Reconnaissance geology of the Harrat Hutaymah Quadrangle, sheet 26/42A, Kingdom of Saudi Arabia
Harrat Hutaymah quadrangle is underlain mostly by Proterozoic rocks that are intruded and partly covered in the north by the Quaternary extrusive products of the Harrat Hutaymah basalt field. The Proterozoic basement of the quadrangle may be broadly divided into the older (circa 650 Ma and older) Laban dioritic complex in the east and the younger (to possibly coeval) Kilab monzogranite batholith in the west. The dioritic complex is overlain by a monoclinal section of mostly intermediate composition volcanic and sedimentary rocks in the southeast (the Hibshi formation - dated at circa 632 Ma). The source plutons for the Hibshi volcanic rocks are not well constrained; the volcanics may be comagmatic with late phases of the Kilab batholith. All of these rocks are intruded by a younger (post-orogenic and circa 600 Ma) suite of more evolved granitic plutons and dikes. Several of these evolved granites may represent the root-zones of volcanic caldera complexes. The Hibshi formation is divided into several members that are described in detail based on data from measured sections at the type section (Jibal Hibshi). There is considerable variation in composition and facies within the formation; individual members thicken and thin laterally, and some pinch out entirely over distances of only a few kilometers. Volcanic members at Jabal Hibshi are mostly andesitic to dacitic pyroclastic rocks, but at Jabal Aba al Liqah, basaltic and rhyolitic flow rocks predominate. The Hibshi formation consists of clastic sediments of volcanic and plutonic provenance intertonguing with the varied products of multiple subaerial volcanic centers. For these reasons, regional correlation based only on lithology is discouraged. The Harrat Hutaymah basalt field contains abundant basaltic tephra as well as alkali olivine basalt flows. Tuff rings formed by explosive eruptions above mantle diatremes. Peridotite nodules are common in the basaltic tuff and in some flow rocks. Basaltic volcanism in the region began at a later time (circa 1.8 Ma) than at the other Saudi Arabian harrats; probably due to progressive mantle underflow related to Red Sea rifting. Areas having mineral resource potential include the ancient Samirah gold mine, a quartz-vein deposit in Laban complex dioritic rocks, and areas with tin-tungsten potential in the evolved plutons. Aggregate and pozzolan deposits occur in the basaltic tephra of the Harrat Hutaymah volcanic field and unusually deep groundwater reservoirs are present in the filled tuff-ring craters of the volcanic field.