Clues from Rodingitization of Tectonic Fragments in the Neoproterozoic Ophiolites of the Eastern Desert, Egypt

Lithos 342–343 (2019) 18–30

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Mid-ocean ridge vs. forearc and subduction settings: Clues from rodingitization of tectonic fragments in the Neoproterozoic ophiolites of the Eastern Desert, Egypt

Adel A. Surour ⁎

Geology Department, Faculty of Science, Cairo University, Giza 12613, Egypt article info abstract

Article history: Tectonic mélange fragments (metabasalt and metadiabase) in the ophiolitic serpentinites of the Egyptian Eastern Received 12 July 2018 Desert show peculiar mineral assemblages and they provide useful insights into metamorphism and metasoma- Accepted 17 May 2019 tism. New field and mineralogical data indicate the formation of rodingite at the expense of mid-ocean ridge ba- Available online 23 May 2019 salt (MORB) at three localities, namely Wadi Sikait, Wadi Abu Rusheid and Hafafit (SRH belt). These rocks occupy a lower structural horizon in the tectonic mélange zone(s), whereas ocean-island and arc basalt and diabase Keywords: Tectonic fragments, rodingite (OIB) is relatively younger with no evidence of rodingite formation. Rodingitization starts at slow-ultraslow Metasomatism spreading centers and continued until subduction and exhumation at an accretionary wedge. It is a process MORB that is characterized by Ca-rich fluids and the development of a complicated “blackwall” due to superimposed Island-arc basalt K+ and Mg2+-metasomatism. Oxygen fugacity is fluctuating where it is high for Hafafit and low for Wadi Abu Ophiolitic mélange Rusheid rodingites. The Ta/Yb-Th/Yb ratios suggest that rodingitization is not contemporaneous with Northeast Africa (Egypt) serpentinization in an island-arc environment but with serpentinization near seamounts at the spreading centers. Combined field observations, whole-rock geochemistry and mineral chemistry data prove that rodingites are formed at the expense of a MORB (mid-ocean basalt) protolith and occupies a lower structural position than unrodingitized IAT (island-arc tholeiite) metadiabases. © 2019 Elsevier B.V. All rights reserved.

1. Introduction (UHP) reactions occur at greater depths and produce a dense solute- rich transitional fluid that is intermediate between hydrous silicate The ophiolite sequence and ophiolitic mélange in the Eastern Desert melts and aqueous fluids (Hack et al., 2007; Hermann et al., 2006; of Egypt are characterized by the presence of low-temperature Alpine- Klimm et al., 2008; Plank et al., 2009). HP and UHP rocks representing type serpentinized ultramaficrocks(Salem et al., 2012; Surour, 2017). parts of ancient subducted slabs are targets studying P-T conditions of Although serpentinites represent 3–4% only of the earth's crust metamorphism as well as composition and type of slab-derived fluids (Guillot and Hattori, 2013), they serve as significant petrological and responsible for metasomatism in subduction zone environments. geodynamic indicators, as they most often represent hydrous fragments Rodingites in the Pan-African ophiolites of northeast Africa and the of ancient oceanic lithosphere with ~12% H2O(Mével, 2003). Metaso- Arabian Peninsula are uncommon. The term “rodingite” was introduced matism of ultramafic rocks continues from sea-floor serpentinization for the first time by Bell et al. (1911) to describe an assemblage of followed by the formation of some peculiar rock varieties such as dillage-prehnite-grossular garnet in the form of veins in the rodingites, blackwall rocks, and listvenites (sometimes spelled as serpentinites of the Roding area in the Dun Mountains of New “listwaenite”). Generally, metasomatism helps to understand several Zealand. This was followed by a note on another occurrence in New aspects about magma generation and geodynamic events, e.g. at con- Zealand at the Nelson area by Grange (1927). Gresens (1969) was the vergent and divergent margins and intracontinental rift systems. Meta- first to connect the formation of rodingites with mobility of Ca2+ in somatic processes lead to the mobilization and redistribution of major Ca-rich fluids, together with the association of serpentinites and high- and trace elements. The common reactions in subduction zones are pressure metamorphic rocks such as blueschists. Coleman (1966) and high-pressure (HP) metamorphic devolatilization that causes trace ele- O'Brien and Rodgers (1973) connected release of Ca2+ to the processes ment transfer between the subducting slab and the mantle wedge of serpentinization in Alpine-type ophiolites. The formation of (Hacker, 2008; Peacock, 1990). On the other hand, ultrahigh-pressure rodingites at modern mid-ocean ridges (MOR) is also known (Aumento and Loubat, 1971; Honnorez and Kirst, 1975; Yurkova, 1978). Literature ⁎ Corresponding author. review documents the occurrence of world-wide rodingites in ophiolite E-mail addresses: [email protected], [email protected]. and ophiolitic mélanges (e.g. Frost, 1975; Evans et al., 1979 and 1981;

Rice, 1983; Schandl et al., 1989; Mittwede and Schandl, 1992; Schandl older ophiolites of Ghana compared to the age of the Pan-African and Mittwede, 2001, El-Shazly and Al-Belushi, 2004; Li et al., 2004; ophiolites in northeast Africa (Paleoproterozoic and Neoproterozoic, Amato et al., 2007; Frost et al., 2008; Bach and Klein, 2009; Li et al., respectively). 2017). Surour (1990, 1993) reported the first occurrences of rodingites The present paper presents the first detailed characteristics of some at the Sikait-Abu Rusheid area in the Egyptian Eastern Desert. Takla et al. metamorphosed basalt-diabase fragments and their alterations in the (1992) described the mineral paragenesis, textures and possible genesis Eastern Desert of Egypt. Based on the materialized data in the present of these rodingites. Attoh et al. (2006) reported rodingites in relatively work, clues about the status and conditions of rodingitization of mafic

Fig. 1. a) Location map and simpliﬁed geology of the Neoproterozoic basement rocks in the Eastern Desert of Egypt from Azer and Stern (2007). b) Generalized geological map of the Hafaﬁt-El Gemal area (Rashwan, 1991; El Ramly et al., 1993). c) Detailed geological map of the Wadi Ghadir area (Basta, 1983). 20 A.A. Surour / Lithos 342–343 (2019) 18–30

pebble-like (down to ~5 cm wide) to mountains like those at the Barramiya area (Umm Salim and Um Salatit ranges, up to ~17 km long). They are commonly fractured with some asbestiform chrysotile along fractures. Along some faults, the serpentinites are extensively si-

licified and altered to listwaenite whereas CO2-metasomatism produces frequent talc‑carbonate rocks in which magnesite and dolomite are vis- ible on the megascopic scale. In all of the studied localities, serpentinites are tectonically included into a metasedimentary matrix metamorphosed to the upper greenschist to lower amphibolites facies forming prominent tectonic mélange(s). The metasedimentary matrix in some localities is represented by chlorite and graphite phyllite like the case of Abu Fannani. In some other localities, they are meta-mudstone, metagreywacke or generally turbidite facies (Wadi Ghadir) or garnet- mica schists (Wadi Sikait-Wadi Abu Rusheid area). The contact between the mélange and possible older continental fragments is defined by well-defined thrust fault(s). The current field observations distinguish two stratigraphic horizons in the tectonic mélange of the Eastern Desert ophiolites. The contact between them is also defined by low-angle thrusts and in both horizons the serpentinites enclose tectonic fragments such as metabasalt, metadiabase and rarely metagabbros. The imbricated tectonic slices of the investigated mélange (upper horizon) have serpentinite fragments with unrodimgitized metadiabases in the form of deformed dykes (Fig. 2a). The unrodingitized metadiabase fragments are frequent and occur as blocks with spheroidal weathering (Fig. 2b). On the other hand, the fragments in the serpentinites of the lower horizon are metabasalt with complete transformation into rodingite (Fig. 2c). This is very common at the Wadi Sikait, Wadi Abu Rusheid and Hafafit areas or the SRH belt. Vermiculitization is a common feature at the contact of rodingite-bearing serpentinites and pegmatites at the Hafafit

