Introduction

The wehrlitic lithologies, consisting mainly of olivine and clinopyroxene, are classically interpreted as a constituent of the earth’s mantle (e.g., Nicolas 1989). The wehrlitic lithologies were recorded in ophiolites (e.g., Oman; Koga et al. 2001 and references therein), oceanic core complexes (e.g., Atlantis Massif; Ildefonse et al. 2006) and Alaskan-type plutonic complexes (e.g., Kamchatka wehrlites; Kepezhinskas et al. 1993).

Detailed petrological characteristics of ophiolitic ultramafic/wehrlite cumulates have been almost exclusively available from the Phanerozoic ophiolites (e.g., England and Davies 1973; Benn and Laurent 1987; Benn et al. 1988; Juteau et al. 1988; Hébert and Laurent 1990; Parlak et al. 1996; Koga et al. 2001; Uesugi 2004; Koepke et al. 2004). They have been known as one of the main components of the Phanerozoic ophiolites (e.g., Benn et al. 1988). For example, they occupy up to 30% of the crustal section exposed in the central part of the Oman ophiolite (Juteau et al. 1988). By contrast, almost no studies have been done on the ophiolitic wehrlites of the Precambrian age, at least in the Arabian Nubian Shield (ANS). However, several ANS ophiolites have well-preserved Moho transition zones characterized by interlayered wehrlites, pyroxenites, lherzolites, dunites and chromitites at the base grading upward into gabbros (e.g., Stern et al. 2004; Johnson et al. 2004). The wehrlite cumulates were recorded in the following ANS (thick Moho transition zone) ophiolites: Onib (Kröner et al. 1987; Hussein et al. 2004) and Oshib (Abdel-Rahman et al. 1990), Sudan; Sol-Hamed, Egypt (Fitches et al. 1983; Gahlan et al., in preparation); Thurwah (Nassief et al. 1984) and Ess (Shanti and Roobol 1979), Saudi Arabia.

It has been controversial whether the formation of wehrlitic lithologies is related to ridge magmatism (e.g., Nicolas 1989; Koga et al. 2001) or to subduction-related magmatism during ophiolite obduction (e.g., Arai et al. 2004, 2006). A variety of origins have been proposed for wehrlitic lithologies; low-pressure primitive cumulates (e.g., Koga et al. 2001; Ildefonse et al. 2006; Koepke et al. 2004), magmatic impregnations into mantle peridotites (e.g., Nicolas and Prinzhofer 1983; Lorand 1987), melt/peridotites interaction products (e.g., Girardeau and Francheteau 1993; Boudier and Nicolas 1995; Suhr et al. 2008; Drouin et al. 2009) and/or high-pressure crystallization products of basaltic melts (e.g., Elthon et al. 1982; Parlak et al. 1996). Based on the mode of occurrence, the wehrlitic lithologies are divided into common crustal (to MTZ) wehrlites and rarer mantle wehrlites. Crustal wehrlites are hosted by the magmatic units (mainly lower crustal gabbros) (e.g., Oman; Benn et al. 1988; Juteau et al. 1988; Koga et al. 2001; Koepke et al. 2004; Troodos; Benn and Laurent 1987; Mersín; Parlak et al. 1996; Kizildağ; Bagci et al. 2005). Mantle wehrlites are hosted by the residual unit (mantle) at convergent margins (e.g., Peslier et al. 2002; Parkinson et al. 2003), continents (e.g., Shaw et al. 2005; Beard et al. 2007) and the ocean floor (Arai and Takemoto 2007). According to Pallister and Hopson (1981), Smewing (1981), Juteau et al. (1988) and Koepke et al. (2009), wehrlitic lithologies have been formed by crystallization of a mantle-derived melt compositionally different from that formed the gabbroic rocks upsection. By contrast, others proposed that wehrlitic lithologies are genetically related to overlying gabbros (e.g., England and Davies 1973; Boudier and Nicolas 1995; Koga et al. 2001; Yamasaki et al. 2006).

The mechanism of wehrlite emplacement has been a matter of discussion, but the model of Rabinowicz et al. (1987) received a wide consensus (e.g., Benn and Laurent 1987; Benn et al. 1988; Juteau et al. 1988; among others). This model ascribes the wehrlitic magma emplacement into crustal levels from the transition zone to the compaction process related to diapirism and crustal accretion. Furthermore, Juteau et al. (1988) and Ernewein et al. (1988) proposed that wehrlite magmas could ascent through the crust only in the compressive regime that occurred at the time of first detachment of the oceanic lithosphere.

This paper aims to discuss the origin of the Ras Salatit wehrlites based on detailed field, petrographical, mineral chemical and petrological studies. According to our perusal, this is the first detailed petrological document on wehrlites from the Precambrian ophiolites.

Geological outline

The Ras Salatit area is located about 100 km west of Marsa Alam City, within the central part of the Eastern Desert of Egypt (Fig. 1). The Eastern Desert of Egypt is a part of the Arabian Nubian Shield (ANS), a complex amalgam of arc, ophiolite and microcontinental terranes that makes up the northern half of the East African Orogen (e.g., Stern 1994, 2002; Kröner and Stern 2004). The ANS ophiolites represent remnants of Neoproterozoic oceanic lithosphere (~870–690 Ma) obducted along destructive plate boundaries during the Pan-African orogeny (~750–650 Ma) (Stern 1994). Contrary to the northern and southern, the Central Eastern Desert (CED) of Egypt exhibits a complete succession of the Egyptian Pan-African basement complex (e.g., Ries et al. 1983; El Gaby et al. 1984, 1990; El Ramly et al. 1993; Greiling et al. 1994).

