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The Central Eastern Desert (CED) is characterized by the widespread distribution of Neoproterozoic intra-oceanic island arc ophiolitic assemblages. The ophiolitic units have both back-arc and forearc geochemical signatures. The forearc ophiolitic units lie to the west of the back-arc related ones, indicating formation of an intra-oceanic island arc system above an east-dipping subducted slab (present coordinates). Following final accretion of the Neoproterozoic island arc into the western Saharan Metacraton, cordilleran margin magmatism started above a new W-dipping subduction zone due to a plate polarity reversal. We identify two belts in the CED representing ancient arc–forearc and arc–back-arc assemblages. The western arc–forearc belt is delineated by major serpentinite bodies running ∼NNW–SSE, marking a suture zone. Ophiolitic units in the back-arc belt to the east show an increase in the subduction geochemical signature from north to south, culminating in the occurrence of bimodal volcanic rocks farther south. This progression in subduction magmatism resulted from diachronous opening of a back-arc basin from north to south, with a bimodal volcanic arc evolving farther to the south. The intra-oceanic island arc units in the CED include coeval Algoma-type banded iron formations (BIFs) and volcanogenic massive sulphide (VMS) deposits. Formation of the BIFs was related to opening of an ocean basin to the north, whereas development of the VMS was related to rifting of the island arc in the south. Gold occurs as vein-type mineral deposits, concentrated along the NNW–SSE arc–forearc belt. The formation of these vein-type gold ore bodies was controlled by the circulation of hydrothermal fluids through serpentinites that resulted in Au mobilization, as constrained by the close spatial association of auriferous quartz veins with serpentinites along the western arc–forearc belt. [ABSTRACT FROM AUTHOR]

Abstract: We present new petrographic, and major and trace element data for the Um Hassa Greywacke Member, the uppermost unit of the Hammamat Group, at Wadi Hammamat area, the Eastern Desert of Egypt. The Neoproterozoic Hammamat sedimentary rocks are immature molasse-type rocks that were deposited at the end of the Pan-African Orogeny. The Um Hassa Greywacke Member is bedded, weakly deformed and lies conformably on the polymictic conglomerate of the Um Had Member. The Um Hassa greywackes are poorly sorted and composed mainly of quartz, lithic fragments, and feldspar grains, with an average modal composition of QFL=53:17:30. Quartz is mostly monocrystalline, feldspars are mainly plagioclase, and lithic fragments are predominantly intermediate to felsic volcanic rocks. The Um Hassa greywackes are characterized by low chemical index of alteration (46–65) and SiO2/Al2O3 (4.6–6.1) that indicate limited chemical weathering of the source rocks and deposition close to the source area. Both point-counting modal analyses and geochemical data suggest that the Um Hassa greywackes were derived from mixed sources. In terms of provenance, continental arc volcanic rocks and oceanic island arc-ophiolitic sources are the main contributors to the sediments of the Um Hassa greywackes. These rock types are exposed in the Eastern Desert as Dokhan-type continental arc volcanic rocks and accreted oceanic arc-ophiolitic mélange, respectively. Considering the geodynamic evolution of the Central Eastern Desert, the Um Hassa greywackes in the Wadi Hammamat area appear to have been deposited in a retroarc foreland basin behind the Dokhan continental arc developed over a west-dipping subduction zone. [ABSTRACT FROM AUTHOR]

