Reconnaissance geology of the Al Hufayr Quadrangle, sheet 27/41A, Kingdom of Saudi Arabia

Abstract

The Al Hufayr quadrangle (27/41 A) lies at the northern edge of the Arabian Shield between lat 27 30' and 28 00' N. and long 41 00' and 41 30' E. A cataclastically foliated syenogranite, the oldest rock exposed in the quadrangle, crops out in a restricted area in the south-central part of the quadrangle. Younger, weakly metamorphosed metarhyolite and minor metabasalt, arkosic sandstone, and conglomerate crop out north of the syenogranite and are correlated with lithologically similar rocks exposed to the south that comprise the Hadn formation. Most of the intrusive rocks in the quadrangle are peralkaline or have a peralkaline affinity and are characterized by the presence of soda pyriboles. A large mountain known as Jibal Aja extends into the southeast corner of the quadrangle and is composed of postorogenic peralkaline alkali granite plutons that form concentric rings around a core granophyre. A small plug of Tertiary alkali basalt crops out near the southeast corner of the quadrangle. Eighty percent of the quadrangle is covered by a thick accumulation of eolian sand that forms the southern edge of the An Nafud dune field. The plutons of the Aja suite represent samples of magma that depict the evolution of a batholith-scale magma body that solidified to the Aja suite. The major- and rare-earthelement chemical variation observed among the components of the suite is a consequence of a discontinuous process that involved chemical evolution via separation of silicate liquid from earlier formed crystals, and emplacement of batches of magma, whose compositions represent stages of the process. Resource potential in the quadrangle is low. No ancient mines are reported in this quadrangle. Geochemical data for a limited number of pan concentrates of wadi sediment from wadis draining the peralkaline rocks of Jibal Aja show distinctive associations of incompatible lithophile elements. A similar association of elements has previously been shown to be diagnostic of peralkaline granite in the northeast part of the Shield but does not imply high resource potential.