Geology of the Jabal Al Qarah Quadrangle, Sheet 20/43 C, Kingdom of Saudi Arabia

Abstract

The Jabal al Qarah quadrangle (sheet 20/43 C) is a 30-minute quadrangle mapped at 1:100,000 scale in the southern Najd Province of Saudi Arabia. The quadrangle is located between lat 20°00'N. and 20°30'N. and long 43°00'E. and 43°30'E. It is underlain by volcanic, volcaniclastic, and plutonic rocks of late Precambrian age. Most of the quadrangle consists of a large, asymmetric gneiss dome that extends far to the north and south of the quadrangle. Parallel with the eastern border of the quadrangle, part of the extensive, serpentinite-bearing, Nabitah fault zone probably represents a major suture between crust of Jiddah age to the west and younger crust of Halaban age to the east. An excellent section of the Halaban-age crust is exposed in and adjacent to the gneiss dome. At the eroded top of the section to the east, calc-alkalic volcanic rocks of the Halaban group are dominantly andesite but range from basalt to dacite. Beneath the volcanic rocks, comagmatic, subvolcanic plutonic rocks include predominantly diorites but range from gabbro to trondhjemite. These rocks were formed during island-arc volcanism and plutonism from about 780 m.y. to 720(?) m.y. ago. About 725 m.y. ago this primitive crust was thickened by emplacement of large intermediate-crustal depth plutons of hornblende tonalite and hornblende granodiorite. Compressional tectonism produced folds and faults with northern trends and accompanying greenschist metamorphism. Granitic plutonism began 625 m.y. ago, perhaps in response to a major continental collision between the Halaban crust and a continental mass to the east. Granodiorite-monzogranite comprising the Wadi Musayrah batholith formed the core of the Jabal al Qarah gneiss dome as the dome rose. The low-density, intermediate-depth, plutonic rocks of the Halaban crust were converted to orthogneisses as they rose. Strong compression resulted in a large west-directed thrust, the Nabitah fault zone, containing serpentinite. Subsequently, plutons of monzogranite, syenogranite, and alkali-feldspar granite intruded the eroding crust at progressively shallower levels until about 600 m.y. ago. After this time the polymictic conglomerate of the Murdama group was deposited over the youngest granite--the alkali-feldspar granite at Jabal ash Shawhatah al Hamrah; the conglomerate was subsequently deformed. Continued compression of the now relatively thick continental crust resulted in the Najd faulting event about 570 m.y. ago. Two northwest-trending faults in the quadrangle have left-lateral displacement; one has 7 km of offset. These two faults are small but are characteristic of the large Najd fault zones farther north. Throughout the quadrangle, northwest-trending Najd fracturing was pervasive and many of these secondary fractures were filled with diabase dikes of Najd age. The diabase dikes and the deeper, non-exposed mafic plutons induced small-scale, but widespread hydrothermal convection and alteration that produced small quartz veins locally mineralized with gold. Quartz-vein gold was mined on a small scale by ancient miners in several areas. An extensive gold placer was worked about 2,600 years ago in a 50 km 2 area at Jabal Mokhyat. None of these deposits are economical by present-day standards.