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Banded iron formations (BIF) in Um Nar, central eastern desert of Egypt, occur intercalated with schists of volcaniclastic and epiclastic origins within "ophiolitic-island arc rocks" of the Arabo-Nubian Shield. The BIF and its host rocks were affected by folding, thrusting, and regional metamorphism during the Pan-African Orogeny resulting in the development of north-verging overturned folds and E-W striking, S-dipping thrusts. Following the intrusion of granitoids, the entire sequence was refolded into south-plunging folds with NW-SE trending fold axes. Peak mineral assemblages of hornblende + plagioclase, and garnet + biotite + plagioclase + quartz in the host rocks, and andradite-rich garnet + epidote + hematite + magnetite + quartz in the BIF indicate metamorphism under epidote amphibolite facies conditions. Using the multiequilibrium approach of Thermocalc, and conventional thermobarometry, peak P-T conditions of metamorphism are estimated at 520 +/- 30 degrees C, 5 +/- 2 kbar. Fluids attending peak conditions in the oxide facies layers of the BIF were characterized by XCO2 similar to 0.03 and log fO(2) similar to -40. Textural and mineral chemical criteria suggest that, following peak conditions, the rocks underwent a stage of near-isobaric cooling or cooling and compression characteristic of a counter-clockwise P-T path.

These results are consistent with a model in which the BIFs formed by hydrothermal activity from off-axis submarine vents in several pulses during a protracted event of oceanic crust generation in an inter-arc basin. Concomitant arc volcanism supplied the basin with pyroclastic material that imposed suboxic conditions on the basin leading to increased concentrations of soluble Fe2+. During periods of arc quiescence, Fe2+ was likely oxidized, leading to deposition of several layers of BIF. During the Pan-African Orogeny, the BIFs, tuffs, oceanic crust and lithospheric mantle were deformed and metamorphosed while being emplaced onto the continental margin. (C) 2014 Elsevier B.V. All rights reserved.