Fig. 1
figure 1

Regional geological map of the Central Eastern Desert (CED) of Egypt (modified after EGPC/CONOCO Coral 1987) showing the distribution of ophiolitic masses and the location of the Ras Salatit ophiolite

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The ENE-trending Ras Salatit ophiolite (serpentinized ultramafic–mafic complex) (Fig. 2) forms elongate fault-bounded sheets that have been structurally controlled by the NE–SW trending regional folds (e.g., Ries et al. 1983; El Gaby et al. 1984; El Ramly et al. 1984). The Ras Salatit area comprises a tectonic admixture or mélange of allochthonous ophiolitic fragments thrust over deep-oceanic graphitic schistose metasediments. Island-arc metagabbro/metadiorite complex and syn- to late-orogenic calc-alkaline granites (Fig. 2) intrude the schistose metasediments and ophiolitic fragments, respectively. Accordingly, the Ras Salatit area well represents the suprastructure of El Gaby et al. (1990) or Tier II of Greiling et al. (1994). According to Penrose Conference (Anonymous 1972), the Ras Salatit ophiolite is only weakly dismembered and offers a continuous sequence/sheet from the mantle section through the MTZ dunite (up to 1 km thick) upward to the mafic volcanics overlying the layered gabbros (Fig. 3).

Fig. 2
figure 2

Detailed geological map highlighting filed and structural relations of the Ras Salatit ophiolite

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Fig. 3
figure 3

Pseudo-lithostratigraphic column of the Ras Salatit ophiolite complex (not to scale)

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High-relief massive lenses and sheet-like bodies of serpentinites, being well preserved in the trough of synforms (Fig. 2), represent the Ras Salatit mantle section. The latter is homogenous and dominated by completely serpentinized harzburgite screen with less abundant concordant dunite veins, lenses and bands (Fig. 3). The serpentinized harzburgite exhibits a tectonic fabric; foliations strike ENE–WSW to E–W with moderate to steep dipping, parallel to the major structural trend of the Ras Salatit ophiolite (Fig. 2). At the uppermost part of the mantle, large masses of serpentinized dunites (up to 1 km thick) represent the so-called Moho transition zone (MTZ) (Fig. 4a). Concordant to sub-concordant chromitite pods are found in the shallowest part of the mantle, the MTZ dunite (Fig. 3). Interlayered with dunite, gabbroic sills and layers (~1 m thick, on average) are demarcating the uppermost part of the Ras Salatit MTZ (Fig. 3). The thickness and proportion of gabbro sills increase upsection by moving toward the base of the layered gabbros. The Ras Salatit pyroxenites are the least abundant ultrabasic rocks in the complex; they are restricted to the contact between MTZ dunite and layered gabbros (Fig. 3).

Fig. 4
figure 4

Selected field photographs (a, b) and photomicrographs (c–f) of wehrlites from the Neoproterozoic Ras Salatit ophiolite, CED, Egypt. Abbreviations: LG layered gabbro, Ol olivine, Cpx clinopyroxene, Tr tremolite. a Boudinaged lensoidal tabular wehrlite body concordantly embedded within thick MTZ dunite. b Wehrlite layers, interdigitated with layered gabbro, striking E–W and dipping nearly vertical. c A well-developed poikilitic texture has been made up by rounded olivine enclosed in clinopyroxene oikocryst. Note the occasional embayments in clinopyroxene suggest a relict magmatic history. d A well-developed orthocumulate texture. Note the triple point between the constituting minerals. e Clinopyroxene being replaced by tremolite along grain boundaries. f Clinopyroxene is completely replaced by carbonate, with magnetite striations fill its original cleavage traces

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The contact between MTZ dunites and the layered gabbros of the Ras Salatit ophiolite is, however, initially magmatic (stratigraphic) and has been disrupted by severe tectonism. The Ras Salatit layered gabbro section is mainly represented by layered and foliated metagabbros. The igneous layering is locally represented by dominant isomodal-type layering (striking N70E–SW and dipping nearly vertical), being concordant with the major mantle harzburgite foliation. As usual, textural variations between individual layers are clearly visible.

Ultramafic cumulates, mainly poikilitic true crustal wehrlites (e.g., Koga et al. 2001; Koepke et al. 2004), are discovered and described for the first time within the Ras Salatit ophiolite. They are restricted to the MTZ dunite and the lowermost layered gabbro sequence and never observed in the serpentinized mantle harzburgite (Fig. 3). They form either boudinaged lensoidal tabular bodies (Fig. 4a) or interdigitated layers striking E–W and dipping nearly vertical (Fig. 4b). Accordingly, they occur as sills, often concordant with the planolinear fabrics of the Ras Salatit ophiolite. Relative to the total volume of the Ras Salatit MTZ, the wehrlite cumulates are moderately significant in volume (~10–15 vol.%). The contact between the wehrlites and the host MTZ dunite or layered gabbro is razor sharp, lobate and/or sinuous, without any chilled margins, xenoliths from the host and/or visible deformations (Fig. 4b). Clinopyroxenites occasionally develop along the margins of Ras Salatit wehrlites.

The Ras Salatit wehrlites range in size from few meters to ten meters thick and few tens of meters in length along strike. All of them are too small to map and exhibit a well-developed pine-tree shape (Fig. 3). On the outcrop pattern, they are coarse-grained, dark-colored, massive, smooth on the surface, uniform and compositionally non-layered (Fig. 4b).

Black marble lenses (up to 100 m length) are observed in the westernmost part of the Ras Salatit ophiolite ridge and further west beyond the limits of Fig. 2. The marble lenses (Fig. 3) strike E–W and dip nearly vertical and show no thermal effects on the host serpentinites. Graphite bands (10–30 cm thick) are commonly observed near the basal structural contact between the serpentinites and the underlying schistose metasediments. Doleritic and granitic dykes (~0.5 m thick, on average) with chilled margins and quartz veins (up to 2 m thick) striking NE–SW to N–S crosscut discordantly the schistose metasediments and the overlying serpentinites (Fig. 3).

