Rare Earth Element (REE) Geochemistry of the Cretaceous Tizkuh Formation Limestones, Northwestern Iran: Mineralogical Controls and Paleoenvironmental Implications

The limestones of the Tizkuh Formation, deposited along the southern margin of the Neo-Tethys Ocean during the Cretaceous, provide an opportunity to investigate REE behavior, depositional processes, and the extent to which primary marine geochemical features may have been retained despite post-depositional modification. In this study, integrated mineralogical (XRD and SEM–EDS) and geochemical (ICP–AES and ICP–MS) data from 15 limestone samples were used to evaluate the controls on major- and trace-element and REE distributions and to assess the preservation of selected marine and redox-related features. The limestones are dominated by calcite, with subordinate kaolinite, quartz, dolomite, illite, hematite, goethite, plagioclase, and locally identified apatite. Negative correlations of CaO with SiO2, Al2O3, TiO2, Ga, Nb, Zr, Ta, and Th indicate that variations in the relative proportions of carbonate and non-carbonate materials were an important control on whole-rock geochemical variability. Positive associations among Ti, Zr, Nb, Ta, and Ga further support a contribution from siliciclastic materials to the non-carbonate fraction. Total REE concentrations (ΣREE = 31.54–62.18 ppm) are within the range of those commonly reported for marine carbonate rocks and do not indicate anomalous bulk-rock REE enrichment. The positive correlations of ΣREE with Al2O3, SiO2, Th, Zr, Y, and P2O5 denote that both siliciclastic and phosphate-bearing components contributed to the bulk-rock REE inventory. The positive relationship between ΣREE and Al2O3 (r = 0.55), together with the broadly similar Al-normalized REE patterns among the studied samples, signifies that variations in absolute REE abundances were partly associated with differences in relative proportions of the carbonate and non-carbonate components. However, the consistency of the Al-normalized patterns suggests that the overall REE fractionation pattern cannot be attributed solely to variations in terrigenous dilution. The SEM–EDS data support the occurrence of apatite in the selected samples. The PAAS-normalized REE patterns show limited LREE enrichment relative to HREE, indicating that the REE system does not represent an unmodified seawater signature and was influenced by the non-carbonate components and post-depositional processes. Weak negative Ce anomalies (Ce/Ce* = 0.76–0.94), together with Pr/Pr* values close to unity and relatively low U/Th (0.04–0.11) and V/Cr (0.77–1.90) ratios, are compatible with relatively oxidizing marine depositional conditions. However, the preservation of a primary seawater Ce signal remains tentative because Ce/Ce* is associated with the non-carbonate and REE-bearing components, while U and V may be affected by diagenetic redistribution and other post-depositional processes. Positive Eu anomalies (Eu/Eu* = 1.44–1.98) are interpreted cautiously and may reflect a combination of mineralogical controls, limited detrital feldspar contribution, and possible early diagenetic and/or fluid–rock interaction. Overall, the Tizkuh limestones represent a mixed and partially modified carbonate geochemical archive in which selected depositional and redox-related features may have been retained. Consequently, reconstruction of the Cretaceous paleoenvironment requires a multi-proxy approach integrating REE patterns, Ce and Eu anomalies, Y/Ho ratios, U/Th, V/Cr, detrital and phosphate indicators, Al-normalized REE patterns, geochemical relationships, and mineralogical evidence.