Major, trace element, and Nd isotopic compositions of banded iron formation and shales from the Sirsi shelf, Dharwar Craton, India: Implications for paleo-seawater chemistry, post-depositional alteration, and provenance

•REE compositions distinguish between unaltered and altered BIF in Sirsi Shelf.•εNd (2.5Ga) of unaltered BIF overlap with contemporaneous global BIFs.•Negative Ce anomaly in the altered BIF represents post-depositional fluid overprint.•Alteration dated at ∼ 0.5 Ga, which coincides with the Pan-African orogeny.•Shales, affected by fluid alteration, derived from Goa Dharwar Sector granitoids. We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb)SN (0.37–0.53) and (Pr/Yb)SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*)SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb)SN (1.73–18.48) and (Pr/Yb)SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*)SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial εNd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(Xelement/XTi)sample/(Xelement/XTi)reference-1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-