Syn-to post-depositional processes related to high grade metamorphic BIFs: Geochemical and geochronological evidences from a Paleo to Neoarchean (3.5-2.6 Ga) terrane in NE Brazil

In the Borborema Province, iron formations occur in Archean-Paleoproterozoic nuclei including the highly metamorphosed and deformed Granjeiro Complex. In this work, we analyze the BIFs of the Granjeiro Complex from the petrographic, geochemical, and isotopic viewpoints and discuss their depositional conditions. Also, we evaluate the post-depositional processes that may have suppressed their original geochemistry. Petrographic and major element analysis divides the BIFs into grunerite magnetite meta-BIF (Grunerite BIF), magnetite/hematite meta-BIF (Magnetite BIF), and garnet magnetite meta-BIF (Garnet BIF). REY and Nd isotope analysis exhibit a distinct pattern and display two groups, according to Sigma REY. Group I (low Sigma REY) shows REY ratios consistent with sediments precipitated from seawater, such positive anomalies of La-SN , LREE-MREE depletion, and superchondritic Y/Ho wt.-ratios. In contrast, Group II has parameters that are different from seawater, including negative or absent La-SN anomalies, LREE-MREE enrichment, and subchondritic Y/Ho, that suggest the influence of other REY sources and post-depositional processes. Both groups present negative Ce anomalies, with higher values on Group II. Considering the results of the Nd isotopes, the Group I presents negative eNd and higher TDM ages, while Group II has positive eNd and lower TDM ages. An isochron could be defined, but the age obtained (635 +/- 49 Ma) is probably metamorphic. Besides, five representative samples of the terrane where the BIFs are enclosed were dated using the LA-ICP-MS U-Pb zircon method. The resulting ages range 3535 +/- 14 Ma to 2384 +/- 35 Ma, with the amphibolite of the metavolcano-sedimentary sequence dating back to 2654 +/- 26 Ma. According to the results, it is possible to infer that the Granjeiro BIFs deposited in an environment with little contribution of terrigenous sediments attested by the low concentration of immobile elements. The depositional environment was probably distant from the continent, under the influence of high-temperature hydrothermal fluids. In contrast, the Garnet BIFs may represent sporadic terrigenous events or more proximity to the shore. Inconsistent Nd isotopes and REY data from the Group II rocks suggest the influence of processes after deposition. Our data show that, although REY and Nd isotopes are considered to be relatively immobile, metamorphism and hydrothermal alteration in areas of intense deformation can lead to remobilization. These should be analyzed before the interpretation of the depositional environment of the BIFs.