Gabal Abu Khruq is a conspicuous peak surrounded by semi-annular ridges and wadis. The complex cuts basement gneisses and schists and is partly overlain by lavas and agglomerates. Only the central area has been described in detail (El Ramly et al., 1969). Trachytes, rhyolites, phonolites and agglomerates extend over much of the western part of the complex and form the central summit; they form a roof and are fenitized close to the underlying syenites. The rhyolites contain abundant alkali feldspar phenocrysts in a groundmass of feldspar, quartz and opaque pseudomorphs. Sodic amphiboles and pyroxenes are concentrated along fractures and are probably the result of fenitization; trachyte porphyries are similar. The intrusive rocks are stock-like intrusions of nepheline syenite in the central area surrounded by a series of incomplete rings of nepheline syenite, syenite and essexitic rocks. The central nepheline syenites are described mainly as foyaites and ditroites but with varying degrees of zeolitization and cancrinitization; some urtite veins occur. The foyaites consist of Na- and K-feldspar, 25-30% nepheline, partly altered to cancrinite and replaced by analcime, alkali pyroxene and amphibole, biotite, zeolite, carbonate and accessories. Ditroites are similar mineralogically but lack a fluidal texture. Both rock types are widely sheared with alteration to cancrinite, zeolite, carbonate and iron oxides concentrated along these zones. The syenites vary from umptekite with up to 9% nepheline to quartz syenite (nordmarkite) with up to 20% quartz; the rest of the mineralogy is similar to that of the nepheline syenites. Small areas of what are called 'essexite gabbros' are found on the northeast side of the central area. These variable rocks consist of andesine-labradorite with some replacement by alkali feldspar, augite, sometimes with alkali pyroxene and amphibole rims, biotite and titanomagnetite. Dyke rocks are numerous and include solvsbergite, tinguaite, nepheline syenite porphyries and pegmatites, bostonite, albitite, trachydolerite and urtite consisting of 70% nepheline, 5% alkali feldspar, analcime, cancrinite and 5% aegirine-augite. Xenoliths are abundant throughout the complex and comprise various types of nepheline syenite together with rocks described as of the 'fenite-tveitasite series’. Chemical and modal analyses of all the principal rock types are to be found in El Ramly et al (1969a) and analyses, including trace element data, are given by Gindy et al. (1978); there are some trace element data in Soliman (1987). A very full whole rock data set is given by Landoll et al. (1994), including trace element and Nd, Sr and O isotopic data, and they conclude that the parental magma was derived from depleted mantle but that there was an input of crustal material to generate the quartz syenites. Hydrothermal activity at Abu Khruq has been investigated by Lutz et al. (1988) using Sr and O isotopes.
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