Age and significance of Neoproterozoic diamictite and metavolcanic rocks in Egypt and Saudi Arabia.

摘要

Neoproterozoic diamictite and metavolcanic rocks along Central Eastern Desert (CED) in Egypt and NW Saudi Arabia associated with banded iron formations (BIFs) provide new insight into the ∼750 Ma crust forming event and record important evidence for Cryogenian "Snowball Earth" in the Arabian-Nubian Shield. Stratigraphic relations and U-Pb zircon ages suggest that the Atud/Nuwaybah diamictite deposition occurred ∼750 Ma ago or slightly later. The distribution of zircon ages characterized by minor Mesoproterozoic and more abundant Paleoproterozoic and Neoarchean ages suggested that the diamictite clasts have been transported hundreds of kilometers, may be by ice-rafting. New geochemical, isotopic, geochronological data and stratigraphic relationship from CED metavolcanics and the Ghawjah formation NW Saudi Arabia indicate that these are tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed to greenschist facies. Tectonic discrimination diagrams show affinities with modern convergent margin magma, suggesting that the metavolcanics formed in an arc/back-arc basin system. Nd isotopic compositions indicate that the crust is juvenile and was extracted from a depleted mantle source, however abundant pre-Neoproterozoic zircons suggest that the juvenile magmas assimilated older continental materials or sediments. Combined field investigation, petrographic study, geochemistry and geochronology results from the diamictite and metavolcanics suggesting that associated BIFs formed in an active tectonic basin may be during glaciation.