THE FIFTH INTERNATIONAL CONFERENCE ON THE OF Vol. (1), P-P V-181 - V- 213 (OCT. 2007) ASSIUT-

PLAGIOCLASE FROM , , AND THEIR BEARING ON THE EVOLUTION OF LITHOSPHERIC MANTLE BENEATH AN EMBRYONIC OCEAN

Mohamed M. A. Abu El-Rus Geology Department, Assiut University, Assiut 71516, Egypt [email protected]

ABSTRACT

The Southern Mass in Zabargad Island represents a fragment of subcontinental mantle tectonically exhumed during the early rifting of the Red Sea. It is dominated by foliated plagioclase-lherzolites, enclosing relics of older un-deformed protogranular -lherzolites. Dunite is a minor variety and is reported only along the fault zone bounding the mass from the north. The textural relationships, mineral compositions and bulk chemistry data indicate that the Southern Peridotite Mass had equilibrated at high temperatures in the garnet-stability field before ascending to shallower depths. During uplifting, the mass had been subjected to deformation resulting in the development of foliation fabrics observed in most of the samples. Thermobarometric calculations indicate that the deformation of the peridotite mass had started in the garnet-stability field and ended within the plagioclase-stability field (at ~ 5.1 kbar). The crystallization of traces of amphibole in the granoblastic domains and the presence of secondary fluid trails cutting the and exsolved pyroxenes (pristine phases) in the highly deformed samples reinforce the invasion of H2O-bearing fluids in the deformed parts of the mass. The presence of hydrous phases could lower the solidus temperatures of the peridotite mass, causing further partial melting and depletion during ascent at relatively low pressure. During late stage and after deformation, the Southern Peritodite Mass was variably impregnated with hot silicate melt crystallizing calcic plagioclase (An76.7 – An92.4) and unexsolved pyroxenes (Mg# 0.88- 0.91). The impregnation had started at ~ 10.5 kbar and continued to ~ 3.8, considerably below the dry solidus. The P-T path obtained for the peridotite mass indicates subsolidus uplift which is particularly consistent with the thermal history expected for the footwall of a lithosphere-scale, gently dipping extensional shear zone, and is different from the P-T trajectories of mantle diapirism. This interpretation is consistent with the overall asymmetric architecture of the coeval passive margins bordering the Red Sea. KEY WORDS: Lithospheric mantle, Subcontinental mantle, Embryonic ocean, Plagioclase lherzolites, Protogranular, P-T trajectory, Simple shear rifting, Mantle diapirism, Sub-solidus.

INTRODUCTION Three mantle peridotite masses crop out in Zabargad Island. The Southern Peridotite Mass is peculiarly enriched in plagioclase content. Three genetic models have been proposed to explain the formation of plagioclase in the mantle : (1) Crystallization from melt pockets that resulted from partial melting of upper mantle diapirs with incomplete melt extraction (e.g. Boudier & Nicolas 1972; Menzies 1973; Quick 1981; Boudier & Nicolas 1986; Nicolas 1986a, Takahashi 2001). (2) Accumulation from exotic melts that impregnated the lithospheric mantle (e.g. Dick 1992; Elthon 1992; Dijkstra et al. 2001; Müntener et al. 2004; Piccardo et al. 2007). (3) Subsolidus transformation of spinel to plagioclase during mantle upwelling (e.g. Hamlyn & Bonatti 1980; Vissers al. 1991; Hoogerduijn Strating et al. 1993; Rampone et al. 1993). Therefore, recognition of the genesis of plagioclase in mantle peridotites is important to constrain on the evolution of the lithospheric mantle beneath Zabargad Island, V-182 Mohamed M. A. Abu El-Rus

whether it represents uplifts under essentially subsolidus conditions or follows the P-T path across the lherzolite dry solidus (i.e. mantle diapirism). This paper presents new petrological and geochemical data on the Southern Peridodite Mass in Zabargad Island. The data are used (1) to constrain on the origin of plagioclase in the mantle peridotites whether it is a subsolidus phase or crystallized from in situ melt fractions or from incorporated exotic melt fractions; (2) to constrain on the P-T path for the evolution of the Southern Peridotite Mass and assess whether this evolution is consistent with uplift of subcontinental lithospheric mantle or with upwelling asthenosphere diapirism, and (3) to discuss the geodynamic significance of Zabargad peridotites in the context of the opening of the Red Sea.

GEOLOGICAL BACKGROUND

The Zabargad Island (St.John's Island) is located in the Red Sea slightly to the north of Latitude 23o 37' N, where a transition zone between the southern Red Sea, with a nearly continuous axial rift valley, and the northern Red Sea, where the axial valley is not yet developed (Bonatti et al., 1981). Geological and geophysical data indicate that the lithosphere in this sector of the Red Sea consists of thinned and extended continental lithosphere, most probably intruded by abundant basaltic dykes (Bonatti et al., 1983, 1986). The study of Zabargad peridotites therefore provides a unique opportunity to understand the nature of mantle processes that operate during break-up of continents and formation of new oceanic basins.

Fig. 1. Simplified geological map of Zabargad Island (modified after Bonatti et al., 1983). 1: Gneisses complex; 2: Mantle peridotites; 3: Basalt and dolerite dikes and shallow basic intrusions; 4: Zabargad Formation; 5: Evaporites; 6: Breccias and conglomerates; 7: Reefal limestones; 8: Principal faults.

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-183

The island exposes three mapable masses of mantle-derived peridotites (Fig. 1) brought to the surface during the early stages of the Red Sea opening (e.g. Bonatti 1985; Bonatti & Michael 1989; Bonatti et al. 1981, 1983, 1986; Nicolas et al. 1987, 1994; Piccardo et al. 1988, Brueckner et al. 1988, 1995; Voggenreitter et al. 1988a; Bohannon 1989; Lancelot & Bosch 1991, Takla et al. 1997, Bosch & Bruguier 1998). Previous studies have revealed a great diversity in the texture, mineralogy and chemistry of the Zabargad peridotites (e.g. Bonatti et al. 1986; Brueckner et al. 1995, Brooker et al. 2004). This diversity was attributed to a complicated evolutionary history involving processes of decompressional partial melting (Bonatti et al. 1986; Piccardo et al. 1988); retrograde metamorphism associated with uplift (Boudier et al. 1988; Agrinier et al. 1993); intrusion by a large number of and gabbro dykes (Piccardo et al.,1988, 1993; Vannucci et al., 1991); two cycles of hydrous metasomatism (Brueckner et al., 1988; Dupuy et al., 1991); an episode of hydrothermal metsomatism related to the intrusion of MORB-type basaltic dikes (Petrini et al., 1988) and two or three deformation events (Bonatti et al., 1986; Nicolas et al., 1987). A special feature is the prevalence of plagioclase lherzolites in the Southern mass whereas the Northern and Central masses are dominantly composed of spinel peridotites. Genetically, the Southern Peridotite Mass is considered as a recent asthenospheric diapiric mantle piercing an ancient subcontinental lithospheric mantle now exposed as the Northern and Central Peridotite Masses (Dupuy et al., 1991; Boullier et al., 1997; Snow & Schmidt, 1999). Nicolas et al. (1987) and Boudier & Nicolas (1991) considered that the three peridotite masses represent parts of an asthenospheric diapir where the Southern Peridotite Mass was derived from the inner part of the mantle diapir whereas the Central and Northern Peridotite Masses were derived from outer shell of this diapir. By contrast, several authors (Piccardo et al., 1988, 1993; Brueckner et al., 1988, 1995) rule out this interpretation and attributed the difference in the mineral assemblages between the peridotite masses to local conditions during mantle intrusion. Furthermore, timing of events in the Zabargad peridotites is controversial. Piccardo et al. (1988, 1993) considered all Zabargad peridotites represent pre-rift lithospheric sub- continental mantle which was accreted from asthenosphere to the Arabian-Nubian lithosphere prior to and coeval with the Pan-African evolution. Brueckner et al. (1988, 1995), based on the isotopic data, concluded that both peridotites and granulite-facies gneisses of Zabargad Island differentiated from a common depleted mantle source, juxtaposed and brought to relatively shallow crustal levels during Pan-African Orogeny and were exhumed to the surface possibly during the lower Cretaceous, but not later than the Miocene.

THE SOUTHERN PERIDOTITE MASS

The Southern Peridotite Mass constitutes the highest relief (265 m above sea level) in the Zabargad Island. It is located at the southeastern shoreline of the Island faulted to the NW against the Zabargad Formation of Cretaceous or Palaeocene age (Bonatti et al. 1983; Omran 1996) and overlain from the NE by Reefal limestones of Pleistocene age (Bonatti et al. 1983) (Fig.1). The mass lacks direct contact with Pan-African polymetamorphic gneisses (Bonatti et a.l. 1986; Boudier et al. 1988; Lancelot & Bosch 1991; Brueckner et al. 1995; Bosch & Bruguier 1998). The Southern Peridotite Mass is exceptionally fresh except along the northern fault zone, where the peridotite rocks are strongly altered to green serpentinites in which Ni-rich garnierite and Ca-rich cancrinite and scapolite were recorded (El Shazly & Saleeb 1979; Takla & Griffin 1980). Large gem-quality peridote crystals have been also mined along this fault zone. The growth of these crystals has been attributed to the influence of the hydrothermal solutions (El Shazly & Saleeb 1979; Bonatti et al. 1983, 1986) probably during the uplift and exposure of the peridotite mass (Snow & Schmidt 1999). In contrast, Takla et al. (1997) considered that the peridote crystals are of primary magmatic origin and were formed within the upper mantle, while the hydrothermal soultions caused only the serpentinization of the host rock and loosening of the crystals. The Southern peridotite mass includes black anhydrous pyroxenite bands, pokects or lenses, up to 1 m thick, commonly trending N 120o -140o. The pyroxenite bands may locally constitute

V-184 Mohamed M. A. Abu El-Rus

up to 10 vol% of the mass outcrop. They occasionally show compositional zonation due to variation in the modal abundances of pyroxenes and spinel minerals; clinopyroxene tends to concentrate towards the margins of the lenses, while orthopyroxene and spinel cluster in the center. The peridotites and usually exhibit a mild NW-SE foliation which is commonly defined by the alignment of thin white plagioclase pockets (< 5 mm thick). This fabric (D1) was thought to be developed while the peridotites and pyroxenites were still deep in the mantle (Nicolas et al. 1987; Piccardo et al. 1988). The deep-seated foliation is locally o transected by centimeters-thick mylonitic shear zones (D2.) trending about N 120 . The peridotites and pyroxenites are transformed to mylonites to blastomylonites along these shear zones. Boudier et al. (1988) described apparently similar shear zones in the Pan-African gneisses. These shear zones were, therefore, most probably developed after juxtaposition of the mantle peridotite masses and gneisses at shallow levels. The Southern Peridotite Mass is intruded by basaltic and doleritic dykes (up to several meters thick), trending mainly N-S and sometimes displaced by the mylonitic shear zones D2. The age of the basalt dikes has been determined to be between 0.9 and 1.7 Ma ( El-Shazly et al. 1974). Petrini et al. (1988) proposed that the basaltic dykes were derived from a chemically heterogeneous mantle source during various stages of the Red Sea rifting. Subsequent brittle deformation is demonstrated by systems of fractures with prevailing E-W direction parallel to the major fault bounding the Southern Peridotite Mass from the north.

