, (2) a calc-alkaline assemblage, includingmolasse-type sedi-ments, Dokhan volcanics, granodiorite, metagabbros, metapyroclastics,and meta-andesites, and (3) an ophiolitic assemblage, including meta-basalts–metadolerites, tholeiitic metagabbros, coarse- and medium-grained amphibolites, and serpentinites–talc-carbonate rocks (Fig. 2).The serpentinized ultramafic rocks of Gebel Mudarjaj and surrounding
Fig. 2. Simplified geological map of Gebel Mudarjaj area (modified from a
areas are found as disrupted masses and are considered to representthe oldest rock units in the area (El Ramly et al., 1993). These arefollowed by similar, but less altered, varieties at Wadi El Lawi andWadi Ghadir areas, just east of the study area (e.g., Abd El-Rahman etal., 2009b; Ahmed et al., 2001).
The ophiolitic serpentinized mafic–ultramafic associations inGebel Mudarjaj area form the extensive ridges of Gebel Mudarjaj ElAswad (Fig. 2), where the serpentinites form an elongated ridge strik-ing roughly E–W with an average width of about 900 m and extend~10 km eastward as a discontinuous chain. The pink biotite graniteof Gebel Mudarjaj El Ahmar occupies the northwestern side of thestudy area, and intrudes northwest below the serpentinized mafic–ultramafic rocks (Fig. 2). It does not show any deformation featuresand probably represents a late facies among the Pan-African youngergranitoid intrusions (e.g., El Ramly et al., 1993). The contact betweenthe pink biotite granite and overlying serpentinized mafic–ultramaficassociations is very sharp and some serpentinite and amphibolite xe-noliths have been found within the granite.
Coarse- to medium-grained amphibolite is distributed over a dis-tance of 4 km from the northern side of Gebel Mudarjaj El Aswad to-wards the NE and S–SE sides (Fig. 2). Amphibolite of the study area ishighly sheared when it is in contact with the pink biotite granite in-trusion (Fig. 3-1) and consists mainly of hornblende and plagioclasewith subordinate epidote, chlorite, biotite and garnet (El-Gharbawi,1988). Basic metavolcanics outcrop to the east and form the southerncontinuation of amphibolite (Fig. 2).
Due to a series of waterfalls in the Gebel Mudarjaj El Aswad area acomplete ophiolitic mafic–ultramafic sequence exposed. This se-quence is represented by several rock units starting from the highlyserpentinized peridotites (serpentinites) as the basal ultramaficunit. This is overlain by a zone of dunitic harzburgite and massive du-nite, and then a layered zone of dunite–gabbro–troctolite which hostsirregular bodies and dike-like masses of pyroxenite rock, before amassive pegmatitic gabbro becomes predominant at the top of thesection. Abundant plagiogranite lenses and dikes are also present inthe gabbroic sequence. The rock sequence in the study area is similarto a typical MTZ in many ophiolites (e.g., Nicolas, 1989; Boudier et al.,
1:100,000 geological map of Wadi Hafafit area, El Ramly et al., 1993).
Fig. 3. Idealized lithologic column of the studiedMTZ rocks with field photographs of various rock types of Gebel Mudarjaj El Aswad. (1) Sharp contact between the pink biotite gran-ite and the overlying serpentinites. (2) Sharp contact of intrusive pink biotite granite and sheared amphibolites. (3) Gradual change from serpentinized harzburgite (Hz) to duniticharzburgite within the massive Moho transition zone (MTZ) dunites. (4) Hour-glass (mesh) texture of massive MTZ dunite overlying dunitic harzburgite. (5) Interlayering of MTZrocks; dunite (D), dunitic Hz, gabbro-troctolite (Gb-troctolite) and gabbro (Gb). (6) Orthopyroxenite (Opxenite) mass in the lower part of crustal layered gabbro. (7) Irregular pla-giogranite mass in the upper part of layered gabbro. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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1989). It is also identical to the rocks drilled from the Hess Deep, EastPacific Rise, which are possibly representative of the Pacific MTZ (Araiand Matsukage, 1996, 1998; Hebert et al., 1983).
The exposed basal ultramafic unit of Gebel Mudarjaj El Aswad,about 150 m thick, is extensively serpentinized and/or highlysheared. It displays a massive fine- to medium-grained appearanceand grayish to greenish colors away from the contact zone, while atthe contact zone with the granite intrusion it appears highly sheared,schistose, and grayish or yellowish brown in color (Fig. 3-1). There isa gradual change from the basal serpentinized harzburgite to a zoneof dunitic harzburgite followed by massive dunites. This zone isabout 30 m thick, starting with schlieren-like dunite impregnationwithin harzburgite (Fig. 3-3), which varies upward to massive MTZdunites with black network veinlets of serpentine/magnetite andiron staining (Fig. 3-4).
Upward from the MTZ dunites, a ~15 m thick zone composed ofgabbroic rocks (pegmatitic gabbros and troctolite thin layers) is wellexposed. This zone is further overlain by a much thicker zone(~250 m thick) of gabbroic rocks, locally layered at the bottom andpoorly layered at the top (Fig. 3-5). The gabbroic rocks, alternatingwith troctolite and dunite layers, are hornblende-rich and sometimes
Fig. 4. Photomicrographs (A–C, E–H) and a thin section photograph (D) of the serpentinizedoxene (Opx) in the basal serpentinized harzburgite. Crossed-polarized light (CP). (B) Needleserpentinized harzburgite. CP. (C) Small homogeneous metamorphic olivine (Ol) replacing ashowing the hour-glass (mesh) texture of polygonal granular orthopyroxene and olivine cro(E) Granular euhedral orthopyroxene (Opx) crystals associated with elongated aggregates otial to Opx and olivine. CP. (F) Polygonal granular orthopyroxene (Opx) hosting euhedral chlight (PL). (G) Anhedral grains showing relics of primary chromian spinel (Cr-Sp) and magEuhedral grains of primary chromian spinel (Cr-Sp) and its ferritchromite-magnetite (Fchr
display a heterogeneous pegmatitic appearance. In the upper half ofthis MTZ-like sequence, several lensoidal bodies of dark green tograyish green coarse-grained pyroxenites are randomly scatteredwithin the gabbroic rocks (Fig. 3-6). These pyroxenites are composedof coarse interlocking grains, protruding on the weathered surface.The pyroxenite bodies are several meters across, and the pyroxenecrystals reach up to few centimeters in size. Few lensoidal, or some-times dike-like, bodies of leucocratic plagioclase-rich rocks (plagio-granites) were also recognized (Fig. 3-7) within the layered gabbrosslightly higher in the sequence than pyroxenites. They vary in sizefrom 2 m to less than 20 m in length and about 3 m in width. The pla-giogranite is fine to medium-grained and composed mainly of plagio-clase, quartz and few mafic minerals like amphibole and biotite.
4. Petrography of the MTZ rocks
4.1. General remarks
The mineral assemblages of the unmetamorphosed and thermallymetamorphosedmafic–ultramafic rocks can be divided into three cat-egories: (1) primary minerals, including orthopyroxene (in
harzburgites and MTZ dunites of Gebel Mudarjaj area. (A) Pseudomorph of orthopyr--like asbestiform anthophyllite (Anth) replacing antigorite serpentine (Ant) of the basalntigorite serpentine in the basal serpentinized harzburgite. CP. (D) Massive MTZ dunitess-cut by thin black veinlets of magnetite. Euhedral small spinel grains are also shown.f small granular olivine in MTZ dunite. Serpentine and chlorite are also shown intersti-romian spinel (Sp) and cross-cut by chrysotile filaments of MTZ dunite. Plain-polarizednetite alteration (Mg) in the basal serpentinized harzburgite. Reflected light (RL). (H)-Mg) alteration rims in serpentinized MTZ dunite. RL.
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pyroxenites) and chromian spinel (in serpentinized harzburgites,MTZ dunites and pyroxenites), (2) metamorphic minerals formedduring the contact metamorphism, including olivine, tremolite, talc,chlorite, anthophyllite, orthopyroxene, antigorite and Al-spinel
(pleonaste), and (3) secondary (low-temperature) minerals pro-duced by alteration of primary and metamorphic minerals, either be-fore or after the contact metamorphism, including ferritchromite, Cr-rich magnetite, talc, chlorite, low-temperature serpentine and few
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carbonate minerals. The studied MTZ rocks can be divided into fourunits starting from the base upward: (1) basal meta-serpentinites,(2) MTZ meta-dunites, (3) gabbroic rocks (meta-gabbro/meta-trocto-lite), and (4) meta-pyroxenites.
4.2. Basal meta-serpentinites
The basal unit comprises fully serpentinized harzburgite, wherealmost all primary silicate minerals (i.e., olivine and pyroxenes)have been converted to secondary and metamorphic minerals (e.g.,serpentine, olivine, tremolite, anthophyllite, talc, and chlorite). Chro-mian spinel is the only primary mineral that survives alteration andmetamorphism. Bastites, pseudomorphs after orthopyroxene, arestill preserved in serpentinites away from the contact zone(Fig. 4A), and can be used to testify the harzburgite protolith. Themetamorphic minerals are represented mainly by olivine, tremolite,and anthophyllite, and rarely by flaky chlorite. They show texturalfeatures (irregular anhedral habits, absence of precursor mineral rel-icts and chemical zoning, poikiloblastic and intergrown associations)similar to those observed in other thermally metamorphosed ultra-mafic rocks (e.g., Arai, 1975; Melcher and Meisel, 2004; Springer,1974; Trommsdorff and Evans, 1972). Fine- to medium-grainedplaty antigorite with its characteristic plumose texture is the mainserpentine mineral of the serpentinized harzburgite away from thecontact with the granitoid intrusion (Fig. 4A and B). Anthophylliteas long thin needles (Fig. 4B), is very common near the contactzone, forming more than 5% of the serpentinites. Tremolite also exists(up to 5–7 vol.%) as acicular bundles (about 3 mm long) within anti-gorite serpentine groundmass near the contact zone. Minute aggre-gates of recrystallized olivine, ranging in size between 10 and100 μm, are also common within the antigorite groundmass, andsometimes show parallel elongation resembling a spinifex texture(Fig. 4C). Olivine grains in meta-serpentinites are equigranular,rounded, rarely cracked, and homogeneous in appearance. Chromianspinel is an accessory phase in the basal meta-serpentinites; it is com-monly anhedral to rounded (Fig. 4C) and sometimes vermicular inshape (Fig. 4C) with uniformly brown color under the microscope.Chromian spinel grains in meta-serpentinites are usually rimmed, orsometimes completely replaced, by ferritchromite and/or Cr-richmagnetite.
4.3. Transition zone meta-dunites
The MTZ meta-dunites are fully recrystallized to polygonal granu-lar orthopyroxene and fine olivine crystals (Fig. 4E). Antigorite andrare chrysotile filaments are the main serpentine minerals interstitialto granular orthopyroxene and olivine. The smooth weathered sur-face and the network texture (see Figs. 3-3, 4) with euhedral grainsof chromian spinel confirm the dunite protolith. Recrystallized ortho-pyroxene, consisting of more than 60 vol.% of the rock, occurs ascoarse granular porphyroblasts, 0.5–3 mm across, occasionallyenclosing euhedral chromian spinel grains (Fig. 4F). Chromian spinelis muchmore common (euhedral to subhedral and up to 5 vol.%) thanin the basal meta-serpentinites (Fig. 4F, G and H). The chromian spi-nel grains are usually fractured and sometimes highly deformed andfragmented to parallel rod-like parts of ferritchromite or Cr-rich mag-netite associated with fresh cores (Fig. 4G). The chromian spinel, bothin meta-serpentinites and MTZmeta-dunites, commonly displays dis-tinct compositional zoning with a darker grey core and brighter rimunder reflected light (Fig. 4H).
