EVOLUTION OF THE SUNSÁS OROGEN, A GRENVILLIAN EQUIVALENT IN SOUTH AMERICA, BASED ON DETRITAL ZIRCON U-PB GEOCHRONOLOGY

The Sunsás Orogen in western Amazon Craton extends from northeast Argentina to Eastern Venezuela and is best exposed along the Brazil-Bolivia border. We integrate earlier U-Pb SHRIMP data with 16 samples of sandstones and meta-sandstones from three groups of basins: pre-Sunsás, late-Sunsás, and post-Sunsás basins. The post-Sunsás sandstones are the Palmeiral and Apiacás formations (five samples, 133 grains), the late-Sunsás sandstones come from the Nova Brasilândia, Huanchaca, and Aguapeí groups (nine samples, 195 grains), and the underlying sandstones come from the Beneficente Group (two samples, 57 grains). We found that: 1) The sandstones of the underlying Beneficente Group are mostly derived from Archean crust and from rocks formed between 1790-1740 Ma. These ages are comparable to the Roosevelt Group and the Jamari Complex (1740 Ma) exposed to the south and southwest, and to those of the Colíder-São Pedro magmatic arc of Juruena Domain (1790 Ma) to the southeast. 2) The late-Sunsás sandstones are chiefly derived from rocks of the Candeias orogeny (1360-1325 Ma), followed by some zircons from the Santa Helena orogeny (1470-1420 Ma), the Serra Providência Suite (1550-1530 Ma), the San Andrés orogeny (1280 Ma), and a few from the Rondônia-Juruena Province (1750-1720 Ma). 3) The post-Sunsás sandstones are derived from granites of the Serra Providência Suite (1570-1520 Ma, 45 grains) and the Candeias Orogeny (1390-1320 Ma, 26 grains). Other sources are rocks of the Santa Helena orogeny (1470-1420 Ma, 14 grains), the Juruena magmatic arc (1810-1760 Ma, 14 grains), the San Andrés orogeny (1285-1240 Ma, ten grains), and the Rondônia suite (1049 -1020 Ma, five grains). Our main conclusions are: a) Archaean crust was exposed in the vicinity of the Beneficente basin between 1740-1700 Ma. b) Almost all the detrital zircon from late-Sunsás and post-Sunsás sandstones is from nearby sources or from within the orogen itself. This suggests that the Sunsás orogen was a convergent margin free of drifting microcontinents. c) The post-Sunsás sandstones are almost entirely derived from the Sunsás orogen and imply that they were deposited in foreland basins. d) The scarcity of zircon younger than 1100 Ma suggests that the equivalent to the Ottawan orogeny of Laurentia is not well developed or did not occur in the western Amazonia.