TECTONIC AND PALEOGEOGRAPHIC RELATIONSHIPS OF PRECORDILLERA OF WESTERN ARGENTINA DETERMINED FROM U/PB GEOCHRONOLOGY OF DETRITAL ZIRCONS FROM UPPER ORDOVICIAN SANDSTONES

With its Lower Cambrian to Middle Ordovician carbonate platform succession and the biogeographic affinities of its benthic faunas, the Argentine Precordillera, part of the larger Cuyania terrane, is considered to have rifted from Laurentia in the early Cambrian, drifted as a microcontinent across the Iapetus Ocean, and collided with the proto-Andean margin of Gondwana in the Mid Ordovician. Lower Upper Ordovician strata deposited above the carbonate succession include coarse siliciclastic sediment, the provenance of which is considered to be the Famatinian magmatic belt that developed in response to the convergence of the Cuyania terrane and the subduction of intervening lithosphere. U/Pb geochronology of 328 detrital zircon grains from four samples reveals that ages coeval with Famatinian magmatism are virtually absent from the lower Upper Ordovician strata. Instead, the samples are characterized by unimodal populations of Mesoproterozic age that are coeval with the Rondonian-San Ignácio and Sunsás geochronological provinces within the Amazonian craton, a likely candidate for their ultimate provenance. In addition, the sandstone beds are composed of first-cycle immature sands. Accordingly, we conclude that sediment was dispersed directly from the cratonic source area to the depositional basins without sediment input from younger magmatic belts. This relationship is incompatible with the Laurentian microcontinent model for which, during early Late Ordovician time, the Cuyania terrane was separated from Proterozoic cratonic source areas by the early to mid Ordovician Famatinian and early Cambrian Pampean magmatic belts. Instead, the Cuyania terrane must have occupied a very different position on the margin of Gondwana and migrated to its present position adjacent to, and outboard of, the Famatinian belt after Ordovician time. Accordingly, paleogeographic and tectonic interpretations based largely on paleobiogeograpic evidence must be seriously re-evaluated.