4.4. Gabbroic rocks
The MTZ dunites gradually change to highly altered, locally lay-ered, coarse-grained heterogeneous hornblende-rich gabbros(Fig. 5A). They are intercalated with dark green to black colored
layers (Figs. 3-5, 5B), which are interpreted as green spinel (pleo-naste)-rich meta-troctolites. Yellow dunitic patches are also commonwithin the heterogeneous leucocratic plagioclase- and hornblende-rich parts (Fig. 5A, B). Hornblende is very coarse and shows two dis-tinct sets of cleavage (Fig. 5C) and pale yellow to dark green pleoch-roism. The leucocratic parts of meta-gabbroic layers have a dustyappearance due to alteration of euhedral to subhedral plagioclasecrystals, which are sometimes poikilitically enclosed within horn-blende and clinopyroxene in thin section. Chlorite is the most com-mon alteration product associated with plagioclase and hornblende.Small rounded chromian spinel and skeletal ilmenite grains are alsocommon within, and interstitial to, hornblende and clinopyroxenecrystals.
Macroscopically, the meta-troctolites form very distinctive het-erogeneous dark green to black colored layers sharply intercalatedwith meta-gabbros (Fig. 5B). The primary olivine and plagioclaseare completely altered to serpentine, chlorite, and Al-spinel (pleo-naste), which gives the rock a stippled appearance of green, brown,yellow, and black spots on a pale green groundmass (Fig. 5B). Micro-scopically, the meta-troctolites are characterized by enrichment ofsmall (b100 μm) globular green spinel (pleonaste) which constitutesabout 30 vol.% of the rock (Fig. 5D). Pleonaste grains commonly formmillimeter-sized aggregates with a skeletal or vermicular shape(Fig. 5E) and are surrounded by reaction corona of chlorite (Fig. 5F)between olivine and plagioclase. Metamorphic olivine in the meta-troctolites is homogeneous, rounded and small in size, up to 100 μmacross, but rarely found as coarse crystals (Fig. 5F). These olivinesare sometimes aligned parallel to each other to form a spinifex-liketexture. Kinked chlorite is very common in the fine-grained part ofthe meta-troctolites. Rare small orthopyroxene and amphibole crys-tals are found in the plagioclase-poor part of the meta-troctolites.Very rare grains of small-altered rounded chromian spinel and ver-micular ilmenite also occur in the mafic parts of the meta-troctolites.
4.5. Meta-pyroxenites
The meta-pyroxenites are predominantly composed of interlock-ing coarse pyroxene pseudomorphs, with small relics of orthopyrox-ene (Fig. 5G) being the only primary silicate mineral. Their mostcommon metamorphic mineral assemblage is tremolite+talc+antigorite±anthophyllite±chlorite (Fig. 5G). The pseudomorphsafter pyroxene, which are now mainly tremolite, reach up to 5 cmlong and several millimeters wide, show an asbestiform shape(Fig. 5H). Short tremolite flakes (b0.5 mm) together with antigorite,entirely replace the former orthopyroxene crystals (Fig. 5G). Talcsometimes is found along the grain boundaries of former pyroxenecrystals (Fig. 5H). Small chromian spinel grains (b0.5 mm) with sub-hedral to rounded shape are included in, or interstitial to, relics oflarge pyroxene crystals.
5. Geochemistry
5.1. Analytical methods
Representative samples from the Gebel Mudarjaj El Aswad area,including basal serpentinized harzburgites and MTZ dunites (23 sam-ples), meta-gabbros (19 samples), troctolites (3 samples), and pyrox-enites (3 samples) have been analyzed for major-elementcomposition, and some further selected samples were analyzed fortrace and rare-earth elements (REE). Each analysis was performedon a 0.2 g sample by inductively coupled plasma-mass spectrometry(ICP-MS) following a LiBO2 fusion and dilute nitric acid digestion.All analyses were carried out in ACME analytical laboratories (Cana-da). Duplicate samples, reference materials and blank samples werealso analyzed to assess the accuracy and precision of the data.
Fig. 5. Thin section photographs (A, B) and photomicrographs (C–H) of the MTZ rocks of Gebel Mudarjaj area. (A) Pegmatitic meta-gabbro alternating with troctolite and dunite. (B)Contact between pegmatitic meta-gabbro and troctolite. The latter is made up of numerous globules of Al-spinel (pleonaste). (C) Anhedral hornblende (Hb) with intercumulus pla-gioclase (Plg) and clinopyroxene relics (Cpx) of the pegmatitic meta-gabbros. CP. (D) Numerous globular green spinel (pleonaste) associated with metamorphic olivine and chloriteof the troctolite layer. PL. (E) Aggregates of globular green pleonaste (Ple) and granular olivine (Ol) replacing chlorite of the troctolite layer. PL. (F) The same as (E). CP. Note smallolivine grains interstitial to green spinel aggregates and replacing chlorite. (G) Relics of orthopyroxene (Opx) crystals associated with talc and tremolite of the pyroxenites. CP. (H)Interlocking pseudomorphs of orthopyroxene (now tremolite) replaced by talc (Tc) along the grain boundaries of pyroxenites. CN. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)
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Table 1Representative whole rock analysis (major and trace elements) of thermally metamorphosed and non-metamorphosed mafic–ultramafic rock samples from Gebel Mudarjaj area, south Eastern Desert, Egypt. n.d. = not detected.
Not thermally metamorphosed (Far from the contact zone) Thermally metamorphosed
Serpentinites Meta-gabbros Meta-serpentinites MTZ dunites Meta-gabbros Meta-troctolites Meta-pyroxenites
S.No. 1_4 1_9 2_3 1_5 2_8 2_13 3M 5M 6M 9M 10M 6M 19M 18M 7M 8M 11M 20M 21M 22M
SiO2 41.05 39.25 41.1 46.15 49.32 50.94 48.98 44.51 45.76 37.84 38.25 47.27 47.51 47.3 30.85 33.76 34.67 52.63 52.96 53.48TiO2 0.15 n.d. 0.02 2.75 2.01 0.62 0.01 0.06 0.04 0.11 0.12 0.59 0.3 0.05 0.25 0.12 0.11 0.09 0.08 0.07Al2O3 0.49 0.55 0.45 17.35 16.72 19.35 9.95 8.32 9.58 6.18 5.86 9.53 14.68 20.45 17.13 13.91 12.68 5.17 4.82 4.41Fe2O3 6.25 8.09 7.25 9.95 8.21 6.02 9.07 9.05 9.36 10.09 11.02 13.99 10.92 5.84 13.87 13.45 12.22 12.27 12.53 12.03MnO 0.17 0.11 0.06 0.17 0.15 0.12 0.14 0.15 0.16 0.15 0.17 0.21 0.15 0.11 0.19 0.15 0.2 0.19 0.17 0.16MgO 37.19 38.24 38.51 6.45 7.52 6.12 24.49 27.88 25.85 32.86 33.03 14.18 9.98 7.77 26.36 28.01 28.84 23.71 24.27 24.43CaO 0.76 0.03 n.d. 10.18 6.84 8.92 2.44 1.36 1.83 0.21 0.24 9.7 11.37 12.5 0.37 1.01 1.86 2.46 2.01 1.97Na2O 0.04 0.05 n.d. 3.37 4.71 3.71 0.1 0.07 0.19 0.01 0.01 2.03 2.1 2.04 0.03 0.05 0.07 0.2 0.16 0.17K2O 0.03 n.d. n.d. 0.68 1.3 0.96 0.61 0.61 0.57 0.01 0.02 0.15 0.38 0.76 0.02 0.02 0.02 0.09 0.07 0.04P2O5 n.d. n.d. n.d. 0.9 0.53 0.13 0.01 0.02 0.02 0.01 0.01 0.02 0.02 0.01 n.d. 0.01 n.d. 0.01 n.d. n.d.L.O.I 12.65 13.79 11.73 1.78 1.85 2.34 3.1 7.8 6.26 11.6 10.95 1.9 2.5 2.8 10.3 9 8.6 2.5 2.3 2.5Total 98.78 100.1 99.12 99.73 99.16 99.23 98.9 99.83 99.62 99.07 99.68 99.57 99.9 99.63 99.37 99.51 99.27 99.32 99.32 99.26Ni 2301 2750 2565 375 45 250 882 801 820 720 675 154 112 69 332 311 289 313 308 303Co 112 115 110 95 78 83 90 104 94 118 115 62 46 30 84 82 80 70 73 75Ba 5 7 4 224 301 225 27 18 22 8 11 20 65 110 25 18 11 21 13 5Sc 20 13 17 150 165 115 7 13 11 18 15 93 66 38 38 45 51 31 31 31Ga 6 9 7 17 19 20 14 9 11 5 7 12 13 14 11 10 9 6 6 6Rb 2 5 4 15 28 23 17 9 13 1 2 3 10 18 1 1 1 3 2 2Sr 7 5 7 775 847 498 120 64 85 8 10 74 594 1114 14 22 31 13 10 7V 47 55 25 250 165 123 66 65 60 64 60 463 279 95 393 289 184 99 95 91La 0.20 0.10 0.10 8.07 3.01 0.76 0.20 0.23 0.20 0.20 0.40 1.40 0.68 0.40 0.40 0.40 0.30 0.30 0.30 0.20Ce 0.20 0.10 0.10 10.15 9.87 1.33 0.30 0.29 0.30 0.20 0.50 6.00 3.01 1.00 0.60 0.60 0.60 0.70 0.70 0.60Pr 0.03 0.02 0.02 2.02 2.01 0.31 0.06 0.06 0.04 0.04 0.05 1.27 0.46 0.13 0.12 0.11 0.08 0.12 0.11 0.09Nd 0.30 0.30 0.30 3.55 9.55 1.98 b0.3 0.30 0.30 b0.3 0.30 8.20 4.91 0.50 0.60 0.50 0.30 0.50 0.50 0.40Sm 0.05 0.05 0.05 1.21 3.02 0.79 0.12 0.10 0.05 0.05 0.05 2.66 1.72 0.18 0.18 0.15 0.08 0.22 0.22 0.21Eu 0.02 0.02 0.02 0.51 0.86 0.47 0.02 0.02 0.02 b0.02 0.02 0.77 0.60 0.13 b0.02 0.04 0.04 0.06 0.06 0.05Gd 0.08 0.05 0.05 1.53 2.35 1.32 0.16 0.14 0.08 0.09 0.08 3.29 1.63 0.26 0.32 0.25 0.13 0.31 0.29 0.25Tb 0.01 0.01 0.01 0.21 0.39 0.22 0.03 0.02 0.01 0.01 0.01 0.49 0.29 0.04 0.05 0.04 0.02 0.04 0.04 0.04Dy 0.05 0.05 0.05 1.35 2.43 1.68 0.21 0.17 0.05 0.09 0.06 2.78 1.67 0.26 0.27 0.23 0.13 0.30 0.28 0.22Ho 0.02 0.02 0.02 0.31 0.43 0.39 0.06 0.04 0.02 0.03 0.02 0.56 0.36 0.05 0.06 0.06 0.05 0.06 0.06 0.05Er 0.03 0.03 0.03 1.04 1.25 1.03 0.20 0.16 0.03 0.07 0.03 1.66 0.75 0.14 0.16 0.16 0.14 0.20 0.16 0.15Tm 0.01 0.01 0.01 0.13 0.19 0.14 0.04 0.03 0.01 0.02 0.01 0.24 0.18 0.04 0.04 0.04 0.03 0.04 0.04 0.03Yb 0.05 0.05 0.05 0.76 1.06 0.92 0.19 0.16 0.05 0.09 0.05 1.47 0.87 0.17 0.17 0.17 0.17 0.24 0.21 0.19Lu 0.01 0.01 0.01 0.12 0.14 0.12 0.03 0.02 0.01 0.01 0.01 0.22 0.20 0.03 0.03 0.04 0.04 0.04 0.04 0.03∑REE 1.06 0.82 0.82 30.96 36.56 11.46 1.62 1.74 1.20 0.90 1.59 31.01 17.33 3.33 3.00 2.79 2.11 3.13 2.99 2.51
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Representative results are shown in Table 1. The procedure, accuracyand precisions of the geochemical data are presented in the OnlineAppendix.
