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Serpentinites in the Eastern Desert, Egypt: Fragments of Forearc Mantle

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Serpentinites in the Eastern Desert, Egypt: Fragments of Forearc Mantle Neoproterozoic (835–720 Ma) Serpentinites in the Eastern Desert, Egypt: Fragments of Forearc Mantle Mokhles K. Azer and Robert J. Stern1 Geology Department, National Research Centre, Al-Behoos Street, 12622-Dokki, Cairo, Egypt (e-mail: [email protected]) ABSTRACT Most Neoproterozoic ophiolites of the Arabian-Nubian Shield show compositions consistent with formation in a suprasubduction zone environment, but it has not been clear whether this was in a forearc or back-arc setting. Ophiolitic serpentinites are common in the Eastern Desert of Egypt, but their composition and significance are not well understood. Here we report new petrographic, mineral, chemical, and whole-rock compositional data for ser- pentinites from Wadi Semna, the northernmost ophiolitic serpentinites in the Eastern Desert, and use these to provide insights into the significance of other Eastern Desert serpentinite locales. The Wadi Semna serpentinites are composed essentially of antigorite, chrysotile, and lizardite, with minor carbonate, chromite, magnetite, magnesite, and chlorite, and they were tectonically emplaced. The alteration of chrome spinel to ferritchromite was accompanied by the formation of chloritic aureoles due to the release of Al from spinel. Major-element compositions indicate that, except for the addition of water, the serpentinites have not experienced extensive element mobility; these were originally ϩ CaO- and Al2O3-depleted harzburgites similar to peridotites from modern oceanic forearcs. High Cr# (Cr/(Cr Al) ) in the relict spinels (average p 0.69 ) indicates that these are residual after extensive partial melting, similar to spinels in modern forearc peridotites. These characteristics of Wadi Semna serpentinites also typify 22 other Eastern Desert serpentinite localities. We infer that Eastern Desert ophiolitic serpentinites, except perhaps Gebel Gerf, originated by forearc seafloor spreading during subduction initiation associated with the closing of the Neoproterozoic Mozam- bique Ocean. Online enhancements: tables. Introduction The Arabian-Nubian Shield (ANS) may be the larg- stabilized as a craton before the development of an est tract of juvenile continental crust of Neopro- extensive peneplain in mid-Cambrian times (∼520 terozoic age on Earth (Patchett and Chase 2002). Ma) and was exhumed in the Neogene as a con- According to Stern (1994), the ANS formed through sequence of Red Sea rifting and flank uplift. four major tectonomagmatic episodes between The abundance of ophiolites and ophiolitic me´- about 900 and 550 Ma. The breakup of Rodinia in langes is a distinctive part of the ANS, sufficiently early to mid-Neoproterozoic time was followed by so that Stern (2004) referred to this region as an seafloor spreading to open the Mozambique Ocean, “ophiolite graveyard.” The abundance of ophiolites attended by formation and accretion of fringing arc is strong evidence that this crust was generated by and back-arc basins (∼870–690 Ma). Fragments of plate-tectonic processes (Price 1984; Pallister et al. 1988; Berhe 1990; Stern et al. 2004). Nevertheless, East and West Gondwanaland collided at ∼610 Ma, the significance of ANS serpentinites is controver- culminating in E-W crustal shortening and escape sial because some researchers contest that these are tectonics, including development of the NW-SE- ophiolitic and those who agree that they are ophi- trending left-lateral Najd shear system. The ANS olitic do not agree on their tectonic significance. Interpretations are further complicated because Manuscript received May 23, 2006; accepted February 19, 2007. ANS ophiolitic complexes are variably dismem- 1 Geosciences Department, University of Texas at Dallas, bered, deformed, and altered. These complications Box 830688, Richardson, Texas 75083-0688, U.S.A. are particularly severe for serpentinized ultramaf- [The Journal of Geology, 2007, volume 115, p. 457–472] ᭧ 2007 by The University of Chicago. All rights reserved. 