Fig. 2. a) Unrodingitized metadiabase dyke (MD) in the serpentinites (S) at Wadi Ghadir. b) Unrodingitized metadiabase fragment (MD) showing bouldery weathering in the serpentinites (S) at Ras Shait. C) Rodingite dyke (R) in the serpentinites at the Hafaﬁtarea. fragments in Egypt are presented. In this respect, the present work suggests a tentative model for the formation of the Egyptian rodingites in an ophiolite suite from the Eastern Desert based on protolith, nature of ﬂuids and tectonic setting. This represents the northern part or “tip” of the so-called “the Pan-African Orogenic Belt” in northeast Africa with famous Neoproterozoic ophiolites, sutures and subduction zones. Finally, the paper tries to investigate if rodingites be used as indicators of high-P and ultrahigh-P metasomatic alterations in subduction zones or not. Such a case study would be helpful for the understanding of rodingitization elsewhere in the world particularly for the timing, tectonic setting and sequence of metamorphic alterations.

2. Geologic setting and ﬁeld observations

Fig. 3. a) Vermiculite (Vrm)-rich zone in the Hafaﬁt serpentinites (S). White batches are ﬁ In all of the studied areas (Fig. 1), ophiolitic ultrama crocksare components of a felsic injection. b) Blastoporphyritic plagioclase (Pl) corroded by mostly serpentinites that occurs as fragments of different size from chlorite (Chl)-rich groundmass in the metadiabase from Wadi Ghadir, cross-nicols. A.A. Surour / Lithos 342–343 (2019) 18–30 21

Table 1 Some varieties from Wadi Sikait contain considerable amount of late a Mineral abundance in rodingites from the SRH belt. hydrothermal tourmaline (up to ~35%). Sample # 3.1. Unrodingitized rocks Mineral SK2 SK3 SD10b RC1 RC2 HF11

Garnet 35 37 43 45 42 38 The metadiabases from upper stratigraphic level have clear relics of Pyroxene 0 0 0 38 37 33 Amphibole 37 24 20 0 2 2 a doleritic (diabasic) texture. They are mainly composed of highly Plagioclase 18 29 11 9 14 15 saussuritized plagioclase, hornblende, actinolite, chlorite and interstitial Ziosite 3 2 2 2 3 8 calcite. Ophitic and sub-ophitic textures are much less common due to Prehnite 0 0 2 1 0 2 regional metamorphism that gives the rock a schistose appearance. In Graphite 1 2 1 1 0 0 some samples, some unaltered plagioclase porphyroclasts are set in a Ilmenite + titanite + rutile 6 6 4 4 2 2 groundmass rich in chlorite (Fig. 3b). Zoisite content increases towards a SK2, SK3 and SD10 from Wadi Sikait, RC1 and RC2 from Wadi Abu Rusheid and HF11 the serpentinite host and this might indicate incomplete rodingitization from Hafafit. b SD10 contains 17% tourmaline. because the essential minerals in rodingite (e.g. garnet and prehnite) are lacking. Coleman (1966) and Leach and Rodgers (1978) also re- ferred to similar rocks as just “altered rocks” rather than proper area (Fig. 3a). It represents a very characteristic K- and Mg- “rodingites”. metasomatism along contact with pegmatite that are connected to A- type granites similar to other localities in the Easter Desert including 3.2. Rodingites the Sikait-Abu Rusheid area as well. Inosilicate in samples from Wadi Sikait is mostly hornblende that is replaced by coarse-grained garnet crystals with poikiloblastic texture 3. Petrography (Fig. 4a). Rodingite from the Hafafit area and Wadi Abu Rusheid is pyroxene-rich and rich in zoisite that forms at the expense of plagio- The rodingite samples collected from the SRH belt were studied mi- clase and finally transformed into garnet. Garnet still shows a croscopically. Summary of their mineral assemblages is given in Table 1. poikiloblastic texture with frequent inclusions (including calcite) or rel- From the tabulated data, it is evident that the investigated rodingites atively early formed diopside (Fig. 4b). Very young silicification took can be sub-divided into pyroxene and amphibole varieties and all sam- place and results in some secondary quartz infiltiration or/and cross- ples contain considerable amounts of garnet crystals. Generally, garnet cutting quartz veinlets. Rodingites from the Sikait area are characterized may reach up to 40% but the average lies in the range of 22–25%. by an overprint of tourmaline (Fig. 4c and d) on the rodingite

Fig. 4. a) Sub-idioblastic garnet (Grt) pushing aside hornblende (Hbl) in amphibole rodingite from Wadi Sikait, plane-polarized light. b) Xenoblastic garnet (Grt) showing sieve-like texture and abundant diopside inclusions (Di) in pyroxene rodingite from Wadi Abu Rusheid. c) Garnet idioblastic crystal (Grt) enclosed by much younger tourmaline (Tur) in tourmaline-bearing amphibole rodingite from Wadi Sikait, plane-polarized light. d) Altered plagioclase (Pl) enclosed by garnet (Grt) and tourmaline (Tur) in tourmaline-bearing amphibole rodingite from Wadi Sikait, plane-polarized light. 22 A.A. Surour / Lithos 342–343 (2019) 18–30

Table 2 assemblage (garnet-zoisite-prehnite-inosilicate). This variety of a Whole-rock composition of rodingites. tourmaline-bearing amphibole rodingite contains preserved relics of il- Sample # SK2 SK3 SD10 RC1 RC2 HF11 menite that is transformed into titanite and rutile contemporaneous to the events of rodingitization and tourmalinization, respectively. In sum- Oxides (wt%) mary, all samples of the studied rodingites are garnet-bearin. The abun- SiO2 37.33 40.28 39.57 55.51 49.57 66.03

TiO2 3.67 3.67 1.08 0.56 1.29 0.43 dance of garnet is variable and becomes more abundant in the Al2O3 16.43 18.19 26.74 13.43 14.16 12.91 amphibole-rich variety that contains two distinct generations of garnet. Fe2O3 21.15 17.23 10.61 5.11 10.26 4.32 The older generation is much ﬁner and is found mostly as sub- MnO 0.72 0.46 0.71 0.45 0.17 0.15 MgO 6.32 5.39 5.94 3.48 9.08 1.62 idioblastic crystals in the idioblastic crystals of the much coarser and CaO 9.86 10.12 10.82 15.82 9.89 13.19 younger generation. Few crystals of the older generation are found out-

Na2O 1.36 1.61 1.21 0.43 1.72 0.74 side the younger garnet. In some instances, the coarse crystals of the K2O 0.71 1.09 0.41 0.66 0.32 0.18 young generation are invaded by thin veinlets of calcite and prehnite. P O 0.31 0.42 0.23 0.33 0.22 0.16 2 5 Also, the coarse garnet occurs as poikiloblastic crystals with abundant L.O·I 0.81 1.03 2.81 2.56 2.65 0.91 Total 98.67 99.49 100.13 98.34 99.33 100.64 silicate inclusions especially at the core.