In-situ analyses of silicate and oxide minerals of selected MTZrocks were carried out using a JEOL electron-probe microanalyzerJXA-8800 at Kanazawa University, Japan. Analytical conditions were15 kV accelerating voltage, 20 nA probe current and 3 μm beam dia-meter, and the raw data were corrected with an on-line ZAF program.The amounts of Fe3+ and Fe2+ in spinel were calculated assumingspinel (AB2O4) stoichiometry, based on the general equation ofDroop (1987) for calculating ferric iron. Standards used for oxideand silicate minerals are quartz for Si, eskolaite for Cr, fayalite forFe, wollastonite for Ca, jadeite for Na, corundum for Al, periclase forMg, manganosite for Mn, and pentlandite for Ni. Selected microprobeanalyses are presented in the Online Table 2–Online Table 6.
5.2. Whole-rock geochemistry
5.2.1. Major elementsThe MTZ rocks show systematic differences in geochemistry
depending on the distance from the contact with granitic intrusion(Table 1). The Al2O3 content is much higher in the samples close tothe contact (6.2–9.9 wt.%) compared with those far from it (0.3–1.4 wt.%). The CaO content also increases in meta-serpentinitesclose to the contact zone (0.2–2.4 wt.%) compared with those awayfrom the contact (0–0.8 wt.%). The MgO content, on the other hand,decreases toward the contact zone, and shows negative correlationswith both SiO2 and Al2O3 from basal meta-serpentinites to the lay-ered meta-gabbros. Due to the presence of more than one type of gab-broic rock (ophiolitic and alkali meta-gabbro) in the study area, thestudied mafic–ultramafic rocks are plotted in the SiO2/MgO versusAl2O3 diagram of Bodinier and Godard (2003), where the meta-gabbros entirely plot within the ophiolitic gabbro field (Fig. 6). Themeta-serpentinites plot within the ophiolitic peridotite field, exceptthose at the contact zone (Fig. 6), perhaps due to the differentialleaching of Mg during metamorphism relative to the more stable Al.The meta-pyroxenites, even when affected by contact metamor-phism, are low in Al2O3 (4.4–5.2 wt.%) and correspond to the Cr-diopside pyroxenite group of Shervais (1979), or the type Ib definedby Frey and Prinz (1978) that includes clinopyroxenites, websterites,and orthopyroxenites containing variable amounts of olivine, chro-mian spinel and/or plagioclase. The remarkably low CaO (2.2 wt.%,on average) and Al2O3 (4.8 wt.%, on average) of the meta-
Fig. 6. Relationship between Al2O3 wt.% and SiO2/MgO of the studied MTZ rocks. TheMORB, ophiolitic gabbros and ophiolitic peridotites are from Bodinier and Godard(2003).
pyroxenites confirm their orthopyroxenite protolith. The meta-troctolites show limited variations in SiO2, Al2O3, Fe2O3, MgO andCaO (Table 1).
5.2.2. Trace and rare-earth elementsAll trace elements show relative enrichment in the rocks away
from the contact zone compared with those close to the granitic in-trusion (Table 1), and this is possibly due to leaching by hydrother-mal fluids associated with the intrusion. The rare-earth element(REE) contents are very low, usually below the detection limit, in allrock types; meta-gabbros have the highest REE contents (Table 1).
5.3. Mineral chemistry
5.3.1. Spinel-group minerals (chromian spinel and pleonaste)
The chromium number [Cr#=Cr/(Cr+Al)] of spinels in the MTZmeta-serpentinites and meta-dunites, overlaps and ranges from0.64 to 0.88 in the former, and from 0.63 to 0.83 in the latter (OnlineTable 1). Chromian spinel in the meta-pyroxenites has the lowest Cr#(0.44–0.51). There is a weak negative correlation between Cr# andMg# of chromian spinel in meta-serpentinites, MTZ meta-dunitesand meta-pyroxenites. Almost all analyzed chromian spinel cores inthe MTZ rocks are slightly higher in Fe3+ content, than spinels in nor-mal ophiolitic mantle peridotites, while the rims have metamorphicferritchromite/magnetite compositions (Fig. 7). The chromian spinelcores of meta-pyroxenites display the highest Mg# and lowest Cr#(Fig. 8A) and slightly lower YFe [= Fe3+/(Cr+Al+Fe3+)] atomicratio (Fig. 8B). The primary chromian spinels of the tectonized peri-dotites from the Phanerozoic Oman ophiolite and the late Proterozoicophiolite of the southern Eastern Desert of Egypt (unpublished dataof the authors) have slightly higher Mg# and lower YFe than the chro-mian spinels of the studied MTZ rocks (Fig. 8A and B).
The olive-green metamorphic spinel (pleonaste) consists mainlyof Al2O3 (56.2–67.1 wt.%), FeOtotal (15.8–22.0 wt.%) and MgO (15.5–17. 9 wt.%) with trace amounts of SiO2, Cr2O3 and MnO (OnlineTable 2). The pleonaste analysis plots in the field of high-grade meta-morphic mafic–ultramafic rocks (Fig. 7).
5.3.2. Olivine
Based on the descriptions of Arai (1975, 1980), on partly thermal-ly metamorphosed peridotites, olivine is a primary phase derivedfrom residual mantle peridotites when it is homogeneous in
Fig. 7. The Cr–Al–Fe3+ ternary diagram (atomic ratios) of the analyzed spinels in thestudied MTZ rocks. Compositional fields of other terrestrial spinels are from Barnesand Roeder (2001).
Fig. 8. Chemical characteristics of spinels in the studied MTZ rocks. (A) Cr/(Cr+Al)atomic ratio vs Fe2+/(Fe2++Mg) atomic ratio. (B) Fe3+/(Cr+Al+Fe3+) atomicratio vs Fe2+/(Fe2++Mg) atomic ratio. Compositional fields of other terrestrial spinels(Barnes and Roeder, 2001) are used for comparison.
Fig. 9. Chemical characteristics of olivines in the studied MTZ rocks. (A) SiO2 vs MgOwt.%. (B) NiO wt.% vs Fo content of the analyzed olivines. Primary olivines of theupper mantle peridotites from Oman and south Eastern Desert of Egypt are used forcomparison.
76 A.H. Ahmed et al. / Lithos 142-143 (2012) 67–83
appearance, and its Fo, NiO, and MnO contents are about 90–92 mol%,0.4 wt.% and b0.1 wt.%, respectively. Petrography and mineral chem-istry have revealed no relicts of primary olivine in all rock types of thestudy area; only metamorphic olivines are present in basal meta-serpentinites, MTZ meta-dunites and meta-troctolites. The MgO andSiO2 contents of the olivines show relatively wide ranges and are con-siderably lower than the primary olivines from upper mantle dunitesand harzburgites of the northern Oman ophiolite and the southernEastern Desert (SED) ophiolite (Fig. 9A). The Fo content of olivinesin the studied MTZ rocks ranges from 75 to 90 and varies dependingon lithology. The meta-peridotites and MTZ dunites, have Fo contentsranging from 86 to 90, while the meta-troctolites range from 75 to 88(Fig. 9B and Online Table 3). The NiO contents do not show any corre-lation with Fo content of olivines in any rock type (Fig. 9B). The meta-troctolite olivine has the lowest NiO in all MTZ rocks, usuallyb0.06 wt.%. In contrast the primary olivines from dunites and harz-burgites of the Neoproterozoic southern Eastern Desert ophiolite ofEgypt and the Phanerozoic Oman ophiolite have significantly higherNiO contents (0.3–0.4 wt.%), and Fo contents (92–93) (Fig. 9B).
5.3.3. Orthopyroxene
Orthopyroxenes from the MTZ meta-dunites, meta-troctolites andmeta-pyroxenites range from 0.83 to 0.89 in Mg#, however, withineach lithology they have a relatively narrow range in Mg#. The Mg#
of orthopyroxene is the highest in the MTZ meta-dunites (0.88 on av-erage), and lowest in the meta-pyroxenites (0.86 on average) whilethe meta-troctolites have intermediate values (0.85 on average) (On-line Table 4). In terms of the Al2O3–CaO relationship (Fig. 10), theorthopyroxenes of MTZ meta-dunites and meta-troctolite are similarto those from well-studied low-P contact metamorphosed ultramaficrocks (Arai, 1975; Frost, 1975; Melcher and Meisel, 2004; Pinsext andHirst, 1977; Springer, 1974). The low contents of CaO and Cr2O3 andvariable contents of Al2O3 in orthopyroxene from Gebel MudarjajMTZ rocks, except meta-pyroxenites, suggest a metamorphic origindue to the thermal effect of the granitic intrusion. The CaO contentof orthopyroxene in MTZ meta-dunites and meta-troctolites is lowcompared with the primary orthopyroxene of the Oman ophioliticperidotites (Fig. 10). The orthopyroxene crystals in the meta-pyroxenites are possibly relics of primary orthopyroxene since theyshow higher Al2O3, CaO and Cr2O3 contents than in the associatedMTZ meta-dunites and meta-troctolites.
5.3.4. AmphibolesAmphiboles analyzed in this study are anthophyllite, tremolite
and Mg-hornblende (Hawthorne and Oberti, 2006; Leake et al.,2004). These amphiboles have homogeneous chemical compositions,and calcic amphiboles follow the prograde metamorphic trend de-fined by Nozaka (2005). Their TiO2 content is low, 0.7 wt.% on aver-age, even in Mg-hornblende, and the Mg#s in anthophyllite,
Fig. 10. Relationship between CaO wt.% and Al2O3 wt.% of the analyzed orthopyroxenes(Opx) in the studied MTZ rocks. Note the difference in Opx composition between theMTZ dunites and the pyroxenites, in comparison with other primary Opx frommantle-wedge xenoliths (Ishimaru et al., 2007), Alpine-type peridotites (e.g. Arai,1975, 1980; Pinsext and Hirst, 1977), Opx of primary mantle peridotites of the Omanophiolite (unpublished data of the authors), contact metamorphic Opx (Arai, 1975;Frost, 1975) and regional metamorphic Opx (Evans and Trommsdorff, 1970; Khedrand Arai, 2010; Trommsdorff et al., 1998).
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tremolite and Mg-hornblende are similar to each other, 0.87, 0.85 and0.86 on average, respectively (Online Table 5). The Al2O3 content isvery low in anthophyllite (0.1–0.8 wt.%) and tremolite (0.6–1.5 wt.%), but significantly higher in Mg-hornblende (8.3–9.7 wt.%)(Online Table 5).