0022-1376/2007/11504-0005$15.00 457 458 M. K. AZER AND R. J. STERN ics, which are the most important and distinctive Egyptian ophiolitic serpentinites and is updated lithology of dismembered ANS ophiolites and me´- here. langes. Serpentinites are usually aligned along NW- Akaad (1996, 1997) and Akaad and Abu El Ela trending Najd shear zones (in the northern ANS, (2002) classified Eastern Desert serpentinites into where this study focuses) or along N-S upright folds (1) allochthonous serpentinites, including boudins and shear zones (in the southern ANS). In the cen- in Hafafit-type gneisses and me´langes, and (2) flow- tral ANS, ophiolites are less disrupted and define intruded serpentinites. The former appear as al- clear suture zones (Abdelsalam and Stern 1997). lochthonous bodies within arc assemblages, Ophiolitic ultramafics of Neoproterozoic age are whereas the latter intrude the surrounding schists. common in the central and southern sectors of the Ahmed (2005) rejected an ophiolite interpretation Eastern Desert of Egypt (fig. 1A), where they occur and, because he inferred high-grade metamorphism as highly altered serpentinites and talc-carbonate near contacts, considered Eastern Desert serpentin- (listwaenite; Johnson et al. 2004) rocks enclosing ites to have originated as intrusions of ultramafic rare relicts of fresher peridotite. These ophiolites magma. This interpretation is not generally ac- were first described by Rittmann (1958), but it was cepted, even though it must be recognized that not until the Wadi Ghadir ophiolite was described Alaska-type layered igneous intrusions, including that the significance of Egyptian ophiolites was ap- ultramafic rocks, exist in the Eastern Desert (e.g., preciated (El Sharkawy and El Bayoumi 1979). Since Dixon 1981; Farahat and Helmy 2006). In general, then, the serpentinites of Egypt have generally been the intrusive appearance of Eastern Desert serpen- interpreted as parts of tectonically emplaced oce- tinites can be attributed to the different structural anic lithosphere (Shackleton et al. 1980; Ries et al. positions of the serpentinites in the surrounding 1983; El Gaby et al. 1984, 1988; Abu El Ela 1996; rocks, coupled with the fact that the serpentinites and many others). are much weaker and easier to deform than are the Ophiolites of the northern ANS are generally in- rocks that enclose them. Our results do not support terpreted to have been generated in suprasubduc- a magmatic, intrusive origin for Egyptian serpen- tion zone tectonic settings (Bakor et al. 1976; Nas- tinites but indicate that they are parts of variably seef et al. 1984; Pallister et al. 1988; Stern et al. dismembered ophiolitic complex. 2004); similar interpretations hold for Egyptian This article reevaluates the significance of East- ophiolites (Abu El Ela 1990; Khudeir and Asran ern Desert ophiolitic serpentinites. Most studies of 1992; El-Sayed et al. 1999; Ahmed et al. 2001; Abdel these have concentrated on their geology, petrog- Aal et al. 2003; El Bahariya and Arai 2003; Farahat raphy, and bulk chemistry. However, information et al. 2004; Azer and Khalil 2005). Seafloor spread- about their protoliths is scarce because of pervasive ing necessary to form suprasubduction zone ophi- alteration; nevertheless, much can be learned with olites occurs in forearcs during the infant arc stage careful work. In this contribution, we show how of subduction initiation or in back-arc basins this can be accomplished. We first present new field (Pearce 2003; Stern 2004). Most researchers infer a and geochemical data for the serpentinites of Wadi back-arc basin setting for Egyptian ophiolites (Khu- Semna. We then integrate these results with pub- deir and Asran 1992; El-Sayed et al. 1999; Ahmed lished whole-rock, spinel, pyroxene, and olivine et al. 2001; Abdel Aal et al. 2003; El Bahariya and compositional data for Eastern Desert serpentin- Arai 2003; Farahat et al. 2004; El Gaby 2005); a ites. This synthesis yields new constraints on the forearc setting is rarely considered, partly because tectonic setting of Egyptian Neoproterozoic ser- the hypothesis of forearc spreading during subduc- pentinites and ophiolites. tion initiation is relatively new (Shervais et al. 2004; Stern 2004). Assessments of tectonic setting Wadi Semna Serpentinites for ANS ophiolites in general and Eastern Desert ophiolites specifically focus mostly on the trace- Field Observations. The Wadi Semna study area element composition of lavas and rarely consider lies in the extreme northern part of the central East- the abundant serpentinites. Stern et al. (2004) rec- ern Desert (fig. 1A) and is occupied by multiply ognized that ANS ophiolitic ultramafics are mostly deformed serpentinites, metasediments, and meta- harzburgitic, containing magnesian olivines and volcanics along with ∼600-Ma Hammamat sedi- spinels (Cr#; molarCr/(Cr ϩ Al) mostly 10.60), ments and younger gabbro (fig. 1B). Metasediments comparable to spinels from modern forearcs and are dominated by pebbly metamudstones that con- distinctly higher than spinels from mid-ocean ridge stitute the matrix of the ophiolitic me´lange. Pri- (MOR) and back-arc basin peridotites. However, the mary structures such as sedimentary layering are review of Stern et al. (2004) was incomplete for preserved. Layering is characterized by alternation Figure 1. A, Distribution of ophiolitic rocks in Eastern Desert of Egypt (modified after Shackleton 1994). Localities of the serpentinite bodies (in which the chemical analyses of chrome spinels are available) are shown on the map. B, Geological map of Wadi Semna area (modified
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