Trace elements in ppm 4. Analytical techniques Ba 163 124 515 996 204 15 Rb b8291213b8 b8 Sr 149 370 1151 671 268 187 Based on extensive ﬁeld works, systematic sampling was done. The Nb 13 33 b4 b4 b47 selected samples of rodingites and unrodingitized rocks from the b b b b La 48 68 20 20 20 20 ophiolites in the Eastern Desert of Egypt were examined microscopi- Ce 104 90 b15 19 b15 34 Nd 51 51 b25 b25 b25 b25 cally. Major elements chemical composition of minerals were carried Y 44422324343 out at the Department of Earth Sciences at the ETH-Zürich (Federal Zr 286 255 105 111 88 154 Swiss Institute of Technology). The electron X-ray microanalyzer V 183 197 192 105 198 113 (Cameca SX50 machine) works at accelerating voltage of 30 kV and Cr 108 91 128 113 534 45 20 nA sample current. The used standards represent a set of natural ox- Ni 103 68 114 78 92 77 Co 99 93 34 b45418 ides and silicates, with the exception of synthetic nickel metal for Ni. Zn 150 133 55 47 91 41 The obtained data were normalized using oxygen and cation NORM Hf 19 23 26 2 12 2 programs (GENORM of Currie, 1991) to calculate cations in the struc- Sc 48 45 33 6 41 5 tural formula. Whole-rock composition was measured by the XRF tech- RC1 and RC2 from Wadi Abu Rusheid. nique in the Swiss Federal Laboratories for Materials Science and HF11 from Hafaﬁt. Technology at Dübendorf using a set of basaltic standards. After a SK2, SK3 and SD10 from Wadi Sikait. crushing, grinding and use of an agate mortar powder of samples (b50 μm grain size) are prepared pressed powder pellets are prepared and

Table 3 Whole-rock composition of unrodingitized metadiabases.a

Sample # GHA1 GHB1 GHB2 SHB1 SHB3 SHC1 SHC4 SHC5 BR1 BR11 AFA13 AFC5 AFH4 AFH6 AFH7

Oxides (wt%)

SiO2 55.49 56.03 58.38 50.27 35.84 26.94 43.39 47.92 56.76 43.53 30.37 43.51 52.71 48.92 61.95

TiO2 1.38 1.42 1.46 0.71 1.21 0.67 0.86 0.69 0.78 0.85 1.58 0.53 0.77 0.36 0.57

Al2O3 13.99 15.14 15.31 14.54 26.16 18.77 14.48 14.52 10.84 19.17 14.85 12.16 14.72 12.22 16.36

Fe2O3 7.34 6.93 7.13 9.39 10.51 8.91 11.77 10.72 7.92 12.08 13.95 8.68 9.25 6.15 4.89 MnO 0.13 0.08 0.08 0.16 0.19 0.21 0.21 0.18 0.19 0.21 0.21 0.17 0.13 0.14 0.09 MgO 7.35 6.93 4.21 8.94 5.87 9.44 12.44 10.93 10.07 7.11 21.31 6.68 6.23 6.32 3.86 CaO 5.23 4.22 3.38 11.25 8.71 19.95 9.76 9.06 9.97 10.37 1.17 12.41 8.33 14.13 2.43

Na2O 3.38 4.16 4.56 3.16 0.74 0.29 2.21 2.51 2.01 1.72 0.66 2.28 3.29 2.16 4.61

K2O 1.26 1.62 2.18 0.15 3.95 0.02 0.16 0.11 0.21 2.08 0.08 0.03 0.16 0.02 0.76

P2O5 0.33 0.52 0.53 0.09 0.14 0.09 0.08 0.06 0.12 0.16 0.15 0.05 0.07 0.23 2.11 L.O.I 4.72 5.41 4.12 1.65 7.53 14.44 5.45 3.86 1.24 2.85 15.34 12.93 2.47 8.23 2.28 Total 100.6 102.46 101.34 100.31 100.85 99.73 100.81 100.56 100.11 100.13 99.67 99.43 98.13 98.88 99.91

Trace elements in ppm Ba 1427 421 617 26 1720 b10 18 21 47 971 b10 10 40 b10 244 Rb 12 17 25 b861b8 b8 b8 b842b8 b8 b8 b89 Sr 466 373 366 185 675 424 111 138 273 539 b15 115 200 72 307 Ce 51 30 34 b15 20 b15 b15 b15 b15 b15 b15 b15 b15 b15 b15 Nd 26 b25 b25 b25 b25 b25 b25 b25 b25 b25 b25 b25 b25 b25 b25 Y 269 10181161315232017b311b310 Zr 218 203 201 32 147 34 33 27 97 40 93 15 31 11 110 V 116 163 164 197 69 161 199 154 248 347 158 159 146 129 97 Cr b6 77 81 260 52 32 322 297 461 189 228 102 492 370 36 Ni 10 140 41 159 27 44 101 94 304 71 290 41 151 73 31 Co b45 b448394680603556284037915 Cu b3136 34 b3 17 100 36 b3 b3193754375 Zn 90 68 88 64 74 55 93 103 85 85 99 50 59 38 55 Hf b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b27511 Sc 11 14 15 59 25 25 39 42 29 54 43 70 39 29 18

SHB1, SHB3, SHC1, SHC4 and SHC5 from Ras Shait. BR1 and BR2 from Barramiya. AFA13, AFC5, AFH4, AFH6 and AFH7 from Abu Fannani. a GHA1, GHB1 and GHB2 from Wadi Ghadir. A.A. Surour / Lithos 342–343 (2019) 18–30 23 thickness of the pellet never exceeds about 3 mm, in order to measure the concentrations of major elements. About 7 g of the powder sample is used to prepare fused glass beads (for trace elements) after mixing with a similar amount of Li-tetraborate taking in consideration the pro- cedure ensure to avoid inhomogeneity and agglomeration. Fusion takes place at about 1050 ᴏC using some of the remained powder from ﬁring to measure the loss on ignition (L.O·I.).

