The Al Makhul quadrangle (26/42 B) lies in the northern part of the Arabian Shield between lat 26°30' and 27°00' N. and long 42°30' and 43°00' E. Amphibolite, the oldest metamorphic rock exposed in the quadrangle, crops out in a restricted area in the south-central part of the quadrangle. Younger, weakly metamorphosed rhyolite, basalt, and graywacke sandstone on the west edge of the quadrangle are correlated with lithologically similar rocks exposed to the southwest that comprise the Hibshi formation. The Turmus formation, located in the north-central part of the quadrangle, is composed of weakly metamorphosed andesite flow rocks, andesitic agglomerate, and related volcanic graywacke and is similar in metamorphic grade, age relations with intrusive rocks, and structural character to the rocks on the west edge of the quadrangle; it is thought to be approximately coeval with them.
Seven plutonic units, ranging in composition from gabbro and diorite to alkali-feldspar granite, intrude the weakly metamorphosed rocks. Some of these plutons are offset along a northwest-trending fault of the Najd fault system but their emplacement postdates all other major tectonic episodes in the area. The weakly metamorphosed rocks were deposited on an erosional surface underlain by a pluton composed of quartz diorite.
Quaternary alkali basalt crops out in the northwest part of the quadrangle. These rocks are at the southeast edge of Harrat al Hutaymah, a basalt field composed of flow rocks, tuff rings, and cinder cones.
Mineral potential in the quadrangle is low. At a very small prospect pit in the north-central part of the quadrangle, massive, milky quartz veins cutting weakly metamorphosed volcanogenic sedimentary rocks are stained blue and green by copper minerals. A previously reported mine site in the southern part of the quadrangle was not relocated.
