干旱区生态环境知识资源中心

Origin and tectonic setting of the late Neoproterozoic Dokhan Volcanics (ca. 610-560 Ma by whole rock Rb-Sr and ca. 600-590 Ma by SHRIMP U-Pb zircon) in the Egyptian Eastern Desert is debated. Debate concerns the tectonic setting they formed in, during transition between convergent to extensional, during or after collision of E and W Gondwana similar to 600 Ma. In order to solve this problem. we studied the geology and analyzed major and trace elements of lavas from Wadi Um Sidra and Urn Asmer in the northern Eastern Desert. These range from medium- to high-K basalt to rhyolite. They comprise voluminous medium- to high-K cale-alkaline lavas, subordinate adakitic lavas, and minor alkali basalt. Adakitic lavas contain very low Y (< 18 ppm) and heavy REE (HPEE) (Y-b <= 1 ppm) contents, with high Sr (> 767 ppm) and Sr/Y (> 40). They also have high Cr and Ni and low Nb, Rb, and Zr contents compared to coexisting calc-alkaline lavas. The adakitic lavas exhibit a fractionated rare earth element (REE) patterns with HREE depletion (La-N/Lu-N = 11.1-14.4), while calc-alkaline rocks have less fractionated REE patterns (La-N/Lu-N = 6.3-9.4). REE patterns of the alkali basalt are also similar to that of adakitic lavas. These alkali basalt, calc-alkaline, and adakitic lavas all show enrichment in large ion lithophile elements (LILEs) and show negative Nb-Ta anomalies with high Th/Zr, which suggest involvement of subducted slab melts in their petrogenesis. Our proposal is subduction of hot oceanic ridge before collision and later melting of the hot oceanic slab beneath Dokhan even after collision event. (c) 2006 Elsevier B.V. All rights reserved.