Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)

Geologic investigations of the Arabian-Nubian Shield (ANS) are supported by a valuable, expanding dataset of geochronologic ages and isotopic analyses. U-Pb zircon SIMS, TIMS, and evaporation data provide robust temporal control on depositional, volcanic, and intrusive events. Nd, Sr, O and, increasingly, Hf isotopic studies provide insights into magma sources and constraints on crustal recycling. Apart from well-established exceptions where Paleoproterozoic and Archean rocks intercalated in the ANS underwent Neoproterozoic reworking, the data elegantly confirm the Neoproterozoic age and juvenile character of the bulk of the ANS. The geochronologic and isotopic data reveal that some ANS sedimentary, volcanic, and igneous rocks contain pre-Neoproterozoic material as xenocrysts and possible glaciogenic clasts, but give no evidence that the ANS is otherwise underlain by any extensive tracts of older crust. Together with structural mapping, the expanding dataset confirms ANS boundaries where long-inferred in the west and suggests the placement of hitherto uncertain boundaries elsewhere. The suggested boundaries imply that the ANS is of a somewhat smaller size than considered in published discussions, which in turn impacts assessment of Neoproterozoic crustal growth rate. The Arabian-Nubian Shield (ANS) is an accretionary orogen at the northern end (present-day coordinates) of the East African Orogen. It is one of the largest expanses of mantle-derived, juvenile Neoproterozoic crust on Earth, underlying an area of ~2.7 x 10 km. Its western boundary is a contact with pre-Neoproterozoic crust belonging to the Saharan Metacraton and Congo-Tanzania Craton. Farther south, the ANS is in contact with the Congo-Tanzania Craton and Mozambique Belt. A southeastern ANS margin is recognized as a contact with pre-Neoproterozoic gneiss belonging to a crustal block referred to as Azania. An eastern ANS margin, long a topic of debate, is suspected beneath Phanerozoic cover in central Arabia, and a northwestern margin with late Mesoproterozoic crust is suspected in Sinai.

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An Expanding Arabian-Nubian Shield Geochronologic and Isotopic Dataset: Defining Limits and Confirming the Tectonic Setting of a Neoproterozoic Accretionary Orogen

Neoproterozoic arc–back–arc system analog to modern arc–back–arc systems: evidence from tholeiite–boninite association, serpentinite mudflows and across-arc geochemical trends in Eritrea, southern Arabian-Nubian shield

The Nubian Shield is a large region of juvenile Neoproterozoic rocks that, together with its counterpart in the Arabian Shield, is part of a major accretionary orogen. It originated as late Tonian-Cryogenian island arcs on the site of the Mozambique Ocean formed by breakup of Rodinia. Arc collisions, subsequent magmatism, volcanism, sedimentation, and orogeny, associated with Cryogenian-Ediacaran convergence of cratonic blocks during the assembly of Gondwana, converted the region into the East African Orogen. The Nubian Shield region also has important Paleozoic-Neogene strata, including significant flood basalt, that conceal large areas of the basement. The Shield contains hundreds of gold occurrences and evidence of a 5,500-year history of gold mining. The main deposit types are orogenic gold and gold associated with volcanogenic massive sulfides (VMS). The juvenile subduction-related origin of the Shield rocks and the pervasiveness of shearing and greenschist-to-amphibolite facies metamorphism associated with late Neoproterozoic orogeny are geologic features highly favorable for the development of these types of deposits, and make the combined Arabian-Nubian Shield the Earth’s largest Neoproterozoic gold resource. Gold-bearing VMS deposits are currently mined at Bisha (Eritrea) and are soon to be mined at Hassai (Sudan). Orogenic gold is mined at Sukari and Hamash (Egypt), Qbgbih and Kamoeb (Sudan), Koka (Eritrea), and Lega Dembi and Sakaro (Ethiopia), and mining is due to start at Gupo (Eritrea) and Tulu Kapi (Ethiopia) in the near future. More than twenty companies are actively exploring for gold in the region and potentially important deposits are known in Egypt (Hamama-VMS), in northern Sudan (orogenic gold and VMS), along strike from Bisha and in the Asmara area, Eritrea (VMS and orogenic gold), in northern Ethiopia (VMS), and in western and southern Ethiopia (orogenic gold and sparse VMS).

Gold-bearing volcanogenic massive sulfides and orogenic-gold deposits in the Nubian Shield

Publisher Summary This chapter reviews the field and geochemical characteristics of recently recognized Precambrian boninites, adakites, MA, NEB, and related arc volcanic rocks. Boninites of Phanerozoic age occur in ophiolites or intra-oceanic island arcs, such as the Izu-Bonin-Mariana arc system. These primary liquids are interpreted as second-stage high-temperature, low-pressuremelting of a depleted refractorymantle wedge fertilized by fluids and/or melts, above a subduction zone. Recent geochemical investigations of Precambrian orogenic belts have documented a great compositional diversity in subduction-related volcanic rocks. The association of adakite, MA, and NEB flows with tholeiitic to calc-alkaline basaltsandesites in intra-oceanic arcs has recently been recognized in two 2.7 Ga greenstone belts of the Superior Province. The original stratigraphic relations between mafic and ultramafic units have been disrupted, and no primary volcanic textures such as spinifex have so far been recognized in ultramafic units. The sedimentary units consist mainly of banded iron formations, cherts, siliciclastic turbidites, and conglomerates.

J. Watson

Papers

Geochemistry and Geochronology of a Neoproterozoic Low-K Tholeiite-Boninite Association in Central Eritrea