Ediacaran banded iron formations and carbonates of the Cachoeirinha Group of NE Brazil: Paleoenvironment and paleoredox conditions
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The Ediacaran Santana dos Garrotes Formation Cachoeirinha Group of the Transversal Zone Domain (TZD) in the Borborema Province, NE Brazil, comprises a metaturbiditic sequence with carbonates, banded iron formations (BIF), and metavolcanic intercalations associated with a marine basin. Rare earth element patterns allow the identification of chemogenic and detrital (terrigenous) components in the BIF. The associated carbonates exhibit a significant terrigenous component and show δ13C values between +3.1 and + 5.5‰. As the U–Pb maximum depositional ages indicate that the carbonates are from the early Ediacarian, the 0.70792 to 0.70927 87Sr/86Sr ratios are considered near-primary. δ53Cr of pure, chemogenic BIF and one carbonate sample display positive fractionation (+0.15 to +1.04‰), indicative of oxidative weathering in the source area. The ironstones and carbonates with significant detrital fractions have negative, unfractionated δ53Cr within the range of high-temperature igneous reservoirs. Whereas the redox diagnostic Ce/Ce* values for Pedreira carbonates are dominantly negative, Oití BIF yielded positive Ce anomalies. These values suggest precipitation from an oxygenated water column for the carbonates and the accumulation of BIF under oxygen-deficient conditions in a deeper environment just below the redoxcline. The above interpretations are reinforced by the enrichment of the redox-sensitive trace elements Cr, U and Mo, which are not enriched in the Pedreira carbonates, and are little enriched in Oití BIF with ratios of Mo and U enrichment factors (MoEF/UEF) up to 6.8. Oití carbonates display no Ce anomaly and varied Mo enrichment, from low (<6) to anomalously high (>13 up to 59) MoEF/UEF ratios. They were probably deposited in a shallow environment with episodic euxinic (sulfidic) conditions. As for the BIF, a lack of glacial features, association with carbonates, lack of a positive Eu anomaly, and positive δ53Cr values allow to assign them to the Superior type, similar to other Neoproterozoic iron formations in Brazil and elsewhere.
|Journal||Journal of South American Earth Sciences|
|Number of pages||6|
|Publication status||Published - 1 Aug 2021|