AGE AND EVOLUTION OF THE SOUTHERN PART OF THE ARABIAN SHIELD

Rb–Sr studies of Precambrian volcanic and plutonic rocks of the Arabian Shield document an early development of continental lithosphere in the Arabian Peninsula between 900 and 680 Ma ago. Geologic studies indicate an intraoceanic island-arc environment characterized by andesitic (dioritic) magmas, volcaniclastic sedimentation, rapid deposition, and contemporaneous deformation along north- or N.W.-trending axes. The earliest plutonic units are diorite to trondhjemite batholiths ranging from 900 to 800 Ma in age that occur in the western and southwestern parts of Saudi Arabia. Younger units, ranging from 750 to 680 Ma in age, vary in composition from quartz diorite to granodiorite and become more abundant in the south-central and eastern parts of the Arabian Shield. Initial 87Sr/86Sr ratios of rocks of both groups range from 0.7023 to 0.7030, averaging 0.7027. The absence of sialic detritus in sedimentary strata and the evidence of an island-arc environment suggest that the early development of the Arabian continental crust occurred at a convergent plate margin between plates of oceanic lithosphere. Subsequent to this subduction-related magmatism and tectonism, called the Hijaz tectonic cycle, Arabia was sutured to the Proterozoic African plate during a collisional event represented in Arabia and eastern Africa by the Pan-African event, extending from prior to 650 to at least 540 Ma and perhaps 520 Ma ago. Although these tectonic processes of subduction and continental collision during the 900 to 500 Ma period are represented here by a single plate-tectonic model of plate convergence, the differences in age and in magmatic and tectonic styles of the Hijaz orogenesis from those of the Pan-African support division into at least two events. The distribution in time of the major plutonic intrusions indicates that the axis of magmatic and tectonic activity migrated east or N.E. during the Hijaz cycle. The granodioritic to granitic plutonism of the Pan-African event, however, shows no definitive geographic variation with age, and plutons are distributed throughout the Arabian Shield.