The area around G. Um Khors (150 Km 2 ) is located in the middle of the basement complex of the Central Eastern Desert, Egypt, south of the Qift-Quseir road. Um Khors trachyte plug (bostonite sill) represents the youngest major rock unit in the district and was intruded into heterogenous country rocks pertaining to the ultramafics, metagabbros, volcanogenic metasedimentary and molasse-type sediments (Hammamat group). The contacts of this plug against the above country rocks are invariably sharp. They include mainly porphyritic sanidine and aegirine-ribeckite trachytes. Both exhibit pronounced porphyritic and trachytic textures. They consist essentially of phenocrysts of sanidine and...