Petrography

The Southern Peridotite Mass is made up chiefly of plagioclase-lherzolite containing residual spinel, sometimes enclosed within feldspar aggregates. Lherzolite assemblages lacking plagioclase are rare and sporadic. Dunites are rare and were reported only along the fault bounding the mass from the north.

1- Lherzolites Most of the lherzolite samples collected form the Southern Peridotite Mass are deformed and exhibit porphyroclastic or porphyroclastic-protogranular transitional textures (nomenclature after Mercier & Nicolas 1975). Few samples are relatively undeformed and essentially displaying protogranular textures (Fig. 2a). Besides the four common mineral phases (ol + cpx + opx+ Cr-sp), the lherzolite samples contain variable amounts of plagioclase, up to > 5 vol%. The amount of plagioclase is notably correlated to the degree of deformation of the samples; undeformed lherzolite samples exhibiting, protogranular texture usually lack or contain traces of plagioclase. Minor amounts of amphibole minerals partly replace the clinopyroxene porphyroclasts or form fine grains in the granoblastic domains in the deformed samples. The unstrained protogranular lherzolite samples might, therefore, represent the earliest and pristine assemblages in the Southern Peridotite Mass that have not been modified by deep seated deformation and plagioclase enrichment processes. Several lherzolite samples show late stage cataclasis and mylonitization and. the degree of mylonitization can vary greatly over short distances (few meters across). a- Protogranular sp-lherzolites Protogranular sp-lherzolite samples are characterized by large grains of olivine and orthopyroxene (up to 5 mm across) coexisting with smaller grains of clinopyroxene (<1.5 mm across) and Cr-spinel (< 1 mm). The olivine and orthopyroxene grains are commonly undeformed or mildly strained with curvilinear to straight boundaries with 120o triple junctions (Fig. 2a), except locally where the grains re-crystallized into strain-free granoblastic aggregates. The mildly strained grains occasionally show wavy extinction or kink bands. The orthopyroxene grains usually show exsolved cores, containing thin lamellae of clinopyroxene ± spinel, surrounded by narrow exsolution-free rims. Locally, the orthopyroxene grains are recrystallized along their boundaries into fine rounded to subrounded exsolution-free orthopyroxene and olivine granoblasts. The clinopyroxene grains do not commonly preserved their original boundaries; they are variably recrystallized to smaller granoblastic aggregates (Fig. 2b) consisting of olivine, clinopyroxene,

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-185

orthopyroxene and rarely spinel. The clinopyroxene grains, are mildly strained and posses rich exsolved cores, comprise lamellae of orthopyroxene ± spinel. In the more deformed samples, the clinopyroxene porphyroclasts are partly replaced by pale green amphibole that may form spindle- shaped lamellae and blebs within the clinopyroxene porphyroclasts. The clinopyroxene granoblasts are rounded to sub-rounded (< 0.2 mm), strain-free and without exsolution lamellae. Cr-Spinel forms brown to greenish brown interstitial grains mostly along the boundaries of orthopyroxenes (Fig. 2a). Plagioclase, if present, forms thin rims around the brown Cr- spinel grains or occurs as fine exsolution lamellae within the clinopyroxene grains. The late stage, low temperature mineral association consisting of secondary amphibole, chlorite, serpentine and magnetite fill sometimes cracks or fractures in some samples (Fig. 2b). b- Plagioclase- lherzolites

Plagioclase lherzolites are variably strained and usually foliated (tectonite foliation) due to the preferred orientation of deformed olivine, pyroxene and spinel grains. In the weakly strained samples, the mineral grains show undulatory extinction, lose their curvilinear boundaries and partially re-crystallize into fine granular aggregates (Fig. 2c). In the highly-strained samples, the large porphyroclasts are no longer preserved and are recrystallized into granular mosaic assemblages (Fig.2d). The highly-strained rocks, are commonly composed of small elongate to tabular grains of olivine, pyroxene and Cr-spinel porphyroclasts (<1.5 mm long) set in a fine granoblastic matrix (<1 mm) (Fig.2d). This texture is comparable with the porhyroclastic texture defined by Mercier & Nicolas (1975). Secondary fluid inclusion trails are common in the olivine and orthopyroxene porphyroclasts in highly strained rocks. a b chl

opx ol ol ol sp cpx opx ol

ol opx opx 670 350 µm µm c opx opx d opx ol ol opx pl opx

cpx ol ol

670 µm 670µm

Fig. 2. (a-b) Textures in protogranular spinel-peridotite. (a) Unstrained grains of olivine (ol), orthopyroxene (opx) and Cr-spinel (sp) exhibiting curvilinear boundaries and triple junctions inbetween. (b) Highly exsolved clinopyroxene protogranular, re-crystallized along the peripheries to fine ol+cpx + opx granobalstic aggregates. The sample is slightly altered along the cracks and grain boundaries to green chlorite (chl). (c-d) Texture in foliated plagioclase-lherzolites. (c) A weakly strained sample exhibiting mineral grains losing their curvilinear boundaries and partially re-crystallized into fine granular aggregates. Plagioclase occurs as interstitial mass or fine granoblasts between olivine and pyroxenes porphyroclasts. (d) A highly foliated sample showing all mineral phases as small elongate and highly strained porphyroclasts set in fine granoblastic matrix composed of ol+ opx + cpx + Cr-sp+ pl. All photographs are between crossed polarizers.

V-186 Mohamed M. A. Abu El-Rus

Olivine, in low-strained samples mainly forms large, strongly to mildly kinked porphyroclasts (up to 7.5 mm long) that are usually surrounded by fine olivine granules. With increasing strain, the olivine porphyroclasts are broken into irregular crystal fragments or recrystallized into clusters of unstrained grains (up to 2 mm across) with interlocking boundaries. Orthopyroxenes are discriminated according to appearance and mode of occurrence, into: (i) strongly strained, exsolved porphyroclasts (residual orthopyroxenes) (Fig. 2 c&d) and (ii) un- to mildly deformed and unexsolved orthopyroxenes present in the interstices between the plagioclase clusters, and between olivine and pyroxenes porphyroclasts (Fig. 3f). They may occur as large poikilitic grains (up to 5 mm in diameter) enclosing several inclusions of plagioclase ± olivine ± orthopyroxene ± clinopyroxene (Fig. 3g) or as corona rims around olivine porphyroclasts in contact with plagioclase (Fig. 3h; j). Importantly, the exsolved orthopyroxene porphyroclasts are variably embayed by granoblastic clusters of ol+ opx ± cpx ± Cr-spinel (Figs. 2c; 3a). Clinopyroxenes form highly deformed, exsolved porphyroclasts (up to 6 mm across) often elongated parallel to the foliation. They are commonly re-crystallized to fine-grained granoblastic aggregates of olivine+ orthopyroxene + clinopyroxene ± plagioclase in the highly strained samples. The porphyroclasts, sometimes show thick plagioclase lamellae which are in optical continuity with the plagioclase in the granoblastic aggregates at the boundaries of the host porphyroclasts (Fig. 3b). However, the presence of plagioclase exsolution lamellae within the clinopyroxene prophyroclasts, suggests a subsolidus re-crystallization of an early super- silicic clinopyroxene enriched in the Ca-Eskola component (Ca0.5AlSi2O6) (Ozawa 2004). Clinopyroxene, similar to orthopyroxene, also forms large unexsolved mildly to strongly deformed interstitial grains (up to 6 mm in diameter), commonly enclosing poikilitically numerous fine inclusions of olivine+ plagioclase+ orthopyroxene± Cr-spinel. Frequently, the inclusions are oriented in parallel trails parallel to the foliation of the granoblastic matrix (Fig. 3i). Unlike the clinopyroxene porphyroclasts, the unexsolved clinopyroxene grains are free of fluid inclusions and show no signs of recrystallization. Plagioclase occurs essentially as discrete crystals (up to 2.0 mm in diameter) or in polycrystalline clusters (4.0 mm across), sometimes stretched parallel to the elongated porphyroclasts and the foliation in the matrix (Fig. 3h, j, k, m). The magmatic origin of these plagioclase crystals is indicated by their relationships with neighbouring mineral grains. They frequently show a reaction rim against adjoining olivine porphyroclasts composed of thin layer of orthopyroxene (Fig. 3h) or othopyroxene + green spinel intergrowth (Fig. 3j), dissolve the Cr- spinel grains (Fig. 3m) and fill the cracks crosscutting the large deformed olivine and orthopyroxene porphyroclasts. Moreover, the close association of plagioclase and unexsolved interstitial pyroxenes suggests that both plagioclase and pyroxenes crystallized from the same melt. Most of the plagioclase crystals show evidence of plastic deformation. The plastically deformed plagioclase crystals exhibit strong undulose extinction, crystal bending, narrow and tapering deformational lamellar twins and abundant re-crystallization to small grains with irregular boundaries (Fig.3k). These textural features indicate the primary crystallization of plagioclase during and after onset of the tectonite foliation. Cr-spinel occurs in weakly strained samples as discrete undeformed rounded to subrounded grains (up to 2 mm in diameter) between the porphyroclasts. The grains are commonly mantled by thin plagioclase rims (Fig. 3c; e) and sometimes show reaction against the orthopyroxene grains resulting in formation of olivine-plagioclase intergrowths (Fig. 3e). The latter microstructure implies the sub-solidus reaction sp + pxs = ol + pl at transition from the spinel- stability field to the plagioclase-stability field (e.g. Rampone et al., 1993; Piccardo et al., 2007). In the highly-strained samples, Cr-sp commonly occurs as small grains flattened parallel to the tectonite foliation. Cr-spinel may also form small subrounded intergrowths (< 1 mm diameter) with orthopyroxene, clinopyroxene and olivine. These symplectite-like domains may develop in embayments in adjacent deformed olivine porphyroclasts. Similar intergrowths in mantle rocks are usually interpreted as a breakdown product of pre-existing mantle garnets (e.g. Hoogerduijn et al., 1993; Vannucci et al., 1993; Piccardo et al. 2007).

