Jebel Qeili

Forming a prominent feature rising 260 m above the surrounding plain the complex covers 6.25 km2 and is emplaced in the Precambrian Butana green schists. The intrusion is formed principally of syenites and granites but inclusions and rafts of gabbro and dolerite are found in all the major rock types. Rhyolitic lavas were extruded prior to the emplacement of the intrusive complex and a strip of these is preserved, probably as a result of faulting, along the eastern margin. The rhyolite contains small phenocrysts of alkali feldspar in a dense matrix of alkali feldspar, quartz and sodic pyroxene and amphibole which are partly replaced by iron oxides. The outermost intrusion of the complex, which defines a complete ring, is a biotite granite of varying texture, deformation and mineralogy. The Qalat Busta fayalite syenite consists of perthite, very minor andesine and ferromagnesian aggregates of aegirine, largely replaced by ferrohastingsite, riebeckite, biotite and occasional grains of fayalite. The El Hawya hastingsite syenite is similar to the Qalat Busta syenite but is free of fayalite and contains more ferrohastingsite. The central Qalat Qeili syenite contains quartz and nearly 10% modal riebeckite. Narrow dykes of microgranite, microsyenite and trachyte cut all the major intrusions. Four chemical analyses are given by Ahmed (1975) who also describes a reconnaissance gravity and magnetic survey of the complex for which Delaney (1955) gives an aerial photograph.