The Arabian–Nubian Shield, an Introduction: Historic Overview, Concepts, Interpretations, and Future Issues

The Arabian–Nubian Shield (ANS) is widely viewed as a crustal block of juvenile Neoproterozoic rocks in Northeast Africa and the western Arabian Peninsula. Exposed in mountainous terrain on either side of the Red Sea and Gulf of Aden or concealed beneath Phanerozoic cover, the rocks make up Earth’s largest block of juvenile Neoproterozoic crust. The ANS has been geologically investigated for many decades from prior to the development of plate tectonics to the application of modern concepts and methods such as microprobe geochronology, isotopic analyses, and geochemical and petrologic techniques that allow sophisticated examination of suprasubduction and subduction-modified magmatic processes. During this time, interpretations of the ANS expanded from a simple inference of a single subduction zone across the entire region to the recognition of multiple arcs of different ages and tectonic settings. The ANS formed over a period of about 450 million years, between ~1000 Ma and 530 Ma, and its growth reflects complex crustal evolution as part of the Rodinia–Gondwana supercontinent cycle. The shield originated with the rifting of Rodinia and the onset of intraoceanic arc magmatism in the Mozambique Ocean in parts of the ocean proximal to the Saharan Metacraton and the Congo–Tanzania Craton. Development of the shield continued with episodic convergence and accretion of arcs, syntectonic intrusion and metamorphism, and deposition of volcanosedimentary successions in basins developed on the newly accreted arcs. Shield development terminated with the growth of stable continental crust following a process of cratonization involving crustal thickening and metamorphism of accreted arcs; mantle delamination; asthenospheric upwelling and crustal melting; emplacement of an exceptional suite of late calc-alkaline to alkaline and A-type granites reflecting late- to post-tectonic and post-collisional settings; and accretion of stable crust to western Gondwana blocks. The relationship of the ANS in the western part of the Arabian Peninsula to Tonian arc and younger Cryogenian–Ediacaran mainly sedimentary rocks exposed in the eastern part of the Arabian Peninsula is not certain. The rocks in the east have a shared history with typical ANS rocks in having originated in the Mozambique Ocean, but may represent deposits proximal to the Indian Craton rather than to the Saharan Metacraton and Congo–Tanzania Craton. In central Egypt and Sudan, the ANS is in sheared contact with the Saharan Metacraton. However a clear distinction between the ANS and Metacraton as separate geologic entities is complicated by the presence of Ediacaran granitoid inrusions in the eastern part of the Metacraton implying extensive Neoproterozoic rejuvenation of Archean-Paleoproterozoic crust in the Metacraton contemporary with final events in the ANS. In southern Kenya and northern Mozambique, the ANS has a sheared contact with the Mozambique Belt, but the original depositional relationship between the ANS and Mozambique Belt is debated. Archean–Paleozoic rocks, part of the north-trending Azania ribbon continent, are present in Ethiopia, Somali, and Yemen and probably extend into central Saudi Arabia. They are in sheared or possibly depositional contact with typical ANS rocks on the east and the west suggesting that Azania was a relatively narrow continental protrusion into the Mozambique Ocean. As generally used, the term “Arabian–Nubian Shield” refers exclusively to juvenile Neoproterozoic rocks, but the presence of extensively reworked Archean–Paleoproterozoic crust on the flanks of, and as enclaves within, the region of typical ANS juvenile rocks suggests that Neoproterozoic tectonic processes affected not only the Mozambique Ocean but surrounding regions. In other words, the “story” of the ANS encompasses more than merely the development of juvenile rocks within an ocean basin. On this basis, it appears to this author that discussion of the geology of the region would be better served by reverting to the original geographic meaning of the term “shield”, in which sense the ANS would be viewed as comprising a crustal block in Northeast Africa and Arabia composed of juvenile Neoproterozoic rocks (as its main constituent) as well as Neoproterozoic rocks containing variable amount of older continental material and Archean–Paleoproterozoic crust present as enclaves within or as continental margins on the flanks of Neoproterozoic crust.