2013

PETR0L0GICAL AND GEOCHEMICAL CONSTRAINTS ON THE EVOLUTION

OF EL KAHFA ALKALINE RING COMPLEX, SOUTH EASTERN DESERT,

EGYPT.

A.M.BISHADY*, A.M.ELSHERIF**, M.E.DARWISH**

*Geology Dep.,Fac.Sci.,Menoufiya Univ. **Nuclear Materials Authority

El Kahfa complex locates at the intersection of Lats 24˚8̍ 18̋ N and 14˚38̍55̋ E South Eastern Desert, Egypt. It comprises an outer ring composed of alkaline syenites, ranging from barren-to quartz-bearing syenites, an inner ring vary in composition from leucocratic to melanocratic syenites, and a central stock formed essentially of alkali gabbros. These alkali gabbros are cut by small bodies of alkaline syenites, which occasionally grade into nepheline syenites. The country rocks are represented mainly by metavolcanics and epidiorites. El Kahfa syenites lack any primary hydrous mafic silicates indicating that their magma was anhydrous. Mjor and trace elements geochemistry of El Kahfa rocks reflects the crystallization of early plagioclase followed by Fe-Ti oxides salic members in the complex are co-magmatic and related to crystal liquid fractionation. The REEs tetrad is obvious on the primitive mantle normalized pattern of El Kahfa rocks. The W-type appears in the less evolved varieties, where the M-type in the more evolved rocks. These tetrad effects are either a feature of magma-fluid system before crystallization, or inherited from external fluid during or after the emplacement of the magma.

Generally, El Kahfa rocks parental magma was probably derived from Nb-enriched or at least under-plate mantle source within the continental East-Africa rift zone, where it may be subjected to minor contamination through their fractional crystallization.