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 $\mathrm{Cr}\,/ ( \mathrm{Cr}\,+\mathrm{Al}\,) $ \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 $\mathrm{average}\,=0.69$ \end{document} ) indicates that these are residual after extensive p