Petrography

The Ras Salatit ophiolite was metamorphosed under the greenschist–lower amphibolite facies conditions during the Pan-African deformation (~750–650 Ma; Stern 1994). Accordingly, almost all the primary silicates have been converted to secondary minerals, mainly serpentine minerals and magnetite, in ultramafic rocks. The altered phases can be assigned to their parental minerals based on relic textural characteristics. Definitions of Streckeisen (1976) have been adopted for classification and nomenclature of the studied rocks. The Ras Salatit wehrlites are orthopyroxene-free, typical wehrlites composed mainly of olivine (45–60 vol.%) and clinopyroxene (40–55 vol.%), with trace amounts of chromian spinel and pentlandite. Serpentine, tremolite, magnetite, chlorite and carbonates are products of retrograde/low-temperature alteration. The Ras Salatit wehrlites were texturally equilibrated and show a characteristic poikilitic texture (Fig. 4c), as well as an orthocumulate texture (Fig. 4d). Triple points between olivine crystals are very common.

The olivine (0.2–3 mm across) forms anhedral to subhedral turbid crystals. It is either included in clinopyroxene (Fig. 4c) or constituting olivine-rich pools among clinopyroxene grains (Fig. 4d). The clinopyroxene (0.5–5 mm across) is subhedral and homogenous and lacks any exsolution lamellae (Fig. 4d). It is altered and replaced by tremolite, carbonate and/or serpentine to various extents along grain boundaries and cleavage (Fig. 4e). Sometimes magnetite striations fill the original cleavage traces of clinopyroxene (Fig. 4f). The chromian spinel is only in trace amounts (a characteristic of orthopyroxene-free wehrlites; Ionov et al. 2005). It is euhedral to subhedral and disseminated in the other phases as inclusions. The magnetite grains are typically disseminated throughout the rock. Neither plagioclase nor orthopyroxene has been observed in the Ras Salatit wehrlites. The crystallization sequence, olivine/spinel followed by clinopyroxene, can be recognized in the studied wehrlites. The olivine/clinopyroxene ratio and degree of alteration vary from sample to sample.

Analytical techniques

A set of 27 representative wehrlite samples have been collected from 5 outcrops in the area of Ras Salatit. Based on the degree of preservation of primary silicates, 15 polished thin sections representing outcrops and samples have been selected for electron microprobe and LA-ICP-MS analyses for major and trace elements, respectively. At least three grains of olivine and/or clinopyroxene were analyzed for each sample. The core-rim analyses and compositional profiles across crystals had been done to reveal chemical homogeneity.

Minerals on polished thin sections were analyzed by a JEOL JXA-8800 (WDS) electron microprobe at Kanazawa University, Kanazawa, Japan. Analyses were done at 20-nA (20 × 10−8) probe current intensity, 20-keV accelerating voltage and 3-μm probe diameter with counting times 20 and 30–50 s, respectively, for major and minor elements. Various natural and synthetic standards have been used. The raw data were corrected with an online ZAF program. All iron in silicates and carbonates were assumed to be ferrous. Ti was assumed to be present as an ulvöspinel molecule in spinel phases. Fe3+ and Fe2+ in spinel phases were calculated assuming spinel stoichiometry. Mg# and Cr# are Mg/(Mg + Fe2+) and Cr/(Cr + Al) atomic ratios, respectively. YCr, YAl and YFe are, respectively, atomic ratios of Cr, Al and Fe3+ to ∑ (Cr + Al + Fe3+). The mineral chemistry of chromian spinel and silicates and carbonates is summarized, respectively, in Tables 1 and 2.

Table 1 Representative microprobe analyses (wt. %) of chromian spinel phases in wehrlites from the Ras Salatit ophiolite
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Table 2 Representative microprobe analyses (wt. %) of silicates and carbonates in wehrlites from the Ras Salatit ophiolite
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Trace element concentrations in clinopyroxene (diopside) were determined in situ by a laser ablation (193-nm ArF excimer: Microlas GeoLas Q-plus) inductively coupled plasma mass spectrometer (Agilent 7500S) (LA-ICP-MS) at the Kanazawa University (Table 3). Analytical details are given in Morishita et al. (2005). Each analysis was performed by ablating 60-μm-diameter spots for clinopyroxene, avoiding cleavage and paring planes, at 6 Hz with energy density of 8 J/cm2 per pulse. The primary calibration standard was NIST SRM 612 glass, with selected element concentrations from the preferred values of Pearce et al. (1997). The standard was analyzed at the beginning of each batch of <3–4 unknowns, with a linear drift correction applied between each calibration. The raw data were quantified by using the SiO2 contents (previously determined by EPMA) as an internal standard, following a protocol essentially identical to that outlined by Longerich et al. (1996). Cores and rims of the clinopyroxene grains had been analyzed to reveal chemical zoning in trace element contents.

Table 3 Representative LA-ICP-MS trace element analyses (ppm) of clinopyroxene in wehrlites from the Ras Salatit ophiolite
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Mineral chemistry

Chromian spinel

The chromian spinel of Ras Salatit wehrlites is compositionally similar to that of the Oman wehrlites (Koga et al. 2001), the oceanic ultramafic cumulates (Hébert and Laurent 1990) and the Hess Deep impregnated dunites (Arai and Matsukage 1996) (Fig. 5). Relics of primary spinel in the Ras Salatit wehrlites contain 38–39 wt. % Cr2O3 and 22.7–23.2 wt. % Al2O3, equivalent to moderate and highly restricted Cr# (0.52–0.53) (Fig. 5; Table 1). Their Mg# is also highly restricted, mostly around 0.52, and shows a weak negative correlation with FeO* (Fig. 5). The YFe is lower than 0.07 (Table 1). TiO2 (0.44 wt. %) and NiO (0.06 wt. %) are lower, and MnO (0.7 wt. %) is slightly higher, on average, than those of chromian spinel of the Oman wehrlites (Koga et al. 2001) and the Hess Deep impregnated dunites (Arai and Matsukage 1996) (Fig. 5; Table 1).

