It is thought that continental crust existed as early as 150 million years after planetary accretion, but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records. For example, the striking global peaks in juvenile igneous activity 2.7, 1.9 and 1.2 Gyr ago imply rapid crustal generation in response to the emplacement of mantle 'super-plumes', rather than by the continuous process of subduction. Yet uncertainties persist over whether these age peaks are artefacts of selective preservation, and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks. Detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle. These 'model' ages are only meaningful if the host magma lacked a mixed or sedimentary source component, but the latter can be diagnosed by oxygen isotopes, which are strongly fractionated by rock-hydrosphere interactions. Here we report the first study that integrates hafnium and oxygen isotopes, all measured in situ on the same, precisely dated detrital zircon grains. The data reveal that crust generation in part of Gondwana was limited to major pulses at 1.9 and 3.3 Gyr ago, and that the zircons crystallized during repeated reworking of crust formed at these times. The implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts.

Episodic growth of the Gondwana supercontinent from hafnium and oxygen isotopes in zircon

The Saharan metacraton

Late Cryogenian-Ediacaran history of the Arabian-Nubian Shield: A review of depositional, plutonic, structural, and tectonic events in the closing stages of the northern East African Orogen

https://www.utdallas.edu/~rjstern/pdfs/crustalevolution2002.pdf

Crustal evolution in the East African Orogen: a neodymium isotopic perspective

Abstract The Arabian-Nubian Shield froms the suture between East and West Gondwana at the northern end of the East African Orogen (EAO). The older components of the shield include Archaean and Palaeoproterozoic continental crust, and Neoproterozoic (c.870–670Ma) continental-marginal and juvenile intraoceanic magmatic-arc terranes that accumulated in an oceanic environment referred to as the Mozambique Ocean. Subduction, starting c. 870 Ma, and initial arc-arc convergence and terrane suturing at c. 780 Ma, marked the beginning of ocean-basin closure and Gondwana assembly. Terrane amalgamation continued until c. 600 Ma, resulting in the juxtaposition of East and West Gondwana across the deformed rocks of the shield, and final assembly of Gondwana was achieved by c. 550 Ma following overlapping periods of basin formation, rifting, compression, strike-slip faulting, and the creation of gneiss domes in association with extension and/or thrusting. Most post-amalgamation basin contain molasse deposits, but those in the eastern Arabian Shield and Oman have marine to glaciomarine deposits, which indicate that seaways penetrated the orogen soon after orogeny. The varied character of the post-amalgamation events militate against any simple tectonic model of final Gondwana convergence at the northern end of the EAO, and requires that models accommodate alternating periods of Late Neoproterozoic extension and shortening, uplift and depression, deposition and erosion.

Development of the Arabian-Nubian Shield: perspectives on accretion and deformation in the northern East African Orogen and the assembly of Gondwana

Serpentinite masses in late Precambrian sediments of the eastern desert of Egypt are in a melange with associated metagabbros and pillow lavas; they represent pieces of Precambrian oceanic crust. The melange is of regional extent and is interpreted as a olistostrome which has been thrust northwestwards over shelf sediments and is overlain by island arc volcanics: thus the observed sequence is a structural rather than a stratigraphic one. A plate tectonic model is implied with a suture to the south-east. The interpretation of the late Precambrian (Pan-African) fold belts in Africa is controversial. It is uncertain whether, like Phanerozoic fold belts, they were formed by plate tectonic processes1–5, or whether they are ensialic and unrelated to subduction of oceanic crust6,7 or collision of continental plates. Here we present evidence relating to this problem from the part of the Pan-African belt exposed in the eastern desert of Egypt.

Late Precambrian ophiolitic mélange in the eastern desert of Egypt

The relationships between the various crustal provinces that comprise north-east and east Africa are far from clear and reflect diverse tectonic and magmatic processes which took place during the Proterozoic. Here we present the results of an Nd isotope study on the major rock units of the Pan-African (1,100–500 Myr BP) terrane. Charnockites from Jabel Uweinat, a basement inlier at the junction of Egypt, Libya and the Sudan, yield middle Archaean model Nd ages, whilst model ages of <1,200 Myr have been obtained in a belt from the Eastern Desert of Egypt to north-west Kenya. This, therefore, represents a westward extension of the juvenile Pan-African crust of the Arabian Shield (Fig. 1). The shield itself is characterized by comparatively rapid crustal growth1, probably in an accreting island arc environment2. Between the two areas, there is a marginal zone of Pan-African rocks with relatively low ɛNd(T) values (see Table 1) due to increased contributions from the pre-Pan African crust. Overall, the Pan-African rocks from north-east and east Africa and those from the Damara of Namibia3 exhibit a wide range of ɛNd(T) from +7.5 to −18.0 which reflects regional changes in tectonic style and is not readily reconciled with simple models for the evolution of average continental crust.

Crustal evolution in north-east and east Africa from model Nd ages

The Late Proterozoic ophiolite of Sol Hamed, NE Sudan

Geochronology, geochemistry and tectonics of the NE Bayuda Desert, N Sudan: Implications for the western margin of the late Proterozoic fold belt of NE Africa

A. C. Ries

Papers

Late Precambrian ophiolitic mélange in the eastern desert of Egypt

Crustal evolution in north-east and east Africa from model Nd ages

The Late Proterozoic ophiolite of Sol Hamed, NE Sudan

Geochronology, geochemistry and tectonics of the NE Bayuda Desert, N Sudan: Implications for the western margin of the late Proterozoic fold belt of NE Africa