Petrogenesis of some alkaline and sub alkaline basalts from Egypt

Hussein Aziz Hegazy

Geology Department, Assiut University,Assiut, Egypt

Email: hegazy512000@yahoo.com

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.