Fig. 5
figure 5

Relations between Mg# and Cr#, FeO* (total iron), NiO, MnO, Fe3+/(Cr + Al + Fe3+) atomic ratio and TiO2 of chromian spinels from the Ras Salatit wehrlites. Note the well-developed chemical gap between chromian spinel cores, and ferritchromite and magnetite rims. Fields of chromian spinels from the Oman wehrlites (Koga et al. 2001), the Hess Deep impregnated dunites (Arai and Matsukage 1996) and the oceanic ultramafic cumulates (Hébert and Laurent 1990) are given for comparison

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Compared with the primary/intact spinel of the Ras Salatit wehrlites, ferritchromite and Cr-magnetite are depleted in Al, Mg and Cr and enriched in Fe (Fig. 5; Table 1). They are lower in Mg# (<0.3) and YAl (0.06) (Fig. 5; Table 1). The ferritchromite and Cr-magnetite are relatively higher in MnO, up to 1.7 and 1.1 wt. %, respectively, than the primary spinel (Fig. 5; Table 1). The ferritchromite is higher in TiO2, up to 0.8 wt. % (Fig. 5; Table 1). The Cr-magnetite (YCr, ~ 0.1) is nickeliferous, with up to 0.6 wt. % NiO (Table 1). A well-developed chromite–magnetite miscibility gap has been observed between spinel cores (YFe poor) and rims (YFe rich) (Fig. 5). We observed a Ti, Mn-rich ferritchromite phase in the more serpentinized varieties (Table 1; no. 7).

Silicates and carbonates

The olivine (ol) of Ras Salatit wehrlites is highly magnesian and varies in composition from Fo90 to Fo94, mostly around Fo93 (Fig. 6; Table 2). Detailed profiling yielded no significant compositional zoning from core to rim, and grains mostly show a compositional homogeneity, although iron is mildly higher in olivine cores than in rims, 7.2 and 6.6 wt. %, on average, respectively (Fig. 6; Table 2). The NiO (0.15–0.26 wt. %) shows no systematic relationship with Mg# in olivine (Fig. 6a), and MnO content is relatively high (0.30–0.64 wt. %). The Ras Salatit wehrlites are peculiar in Mg#-NiO relation of olivine; the Ras Salatit wehrlites olivine is high in Mg# but relatively low in NiO (Fig. 6a). The olivine of Ras Salatit wehrlites is similar in NiO content but is higher in Mg# (Fo content) than the Oman wehrlites (Koga et al. 2001), Mersín wehrlites (Parlak et al. 1996), MAR olivine-rich troctolites (Suhr et al. 2008; Drouin et al. 2009) and the oceanic ultramafic cumulates (Hébert and Laurent 1990) (Fig. 6a). The Mg# (Fo content) of the Ras Salatit wehrlites olivine is as high as some mantle peridotites but is distinctly lower in NiO content than the latter (Fig. 6a). The concentration of CaO (0.02 wt. %, on average) is close to the detection limit of the microprobe. The Ras Salatit wehrlites are again peculiar in Mg# (olivine)-TiO2 (spinel) relation; the spinel is relatively high in TiO2 content although the olivine is highly magnesian (Fig. 6b). The Ras Salatit wehrlites (Fig. 6b) are clustered and plot close to the Oman harzburgites (Kelemen et al. 1995) and the Hess Deep mantle wehrlites (Arai and Takemoto 2007). Being away from the Oman wehrlites’ differentiation trend with crustal height (Koepke et al. 2004), and to lesser extent the Hess Deep impregnated dunites (Arai and Matsukage 1996) (Fig. 6b).

Fig. 6
figure 6

Relationships of Mg# in olivine with NiO in olivine (a) and with TiO2 content of spinel (b) from the Ras Salatit wehrlites. The fields of primary mantle olivine (Takahashi 1987; Rudnick et al. 1993), oceanic ultramafic cumulates (Hébert and Laurent 1990), Oman wehrlites (Koga et al. 2001), Mersín wehrlites (Parlak et al. 1996), MAR olivine-rich troctolites (Suhr et al. 2008; Drouin et al. 2009), EPR Hess Deep mantle wehrlites (Arai and Takemoto 2007), Oman harzburgite (Hz) (Kelemen et al. 1995) and EPR Hess Deep impregnated dunites (Arai and Matsukage 1996) and the Oman wehrlites differentiation trend with crustal height (Koepke et al. 2004) are all given for comparison

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The clinopyroxene (diopside) of Ras Salatit wehrlites has very restricted compositional variations, mostly around Wo47En48Fs5 (Table 2). It shows a chemical homogeneity and no core-rim compositional zoning (Fig. 7; Table 2). The Mg# (0.90–0.93) of clinopyroxene is high and restricted and almost positively correlated with that of the associated olivine (Fig. 7). The SiO2 content (52.1–54.6 wt. %) is relatively high, and Al2O3 and Cr2O3 contents are low, 2.3 and 0.5 wt. %, on average, respectively. The Al2O3 content shows a sharp negative correlation with the Mg# in clinopyroxene (Fig. 7). The Cr2O3 content of Ras Salatit wehrlites clinopyroxene (Fig. 7) is largely similar to the equivalents from the Mersín (Parlak et al. 1996) and Oman ophiolites (Koga et al. 2001) and to lesser extent from the Bay of Islands ophiolite (Elthon et al. 1982). The TiO2 content (0.15 wt. %, on average) is low and shows a negative correlation with the Mg# (Fig. 7). Compared with clinopyroxenes of the MAR olivine-rich troctolites (Suhr et al. 2008), the Ras Salatit wehrlite clinopyroxene is markedly lower in Cr2O3 and TiO2 and higher in Mg# (Fig. 7). The Ras Salatit wehrlites clinopyroxene is similar in chemistry to that of the Mersín wehrlites (Parlak et al. 1996) (Figs. 7, 8). Compared to the averaged clinopyroxene composition of the Oman wehrlites (Koga et al. 2001), the clinopyroxene of Ras Salatit wehrlites shows some similarities in Si, Fe, Mn, Mg, Ca and Na contents, but is slightly lower in Ti, Al and Cr contents (Fig. 8).

