Geoquímica de tierras raras y fosfogénesis en un engolfamiento marino del Cretácico Tardío-Paleoceno de Patagonia, Provincia del Chubut, Argentina
Major and rare earth elements (REE) in authigenic phosphatic concretions of the siliciclastic Lefipán Formation (Late Cretaceous – Paleocene of Chubut Province, Patagonia, Argentina) were analyzed. This geochemical study aimed to reveal the processes that led to the formation of the concretions under the particular paleoenvironmental and diagenetic conditions in the sediments. Statistical principal component analysis (PCA) was applied to the compositional data of the concretions. Two groups of elements with high internal correlation were classified from the PCA analysis: a first group associated with the clastic detritic components (SiO2, Al2O3 y TiO2), and a second group associated with the authigenic components (P2O5 y CaO). K and Mn plotted out of both groups. The abundance of K-rich authigenic glauconite and the high mobility of Mn justify this geochemical behavior. REE abundances, normalized to Post-Archean Australian Shale (PAAS) display slightly convex patterns, consistent with an estuarine coastal marine environment, and also with early diagenetic processes linked to high concentrations of dissolved Fe in pore water. The abundance of this metal in pore waters resulted in ferruginous cements and coated clasts in rocks of the Lefipán Formation, and was probably caused by intense chemical weathering on the continent under humid, temperate to warm climate. Zoned concretions show higher REE content in the outer layer and a correlation between total REE content and medium REE enrichment are also observed. This process is linked to post-depositional changes which led to incorporation and partition of REE from circulating fluids during the early diagenesis. Fluctuation in the Ce anomaly reveals variable oxygenation and salinity conditions. Minor changes in Y and La anomalies are the result of the early diagenesis.
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