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-187

Amphiboles occur in trace amounts replacing the clinopyroxene porphyroclasts and rarely as fine granoblastic domains comprising cpx + ol + opx ± Cr-spinel ± pl in the highly strained samples. a b opx pl opx pl ol

ol cpx

opx 350 µm 350 µm c d ol sp ol opx opx opx sp sp opx cpx cpx pl sp ol ol opx

165 µm 165 µm e opx f ol

sp

ol + pl ol opx

ol opx 165 µm 165 µm

Fig. 3. Photomicrographs documenting textural and mineralogical evolution in the Southern Peridotite Mass. (a-e) Textures related to partial melting event and sub-solidus reactions. (a) Heavy strained exsolved orthopyroxene porphyroclasts embayed by granoblasts clusters of ol+ opx ± cpx ± cr-spinel, implying incongruent partial melting predating or during deformation. (b) The clinopyroxene porphyroclasts show thick plagioclase exsolution lamellae, sometimes in optic continuity with plagioclase granoblasts corroding the peripheries of the porphyroclast. (c) Thin plagioclase rim armoring the Cr-spinel. (d) Rounded to subrounded fine granoblast aggregates formed mainly of Cr-spinel + orthopyroxene + subordinate clinopyroxene + olivine. These clusters may represent breakdown products of pre-existing mantle garnets (see text). (e) Olivine-plagioclase intergrowths developed along the boundary between Cr-spinel and orthopyroxene porphyroclasts. This texture was most probably formed by the subsolidus reaction sp + pxs =ol + pl due to transition from the sp-stability field to the pl- stability field. (f-m) Textures related to melt impregnation and solid-melt reactions. (f) The interstitial mass of unexsolved orthopyroxene between the olivine porphyroclas.

V-188 Mohamed M. A. Abu El-Rus g opx h cpx pl ol ol pl pl opx pl opx ol pl ol pl opx ol ol sp pl 350 µm 165 µm i pl j pc

cpx pl

opx opx +sp pl ol cpx

670 µm 670 µm k m e ol pl ol pl pl

sp pl opx pl pl ol sp ol pl 670 µm 350 µm

Fig. 3. Continued. (g) Large irregular poikilitic grains of the unexsolved orthopyroxene enclosing fine grains of plagioclase. (h) An orthopyroxene reaction rim developed along the boundaries of olivine grains in contact with the polycrystalline plagioclase pockets; note that plagioclase crystals show no or weak strain effects. (i) A large unexsolved poikilitic clinopyroxene grain enclosing numerous fine inclusion trails parallel to the foliation in the matrix; note that the rims of the grain are often free of the inclusions. (j) An opx – green spinel intergrowth developed along the boundaries of olivine porphyroclast in contact with the plagioclase polycrystalline pocket; note the partly re-crystallization of plagioclase to smaller grains cluster (pc). (k) Plagioclase discrete crystals elongated parallel to the foliation in the high-strained samples; note that the plagioclase crystals exhibit plastic deformation effect represented by wavy extinction crystal bending, narrow and tapering lamellar twins. (m) Corroded Cr-spinel surrounded by plagioclase polycrystalline aggregates. All photographs are between crossed polarizers.

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2-Dunites Dunites exhibit protogranular texture, and consist mainly of moderately to highly strained megascopic grains of olivine (up to 30 mm long) together with minor interstitial grains of altered pyroxenes and brown Cr-spinel. Plagioclase is notably absent while pyrrhotite and pyrite may be present as fine grains in and along the boundaries of the olivine grains. The olivine grains are usually traversed by a network of carbonate-serpentine (crysotile) veinlets. Their boundaries are sometimes fragmented or re-crystallized to fine anhedral to subhedral mildly strained granoblasts.

ANALYTICAL METHODS

Mineral analyses were carried out using a CAMECA CAMEBAX Sx100 electron microprobe fitted with a LINK energy-dispersive system at the Department of Geosciences, University of Oslo, Norway. Accelerating voltage was 15 kV and peaks were counted for 10s and backgrounds for 5s using a beam current of 20 nA for Na-poor phases (olivine, pyroxenes, ) and 10 nA for plagioclase. A focused beam was used, except for traverse analyses across pyroxenes, where a defocused beam was used to retrieve the bulk composition of the pyroxenes before subsolidus exsolution. A beam, 10 µm in diameter, was used for analyzing pyroxene grains containing exsolution lamellae < 1 µm thick. The pyroxene grains having ~ 1 to 10 µm thick exsolution lamellae were analysed using a beam 50 µm in diameter. The defocused analyses were carried out at equally spaced spots in the direction perpendicular to the exsolution lamellae of pyroxene. Light elements (Na, K) were counted first in all analyses to preclude loss by volatilization. Oxides, as natural and synthetic standards, have been used. Matrix corrections were performed by the PAP-procedure in the CAMECA software. Analytical precision (2σ) evaluated by repeated analyses of individual grains is better than ± 1% for elements in concentrations of > 20 wt% oxide, better than ± 2% for elements in the range of 10-20 wt% oxide, better than 5% for elements in the range of 2-10 wt % oxide and better than ± 10% for the elements in the range of 0.5-2 wt% oxide. Fe2+ and Fe3+ were distributed according to the stoichiometry. Representative mineral analyses are presented in Tables 1-6. Major elements in bulk rocks were determined on a Phillips PW 1220 X-ray fluorescence spectrometer (XRF) at the Department of Earth Sciences, Aarhus University, Denmark, using glass fusion discs and pressed powder tablets and international standards for calibration. Analyses of SiO2, TiO2, Al2O3, Fe2O3 (total), CaO and K2O were performed on glass discs, prepared by fusion with a lithium tetraborate and lithium metaborate mixture. P2O5 was analyzed on pressed powder pellets, whereas Na2O, MnO and MgO were analyzed by atomic absorption. FeO was determined by titration. With the exception of dunite samples, all the analyzed samples have L.O.I. < 2.5 wt%. The dunite samples gave L.O.I. up to 11.5 wt% consistent with the partial serpentinization of these samples. Analytical precision (2σ) is better than 0.5%. The analytical results a represented in Table 7.

MINERAL CHEMISTRY

1- lherzolites

Olivines in spinel-lherzolites show a restricted compositional range in the cores of the olivine porphyroclasts (Fo90.7-91.4, 0.38-0.45 wt% NiO). Olivine is less in magnesium (Fo 88.23-90.66) and NiO (0.27-0.40 wt%) in the plagioclase –lherzolites than in the spinel –lherzolites (Table 1; Fig. 4). CaO is usually below the detection limit, except in the highly-strained samples Zb10B and Zb12B, where it reaches up to 0.15 wt%. The olivine porphyroclasts are compositionally homogeneous or possess rims lower in content than the cores (Table1). Olivine neoblasts are indistinguishable chemically from the porphyroclasts (Table 1; Fig. 4). Orthopyroxenes are comparatively higher in Mg#, Al2O3 and Cr2O3 and lower in TiO2 and CaO in the spinel lherzolites than in the plagioclase-lherzolites (Fig. 5; Table 2). However, the orthopyroxene porphyroclasts vary in composition with texture, where the highest amounts of Cr and Ti and the lowest amounts of Al are observed in the highly-strained samples (e.g. samples Zb-7 & Zb-17; Table 2). The unexsolved interstitial orthopyroxene grains are relatively lower in Mg# and Al2O3 and higher in TiO2 compared with the orthopyroxene porphyroclasts in

V-190 Mohamed M. A. Abu El-Rus

the spinel-lherzolites (Fig. 5; Table 2). However, both of the orthopyroxene porphyroclasts and interstitial grains show no distinct compositional differences between core and rim, except for a lower content in Al2O3 in the rims (Table 2).

Table 1: Representative olivine analyses (wt%) from the Southern Peridotite Mass

Sp-lherzolite Pl- lherzolite Zb-74 Zb-75 Zb-66 Zb-17 Porph Porph Porph Porph Gran Gran Gran core rim core rim core rim core rim

SiO2 41.32 41.02 41.10 41.21 41.03 41.06 41.04 41.08 40.83 40.88 41.03 FeO 8.45 9.11 9.18 8.98 10.22 10.07 10.05 10.38 9.64 9.60 9.18 MnO 0.13 0.14 0.13 0.11 0.11 0.15 0.14 0.14 0.06 0.01 0.15 MgO 49.63 49.36 49.12 49.14 49.07 49.15 49.07 48.79 49.16 49.17 49.26 NiO 0.41 0.36 0.31 0.38 0.23 0.27 0.30 0.29 0.31 0.29 0.24 Total 99.94 99.99 99.84 99.82 100.66 100.7 100.6 100.68 100.0 99.95 99.86 Fo 91.27 90.63 90.51 90.70 89.54 89.69 89.69 89.34 90.09 90.13 90.53

Pl-lherzolite Dunites Zb-10B* Zb-12B* Zb-93 Zb-95B Porph Porph Porph Porph Gran Gran Gran core rim core rim core rim core rim

SiO2 40.48 40.61 40.43 41.09 41.08 41.16 41.08 41.27 41.07 41.02 41.02 FeO 11.33 11.41 11.46 9.14 9.30 9.15 9.22 9.78 9.73 9.49 9.69 MnO 0.19 0.19 0.18 0.13 0.10 0.12 0.11 0.16 0.09 0.12 0.13 MgO 47.67 47.88 47.7 49.73 49.34 50.03 49.52 49.37 49.34 49.42 49.40 NiO 0.28 0.31 0.31 0.31 0.28 0.30 0.37 0.32 0.31 0.35 0.33 Total 99.95 100.4 100.08 100.4 100.1 100.76 100.3 100.9 100.54 100.4 100.57 Fo 88.23 88.18 88.12 90.66 90.43 90.69 90.55 90.00 90.04 90.28 90.09

* Samples contain large unexsolved pyroxene grains; Porh: porphyroclast; Gran: fine granoblast, Fo: Forsterite content = Mg*100/(Mg+Fetotal). The samples of plagioclase- lherzolites are arranged from left to right with increasing the tectonite deformation. Most points represent the average of three analyses of three porphyroclast grains or the average of five to six analyses of granoblasts in the sample.

Orthopyroxene granoblasts vary significantly in composition both within and between the samples, with a clear tendency of increasing Mg# and decreasing Al2O3 in the granoblasts in the samples containing large unexsolved pyroxene grains (Fig. 5, Table 2). In general, the granoblasts exhibit lower TiO2, CaO and Al2O3 contents (except sample Zb-7) when compared with the porphyroclast cores in the same sample. Orthopyroxenes in the corona rims around olivine are distinctly richer in Al2O3 and poorer in TiO2 and Cr2O3 than the porphyroclast cores in the same sample (Table 2). Clinopyroxene porphyroclasts in the plagioclase lherzolites cover a wide compositional range compared with the composition of the clinopyroxene protogranules in the spinel- lherzolites, where the latter tend to be lower in TiO2, Cr2O3 and Na2O and higher in Mg# and Al2O3 (Table 3; Fig. 6). Both clinopyroxene protogranules and porphyroclasts are chromian dioposide-endioposide with Mg# and Cr# (Cr/(Cr+ Al) ranging from 0.897 to 0.919 and from 0.066-0.152, respectively (Table 3). The clinopyroxene protogranules show significant intra- grain compositional heterogeneity. They exhibit an overall M-shaped Al zoning profile where Al-content increases from the Al-poor core toward the margin, but decreases again at the rims (Fig.7). The variation along the Al profile is clearly compensated by variations in the abundance of Si and Mg, suggesting a Mg-tschermakite substitution Si +Mg ↔2Al. Abundances of Ti and Cr monotonously decrease from the core to the rim whereas Ca abundance is relatively uniform throughout the grain. Similar M-shaped Al zoning in the pyroxenes from mantle peridotites is attributed to decompression of mantle peridotites when passing from the garnet, through spinel, into the plagioclase stability fields (Ozawa & Takahashi 1995; Takazawa et al. 1996; Ozawa 2004). In contrast, the clinopyroxene porphyroclasts in the plagioclase-lherzolites are homogeneous in composition or exhibit rims enriched in Si and depleted in Al relative to the cores (Fig. 7).

