Reconnaissance geologic map of the Jabal Zain Quadrangle, sheet 20/44A, Kingdom of Saudi Arabia
Late Precambrian rocks of the Jabal Zain quadrangle comprise two distinct terranes separated by a broad cataclastic zone that formed during the latest Precambrian Najd strike-slip faulting. Only minor gold-silver prospects have been located in the quadrangle, and the potential for larger depostts is considered low. North of the Najd fault zone, remnants of amphibolite-grade metamorphic rocks (Halaban (?) group) and quartz diorite, tonalite, and trondhjemite are unconformable overlain by a 3- to 5-km thick unit of conglomerate, arkose, and intermediate volcanic rocks of the Zain formation (Murdama group). The Zain appears to be conformably overlain by more intensely deformed metabasalt and immature sedimentary rock of the Masamah formation. Southwest of the Najd fault zone, the Murdama group contains immature fine-grained greywacke and minor flows (Hofoor formation) and more mature arkose and conglomerate (Al Ghabiyah formation). The Yafikh formation of calcareous sedimentary rock and marble is confined to the Najd fault zone, and its stratigraphic context in this quadrangle is unclear. Abundant post-Murdama plutonic rocks consist of small bodies of gabbro and diorite, and an expansive compound batholith of granodiorite-granite, and discrete ovoid plutons of leucocratic granite. Cataclastic rocks in the Najd fault zone contain variable granitoid gneiss that may have been formed from diapirically emplaced pre-Murdama plutonic basement rocks.