Reconnaissance geology of the Qufar Quadrangle, sheet 27/41 D, Kingdom of Saudi Arabia

Abstract

The Qufar quadrangle, south of the city of Ha' il in the northern Arabian Shield is underlain by late Proterozoic granitic and dioritic rocks and two volcano-sedimentary sequences. Phanerozoic rocks include a few outcrops of the Cambrian and Ordovician Saq Sandstone and small remnants of Miocene basalt flows and plugs. The oldest rocks in the quadrangle comprise the Nuf formation, a layered sequence of submarine, tholeiitic metabasalt and meta-andesite, and interbedded metagraywacke and marble. The Nuf formation may correlate with rocks mapped as Halaban or Hulayfah group (approximately 780-720 Ma old) to the south of the quadrangle. Cogenetic subvolcanic rocks include gabbro and diorite. Voluminous plutonic rocks of approximately monzogranite composition intruded and dismembered the Nuf formation, gabbro, and diorite, which were simultaneously metamorphosed and internally deformed. Metamorphism of the Nuf formation was variable, but generally' upper greenstone-facies assemblages were produced. Following a period of extensive erosion, the Hadn formation, dacitic to rhyolitic ignimbrite and flow breccia, and interbedded subgraywacke, arkose, and minor conglomerate was deposited. The Hadn formation may be a continental equivalent of the Murdama group, which is mapped to the south of the quadrangle and is approximately 650 to 610 Ma old. Numerous plutons, predominantly monzogranite, but ranging from gabbro to alkali-feldspar granite, post-date the Hadn formation. Of these, the Malayhah granite is particularly noteworthy because it has a broad zone of cataclasis along the western and southern border. Country rock within several kilometers of the western contact is also highly sheared, predominantly along northerly trends. Locally, a melange of several rock types was produced. Contact metamorphism to garnet-amphibolite hornfels facies occurred at this time, and may be responsible for the formation of small sub-economic magnetite lenses interlayered with some of the marbles of the Nuf formation. In the northern part of the quadrangle, southeast dipping imbricate thrust faults probably closely post-date the emplacement of the Mulayhah granite. These thrusts were followed in time by predominantly northeast-trending high-angle faults. The last major plutonic event in the area is the intrusion of the alkalic granite complexes at Jabal Aja and Jabal Sal ma about 580 Ma ago. Of particular note is a per alkalic border facies of the Jabal Aja complex that is associated with pegmatites enriched in thorium, niobium, and rare-earth elements.