PETROLOGY AND GEOCHEMISTRY OF UM ESH OPHIOLITE SERPENTINITES, EASTERN DESERT, EGYPT: A NEOPROTEROZOIC METAMORPHOSED SSZ MANTLE

Abstract

The Um Esh Neoproterozoic ophiolite serpentinites, Eastern Desert of Egypt, comprise massive and foliated varieties which are composed mainly of antigorite with accessory Cr-spinel. Carbonates are variable in abundance and represented by magnesite aggregates and veinlets. The Cr-spinels are usually zoned with fresh cores or relics surrounded by ferritchromite and chromian magnetite rims. The very low TiO2 contents (<0.025 wt%) of primary Cr-spinel and the low Al2O3/SiO2 values (0.010-0.016) of serpentinite samples are consistent with residual mantle which experienced high degree of partial melting. The Al2O3 (16-25 wt%) and TiO2 (average = 0.007 wt%) contents of the primary Cr-spinel overlap those of spinels of suprasubduction zone (SSZ) and mid-oceanic ridge (MOR) peridotites. However, the Cr# (0.55-0.66) and Mg# (0.39-0.59) of the fresh Cr-spinels together with depleted nature of the Um Esh serpentinites are similar to those of modern SSZ fore-arc peridotites. Profiles along zoned Cr-spinels revealed that Cr2O3, Al2O3 and MgO abundances decreased abruptly, whereas FeO contents and Fe# (Fe/Cr+Al+Fe) increased rapidly from fresh Cr-spinel through ferritchromite to chromian magnetite, with gradual change within the individual metamorphic spinel zones. On the other hand, the variation of MnO and NiO abundances is not regular from core to rim. The dominance of antigorite and the presence of metamorphic spinel rims suggest that Um Esh serpentinites have experienced prograde metamorphism up to lower amphibolite facies under oxidizing conditions.