Fig. 7
figure 7

Mg# of clinopyroxene versus Mg# of coexisting olivine and Al2O3, TiO2 and Cr2O3 of clinopyroxene from the Ras Salatit wehrlites. Fields of the experimentally determined 1-atm phase equilibria boundaries of MORBs, Bay of Islands ophiolite, oceanic ultramafic and mafic cumulates and the San Carlos mantle xenoliths are after Elthon et al. (1982) and references therein. In addition, fields of the Oregon high-pressure peridotites (Medaris 1972), low-pressure Cpx 1-atm experimental studies of N-MORB (Elthon 1987), the Mersín wehrlites (Parlak et al. 1996), MAR olivine-rich troctolites (Suhr et al. 2008; Drouin et al. 2009) and the Oman wehrlites (Koga et al. 2001) are all given for comparison. Differentiation trends A (without plag.) and B (with plag.) for typical arc cumulates of the Chilas complex are after Jagoutz et al. (2007)

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Fig. 8
figure 8

Major elements composition of clinopyroxenes from the Ras Salatit wehrlites normalized to that of the Oman wehrlites (Koga et al. 2001) and compared with that of the Mersín wehrlites (Parlak et al. 1996)

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Tremolite is rather homogenous in chemistry, with Mg# of ~0.97, and shows a marked depletion in Al2O3 (0.3 wt. %, on average) (Table 2). Chlorite, classified as Mg-chlorite (e.g., Weiss 1998), is penninite (e.g., Hey 1954), with 0.95 as an average Mg# (Table 2). Carbonate is dolomite, with Mg# mostly around 0.98 (Table 2).

Trace elements in clinopyroxene and in calculated equilibrium melt compositions

Trace element abundances (Table 3) have been normalized to the CI chondrite values (subscript N) (McDonough and Sun 1995). No appreciable difference in trace element character of the Ras Salatit wehrlites clinopyroxene has been recognized between core and rim (Table 3). The clinopyroxene of Ras Salatit wehrlites is characterized by a slightly convex-upward LREE-depleted [(La/Sm)N = 0.12–0.16] chondrite-normalized pattern with a flat middle- to heavy-REE level [(Gd/Yb)N ~ 1] (Fig. 9). This pattern is quite similar to that of the Oman wehrlites clinopyroxene (Koga et al. 2001), except in a very slight depletion in the Eu (Eu/Eu* = 0.9) of Ras Salatit wehrlites clinopyroxene (Fig. 9). SrN and YN levels of the Ras Salatit wehrlites clinopyroxene, respectively, 1.5 and 2.9, on average, are similar to those of the Oman one (Koga et al. 2001), but ZrN (0.6, on average) is lower (Table 3). Notably, the chondrite-normalized REE pattern of the Ras Salatit wehrlites clinopyroxene (Fig. 9) is parallel to but lower in level than that of the MAR olivine-rich troctolites clinopyroxene (Drouin et al. 2009). The primitive mantle (subscript PM)-normalized (Sun and McDonough 1989) multi-element pattern of the Ras Salatit wehrlites clinopyroxene shows a marked depletion in incompatible trace elements (Fig. 10). It shows a depletion in Zr and Hf relative to MREE [(Zr/Sm)PM ~ 0.2] and a slight Sr negative anomaly [(Sr/Nd)PM ~ 0.9] as well as variable levels in Pb relative to neighboring elements (Fig. 10). In terms of trace element character, the Ras Salatit wehrlites clinopyroxene shows no core-rim compositional zoning (Table 3).

Fig. 9
figure 9

Chondrite-normalized REE patterns of clinopyroxene from the Ras Salatit wehrlite cumulates and calculated melts in equilibrium. Chondrite abundances are after McDonough and Sun (1995). Partition coefficients of clinopyroxene/melt are after Hart and Dunn (1993). Fields of the Oman wehrlites (Koga et al. 2001), the Izu-Bonin forearc (Pearce et al. 1992), magnesian MORB-like melts (Allan et al. 1996; Perfit et al. 1996) and MAR olivine-rich troctolites (Drouin et al. 2009) are all given for comparison

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Fig. 10
figure 10

Primitive mantle-normalized multi-element diagram for the Ras Salatit wehrlites and the calculated melts in equilibrium. Chondrite abundances are after McDonough and Sun (1995). Partition coefficients of clinopyroxene/melt are after Hart and Dunn (1993). Fields of the Izu-Bonin forearc lava (Pearce et al. 1992) and N-MORB (Sun and McDonough 1989) are given for comparison

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We calculated the trace element composition of the melt (Table 3) that could has been in equilibrium with the Ras Salatit wehrlites clinopyroxene by using the experimentally determined cpx/melt partition coefficients of Hart and Dunn (1993). The calculated melt is almost similar to some lavas from the Izu-Bonin forearc (e.g., Pearce et al. 1992) rather than magnesian MORB-like melt (e.g., Allan et al. 1996) or a primitive picritic MORB melt (e.g., Perfit et al. 1996) (Figs. 9, 10). It is noteworthy that the Ras Salatit melt shows a feature clearly different from the N-MORB signature (Sun and McDonough 1989) (Fig. 10).

