Mineralogical and geochemical aspects of the petrogenesis of Pan African Dokhan Volcanics at Esh El Mellaha Area—NE Desert, Egypt

The volcanic activities in the Eastern Desert of Egypt define two major magmatic episodes separated in time. The Dokhan Volcanics is the main representative of the later episode which commenced about 600 Ma and continued for about 25 Ma. These volcanics have been found to vary lithologically from mafic to felsic rocks. Mineralogical and geochemical signatures were used to examine a suite of Dokhan lithology from the Esh El Mellaha area of Egypt Eastern Desert with the aim of clarifying their petrogenesis and tectonic significance. The primary phases (e.g. pyroxene, amphibole, and plagioclase) of the Dokhan volcanics illustrate a progression from basalt to andesite to dacite. Enstatite component (En) of pyroxenes and anorthite (An) component of plagioclase decrease in modes from basalts to andesites to dacites. Whole rock geochemistry records similar trends. There is a wide range of SiO₂ (43.62–63.91 wt.%), CaO (2.34–10.56 wt.%), Sr (110–908 ppm), and Zr (120–296 ppm) contents, and a moderate enrichment in incompatible elements. The fractionation index (FeO_total/MgO) increases gradually from basalts through andesites to dacites (2.72, 2.89, and 4.03, respectively). These chemical variations support a model of fractional crystallization playing an essential role during the evolution of the Dokhan volcanic magma series. The enrichment of LILE (e.g. Rb, Ba, K, and Sr) and the relative depletion of HFSE (e.g. Nb, Zr, Y, and Ti) appear to be inherited from a mantle source. Compiling field relationships with mineralogical and geochemical data confirm three magmatic assemblages: tholeiitic basalts, calc-alkaline andesites, and dacites. The data is interpreted to indicate an island arc tectonic setting that may have been transitional to an active continental margin in a subduction-related environment.