PROVENANCE OF A LATE PROTEROZOIC-EARLY CAMBRIAN BASIN, SIERRAS DE CÓRDOBA, ARGENTINA: DETRITUS FROM GONDWANA OR LAURENTIA?

Detrital metasediments of Cambrian or older age are the oldest rocks of the Pampean orogen in the Sierras de Córdoba. Previous studies have suggested that these pelitic to psammitic sediments were deposited marginal to the Rio de La Plata craton, but considerable uncertainty surrounds this assumption due to the fact that no Precambrian basement rocks are exposed in or near the Sierras de Córdoba or other Pampean ranges. Moreover, the strong tectonic and metamorphic overprints associated with the Pampean orogeny leave open the possibility that these rocks had a more exotic origin and were accreted during later tectonic events. We determined the ages of detrital zircons from the Sierras de Córdoba in order to gain insight into their provenance. The results indicate dominant zircon populations of Neoproterozoic (~600-700 Ma) and Middle Proterozoic (950-1050 Ma) age, and a limited presence of Paleoproterozoic (~1900 Ma) zircon.

We interpret the sources of the Neoproterozoic and Paleoproterozoic zircons to be Gondwanan, most probably derived from adjacent South America. The large Middle Proterozoic population, constituting more than half the analyzed grains, is more equivocal in origin. They could be derived from ~1.0 Ga rocks exposed in the Sunsas belt on the western margin of the Amazon craton and/or its rifted equivalents (e.g., Arequipa massif), or from the similarly aged Grenville province in Laurentia or its rifted equivalents (e.g., the Argentine Precordillera). The Pb isotopic composition of feldspars from Córdoba gneisses were determined to distinguish between these possibilities, as Gondwanan and Laurentian sources are known to have distinctive Pb isotopic signatures. The results fall within the ²⁰⁷Pb-rich Gondwana provinces and therefore favor a Gondwanan origin.

Based on these results, the high-grade metasediments of the Sierras de Córdoba share compositional and chronological similarities with the low-grade Puncoviscana Formation to the north. These similarities suggest that these two assemblages formed contemporaneously on the Gondwanan side of the Neoproterozoic rift basin associated with the break up of Rodinia.