5. Whole-rock geochemistry

Major oxide and trace elements composition of rodingites is presented in Tables 2 and 3, respectively. Trace elements, especially the incompatible ones, are useful for petrogenesis of maﬁc and ultramaﬁc rocks in opholites. Both rodingites and unrodingitized metadiabases are exclusively tholeiitic (Fig. 5a) using the Zr-P2O5 binary relationship. The Ni-TiO2 binary plots show inherited imprints of fractional crystallization in unrodingitized metadiabases which is missed in rodingitized samples (Fig. 5b). Some of the unrodingitized samples follow a similar trend

Fig. 6. a) and b) MORB precursor for rodingites and IAT for unrodingitized rocks (Pearce, 1980).

of metadiabases from other world ophiolites, e.g. in Oman, the Alps and elsewhere (Boudier and Nicolas, 2011; Dilek and Furnes, 2014; Gale et al., 2013). Such similarity is also indicated by the plots of the investigated Egyptian rocks on the Ti/100 vs. V binary diagram (Fig. 5c) of Shervais (1982). Fig. 6 uses some trace elements to show the possible tectonic setting which is typical MORB for rodingites and island-arc tholeiites for the unrodingitized metadiabases. Fig. 7 shows spider diagrams of the investigated rodingites and unrodingitized metadiabases normalized to average MORB composition of Pearce (1980). Generally, the patterns of either rodingites or unrodingitized rocks show enrichment trend from Cr towards Sr (Fig. 7). The patterns are somehow serrated in case of unrodingitized samples (Fig. 7b) due to the metasomatic changes that are discussed in the following section. Rodingites are relatively enriched in Nb, Ce, Y and Zr. In both rock varieties, Sc remains the same with few excep- tions, e.g. the Hafaﬁt rodingite with typical SSZ (Surour, 1993). Such composition suggests again derivation of rodingites from MORB. In

rodingites, TiO2 is variable (0.43–3.57 wt%) and then correlatable with metasomatic changes during rodingatization. At the contrary, unrodingitized samples are remarkably depleted in Ti which is the case of island-arc tholeiites in the Egyptian ophiolites that are possibly formed back-arc environment (Greiling et al., 1988) that is a matter of Ti mobility. In the Eastern Desert of Egypt, one can distinguish between basaltic rocks of the MORB-intact ophiolites (MIO), blocks of metabasalts in the mélanges that display N-MORB affinity and relatively younger arc basalt with of typical SSZ tectonic setting (El Bahariya, 2018). Nevertheless, several authors (e.g. Abd El-Rahman et al., 2012; El Bahariya and Arai, 2003; Farahat, 2010; Gamal El Dien et al., 2016) believe that the ophiolites of the Central Eastern Desert (particulary in its central part) Fig. 5. a) Magma type of rodingites and unrodingitized rocks (Elliot, 1973). b) c) fall into two groups, namely MORB (to somehow Back-arc) and SSZ Rodingites and unrodingitized rocks from the Eastern desert of Egypt with ~Ti/V = 50 ophiolites that are spatially and temporally unrelated. They are not (Shervais, 1982). In b & c, field of metamorphosed basalt-gabbro association from the Egyptian ophiolites is given for comparison based on datasets from Abd El Rahman et al. petrogenetically related, and therefore the Egyptian ophiolites repre- (2012) and El Bahariya (2018). sent a change of the tectonic setting from MORB to SSZ with time. The 24 A.A. Surour / Lithos 342–343 (2019) 18–30 dataset of the studied rodiningites and unrodingitized samples of the (65–67%) in much younger idioblastic garnet. The early garnet phase present work are in harmony of such two different tectonic settings is much finer and can be observed at core and intermediate core of based on their trace element composition (Fig. 5c). This figure suggests the coarse idioblastic garnet so the former might represent substratum a P (plume)-like composition of the MORB magma especially in the for more advanced crystallization during rodingitization viz. metasoma- rodingitized samples with slight redistribution of trace elements. Thus, tism. The early garnet phase in the amphibole rodingite from Wadi the fractionation of incompatible element patterns in the rodingites Sikait has very distinct pyrope component in comparison to the ana- higher than typical MORB (Figs. 6 and 7), and accordingly the lyzed garnets from all varieties and among all localities. Fig. 8a shows rodingitization process played a role in the modification of the typical a trend of decrease of pyrope content from the early cores to the more MORB composition. spessartine rich rims that is consistent with the increase of the pyralspite component at nearly constant grandite content as indicated 6. Mineral chemistry by the microanalyses (Tables 4 and 5). Fig. 8b provides a chemical profile in a single garnet crystal from the Wadi Sikait tourmaline bearing This section is devoted to the chemistry of silicate and not-silicate amphibole rodingite. The figure suggests distinct crypto-zoning in minerals in the studied rodingites from the SRH belt in the Eastern De- which grossular and spessartine components increases towards the sert of Egypt. The chemical analyses by the electron microprobe are pre- core as CaO and MnO increase. At the contrast, FeO and MgO increase to- 2+ 2+ sented in Tables from 4 to 7. Ferric oxide for some silicate minerals was wards the rim suggesting a coupled ionic substitution (Ca +Mn = 2+ 2+ calculated using the method of Droop (1987). Fe +Mg ) with dominancy of Mg over ferrous Fe. Similarly, Evans et al. (1979) reported a decrease of Fe2+/Mg ratio towards the rims of 6.1. Garnet garnet from rodingites. Garnet from both pyroxene and amphibole rodingites has low TiO2 content (0.02–0.40 wt%). Nevertheless, the Table 4 shows that garnet from the pyroxene rodingite from early garnet phase is more enriched in titania where it amounts – the Hafafit area is grossular-andradite which can be also seen on the 0.40 0.71 wt%. Because water in the structure of garnet is not measured mole% ternary diagram (Fig. 8a). The grandite content of this garnet by the electron microprobe, estimation of H2O content in garnet crystals + ranges from 85.1% to 88.3% whereas the grossular component greatly is based on stoichiometry. Four H atoms are needed to replace a single 4+ exceeds that of andradite (62.6–66.4% and 19.3–25.7%, respectively). Si atom in the tetrahedral sites. Basso et al. (1984) considered this “ ” Its pyralspite content ranges from 10.2% to 12.5% while the spessartine garnet species as hydrogarnet that is slightly anisotropic, and with a “ ” + 4+ in solid solution is limited (1.4–4.9%). On the other hand, garnet in the defective structure in which the 4H =Si substitution is accompa- ‑ pyroxene rodingite from Wadi Abu Rusheid has lesser grandite content nied by increase of the d oxygen distance in the crystal structure and ex- (42.5–47.2%). In this occurrence, the grandite content increases towards pansion of the unit cell (Larger et al., 1987; Schandl and Mittwede, rims, while its spessartine+almandine component ranges from 31.4% to 2001). The grossular-andradite in the pyroxene rodingite from the fi – 35.8% with remarkable enrichment at the core (up to 44.8%). Hafa t area has presumably the highest H2O content (~0.75 1.08 wt%) fl 4+ Garnet from the amphibole rodingite from Wadi Sikait and Wadi and can be considered as hydrogarnet. This is re ected in its Si – – Abu Rusheid is enriched in the pyralspite component (~30–40% in the atoms that amount 2.91 2.93 whereas it lies in the range of 2.96 3in early phase of garnet especially at cores) whereas it is much higher the rest of investigated garnets from other localities and varieties.