5.3.5. Serpentines, talc and chloritesSerpentines, talc and chlorite are highly magnesian, with Mg#
ranging from 0.86 to 0.93 (Online Table 6). Antigorite serpentineshows a wide compositional variation in SiO2 (37 to 45 wt.%) andMgO (33 to 43 wt.%). Talc shows inter-grain chemical homogeneity,but chlorites have significantly higher Cr2O3 contents (2.8 wt.% on av-erage) in the meta-serpentinites than in the meta-troctolite(b0.05 wt.% on average). Meta-serpentinites contain two types ofchlorite (based on classification of Hey, 1954; Zane and Sassi, 1998):penninites with low Al2O3 (12.9 wt.% on average) and FeO (4.0 wt.%on average), and clinochlores with high Al2O3 (24.4 wt.% on average)and FeO (8.1 wt.% on average). In the meta-troctolites, chlorite has ahomogeneous clinochlore composition (Online Table 6).
6. Discussion
6.1. Petrogenesis of the MTZ rocks
6.1.1. Basal serpentinites and MTZ dunitesThere is now a general consensus that many ophiolite complexes
are formed in a mid-ocean ridge setting and are subsequently modi-fied by substantial amounts of arc components (e.g., Ahmed andArai, 2002; Arai et al., 2006; Zhou et al., 1996). Chromian spinel inmafic–ultramafic rocks can be used as a petrogenetic indicator to dis-criminate between magmas involved in different tectonic settings(e.g., Ahmed et al., 2005, 2009; Arai, 1992; Arai and Yurimoto, 1994;Arai et al., 2011; Bonatti and Michael, 1989; Dick and Bullen, 1984).For example, chromian spinels with high Cr# (>0.7), or low Al2O3
content, are frequently found in magmatic rocks of boninitic or Mg-rich tholeiitic affinity that occur in the subduction zone
environments, while the rocks from mid-ocean ridge settings rarelyhave chromian spinel with Cr# higher than 0.6 (Arai, 1994, 1997;Arai and Yurimoto, 1995; Dick and Bullen, 1984). The Cr# of spinelsappears not to be significantly affected by variations in fO2 or by litho-static pressure of the magma under most natural conditions (Roederand Reynolds 1991). Therefore, the only significant variable control-ling Cr-spinel composition, is the chemistry (especially the Cr/Alratio) of the equilibrated melt. Abyssal peridotites, which are closelysimilar to peridotites from back-arc basin settings (Ohara et al.,2002), have a Cr# range of spinel from 0.1 to 0.6 and possibly repre-sent mantle residues of fractional melting that formed mid-oceanridge basalt (MORB). On the other hand, arc (supra-subductionzone) peridotites are characterized by much higher-Cr# (0.4–>0.8)spinels reflecting higher degrees of partial melting in arc settings(e.g., Arai, 1994; Choi et al., 2008; Gaetani and Grove, 1998; Pearceet al., 2000).
The abundance of trivalent (Cr, Al) and tetravalent (Ti) cations inprimary Cr-spinels of mantle peridotites displays very little changeduring post-magmatic processes (e.g., cooling) due to minimal ex-change with olivine (e.g., Barnes, 1998; Kamenetsky et al., 2001;Roeder and Campbell, 1985). Therefore, in spinels these elementscan be used to characterize the tectonic provenance of the studiedMTZ rocks. Using the Mg# of spinel, on the other hand, to characterizethe tectonic setting of formation of the mafic–ultramafic plutonics issometimes unreliable, because the Mg# is strongly dependent onthe post-magmatic subsolidus re-equilibration with olivine andother silicates (Arai et al., 2011).
It is noteworthy that the chemical composition of the accessoryspinels in both basal serpentinites and MTZ dunites shows the samechemical characteristics, which suggests a genetic link betweenthem. The compositional characteristics of spinel (Cr#–TiO2 andYFe–TiO2) (Fig. 11A and B) clearly indicate that both the basal serpen-tinites and MTZ dunites are arc-related (Arai et al., 2006, 2011). TheCr#–TiO2 relationship (Fig. 12) in spinels shows also that the serpen-tinites and MTZ dunites plot along and/or around the trend of boni-nite melts and are distinct from the trend of abyssal peridotiteMORB melt.
In conclusion, the homogeneous composition of chromian spinelsfrom the studied basal serpentinites in terms of low YFe–Al2O3–TiO2
contents and high Cr# reveals that these rocks represent mantle res-idues after high degrees of partial melting within an arc setting ratherthan the more fertile abyssal peridotites of a MORB setting. The high-Cr#, low-TiO2 characters of chromian spinels in the MTZ dunites sug-gest an important role of peridotite/melt interaction to form depletedhigh-Mg melts producing these dunites. This conclusion is also sup-ported by the extremely low trace element and REE contents of thebulk MTZ rocks which is a characteristic feature of an arc setting(Table 1) (e.g., Choi et al., 2008; González-Mancera et al., 2009).
6.1.2. PyroxenitesThree genetic types have been proposed for pyroxenites includ-
ing: (1) veins and/or dykes of different sizes in various mantle perido-tite lithologies (Bodinier et al., 1987; Spray, 1989), (2) cumulatesformed in a lower crustal magma chamber from fractionated meltnear the mantle–crust boundary (DeBari and Coleman, 1989;Melcher and Meisel, 2004; Müntener et al., 2001), and (3) productsof melt–rock and/or metasomatic reactions within the mantlewedge in SSZ settings (Arai et al., 2004; Berly et al., 2006; Ishimaruet al., 2007; Kelemen et al., 1992; Varfalvy et al., 1996). These varioustypes of pyroxenites have different processes of formation which canbe distinguished from each other using field relationships along withpetrological, mineralogical and geochemical characteristics.
The very low bulk-rock CaO and Al2O3 contents indicate that theorthopyroxenite was the precursor protolith for the studied pyroxe-nites (see Table 1). The pyroxenites contain detectable amounts ofCr2O3 (0.25 wt.% on average), and the relict orthopyroxene shows
Fig. 11. Discrimination diagrams of (A) spinel Cr# (= Cr/(Cr+Al) atomic ratio) vs TiO2
wt.% and (B) spinel YFe (= Fe3+/Cr+Al+Fe3+) atomic ratio) wt.% vs TiO2 wt.%, of thestudied MTZ rocks. Compositional fields of abyssal peridotites, island-arc and fore-arcperidotites of (A) are from Arai (1992) and Arai et al. (2006), and the fields of MORB,MOR plutonics, arc and hotspot peridotites of (B) are from Arai et al. (2011).
Fig. 12. Plots of spinel Cr# vs TiO2 wt.% of the studied MTZ rocks. The compositionalfield for abyssal peridotites and trends of the effect of MORBmelt reaction on refractoryabyssal peridotite spinels and of boninite melt reaction on refractory SSZ peridotite spi-nels are from Choi et al. (2008). The average abyssal basalt and the trend of partialmelting degrees of the Fertile MORB Mantle (FMM) are also shown.
78 A.H. Ahmed et al. / Lithos 142-143 (2012) 67–83
high Al2O3 contents compared with the normal mantle values(Fig. 12), which supports modification of the original mantle melt ina lower crustal magma chamber. The chromian spinels from pyroxe-nites have high TiO2 contents (0.49–0.93 wt.%) and low Cr# (0.44–0.51) compared with the underlying serpentinized harzburgites andMTZ dunites and plot between arc and MORB peridotite fields(Fig. 11A) in terms of TiO2–Cr# relationships. In addition, the pyrox-enite chromian spinels show slightly high YFe (≥0.2) comparedwith the MORB plutonics (Fig. 11B). They plot on the extension ofthe MORB-melt reaction trend of the abyssal peridotites close to aver-age MORB spinels in the Cr#–TiO2 variation diagram (Fig. 12). Fur-thermore, the primary orthopyroxene in pyroxenites, in terms ofAl2O3–Mg# variation, shows strong affinities with those formed inarc-related crustal cumulates that have distinctly lower Al2O3 andMg# (Berly et al., 2006) than the mantle pyroxenite cumulates.
All of the above features along with its presence in the lower partof the layered meta-gabbros, close to the crust–mantle boundary,suggest that the pyroxenites of the study area represent arc-relatedcrustal cumulates that possibly formed by the assimilation of somecrustal materials in the arc-related magmas. The resultant mixed
melt was richer in Al, Ti and H2O produced more Al- and Ti-rich chro-mian spinel (Bédard and Hébert, 1998; González-Jiménez et al., 2011;Proenza et al., 1999). This model is supported by the high Al2O3 andTiO2 contents of spinels, as well as lower Cr#–Mg# values thanthose found in the associated basal serpentinites and MTZ dunites.
6.2. Parental melt composition of the MTZ rocks
The elements more resistant to post-magmatic re-equilibration(Al2O3 and TiO2) have been used to calculate the parental melt com-position due to their low diffusivity between spinel and surroundingsilicates (e.g., Barnes, 1998; Kamenetsky et al., 2001; Roeder andCampbell, 1985). The Al2O3 contents of the parental melt were calcu-lated using the equation of Maurel and Maurel (1982) for spinel–meltequilibrium at 1 bar where the (Al2O3)spinel=0.035(Al2O3)2.42melt.The TiO2 content of the parental melt was computed from the distri-bution coefficient of Ti between spinel and melt (Kamenetsky et al.,2001). Table 2 shows the minimum, maximum and average contentsof the parental melt composition in terms of its Al2O3 and TiO2 con-tents compared with some other well-known ultramafic–maficmagmas. The degree of partial melting (F) was also calculated usingthe chemistry of unaltered chromian spinel, based on the empiricalequation proposed by Hellebrand et al. (2001), where F=10ln(Cr#)+24.
Compared with the literature (e.g., Barnes, 1989; Falloon et al.,2008; Hicky and Frey, 1982; Kamenetsky et al., 2001; Wilson, 1989,and references therein), the parental melt composition in equilibriumwith the basal harzburgites contains 7.9–12.4 wt.% Al2O3, and 0.16–0.36 wt.% TiO2 (Table 2). The melt composition for the MTZ dunitesshows a similar composition to that of the basal harzburgites, con-taining 9.11–12.42 wt.% Al2O3, and 0.09–0.47 wt.% TiO2 (Table 2).The low Al2O3 and TiO2 character of the melt that was in equilibriumwith the basal harzburgites and MTZ dunites is consistent with thecomposition of boninites and high-Mg tholeiite magmas, which
Table 2Minimum–maximum (range) and average contents (wt.%) of Al2O3 and TiO2 of melts in equilibrium with the accessory chromian spinel in MTZ rocks of Gebel Mudarjaj area, southEastern Desert, Egypt, in comparison with primitive melts of different tectonic settings.
Al2O3a TiO2
b Reference
Range Average Range Average
Serpentinized Hz 7.90–12.43 10.32 0.16–0.36 0.25 This studyMTZ dunites 9.11–12.42 10.75 0.09–0.47 0.28 This studyPyroxenites 13.62–15.15 14.64 1.20–1.95 1.65 This studyBoninites 10.60–14.40 12.50 0.10–0.52 0.32 Wilson (1989), Hicky and Frey (1982)MORB 14.00–17.00 ~16 0.32–2.20 1.26 Wilson (1989), Kamenetsky et al. (2001)Back-arc basin basalt 4.50–13.00 ~17 0.45–1.45 0.95 Kamenetsky et al. (2001)Island arc basalt 8.75 0.07–0.75 0.41 Kamenetsky et al. (2001)
a Maurel and Maurel (1982).b Kamenetsky et al. (2001).