Discussion

General petrological characteristics

The Ras Salatit ophiolite is composed of two petrogenetically distinct components, lower residual and upper magmatic units, corresponding to the mantle and crustal (and MTZ) sections, respectively (Fig. 3). The magmatic unit is composed of: (a) a lower ultramafic and mafic plutonic section and (b) an upper basaltic volcanic carapace. The Ras Salatit wehrlites are restricted to the lowermost part of the magmatic unit (the thick MTZ dunite and the base of layered gabbro sequence).

The wehrlite bodies are commonly restricted in space to thick MTZ sections in ophiolites, such as Albania (Nicolas et al. 1999), Bay of Islands (Casey and Karson 1981; Suhr 1992; Bédard and Hébert 1996), Cyprus (George and George 1978; Benn and Laurent 1987), Oman (Juteau et al. 1988; Jousselin and Nicolas 2000) and Mersín in Turkey (Parlak et al. 1996). In consensus, the ANS ophiolites that are devoid of thick MTZ dunite at the fossil Moho (e.g., Fawakhir ophiolite; El-Sayed et al. 1999) are commonly free of wehrlites, probably because rooting of wehrlites in the MTZ may efficiently drain large amounts of melt that would have formed the MTZ dunite (e.g., Benn et al. 1988; Juteau et al. 1988; Ernewein et al. 1988).

Absence of chilled margins and deformation around the Ras Salatit wehrlite bodies coupled with sharp, lobate and/or sinuous contacts (Fig. 4) may indicate their contact at some high-temperature condition. We suggest therefore that the Ras Salatit layered gabbros have not yet consolidated at the time of wehrlites emplacement. Textural relations indicate that olivine and chromian spinel were co-liquidus phases and had crystallized prior to clinopyroxene (Fig. 4) (e.g., Jaques 1981). The uniform composition of the Ras Salatit wehrlites-forming minerals (Figs. 6, 7), olivine and clinopyroxene, reflects rather complete subsolidus re-equilibration. The apparent Mg–Fe coefficient between the two phases is around unity (Fig. 7), indicating a high temperature (cf. Obata et al. 1974). Hence, we suggest that olivine and clinopyroxene in the Ras Salatit wehrlites represent primary igneous compositions.

Spinels are recognized as important petrogenetic indicators (e.g., Irvine 1967; Dick and Bullen 1984; Arai 1992) and can be used to investigate the conditions of magma evolution and emplacement of plutonic bodies (e.g., Agata 1988; Jan and Windley 1990). In terms of spinel chemistry, the Ras Salatit wehrlites show clear differences from wehrlites of Alaskan-type plutonic complex (e.g., Kepezhinskas et al. 1993; Farahat and Helmy 2006). Spinels from wehrlites of the Alaskan-type complexes have significantly higher Fe3+ contents coupled with lower Mg, Cr and Al contents (e.g., Kamchatka wehrlites; Kepezhinskas et al. 1993) than spinels from the ophiolitic wehrlites. Chromian spinel of the Ras Salatit wehrlites (Fig. 5) is comparable in chemistry to that from the Oman wehrlites (Koga et al. 2001), the Hess Deep impregnated dunites (Arai and Matsukage 1996) and the oceanic (Hébert and Laurent 1990) and Outokumpu (Vuollo et al. 1995) ultramafic cumulates. It is markedly more Al rich, much smaller in grain size and lower in mode than that from the associated MTZ dunites, probably due to a somewhat cession in spinel crystallization (e.g., Jaques 1981). It is lower in TiO2 than that of the Hess Deep impregnated dunites (Fig. 5), suggesting lack of reaction with an evolved melt or less involvement of the melt interaction process in formation of the Ras Salatit wehrlites. Chromian spinel from the Ras Salatit wehrlites is altered along grain boundaries and cracks, showing discontinuous compositional variations (chemical gap) from intact core to ferritchromite and magnetite rims (Fig. 5). The latter compositional trend (Fig. 5) can be attributed to low-temperature alteration (e.g., Roeder 1994).

MnO contents in excess of 0.5 wt. % are rather unusual for chromian spinels in ultramafic rocks (e.g., Paraskevopoulos and Economou 1981; Barnes 2000). Considerable amounts of Mn and Ni have been recorded in the Ras Salatit wehrlites chromian spinel (Fig. 5; Table 1), particularly in ferritchromite and magnetite rims. Serpentinization can redistribute Fe of the primary silicates (Ramdohr 1967; MacFarlane and Mossman 1981; Gahlan et al. 2006), Ni (Bliss and MacLean 1975) and Mn (Lehmann 1983; Gahlan and Arai 2007). Hence, Mn and Ni enrichments in the Ras Salatit wehrlites chromian spinel are of secondary origin, being associated with low-temperature alteration (e.g., Michailidis 1990; Barnes 2000; Gahlan and Arai 2007). Mn and Ni released from olivine on decomposition were incompatible with serpentine minerals and were mobile through hydrothermal solutions involved in serpentinization and added to chromian spinel (e.g., Gahlan and Arai 2007).

Bender et al. (1978), Bence et al. (1979) and Elthon et al. (1982, 1992) suggested that high-Mg# clinopyroxenes are early precipitates in the MORB-crystallization sequence at pressures (up to 10 kbar) higher than the generally proposed oceanic crustal pressures (~2 kbar). Others have suggested that high-Mg# clinopyroxenes are the product of chemical re-equilibration between pre-existing cumulates and percolating melts, namely, melt/rock interaction (e.g., Meyer et al. 1989) or crustal assimilation (e.g., Kvassnes and Grove 2008; Lissenberg and Dick 2008). We believe that higher Mg#s of olivine (Fig. 6; Table 2) and clinopyroxene (Fig. 7) are probably due to prevailing higher oxidizing conditions. Presence of water in the system has strongly oxidizing effects, resulting in higher Fe3+/Fe2+ ratio in the melt, and therefore higher Mg#s in precipitating mafic phases (e.g., Berndt et al. 2005; Botcharnikov et al. 2005; Feig et al. 2006). An earlier fractionation stage of much higher-Mg# and -Ni olivines may be required to form the melt that precipitated the Ras Salatit olivine rich in Mg but relatively low in Ni (Fig. 6a). According to Sato (1977), the NiO content of both magma and olivine decreases by 50% after fractional crystallization of 6–12% of olivine.

The Ras Salatit wehrlites clinopyroxene shows a negative correlation between Mg# and TiO2 content (Fig. 7). This relation has been observed in clinopyroxenes from ultramafic/wehrlite cumulates of the following ophiolites: Bay of Islands (Elthon et al. 1982, 1984; Komor et al. 1985), Troodos (Hébert and Laurent 1990), Mersín (Parlak et al. 1996) and Oman (Koga et al. 2001) (Fig. 7). The TiO2 content of clinopyroxene is partly dependent on cooling rate, sympathetically changing with cooling rate (e.g., Coish and Taylor 1979; Gamble and Taylor 1980; Elthon 1987). Combined with the effect of fractional crystallization, the TiO2 content of clinopyroxene from the wehrlite cumulates is expected to be lower than that of the associated ophiolitic volcanics (e.g., Mersín ophiolite; Parlak et al. 1996). Based on the Ti content of clinopyroxene (Fig. 7) as a petrogenetic indicator to the magma source (Pearce and Norry 1979), the parent magma from which the Ras Salatit wehrlites were crystallized was depleted and Ti poor. The low Ti content and absence of zoning in the Ras Salatit wehrlites clinopyroxene (Fig. 7; Table 2) may indicate slow cooling rates to attain equilibrium (e.g., Burns 1985; Parlak et al. 1996; Koepke et al. 2009).

The low Al content (Fig. 7; Table 2) of the Ras Salatit wehrlites clinopyroxene suggests relatively high SiO2 activity in the parental and derivative melts (Hébert and Laurent 1990). This indicates that the magma had a tholeiitic or calc-alkalic affinity (Le Bas 1962).

The faint Eu negative anomaly (Eu/Eu* = 0.9) or its lack in the Ras Salatit wehrlites clinopyroxene (Fig. 9) indicates its precipitation from the melt that was little fractionated by plagioclase crystallization (e.g., Drouin et al. 2009). Such very faint Eu negative anomaly in clinopyroxene is absent in the Oman wehrlites (Fig. 9), which may indicate higher degrees of crystal fractionation for the Ras Salatit wehrlites. The calculated REE characteristics of the melt in equilibrium with clinopyroxene from the Ras Salatit wehrlites (Fig. 9) are largely similar to REE contents of lavas from the Izu-Bonin forearc (e.g., Pearce et al. 1992). Moreover, the primitive mantle-normalized calculated melt (Fig. 10) broadly indicates a subduction-related melt signature and in turn argue against a MORB-like melt.

Given the above characteristics, the primary melt for the Ras Salatit wehrlites has been probably derived from re-melting of a refractory source (the underlying harzburgites) that had a potential to produce high-Mg, and low-Ti, Al and LILE melts (e.g., Duncan and Green 1980; Hébert and Laurent 1990; Koepke et al. 2009).

Origin of the Ras Salatit wehrlites

Shifts of the phase equilibria of basalts at low pressures (1-atm–2.5 kbar) are limited (e.g., Green et al. 1979), but a significant shift does occur either at high pressures (e.g., O’Hara 1965, 1968) or at hydrous conditions (e.g., Feig et al. 2006). Water alone has a potential to shift liquidus boundary lines and influence differentiation trends based on mineral compositions significantly, irrespective of the involved geotectonic setting (e.g., Koepke et al. 2009).

The Ras Salatit ultramafic–mafic cumulate section around the possible MTZ has the following rock types: dunites, wehrlites, clinopyroxenites and gabbros. Corresponding to the following crystallization order: olivine/spinel, clinopyroxene and eventually plagioclase, if any (e.g., Mersín ophiolite; Parlak et al. 1996). Such crystallization order implying that the parental melts of the Ras Salatit cumulate sequence were hydrous. Based on experimental results, the plagioclase crystallization is suppressed relative to olivine and clinopyroxene in water-rich tholeiitic systems (e.g., Gaetani et al. 1993; Feig et al. 2006). This is favorable for subduction-related ophiolitic wehrlites, where hydrous parental melts are given by fluid-enhanced melting due to the subducted slab (e.g., Shervais et al. 2004; Koepke et al. 2009).

