A NON-GLACIOGENIC ORIGIN FOR NEOPROTEROZOIC BANDED IRON FORMATIONS FROM EGYPT
CL images of 44 zircons from 7 samples from Um Nar, G Hadeed, Um Ghamis, and Wadi Kareim BIFs show complex oscillatory zoning with some hydrothermal/ metamorphic overgrowths. LA-ICP-MS isotopic analyses of 36 crystals from tuffaceous metasediments from Um Nar and Wadi Kareim yield several age populations of 1.1 – 2.2 Ga, 820 – 850 Ma, 721 ± 35 Ma, 630 Ma, 600 Ma, and 550 Ma, and a distinct age population of 670 – 696 Ma for some rim overgrowths. Two acidic intrusives yield ages of 645 ± 7 and 606 ± 24 Ma. These results suggest that the BIFs formed by hydrothermal activity from off axis submarine vents in several pulses during a protracted event (850 – 700 Ma) of oceanic crust generation in an inter-arc basin. Concomitant arc volcanism supplied the basin with pyroclastic material that included zircon xenocrysts from an Archean crust. This debris imposed suboxic conditions on the basin by inhibiting growth of photosynthetic organisms, leading to increased concentrations of soluble Fe2+. Recovery of biota during periods of arc quiescence led to oxidation of Fe2+ and deposition of several layers of BIF. Hydrothermal alteration related to serpentinzation of lithospheric mantle rocks likely occurred at ~ 690 Ma, resulting in Ca-metasomatism and/or Fe-enrichment of crustal rocks. During the Pan-African orogeny, the BIFs, tuffs and oceanic lithosphere were thrust northward onto the continental margin as ophiolitic material and were regionally metamorphosed. Intrusion of syntectonic granites and gabbros led to localized contact metamorphism and more hydrothermal alteration ~ 640 – 550 Ma.