79A.H. Ahmed et al. / Lithos 142-143 (2012) 67–83
were produced by high degrees of partial melting in an arc setting.The calculated degrees of partial melting of the original harzburgitesusing the equation of Hellebrand et al. (2001) are between 20–23%,and are entirely within the range of peridotites from a supra-subduction zone (SSZ) (Table 3). Such high degrees of partial meltingcould result in the formation of boninites or high-Mg tholeiiticmagmas at arc settings.
The parental melt composition that was in equilibrium with thepyroxenites, on the other hand, shows high contents of both Al2O3
(13.62–15.15 wt.%, with an average of 14.64 wt.%) and TiO2 (1.2–1.95 wt.%, with an average of 1.65 wt.%) (Table 2). This compositionis very close to the melt composition of MORB and/or back-arc basinbasalt (BABB) magmas where the degree of partial melting is relative-ly low. The melt that precipitated the original pyroxenite may there-fore have been the result of a relatively low degree of partial melting(16–17%), which is still in the lower range of the extent of partialmelting represented by arc peridotites (Table 3). The chemical char-acteristics of spinels (Figs. 11 and 12) show an arc tectonic settingfor the studied MTZ rocks, except for the pyroxenites, which have in-termediate compositions between arc and MORB. These transitionalfeatures between arc and MORB settings could have formed in a na-scent spreading center such as a back-arc environment (e.g., Araiand Yurimoto, 1995; Dick and Bullen, 1984; Niu et al., 2003;Proenza et al., 1999).
6.3. Evolution of the MTZ rocks
The basal harzburgites, MTZ dunites, pyroxenites and their associ-ated gabbros are closely similar to the MTZ of ophiolites worldwide,in that they show evidence of intense magmatic activity (Boudierand Nicolas, 1995; Dick, 1989; Seyler et al., 2001, 2007; Takazawa etal., 2007). The thickness of the MTZ (including MTZ dunites and over-lying gabbros) in the study area is remarkably thin (30–50 m) whencompared with Phanerozoic complexes such as those in Oman ophio-lite (e.g., Boudier and Nicolas, 1995; Jousselin and Nicolas, 2000; Kogaet al., 2001), or even compared with some of the Neoproterozoic
Table 3Minimum–maximum (range) and average degrees of partial melting calculated for MTZ roctectonic settings.
Cr#
Range Average
Serpentinized Hz 0.64–0.88 0.77MTZ dunites 0.63–0.83 0.74Pyroxenites 0.44–0.51 0.46SSZ peridotites 0.30–0.80Passive margin peridotites 0.14–0.44Abyssal peridotites 0.10–0.50
Cr#=Cr/(Cr+Al) atomic ratio.
Arabian Nubian Shield ophiolites (e.g., Hussein et al., 2004; Kröneret al., 1987). The high-Cr#, low-TiO2 characteristics of the studiedbasal harzburgites and MTZ dunites and the similarity of their paren-tal melt composition to boninitic melts, suggest that they formed bysubstantial modification of initial MORB oceanic lithosphere by sec-ond stage melting in an arc environment.
The basal serpentinized harzburgite beneath the MTZ probablyrepresents residual mantle residues after high degree partial meltingin an arc setting (e.g., Arai, 1994; Dick and Bullen, 1984; Gaetani andGrove, 1998). The MTZ dunites, on the other hand, are interpreted as“replacive” dunites formed by reaction between ascending MORB/arcmelts and the surrounding depleted mantle peridotites (Kelemen andDick, 1995; Kelemen et al., 1995; Nicolas, 1986; Spiegelman et al.,2001; Suhr, 1999). At the crust–mantle boundary, the olivine field ex-pands resulting in dissolution of pyroxenes in wall-rock peridotitesand the precipitation of olivine to produce dunites (e.g., Kelemenand Dick, 1995; Kelemen et al., 1995). Furthermore, as the melt isfractionated, the percolated residual melt would crystallize plagio-clase and clinopyroxene to produce the gabbroic rocks (gabbros andtroctolite) overlying the MTZ dunites.
The occurrence of pyroxenites at the crust–mantle boundary (inthe lower crustal section) indicates that these pyroxenite bodies aremedium- to high-pressure, ultramafic cumulates formed in mid- tolower crustal magma chambers (e.g., Berly et al., 2006; DeBari andColeman, 1989). The pyroxenites are characterized by a narrowrange of low CaO and Al2O3 contents similar to that of arc-relatedcrustal pyroxenites. Mixing of the evolved mantle melt with thecrustal materials would increase the TiO2 and Al2O3 contents of theresultant melt to produce arc-related cumulate pyroxenites at thebase of the crustal magma chamber. The calculated degrees of partialmelting along with the intermediate Cr#, high TiO2 and Al2O3 con-tents of spinels suggest that the pyroxenites formed by crustal con-tamination of an arc-related melt. Similar pyroxenites have beendescribed from a number of arc-related environments such as theAlaskan complexes (DeBari and Coleman, 1989), arc-related cumu-lates in southern New Zealand (Spandler et al., 2003), the
ks of Gebel Mudarjaj area, south Eastern Desert, Egypt, compared with other different
Degrees of partial melting (%) Reference
Range Average
20–23 21 This study– – This study16–17 16 This study15–40 Pearce et al.(2000), Mellini et al. (2005)5–15 Pearce et al.(2000)10–20 Dick and Bullen (1984)
80 A.H. Ahmed et al. / Lithos 142-143 (2012) 67–83
pyroxenite-rich peridotites from Cabo Ortegal, Spain (Santos et al.,2002), and the Beni Bousera massif, Morocco (Pearson et al., 1993).
6.4. Metamorphic evolution of the MTZ rocks
In the study area, ultramafic rocks away from the contact zonewere not affected by contact metamorphism but have been fully ser-pentinized to form antigorite due to regional metamorphism duringor after emplacement. It is significant that the assemblage of newlyformed metamorphic minerals depends not only on the metamorphictemperature but also substantially on the composition of the rocks.The basal serpentinized harzburgites and MTZ dunites, for example,are more magnesian than the overlying gabbroic/troctolitic rocks,which lead to the formation of different metamorphic productsunder the same temperature–pressure conditions. The most notablefeature of the bulk composition of the rocks in the metamorphic aure-ole is the enrichment in Al2O3 and depletion in MgO compared withthe same rocks away from the contact zone.
In general, olivine and tremolite are stable in the entire amphibo-lite facies in Ca-bearing ultramafic rocks, while orthopyroxene is onlystable at mid-amphibolite facies and higher grades. In the peridotitesystem, chlorite is stable in low to mid amphibolite facies, and be-comes unstable in the upper amphibolite facies where it is replacedby picotite (Cr-hercynite) and/or Al-spinel (pleonaste) (e.g., Gotoand Tatsumi, 1990). Prior to the effect of thermal metamorphism, ser-pentinized ultramafic rocks mainly contain antigorite serpentine as astable mineral at metamorphic temperatures of greenschist faciesduring the emplacement of ophiolite (Evans and Trommsdorff, 1970).
The basal serpentinized harzburgites are composed mainly of, indecreasing order of abundance, antigorite, olivine, tremolite, antho-phyllite, talc, ferritchromite and chlorite. The metamorphic olivinesfrom all the studied rock types display a low Fo content (Fo75–90,see Online Table 3); the basal serpentinized harzburgites and MTZdunites contain olivines with Fo content higher than those of trocto-lites, possibly depending on the Mg/(Mg+Fe2+) ratio (Fig. 9 and On-line Table 3). The highly magnesian metamorphic olivines (Fo92–97),which are not found in our study, could be produced by the dehydra-tion of serpentine minerals under relatively low-temperature condi-tions (Evans and Trommsdorff, 1970). Antigorite is the main stableserpentine phase formed from olivine under equilibrium conditions(Dungan, 1977; Nozaka, 2003;Wicks andWhittaker, 1977). However,antigorite does not contain significant amounts of Fe (see OnlineTable 6), thus its normal dehydration should produce metamorphicolivine with Fo content well over 90 (Nozaka, 2005). In our case, allolivines are of Fe-rich type, which may need an additional Fe-richphase(s) in the metamorphic system. Because magnetite is the mostabundant Fe-rich phase and is produced by alteration of chromianspinel and the breakdown of primary olivines, it is the most appropri-ate Fe source for the Fe-rich olivines via antigorite breakdown. Theformation of Fe-rich olivines by the involvement of magnetite occursby the following reaction (Frost, 1975):
6Mg3Si2O5 OHð Þ4antigorite
þ2Fe3O4magnetite
¼ 12 Mg; Feð Þ2SiO4olivine
þ12H2Oþ O2:
Thus, the initial reactions upon progressive thermal metamor-phism in the studied mafic–ultramafic rocks would have producedFe-rich olivine below about 400 °C (Evans and Trommsdorff, 1970;Tracy et al., 1984). At about 500 °C, any remaining serpentinemineralsshould breakdown to produce talc and olivine (serpentine=talc+olivine+vapor, Tracy et al., 1984). This can explain the associationof small amounts of talc with olivine in the basal serpentinizedharzburgites.
Although less common than other mafic minerals, such as olivine,orthopyroxene and talc in the Gebel Mudarjaj MTZ rocks, anthophyl-lite is a common metamorphic mineral in ordinary low-pressure
meta-peridotites (Arai, 1975; Evans and Trommsdorff, 1970; Nozaka,2011). The presence of anthophyllite in equilibrium with olivine,tremolite and talc, as well as the virtual absence of clinopyroxene, inthe basal serpentinites suggests a temperature of about 500–550 °C,or slightly higher, which is characteristic of themid-amphibolite facies(e.g., Evans and Trommsdorff, 1970; Oliver et al., 1972). The antho-phyllite stability field depends mainly on pressure and Fe/Mg ratio ofthe metamorphosed rock (Frost, 1975; Nozaka, 2003; O'Hanley,1996; Trommsdorff and Evans, 1974). Anthophyllite seems to be ab-sent in the most metamorphosed magensian-rich rocks at lower pres-sures (Nozaka, 2003); instead it is stable under relatively highpressures in Fe-bearing natural peridotites (Evans, 1977; Evans,2008; Evans and Guggenheim, 1988; Katzir et al., 1999). Furthermore,anthophyllite is rarely found in highly magnesian meta-peridotitesbecause its stability is reduced with increasing Mg# of peridotites(Frost, 1975; Nozaka, 2011). Formation of anthophyllite in equilibri-um with Fe-rich olivine within the studied basal serpentinized harz-burgites probably occurred around 650 °C and pressures b7 kbar(Evans, 1977; Evans and Trommsdorff, 1970; Melcher and Meisel,2004; Nozaka, 2011). The homogeneous Fo content of Fe-rich olivinesalongwith the formation of anthophyllite in the studied serpentinizedharzburgites suggests a sufficient duration of homogenization duringhigh-temperature contact metamorphism (e.g., Nozaka, 2011).
