Archean crust in the Rio de la Plata Craton, Uruguay - SHRIMP U-Pb zircon reconnaissance geochronology

Two Archean units were detected in the center of the Nico Perez Terrane of the Rio de la Plata Craton in Uruguay, the La China Complex, a deformed granite-greenstone belt and the Las Tetas Complex, a deformed sedimentary platform cover. Reconnaissance dating of zircons from three samples by U-Pb sensitive high-mass resolution ion microprobe (SHRIMP) indicates that the two units are amongst the oldest Archean continental remnants in South America. The La China metatonalite has zircons with igneous cores (3.41 Ga) with metamorphic rims between 3.10 and 2.7 Ga. A muscovite quartzite from the Las Tetas Complex. has detrital zircons with ages between 3.26 and 3.14 Ga, while the main source of the Las Tetas metaconglomerate was dated by zircon at about 2.76 Ga. The interpretation is that a similar to 3410 Ma greenstone belt was thrust-stacked with a 2.7 Ga carbonate-quartzite -pelite-conglomerate platform cover, so that granite-greenstone piles are positioned on top of cover rocks. The La China greenstone belt has intercalations of subvertical decameter-sized layers of talc-chlorite-tremolite schists with mafic and ultramafic amphibolites, a presumed volcanic komatiite/basalt sequence. The Las Tetas platform cover is a layered sequence of metamorphosed limestones, quartzites, conglomerates and pelites. Stomatolites occur in the carbonate sequence, whereas the pelites are now staurolite schists. The characterization of Archean continental remnants in the center of the Nico Perez Terrane requires a reevaluation of the internal structure of the Rio de la Plata Craton. Important consequences emerge from this investigation for the understanding of the oldest environments in the South American crust and for the reconstruction of Archean paleocontinents. (C) 2001 Elsevier Science Ltd. All rights reserved.