A question is raised whether the Ras Salatit wehrlites represent primitive cumulates or modified mantle rocks. Argue against a melt-peridotite interaction origin for the Ras Salatit wehrlites are the absence of the following: (1) relic harzburgite blocks within the wehrlite bodies (e.g., Arai and Matsukage 1996); (2) dunite and/or isotropic gabbro halos, veins and/or reaction zones around the wehrlite bodies (Fig. 4) (e.g., Jousselin and Nicolas 2000; Arai and Takemoto 2007; Suhr et al. 2008); (3) fine-grained olivine microstructures, grain-scale veining, any former link between at least some adjacent grains and/or locally occurring much larger olivine grains that all indicating dissolution (e.g., Suhr et al. 2008); (4) part of the olivine preserves a weak intra-crystalline evidence of deformation while clinopyroxene is undeformed or vice versa (e.g., Drouin et al. 2009); (5) olivine and clinopyroxene zoning that indicates further reaction with melts (Figs. 6 and 7) (e.g., Suhr et al. 2008); (6) clinopyroxene resorption (e.g., Xu et al. 1996); (7) high Cr content of clinopyroxene (Fig. 7) as a result of orthopyroxene and spinel dissolution (e.g., Suhr et al. 2008; Drouin et al. 2009); (8) high Ti contents of spinel and clinopyroxene (Figs. 5, 7) as a result of the late migration and crystallization of more evolved melts (e.g., Arai and Matsukage 1996; Suhr et al. 2008); and (9) enrichment of LREE in the melt that involved in genesis of wehrlites (Fig. 9) (e.g., Kelemen et al. 1997; Shimizu 1998). Furthermore, it is well known that significant impregnation of peridotites by basaltic melts resulted in lower olivine Mg# and higher spinel TiO2 (e.g., Arai and Matsukage 1996). By contrast, the Ras Salatit wehrlites show markedly high-Mg# olivine and low TiO2 spinel (Fig. 6b). In Fig. 6b, the Ras Salatit wehrlites plot far away from the Oman wehrlites differentiation trend (Koepke et al. 2004) and close to the Oman harzburgites (Kelemen et al. 1995) and the Hess Deep mantle wehrlites (Arai and Takemoto 2007), suggesting a somewhat link to a mantle derivation (e.g., Benn et al. 1988; Jousselin and Nicolas 2000) and a lack of reactions with melt.

The following criteria favor a shallow-formed (low pressure) cumulate origin of the Ras Salatit wehrlites: (1) absence of orthopyroxene, which is a very dominant phase in deeply formed cumulates; (2) absence of high-Al clinopyroxene (>6 Al2O3 wt. %; Stolper 1980), frequently formed at high pressures; only low-Al clinopyroxene (≤2.5 wt. % Al2O3) is available from the Ras Salatit wehrlites (Fig. 7); and (3) absence of the characteristic LILE enrichment in the calculated melts in equilibrium (Fig. 10).

The good similarity in geology between the Ras Salatit wehrlites and impregnated dunites of the MTZ in ophiolites combined with the absence of wehrlites in the mantle harzburgite suggests that the Ras Salatit wehrlites are equivalent to the MTZ facies (e.g., Benn et al. 1988; Jousselin and Nicolas 2000). Consequently, a shallow subduction zone model (Boudier et al. 1985; Hacker et al. 1996; Ishikawa et al. 2002; Boudier and Nicolas 2007) can be adopted for the origin of the Ras Salatit wehrlites. In the shallow subduction zone scenario (~2 kbar) (Boudier et al. 1985; Boudier and Nicolas 2007), water-rich fluids, deliberated by dehydration of subducted slab, penetrate upward into the upper wedge and initiate a late wet magmatic phase. Early olivine and clinopyroxene may accumulate in wet tholeiitic magma near the MTZ, forming a primitive wehrlitic crystal mush. The latter mush may either freeze within the MTZ dunites or intrude into the surrounding layered gabbros.

Experimental phase equilibrium studies on hydrous tholeiitic systems show that the formation of wehrlitic lithologies is promoted by the presence of water (Koepke et al. 2004; Feig et al. 2006). Based on the experimental results of Koepke et al. (2009), the characteristic wehrlite paragenesis (ol + cpx) can be produced from a wet primitive basaltic melt at temperatures between 1,040 and 1,080°C at crustal pressure by simple accumulation of fractionated crystals. Temperature interval (1,040–1,080°C) of the intruding wet wehrlitic mush is consistent with the absence of any anatexis in the surrounding Ras Salatit layered gabbros (e.g., Koepke et al. 2004). Furthermore, since the magmatic activity in the upper wedge is just ceased, temperatures in the lowermost crust of the upper wedge are still high enough to prevent formation of any chilled margins around wehrlite bodies. This hypothesis supports slow cooling rates manifested in the previously observed absence of any exsolution lamella and/or zoning in the studied mafic mineral phases.

Conclusions

  1. 1.

    The Neoproterozoic Ras Salatit crustal true wehrlites are hosted by thick MTZ section (~1 km thick). They show concordant relations with the host MTZ dunite and/or layered gabbro. Neither chilled margins, xenoliths from the host nor any visible deformations have been observed along the wehrlites’ contacts, indicating their contact at some high-temperature condition.

  2. 2.

    The higher Mg#s of olivine and clinopyroxene in the Ras Salatit wehrlites and changing the order of crystallization (ol → cpx with the absence of plag) are probably due to prevailing higher oxidizing conditions. The latter conditions are due to water deliberated from dehydration of the subducted slab (e.g., Koepke et al. 2009). This is consistent with the slightly higher Mg# in olivine than in clinopyroxene; this is quite unusual in plutonic rocks and is possibly due to small amount of Fe3+ in the former phase.

  3. 3.

    Having unzoned and compositionally uniform clinopyroxene and olivine from the Ras Salatit wehrlites, coupled with low Ti of clinopyroxene, possibly indicate slow cooling rates near the MTZ of the upper wedge.

  4. 4.

    The Ras Salatit wehrlites primary melt (hydrous, high-Mg, and low-Ti, Al and LILE melt) has been probably derived from re-melting of a refractory source (the underlying harzburgites) in a somewhat shallow sub-arc environment. The melt involved in the Ras Salatit wehrlites precipitation possibly experienced fractional crystallization because the NiO content of their olivine is clearly low at the high Mg# for equilibration with the mantle peridotite.

  5. 5.

    The Ras Salatit wehrlites are produced from crystallization of a hydrous depleted tholeiitic melt corresponding to temperatures between 1,000 and 1,100°C at the oceanic crustal pressure (~2 kbar) by simple accumulation of fractionating crystals.