Reconnaissance geology of the Al Ufayriyah quadrangle, sheet 20/42 A, Kingdom of Saudi Arabia

Abstract

The Al Ufayriyah quadrangle is located in the Najd Plateau of southwestern Saudi Arabia between lat 20?30' and 21?00' N.,. long 42?00' and 42?30' E. Two-thirds of the quadrangle is underlain by Precambrian metasedimentary, metavolcanic, plutonic, and hypabyssal intrusive rocks; this area is in part mountainous and in part uneven plains. One-third is underlain by Cenozoic basalt and is part of Harrat Buqum. Flows, which surround volcanic cones, form an east-dipping surface of low relief. Precambrian metavolcanic and metasedimentary rocks are mostly metamorphosed andesite and basalt flow rocks and turfs and are divided into two principal belts in the eastern and central to western parts of the quadrangle. Plutonic rocks, which are part of the An Nimas batholith, consist of tonalite, diorite, and some quartz diorite. These rocks are postdated by younger plutonic and hypabyssal rocks that include extensive graphic granite and rhyolite, several small bodies of granite, and meta-andesite dikes. Volcanoes on Harrat Buqum are cinder cones, composite cones, shield volcanoes, and lava cones; basalt flows originating in some of the volcanoes can be traced down the slope of the harrat, and some have blocked Wadi Ranyah causing deposition of silt, sand, and gravel. Radiometric age dating of Precambrian rocks from adjacent quadrangles suggests that the layered rocks of the Al Ufayriyah quadrangle may have formed about 900 Ma ago, plutonic rocks of the An Nimas batholith 835 to 800 Ma ago, and younger plutonic rocks from 640 to 570 Ma ago. Basalts of Harrat Buqum range in age from 3.5 to 1.72 A zone in the eastern part of the quadrangle contains small gold-quartz veins, a quartz breccia reef, and a contact metasomatic iron deposit.