Paleozoic alkaline volcanism: geochemistry and petrogenesis of Um Khors and Um Shaghir trachytes of the central Eastern Desert, Egyptبركانيات الباليوزويك القلوية: جيوكيميائية ونشأة صخور التراكيت بمنطقتى أم خرس وأم شغر، وسط الصحراء الشرقية- مصرأ.م.د. مصطفى إسماعيل غريب – قسم الجيولوجيا – كلية العلوم- جامعة حلوانأ. د. محمد عبد الفتاح عبيد– قسم الجيولوجيا – كلية العلوم- جامعة الفيومأ.م.د. أحمد حسن أحمد – قسم الجيولوجيا – كلية العلوم- جامعة حلوان

The Um Khors and Um Shaghir trachyte (UKT and UST) plugs and sheets represent two conspicuous outcrops of Paleozoic alkaline volcanism in the central Eastern Desert of Egypt. The trachyte magmatisms erupted along the Pan-African NW-trending shear zone (302 ± 15 and 273 ± 15 Ma, respectively) and intruded the Late Proterozoic rocks of the studied area. The trachyte rocks consist mainly of sanidine, anorthoclase, albite, and quartz with a noticeable amount of aegirine–augite, aegirine, hedenbergite, and arvedsonite. The studied trachytes are moderately evolved in composition (with 62–67.5 wt.% SiO2) and exhibit a limited compositional range in most of the major elements. They are alkaline in nature and considered as silica-oversaturated rocks. The rare earth elements (REE) patterns are somewhat uniform and highly fractionated, being enriched in light REE over heavy REE and show prominent negative Eu anomalies. The UKT and UST are enriched in high field strength elements Nb, Zr, and Y, consistent with typical within-plate alkaline magmatisms of extensional tectonic regimes. They were generated through the fractional crystallization of mantle-derived magmas. Although the UST is younger than the UKT, they show approximately similar chemical compositional ranges of the most major and trace elements, with somewhat higher MgO, Cr, Ni, and Ba contents in the former. This may argue against the evolution of the UST via a continuous fractional crystallization of the residual magmatic melt of the UKT. Thus, the UKT and UST are genetically related but could be emplaced through two various magmatic pulses of the same parent source (i.e., asthenospheric mantle source) at different times. The ascending magmatisms were subjected to variable significant degrees of crustal contamination during their generation.