Petrology and Geochemistry of Some Ophiolitic Metaperidotites from the Eastern Desert of Egypt: Insights into Geodynamic Evolution and Metasomatic Processes

Abstract

Ophiolitic peridotites exposed in the Eastern Desert (ED) of Egypt record multiple stages of evolution, including different degrees of partial melting and melt extraction, serpentinization, carbonatization and metamorphism. The present study deals with metaperidotites at two selected localities in the central and southern ED, namely Wadi El‐Nabá and Wadi Ghadir, respectively. They represent residual mantle sections of a Neoproterozoic dismembered ophiolite that tectonically emplaced over a volcano‐sedimentary succession that represents island–arc assemblages. The studied metaperidotites are serpentinized, with the development of talc‐carbonate and quartz‐carbonate rocks, especially along shear and fault planes. Fresh relics of primary minerals (olivine, orthopyroxene and Cr‐spinel) are preserved in a few samples of partially‐serpentinized peridotite. Most of the Cr‐spinel crystals have fresh cores followed by outer zones of ferritchromite and Cr‐magnetite, which indicates that melt extraction from the mantle protolith took place under oxidizing conditions. The protoliths of the studied metaperidotites were dominated by harzburgites, which is supported by the abundance of mesh and bastite textures in addition to some evidence from mineral and whole‐rock chemical compositions. The high Cr# (0.62–0.69; Av. 0.66) and low TiO2 (<0.3 wt%) contents of the fresh Cr‐spinels, the higher Fo (89–92; Av. 91) and NiO (0.24–0.54 wt%, Av. 0.40) contents of the primary olivine relics, together with the high Mg# (0.91–0.93; Av. 91) and low CaO, Al2O3 and TiO2 of the orthopyroxene relics, are all comparable with depleted to highly depleted forearc harzburgite from a suprasubduction zone setting. The investigated peridotites have suffered subsequent phases of metasomatism, from ocean‐floor hydrothermal alteration (serpentinization) to magmatic hydrothermal alteration. The enrichment of the studied samples in light rare earth elements (LREEs) relative to the heavy ones (HREEs) is attributed to most probably be due to the contamination of their mantle source with granitic source hydrothermal fluids after the obduction of the ophiolite assemblage onto the continental crust. The examined rocks represent mantle residue that experienced different degrees of partial melting (∼10% to 25% for W. El‐Nabá rocks and ∼5% to 23% for W. Ghadir rocks). Variable degrees of partial melting among the two investigated areas suggest mantle heterogeneity beneath the Arabian‐Nubian Shield (ANS).

An Early Paleozoic Ultramafic Complex in the North Wulan Metamorphic Complex, North Qaidam: Contraints on the Nature of the Alaskan‐type Continental Arc Root

“…4b) and the low Ni/Mn ratios (0.7-1.0) of olivine are different from those of replacive peridotites, which are characterized by the replacement of orthopyroxene by olivine neoblasts. The relationship between spinel Cr # and olivine Fo values can be used to deduce the petrogenetic environment, the degree of partial melting, and estimate the formation pressure (Dick and Bullen, 1984;Pearce and Parkinson, 2000;Abdel-Karim et al, 2021). On an olivine Fo contents versus spinel Cr # diagram (Fig.…”

Tectonic Evolution of the Fawakhir Ophiolite, Central Eastern Desert of Egypt: Implications for Island Arc Amalgamation and Subduction Polarity during the Neoproterozoic

“…Ophiolitic rocks are overlain unconformably from south and west by the post-collisional Dokhan volcanic rocks, which are not metamorphosed and have intermediate to silicic whole-rock composition (630-590 Ma) [23][24][25], and by the Hammamat sediments of clastic molasses-facies (632-590 Ma) [26,27]. A geologic map of the study area [28]. Fawakhir ophiolite sequence and post-collision related granitoid rocks are located in the west, and the ophiolitic mélange occurs in the east.…”

“…Figure 4.A geologic map of the study area[28]. Fawakhir ophiolite sequence and post-collision related granitoid rocks are located in the west, and the ophiolitic mélange occurs in the east.…”