Reconnaissance geology of the Al'Awshaziyah Quadrangle, sheet 26/41 B, Kingdom of Saudi Arabia

Abstract

The Al 'Awshaziyah quadrangle (26/41 B) is located between lat 26°30' and 27°00 f N. and long 41°30' and 42°00' E. in the northeastern part of the Arabian Shield. The quadrangle contains surficial Quaternary deposits and locally exposed underlying upper Proterozoic intrusive, volcanic, and volcaniclastic rocks. The oldest rocks, gabbro and pyroxenite, are tentatively correlated with the Ha'il mafic-ultramafic complex to the north of the quadrangle. Two younger volcanic sequences, structurally more or less distinct but compositionally transitional, have been identified. The Aqab formation (about 640-610 Ma old) consists of slightly metamorphosed, moderately to strongly folded flows and tuffs of basalt, dacite, and minor rhyolite. An interlayered subaerial conglomerate and a lack of deep-marine sediments distinguish the Aqab from the older and distinctly oceanic Nuf formation. The.Aqab formation is overlain by felsic ash-flow tuffs and related fragmental rocks of the Al 'Awshaziyah formation whose major source is a large caldera in the western part of the quadrangle. Plutonic rocks include granites that predate and postdate both the Aqab and Al 'Awshaziyah formations. The youngest granites, dated at about 580 Ma, are the Salma and Ar Rumman batholiths and are more -alkaline and more silicic than the older granites. The Ar Rumman granite is peralkaline. Even the youngest intrusive rocks have been cut by faults that are probably related to the northwest-trending Najd fault system. No metallic mineralization has been recognized in the quadrangle. A small quarry near the western boundary of the quadrangle produces crushed rock.