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 gra nite-greenstone belt and the Las Tetas Complex, a deformed sedimentary plat form cover. Reconnaissance dating of zircons from three samples by U-Pb sen sitive 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 me tamorphic 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, whi le 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-conglomer ate platform cover, so that granite-greenstone piles are positioned on top of cover rocks. The La China greenstone belt has intercalations of subvertical decameter-si zed layers of talc-chlorite-tremolite schists with mafic and ultramafic amp hibolites, a presumed volcanic komatiite/basalt sequence. The Las Tetas pla tform cover is a layered sequence of metamorphosed limestones, quartzites, conglomerates and pelites. Stomatolites occur in the carbonate sequence, wh ereas the pelites are now staurolite schists. The characterization of Archean continental remnants in the center of the N ico Perez Terrane requires a reevaluation of the internal structure of the Rio de la Plata Craton. Important consequences emerge from this investigati on for the understanding of the oldest environments in the South American c rust and for the reconstruction of Archean paleocontinents. (C) 2001 Elsevi er Science Ltd. All rights reserved.