Sr isotopic composition and Pb-Pb age of Neoproterozoic-lower Paleozoic carbonate sequences at Salinas hill and Pie de Palo range, Western Argentina

Abstract

It is weil known that isotope stratigraphy is one of the most powerful tools in Upper Precambrian to Lower Paleozoic chronostratigraphy. The Sr isotope composition of seawater for this age interval is characterized by a contm. uous .mcrease 0f 87Sr/86Sr that I.S I. nterrupted, by sharp rises, which represent important changes in the Earth history. During Late Cambrian to Early Ordovician LUlle the 87Sr/86Sr seawater ratio is high and close to the modern value of 0.70907 (Kaufman et al., 1993; Veizer et al., 1997; Jacobsen & Kaufman, 1999; Montafiez et al., 2000). The Pb-Pb isotopic dating technique have been refined and successfully applied to a variety of geological problems (Jahn & Cuvellier, 1994, Babinski et al., 1999). We examine in this paper carbonate sequences from the Precordillera or Cuyania Terrane that are located at the Salinas Hill and Pie de Palo Range (Fig. 1), as a part of the "Angaco Bel!" (Vujovich & Ramos, 1994). Also for comparison, we use two samples from the fossiliferous :.uwer to Middle Cambrian carbonates in the Eastern Precordillera (La Laja Fm.). We have studied their 87Sr/86Sr composition and compare it to global Sr isotope secular variation curves and we present new Pb-Pb isotope analyses on carbonate samples from Salinas Hill. The aims of the study were to constrain the timing of deposition/diagenesis and of subsequent metamorphisrn/ deforrnation of the carbonate levels from the Caucete Group.