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-191

Fig. 4. Compositional variation among from the Southern Peridotite Mass. Fo is the forsterite content {Mg/(Mg +Fetotal)}. proto: protogranular; porph: porphyroclast; gran: fine granoblasts. Filled symbols are data from the pl-lherzolite samples containing large unexsolved pyroxene grains. Each point represents the average of three to six analyses in each sample. The grey field outlines the spinel-peridotite samples.

The Unexsolved clinopyroxene grains are Al-dioposide (Cr# < 0.05), however varying within a considerable compositional range both within and among the samples. They display in general higher TiO2 (< 0.89 wt%) and lower Cr2O3 (< 0.5 wt%) and Na2O ( <0 .7 wt%) contents than the exsolved clinopyroxenes (Fig. 6; Table 3). The unexsolved clinopyroxenes show gradual decrease in Al and increase in Mg#, Si, Ti and Cr toward the rims. Na content is almost constant throughout the grains. Clinopyroxene granoblasts cover significant compositional ranges both within and between the samples (Fig.8; Table3). They usually exhibit higher Mg-number than the cores of the associated clinopyroxene porphyroclasts. Cr-spinels show significant compositional variation strongly correlated with the deformational texture of the sample. The Cr-spinels define a narrow compositional range in the protogranular spinel-lherzolites (Mg# 0.7-0.71, Cr# 0.102-0.104, TiO2 < 0.03 wt%) and plot close or within the more fertile (low Mg# and Cr#) part of the abyssal peridotite field (Fig, 8). In comparison, Cr-spinel from foliated plagioclase-lherzolite covers a wider compositional range (Mg# 0.48-0.73, Cr# 0.07- 0.37, TiO2 0.04-29 wt%); with more depleted compositions (Al-poor and Cr-rich) have been observed in the highly-strained samples (e.g. sample Zb-12b in Table 4). Composition variation along traverses across Cr-spinel grains indicate that they are largely homogeneous in the least deformed samples (e.g. samples Zb-74 & 66; Table 4) whereas they show a clear tendency for increasing Cr# and decreasing Mg# toward the rims in the highly deformed samples (e.g. samples Zb-7 & 12B; Table 4). Fine Cr- spinel granoblasts show compositional variation in the same thin section. They are more or less depleted in MgO and Cr2O3, but usually enriched in TiO2, than the cores of the associated Cr-spinel porphyroclasts.

V-192 Mohamed M. A. Abu El-Rus

Table 2: Representative orthopyroxene analyses (wt%) from the Southern Peridotite Mass

Sp-lherzolite Pl- lherzolite Zb-74 Zb-75B Zb-66 Zb-17 Porph Porph Porph Porph Gran Gran core rim core rim core rim core rim

SiO2 56.00 56.48 55.74 56.45 55.97 56.80 54.08 56.30 56.74 57.95 TiO2 0.06 0.03 0.05 b.d.l. 0.18 0.08 0.07 0.20 0.21 0.09 Al2O3 3.23 2.58 4.10 2.84 1.87 0.64 2.66 1.03 0.85 1.10 Cr2O3 0.15 0.09 0.20 0.08 0.33 0.19 0.59 0.47 0.37 0.50 FeO 6.28 6.12 5.85 6.14 7.14 7.26 9.33 6.50 6.53 6.15 MnO 0.12 0.13 0.11 0.12 0.15 0.16 0.19 0.13 0.15 0.14 MgO 33.76 33.89 33.07 33.75 33.05 33.83 32.29 34.24 34.20 34.03 CaO 0.55 0.24 0.65 0.39 0.48 0.40 0.62 0.45 0.68 0.46 NiO 0.05 0.06 0.06 0.05 0.05 0.06 0.03 0.06 0.06 0.07 Na2O 0.01 0.01 b.d.l. 0.01 0.08 0.02 0.08 0.01 0.10 b.d.l. Total 100.21 99.62 99.83 99.83 99.30 99.43 99.94 99.39 99.89 99.49 Mg# 0.906 0.908 0.910 0.907 0.892 0.893 0.861 0.904 0.903 0.908 Cr# 0.030 0.022 0.031 0.019 0.106 0.164 0.130 0.234 0.236 0.234

Pl-lherzolite Zb-7 Zb-10.B Zb-12.B Porph Porph* Porph* Gran Cor Gran Cor core rim core rim core rim

SiO2 55.96 56.15 55.26 54.99 55.64 55.03 55.97 56.82 58.05 54.73 TiO2 0.21 0.12 0.17 0.01 0.22 0.19 0.21 0.26 0.06 0.08 Al2O3 1.22 1.16 1.65 2.45 3.17 2.89 1.95 0.85 b.d.l. 2.64 Cr2O3 0.54 0.51 0.58 b.d.l. 0.25 0.37 0.59 0.35 0.06 0.10 FeO 6.55 6.64 7.95 7.05 8.06 7.91 6.47 6.44 6.06 7.48 MnO 0.18 0.15 0.22 0.34 0.13 0.20 0.15 0.15 0.14 0.24 MgO 34.28 34.27 33.55 34.90 32.82 32.86 34.04 34.10 35.04 33.7 CaO 0.53 0.57 0.71 0.34 0.39 0.35 0.55 0.37 0.27 1.06 NiO 0.11 0.08 0.08 0.04 b.d.l. b.d.l. 0.08 0.07 0.07 0.01 Na2O 0.01 0.02 0.05 0.08 b.d.l. 0.01 0.01 0.02 0.02 0.04 Total 99.59 99.64 100.41 100.2 100.68 99.81 100.02 99.43 99.77 100.08 Mg# 0.903 0.902 0.883 0.898 0.879 0.881 0.904 0.904 0.912 0.889 Cr# 0.229 0.229 0.191 ------0.050 0.079 0.169 0.104 ------0.025 Porh: porphyroclast; Gran: fine granoblast; Cor: coronatic opx; b.d.l.; below detection limit; Mg#: mg-number (Mg/Mg +Fe total); Cr#: Cr/ (Cr+AL); Porph*: unexsolved orthopyroxene; Most points represent average of three analyses of three porphyroclast grains or average of five to six analyses of granoblasts in the sample.

Plagioclases of subsolidus origin form thin rims around Cr-spinel and exsolved lamellae in clinopyroxene porphyroclasts, as well as intergrowths with olivine along the contact between Cr-spinel and orthopyroxene. They possess a restricted composition range from An48.3 to An53.3. No correlations have been observed between An content and deformation textures. Plagioclases of magmatic origin forming discrete grains or crystal aggregates show a relatively wider composition range and are more calcic in composition (An76.7 – An92.4) than the subsolidus plagioclase. The magmatic plagioclase grains are homogenous, or may exhibit Ca-enriched rims relative to the core (Table 5). Amphiboles have similar composition in both spinel- and plagioclase-lherzolites. They range from edenitic hornblende to Ti-pargasite according to the nomenclature scheme of Leake (1978) (Fig. 9). No correlation has been found between the composition of amphiboles and their mode of occurrence (i.e. fine grains in the granoblastic domains, rims or exsolved spindle- shaped bodies in the clinopyroxene porphyroclasts). Amphiboles in the plagioclase- lherzolites (e.g. samples Zb 7 & Zb-14) are usually lower in Ti2O than those in the spinel-lherzolites (e.g. Zb-74& Zb-66; Table 6). All amphiboles are low in K2O content (<0.4 wt%).

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-193

Fig. 5. Compositional variations among orthopyroxenes from the Southern Peridotite Mass. porph.: porphyroclasts; gran: fine granoblasts; gran*: fine granoblasts from samples containing unexsoved orthopyroxene grains; corona: orthopyroxene rims overgrown on the peripheries of olivine grains adjacent to plagioclase pockets. Each point represents the average of three to six analyses in each sample. The grey fields outline the spinel-peridotite samples.

2- Dunites

Olivine porphyroclasts in dunite are compositionally almost indistinguishable from those in the Sp-lherzolites. They have high Fo-content (90.28 -90.55), with NiO content ranging from 0.37-0.31 wt% and CaO content of about 0.1 wt%. The porphyroclast rims and the granoblasts are slightly poorer in Fo and NiO relative to the cores of the porphyroclasts. Cr-spinel granoblasts posses the most refractory composition (high Cr# and Low Mg#, Table 4) and the highest TiO2 (up to 0.33 wt%) in the Southern Peridotite Mass.

WHOLE ROCK CHEMISTRY

Spinel lherzolites from the Southern Peridotite Mass exhibit a narrow compositional range in all major oxides (e.g. Mg# 89.2-89.8; 2.9-3.7 wt% Al2O3; 0.11-0.13 wt% TiO2; 2.4-3.0 wt% CaO; Table 7; Fig.10 &11). Comparatively, plagioclase lherzolites cover a wider compositional range (e.g. Mg# 88.6-90.6; 1.8-5.7 wt% Al2O3; 0.04-0.135 wt% TiO2; 1.57-3.33 wt% CaO; Table 7; Fig.10 &11). Together, the spinel-lherzolites and plagioclase-llherzolites define clear trends of increasing SiO2, Al2O3, CaO and Na2O with decreasing MgO (Fig. 10 & 11). Generally, the MgO increases, while Al2O3 and CaO contents decrease with increasing intensity foliation development in the samples (Table 7). This indicates that the deformation of the

V-194 Mohamed M. A. Abu El-Rus

Zabargad mantle peridotites was most probably accompanied by partial melting event. The Southern Peridotite Mass defines trends on the MgO-Al2O3 and MgO-SiO2 variation diagrams (Fig. 10 a & b), trends that are mostly coincident with residues formed by initial melting at 2-3 GPa and final melting at 1- ~2.5 GPa with melt fractions ranging from ~ 0.02- 0.1 for spinel.- lherzolites to ~0.02-0.2 for plagioclase-lherzolites. The plagioclase-lherzolite samples of Bonatti et al. (1986) are very enriched in plagioclase (average modal abundance 16%) and their bulk compositions, therefore, seem to be profoundly modified by melt infiltration. The analyzed lherzolite samples on the FeO-MgO plot (Fig. 10 c) define an apparent high initial melting at ~ 5 - > 10 GPa, due to their relatively low FeO contents. The reduction in the FeO content in the analyzed samples is best explained by elevated oxygen fugacity during melt impregnation that has increased the content of Fe2O3 at the expense of FeO. The dunites samples fall within the Mg# range 88.6-90.1 and are distinctly depleted in SiO2, Al2O3 and CaO compared with the lherzolites (Fig. 10& 11; Table 7). Misfits of most dunite samples with melting grids in Figures 8 a-c indicate that the dunites do not represent residues formed by very high degrees of partial melting of the primitive mantle.

Table 3: Representative clinopyroxene analyses (wt%) from the Southern Peridotite Mass

Sp-lherzolite Pl- lherzolite Zb-74 Zb- Zb-66 Zb-17 75B Porph Porph Porph Porph Gran Gran core rim core core rim core rim rim*

SiO2 50.69 52.20 51.14 50.64 51.30 53.65 50.92 50.88 52.34 52.60 TiO2 0.68 0.39 0.52 0.87 0.63 0.17 0.87 1.17 1.05 1.05 Al2O3 6.67 5.48 6.13 6.58 5.85 2.81 4.50 4.43 2.21 2.01 Cr2O3 0.74 0.65 0.73 0.69 0.63 0.54 0.96 0.89 0.63 1.25 FeO 2.78 2.54 2.96 3.08 3.02 2.64 2.47 2.15 2.12 2.56 MnO 0.05 0.05 0.06 0.07 0.09 0.11 0.05 0.08 0.06 0.06 MgO 15.18 15.24 16.20 15.16 14.41 16.94 15.79 16.09 16.50 16.47 CaO 20.76 22.18 20.49 20.50 22.06 23.02 22.9 23.02 24.37 23.43 NiO 0.03 0.10 0.13 0.04 0.03 0.07 0.02 0.02 0.04 0.05 Na2O 1.65 1.01 1.50 1.81 1.23 0.55 0.85 0.40 0.43 0.62 Total 99.24 99.84 99.86 99.44 99.24 100.5 99.33 99.13 99.75 100.1 Mg# 0.907 0.915 0.907 0.897 0.896 0.920 0.919 0.930 0.933 0.920 Cr# 0.069 0.074 0.074 0.066 0.067 0.114 0.125 0.119 0.161 0.294

Pl-lherzolite Zb-7 Zb-10.B Zb-12.B Porph Porph* Porph* Porph* Gran Gran Gran Gran core rim core rim core core rim

SiO2 51.24 56.15 53.24 52.04 50.05 50.78 51.33 49.55 50.68 51.58 50.03 TiO2 0.73 0.12 0.36 0.36 0.96 1.42 1.01 0.89 1.11 1.26 1.45 Al2O3 4.30 1.19 1.96 3.96 7.57 4.90 5.29 7.64 5.16 3.96 5.69 Cr2O3 0.907 0.51 0.46 0.96 0.29 0.54 0.28 0.28 0.23 0.36 0.27 FeO 3.24 6.64 1.61 1.91 3.62 3.25 3.25 3.66 3.55 3.20 3.24 MnO 0.08 0.15 0.08 0.08 0.13 0.10 0.09 0.12 0.09 0.07 0.04 MgO 16.02 34.27 16.84 16.90 15.27 15.28 15.37 14.41 14.94 15.60 14.87 CaO 22.77 0.65 24.02 22.90 20.93 23.08 22.78 22.02 22.73 22.76 23.87 NiO 0.05 0.08 0.04 0.04 0.11 0.12 b.d.l. 0.07 0.04 0.05 0.01 Na2O 0.50 0.05 0.39 0.25 0.48 0.40 0.42 0.50 0.68 0.61 0.63 Total 99.84 99.64 99.14 99.40 99.41 99.87 99.81 99.14 99.21 99.45 100.10 Mg# 0.898 0.902 0.949 0.940 0.883 0.893 0.894 0.875 0.883 0.899 0.891 Cr# 0.124 0.223 0.136 0.140 0.025 0.069 0.034 0.024 0.029 0.057 0.031

Porh: porphyroclast; Gran: granoblast; Mg#: mg-number (Mg/Mg +Fe total); Cr#: Cr/ (Cr+AL) rim*: rim-free exsolution; .Porph*: unexsolved clinopyroxene; b.d.l.; below detection limit. Most points represent average of three analyses of three porphyroclast grains or average of five to six analyses of granoblasts in the sample.

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-195

Fig. 6. Compositional variations among clinopyroxenes from the Southern Peridotite Mass. symbols are as in Fig. 5.

V-196 Mohamed M. A. Abu El-Rus

Fig. 7. Compositional profiles through clinopyroxene grains in the protogranular spinel- peridotite (sample Zb-74) and foliated plagioclase-lherzolite (sample Zb-7). The analyses were performed with defocused beams to retrieve the original composition. Al, Si, Mg and Na cations are calculated on the basis of 6 oxygens.

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-197

Table 4: Representative spinel analyses (wt%) from the Southern Peridotite Mass

Sp- lherzolites Pl- lherzolites Dunite Zb-74 Zb-66 Zb-7 Zb-10.B* Zb-95B Porph Porph Porph Porph Porph Gran Gran Gran core rim core rim core rim core rim core

TiO2 0.03 b.d.l. 0.09 0.13 0.14 0.16 0.12 0.17 0.04 0.09 0.06 0.33 Al2O3 56.88 56.83 55.39 55.77 54.33 33.06 31.14 36.20 49.77 49.41 53.6 30.87 Cr2O3 9.65 9.87 9.86 9.87 10.84 28.87 29.86 26.23 14.08 14.29 9.97 33.61 Fe2O3 2.94 2.96 3.323 2.994 3.16 7.435 8.628 7.09 5.99 6.04 5.457 5.01 FeO 9.60 9.12 10.85 10.94 12.52 17.60 18.03 14.35 9.81 10.31 10.27 17.37 MnO 0.05 b.d.l. 0.11 0.11 0.15 0.24 0.27 0.21 0.15 0.18 0.14 0.16 MgO 20.33 20.39 19.29 19.21 18.03 12.84 12.26 15.13 19.22 18.88 19.22 12.65 NiO 0.09 0.22 0.27 0.27 0.29 0.22 0.21 0.16 0.29 0.35 0.4 0.11 Total 99.58 99.39 99.18 99.29 99.53 100.4 100.5 99.53 99.35 99.55 99.12 100.11 Mg# 0.747 0.755 0.713 0.715 0.748 0.485 0.459 0.566 0.693 0.681 0.693 0.508 Cr# 0.102 0.104 0.107 0.106 0.118 0.369 0.391 0.327 0.160 0.163 0.111 0.422

Porh: porphyroclast; Gran: negranoblast; b.d.l.; below detection limit; Mg#: mg-number (Mg/Mg +Fe total); Cr#: Cr/ (Cr+AL); Fe2O3 is estimated on the basis of 3 cations and 4 oxygen in the structural formula.* sample contains large unexsolved pyroxenes grains. Most points represent average of three analyses of three porphyroclast grains or average of five to six analyses of granoblasts in the sample.

Fig. 8. Compositional variation of Cr-spinel from the Southern Peridotite Mass and associated dunites. Cr# is the cation ratio of Cr/ (Cr+Al) and Mg# is the cation ratio of Mg/(Mg+ Fetotal). Also shown is the compositional field of spinels from abyssal, plagioclase-free, spinel-peridotites that show different degrees of partial melting (up to 18%; Hellebrand et al. 2002). Filled symbols are data from the pl-lherzolite samples containing large unexsolved pyroxene grains.

V-198 Mohamed M. A. Abu El-Rus

Table 5: Representative plagioclase analyses (wt%) from the Southern Peridotite Mass

Sp-Lherzolites Plagioclase-lherzolites Zb- Zb-74 Zb-75B Zb-66 Zb-7 Zb-10B* Zb-12B* 17 r ex r int fg ex gr gr gr(core) gr(rim) gr

SiO2 54.3 55.7 55.71 55.53 55.3 55.1 47.10 46.02 45.50 45.25 47.17 Al2O3 29.1 28.1 28.10 28.35 28.3 28.3 34.04 34.72 34.84 35.04 34.02 FeOtotal 0.13 0.05 0.14 0.24 0.09 0.10 0.09 0.19 b.d.l. b.d.l. 0.10 CaO 10.9 10.0 10.17 10.29 9.89 9.91 17.56 17.95 18.35 18.55 17.14 Na2O 5.19 5.90 5.78 5.71 5.77 5.73 1.55 1.21 0.96 0.85 1.75 K2O 0.04 0.03 0.03 0.01 0.02 0.03 0.06 0.01 b.d.l. b.d.l. b.d.l. Total 99.7 99.8 99.93 100.1 99.4 99.2 100.4 100.1 99.65 99.69 100.18 An 53.6 48.3 49.21 49.87 48.6 48. 8 85.93 89.08 91.35 92.34 84.41 Ab 46.2 51.5 50.62 50.07 51.3 51.0 13.72 10.87 8.65 7.66 15.60 Or 0.22 0.20 0.17 0.06 0.11 0.20 0.35 0.06 ------

* samples contain large unexsolved pyroxene grains; b.d.l.: below detection; r: rim around spinel grain; ex: exsolution in the clinopyroxene porphyroclasts; int: plagioclase-olivine intergrowth along the contact between orthopyroxene porphyroclast and Cr-spinel grain; fg: fine ganoblast grains along the peripheries of the clinopyroxene porphyroclasts; gr: discrete grains or in polycrystalline clusters, An, Ab and Or are the anorthite, albite and orthoclase molecules in the plagioclase.

Table 6: Representative amphibole analyses (wt%) from the Southern Peridotite Mass

Sp-lherz Pl-lherzolites Zb-74 Zb-66 Zb-7 Zb-14 Zb-91A fg fg spind fg fg r fg

SiO2 43.48 43.5 43.34 44.64 44.45 44.16 44.27 TiO2 3.83 3.91 3.18 0.33 0.79 0.75 0.81 Al2O3 11.91 11.65 13.06 11.83 11.83 12.08 10.29 Cr2O3 1.34 1.26 0.62 0.6 1.19 1.25 1.11 FeO 4.68 4.89 4.67 5.69 5.68 5.44 5.69 MnO 0.07 0.09 0.07 0.07 0.02 0.06 0.05 MgO 16.59 17 16.9 17.81 18.25 17.67 20.65 CaO 12.48 12.03 12.16 12.55 12.38 12.22 11.54 NiO 0.07 0.07 0.09 0.06 0.04 0.13 0.09 Na2O 2.43 2.2 2.72 2.66 2.52 2.67 2.39 K2O 0.01 0.02 0.03 0.34 0.23 0.2 0.25 Total 96.89 96.62 96.84 96.58 97.38 96.63 97.14 Mg# 0.863 0.861 0.866 0.848 0.851 0.85.3 0.866

Fg: fine grain in domains together with ol+ cpx ± cr-sp ± pl; spind: spindle-like bodies within clinopyroxene porphyroclasts; r: thin rims replacing the margins of the clinopyroxene porphyroclasts; Mg#: mg-number (Mg/Mg +Fe total).

Fig. 9. Compositional variation of amphiboles from the Southern Peridotite Mass and associated dunites. Nomenclature of amphiboles is after Leake (1978). The chemical formula .is estimated VI on the basis of 16 cations and 23 oxygen. (Na+)A refers to A site occupancy whereas Al refers to Al content in C site. Fg = fine grains in the granoblasts domains comprising cpx+ ol + opx ± pl ± Cr-spinel.

