Neoproterozoic formation and evolution of Eastern Desert continental crust – The importance of the infrastructure-superstructure transition

Abstract

Neoproterozoic crust is beautifully exposed in the Eastern Desert (ED) of Egypt as an unbroken rift-flank uplift extending ∼800 km between the Nile and the Red Sea, from near Cairo to the Sudan border. Different but related Neoproterozoic rocks are exposed in the northern, central and south Eastern Desert (NED, CED, and SED). Teleseismic data from four ED stations suggest that the crust is thin (28–33 km) and contains a mafic underplate ∼5 km thick at its base. The CED is the best known of the three blocks because of low relief, good roads, and mineral deposits. The CED is mostly composed of Late Tonian-Cryogenian (∼700–750 Ma) superstructure, of largely ensimatic character: disrupted ophiolitic crust with associated immature clastic sediments, diamictite, banded iron formation, and arc sequences. CED superstructure was deformed, metamorphosed to greenschist facies and especially ultramafic rocks pervasively altered by interaction with CO2 –rich fluids of mostly mantle origin. Several domes bring deeper rocks of the gneissic infrastructure to the surface; these generally give younger radiometric ages than superstructure rocks, demonstrating that the infrastructure was hot, partially molten, and weak while the superstructure deformed above it. Special emphasis is given to a subhorizontal shear zone termed the Eastern Desert Decollement (EDD), localized at the boundary between brittle, greenschist-facies superstructure and ductile, partially molten infrastructure. The EDD was active during Najd strike-slip deformation ∼600 Ma, but the infrastructure began forming in the south ∼710 Ma, younging northward to the Ediacaran magmatic culmination in the NED. Melting of the infrastructure reflected magmatic underplating, now apparent as a 5 km –thick mafic lower crust. Fluid infiltration accompanying superstructure deformation and infrastructure magmatism were responsible for generating most of ED gold deposits. The SED is broadly similar to the CED but is poorly known. The NED is very different than the CED or SED and is almost entirely an Ediacaran magmatic construction, although minor relicts of Late Tonian –Cryogenian superstructure have been identified. Some suggestions for future studies are provided.