DETRITAL ZIRCON AGES IN NEOPROTEROZOIC SEDIMENTARY UNITS OF THE RÍO DE LA PLATA CRATON: UNRAVELLING ITS COMPLEX GEOLOGICAL EVOLUTION

Although the Río de la Plata craton is exposed over a large area in Uruguay, eastern Argentina, and southernmost Brazil, its composition is poorly known because of great geological complexity. We report here U-Pb ages of detrital zircons from samples of five Neoproterozoic sandstone units deposited on the Río de la Plata Craton (RPC) in Uruguay and Argentina. The zircon ages provide definitive evidence of a great range of ages of rocks composing the craton - ages that would not have been known without the geochronology of detrital zircons. In turn, these new ages allow for a much fuller understanding of the nature and history of the craton.

Quartz-arenites of the Piedras de Afilar Formation show typical Transamazonian ages, with peaks at 2.07, 1.87 and 1.78 Ga. However, the most important zircon population is Mesoproterozoic, showing maxima at 1.49, 1.35, 1.25 and 1.0 Ga. Archean zircons are subordinate. On the other hand, zircons recovered from two sandstone levels in the Arroyo del Soldado Group (Yerbal and Cerros San Francisco formations) are mostly Archean in age, with maxima at 3.2 and 2.7 Ga. Palaeoproterozoic zircons are also prominent in this unit, with peaks at 2.45 and 2.19, with the latter a typical Transamazonian age. Two samples from the Sierras Bayas Group in Tandilia (Argentina) show different age spectra. Sandstones of the Villa Mónica Formation show a unimodal zircon population of Transamazonian age (peak at 2.14 Ga). Sandstones of the Cerro Largo Formation are characterized by a dominant Transamazonian zircon population (peaks at 2.15, 2.0 and 1.79), but also important Archean-lowermost Palaeoproterozoic (3.33, 2.99, 2.7, 2.47 Ga) and Mesoproterozoic peaks (1.56, 1.24 and 1.07).

The abundance of Mesoproterozoic detrital zircons is surprising, given the limited outcrop area of Mesoproterozoic units in the RPC. Thus, the Mesoproterozoic orogenic event responsible for the generation of the dextral Sarandí del Yí megashear was much more important than previously assumed. Archean rocks of the RPC crop out only in the Nico Pérez Terrane in Uruguay. The presence of Archean zircons in sandstones of the Sierras Bayas Group in Argentina suggest that the Nico Pérez Terrane was much closer to Tandilia than it is today. Finally, the conspicuous absence of Neoproterozoic zircons confirm other lines of evidence suggesting that the studied units were deposited in a stable continental margin.