Petrogenesis of some alkaline and sub alkaline basalts from Egypt

Hussein Aziz Hegazy

Geology Department, Assiut University,Assiut, Egypt


Phone.002088 2321015


In an effort to understand the petrogenesis of mid-certiary basalts in Egypt, basalts samples from Sharm El-Qibli and Sharm El-Bahari (Red Sea Coast), Mir (NW of Assiut) and Abu Roash (W. of Cairo) were studied mineralogically and geochemically. Chemical analysis shows that they include oversaturated tholeiites and undersaturated, nepheline √ normative alkali basalt. Discrimination diagrams indicate that the basalts, erupted in anorogenic, within-plate environment. The oceanic affinities apparent on the initial rifting stage of the Red Sea.

The content of P2O5 and K2O indicate that the tholeiitic basalt from Abu Roash and Sharm El-Bahari were generated by 21-23% partial melting of pyrolite mantle at pressures less than 15 Kb, whereas the alkali basalt magma of Sharm El-Qibli was formed at greater depth at 15-25 Kb. The crystallization temperature, deduced from the MgO content was 1060ºC for the basalt from Mir, and between 1100ºC to 1200ºC for the basalts from the other localities.

Fe/Mg index, Mg/Mg+Fe and mafic/Felstic ratio indicate an increasing degree of fractionated prior to extrusion in the following order: Sharm El-Bahari, Abu Roash and Mir. The close occurrence of alkali basalt at Sharm El-Qibli and tholeiitic basalts at Sharm El-Bahari, might suggest the presence of deep fractures of varying extension tapping magmas originating at different depths.

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