∼750 Ma banded iron formation from the Arabian-Nubian Shield—Implications for understanding neoproterozoic tectonics, volcanism, and climate change

Abstract

Neoproterozoic Banded Iron Formation (BIF) from Sawawin, NW Saudi Arabia and the Central Eastern Desert of Egypt define the 200×100km Arabian-Nubian Shield (ANS) BIF basin. ANS BIF formed ∼750Ma, prior to the Sturtian glacial episode (which began ∼716Ma). BIF deposition occurred in a marine basin associated with arc/backarc basin volcanism and immature clastic sedimentation. Beds are composed of alternating iron- and silica-rich laminae, which may reflect seasonal changes in deposition of Fe vs. Si. Fe-rich layers are dominantly composed of primary fine-grained hematite “dust” and minor apatite, with abundant secondary magnetite. Rapid deposition is revealed by: (1) major and trace element data indicating that ANS-BIF are very pure (<20% detrital input) chemical sediments in spite of being deposited in a basin with high sedimentation rates, and (2) pervasive evidence for soft-sediment deformation, suggesting that rapid sedimentation of dense, weak materials resulted in slumping. Nd and Pb isotopic compositions are predominantly mantle-like, indicating the dominance of hydrothermal sources or weathering of juvenile ANS crust for these elements. REE data show HREE-enriched patterns typical of modern seawater, with small positive Eu and small negative Ce anomalies. Low abundances of transition elements that are commonly abundant in proximal hydrothermal deposits of modern oceans may indicate that ANS-BIF formed at some distance from hydrothermal vents, or may reflect prior sulfide scavenging by euxinic and sulfidic deep ocean waters. REE data and Zn/Co share characteristics of both modern seawater and hydrothermal vent fluids suggesting derivation from a mixture of shallow suboxic seawater with a dilute, low-T hydrothermal vent fluid. Considered in conjunction with BIF of similar age on other paleocontinents, these observations support that rapid and widespread re-oxygenation of Fe+2 in previously anoxic or suboxic seawater led to rapid precipitation of hematite “dust” and BIF deposition ∼750Ma. Sulfate limitation or diminished bacterial sulfate reduction, required to form BIF instead of pyrite, may reflect large-scale glaciation but evidence for deep ferruginous conditions prior to Cryogenian glaciations suggests than any scenario that substantially reduced continental weathering (i.e., hard snowball or slushball) could have primed the oceans for BIF deposition. The likely short duration of ANS BIF deposition (a few to 10s of kyr) and apparent timing well before the “Sturtian” glaciation suggest that conditions favoring Neoproterozoic BIF formation could have existed over an extended period (10s of myrs) and that Neoproterozoic Oxidation Event began during the “Kaigas-Sturtian” time frame.