Abstract

Abdel-Aal M. Abdel-Karim

Five minor isolated intrusions of younger gabbros crop out in Southeastern-, Central- and Southeastern Sinai are presented. The gabbros range in composition from normal, pyroxene-hornblende, hornblende, and leucogabbro and pyroxenite (low Ti content) to normal, pyroxene-hornblende, hornblende, olivine and uralitized gabbro, anorthosite and hornblendite (high Ti content). They are derived from two magma types (tholeiitic and calc-alkaline) closely related in time and space, but with different geochemical trends. The tholeiitic younger gabbros (THYG) at W. Tweiba, W. Rahaba and El-Khamila areas are generally characterized by high FeOt , TiO2, MnO, Zr, Y, Nb, Th and total REE contents as compared with the calc-alkaline younger gabbros (CAYG) at Imliq and W. Nakhil areas. The parental magma of the THYG was controlled by multistage processes including varying degrees of crystal fractionation and differentiation with prominent crustal contamination. The parental magma of the CAYG was controlled by differentiation and crystallization of anhydrous minerals which accompanied by high oxygen fugacity and water activity. The present gabbros were suffered a minor crustal materials involvement. The gabbros of all localities have LREE enrichment; HREE depletion and a small positive Eu anomaly suggest their moderate differentiation. The REE patterns reveal the dominant role of plagioclase over the pyroxene. The absence of a positive Eu anomaly of the cumulus plagioclase is due to pressure fractionation before emplacement. A slight positive Eu anomaly in some gabbros is attributed to plagioclase accumulation. The diversities of both kinds of gabbros are ascribed to the varying degrees of partial melting, crustal contamination and/or subduction related contamination by subcontinental lithosphere. The existence of the two magma types reveals a heterogeneous mantle or upper mantle-lower crust source. They were emplaced in continental crust and tend to be formed during the final stage of arc to active continental margin.