Mokhles K. Azer

Neoproterozoic (835-720 Ma) Serpentinites in the Eastern Desert, Egypt: Fragments of Forearc Mantle

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 serpentinites 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 CaO‐ and Al2O3‐depleted harzburgites similar to peridotites from modern oceanic forearcs. High Cr# ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Serpentinized Peridotites at the North Part of the Wadi Allaqi District (Egypt): Implications for the Tectono‐Magmatic Evolution of Fore‐arc Crust

\mathrm{Cr}\,/ ( \mathrm{Cr}\,+\mathrm{Al}\,)

An example of post-collisional appinitic magmatism with an arc-like signature: the Wadi Nasb mafic intrusion, north Arabian–Nubian Shield, south Sinai, Egypt

\end{document} ) in the relict spinels ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Geochemistry and petrogenesis of post-collisional alkaline and peralkaline granites of the Arabian-Nubian Shield: a case study from the southern tip of Sinai Peninsula, Egypt

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Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the Allaqi–Heiani Suture, South Eastern Desert of Egypt

\end{document} ) 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 Mozambique Ocean.

A-type volcanics in Central Eastern Sinai, Egypt

ABSTRACT Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and 87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.