The next reaction occurs at temperatures higher than 650 °C toproduce chiefly orthopyroxene and olivine in the MTZ dunites,which is characteristic of the upper amphibolite facies (Evans, 1977;Greenwood, 1971; Tracy et al., 1984). The MTZ dunites are composedessentially of granular metamorphic orthopyroxene and olivine withaccessory serpentine, magnetite, chlorite, chromian spinel and its fer-ritchromite alteration products. Orthopyroxene (and olivine) can beformed directly by breakdown of antigorite serpentine, at higher tem-peratures between 640 and 720 °C (Trommsdorff et al., 1998):
Higher in the section, where the gabbroic/troctolitic rocks are ex-posed, the observed mineral assemblage is olivine+aluminous spinel(pleonaste)+chlorite+magnetite±orthopyroxene. Throughout thetroctolitic layers, chlorites were occasionally found surrounding alu-minous spinel and olivine. Aluminous spinel is a characteristic miner-al of this assemblage. Theoretically, aluminous spinel (pleonaste) isformed in the upper amphibolite facies at about 700 °C in ultramaficrocks (Frost, 1975; Tracy et al., 1984). However, the composition ofmetamorphosed rocks essentially controls the metamorphic prod-ucts. The troctolitic rocks are less magnesian than the underlyingultramafic rocks, so aluminous spinel in troctolite could be formedat least in the same temperature condition, or slightly higher, asthat for the underlying ultramafic rocks, around 700 °C. Several hy-potheses have been proposed for the formation of aluminous spinelin mafic–ultramafic rocks (Springer, 1974): (1) breakdown of magne-sian chlorite, (2) unmixing of a single spinel phase, and (3) metamor-phic reactions involving chlorite. The virtual absence of chromianspinel in association with aluminous spinel throughout the troctoliticlayers argues against the formation of the latter by unmixing of a sin-gle Cr–Al spinel phase.
Experimentally, the breakdown of Mg-rich aluminous chloriteabove 5 kb should occur in the stability field of the assemblageolivine+orthopyroxene (Chernosky, 1974; Goto and Tatsumi,1990; Staudigel and Schreyer, 1977). This is consistent with theobserved assemblages of the study area where the breakdown ofAl-rich chlorite took place in the presence of olivine and, to a lesserextent, orthopyroxene at a peak temperature of about 700 °C (Frost,1975; Tracy et al., 1984). Furthermore, two metamorphic reactionshave also been suggested as a possible way to form aluminous spi-nel in mafic–ultramafic hornfels (e.g., Springer, 1974): (1) Al-richchlorite+magnetite=Al-poor chlorite+aluminous spinel, and(2) serpentine+olivine+Al (introduced)=chlorite+aluminousspinel. However, the lack of chlorite compositional heterogeneity(Al-rich and Al-poor ones) in troctolites, and the virtual absence
81A.H. Ahmed et al. / Lithos 142-143 (2012) 67–83
of aluminous spinel in other serpentine-bearing mafic–ultramaficrocks in the study area argue against these metamorphic reactions.
7. Conclusions
1. A rare complete mantle–crust boundary (MTZ) of a Neoprotero-zoic ophiolite complex at Gebel Mudarjaj, south Eastern Desert ofEgypt is petrographically and geochemically described, and itsmetamorphic history is highlighted. The studied MTZ consistsmainly of MTZ dunites, gabbroic/troctolitic rocks and pyroxenitesoverlying the basal serpentinites.
2. The Cr# of spinels in the studied basal serpentinized harzburgitesand MTZ dunites is similar to the range in arc peridotites. Themelt composition inferred from the unaltered chromian spinelcores that was in equilibrium with these rocks is compositionallysimilar to boninitic magmas produced by high degrees of partialmelting. The basal serpentinized harzburgites formed by 20–23%partial melting, similar to the range for arc peridotites. The gabbro-ic rocks (gabbros and troctolites) could be a continuous spectrumfrom the melt involved in the formation of the MTZ dunites.
3. Chromian spinels of pyroxenites show intermediate Cr# of 0.47 onaverage. The inferred parental melt composition that was in equi-librium with these pyroxenites was formed by 16–17% partialmelting of the source rocks; and is transitional in composition be-tween MORB and arc peridotites. The occurrence of pyroxenites ina lower crustal section suggests that medium- to high-pressure, ul-tramafic cumulates formed in mid- to lower crustal magma cham-bers. The narrow range of low CaO and Al2O3 contents ofpyroxenites is similar to that of arc-related crustal pyroxenites.The computed degrees of partial melting (16–17%) along withthe intermediate Cr# and high TiO2 of chromian spinels suggestcontamination of arc-related melt by crustal materials during theformation of the pyroxenites.
4. The studied MTZ rocks were highly affected by thermal metamor-phism of the late granitic intrusion. The original serpentinizedmafic–ultramafic rocks were affected by high-temperaturemoderate-pressure thermal metamorphism with peak metamor-phism of about 650°–700 °C and b7 kb. The mineral assemblagesin the gabbroic/troctolitic rocks, are characteristic of upper am-phibolite facies metamorphism.
Supplementary materials related to this article can be found on-line at doi:10.1016/j.lithos.2012.02.014.
Acknowledgments
The first author would like to express his deep thanks to A. Tamuraand T. Morishita for their assistant in the laboratory works during hisstay in Kanazawa University. The authors are also indebted to Dr.Abou El-Hajjaj for his guidance during the field trip. Comments ofDr. Sobhi Nasr and anonymous reviewer are highly appreciated to im-prove the earlier version of the manuscript. Dr. K. Ali is highly indebt-ed for the discussion and checking the revised version of themanuscript. Dr. Andrew Kerr is highly thanked for his valuable com-ments and handling the manuscript.
References
Abd El-Rahman, Y., Polat, A., Dilek, Y., Fryer, B.J., El-Sharkawy, M., Sakran, S., 2009a.Geochemistry and tectonic evolution of the Neoproterozoic incipient arc–forearccrust in the Fawakhir area, central Eastern Desert of Egypt. Precambrian Research175, 116–134.
Abd El-Rahman, Y., Polat, A., Dilek, Y., Fryer, B.J., El-Sharkawy, M., Sakran, S., 2009b.Geochemistry and tectonic evolution of the Neoproterozoic Wadi Ghadir ophiolite,Eastern Desert, Egypt. Lithos 113, 158–178.
Ahmed, A.H., Arai, S., 2002. Unexpectedly high-PGE chromitite from the deeper mantlesection of the northern Oman ophiolite and its tectonic implications. Contributionsto Mineralogy and Petrology 143, 263–278.
Ahmed, A.H., Arai, S., Attia, A.K., 2001. Petrological characteristics of podiform chromi-tites and associated peridotites of the Pan African ophiolite complexes of Egypt.Mineralium Deposita 36, 72–84.
Ahmed, A.H., Arai, S., Yaser, M.A., Rahimi, A., 2005. Spinel composition as a petrogeneticindicator of the mantle section in the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. Precambrian Research 138, 225–234.
Ahmed, A.H., Arai, S., Yaser, M.A., Ikenne, M., Rahimi, A., 2009. Platinum-group ele-ments distribution and spinel composition in podiform chromitites and associatedrocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite,Anti-Atlas, Morocco. Journal of African Earth Sciences 55, 92–104.
Akizawa, N., Arai, S., 2009. Petrologic profile of peridotite layers under a possible Mohoin the northern Oman ophiolite: an example fromWadi Fizh. Journal of Mineralog-ical and Petrological Sciences 104, 389–394.
Amri, I., Benoit, M., Ceuleneer, G., 1996. Tectonic setting for the genesis of oceanic pla-giogranites: evidence from a paleospreading structure in the Oman ophiolite. Earthand Planetary Science Letters 139, 177–194.
Arai, S., 1975. Contact metamorphosed dunite–harzburgite complex in the Chugokudistrict, western Japan. Contributions to Mineralogy and Petrology 52, 1–16.
Arai, S., 1980. Dunite–harzburgite–chromitite complexes as refractory residue in theSangun-Yamaguchi zone, western Japan. Journal of Petrology 21, 141–165.
Arai, S., 1992. Chemistry of chromian spinel in volcanic rocks as a potential guide tomagma chemistry. Mineralogical Magazine 56, 173–184.
Arai, S., 1994. Characterization of spinel peridotites by olivine-spinel compositional re-lationships: review and interpretation. Chemical Geology 113, 191–204.
Arai, S., 1997. Control of wall-rock composition on the formation of podiform chromi-tites as a result of magma/peridotite interaction. Resource Geology 47, 177–187.
Arai, S., Matsukage, K., 1996. Petrology of the gabbro–troctolite–peridotite complexfrom Hess Deep, equatorial Pacific: implications for mantle–melt interaction with-in the oceanic lithosphere. Proceeding of Ocean Drilling Program, Scientific Results147, 135–155.
Arai, S., Matsukage, K., 1998. Petrology of a chromitite micropod from Hess Deep, equa-torial Pacific: a comparison between abyssal and alpine-type podiform chromitites.Lithos 43, 1–14.
Arai, S., Yurimoto, H., 1994. Podiform chromitites of the Tari-Misaka ultramafic com-plex, southwestern Japan, as mantle–melt interaction products. Economic Geology89, 1279–1288.
Arai, S., Yurimoto, H., 1995. Possible sub-arc origin of podiform chromitites. Island Arc4, 104–111.
Arai, S., Takada, S., Michibayashi, K., Kida, M., 2004. Petrology of peridotite xenolithsfrom Iraya Volcano, Philippines, and its implication for dynamic mantle wedgeprocesses. Journal of Petrology 45, 369–389.
Arai, S., Kadoshima, K., Morishita, T., 2006. Widespread arc-related melting in themantle section of the northern Oman ophiolite as inferred from detrital chromianspinels. Journal of the Geological Society of London 163, 869–879.
Arai, S., Okamura, H., Kadoshima, K., Tanaka, C., Suzuki, S., Ishimaru, S., 2011. Chemicalcharacteristics of chromian spinel in plutonic rocks: implications for deep magmaprocesses and discrimination of tectonic setting. Island Arc 20, 125–137.
Barnes, S.J., 1998. Chromite in komatiites, 1. Magmatic controls on crystallization andcomposition. Journal of Petrology 39, 1689–1720.
Barnes, S.J., Roeder, P.L., 2001. The range of spinel compositions in terrestrial mafic andultramafic rocks. Journal of Petrology 42, 2279–2302.
Bédard, J.H., Hébert, R., 1998. Formation of chromitites by assimilation of crustal pyroxenitesand gabbros into peridotitic intrusions: North Arm Mountain Massif, Bay of IslandsOphiolite, Newfoundland, Canada. Journal of Geophysical Research 103, 5165–5184.
Benn, K., Nicolas, A., Reuber, I., 1988. Mantle–crust transition zone and origin ofwehrlitic magmas: evidence from the Oman ophiolite. Tectonophysics 151, 75–85.
Berly, T.J., Hermann, J., Arculus, R.J., Lapierre, H., 2006. Supra-subduction zone pyroxenitesfrom San Jore and Santa Isabel (Solomon Islands). Journal of Petrology 47, 1531–1555.
Bodinier, J.-L., Godard, M., 2003. Orogenic, ophiolitic, and abyssal peridotites. In:Carlson, R.W. (Ed.), Treatise on Geochemistry. Mantle and Core. : Treatise onGeochemistry, 2. Elsevier Science Ltd, pp. 103–170.
Bodinier, J.L., Guiraud, M., Fabriès, J., Dostal, J., Dupuy, C., 1987. Petrogenesis of layeredpyroxenites from the Lherz, Freychinède and Prades ultramafic bodies (Ariège,French Pyrénées). Geochimica et Cosmochimica Acta 51, 279–290.
Bonatti, E., Michael, P.J., 1989. Mantle peridotites from continental rifts to ocean basinsto subduction zones. Earth and Planetary Science Letters 91, 297–311.
