Pan African volcanism: Geochemistry and petrogenesis of Um Khariga metavolcanics, central Eastern Desert, Egypt

The Um Khariga metavolcanics (UKMV) represent one of the biggest metavolcan ic suites in the central Eastern Desert of Egypt. Based on field, petrograp hic and geochemical considerations the UKMV range in composition from basal ts to rhyodacites. Geochemically, the volcanics have a strong calc-alkaline to weak tholeiitic affinity, exhibit enrichment in LILE, depletion in HFSE and have negative Nb anomalies, together with high Zr/Y ratios. These feat ures are characteristic for basalts from active continental margins. The lo w Mg# (45), Ni (57 ppm) and Cr (113 ppm) contents of the Um Khariga basalts , together with the curvilinear trends displayed by some element variations indicate that the rocks evolved by fractional crystalization processes. Th e evolution of the UKMV andesites and rhyodacites can be modelled by separa tion of 29% and 34% from basaltic-andesites and andesites parents, respecti vely. The subduction zone is the dominant source of Ba, Rb, Sr, K and P for the genesis of the Um Khariga basalts, while Nb, Zr, Ce, Ti and Y are inhe rited from small scale partial melting of the subducted slab. The UKMV most probably originated in a convergent plate margin setting, possibly a primi tive continental are, similar to many contemporaneous calc-alkaline converg ent plate margin basalts.