Mineralogy and geochemistry of serpentinized peridotites of the Northern Nubian Shield: The origin of compositionally banded olivine and implications for Neoproterozoic supra-subduction zone metasomatism

A sequence of Neoproterozoic ultramafic rocks - serpentinized peridotite, serpentinite, and clinopyroxenite - outcrop in Wadi El-Mireiwa in the south Eastern Desert of Egypt. They represent the mantle and lowermost crustal section of a fragmented ophiolite, emplaced by thrusting above metasedimentary rocks and an island arc assemblage and later intruded by a granite mass. In samples of the serpentinized peridotite, back-scattered electron images of olivine reveal banded zoning, defined by straight and parallel Mg-enriched (Fo - 96) lamellae within typically magnesian host crystals (Fo - 89). The presence of clinopyroxene, the relatively low whole-rock Mg# (-88), the low NiO content of olivine (0.1-0.32 wt%), and the relatively high TiO2 content of fresh Cr-spinel (-0.3 wt%) are all consistent with a cumulate origin for the serpentinized peridotite and serpentinites. On the other hand, some associated serpentinite has higher whole-rock Mg# (92) and low TiO2 in Crspinel (-0.01 wt%), consistent with a residual origin by high-degree melt extraction in the fore-arc region of a supra-subduction zone (SSZ) environment. The formation of Mg-enriched bands in olivine is attributed to the enhancement of metasomatic Mg-Fe exchange with high-Mg/Fe2+ fluids along crystallographically-oriented subgrain boundaries produced by high strain-rate dislocation creep. Such high-Mg/Fe2+ fluid metasomatism also explains reverse zoning of MgO in Cr-spinel, whereas the low whole-rock concentrations of REE and other trace elements exclude a significant role for melt-rock interaction. The evidence for high-Mg/Fe2+ fluid metasomatism in this suite indicates that SSZ fluids and high fore-arc strain rates affect not only the mantle wedge but also overlying ultramafic cumulates. The oxygen fugacity [Delta log integral O2(FMQ)] values calculated from unmetasomatized (+2.48 to +2.67) and metasomatized (+3.75 to +4.53) samples reflect the oxidized nature of Neoproterozoic SSZ magma and the even more oxidizing character of the metasomatic fluids. Banded zoning has also been recorded in olivine from Mariana fore-arc peridotite and Phanerozoic SSZ ophiolites; the Neoproterozoic Wadi El-Mireiwa case shows that rapid mantle flow has been a persistent feature of the tectonic environment where fore-arc ophiolites develop.

Automated summary

This study in the Neoproterozoic region of eastern Egypt reveals the existence of ultramafic rock sequences, and through measuring the minerals and elements within them, insights can be gained into the formation process of the minerals and the characteristics of magmatic activity.