Reconnaissance geology of the Ghazzalah Quadrangle, sheet 26/41 A, Kingdom of Saudi Arabia

Open-File Report 83-331




The Ghazzalah quadrangle is located in the northern Precambrian shield of Saudi Arabia between lat 26?30' and 27?00' N. and long 41?00' and 41?30' E. The area is underlain by two lithologically distinct, Precambrian volcanosedimentary units and a wide range of dioritoid and granitoid plutonic intrusive rocks. The only Phanerozoic rocks consist of one outcrop of Tertiary(?) basalt and widespread but thin deposits of Quaternary detritus. The Banana greenstone, the oldest rock in the quadrangle, consists of intermediate volcanic and subvolcanic rocks and minor interbedded marble, which have been metamorphosed to greenschist-facies assemblages. Volcanic rocks mainly range in composition from basalt to andesite, and subvolcanic rocks consist of diorite and diabase. The Banana greenstone is unconformably overlain by silicic volcanic rocks and minor arkosic sandstone and breccia of the Hadn formation. Preservation of delicate volcanic textures suggests that the rocks have been only incipiently metamorphosed. Unpublished rubidium/strontium isotopic data for the Hadn formation suggest an age of 620 to 610 Ma. Intrusive rocks are separable according to their ages relative to the Hadn formation. Those that are unconformably overlain by the Hadn formation consist of hornblende quartz diorite and gabbro, which may be consanguineous with the Banana greenstone, and younger tonalite, biotite-hornblende granodiorite, syenogranite, and monzogranite. Plutons of monzogranite, alkali-feldspar g,ranite, syenbgranite, peralkaline granite, and hypabyssal intrusions of granophyre were probably emplaced during a period coincident with and (or) following Hadn volcanism. Uranium-lead and rubidium/strontium isotopic data for two plutons in the adjacent Al Qasr quadrangle suggest that plutonic activity persisted in the region until about 580 to 570 Ma. Faulting appears to postdate all of the plutonic rocks. The dominant faults belong to a northeast-trending system of right-lateral shears; a subordinant system consists of mainly north- to northwest-trending faults. The peralkaline-granite plutons underlying Jibal Ba'gham and Jibal ar Rumman have the most economic potential. Wadi samples from these areas show an anomalous concentrations of tin, lead, niobium, and yttrium. Localized, intense radiometric anomalies in the Ba'gham intrusive complex are associated with high concentrations )f thorium, uranium, andrare-earth elements.

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Reconnaissance geology of the Ghazzalah Quadrangle, sheet 26/41 A, Kingdom of Saudi Arabia
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Open-File Report
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U.S. Geological Survey,
48 p. ill., maps ;28 cm.