NEOPROTEROZOIC MAGMATISM IN THE NW SINAI, EGYPT: MAGMA SOURCE AND EVOLUTION OF COLLISION-RELATED INTRACRUSTAL ANATEXIS LEUCOGRANITE

The study area is located near the northwest corner of exposed basement in Sinai and is composed of a gabbroic complex, amphibolite, post-orogenic leucogranite and Feiran gneisses. The amphibolite and gabbroic suites, generated in an island arc environment, have a high Fe-tholeiitic affinity and derived from two different independant magmas. On the basis of geochemical data, The gabbroid melts are assumed to have been generated from a garnet- and amphibole-bearing, enriched mantle modified by fractionation of pyroxene and amphibole, whereas the amphibolite melts could be derived from garnet-free depleted mantle.

The leucogranite has high Al2O3 content, with alumina saturation index mostly greater than 1 and normative corundum indicating a high degree of peraluminosity. REE pattern of the leucogranite shows LREE enrichment, with general flattening out in the HREE and has a negative Eu anomaly. The peraluminous nature and enrichment of the incompatible element in the leucogranite strongly suggest derivation from a crustal source. The most probable source for the leucogranite is the adjacent Feiran gneisses. It appears likely that the restite unmixing model is responsible for the chemical variation of the leucogranite. In accordance with this model, the chemical variation of the leucogranite can be attributed to varing degrees of separation of restitic material from the melt during its emplacement and solidification. Fractional crystallization may play some role during magma ascent through the crust.