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-199

Table 7: Representative whole rock major elements (wt%) for peridotites and associated dunites in the Southern Peridotite Mass

Sp-lherzolite Pl- lherzolite Zb-74 Zb-75 Zb-64 Zb-67B Zb-66 Zb-6 Zb-91.A Zb-22

SiO2 44.15 44.34 44.14 44.40 43.55 44.54 44.56 44.15 TiO2 0.11 0.11 0.12 0.12 0.15 0.11 0.08 0.08 Al2O3 3.01 3.18 2.92 3.19 3.61 3.63 3.09 2.01 Fe2O3 1.25 1.44 1.68 1.54 1.37 1.09 1.19 1.27 FeO 6.98 6.74 6.85 6.81 7.35 6.86 6.65 7.14 MnO 0.14 0.14 0.15 0.14 0.15 0.14 0.14 0.14 MgO 39.73 39.94 39.62 39.82 37.94 39.04 40.03 41.82 CaO 2.58 2.40 2.44 2.61 3.22 2.36 2.86 2.00 Na2O 0.20 0.20 0.08 0.21 0.21 0.01 0.07 0.11 K2O 0.02 0.02 0.02 0.02 0.01 <0.01 <0.01 <0.01 P2O5 0.01 0.01 0.01 0.02 0.01 <0.01 <0.01 0.01 L.O.I. 1.58 1.65 1.45 1.37 2.08 2.23 1.29 1.09 Total 99.76 100.17 99.48 100.25 99.65 100.03 99.96 99.82 Mg# 0.897 0.899 0.894 0.896 0.887 0.904 0.902 0.900

Pl- lherzolite Dunite Zb- 17 Zb-7 Zb-38C Zb-14 Zb-12.B* Zb-18.A* Zb-93 Zb-95B

SiO2 43.78 44.47 44.09 43.74 43.86 44.20 37.75 36.06 TiO2 0.12 0.13 0.10 0.09 0.07 0.07 0.04 0.01 Al2O3 2.17 2.99 2.57 1.82 2.08 2.73 0.61 0.01 Fe2O3 1.16 1.20 1.26 1.53 1.17 1.33 4.85 4.22 FeO 7.24 7.07 7.00 6.92 6.90 6.87 4.18 5.12 MnO 0.14 0.14 0.14 0.13 0.13 0.14 0.12 0.13 MgO 42.91 40.01 41.11 42.41 42.76 40.84 39.03 42.92 CaO 1.57 2.62 2.23 1.79 1.72 1.94 0.75 0.95 Na2O 0.04 0.12 0.08 0.05 <0.01 0.14 0.53 0.05 K2O 0.01 0.03 0.01 <0.01 <0.01 0.01 0.18 0.06 P2O5 0.01 0.01 0.01 0.01 <0.01 <0.01 0.07 0.04 L.O.I. 1.32 1.13 1.42 1.79 1.21 1.40 11.57 10.69 Total 100.47 99.92 100.01 100.28 99.90 99.67 99.65 100.26 Mg# 0.902 0.897 0.900 0.901 0.906 0.900 0.896 0.896

* Samples containing large unexsolved pyroxene grains; the samples of plagioclase-lherzolites are arranged from left to right with increasing intensity of foliation.

P-T ESTIMATES

Equilibrium temperatures for the analyzed samples from the Southern Peridotite Mass were estimated on the basis of partitioning of Fe2+, Mg and Ca between opx and cpx (Wood & Banno 1973; Wells 1997); CaO in enstatite coexisting with dioposide (Lindsley & Dixon 1976).; Cr-Al in orthopyroxene and clinopyroxene in the assemblage En-Di-Fo-Sp (i.e. single-pyroxene thermobarometry; Mercier 1980); Al-solubility in orthopyroxene in the system En-Fo-Sp (Sachtleben & Seck 1981) and partitioning of Mg and Fe2+ between olivine and spinel (Fabriès 1979; Roeder et al. 1979). Equilibrium pressures for residual assemblages (i.e. exsolved pyroxenes + spinel + olivine) were estimated based on the single-pyroxene thermobarometry of Mercier (1980) and partitioning of Ca in coexisting ortho- and clinopyroxene (Mercier et al. 1984), whereas the composition of the un-exsolved clinopyroxene combined with the clinopyroxene geobarometer of Nimms (1995) were used to estimate the crystallization pressure of the impregnated melt. According to Sen & Jones (1989), the inferred effect of Fe3+ (acmite) in clinopyroxene on the Mercier et al. (1984) barometer is neglected, as the acmite contents in the analyzed clinopyroxenes are invariably low (XAc ≤ 0.05). For each sample, separate temperatures and pressures were calculated for cores and rims of residual phases, for fine granoblastic phases, and for cores and rims of the unexsolved pyroxene grains (samples Zb-10B & Zb-12B). The P and T estimates are listed in Table 8. The pressure and temperature estimates for exsolved pyroxenes represent the subsolidus re-equilibration conditions. Explicitly, the P-T

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conditions obtained by different equations for the same assemblage are widely scattered with differences up to more than 200oC and 8 kbars. This considerable range in the estimated P-T conditions resulted from the notable variations in the cation diffusion rates. For example the diffusion rates of Fe and Mg in pyroxenes are much faster than those of Ca and Al (Freer 1981; Sautter & Fabriès 1990). This implies that equilibrium exchange between Fe and Mg may effectively continue to lower temperatures than in the case of Ca- and Al diffusions (Fabriès 1979; Kretz 1982; Hoogerduijn Strating et al. 1993). The Fe-Mg exchange thermometers, therefore, would give lower temperature estimates compared to the thermometers based on the diffusion of Ca and Al. Besides, the different models embody different assumptions concerning the activity of the other elements such as Cr and Fe3+, deviations from ideal cation ordering on sites of coexisting mineral phases, cooling rate, role of fluids and influence of deformation on diffusion kinetics. Each of these variables should affect the blocking of a given cation-exchange reaction (Fabriès 1979; Hoogerduijn Strating et al. 1993).

Fig. 10. The Southern Peridodite Mass samples compared with model residues formed by fractional melting of fertile peridotite (FP) after Herzberg (2004). Bold lines labeled with squares: initial melting pressures; light lines labeled with circles: the final melting pressures; light dashed lines: melt fractions, grey shaded fields: compositions of residual harzburgite designated as (ol +opx+L). Field symbols are data from Bonatti et al. (1986) for spinel– and plagioclase- peridotites and from Takla et al. (1997) for dunite samples. All analyses are plotted on anhydrous basis. Dunite samples of Takla et al. (1997) that contain < 4 wt% FeO are not shown in Fig. 10c.

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-201

Fig. 11. Whole rock variations of the Southern Perodotite Mass plotted on TiO2, CaO and Na2O versus MgO. Symbols are as in Figure 10. Dunite samples of Takla et al. (1997) that contain > 0.6 wt% Na2O are not shown in Fig. 10c

The thermometers of Fabriès (1979) and Sachtleben & Seck (1981) estimate relatively low equilibrium temperatures, while other methods yield mean equilibrium temperatures falling within a range of 150oC, for the same composition. The thermometers of Wood and Banno (1973) and Wells (1977) always mark the upper and lower bounds of this T range respectively. In spite of the controversial validity of barometers for spinel lherzolites (e.g. Hoogerduijn Strating et al., 1993; Rampone et al. 1995), pressure estimates obtained by the above barometers are consistent with the microstructures and compositional zoning in the clinopyroxene porphyroclasts that indicate the transition from the garnet-stability field through the spinel- stability field to the plagioclase-stability field. Therefore, the mean of the pressure values obtained from the three barometers are used to constrain on the P-T trajectory of the Southern Peridotite Mass. Thermo-barometric calculations show that the cores of the residual phases had equilibrated over a wide range of pressure and temperature (Table 8). However, most thermometers and barometers define a distinct trend from equilibration at high temperatures and pressures (garnet- stability field; Fig. 12) for the least- deformed samples (Zb-74, 75, 66) to equilibration at low temperatures and pressures (plagioclase-stability field) for the highly foliated samples (Table 8, Fig. 12), indicating that deformation of the Southern Peridotite Mass had occurred during its cooling and uplifting to shallower depths. The calculations also show that the rims of the residual phases (and granoblasts) equilibrated at lower temperatures and pressures compared to the cores of the residual phases in the spinel-lherzolite samples (Table 8). This is not true for the highly deformed samples that contain notable amounts of magmatic plagioclase (± unexsolved pyroxenes), where the rims of the residual phases and granoblasts show tendency for equilibration at higher temperature and lower pressure than the cores of the residual phases

V-202 Mohamed M. A. Abu El-Rus

(Table 8). This finding is a strong evidence that the Southern Peridotite Mass had re- equilibrated partly to higher temperatures during the ascent of hot melts crystallizing plagioclase and unexsolved pyroxenes. However, the thermobarometric calculations based on the composition of the cores and rims of the large unexsolved pyroxene grains (Table 8) indicate that impregnation with the hot melt started at ~ 10.5 kbar and continued to ~ 3.8 kbar, considerably below the lherzolite dry solidus (Fig. 12).

Table 8: Equilibrium P-T conditions for samples of peridotites and associated dunites from the Southern Peridotite Mass

Equilibrium Temperature (oC) L&D M-Cpx M-Opx W &B Wells S &S F R et al. (1976) (1980) (1980) (1973) (1977) (1981) (1979) (1979) A- Protogranular spinel- lherzolites (samples Zb-74 & 75B): 950 - Cores of residual phases 969 - 998 988 - 1072 993 - 1077 1025 - 1082 925 - 992 879-980 790 - 827 1002 950 - Rims of residual phases 884 - 898 868 - 870 876 - 994 920 - 925 797 - 805 777 - 810 764 - 829 1008 B- Least deformed plagioclase- lherzolites (sample Zb-66): Cores of residual phases 951 1014 972 1009 909 675 832 989 Rims of residual phases 928 866 950 939 828 630 800 957 Granoblasts associations 984 973 1017 985 902 850 725 848 C- Highly deformed plagioclase-lherzolites

(samples Zb-7 &17 & 14 & 91.A& 10B &12B): Cores of residual phases 940 - 986 848 - 950 975 - 1022 937 - 1002 818 - 903 609 - 802 795 - 830 832 - 879 1005 - Rims of residual phases 970 - 1000 876 - 994 957 - 1022 842 - 927 561 - 808 733 - 827 637 - 885 1034 Granoblasts associations 945 - 1004 910 -954 980 - 1038 942 - 1033 820 - 945 660 - 814 818 - 898 782 - 884 Cores unexsolved pyroxenes 925 - 967 958 - 1082 881 -1008 Rim unexsolved pyroxenes 914 -920 968 - 976 862 - 868 D- Dunites (sample 95B) Cores of olivine and Cr-spinel 720 - 750 633 - 651

Equilibrium Pressure (kbar) M-Cpx M-Opx M et al. Nim (1980) (1980) (1984) (1995) A- Protogranular spinel-lherzolites (samples Zb-74 & 75B): Cores of residual phases 16.7 -20.9 20.3 – 21.9 20.9 – 24.9 Rims of residual phases 9.6 – 9.7 17.3 – 17.7 15.2 – 16.4 B- Least deformed plagioclase-lherzolites

