Petrogenetic and geotectonic significance of Neoproterozoic suprasubduction mantle as revealed by the Wizer ophiolite complex, Central Eastern Desert, Egypt

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 (TiO 2 = *\0.07-0.22 wt%; Al 2 O 3 = 9.4-10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO 2 = \0.09-0.28 wt%; Al 2 O 3 = 11.2-14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO 2 = *\0.38-0.51 wt%; Al 2 O 3 = 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 TiO 2 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.

The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt: petrological characteristics and tectonomagmatic evolution

“…5a) is characteristic Fig. 8 Chromian spinel of the Abu Dahr serpentinized peridotites: a Mg# vs. Cr# diagram, tectonic setting compositional fields for likely ophiolitic analogs of Cenozoic age are from Stern et al (2004), the calculated percentages of partial melting arrow of the host peridotite are after Hirose and Kawamoto (1995); b Fields of spinel compositions from Oman boninite (Python et al 2008), South Eastern Desert ophiolites (SED), Egypt (Ahmed et al 2001;Ahmed 2013), Central Eastern Desert ophiolites (CED), Egypt (Ahmed et al 2001;Farahat et al 2011), and Arabian Shield ophiolites (AS), Saudi Arabia (Ahemd and Hariri 2008), are all given for comparison; and c CrAl-Fe 3+ trivalent cation ratios, fields of mid-ocean ridge and fore-arc peridotites are from Arai et al (2006), note the welldeveloped chemical gab between chromian spinel core and rim of highly refractory harzburgite and dunite, and reflects a high degree of partial melting and a high melt/rock ratio (e.g. Bodinier and Godard 2003).…”

“…8a), the Eastern Desert of Egypt (Ahmed et al 2001;Farahat et al 2011;Ahmed 2013) and the Arabian Shield (Ahmed and Hariri 2008), and Oman boninite (Python et al 2008) (Fig. 8b).…”

Petrological studies of neoproterozoic serpentinized ultramafics of the nubian shield: spinel compositions as evidence of the tectonic evolution of egyptian ophiolites

“…The melting trend of experimental equilibrium (melting %) is from Hirose and Kawamoto (1995). Field for chromites in the Egyptian serpentinites of the Eastern Desert is adopted from Farahat et al (2011) Text- fig. 7.…”

“…The ultramafic rocks of the ANS ophiolites are essentially represented by harzburgites and dunites (e.g. Farahat et al 2011;Azer et al 2013). On the other hand, the ultramafic rocks of layered mafic-ultramafic intrusions show a wide variation in rock types, including wehrlite, dunite, lherzolite and pyroxenites with minor harzburgite.…”

Petrogenetic Significance of Spinels From Serpentinised Peridotites From the Verianaousa Ophiolite

“…Unlike the Al-spinels, which are depleted in TiO2 and ZnO, Cr-spinels (including magnesiochromites) have rather high amounts of these oxides, which reflects the involvement of impregnating melts during the evolution of both peridotites (Farahat et al, 2011). The development of subhedral to euhedral Cr-spinel grains in the serpentinised harzburgite, as well as lack of significant correlation between Cr# and Mg# in the magnesiochromite and Cr-spinel are also consistent with a melt-peridotite interaction episode and their crystallisation from magmatic processes (Zhou et al, 1994(Zhou et al, , 2005.…”