Fig. 7. Spider diagrams of rodingites (a) and unrodingitized rocks (b). Normalization values are based on datasets from Pearce (1980) and Gale et al. (2013). A.A. Surour / Lithos 342–343 (2019) 18–30 25

Table 4 Electron microprobe analyses of garnet in rodingite from Wadi Sikait.a

Spot # SD10-1 SD10-2 SD10-3 SD10-4 SD10-5 SD10-6 SD10-7

SiO2 37.46 37.41 37.31 37.46 37.35 37.59 37.49

TiO2 0.04 0.04 0.15 0.17 0.12 0.07 0.06 b Cr2O3 nd nd 0.01 0.01 nd 0.01 nd

Al2O3 21.72 21.36 21.18 21.21 21.34 21.58 21.6

Fe2O3 1.14 0.42 1.48 1.37 0.82 1.11 1.14 FeO 26.52 25.01 20.93 21.07 24.24 25.84 26.66 MnO 1.01 3.71 8.34 8.98 5.28 3.61 1.11 MgO 3.83 2.41 1.37 1.32 1.95 2.61 3.59 NiO nd nd nd 0.03 nd 0.06 nd CaO 8.05 9.03 9.98 9.63 9.08 8.35 8.35

Na2O 0.02 0.02 0.03 0.01 0.01 0.03 nd

K2O 0.02 0.01 nd nd nd nd 0.01 Total 99.81 99.42 100.78 101.26 100.19 100.86 100.01

Cations on the basis of 24 oxygen atoms Si 2.9563 2.983 2.9592 2.9614 2.9696 2.9624 2.9584 Ti 0.0022 0.0025 0.0088 0.0101 0.0073 0.0041 0.0039 Cr 0.0002 0.0001 0.0009 0.0008 0.0001 0.0006 0.0001 Al 2.0201 2.0077 1.9802 1.9764 1.9997 2.0046 2.0088 Fe3+ 0.068 0.0255 0.0882 0.0818 0.0488 0.0656 0.0674 Fe2+ 1.7503 1.6676 1.3881 1.393 1.6115 1.7027 1.7592 Mn 0.0667 0.2506 0.5602 0.601 0.3559 0.2409 0.047 Mg 0.4505 0.2869 0.1614 0.1557 0.2311 0.3065 0.4216 Ni 0 0 0 0.0022 0 0.0037 0 Ca 0.6807 0.7717 0.8477 0.8156 0.7737 0.7049 0.7058 Na 0.0035 0.0029 0.0049 0.002 0.0018 0.004 0 K 0.0016 0.0014 0.0004 0 0.0005 0 0.0008 Σ Cations 8.0001 7.9999 8 8 8 8 7.973

Endmembers Grossular 0.195 0.244 0.236 0.228 0.231 0.202 0.202 Almandine 0.593 0.56 0.469 0.469 0.542 0.575 0.594 Pyrope 0.153 0.096 0.054 0.052 0.078 0.104 0.142 Spessartine 0.023 0.084 0.189 0.203 0.12 0.081 0.025 Andradite 0.035 0.013 0.045 0.041 0.025 0.033 0.034 Uvarovite 0 0 0 0 0 0 0 Ti-Al garnet 0 0 0.002 0.004 0.002 0 0.002 Na-Ti garnet 0.002 0.003 0.005 0.002 0.002 0.004 0.001 Σ Endmembers 1.001 1 1 0.999 1 0.999 1

a SD10 is tourmaline-bearing rodingite. b nd: not detected.

6.2. Clinopyroxene and amphibole 6.3. Zoisite, ilmenite and rutile

Electron microprobe analyses of clinopyroxene from the pyroxene As shown in the petrography section zoisite is a common epidote- rodingites of the Hafaﬁt and Wadi Abu Rusheid areas are given in group mineral that results from alteration of plagioclase. Ziosite, as a Table 6. Clinopyroxene in both localities is Ca-rich as the CaO content transitional phase that leads to garnet (Schandletal.,1989) in the stud- ranges from 23.46 to 23.97 wt% (Wadi Abu Rusheid) and from 23.99 ied rodingite was analyzed (Table 7). Zoisite has a normal CaO content to 24.12 wt% (Hafaﬁt). Fig. 9 shows that they are diopside from the of a metasomatic epidote species (24.09–24.32 wt%) but with variable

Abu Rusheid samples whereas it is intermediate between diopside traces of MnO (0.3–0.36 wt%) and TiO2 (0.04–0.22 wt%) but a constant and hedenbergite in case of the Hafafit ones. Clinopyroxene from the Cr2O3 (0.04 wt%). Ilmenite is Mn-bearing with an expected MnO con- Wadi Abu Rusheid rodingite is more enriched in MgO in comparison tent 2.51–2.63 wt% (Table 7) that is normal for ilmenite of the mafic with its counterpart from the Hafafitarea(8.66–11.71 wt% and protolith. The table also contains two representative analyses of ilmen- 7.28–7.69 wt%, respectively). Mg# is variable from 44.55 to 66.86 ite alteration (rutile) with up to 0.29 wt% FeOt or MgO indicating minor where the maximum value is attributed to the closeness with the replacement of Ti4+ by both Fe2+ and Mg. serpentinite host (Table 6). At the contact with the ultramafic host, t clinopyroxene is more enriched in FeO than those at the core of 7. Discussion rodingite (15.28–16.34 wt% and 10.73 wt%, respectively) suggesting a 2+ 2+ possible simple substitution trend (Mg =Fe ). In some world ex- 7.1. Precursor of rodingites amples, calcic clinopyroxene from rodingites possesses couple ionic 2+ 2+ 2+ 2+ substitution following the trend Mg +Mn =Ca +Fe (Rodgers, Rodingites are exclusively metasomatic rocks that might have differ- 1978). ent parageneses based on chemical characteristics in terms of the bulk Table 7 provides representative spot analyses (each is an average of composition and mineral composition of their protoliths. It is believed 3 spots) of amphibole from the studied rodingites. It is a typical horn- that primitive rodingite forms at the earliest stages of sea-floor meta- t blende with very characteristic and nearly similar contents of FeO , morphism and that the degree of rodinigitization and nature of precur- MgO and CaO (12.17–13.94 wt%, 9.83–12.81 wt%, 11.35–11.71 wt%, re- sor control chemical variations and mineral paragenesis (Li et al., 2004). – spectively). TiO2 of that hornblende is low (0.33 0.58 wt%) with Rodingitization might continue until subducion and the hydrothermal – 40.79 41.05 wt% SiO2 that are both typical for a metamorphic fluids that result from subduction attack mafic fragments and dykes in amphibole. the ultramafic host (Christensen, 2004) to precede the metasomatic 26 A.A. Surour / Lithos 342–343 (2019) 18–30

breakdown of Ca-rich igneous pyroxene and plagioclase during serpentinization under greenschist facies condition. It is a part of seafloor metamorphism of peridotites during the Pan-African orogenic evolution and the development of Alpine-type ophiolite in the Eastern Desert of Egypt. No thermometric data are available for rodingites themselves from the Eastern Desert but there are evidence of amphibolites facies metamorphism at ~ 550–570 ᴏC(Surour, 1993, 1995)forthe ophiolites that host rodingitized rocks at the Wadi Sikait and Hafafit areas for example. Similarly, and in other world examples, fluids