Boudier, F., Nicolas, A., 1995. Nature of the Moho transition zone in the Oman ophiolite.Journal of Petrology 36, 777–796.
Boudier, F., Le Suer, E., Nicolas, A., 1989. Structure of an atypical ophiolite: the TrinityComplex, eastern Klamath Mountains, California. Geological Society of AmericaBulletin 101, 820–833.
Chernosky, J.V., 1974. The upper stability of clinochlore at low pressure and the free en-ergy of formation of Mg-cordierite. American Mineralogist 59, 496–507.
Choi, S.H., Shervais, J.W., Mukasa, S.B., 2008. Suprasubduction and abyssal mantle peri-dotites of the coast range ophiolite, California. Contributions to Mineralogy and Pe-trology 156, 551–576.
Clark, M.D., 1985. Late Proterozoic crustal evolution of the Midyan region, northwest-ern Saudi Arabia. Geology 13, 611–615.
Coleman, R.G., 1977. Ophiolites: Ancient Oceanic Lithosphere? Springer-Verlag, Berlin.229 pp.
DeBari, S.M., Coleman, R.J., 1989. Examination of the deep levels of an island arc: evi-dence from the Tonsina ultramafic–mafic assemblage. Journal of Geophysical Re-search 94, 4373–4391.
Dick, H.J.B., 1989. Abyssal peridotites, very slow spreading ridges and ocean ridge mag-matism. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins:Geological Society Special Publication, London, 42, pp. 71–105.
82 A.H. Ahmed et al. / Lithos 142-143 (2012) 67–83
Dick, H.J.B., Bullen, T., 1984. Chromian spinel as a petrogenetic indicator in abyssal andAlpine-type peridotites and spatially associated lavas. Contributions to Mineralogyand Petrology 86, 54–76.
Droop, G.T.R., 1987. A general equation for estimating Fe3+ in ferromagnesian silicatesand oxides from microprobe analysis, using stoichiometric criteria. MineralogicalMagazine 51, 431–437.
Dungan, M.A., 1977. Metastability in serpentine–olivine equilibria. American Mineral-ogist 62, 1018–1029.
El Ramly, M.F., 1972. “A new geological map for the basement rocks in the Eastern andSouthwestern Desert of Egypt, scale 1:1000 000”. Annals of Geological Survey ofEgypt 2, 1–18.
El Ramly, M.F., Hermina, M.H., 1978. Geologic Map of the Aswan Quadrangle, Egypt(scale 1: 500,000). Egyptian Geological Survey and Mining Authority.
El Ramly, M.F., Greiling, R., Kröner, A., Rashwan, A.A., 1984. On the tectonic evolution ofthe Wadi Hafafit area and environs, Eastern Desert of Egypt. Bulletin of King Abdu-laziz University 6, 113–126.
El Ramly, M.F., Greiling, R.O., Rashwan, A.A., Rasmy, A.H., 1993. Geologic map of WadiHafifit area. Scale 1:100.000. Egyptian Geological Survey 68.
El-Gaby, S., List, F.K., Tehrani, R., 1988. Geology, evolution and metallogenesis of thePan-African belt of northeast Africa and adjacent areas. In: El-Gaby, S., Greiling,R.O. (Eds.), Vieweg, Braunschweig, pp. 17–68.
El-Gharbawi, R.I.A., 1988. Petrological and petrochemical studies of igneous and meta-morphic rocks of Gabal Mudargag area, south Eastern Desert, Egypt. PhD thesis,Faculty of Science, Ain Shams University, Cairo, Egypt.
Evans, B.W., 1977. Metamorphism of alpine peridotite and serpentinite. Annual Reviewof Earth and Planetary Sciences 5, 397–447.
Evans, B.W., 2008. Control of the products of serpentinization by the Fe+2 Mg−1
exchange potential of olivine and orthopyroxene. Journal of Petrology 49,1873–1887.
Evans, B.W., Guggenheim, S., 1988. Talc, pyrophyllite, and related minerals. In: Bailey,S.W. (Ed.), Hydrous Phyllosilicates. : Review of Mineralogy, 19. Mineral. Soc. Amer-ica, Washington, DC, pp. 225–294.
Evans, B.W., Trommsdorff, V., 1970. Regional metamorphism of ultramafic rocks in theCentral Alps: parageneses in the system CaO–MgO–SiO2–H2O. SchweizerischeMineralogische und Petrographische Mitteilungen 50, 481–492.
Falloon, T.J., Green, D.H., Danyushevsky, L.V., McNeill, A.W., 2008. The composition ofnear-solidus partial melts of fertile peridotite at 1 and 1.5 GPa: implications forthe petrogenesis of MORB. Journal of Petrology 49, 591–613.
Farahat, E.S., Hoinkes, G., Mogessie, A., 2010. Petrogenetic and geotectonic significanceof Neoproterozoic suprasubduction mantle as revealed by the Wizer ophiolitecomplex, Central Eastern Desert, Egypt. International Journal of Earth Sciences100, 1433–1450.
Frey, F.A., Prinz, M., 1978. Ultramafic inclusions from San Carlos, Arizona; petrologicand geochemical data bearing on their petrogenesis. Earth and Planetary ScienceLetters 38, 129–176.
Frost, B.R., 1975. Contact metamorphism of serpentinite, chloritic blackwall and rodin-gite at Paddy-Go-Easy Pass, central Cascades, Washington. Journal of Petrology 16,272–313.
Gaetani, G.A., Grove, T.L., 1998. The influence of water on melting of mantle peridotite.Contributions to Mineralogy and Petrology 131, 323–346.
Gahlan, H.A., Arai, S., Abu El-Ela, F.F., Tamura, A., 2012. Origin of wehrlite cumulates inthe Moho transition zone of the Neoproterozoic Ras Salatit ophiolite, Central East-ern Desert, Egypt: crustal wehrlites with typical mantle Characteristics. Contribu-tions to Mineralogy and Petrology 163, 225–241.
González-Jiménez, J.M., Proenza, J.A., Gervilla, F., Melgarejo, J.C., Blanco-Moreno, J.A.,Ruiz-Sánchez, R., Griffin, W.L., 2011. High-Cr and high-Al chromitites from theSagua de Tánamo district, Mayarí-Cristal ophiolitic massif (eastern Cuba): con-straints on their origin from mineralogy and geochemistry of chromian spineland platinum-group elements. Lithos 125, 101–121.
González-Mancera, G., Ortega-Gutiérrez, F., Proenza, J.A., Atudorei, V., 2009. Petrologyand geochemistry of Tehuitzingo serpentinites (Acatlán Complex, SW Mexico).Boletin De la Sociedad Geológica Mexicana 61, 419–435.
Goto, A., Tatsumi, Y., 1990. Stability of chlorite in the upper mantle. American Mineral-ogist 75, 105–108.
Greenwood, H.J., 1971. Anthophyllite. Corrections and comments on its stability. Amer-ican Journal of Science 270, 151–154.
Hawthorne, F.C., Oberti, R., 2006. On the classification of amphiboles. The CanadianMineralogist 44, 1–21.
Hebert, R., Bideau, D., Hekinian, R., 1983. Ultramafic and mafic rocks from the Garrettransform fault near 13°30′S on the East Pacific Rise: igneous petrology. Earthand Planetary Science Letters 65, 107–125.
Hellebrand, E., Snow, J.E., Dick, H.J.B., Hoffmann, A.W., 2001. Coupled major and traceelements as indicators of the extent of melting in mid-ocean-ridge peridotites. Na-ture 410, 677–681.
Hey, M.H., 1954. A new review of the chlorites. Mineralogical Magazine 30, 277–292.Hicky, R.L., Frey, F.A., 1982. Geochemical characteristics of boninite serives volca-
nic: implication for their source. Geochimica et Cosmochimica Acta 46,2099–2115.
Hussein, I.M., Krö ner, A., Reischmann, T., 2004. The Wadi Onib mafic–ultramaficcomplex: a Neoproterozoic supra-subduction zone ophiolite in the northern RedSea hills of the Sudan. In: Kusky, T.M. (Ed.), Precambrian Ophiolite and RelatedRocks. Elsevier, pp. 163–206.
Ishimaru, S., Arai, S., Ishida, Y., Shirasaka, M., Okrugin, V.M., 2007. Melting and multi-stage metasomatism in the mantle wedge beneath a frontal arc inferred from high-ly depleted peridotite xenoliths from the Avacha volcano, southern Kamchatka.Journal of Petrology 48, 395–433.
Johnson, P.R., Kattan, F.H., 2001. Oblique sinistral transpression in the Arabian shield:the timing and kinematics of a Neoproterozoic suture zone. Precambrian Research107, 117–138.
Jousselin, D., Nicolas, A., 2000. The Moho transition zone in the Oman ophiolite. Rela-tion with wehrlites in the crust and dunites in the mantle. Marine Geophysical Re-searches 21, 229–241.
Kamenetsky, V., Crawford, A.J., Meffre, S., 2001. Factors controlling chemistry of mag-matic spinel: an empirical study of associated olivine, Cr-spinel and melt inclusionsfrom primitive rocks. Journal of Petrology 42, 655–671.
Karson, J.A., Collins, J.A., 1984. Geologic and seismic velocity structure of the crust/mantle transition in the Bay of Islands ophiolite complex. Journal of GeophysicalResearch 89, 6126–6138.
Katzir, Y., Avigado, D., Matthews, A., Garfunkel, Z., Evans, B.W., 1999. Origin and meta-morphism of ultrabasic rocks associated with a subducted continental margin,Naxos (Cyclades, Greece). Journal of Metamorphic Geology 17, 301–318.
Kelemen, P.B., Dick, H.J.B., 1995. Focused melt flow and localized deformation in theupper mantle: juxtaposition of replacive dunite and ductile shear zones. Journalof Geophysical Research 100, 423–438.
Kelemen, P.B., Dick, H.J.B., Quick, J.E., 1992. Formation of harzburgite by pervasive melt/rock reaction in the upper mantle. Nature 358, 635–641.
Kelemen, P.B., Shimizu, N., Salters, V.J.M., 1995. Extraction of midocean-ridge basaltfrom mantle by focused ß ow of melt in dunite channels. Nature 375, 747–753.
Khedr, M.Z., Arai, S., 2010. Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O'nemetaperidotites (Japan). Contributions to Mineralogy and Petrology 159, 137–157.
Koepke, J., Berndt, J., Feig, S.T., Holtz, F., 2007. The formation of SiO2-rich melts withinthe deep oceanic crust by hydrous partial melting of gabbros. Contributions toMineralogy and Petrology 153, 67–84.
Koepke, J., Schoenborn, S., Oelze, M., Wittmann, H., Feig, S.T., Hellebrand, E., Boudier, F.,Schoenberg, R., 2009. Petrogenesis of crustal wehrlites in the Oman ophiolite: ex-periments and natural rocks. Geochemistry, Geophysics, Geosystems 10, Q10002.doi:10.1029/2009GC002488.
Koga, K.T., Kelemen, P.B., Shimizu, N., 2001. Petrogenesis of the crust–mantle transitionzone and the origin of lower crustal wehrlite in the Oman ophiolite. Geochemistry,Geophysics, Geosystems 2 2000GC000132.
Kröner, A., Greiling, R., Reischmann, T., Hussein, I.M., Stern, R.J., Durr, S., Kruger, J.,Zimmer, M., 1987. Pan-African crustal evolution in northeast Africa. In: Kröner, A.(Ed.), Proterozoic Lithospheric Evolution: Geodynamic Series, 17, pp. 235–257.
