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

Lithos 342–343 (2019) 18–30 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos 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; https://doi.org/10.1016/j.lithos.2019.05.021 0024-4937/© 2019 Elsevier B.V. All rights reserved. A.A. Surour / Lithos 342–343 (2019) 18–30 19 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 sug- gests 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

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