Provenance of ordovician to silurian clastic rocks of the Argentinean precordillera and its geotectonic implications

D. Phil.A Mesoproterozoic basement and a Cambrian-Ordovician carbonate platform characterize the Precordillera terrane. These characteristics and its distinct geologic history mark a difference between this suspected exotic-to-Gondwana terrane and the Gondwanan autochthonous, leading to speculation that the Precordillera was derived from Laurentia. The surprising similarities of the carbonate sequences between the Precordillera and certain parts of southeast Laurentia suggest a common geological history. However, other models interpret the origin of the Precordillera terrane as being para-autochthonous with respect to Gondwana. All these models are still controversial. A combination of several methodologies including petrography and heavy minerals analysis, geochemistry, Sm-Nd and Pb-Pb isotopes and zircon dating were applied to several Ordovician and Ordovician to Silurian units of the Precordillera terrane. Geochemistry and petrography indicates that all the Formations studied have similar characteristics, with at least two sources providing detritus to the basin. The dominant source has an unrecycled upper continental crust composition whereas the other component is more depleted. The study of detrital chromian spinels suggests that mid-ocean ridge basalts, continental intraplate flood basalts and ocean island basaltrelated rocks were among the sources for the detrital record of the Precordillera terrane. Nevertheless, the mafic sources and their ages remain unknown. Nd isotopes account for negative εNd values and TDM ages in a range of variation found elsewhere within Gondwana and basement rocks of the Precordillera. The Sm/Nd ratios of certain samples indicate fractionation of LREE. Pb isotopes indicate that a source with high 207Pb/204Pb was important, and point to Gondwanan sources. Detrital zircon dating constrain the sources as being dominantly of Mesoproterozoic age (but with a main peak in the range 1.0 to 1.3 Ga), with less abundant populations of Neoproterozoic (with a main peak in the range 0.9 to 1.0 Ga), Palaeoproterozoic, Cambrian and Ordovician ages in order of abundance. i The uniformity shown by the provenance proxies indicate that there were no important changes in the provenance from the Lower Ordovician until the Early Silurian. Several areas are evaluated as sources for the Precordillera terrane. The rocks that fit best all the provenance constraints are found within the basement of the Precordillera terrane and the Western Pampeanas Ranges. Basement rocks from the Arequipa-Antofalla area (Central Andes) also match the isotopic characteristics, but a northern source is less probable, except for the Western tectofacies. On the other hand, areas such as Antarctica, Falklands/Malvinas Microplate, the Natal-Namaqua Metamorphic belt and the Grenville Province of Laurentia can be neglected as sources. The proposal of these areas as sources is in agreement with palaeocurrents and facies analyses and suggests proximity between them and the Precordillera since at least the Late Arenig to Early Llanvirn. This has important implications for the proposed models regarding the geotectonic evolution of the Precordillera terrane. The models would need to be adjusted to the here proposed youngest timing of collision