Geology of the Wadi Ash Shu'bah Quadrangle, Sheet 26 E, Kingdom of Saudi Arabia

The Wadi Ash Shu'bah quadrangle, located in the north-central part of the Arabian Shield, is underlain by nine Precambrian volcano-sedimentary formations and a wide compositional range of Precambrian intrusive rocks. Late Proterozoic to early Paleozoic rocks of the Jibalah Group crop out in the southwestern part of the quadrangle. Paleozoic and younger units are limited to the early Paleozoic Siq Sandstone that crops out in the northwestern part of the quadrangle and small outcrops of Cenozoic basalt and unconsolidated Quaternary sediments.

The Hulayfah group, dated at older than 738 Ma, is the oldest volcano-sedimentary unit in the quadrangle and consists of metamorphosed basaltic to andesitic volcanic and subvolcanic rock, and interbedded limestone, sandstone, and silicic volcanic rock. The Maraghan formation is composed of strongly folded, fine-grained sedimentary rock that was deposited between 677 and 621 Ma. The Hadn and Hibshi formations are composed of rhyolitic to rhyodacitic volcanic rock and interbedded continental sedimentary rock that were deposited between 651 and 600 Ma, but are inferred to postdate the Maraghan formation because they are significantly less deformed. Andesitic flows and volcanic breccia in the southeast corner of the quadrangle are mapped as the Jurdhawiyah group; these rocks unconformably overlie the Maraghan formation. The Zarghat formation is a thick sequence of interbedded sedimentary rock and silicic volcanic rock, and unconformably overlies the Hadn formation. It is inferred to have been deposited between 600 and 580 Ma.

The Precambrian igneous rocks are organized into four tentative magmatic episodes. Greenstone of the Hulayfah group and dioritic intrusive rocks represent the oldest episode, and are believed to have formed in an ensimatic magmatic arc environment. These rocks were intruded during a second magmatic episode by post-Hulayfah monzogranite and granodiorite. A still younger, post-Hadn magmatic episode is represented by alkali-feldspar granite and syenogranite. The most recent magmatic episode is represented by small diabase and gabbro intrusions that are inferred to have been emplaced during a period of crustal rifting coincident with the deposition of the Jibalah group.

The magnesite deposit near the village of Zarghat is the most significant mineral deposit in the quadrangle. However, the Hulayfah group has the most potential for metallic deposits in the area as it contains numerous gossans and ancient mine workings.