الفهرس 
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Abstract
Dismembered ophiolites and ophiolitic mélanges are common in the Central Eastern Desert (CED) of Egypt. One of the most important exposures of the ophiolitic ultramafic rocks is the Idfu-Mersa Alam district along which four representative main localities were selected for detailed study in this work. The current study represents the detailed geological, petrographical, chemical, mineral chemical, oxygen fugacity, XRD, and isotopic studies for a group of ultramafic rocks and their carbonate alteration products from the study localities and some other localities that were included in the study for comparison. The study results revealed that these rocks are composed of serpentinized peridotites, serpentinites and pyroxenites that were derived dominantly from harzburgites protoliths that represent remnants of depleted to highly depleted mantle and evolved in a forearc suprasubduction zone (SSZ) setting. The calculated temperatures using olivine-spinel and Al content in orthopyroxene geothermometery (668-778 ºC and 984 ºC, respectively) are in a broad consistency with those reported for modern forearc mantle peridotites and revealed that these rocks were cooled at slow rates. The preliminary calculated oxygen fugacity (ΔlogƒO2 (FMQ)) values (−3.01 to +0.32) using the raw microprobe olivine-spinel data referred to highly heterogenous SSZ mantle nature in the terms of mantle oxidation state and inferred that the SSZ should not be pervasively oxidised compared with other tectonic settings. Occasional podiform chromitite deposits recorded in the ultramafic rocks of some localities and were inferred to be formed through different subduction stages either by the mantle interaction with forearc basaltic melt (FAB) in the early subduction initiation stage in some localities or with boninitic melt in the late mature arc stage in other localities.
The detailed Mӧssbauer spectroscopy-corrected oxygen fugacity study for samples (harzburgite) from the study localities and some additional Neoproterozoic samples (harzburgite, dunite and chromitite) from Egypt and Saudi Arabia and Cretaceous ones (harzburgite) from Iran revealed strong heterogeneity in the Neoproterozoic upper mantle oxidation state ranging from moderately oxidized (FMQ +0.54) to ultra-reduced (FMQ -4.73) for harzburgites, from highly oxidized (FMQ +1.49) to moderately reduced (FMQ -0.60) for dunites as well as one highly reduced (FMQ -1.61) value for chromitite. Meanwhile, such strong heterogeneity disappears within the Cretaceous mantle fO2 values ranging from slightly oxidized (FMQ +0.45) to moderately reduced (FMQ -0.85). The results of the investigation combined with previously published data reflect the consistency in the mantle oxidation state from the Proterozoic till present day.
Detailed petrological, mineralogical and isotopic studies for serpentinites and their alteration products (listvenites and talc-carbonates) from several localities along Idfu-Mersa Alam road and some other localities from both of the Central and South Eastern Desert revealed several variably carbonated lithologies including Lz- and Atg- serpentinites, talc-carbonates and listvenites and its related carbonate veins. Mode of occurrence, carbonate clumped isotope temperatures (range from 426 °C for the crystalline dolomite vein, 224 to 266 °C for the Atg-serpentinite and talc-carbonates, 90 to 260 °C in listvenites, 25.7 °C in cryptocrystalline magnesite to 37.3 °C in Au-mineralized quartz-carbonate veins) and C-O-Sr isotope compositions of ophiolitic carbonates indicate that carbonation of the ophiolitic ultramafic rocks in the ED most likely occurred through two carbonation stages with three main sources were proposed for carbon in the first stage: 1) deep (infrastructure) mantle-derived carbon, 2) remobilized sediment-derived carbon and 3) shallow levels (suprastructure) organic carbon. During the second carbonation stage, represented by low-T cryptocrystalline magnesite, carbon is proposed to be derived through meteoric fluids.