Stratigraphy, petrology, and geochemistry of a neoproterozoic banded iron sequence in the El-Dabbah Group, central Eastern Desert, Egypt

Abstract

The El-Dabbah Group of the Nubian Shield, Central Eastern Desert, Egypt, contains a Neoproterozoic banded iron sequence within volcaniclastic rocks deposited in an island arc setting. The group contains a volcaniclastic sequence of relatively undeformed lower-greenschist-facies rocks that are unconformably overlain by terrestrial sedimentary strata of the Hamamat Group. The group is > 7000 m thick and contains three formations (the Lower, Middle, and Upper El-Dabbah formations), which include massive metavolcanic rocks, pillow lavas, well-bedded volcaniclastic rocks, black and greenish shale and banded iron sequences. There is no evidence of glaciation, such as diamictites or cap-carbonate beds, in the three formations. The Middle El-Dabbah Formation (2000 m thick) contains well-preserved iron sequences within the volcaniclastic rocks. Most of the iron sequences comprise beds that are a few meters thick. The iron sequence rocks have low contents of Al, Ti, K, and Na, and low Al/(Al + Fe + Mn + Na + K + Ca) ratios, which suggest there was no continental input. Heavy rare earth element enrichment and the absence of Eu anomalies indicate these rocks formed from low-temperature hydrothermal fluids or in a distal setting from a hydrothermal vent system. δ13C values of the black shales are ca. −22‰, suggesting the organic matter was derived from cyanobacterial activity. Given that several black shale units were precipitated below the iron sequences, the cyanobacterial activity might have produced the oxygen at the ocean surface that led to iron oxidation and deposition. Based on the stratigraphic and geochemical characteristics of the El-Dabbah Group, we propose the iron sequences were deposited in a small isolated basin within the central rift zone of an oceanic island arc. The relationship between the El-Dabbah Group and Sturtian glaciation remains unclear.