The exposed part of the Um Risha complex measures 12 km east-west but an unknown area is concealed beneath Nubian sandstone. The intrusive rocks are peralkaline syenites and granites of varying grain size.
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There is no full, general account of the alkaline rocks of Egypt but there are a number giving details of the main groups of 'ring complexes'. The fullest general description, which covers field relationships, petrography and petrochemistry of the alkaline rocks of the southeastern desert, is that of El Ramly et al. (1971) who describe 10 of the major complexes as well as many of the smaller occurrences. A useful compilation of chemical analyses of the igneous rocks of Egypt is that of Aly et al. (1983). Several papers list age data including those of Serencsits et al. (1979) (K-Ar), Hashad and El Reedy (1979) (Rb-Sr) and Omar et al. (1978) (fission-track).
Emplaced between Precambrian volcanic rocks and granites the intrusion comprises two structures, a southern ring and a northern centre which is larger and younger. The southern structure consists of an outer ring separated from an inner one by a wadi.
The Amreit complex (Robertson et al., 1990) comprises peralkaline granite and quartz syenite at Gebel Amreit and, to the south, granite and gabbro at Gebel Akab El Nugum.
This intrusion of peralkaline syenite and granite forms a ridge 3.5 km in length and is intruded into Precambrian metavolcanics, schists and granodiorites.
Emplaced in Precambrian granite gneisses, amphibolites and schists, the complex comprises a central nepheline syenite stock, an inner ring of essexite, and a partial outer ring of peralkaline granite.
This intrusion forms a cone-shaped mountain about 3 km in diameter surrounded by alluvium. The central area consists of peralkaline syenites that pass gradationally into an outer zone of mesocratic syenites and quartz syenites containing abundant xenoliths.
An approximately circular complex 8 km in diameter is capped in places at higher levels by a roof of lavas and agglomerates. The principal rock types are syenites of widely varying grain size which vary from quartz- to nepheline- and analcime-bearing.
Approximately 4 km in diameter, this intrusion consists of peralkaline granite and quartz syenite intruded into Precambrian granite and gneiss, the contacts being either tectonic or concealed beneath alluvium.
El Naga defines a horseshoe-shaped ridge some 4 km in diameter completely surrounded by wadi alluvium so that no outer contacts are exposed. However, on the outer side of the wadi in the northeast and southeast, beyond the area shown on Fig.
This 3.5 km diameter complex is emplaced in Precambrian metavolcanic rocks and has a clearly defined ring structure.