PROVENANCE OF THE MIDDLE–LATE ORDOVICIAN CLASTIC WEDGE IN THE ARGENTINE PRECORDILLERA

Middle–Late Ordovician subduction of the Argentine Precordillera beneath western Gondwana is indicated by: Famatina continental-margin arc on western Gondwana; thermochronology and top-to-west shear in massifs along the eastern Precordillera; down-to-east subsidence of the Precordillera carbonate platform, and westward progression of a synorogenic clastic wedge primarily from sources east of the Precordillera. U-Pb analyses of detrital zircons from 10 clastic-wedge sandstones span a vertical succession (Middle–Late Ordovician) and a cross-strike transect from east (proximal) to west (distal). Results include: (1) ages of 980–1200 Ma dominate all samples, (2) secondary abundances 1200–1500 Ma characterize most samples, and (3) scattered ages in the ranges of 515–940, 1650–1920, and 1970–2180 Ma are common. Clasts from the Las Vacas Conglomerate include: abundant igneous rocks (crystallization ages of 614–647 Ma with xenocrysts of 1000–1200 Ma); white quartzite and red sandstone (similar to the detrital populations); and very rare gneiss (distinctive peaks at 550–700 and 1750–1810 Ma with scattered, rare ages at 770–850, 1010–1320, and 2000–2277 Ma).

Late Mesoproterozoic (1000–1200 Ma) ages are documented in basement of the Precordillera and of the Sierra de Pie de Palo and other massifs east of the Precordillera, as well as older (~1370 Ma) basement clasts in the Precordillera. More importantly, quartzite in Pie de Palo has detrital-zircon age peaks at 1000–1200 and 1300–1450 Ma. The abundance of 1000–1200 Ma zircons in the clastic wedge is compatible with a source in the Pie de Palo and other massifs (basement and cover), which may have been incorporated into a complex accrectionary prism and backthrusted during subduction. The 1000–1500 Ma grains could be from a primary Sunsás or Rondonian source or from recycling of pre-Ordovician sedimentary cover on the upper plate; however, the relative abundance of these grains favors the basement and cover of Pie de Palo and other nearby massifs. Neoproterozoic ages (515–800 Ma) are compatible with upper-plate (e.g., Pampean and Brasiliano) rocks or, alternatively, Iapetan synrift igneous rocks, which have been recognized in Pie de Palo. The distinct Paleoproterozoic grains (2000–2200 Ma) suggest a primary source in Amazonia or Rio de La Plata basement or cover.