EARLY GONDWANIAN AFFINITY OF THE ARGENTINE PRECORDILLERA: EVIDENCE FROM U-PB GEOCHRONOLOGY OF DETRITAL ZIRCON POPULATIONS FROM CAMBRIAN AND ORDOVICIAN SANDSTONES

The widely accepted interpretation that the Argentine Precordillera terrane is of Laurentian (North American) affinity is based on several lines of evidence, one of which is that the basement of the Precordillera is Grenvillian-age with a Pb isotope ratio identical to that of Grenvillian-age rocks of the Llano uplift of Texas. Is this evidence still valid as a paleogeographic pinning point? Recent studies describe Mesoproterozoic basement rocks of Antarctica, the Natal, and the Falkland Islands with Pb isotope ratios like those of Grenvillian age rocks of the Precordillera and Laurentia, casting doubt on the uniqueness of this signature. In addition, recently obtained detrital zircon ages show little evidence of a Laurentian affinity for the Precordillera terrane. The early Cambrian Cerro Totora Formation exhibits age groups correlating with the Brasiliano/Pan-African (0.55-0.80 Ga) and Transamazonian/Birimian/Eburnian (2.00-2.25 Ga) orogenic belts. This age spectrum is characteristic of Gondwana, not Laurentia. In contrast, the lower Upper Ordovician Las Vacas Formation has an age population matching non-Laurentian, Grenvillian-age equivalent (1.0-1.5 Ga) rocks found throughout West Gondwana. Surprisingly, middle Ordovician-age zircons from the Famatina magmatic arc are completely lacking in the Las Vacas sample, suggesting post-Ordovician tectonic juxtaposition of the Precordillera terrane and the Famatina Arc. These data require critical re-evaluation of widely held assumptions regarding the paleogeography of the Argentine Precordillera.