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Yildirim Dilek, Miami University (United States)
Ali Polat, University of Windsor (Canada)
Yasser Abd El-Rahman, University of Windsor (Canada)
Mohamed A. El Sharkawi, Cairo University (Egypt)
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The Neoproterozoic (ca. 750 Ma) Wadi Ghadir Ophiolite in the Eastern Desert of Egypt is composed mainly of pillow basalts, and layered and isotropic gabbros. These ophiolitic subunits are cut by four groups of mafic dykes, although a genuine sheeted dyke complex has not yet been recognized in the ophiolite. Basalts, gabbros, and three groups of dykes are characterized by a tholeiitic affinity, whereas one group of dykes displays a calc-alkaline affinity. All these rocks are intruded by calc-alkaline granites. On the Ta/Yb versus Th/Yb diagram all tholeiitic rock types plot in the field of modern island arcs. In contrast, the calc-alkaline dykes plot in the field of active continental margin arcs on the Ta/Yb versus Th/Yb diagram. Pillow basalts are characterized by LREE-enriched (La/Smcn=1.15-1.90; La/Ybcn=2.20-5.40) and Nb-depleted (Nb/Nb*=0.24-0.36), relative to Th and La, trace element patterns. Gabbros display depleted to moderately enriched REE patters (La/Smcn=0.49-1.03; La/Ybcn=0.63-1.70) and negative Nb (Nb/Nb*=0.28-0.34) anomalies. One group of tholeiitic dykes displays variably depleted LREE patterns (La/Smcn=0.48-0.77; La/Ybcn=0.72-1.73) and negative Nb anomalies (Nb/Nb*=0.28-0.49). The other groups of dykes have variably enriched REE patterns (La/Smcn=1.18-2.83; La/Ybcn=1.90-13.5) and large negative Nb anomalies (Nb/Nb*=0.21-0.54). All these geochemical characteristics are consistent with a subduction zone geochemical signature.
We interpret the geochemical characteristics of tholeiitic pillow basalts, gabbros, and first three generations of dykes as indicating a backarc tectonic setting, and suggest that the Wadi Ghadir ophiolite formed in a back-arc environment to the east of the Nugrus arc, which developed above an east-dipping subduction zone. The geochemical characteristics of the fourth generation (youngest) of calc-alkaline dykes are attributed to magmatism associated with an Andean-type continental setting. Following the collision between the Nugrus arc and the Hafafit passive margin to the west-southwest, the Wadi Ghadir ophiolite was emplaced onto the Nugrus arc, and the Atlantic-type Hafafit margin was converted to an Andean-type margin via the change of subduction polarity.
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