Banded iron formations of Um Nar, Eastern Desert of Egypt: P–T–X conditions of metamorphism and tectonic implications

Abstract 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 °C, 5 ± 2 kbar. Fluids attending peak conditions in the oxide facies layers of the BIF were characterized by XCO2 ~ 0.03 and log fO2 ~ − 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.