(sample Zb-66): Cores of residual phases 19.4 20.81 22.5 Rims of residual phases 8.2 13.2 17.0 Granoblasts associations 15.0 19.9 11.9 C- Highly deformed plagioclase-lherzolites

(samples Zb-7 &17 & 14 & 91.A& 10B &12B): Cores of residual phases 5.3 – 11.5 8.1 -12.4 8.5 - 14.7 Rims of residual phases 6.0 – 11.9 8.0 – 9.4 5.1 -12.8 Granoblasts associations 7.1 – 11.3 8.0 – 12.4 8.0 -14.6 Cores unexsolved pyroxenes 5.1 – 10.5 Rim unexsolved pyroxenes 3.8 - 3.9 D- Dunites (sample 95B) Cores of olivine and Cr-spinel

The temperature estimations are based on the solubility of CaO in enstatite coexisting with (Lindsley & Dixon 1976; L & D), Cr-Al in clinopyroxene (M-Cpx) and in orthopyroxene (M-Opx) in the assemblage en-di-fo-sp (Mercier 1980), portioning of Fe2+ , Mg and Ca between coexisting orthopyroxene and clinopyroxene [Wood & Banno 1973 (W & B); Wells 1977 ], the solubility of Al in orthopyroxene (Sachtleben & Seck 1981; S & S) and the partitioning of Mg and Fe2+ between olivine and spinel [Fabriès 1979 (F); Roeder et al. 1979 (R et al.)]. The pressure estimations are based on solubility of Cr-Al in clinopyroxene (M- Cpx) and in orthopyroxene (M-Opx) in the assemblage en-di-fo-sp (Mercier 1980), mutual solubility of Ca in the coexisting pyroxenes (equation 5 in Mercier et al 1984; M et al.) and the clinopyroxene geobarometer of Nimis (1995). The modal abundance of plagioclase increases with increasing deformation.

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-203

Fig. 12. P-T path inferred for the Southern Peridotite Mass plotted in a phase diagram for reactions in the ultramafic system (after Drury et al. 1990). The continental geotherm of Hoogerduijn Strating et al. (1993) is shown for comparison. For exsolved mineral assemblages, each point represents the mean values of thermometers of Wood & Banno (1973), Wells (1997), Lindsley & Dixon (1976) Mercier (1980) and Roeder et al. (1979) and the mean values of barometers of Mercier (1980) and Mercier et al. (1984). For unexsolved pyroxenes which had accumulated from the impregnated melts, each point represents the mean values of thermometers of Wood & Banno (1973), Wells (1997) and Lindsley & Dixon (1976) and pressure value obtained from the Nimms (1995). barometer. Bars give maximum standard deviation (1σ). Note that the highest P- T conditions plot fairly along or close to the continental geotherm.

DISCUSSION

1- Origin of Plagioclase in the Zabargad peridotites

The microstructural features and chemical data of the samples collected from the Southern Peridotite mass suggest that the protogranular spinel-lherzolites represent the oldest mineral assemblages in the mass that were later variably overprinted by development of abundant plagioclase. Bonatti et al. (1983, 1986) ascribed the formation of plagioclase in Zabargad peridotite either to the crystallization of small fractions of trapped-melt, which could have resulted from localized peritectic melting in the upwelling lithospheric mantle (in situ melt fraction) or to crystallization from pervasive exotic melt fractions. Bonatti et al. (1986) and Brueckner et al. (1988; 1995) favored the latter model (allochthonous melt fractions), while others (Nicolas et al.1987; Piccardo et al. 1988; Dupuy et al. 1991) believe that the crystallization was from in situ melt fractions. In fact, the present textural and chemical data revealed two main types of plagioclase: (a) plagioclase of intermediate composition (An48.3 -

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An53.3) which occurs essentially as rims around Cr-spinel grains, thick exsolutions lamellae in clinopyroxenes porphyroclasts or intergrowth with olivine in small knots on the contact between pyroxene and Cr-spinel grains and (b) calcic plagioclase (An76.7 – An92.4) which occurs as blebs or fine crystal aggregates interstices between the old porphyroclasts. a. Origin of the plagioclase of intermediate composition

The development of plagioclase-olivine intergrowths along the contacts between spinel and orthopyroxene porphyroclasts (Fig. 3e) is a strong evidence for subsolidus transition from the spinel-stability field to the plagioclase-stability field (Rampone et al. 1993; Piccardo et al. 2007) and can be ascribed to the univariant reaction: Orthopyroxene + clinopyroxene + spinel = anorthite + 2 forsterite (Rampone et al. 1993). The decrease of Na-content in the clinopyroxene porphyroclasts toward the rim (Fig. 7) suggests that the clinopyroxenes were the source of Na for the intermediate composition of the intergrown plagioclase (~ An50, Table 3). Similar interpretation has been suggested by Ozawa & Takahashi (1995) for subsolidus plagioclase in the Horoman peridotite complex. Experimentally, the above subsolidus reaction is essentially pressure (P) dependent at high temperature and has been located at about 8 kbar for T > 900oC (Kushiro and Yoder 1966; Presnall 1976; Herzburg 1976, 1978). The transition from the spinel- stability field to the plagioclase-stability field are supported by other petrographic features such as development of plagioclase thin rims around spinel (Fig. 3c) and exsolution lamellae of plagioclase within spinel-facies clinopyroxene (Fig. 3b). Such petrographic features are commonly found in the mantle peridotite massifs that had experienced decompression during uplift such as in the External Ligurid peridotites (Rampone et al. 1993). b. Origin of the calcic plagioclase

The voluminous and widespread occurrence of calcic plagioclase blebs and crystal aggregates which are mainly confined to grain boundaries and sometimes fill cracks cross cutting the porphyroclasts in the deformed samples argue strongly against their formation by subsolidus metamorphic reactions during uplift when transcending from spinel- stability field to plagioclase-stability field. Added to this, the relatively wide composition range of plagioclase (An76.7 – An92.4, Table 5) as well as the presence of calcic plagioclase without spinel in most samples cannot be easily reconciled with the subsolidus origin and suggest crystallization from melt fractions. These melt fractions could be either the melt pockets that resulted from partial melting of the upper mantle diapirs with incomplete melt extraction (i.e. in situ melt fractions) or from impregnation of the lithospheric mantle by an exotic melt. The following are several lines of evidence that are consistent with crystallization of calcic plagioclase in Zabargad mantle rocks from impregnated melt fractions rather than from in situ melt fractions: 1-The calcic plagioclase has never been observed in the protogranular samples (e.g. sample Zb- 74) or in the least deformed samples (sample Zb-66). It always forms lenses and pockets elongate parallel to foliation in the highly deformed rocks (e.g. sample Zb-7; Fig. 3k). The plagioclase crystals in these crystal aggregates are generally intact or much less- deformed than the other minerals in the host rock. Furthermore, Piccardo et al. (1988) and Kurat et al. (1993) reported plagioclase-rich dykelets or veins containing undeformed calcic plagioclase crystals indistinguishable from those in the present samples cross-cutting the foliation planes in the Zabargad peridotites. Plagioclase, therefore, should have crystallized in the late stage of deformation of the Zabargad peridotites. Table 8 shows that the maximum deformation temperature deduced from the compositions of the pyroxene porphyroclasts and neoblasts in the deformed samples is 1038oC (i.e. distinctly below the dry peridotite solidus; Fig. 12). This means that the ambient temperature of the host mantle rocks was below1038oC when plagioclase crystallized. This temperature range is unlikely for crystallization of plagioclase from in situ melt fractions which should have become saturated in plagioclase when the temperature of the host peridotite falls to 1200-1250oC (Bender et al., 1978; Elthon & Scarfe, 1984; Kelemen & Aharonov, 1998). Even, if plagioclase crystallized from in situ melt pockets at temperature > 1038oC, the crystallized plagioclase then should have been deformed during subsequent subsolidus deformation which is inconsistent with petrographic observations that

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-205

the plagioclase crystals are generally undeformed or show less strain effect and were most likely not present at the start of the deep seated deformation event. 2- According to Nicolas (1986a, 1989), the presence of clinopyroxene depleted zones surrounding plagioclase clusters is a criterion to distinguish plagioclase that crystallized from an in situ partial melt. In Zabargad peridotites, these depleted haloes are not observed and in several cases calcic plagioclase clusters are close or in contact with the clinopyroxene porphyroclasts. 3- The modal abundance of plagioclase in several samples is >5% (c.f. 16% in Bonatti et al., 1986). Such high plagioclase content is not consistent with crystallization from an in situ partial melt since the maximum amount of melt that may be stable in partially molten peridotites before permeability range from <1% (Von Bargen & Waff, 1986; McKenzie, 1989) to 3% (Faul, 1997). It seems more plausible to suggest that the calcic plagioclase accumulated from a melt impregnating the mantle lithosphere. 4- The relatively high An content of plagioclase as well as the high Mg# of the unexsolved pyroxenes poikiloitic crystals and those interstitial with plagioclase clusters are inconsistent with experimental data and thermodynamic calculations that the low-degree peridotite partial melts are andesitic in composition characterized by very high normative plagioclase of andesine composition (Baker et al., 1995). Moreover, the homogeneous composition of the calcic plagioclase grains is inconsistent with crystallization from in situ melt pockets, where the plagioclase grains are zoned (Takahashi 2001). The calcic plagioclase clusters, therefore, do not represent frozen-in melt fractions, but rather they crystallized from exotic melt fractions having high Mg# and Ca/Ca+Na ratios. 5- The abrupt chemical variations (within few meters scale) exhibited by the residual phases in the Southern Peridotite Mass are inconsistent with partial melting in an ascending mantle diapir, which would produce more uniformly depleted compositions (Müntener et al. 2004). Instead, they may predominantly form as a consequence of melt infiltration and melt-rock reaction within the stability field of plagioclase. 6- On the scale of thin sections, the residual phases of olivine, pyroxenes and Cr-spinel within or adjacent to the plagioclase clusters exhibit Mg# and Cr# similar to those in the plagioclase- free domains which argues against in situ partial melting that should leave olivine and orthopyroxene residues having higher Mg# (Jaques & Green, 1980) and Cr-spinel residue enriched in Cr (Dick & Bullen 1984). On the basis of the above criteria, it is worth to speculate that the calcic- plagioclase in Zabargad peridotites crystallized from an exotic melt infiltrating the lithospheric mantle mainly at P-T conditions within the plagioclase-stability field (3.8 –10.5 kbar; Table 8). The lithospheric mantle should have been subjected to a previous, near-fractional melting that was initiated at a deep level (~ 2-3 GPa, Fig. 10). This is largely consistent with the hypothesis that re-fertilization of a previously depleted subcontinental mantle by ascending melts is a major process in the formation of embryonic oceans, the re-fertilization events occur at a relatively low pressure whereas the depletion event takes place by near-fractional melting in the garnet- and/or spinel- stability field (Müntener et al., 2004).