attacking mafic fragments and dykes are Ca-rich H2-bearing, reducing brine (8 equiv. wt% NaCl) originating from the serpentinite front at 250–400 ᴏC upon contemporaneous serpentinization (El-Shazly and Al-Belushi, 2004; Ferrando et al., 2010; Palandri and Reed, 2004; Schandl and Mittwede, 2001). Serpentinization itself covers the temperature range of 300–500 ᴏC(Alt and Shanks, 2003; Mével, 2003; Paulick et al., 2006). At the beginning, pressure is high (up to 6 kbar) then it is minimized to ˂3.2 kbar at the late stages of rodingitization when the infiltrating aqueous fluids becomes less saline, and this takes during exhumation and continues after the final emplacement of the ophiolite (El-Shazly and Al-Belushi, 2004). Recent studies suggest that rodingites form within the oceanic crust and then later involved in a subduction-zone environment at high pressures (Li et al., 2017), which is almost the case of the investigated rodingites at the SRH belt. The occurrence of calcite in equilibrium with late-stage garnet in the studied SRH rodingites suggests that rodingitization might continue

until subduction began when subduction-related CO2-rich fluids result from decomposition of oceanic carbonate beds that increase in the pH of the system. A similar conclusion was reached by Tsikouras et al. (2009) for rodingites in the ophiolite complex of Othrys, central Greece. Based on the petrographic evidence, the diopside is exclusively not igneous that forms at elevated pressure during metamorphism and subsequent metasomatism upon subduction. Surour (1995) used the mineral chemistry of garnet amphibolites at the Wadi Sikait area to es- timate medium- and high-pressure metamorphism up to 7.7 kbar Fig. 8. a) Composition of garnet in rodingite from the SRH belt. b) Chemical profile (rim- core-rim) in a garnet crystal in amphibole rodingite from Wadi Sikait. owing to subduction. Oxygen fugacity during rodingitization can be es- timated tentatively from the chemistry of diopside where it is low in case of Wadi Abu Rusheid and high for the Hafafit samples (Fig. 10). process(s) that finally lead to the formation of well-developed Normally, blackwalls are made up of phyllosilicates (Frost, 1975). In rodingite. Fig. 6 shows that the precursors of rodingites in the SRH the Eastern desert of Egypt, blackwalls surround either rodingites or belt were MORB whereas unrodingitized samples are derived from serpentinized Alpine-type ultramafics that show zonal arrangements arc-tholeiites and later both series were emplaced in a mélange within of antigorite, talc, tremolite and chlorite (Surour, 1990; Takla et al., an accretionary wedge. It is believed that early rodingitization of 1992 and 2003). In this respect, one should distinguish the rodingitic MORB is earlier than lawsonite or other eclogitic HP assemblages that blackwalls from phlopgopite, vermiculite and chlorite zones. No doubts needs subduction (Beane and Liou, 2005; Cristi Sansone et al., 2011). the event of phlogopitization (i.e. K-metasomatism) has an impact, even Textural fabrics of rodingites from the SRH belt suggest the formation partial, on the composition of rocks and then it explains the enriched of rodingite mineralogy by an early event prior to subduction when patterns in Fig. 7 in addition to fractionation of incompatible elements proto-garnet is formed (Fig. 8 and Table 4). so that some of the studied rodingites that contain phlogopite, tourma- Outside the SRH belt rodingites are lacking, and the metadiabases of line and more albite are a bit different than other rodingites with no ef- arc affinity show distinct geochemical signature than that of MORB fect of any granitic fluids or such an event of metasomatism. The current (now rodingite) that witnessed sea-floor metamorphism prior to sub- field observations, textural evidence and mineral chemistry indicate duction. Contents of large-ion lithophile elements (e.g. Rb, Ba, K and that phlogopite is a product of K-metasomatism of tremolite-actinolite Sr) in the unrodingitized metadiabases are typical of mafic to interme- (after metapyroxenite pockets and dykes) in serpentinite after a diate rocks of suprasubduction zone-type affinity (Sarıfakıoğlu et al., dunite-peridotite protolith by the intrusion of much younger A-type 2010; Schellekens, 1998). On the other hand, ocean-island basalts granites especially at Wadi Sikait in the vicinity of the old emerald (OIB) with relatively immobile trace and rare-earth elements might mines and the Roman temples. On the other hand, vermiculite zones transform into rodingites same as MORB does (El-Shazly and Al- at the margin of ophiolitic serpentinites at the Hafafit area are products Belushi, 2004). Recently, Harigane et al. (2016) calculated ∼17% as the of Mg-metasomatism and disilicification of adjacent pegmatites degree of partial melting of mantle peridotite beneath the spreading (Fig. 4a). The last peculiar type of blackwall in the studied Egyptian axis to generate MORB, particularly the N-MORB which is the case of example is the magnetite-rich chloritite that forms in the some investigated rodingites from the SRH belt in Egypt (Fig. 6c). metasedimentary mélange juxtaposing the tectonic serpentinite fragments due to the release of Fe2+,Mg2+ and Fe3+ from the serprntinites, 7.2. Process of rodingitization and the formation blackwalls most probably during uplift and exhumation. Fig. 10 gives some significant information about the process of rodingitization in the Egyptian Ferrando et al. (2010) interpreted that the event of rodingitization is shield rocks in terms of pyroxene chemistry and its information about triggered by hydration of olivine, but on the other hand the current min- oxygen fugacity. The figure suggests that rodingite from the Hafafit eralogical and textural criteria of rodingites at the SRH belt is formed by area formed at a relatively higher oxygen fugacity than the Abu A.A. Surour / Lithos 342–343 (2019) 18–30 27

Table 5 Electron microprobe analyses of garnet in rodingites from Hafaﬁt and Wadi Abu Rusheid.a

Spot # HF11-36 HF11-37 HF11-40 HF11-41 HF11-41 RC1-49 RC1-50 RC1-51 RC1-55 RC1-56 RC1-57 RC1-59

SiO2 37.47 37.02 37.54 37.56 37.08 37.7 37.73 38.05 38.26 37.71 37.73 37.95

TiO2 0.29 0.31 0.3 0.21 0.23 0.13 0.03 0.05 0.04 0.02 0.1 0.05

Cr2O3 0.34 0.35 0.2 0.12 0.25 0.02 0.02 0.07 0.09 0.04 0.02 0.03

Al2O3 18.37 18.38 17.9 18.49 17.16 21.14 21.11 21.04 21.41 21.1 21.13 21.3

Fe2O3 6.43 7.03 7.43 7.23 8.48 0.74 1.78 0.42 1.5 0.98 1.44 1.52 FeO 3.42 2.93 3.61 3.39 3.91 15.22 15.69 12.13 10.35 16.16 15.68 16.23 MnO 2.18 2.17 1.24 1.25 0.6 3.1 7.3 5.45 5.53 6.85 7.07 7.1 MgO 0.11 0.12 0.08 0.07 0.01 1.44 1.4 1.12 1.11 1.27 1.42 1.39 NiO 0.03 ndb 0.05 nd nd nd nd 0.02 0.07 nd 0.02 0.03 CaO 30.6 30.6 31.27 31.35 31.19 19.6 15.23 22.27 22.99 15.41 15.4 15.14

Na2O nd nd 0.02 0.04 0.02 0.04 0.02 nd nd 0.01 0.03 0.04

K2O nd nd 0.01 0.02 0.01 0.02 0.01 nd nd nd 0.02 0.02 Total 99.24 98.91 99.65 99.73 98.94 99.15 100.32 100.62 101.35 99.55 100.06 100.8

