Geochemistry, mineralogy, and radioactivity of Upper Cretaceous Abu Khruq Ring Complex, South Eastern Desert, Egypt
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ORIGINAL PAPER
Geochemistry, mineralogy, and radioactivity of Upper Cretaceous Abu Khruq Ring Complex, South Eastern Desert, Egypt A. H. El Afandy & H. A. El Nahas & N. A. Dawood & H. M. Asran
Received: 17 November 2011 / Accepted: 5 March 2012 / Published online: 24 March 2012 # Saudi Society for Geosciences 2012
Abstract The upper Cretaceous Abu Khruq ring complex (ARC) is located in the South Eastern Desert between latitudes 24°00′10′′ and 24°03′15′′ N, and longitudes 33°54′50′′ and 33°58′ E and has a roughly circular shape with a diameter of 7 km. ARC is built up by major extrusion of alkaline volcanic rocks comprising mainly rhyolite porphyry and alkaline trachyte rocks at the center of the ring complex followed by successive intrusions of alkaline gabbro and syenitic rocks comprising quartz syenite (oversaturated), syenite (saturated), and nepheline syenite (undersaturated). Petrographical and geochemical studies were carried out for the rocks of the forming ARC. For mineralogical and radioactive investigations, samples were collected from the most promising locations representing the hematitized nepheline syenite, nepheline syenite pegmatites, and quartz syenite. The most important minerals comprise: phosphuranylite, zircon, monazite, xenotime, plumbopyrochlore, pyrite, huttonite, apatite, REE mineral, rutile, and atacamite. The hematitized nepheline syenite is the most U- and Th-rich rocks, where eU content in this rock ranges from 375 to 788 ppm with an average 502 ppm and the average eTh content is 2,345 ppm ranging from 1,918 to 3,067 ppm. The pegmatite syenite and quartz syenite contain relatively low concentrations of U and Th, where the average eU content are 11 and 16 ppm and average eTh contents are 27 and 327 ppm, respectively. Keywords Abu Khruq ring complex . Petrography . Geochemistry . Mineralogy A. H. El Afandy : H. A. El Nahas (*) : N. A. Dawood : H. M. Asran Nuclear Materials Authority, P.O. Box 530, El Maadi, Cairo, Egypt e-mail: [email protected]
Introduction The Egyptian basement complex evolved during the PanAfrican time and ended in a sequence of post-alkaline magmatic activity in the form of ring complexes, stocks, and volcanic plugs continued till the Cretaceous. Geochronologically, Phanerozoic ring complexes in Egypt can be divided into four groups, the first one is of Paleozoic age (550– 313 Ma), the second is Permo-Triassic in age (230–200 Ma), the third is of Late Jurassic–Early Cretaceous age (160– 120 Ma), and the fourth group of Late Cretaceous age (110–80 Ma) (Serecsists et al. 1979; Serecsists et al. 1981; Hashad and El Reedy 1979; Meneisy and Kreuzer 1974). The Abu Khruq ring complex (ARC) belongs to the youngest group having age 89±2 Ma (Serecsists et al. 1981; Lutz et al. 1988). Garson and Krs (1976) proposed that most of the alkaline ring complexes of the Eastern Desert of Egypt are controlled by two sets of major structural lineaments. One of them trends N 60° E and represents a series of onshore extensions of transform faults. The other lineament trends N 30° W and
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