Petrogenesis of a composite dike system in the Wadi Rimthi area, South Sinai, Egypt

Abstract

Composite dike systems are a common feature in the basement complex of the Egyptian Eastern Desert and Sinai. These dike systems cut several lithological units, but mostly concentrated in post-orogenic calc-alkaline and alkaline granitoid rocks. New field observations, petrographic studies, and whole rock geochemical data are used to reveal the genesis and evolution of these dikes as well as to examine the genetic relationship between the parent mafic and felsic melts. The investigated composite dike system is characterized by a felsic core and mafic margins. The mafic margins are basalt to andesite in composition, whereas the felsic core is mainly rhyolitic in composition. The mafic dike samples are typically enriched in rare earth elements (REEs) (up to 305 ppm) with fractionated patterns ((La/Lu)N = 7.81–9.25) and mildly fractionated heavy rare earth elements (HREEs) ((Gd/Lu)N = 2.53–2.69). The felsic dike samples have low REE contents (up to 234 ppm) with less fractionated patterns ((La/Lu)N = 4.05–5.59) and weak to unfractionated HREE patterns ((Gd/Lu)N = 1.35–1.53). Major and trace element data of the examined mafic and felsic dikes do not show a clear mutual fractionation history. The marked variations in the K/Rb, Rb/Zr, and La/Sm ratios suggest that mafic and felsic rocks were formed from two individual melts and partly contaminated with crustal materials. According to the incompatible trace element patterns, mafic magma is assumed to be generating from the melting of a garnet-bearing peridotite, modified by the fractionation of amphibole, clinopyroxene, olivine, and zircon. The felsic magma, on the other hand, might have been derived from the melting of garnet-free crustal material dominated by feldspar, apatite, and Fe-Ti oxides, followed by pyroxene and amphibole fractionation crystallization.