Abstract: The geodynamic origin of the Neoproterozoic ophiolites of the Arabian-Nubian Shield exposed in the Eastern Desert of Egypt remains controversial. In this study, we present new geochemical and field data from the Fawakhir ophiolite and from some mélange blocks along the Qift-Qusier Road in order to constraint the tectonic evolution of this part of the Central Eastern Desert. The Fawakhir ophiolite contains most lithological units of a Penrose-type ophiolite sequence, and includes ultramafic rocks in the west overlain by isotropic gabbro, sheeted dikes and pillow basalt in the east. These ophiolitic units are enriched in LREE (light rare earth elements) and LILE (large ion lithophile elements) but depleted in high field strength elements (La/Smcn =0.40–1.22, Th/Nbpm =1.7–10.9, La/Nbpm =1.4–6.6). Their magmas appear to have been derived from a depleted (N-MORB-like) mantle source, and their geochemical characteristics are comparable to those of the Izu-Bonin-Mariana forearc oceanic crust formed during the initiation of an intra-oceanic subduction zone. Pillow lava blocks in the eastern mélange have geochemical signatures similar to those of oceanic crust generated in back-arc basins. The decrease in the magnitude of mantle depletion and the change of the geochemical signature along the Qift-Qusier Road from a forearc in the west to a back-arc in the east suggest the formation of the Fawakhir intra-oceanic arc system over an east-dipping subduction zone. With continued subduction and arc migration, this intra-oceanic arc system finally collided with the passive margin of the West Gondwana (the Saharan craton), resulting in the accretion of the Fawakhir arc–forearc units. Following its tectonic accretion onto the West Gondwana continental margin, the Fawakhir ophiolite was intruded by calc-alkaline dikes, whose magmas were derived partly from partial melting of the sub-continental lithospheric mantle. These dikes have geochemical characteristics similar to those of modern active continental margin (Andean-type) rocks (La/Smcn =2.13–2.48, Gd/Ybcn =2.04–4.25, Th/Nbpm =3.2–5.8, La/Nbpm =2.5–4.9), suggesting that the West Gondwana passive margin (Atlantic-type) was converted to an Andean-type margin subsequent to the arc-continent collision. The inferred conversion of the Atlantic-type margin to an Andean-type margin resulted from the collision-induced reversal of the subduction direction. [Copyright &y& Elsevier]

Abstract: We report new geochemical data from the Neoproterozoic ophiolite in the Wadi Ghadir area, Eastern Desert, Egypt. The Wadi Ghadir ophiolite (WGO) is composed of layered and isotropic gabbros and amygdaloidal to porphyritic pillow lavas. Both the gabbroic rocks and the pillow lavas are intruded by dike swarms with different chemical affinities and spatial orientations. The WGO occurs in an ophiolitic mélange (Wadi Ghadir mélange, WGM), and both the WGO and WGM are intruded by granitic rocks to the west. On the basis of Zr and Y variations, units of the WGO are classified as tholeiites (e.g., gabbros, amygdaloidal pillow lavas, D1, and D3 dikes). The late-stage D4 dikes show a calc-alkaline affinity, whereas porphyritic pillow lavas and D2 dikes have a transitional character. All ophiolitic units display subduction zone trace element signatures characterized by the enrichment of LILE over HFSE and negative Nb–Ta anomalies. Tholeiitic rocks are further divided into LREE-depleted (gabbros, D3 dikes) and LREE-enriched (amygdaloidal pillow lavas, D1 dikes) groups. Light REE-depleted tholeiitic rocks were derived from melting of a slightly depleted mantle source, whereas the LREE-enriched tholeiitic rocks were derived from a fertile N-MORB source. Calc-alkaline D4 dikes are characterized by steep REE patterns and have low Zr/Nb ratios (23–29), indicating melt contribution from an enriched mantle source, such as sub-continental lithospheric mantle and/or garnet peridotite. The transitional group has LREE-enriched patterns, but its degree of REE enrichment and Zr/Nb ratios are intermediate between the tholeiitic and calc-alkaline groups. The WGO is interpreted to have formed in a back-arc setting behind the Nugrus volcanic arc developed above a NE-dipping subduction zone. The collision of this arc–back-arc system with the passive margin of the Nubian shield (Hafafit dome) resulted in the accretion of the Nugrus arc and the WGO onto the Nubian continental margin and in the initiation of a new subduction zone dipping SW beneath the newly accreted arc–back-arc crust and Hafafit dome. The establishment of this new Andean-type continental margin produced the calc-alkaline D4 dikes, leucogabbros and granitoid plutons intrude the Hafafit dome, the WGO, and the ophiolitic mélange. [Copyright &y& Elsevier]