Petrogenetic and geotectonic significance of Neoproterozoic suprasubduction mantle as revealed by the Wizer ophiolite complex,Central Eastern Desert,Egypt |
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Authors: | E S Farahat G Hoinkes A Mogessie |
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Institution: | (1) Department of Geology, Minia University, El-Minia, 61519, Egypt;(2) Institute of Earth Science (Mineralogy and Petrology), Karl-Franzens University, Graz, Austria |
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Abstract: | Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the
Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins
these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented
by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the
incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above
a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# 100Cr/(Cr + Al)]
decrease gradually from dunite bodies (Cr# = 81–87) and their host highly depleted harzburgites (Cr# = 67–79) to the less
depleted harzburgites (Cr# = 57–63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents
in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for
the dunite bodies (TiO2 = ~<0.07–0.22 wt%; Al2O3 = 9.4–10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO2 = <0.09–0.28 wt%; Al2O3 = 11.2–14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO2 = ~<0.38–0.51 wt%; Al2O3 = 14.5–15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB–island
arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO2 and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial
melting/melt–rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts
is followed by a second stage melting by interaction with melts of IAT–boninitic affinities in a suprasubduction zone environment
to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within
the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together
with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from
IAT–boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc
system above a southeast-dipping subduction zone. |
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