Paleozoic alkaline volcanism: geochemistry and petrogenesis of Um Khors and Um Shaghir trachytes of the central Eastern Desert, Egypt

Abstract

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.% SiO 2 ) and exhibit a limited compositional range in most of the major elements. They are alkaline in nature and considered as silicaoversaturated 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.

Petrogenesis and geological implications of the trachytic rocks in the South Eastern Desert, Egypt: Mineralogical, geochemical, and radioactivity insights

“…The composition of Palaeozoic and Mesozoic volcanic rocks ranges from olivine basalt to trachyte, whereas tertiary volcanic formations are mostly basaltic. There have been numerous prior research on these alkaline volcanic systems conducted by (Heikal 2003; Dawood et al 2003; Saleh et al 2004, 2007, 2015, 2020, 2021; Ibrahim et al 2005; Abdalla 2001; El Tohamy 2011; Gharib et al 2012; Nabawy and El Sharawy 2018; Sadek et al 2020; Sallam et al 2021; Abuamarah and Nabawy 2021; Kamar et al 2022; Oraby et al 2022; Ali-Bik et al 2022; Abdalla et al 2023; Dessouky and Ali 2018; Dessouky et al, 2023; Khaleal et al 2022, 2023).…”

Geochemistry, Uranium, Thorium and Rare Earth Elements of Trachyte Dykes of Umm Salatit Mountain Area, Central Eastern Desert, Egypt

“…The contents of the major oxides, trace elements in the analyzed trachyte dykes as well as CIPW norms are given in ( Table 1). The average chemical composition of the studied trachyte dykes when compared with world trachyte dykes [6] and trachyte of Umm Shaghir CED, Egypt [7] Table 2). The relative concentrations of trace elements in the studied rocks are shown in the form of a primitive mantle-normalized diagram ( Fig.…”