Leake, B.E., Woolley, A.R., Birch, W.D., Burke, E.A.J., Ferraris, G., Grice, J.D., Hawthorne,F.C., Kisch, H.J., Krivovichev, V.G., Schumacher, J.C., Stephenson, N.C.N., Whittaker,E.J.W., 2004. Nomenclature of amphiboles: additions and revisions to the Interna-tional Mineralogical Association's amphibole nomenclature. The Canadian Miner-alogist 41, 1355–1370.
Maurel, C., Maurel, P., 1982. Étude expérimentale de la distribution de l'aluminiumentre bain silicaté basique et spinelle chromifère. Implications pétrogénétiques:teneur en chrome des spinelles. Bulletin de Minéralogie 105, 197–202.
Melcher, F., Meisel, 2004. A metamorphosed early Cambrian crust–mantle transition inthe eastern Alps, Austria. Journal of Petrology 45, 1689–1723.
Mellini, M., Rumori, C., Viti, C., 2005. Hydrothermally reset magmatic spinels in retro-grade serpentinites: formation of “ferritchromit” rims and chlorite aureoles. Con-tributions to Mineralogy and Petrology 149, 266–275.
Müntener, O., Kelemen, P., Grove, T.L., 2001. The role of H2O during crystallisation ofprimitive arc magmas under uppermost mantle conditions and genesis of igneouspyroxenites: an experimental study. Contributions to Mineralogy and Petrology141, 643–658.
Nicolas, A., 1986. A melt extraction model based on structural studies in mantle perido-tites. Journal of Petrology 27, 999–1022.
Nicolas, A., 1989. Structures of Ophiolites and Dynamics of Oceanic Lithosphere.Kluwer, Dordrecht. 367 pp.
Niu, Y., O'Hara, M., Pearce, J.A., 2003. Initiation of subduction zones as a consequence oflateral compositional buoyancy contrast within the lithosphere: a petrological per-spective. Journal of Petrology 44, 851–866.
Nozaka, T., 2003. Compositional heterogeneity of olivine in thermally metamorphosedserpentinite from southwest Japan. American Mineralogist 88, 1377–1384.
Nozaka, T., 2005. Metamorphic history of serpentinite mylonites from the Happo ultra-mafic complex, central Japan. Journal of Metamorphic Geology 23, 711–723.
Nozaka, T., 2011. Constraints on anthophyllite formation in thermally metamorphosedperidotites from southwestern Japan. Journal of Metamorphic Geology 29, 385–398.
O'Hanley, D.S., 1996. Serpentinites: records of tectonic and petrological history: OxfordMonographs in Geology and Geophysics, 34. 277 pp.
Ohara, Y., Stern, R.J., Ishii, T., Yurimoto, H., Yamazaki, T., 2002. Peridotites from theMariana Trough: first look at the mantle beneath an active back-arc basin. Contri-butions to Mineralogy and Petrology 143, 1–18.
Oliver, R.L., Nesbitt, R.W., Hausen, D.M., Franzen, N., 1972. Metamorphic olivine in ul-tramafic rocks from Western Australia. Contributions to Mineralogy and Petrology36, 335–342.
Patchett, P.J., Chase, C.G., 2002. Role of transform continental margins in major crustalgrowth episodes. Geology 30, 39–42.
Pearce, J.A., Barker, P.F., Edwards, S.J., Parkinson, I.J., Leat, P.T., 2000. Geochemistry andtectonic significance of peridotites from the South Sandwich arc-basin system,South Atlantic. Contributions to Mineralogy and Petrology 139, 36–53.
Pearson, D.G., Davies, G.R., Nixon, P.H., 1993. Geochemical constraints on the petrogen-esis of diamond facies pyroxenites from the Beni Bousera peridotite massif. Journalof Petrology 34, 125–172.
Pinsext, R.H., Hirst, D.M., 1977. The metamorphism of the Blue River ultramafic body,Cassiar, British Columbia, Canada. Journal of Petrology 18, 567–594.
83A.H. Ahmed et al. / Lithos 142-143 (2012) 67–83
Proenza, J., Gervilla, F., Melgarejo, J.C., Bodinier, J.L., 1999. Al- and Cr-rich chromititesfrom the Mayari–Baracoa Ophiolitic Belt (eastern Cuba): consequence of interac-tion between volatile-rich melts and peridotite in suprasubduction mantle. Eco-nomic Geology 94, 547–566.
Rao, D.R., Rai, H., Kumar, J.S., 2004. Origin of oceanic plagiogranite in the Nidar ophio-litic sequence of eastern Ladakh, India. Current Science India 87, 999–1005.
Ries, A.C., Schackelton, R.M., Grahans, R.H., Fitches, W.R., 1983. Pan-African structure,ophiolites and mélange in the Eastern Desert of Egypt, a traverse at 26 N.J. Geolog-ical Society of London 140, 75–95.
Roeder, P.L., Campbell, I.H., 1985. The effect of postcumulus reactions on compositionof chrome-spinels from the Jimberlana Intrusion. Journal of Petrology 26, 763–786.
Roeder, P.L., Reynolds, I., 1991. Crystallization of chromite and chromium solubility inbasaltic melts. Journal of Petrology 32, 909–934.
Santos, J.F., Scharer, U., Ibarguchi, J.I.G., Girardeau, J., 2002. Genesis of pyroxenite-richperidotite at Cabo Ortegal (NW Spain): geochemical and Pb–Sr–Nd isotope data.Journal of Petrology 43, 17–43.
Seyler, M., Toplis, M.J., Lorand, J.P., Luguet, A., Cannat, M., 2001. Clinopyroxene micro-textures reveal incompletely extracted melts in abyssal peridotites. Geology 29,155–158.
Seyler, M., Lorand, J.-P., Dick, H.J.B., Drouin, M., 2007. Pervasive melt percolation reac-tions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15°20′N:ODP Hole 1274A. Contributions to Mineralogy and Petrology 153, 303–319.
Shackleton, R.M., Ries, A.C., Graham, R.H., Fitches, W.R., 1980. Late Precambrian ophio-litic melange in the eastern desert of Egypt. Nature 285, 472–474.
Shervais, J.W., 1979. Ultramafic and mafic layers in the Alpine lherzolite massif at Bal-muccia, N. W. Italy. Proc. 2nd Symp. Ivrea-Verbano: Università di Padova MemorieDi Scienze Geologiche, 33, pp. 133–145.
Shimron, A.E., 1984. Metamorphism and tectonics of a Pan-African terrain in south-eastern Sinai. A discussion. Precambrian Research 24, 173–188.
Spandler, C.J., Arculus, R.J., Eggins, S.M., Mavrogenes, J.A., Price, R.C., Reay, A., 2003. Pet-rogenesis of the Greenhills Complex, Southland, New Zealand: magmatic differen-tiation and cumulate formation at the roots of a Permian island-arc volcano.Contributions to Mineralogy and Petrology 144, 703–721.
Spiegelman, M., Kelemen, P.B., Aharonov, E., 2001. Causes and consequences of flow or-ganization during melt transport: the reaction infiltration instability. Journal ofGeophysical Research 106, 2061–2078.
Spray, J.G., 1989. Upper mantle segregation processes: evidence from alpine-type peri-dotite. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins: Geo-logical Society Special Publication, 42, pp. 29–40.
Springer, R.K., 1974. Contact metamorphosed ultramafic rocks in the western SierraNevada foothills, California. Journal of Petrology 15, 160–195.
Staudigel, H., Schreyer, W., 1977. The upper thermal stability of clinochlore, Mg5Al(AlSi3O10(OH)8, at l0–35 kb PH2O. Contributions to Mineralogy and Petrology 61,187–198.
Stern, R.J., 1985. The Najd fault system, Saudi Arabia and Egypt: a late Precambrian rift-related transform system? Tectonics 4, 497–511.
Stern, R.J., Hedge, C.E., 1985. Geochronologic and isotopic constraints on Late Precam-brian crustal evolution in the Eastern Desert of Egypt. American Journal of Science285, 97–127.
Stern, R.J., Johnson, P.R., Kröner, A., Yibas, B., 2004. Neoproterozoic ophiolites of theArabian–Nubian Shield. In: Kusky, T.M. (Ed.), Precambrian Ophiolites and RelatedRocks: Development of Precambrian Geology, 13, pp. 95–128.
Suhr, G., 1999. Melt migration under oceanic ridges: inferences from reactive transportmodeling of upper mantle hosted dunites. Journal of Petrology 40, 575–599.
Takazawa, E., Abe, N., Seyler, M., Meurer, W., 2007. Hybridization of dunite and gabbro-ic rocks in Hole 1271B from Mid-Atlantic Ridge 15°N: implications for melt flowand reaction in the upper mantle. In: Kelemen, P.B., Kikawa, E., Miller, D.J. (Eds.),Proceeding of Ocean Drilling Program: Scientific Results, 209, pp. 1–9.
Tamura, A., Arai, S., 2006. Harzburgite–dunite–orthopyroxenite suite as a record ofsupra-subduction zone setting for the Oman ophiolite mantle. Lithos 90, 43–56.
Tracy, R.J., Robinson, P., Wolff, R.A., 1984. Metamorphosed ultramafic rocks in the Bron-son Hill anticlinorium, central Massachusetts. American Journal of Science 284,530–558.
Trommsdorff, V., Evans, B.W., 1972. Progressive metamorphism of antigorite schist inthe Bergell tonalite aureole, Italy. American Journal of Science 272, 423–437.
Trommsdorff, V., Evans, B.W., 1974. Alpine metamorphism of peridotitic rocks. Schwei-zerische Mineralogische und Petrographische Mitteilungen 54, 333–352.
Trommsdorff, V., López Sánchez-Vizcaíno, V., Gómez-Pugnaire, M.T., Müntener, O.,1998. High pressure breakdown of antigorite to spinifex-textured olivine andorthopyroxene, SE Spain. Contributions to Mineralogy and Petrology 132, 139–148.
Uesugi, J., 2004. The late-intrusive rocks in the Oman ophiolite: their implications forthe origin of ophiolite and for the formation of intra-oceanic arc. PhD Thesis, Kana-zawa University, Japan.
Varfalvy, V., Hébert, R., Bédard, J.H., 1996. Interactions between melt and upper-mantleperidotites in the North Arm Mountain massif, Bay of Islands ophiolite, Newfound-land, Canada: implications for the genesis of boninitic and related magmas. Chem-ical Geology 129, 71–90.
Wicks, F.J., Whittaker, E.J.W., 1977. Serpentine textures and serpentinization. The Cana-dian Mineralogist 15, 459–488.
Wilson, M., 1989. Igneous Petrogenesis. Unwin Hyman, London, p. 446.Yaliniz, M.K., Floyd, P.A., Gönsüoğlu, M.C., 2000. Petrology and geotectonic significance
of plagiogranite from the Sarikaraman ophiolite (Central Anatolia, Turkey). Ofioliti25, 31–37.
Zane, A., Sassi, R., 1998. New data on metamorphic chlorite as a petrogenetic indicatormineral, with special regard to greenschist-facies rocks. The Canadian Mineralogist36, 713–726.
Zhou, M.-F., Robinson, P.T., Malpas, J., Li, Z., 1996. Podiform chromitites in the Luobusaophiolite (southern Tibet): implications for melt–rock interaction and chromitesegregation in the upper mantle. Journal of Petrology 37, 3–21.
