GEOCHEMISTRY AND U-PB ZIRCON SHRIMP DATING OF NEOPROTEROZOIC VOLCANIC ROCKS FROM THE CENTRAL EASTERN DESERT, EGYPT, ARABIAN – NUBIAN SHIELD

The Arabian – Nubian Shield exposes Neoproterozoic (1000 – 544 Ma) crust on the flanks of the Red Sea but the age and tectonic setting of metavolcanic sequences in Egypt are poorly understood. To understand these events we are investigating the geology of Wadi Kareim and Wadi El Dabbah in the Central Eastern Desert of Egypt. The rocks are dominantly volcanics, ranging from tholeiitic basalt to andesite, and are metamorphosed to greenschist facies. Typically, plagioclase has altered calcic cores and less altered rims and pyroxene is altered to actinolite and chlorite. The magmas are moderately fractionated, as indicated from the averages of Mg# (59), Cr (253 ppm) and Ni (95 ppm). Their REE patterns are mostly slightly fractionated [(La/Yb)N = 1.3 – 2.3], although two samples have (La/Yb)N = 0.75 and 5.5. Multi-element diagrams show Ba, Sr, K and Rb enrichments (some samples are K and Rb depleted perhaps due to metamorphic processes) and Nb, Ta, Zr, Th and Ti depletions. Samples have La/Nb generally > 1.9 (1.9 to 4.0) and low ratios of Ce/Yb, Ta/Yb and Th/Yb. Most samples when plotted on tectonic discrimination diagrams using immobile elements have volcanic arc chemistry consistent with the overall major and trace elements characteristics shown above although they have been somewhat modified by metamorphism. U-Pb SHRIMP zircon ages indicate these sequences are 753 ± 14 Ma (Wadi Kareim) and 749 ± 23 Ma (Wadi El Dabbah) but contain abundant inherited zircons (~ 1.3 Ga, 2.1 Ga, 2.5 Ga and 2.7 Ga). These zircons might be derived from underlying older continental crust or picked up from sediments. These results indicate that an intensive arc volcanism occurred at ~750 Ma was important for forming and modifying the crust in this region.