2- P-T trajectory of the Southern Peridotite Mass

As shown in Figure 10 a&b, the whole-rock compositions of the analyzed samples from the Southern Peridotite Mass define trends that are coincident with residues formed by initial melting at 2-3GPa (~24-36 kbar) and are in good agreement with abyssal peridotites (Herzberg 2004). These P-T estimations are largely consistent with P-T estimates based on the composition of the core of the exsolved pyroxenes in the protogranular and least deformed samples (Table 8) that indicate early equilibration in the garnet- stability field (Fig. 12). The early equilibration in the garnet-stability field is also supported by the occurrence of pyroxene- spinel symplectites in some samples (Fig. 3d) and by the occurrence of pyroxenes with an M- shaped Al zoning profiles in the exsolved clinopyroxene grains in the spinel-lherzolite samples (Fig. 7), which indicate that the mass started to ascend from the deeper, high pressure garnet- stability field into the lower pressure plagioclase-stability field. The increase of Al content from

V-206 Mohamed M. A. Abu El-Rus

the core to the margin is due to decompression and the passage from the garnet-stability field to the spinel-stability field, whereas the following decrease in Al content toward the rim indicates further decompression from the spinel-stability field to the plagioclase-stability field (Ozawa & Takahashi 1995 & Ozawa 2004). Further evidence for the early equilibration in the garnet- stability field is the unusual abundance of HREE, Zr and Sc in the pyroxene porphyroclasts of the Al-Di pyroxenite bands in the Zabargad peridotites which implies inheritance from precursor garnets (Vannucci et al. 1993). Piccardo et al. (1988), based on the chemistry of pyroxenes, suggested that the Al-Di pyroxenite bands represent a major episode of deep-seated magmatism occurring at near solidus temperatures under pressures compatible with the garnet- stability field (T ≥ 1000oC and P ≥ 20 kbar). The thermobarometric calculations based on core and rim compositions of the residual phases in the deformed samples indicate that the deformation of the Southern Peridotite Mass has started within the garnet-stability field (core compositions in the least deformed sample Zb- 66) and ended in the plagioclase-stability field (rim compositions in highly deformed samples) (Fig. 12). The estimated pressure of 5.1 kbar (Table 8) obtained for rim compositions in the highly foliated samples is believed to represent the final re-equilibration pressure of the mass. Re-equilibration within the plagioclase-stability field is also confirmed by the growth of plagioclase of restricted composition (An48.5-53.6) as thin rims around Cr-spinel, as thick exsolved lamellae within clinopyroxene porphyroclasts and as symplectites intergrown with olivine along the contacts of Cr-spinel and orthpyroxenes porphyroclasts. As shown in Figure 10 a&b, the protogranular and least deformed samples exhibit a low melt fraction < 0.1 while the highly foliated samples exhibit higher melt fractions of 0.02 to ~ 0.2. This is an unequivocal evidence that the deformation of the peridotite mass was accompanied by further partial melting. The protogranular samples, therefore, represent survived remnants of mantle peridotite that escaped from both deformation and relatively high degree depletion. Figure 12 shows that the P-T trajectory of the Southern Peridotite Mass is totally below the dry lherzolite solidus during ascent, which implies that the partial melting during the deformation may have occurred at relatively lower temperatures in the presence of volatiles (e.g. Wyllie 1979; Olafsson & Eggler 1983). The presence of secondary fluid inclusion trails cutting the olivine and unexsolved porphyroclasts and the formation of amphibole granoblasts in the matrix in several highly deformed samples, however, confirm the presence of volatiles or H2O-bearing fluids during the deformation of the Southern Peridotite Mass. The residual mineral assemblages in the most deformed (i.e. foliated) samples are overprinted by growth of calcic plagioclase ± unexsolved pyroxene grains that accumulated from the impregnating melt(s). The thermobarometric calculations revealed that the impregnation event took place approximately at the boundary between the spinel- and plagioclase- stability fields and continued to about 3.8 kbar, at pressure slightly lower than the lower pressure limit estimated for the deformation event (~ 5.1 kbar, Table 8). These estimations are largely consistent with textural observations that the calcic plagioclase and the unexsolved pyroxene grains are intact or less deformed than the residual mantle phases in the same sample. The general correlation between the deformation of the sample and the amount of overprinting minerals suggests that melt migration have been enhanced by the presence of deformation foliation in the Southern Peridotite Mass (i.e. melt migration was driven by pre- existing structural discontinuities). The Southern Peridotite Mass plots fairly along or close to the continental geotherm at the highest P-T conditions (Fig. 12). With decreasing pressure (uplift), the re-equilibration conditions moved farther away from the continental geotherm towards higher temperatures. The slope of the P-T trajectory of the Southern Peridotite Mass is relatively steep indicating rapid ascent which is also supported by limited subsolidus plagioclase crystallization. In summary, the Southern Peridotite Mass had equilibrated at high temperatures in the garnet-stability field before ascending to shallower depths. During uplifting, the mass had been subjected to a deformation event resulting in the development of foliation observed in most of the samples. In the later stages of deformation, the mass has been partly re-equilibrated to higher temperatures due to the ascent of hot silicate melts that crystallized plagioclase and unexsolved pyroxenes. The lack of plastic deformation in the plagioclase crystals of some

Plagioclase Lherzolites From Zabargad Island, Red Sea, And Their Bearing,…… V-207

samples indicates that melt impregnation has continued after the cessation of the deformation event. 3- Geodynamic implication A peculiar feature in the Zabargad peridotites is the predominance of a rather fertile lherzolite facies as evidenced by the presence of notable amounts of clinopyroxene in most samples, modest LREE-depleted patterns in clinopyroxene porphyroclasts (Vannucci et al. 1991; Piccardo et al.1993) and relatively low degrees of partial melting estimated for most samples (< 0.1 in the least deformed samples, Fig. 10). Such fertile lherzolites exposed along passive continental margins are interpreted either as (a) an asthenospheric mantle which upwelled adiabatically and intruded into the continental lithosphere to produce the newly formed oceanic lithosphere (i.e. active rifting mechanism) (Nicolas 1984, 1986b; Evans & Girardeau 1988) or (b) a sub-continental lithospheric mantle which was exposed during the ocean opening by passive extensional (i.e. simple shear) mechanisms (Kornprobst & Tabit 1988; Rampone et al. 1993, 1995, 2005). Wernicke (1985), Voggenreitter et al. (1988b) and Piccardo et al. (1993) favored the latter mechanism for exhumation of the Zabargad peridotites. By contrast, others (e.g. Bonatti 1985; Nicolas et al. 1987; 1994; Boudier et al 1988; Dupuy et al. 1991; Boullier et al 1997; Bosch & Bruguier 1998) interpreted the Zabargad peridotites, or parts of them (in particular the Southern Peridotite Mass) within the context of one continuous diapiric upwelling associated with recent (post-Miocene) continental rifting and decompression due to extensional thinning. The Southern Peridotite Mass shows petrological and mineralogical similarities with other sub-continental lithospheric peridotite, such as External Liguride (Rampone et al. 1993, 1995) and Erro-Tobbio peridotites (Hoogerbuijn Strating 1993, Rampone et al. 2005). It is also 143 144 depleted in Nd isotopic composition for clinopyroxene separates ( Nd/ Ndtoday= 0.51283- 0.51319, Brueckner et al. 1988, 1995) which largely coincide with the values reported for Depleted Mid-ocean Mantle (DMM) (e.g. 0.51317- 0.51330, Allegre et al. 1983; 0.551225- 0.51235, McCulloch and Chappell 1982; McCulloch & Black 1984). Therefore, it seems plausible to interpret the Southern Peridotite Mass as a deep, plastic part of the sub-continental lithosphere rather than originating from the asthenosphere. Furthermore, the uplift history of Zabargad peridotites is marked by a subsolidus evolution with slight decreasing temperatures under decompression (Fig. 12). This evolutionary path is consistent with the asymmetric, simple shear-dominated rifting process (Ruppel et al. 1988; Buck et al. 1988; Lattin & White 1990; Hoogerduijn Strating et al., 1993) in which the mantle rocks are uplifted in the footwall of large, detachment, low angle normal faults (Wernicke, 1985; Hoogerduijn Strating et al., 1993; Rampone et al., 1995). The simple shear-dominated rifting process is consistent with the asymmetric nature of the Red Sea system expressed in the localization of volcanics on the Arabian side (Voggenreiter et al., 1988a & b) and in the geophysical features like heat-flow (Cochran et al., 1986) and gravity (Izzeldin 1982). A similar geodynamic process is regarded as the most plausible for the exhumation of some upper mantle sections along the former passive margin of the Tethys Ocean which are still recognizable in the Alps (Lemoine et al. 1987; Florineth & Froitzheim1994; Manatschal & Nievergelt 1997) as well as along the East Atlantic passive margin, namely along the western coast of Spain and Portugal (Boillot et al. 1987 &1988; Abe 2001; Hébert et al. 2001; Chazot et al. 2005).

CONCLUSIONS (1) The Southern Peridotite Mass represents a fragment of the subcontinental lithospheric mantle that was tectonically exhumed during the early rifting of the Red Sea. It is dominated by undepleted to less depleted lherzolites, minor dunites and pyroxenite bands and dykes. The mass exhibits a wide variation in texture, mineralogy and chemistry due to a complicated evolutionary history involving partial melting, sub-solidus recrystallization, deformation, metasomatism, and melt impregnation. (2) The Mass had undergone progressive decompression and uplifting from the P-T conditions of the garnet-stability field to that of the plagioclase-stability field along a subsolidus trajectory

V-208 Mohamed M. A. Abu El-Rus

evolution, consistent with its uplift by asymmetric, simple shear mechanism rather than by mantle diapirism. (3) The Mass was subjected to deformation during ascending to shallow depths. The deformation had started under the P-T conditions of the garnet-stability field and ended at ~ 5.1 kbar within the plagioclase-stability field. (4) The H2O-bearing fluids or volatiles invaded the Southern Peridotite Mass during its ascent, lowering its solidus temperature and causing partial melting and depletion at relatively low pressure. (5) During and after the late stage deformation, the Southern Peridotite Mass was impregnated by hot silicate melt crystallizing calcic plagioclase and unexsolved pyroxenes. The positive correlation between the degree of deformation of the samples and the amount of overprinting minerals indicates that the migration of the hot silicate melt was enhanced by the deformational fabrics in the Southern Peridotite Mass. (6) The evolution history of the Southern Peridotite Mass is consistent with the hypothesis that re- fertilization of previously depleted subcontinental mantle by ascending melts is a major process in the formation of the embryonic oceans (Müntener et al., 2004).

ACKNOWLEDGEMENTS

The geochemical data were obtained at the Geology Department, Oslo University, Norway during the author's tenure of a scholarship from The Research Council of Norway. The author is indebted to Prof. E.-R. Neuman, University of Oslo for availing the Microprobe facilities and Prof. R. Wilson, Aarhus University for the whole rock analyses. The author is also indebted to professor Ali Khudeir for offering the samples and discussing the field observations. Professors E.-R. Neumann, S. El-Gaby and W. Bishara are gratefully thanked for enlightening discussions and improving the first draft of the paper. Comments and constructive criticism by the reviewers and by Prof. Mostafa Youssef (Editor) greatly improved the final version of the manuscript.

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