Cations on the basis of 24 oxygen atoms Si 2.935 2.9108 2.9339 2.926 2.9287 2.9954 2.9676 3.004 2.9713 2.9847 2.9716 2.9705 Ti 0.0172 0.0184 0.0176 0.0122 0.0135 0.0079 0.0016 0.0028 0.0025 0.0012 0.0062 0.0031 Cr 0.0211 0.0221 0.0125 0.0074 0.0154 0.001 0.001 0.0041 0.0055 0.0024 0.0011 0.0015 Al 1.6955 1.7034 1.6491 1.6974 1.5977 1.9725 1.9566 1.9577 1.9592 1.9683 1.9613 1.9649 Fe3+ 0.379 0.4162 0.4368 0.4241 0.5039 0.0442 0.1052 0.0947 0.0878 0.0581 0.0853 0.0897 Fe2+ 0.2239 0.1927 0.2357 0.2207 0.2583 0.9388 1.0322 0.9701 0.9017 1.0692 1.0326 1.0625 Mn 0.145 0.1447 0.082 0.0823 0.0403 0.2432 0.4864 0.4378 0.4855 0.4599 0.4717 0.4705 Mg 0.0132 0.0145 0.0094 0.0077 0.0014 0.2884 0.1642 0.1419 0.1589 0.1501 0.1662 0.1617 Ni 0.0018 0 0.003 0.0003 0 0 0.0002 0.0015 0.0043 0 0.0011 0.0021 Ca 2.5683 2.5772 2.6186 2.6167 2.6394 1.501 1.2837 1.3765 1.4134 1.3065 1.2993 1.2698 Na 0 0 0.0012 0.0035 0.0014 0.0028 0.0012 0 0 0.0006 0.0029 0.0035 K 0 0 0.0002 0.0018 0 0.0016 0 0 0 0 0.005 0 Σ Cations 8 8 8 8.0001 8 7.9968 7.9999 7.9911 7.9901 8.001 8.0043 7.9998

Endmembers Grossular 0.658 0.646 0.651 0.664 0.626 0.379 0.378 0.531 0.508 0.407 0.39 0.38 Almandine 0.076 0.066 0.08 0.075 0.088 0.399 0.348 0.272 0.264 0.358 0.347 0.358 Pyrope 0.004 0.005 0.003 0.003 0 0.098 0.055 0.044 0.043 0.05 0.056 0.054 Spessartine 0.049 0.049 0.028 0.028 0.014 0.093 0.164 0.136 0.137 0.154 0.159 0.159 Andradite 0.193 0.213 0.222 0.217 0.257 0.022 0.053 0.012 0.044 0.029 0.043 0.045 Uvarovite 0.011 0.011 0.006 0.004 0.008 0.001 0.001 0.002 0.003 0.001 0.001 0.001 Ti-Al garnet 0.009 0.009 0.008 0.004 0.006 0.002 0 0.001 0.001 0 0.001 0 Na-Ti garnet 0 0 0.001 0.005 0.001 0.004 0.001 0 0 0.001 0.003 0.003 Σ Endmembers 1 0.999 0.999 1 1 0.998 1 0.998 1 1 1 1

a HF11 from Hafaﬁt and RC1 from Wadi Abu Rusheid. b nd: not detected.

Rushaeid rodingite. This is explained by the fact that the subducted oce- Table 6 anic slab at the Hafafit area was brought to a relatively higher depth and Electron microprobe analyses of clinopyroxene in rodingite.a fi pressure (Surour, 2003). Also, rodingite from the Hafa t area contains Spot # HF11-1 HF11-2 RC1-1 RC1-2 RC1-3 RC2-2 some vermiculite-phlogopite-pyroxene micro-veinlets and such vein- SiO 49.94 49.74 51.81 51.37 52.97 52.82 lets are indication of elevated pressure and oxygen fugacity in subducted 2 TiO2 0.04 0.02 0.13 0.08 0.02 0.12 oceanic slabs after formation from the mantle at a mid-ocean ridge Al2O3 1.04 0.82 0.96 0.84 1.01 0.92 (Franz et al., 2002). It is worthmentioning here that hydrothermal alter- FeOt 16.18 16.35 10.79 15.28 10.37 10.41 ation of seafloor peridotite viz. serpentinization does not contribute to MnO 0.38 0.41 0 0 0 0 modification of oxygen fugacity of the mantle minerals including MgO 7.69 7.29 11.37 8.66 11.71 11.69 CaO 23.99 24.12 23.94 23.65 23.95 23.97 diposide as the most common pyroxene prior to serpentinzation Na2O 0.23 0.18 0.11 0.03 0.01 0.16 b (Binner et al., 2016). This is not the case of the Egyptian ophiolite be- K2Ond 0.01 nd nd nd nd cause subsequent events of rodingitization and K-metaomatism might Total 99.49 98.94 99.11 99.91 100.04 100.09 unbalance oxygen fugacity contemporaneous to the formation of Cations on the basis of 6 oxygen atoms reworked or metasomatic diopside in rodingite. At late stage of Si 1.946 1.954 1.976 1.985 1.999 1.991 rodingitization, diopside might be recrystallized into amphibole and Ti 0.001 0.001 0.004 0.002 0.001 0.003 this can happen when the oxygen fugacity drops, so the redox condition Al 0.048 0.038 0.043 0.038 0.045 0.041 Fe3+ 0.076 0.066 0.005 0.000 0.000 0.000 of the subducted slab and assoited rodingitization is changeable Fe2+ 0.451 0.471 0.339 0.494 0.327 0.328 (Laborda-López et al., 2018). Mn 0.013 0.013 0.000 0.000 0.000 0.000 The formation of blackwall is connected to rodingitization where the Mg 0.447 0.427 0.646 0.499 0.659 0.657 blackwall is a part of the mafic precursor itself like at Przemilów in Ca 1.001 1.015 0.978 0.979 0.968 0.968 Poland (Dubińska and Wiewóra, 1999). In the Polish locality, early Na 0.017 0.014 0.008 0.002 0.001 0.012 2+ K 0 0.001 0 0 0 0 phase of rodingitization received Ca from the serpentinite host but Mg# 49.78 47.55 65.58 50.25 66.86 66.70 released K+ and Al3+ to form phlogopite. Endmembers Wo 52.72 53.07 49.83 49.65 49.55 49.57 7.3. Geodynamic and tectonic implications En 23.51 22.32 32.92 25.30 33.71 33.63 Fs 23.77 24.61 17.25 25.05 16.75 16.80

ﬁ a Alteration processes of fresh ultrama c rocks, e.g. peridotite, results HF11 from Hafaﬁt; RC1 and RC2 from Wadi Abu Rusheid. in the formation of serpentine “polymorphs” and a variety of accessory b nd: not detected. 28 A.A. Surour / Lithos 342–343 (2019) 18–30

