Sr, C and O isotope geochemistry and stratigraphy of Precambrian and lower Paleozoic carbonate sequences from the Western Sierras Pampeanas of Argentina: tectonic implications

Sr, C and O isotope data are presented for carbonate rocks from the Sierra de Pie de Palo and other crystalline outcrops of the Western Sierras Pampeanas. These are used to distinguish three groups of rocks of quite different ages within the nappe pile that constitutes the Sierra de Pie de Palo. Carbonates from the Grenville-age ophiolitic unit at the bottom of the pile probably resulted from the interaction between hot seawater and contemporaneous oceanic crust. These carbonates are tentatively classified as ophicalcites. Carbonates from the Difunta Correa Sedimentary Sequence are part of a cover sequence to a Grenvillian basement, together forming the upper nappes. Isotope stratigraphy suggests a middle to Late Neoproterozoic age (580–720 Ma) for these carbonates. This cover might be correlated with cover sequences to the Archean and Early to Middle Proterozoic cratons that are well preserved over large areas of Brazil. This fact, together with the apparently absence of an equivalent Neoproterozoic carbonate-bearing cover in the Appalachian margin of Laurentia, suggests that the Western Sierras Pampeanas, which are considered part of the exotic Argentine Precordillera terrane of allegedly Laurentian derivation, could be autochthonous or para-autochthonous to Gondwana. Other geochemical and geological arguments reinforce this hypothesis. The Caucete Group carbonates underlie the nappe pile and are separated from it by a first order thrust (the Pirquitas thrust). These latter carbonates are Cambrian in age and isotopically similar to the carbonate platform of the Precordillera.We thus conclude that the Pirquitas thrust is the boundary between the exotic Precordillera terrane and the autochthonous or para-autochthonousWestern Sierras Pampeanas.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu