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.

中文翻译：

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埃及中东部沙漠 El-Dabbah 群新元古代带状铁层序的地层学、岩石学和地球化学

摘要 埃及中东部沙漠努比亚地盾的 El-Dabbah 群在岛弧环境中沉积的火山碎屑岩中包含新元古代带状铁层序。该群包含相对未变形的低绿片岩相岩石的火山碎屑层序，这些岩石不整合地被哈马特群的陆地沉积地层覆盖。该群厚度 > 7000 m，包含 3 个地层（下、中和上 El-Dabbah 地层），其中包括块状变质火山岩、枕状熔岩、层状火山碎屑岩、黑色和绿色页岩和条带状铁层序。在三个地层中没有冰川作用的证据，例如混叠岩或碳酸盐盖层。中 El-Dabbah 组（2000 米厚）在火山碎屑岩中包含保存完好的铁层序。大多数铁层包括几米厚的层。铁层序岩的Al、Ti、K、Na含量低，Al/(Al+Fe+Mn+Na+K+Ca)比值低，表明没有大陆输入。重稀土元素富集和没有 Eu 异常表明这些岩石形成于低温热液流体或位于热液喷口系统的远端环境中。黑色页岩的 δ13C 值约为。−22‰，表明有机质来源于蓝藻活动。鉴于在铁序列下方沉淀了几个黑色页岩单元，蓝藻活动可能在海洋表面产生了氧气，导致铁氧化和沉积。根据 El-Dabbah Group 的地层和地球化学特征，我们建议铁层序沉积在大洋岛弧中央裂谷带内的一个小的孤立盆地中。El-Dabbah 群和 Sturtian 冰川之间的关系仍不清楚。