抄録
Late Neoproterozoic (∼610-560 Ma) volcanic rocks from Wadi Umm Sidra and Wadi Umm Asmer, North Eastern Desert, Egypt belong to the Dokhan Volcanic unit in the northwestern Arabian-Nubian Shield. The studied volcanics are classified into two units: (1) the basic unit comprises basalt, basaltic andesite, and andesite, with very subordinate pyroclastic rocks, and (2) the acidic unit comprises dacite, rhyodacie and rhyolite. Chemical analyses display a continuous series that ranges from 48.8 to 76.9 SiO2 (wt%) with few samples in the 48-55% range and no indication of compositional gap. They show calc-alkaline and metaluminous to slightly peraluminous characters. They define a continuous evolutionary trend and show a decrease of elements compatible with pyroxene, amphibole, feldspar and Fe-Ti oxides (MgO, Fe2O3, CaO, Al2O3, TiO2, Cr, Ni, and Sr) and the increase of incompatible elements such as K2O, Na2O, Rb, Zr, Nb, Ba and Y with increasing SiO2 content. These relations together with the REE trends reveal that the acidic varieties are genetically linked through closed system crystal fractionation processes to the basic varieties. During the late stages of the evolution of these rocks, limited crustal contamination of the overlying island arc rocks might occur. The normalized-trace elements patterns show these rocks are enriched in LILEs relative to HFSEs and negative Nb-Ta anomaly resembling to volcanic rocks generated in subduction-related orogenic setting. Major and trace elements suggest that these rocks were generated by partial melting of subcontinental mantle wedge that inherited the geochemical signatures of long time subduction event before the collision of E and W parts of Gondwana.