Fig. 9. Classification of pyroxene in the studied pyroxene rodingites from Hafafit and Wadi Abu Rusheid (after Morimoto et al., 1988). The thermal isograds are from Lindsley (1983) and Kretz (1982). minerals such as brucite, magnesite, talc, tremolite and magnetite. only tendency to be eclogite-like based on high-pyrope component in These alterations result in remarkable difference in seismic anisotropy the analyzed garnet that associates diopside in the rodingite. Thus, and this lower seismic velocity in forearc mantle wedge (Christensen, MORB of ophiolites in the Egyptian Eastern Desert presumably forms 2004). The present author agrees with Ferrando et al. (2010) that the at slow spreading oceans similar to the “fossilized” or ancient Ligurian tectono-metamorphic history of rodingite is the same of their example in Italy, and the nowadays Atlantic and Indian Oceans sepentinite host and this comprises a scenario that starts by an earlier (Piccardo, 2013; Paulick et al., 2006; Paquet et al., 2016, respectively). high pressure metamorphism, followed by decompression/re- This explains why bluschists and proper eclogite are lacking not only equilibration under greenschist facies conditions and by final cooling. in Egypt but in the entire Pan-African ophiolites of Neoproterozoic age The unrodingitized metadiabases in the Eastern Desert of Egypt in northeast Africa and the Arabian Peninsula. On the other hand, (outside the SRH belt) are the final products of exhumation and incor- rapid ocean-floor spreading and high subduction rates (8.8–7.0 cm/y poration of the metamorphosed arc mafic-ultramafic blocks in a tec- full-rate; Ohara et al., 2003) tend to favor formation and exhumation tonic mélange with a metasidementary matrix. This type of mélange is of blueschist/eclogite belts (Maruyama et al., 1996). Elsewhere in the relatively younger than the serpentinitic mélange of the SRH belt with world, HP and LT rocks formed upon subduction are brought up to complete rodingitization of MORB near the seamounts by sea-floor shallower levels by extensional faults (Hopson and Pessagno, 2005). metamorphism and the associated metasomatic alterations. The In the Eastern Desert of Egypt, ophiolitic serpentinites flow due to this so-called “serpentinitic” mélange is a part of an obducted oceanic litho- buoyancy (Akkad, 1997) and upon exhumation of rodingite-bearing sphere and contains mafic to intermediate blocks of LP and LT metamor- fragmented HP rocks with rodingites, island-arc metadiabses are incor- phic rocks associated with island arc and/or seamount terranes porated into the serpentinites with no evidence of rodingitization. In (Dobretsov and Buslov, 2004). Rock association, mineralogy and geo- this respect, the present work agrees with Austrheim and Prestvik chemical signature support that the Egyptian examples belong to the (2008) that hydration of the oceanic lithosphere occurs at various struc- latter type. Proper blueschist in the shield rocks of Egypt have not tural levels and is associated with Ca-metasomatism also in places known yet but the eclogite-like nature of the Wadi Sikait rodingites is where rodingite is lacking, which is the case of unrodingitized a strong evidence of HP metamorphism. It is worth mentioning here it metadiabases in the serpentinites outside the SRH belt. No evidence is not a typical eclogite that should contain omphacite and pyrope but for the formation of studied rodingites from Egypt outside the MOR

Table 7 Electron microprobe analyses of hornblende, zoisite, ilmenite and rutile in Wadi Sikait rodingites.a

Spot # SK2-7 SD10-19 RC2-52 SD10-78 SK2-85 SK2-86 SK2-88 SD10-101 SD10-104

SiO2 40.79 41.05 39.12 39.12 0.04 0.14 0.02 0.02 0.48

TiO2 0.33 0.58 0.22 0.04 52.88 53.09 52.96 99.69 98.71 b Cr2O3 Nd 0.03 0.04 0.04 nd nd nd nd nd

Al2O3 16.84 19.74 29.71 31.19 0.01 0.03 0.01 nd nd FeOt 13.94 12.17 4.29 2.78 44.72 43.37 43.68 0.26 0.29 MnO nd 0.22 s0.36 0.03 2.51 2.63 2.58 nd nd MgO 12.81 9.83 0.06 nd 0.03 0.04 0.05 0.29 0.07 NiO nd 0.11 nd 0.02 0.09 0.04 0.03 nd 0.01 CaO 11.71 11.35 24.09 24.32 0.12 0.18 0.16 0.02 0.01

Na2O 0.82 2.11 nd 0.19 nd nd nd nd nd

K2O 0.17 0.39 nd 0.01 nd nd nd nd nd Total 97.41 97.58 97.89 97.74 100.4 99.52 99.49 100.28 99.57

RC2-55 and SD10-78 are zoisite from amphibole- and tourmaline-bearing rodingite from Wadi Sikait, respectively. SK2-86 and SK-88 are ilmenite from amphibole-bearing rodingite. SD10-101 and SD10-104: rutile from tourmaline-bearing rodingite. a SK2-7 and SD10-19 are hornblende from amphibole- and tourmaline bearing rodingite from Wadi Sikait, respectively. b nd: not dtected. A.A. Surour / Lithos 342–343 (2019) 18–30 29

events of mafic-ultramafic rocks that take place in MOR and fore-arcs with possible economic potentiality, e.g. Au-Cu-Ag mineralization (Pal'yanova et al., 2018). Prospection for noble metals in rodingitized tectonic fragments in ophiolites attracted attention of workers in other parts of the world during the last three decades, e.g. in Morocco (Leblanc and Lbouabi, 1988) and in Canada (Knight and Leitch, 2001) that acquires alkaline fluids and reducing condition upon serpentinization and consequent metasomatism.

Acknowledgements

Fig. 10. Variation of oxygen fugacity during the formation of rodingites (Schweitzer et al., The author is greatly indebted to the staff members at the ETH- 1979). Zürich for their support, fruitful discussion and access to the mineralogy and petrology laboratories for the analyses needed. Thanks for the Fed- regime. However, some recent studies suggest the formation of eral Swiss Scholarship Program for the logistic support and opportuni- rodingites in the forearc setting based on thallium isotopes which is ties given to the author to finalize the presented work. Alessio the case of the Mariana forearc (Nielsen et al., 2015). In the Egyptian Sanfilippo, Nelson Boniface and an anonymous reviewer are very thank- Eastern Desert of Egypt, the mantle wedge carbonation result in meta- ful for their review and useful comments that greatly improved somatic fluid influx for consumption in rodingitization. It is widely ac- the quality of the submitted work. The author is grateful for Prof. Hafiz cepted that the metasomatic fluids for the process of rodingitization Ur Rehman for the editorial handling, patience and valuable originate from subduction of carbonate-bearing sediments result in de- recommendations. carbonation and production of Ca-rich fluids, e.g. the Greek and Chinese ophiolites (Tsikouras et al., 2009; Chaonan and Santosh, 2017, respec- References tively). Geotectonically, the process of rodingitization can happen at Abd El-Rahman, Y., Polat, A., Dilek, Y., Kusky, T.M., El-Sharkawy, M., Said, A., 2012. three stages with respect to evolution from low-pressure regime Cryogenian ophiolite tectonics and metallogeny of the central Eastern Desert of (b2 kbar) at the seafloor to proper subduction stage, possibly up to Egypt. Int. Geol. Rev. 54, 1870–1884. 10 kbar or higher (Laborda-López et al., 2018). Akkad, M.K., 1997. On the Behaviour of Serpentinites and Its Implications. 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