Deciphering the geochemistry of two key Paleoproterozoic siliciclastic sequences of the Piedra Alta Terrane (Río de la Plata Craton, Uruguay)

Abstract

The geochemistry of two metavolcano-sedimentary sequences deposited in the Piedra Alta Terrane is compared, and their geotectonic evolution is discussed. The Ojosmín Unit (OU) comprises MORB-like basic rocks at the base and a fine-grained siliciclastic sequence interpreted as marine turbidites towards the top. The succession was later obducted during the Orosirian. Whole-rock geochemistry of the metasedimentary rocks of the OU indicates the lack of source rock alteration during deposition (Chemical Index of Alteration, CIA ca. 40-53), implying ice-house climatic conditions in correlation with the worldwide Rhyacian glaciations. Recycling of zircon fractionates the Rare Earth Elements (REE) increasing the amount of HREE, Y, and Hf. Variation ranges of Th/Sc (0.4-4.9), Zr/Sc (30-410), Th/U (2.5-4.3), and of the Eu/Eu * negative anomaly (0.4-0.7) approximate Upper Continental Crust (UCC) values. A new clastic metasedimentary, gently folded unit, the Cerro de la Figurita Formation (CFFm), is erected. The CFFm clastic sedimentation (3,000 m in thickness) represents a deepening upward sequence, starting with polymictic conglomerates deposited in an alluvial fan-dominated environment that evolves to marine turbidites. The CFFm is probably related to a foreland geotectonic setting developed during the Orosirian. The geochemistry of the CFFm reveals similarities to unrecycled UCC, and weathering of the source rocks increases up section (CIA 45-92). Low ratios of Th/Sc (0.3-1.5), Zr/Sc (6-20), Th/U (3-6), high Cr/V (1.1-12.2), and a less pronounced Eu/Eu* negative anomaly of certain samples (0.5-0.9) suggest a contribution from mafic source rocks (probably ophiolitic).