Provenance of the Late Ediacaran Rocha Formation, Cuchilla Dionisio Terrane, Uruguay: Tectonic implications on the assembly of Gondwana

Abstract

A provenance study on the Rocha Formation (Cuchilla Dionisio Terrane, Uruguay) based on elemental and isotope geochemistry and geochronology revealed characteristics typical of turbidites linked to orogenic arcs: textural and mineralogical immaturity, and low weathering and sedimentary recycling indicated by low CIA values, Th/U and Zr/Sc ratios. We separate data into two groups, namely northern and southern sectors. Even though the whole unit shows a provenance of upper continental crust denoted by Th/Sc, Zr/Sc, Th/U ratios around UCC averages, REE patterns parallel to PAAS and negative Eu-anomalies, a more depleted component was detected mainly in the northern sector (NS) showing lower Th/Sc and Th/U ratios, negative Eu- anomalies of up to 0.9, high abundances of compatible elements, and lower εNd (t) and 87Sr/86Sr ratios than the southern sector (SS). U-Pb detrital zircon ages indicate 569 ± 10 Ma as the maximum sedimentation age. The most prominent age peak is Neoproterozoic, and while the NS shows dominance of Tonian and a broad data spread for Paleoproterozoic ages, the southern sector comprises both the youngest and oldest zircon and displays a pattern with a more pronounced Stenian population whereas Paleoproterozoic ages are restrained to Orosirian. A K-S test indicates that the change in provenance shown by detrital zircon patterns is reflecting variations of the sediment supply linked to basinal geomorphic heterogeneities within an active margin. Given detailed whole-rock lithogeochemistry, Nd, Pb and Sr isotopic analyses as well as detrital zircon age patterns, correlations with the Oranjemund Group of the Marmora Terrane and with the Tygerberg Formation and associated units of the Malmesbury Terrane (Western Saldania Belt), raised from previous interpretations, are reassessed. Source areas were most likely located within the Cuchilla Dionisio Terrane and Kalahari Craton, and therefore deposition within a foredeep basin occurred before tangential accretion to the Río de la Plata Craton along the Sierra Ballena Shear Zone.