PETROGRAPHY AND RARE EARTH ELEMENT STUDIES OF NEOPROTEROZOIC BANDED IRON FORMATIONS - EVIDENCE FOR SNOWBALL EARTH IN THE ARABIAN NUBIAN SHIELD?

The largest tract of juvenile crust of Neoproterozoic age is found in the Arabian Nubian Shield (ANS). The presence of Banded Iron Formations (BIFs) in the northern ANS (NW Arabia and E Egypt) provides an opportunity to examine its relation to the Snowball Earth hypothesis. ANS-BIFs are oxide facies and are locally associated with diamictite of probable glaciogenic origin and submarine metavolcanics, a potential source of hydrothermal Fe. The iron rich layers are defined by the iron oxides such as hematite and magnetite whereas the silica-rich layers are composed of chert. Carbonates, aluminosilicates and apatite occur as accessory minerals. Twelve samples from Sawawin, NW Arabia and four samples from Egypt (Wadi Kareim and Wadi Dabbah) were analyzed for rare earth elements (REE). NASC-normalized plots of the REE in the Arabian BIF samples reveal positive Eu anomalies (Eu/Eu^* =1.02 to 1.33, mean = 1.13) characteristic of modern hydrothermal vent. Arabian BIFs also show a clear enrichment of HREE over LREE, characteristic of modern seawater ([La/Yb]_NASC = 0.12 to 0.44). With the exception of the Kareim sample, the Egyptian samples also show clear HREE enrichment over the LREE ([La/Yb]_NASC = 0.19 to 0.36) though none of them show positive Eu anomalies (Eu/Eu^* = 0.91 to 0.95, mean = 0.93). The REE patterns of the BIF in this study indicate that they were deposited following mixing between Neoproterozoic seawater and hydrothermal fluids. These data suggest that the Arabian BIF were deposited closer to the hydrothermal source and that the Egyptian deposits were more distant. The REE results are consistent with the Nd isotopes indicating hydrothermal origin for the iron. The εNd_700Ma value ranges from +2.4 to +5.4 for the Sawawin samples whereas the samples from the Egypt ranges from +2.8 to +5.7. The higher εNd value observed towards the top of the successions might indicate increased hydrothermal input or decrease of the crustal input from the neighboring areas due to more intense glacial conditions.