Phanerozoic Minor Volcanics and Intrusives of the Arabian-Nubian Shield

Abstract

AbstractThis chapter reviews four types of Phanerozoic minor volcanics and intrusives of the Arabian-Nubian Shield (ANS). Outlines of this chapter are: alkaline ring complexes including associated volcanics, Wadi (W) Natash volcanics, felsite dykes and plugs, trachyte and bostonite dykes and concordant sheets. Geographically and from north to south the (ANS) includes the nations of Egypt, Sudan, Erittrea, Ethiopia, Saudi Arabia and Yemen. Gabal (G) Elba and Gabal Shendeib ring complexes (Egypt) form high positive topographic rock units, quite distinct from the surrounding flat low lying Pan-African country rocks (Arc-assemblage and Post-orogenic). Gabal Elba complex consists of intermediate to acidic extrusive rocks including andesites, trachyandesites, dacites, rhyodacites, rhyolites and associated pyroclastics, while the associated intrusive rocks are represented by gabbros, alkaline syenites and alkaline granites. The uranium mineralizations are more common in Gabal Elba ring complex (Fe—subvolcanic rocks and acidic pyroclastics) and in Gabal Shendeib ring complex (alkaline syenites). The Phanerozoic volcanic rocks of the Saudi Harrats of the Arabian-Nubian Shield were generated by the Oligo-Miocene initiation of the Red Sea that produced large amounts of lithospheric melts, including various lava flows of olivine basalt and hawaiite, in addition to mugearite, trachyte, and benmorite. Neogene-Quaternary volcanic rocks are mainly widespread in the north-western part of the Arabian Plate including Jordan. Cenozoic igneous activity near the Red Sea and the Gulf of Aden is the site of youngest continental flood volcanism province associated with formation of new miniature oceanic basins and resulted from the impingement of Afar plume, continental breakup and rifting due to the movement of the Arabian plate away from Africa in the NE direction. The volcanic rocks of Yemen constitute part of the Afro-Arabian continental flood igneous province, and the term Yemen Volcanic Group (YVG) is brought to comprise all the Cenozoic volcanic rocks and their associated intrusions which dated back to late Oligocene-early Miocene and late Miocene to Recent. Moreover, Sinai basalts were emplaced along fissures oriented parallel to the Red Sea approximately 18–31 Ma. The basaltic rocks occur mainly in the form of lava flows, dykes, sills, small irregular stocks, plugs and cinder cones. They occur sporadically over a large area beneath the Nile delta (K-Ar dating suggests ages of 16–28 Ma) and the adjacent parts of the Western Desert. Three major Oligo-Miocene basaltic occurrences in Sinai, along the eastern side of the Gulf of Suez rift (Wadi Tayiba, Wadi Matulla and Gabal Abu Durba), have been chosen to reveal the geochemistry and petrogenesis of the magmatism associated with this newly formed continental rift. The volcanic sequences at Wadi Natash, south Eastern Desert of the Arabian-Nubian Shield are represented by three flow units, separated by two sequences of volcaniclastic sediments. Each of the three flow units shows a gradual change in composition upwards from alkali olivine basalt through trachybasalt, trachyandesite, to trachyte. Two ring structures (Western Ghorfa and Gaziret Khashm Natash) and one ring dyke (Gabal El Ghorfa) have extruded through the volcanic flows. A according to the results revealed from the mineralogical studies, the volcanic rocks contain meta-autunite and kasolite as uranium minerals. Generally Natash volcanics show high U (190–211 ppm) and Th (43–55 ppm) content. The basement rock units of Um Safi area is a part of the Arabian-Nubian Shield. They are represented by serpentinite rocks, metavolcanics and related volcaniclastic, metagabbro-diorite complex and older granitoids. The whole sequence is intruded by different late-to-post tectonic younger granitoids and overlain by Hammamat molasse sediments. These rocks are extruded by felsites and trachyte plugs. The gamma-ray spectrometric survey indicates that the study area possesses gamma radiation ranging between 1200 and 7000 as a D.R. (dose rate), 2–7.9% for K, 13–81 ppm for eU and 290–1670 ppm for eTh. Significant accumulations of felsic volcanic rocks (Wadi El Miyah, Wadi Atalla, Wadi Shait, Wadi Ranga areas, Egypt) and high-level intrusive rocks in the Eastern Desert have calc-alkalic and tholeiitic affinities with low high field strength element (HFSE) contents. The studied volcanics of the Arabian-Nubian Shield at the area are: El Atshan, Wadi Kareem, Um El Khors, Um Domi, Um Doweila and Nasb El Qash volcanics, Egypt. The contacts of the volcanics with country rocks (mostly Hammamat sediments) are sharp and rarely exhibit thermal metamorphism on the host rocks. Radioactivity: It is clear that El Atshan volcanics are the most radioactive rock unit compared to the other studied volcanics. The mineralized volcanic of El Atshan exhibits an expanded enrichment of eU (25–4035 ppm) relative to eTh (8–27 ppm).KeywordsRing complexesPhanerozoic Minor VolcanicsWadi NatashAlkaline rocksGeochemistryEastern DesertEgyptArabian-Nubian Shield