Geochemistry and petrogenesis of dyke swarms in NW Sinai, Egypt: a case of transition from compressional to extensional regimes during the late Neoproterozoic

Dyke swarms emplacement constitutes one of the conspicuous features of the Neoproterozoic Arabian-Nubian Shield (ANS) (~950-540Ma). Based on field investigations, petrographic and geochemical characteristics the dyke swarms in NW Sinai (Egypt) comprise of mafic (dolerite-trachy-basalt-basalt), intermediate (basaltic-andesite trachy-andesite) and felsic (rhyodacite-rhyolite) varieties. Geochemically, all varieties are calc-alkaline with the exception of few mafic varieties that show weak alkaline affinity. Increasing SiO2 is accompanied by enrichment in Na2O, K2O and depletion of TiO2, FeOt, MgO, CaO, P2O5, Co, Sr, Ba, Zr and Y. There is a compositional gap between the mafic, intermediate and the felsic dykes perhaps indicating more than one magma source. Although, Geochemical signatures of the investigated dykes swarms suggest that they are related to subduction processes,but according to tectonics and field relationships the studied dykes represent a post-orogenic or, at least, represent a transitional tectonic setting between Subduction and extension phases. The mafic dykes resulted from a lithospheric mantle-enriched material during a previous subduction event ( > 300 Ma) by small degree of partial melting.The intermediate dykes are most probably produced by partial melting of basaltic magma that followed by fractional crystallization processes. Fractional crystallization processes are dominated during the evolution of the most evolved intermediate samples; due to consumption of pyroxene, amphibole and plagioclase. The felsic dykes most probably evolved by partial melting of a lower mafic crust which led to the formation of a rhyolitic magma. In addition, the investigated samples show remarkable crustal contamination during their formation. The investigate dyke swarms were emplaced at a post-orogenic extensional collapse event transitional between volcanic-arc and within-plate environment during the late stages of the Late Neoproterozoic juvenile Pan-African crust of the Sinai Peninsula.